sensors Review
Advanced Spatial-Division Multiplexed Measurement Systems Propositions—From Telecommunication to Sensing Applications: A Review Yi Weng 1,2, *, Ezra Ip 1 , Zhongqi Pan 2 and Ting Wang 1 1 2
*
NEC Laboratories America, Inc., Princeton, NJ 08540, USA;
[email protected] (E.I.);
[email protected] (T.W.) Department of Electrical & Computer Engineering, University of Louisiana at Lafayette, Lafayette, LA 70504, USA;
[email protected] Correspondence:
[email protected]; Tel.: +1-609-520-1555; Fax: +1-609-951-2481
Academic Editors: Manuel Lopez-Amo, Jose Miguel Lopez-Higuera and Jose Luis Santos Received: 19 June 2016; Accepted: 24 August 2016; Published: 30 August 2016
Abstract: The concepts of spatial-division multiplexing (SDM) technology were first proposed in the telecommunications industry as an indispensable solution to reduce the cost-per-bit of optical fiber transmission. Recently, such spatial channels and modes have been applied in optical sensing applications where the returned echo is analyzed for the collection of essential environmental information. The key advantages of implementing SDM techniques in optical measurement systems include the multi-parameter discriminative capability and accuracy improvement. In this paper, to help readers without a telecommunication background better understand how the SDM-based sensing systems can be incorporated, the crucial components of SDM techniques, such as laser beam shaping, mode generation and conversion, multimode or multicore elements using special fibers and multiplexers are introduced, along with the recent developments in SDM amplifiers, opto-electronic sources and detection units of sensing systems. The examples of SDM-based sensing systems not only include Brillouin optical time-domain reflectometry or Brillouin optical time-domain analysis (BOTDR/BOTDA) using few-mode fibers (FMF) and the multicore fiber (MCF) based integrated fiber Bragg grating (FBG) sensors, but also involve the widely used components with their whole information used in the full multimode constructions, such as the whispering gallery modes for fiber profiling and chemical species measurements, the screw/twisted modes for examining water quality, as well as the optical beam shaping to improve cantilever deflection measurements. Besides, the various applications of SDM sensors, the cost efficiency issue, as well as how these complex mode multiplexing techniques might improve the standard fiber-optic sensor approaches using single-mode fibers (SMF) and photonic crystal fibers (PCF) have also been summarized. Finally, we conclude with a prospective outlook for the opportunities and challenges of SDM technologies in optical sensing industry. Keywords: optical fiber sensors; multiplexing; Brillouin scattering; structural health monitoring; distributed sensors; optical fabrication; birefringence; acoustic wave; fiber Bragg grating; optical time domain reflectrometer (OTDR)
1. Introduction 1.1. Background Introduction From the perception of power consumption per bit, the logarithmical channel capacity scaling of up-to-date wavelength-division multiplexed (WDM) coherent optical communication systems Sensors 2016, 16, 1387; doi:10.3390/s16091387
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has exhausted nearly all possible degrees of freedom, including time, frequency, polarization and phase in single-mode fibers (SMF), and thus can no longer satisfy the ever-increasing demand of exponential global traffic growth [1,2]. To accomplish a further cost-effective scaling in system capacity, space-division multiplexing (SDM) has been proposed as a new paradigm for optical fiber communication research which has attracted loads of attention in the past few years [3,4]. These SDM technologies allow autonomous data streams to be transmitted in parallel spatial channels, which primarily include core multiplexing using multicore fibers (MCF) with a single-strand of fiber with many independent cores to pass through, and mode-division multiplexing (MDM) using multimode fibers (MMF) or few-mode fibers (FMF) whereas one single-core large-area fiber allows a number of spatial guiding modes to travel inside [5,6]. In the intervening time, the optical fiber sensors have been extensively developed owing to their outstanding advantages of high reliability and compactness over the past few decades, whereas SMFs and photonic crystal fibers (PCF) were commonly deployed [7]. Recently SDM-based fiber-optic sensors have attracted broad attentiveness attributable to their potentially higher capacities, sensitivity and flexibilities, compared with conventional SMF-based sensors, via exploring the fifth dimension—the space dimension [8]. Besides, the definition of SDM not only include the spatial mode information in FMF or MCF, but also involve the widely used components with their whole information used in the full multimode constructions, such as the whispering gallery modes for fiber profiling and chemical species measurements, the screw/twisted modes for examining water quality, as well as the optical beam shaping to improve cantilever deflection measurements [9–11]. 1.2. Spatial Division of Information Models for Fiber-Optic Sensing Components This subsection introduces the possible spatial division of information models fit for use in fiber-optic sensing components. Please note that, although the concepts of spatial channels and modes were first utilized in the telecommunications industry as an indispensable solution to reduce the cost-per-bit of optical fiber transmission, these valuable concepts have been explored recently in other areas of science and engineering, from a fundamental principle point of view [12,13]. Particularly, the spatial modes are applied in fiber-optic sensing applications, as high-speed illuminating signals containing fast-varying data distribution, with the returned echo is then analyzed for the collection of essential environmental information [14,15]. To begin with, the transverse field of linearly polarized (LP) modes in the fiber core E (r, θ) is given by [16]: JP ( k r r ) E (r, θ ) = E ( a) · · cos ( pθ ) . (1) JP ( k r a ) where r and θ denote polar coordinates, JP symbolizes the Jones vector, a stands for the fiber core radius, p represents the azimuthal mode number. The non-negative integer kr can be expressed as [17]: 1 2 kr = k2 nCore 2 − β2 .
(2)
where nCore is the refractive index of the fiber core, which determines the material dispersion along the fiber. The propagation constant β determines how fast electric vectors are oscillating during propagation through the optical fiber, which can be written as [18]:
β = ∆−∆·
U2 V2
+ 1 · nCore · k.
(3)
nCore nClad / nCore .
(4)
where nClad denotes the refractive index of the homogeneous cladding. The normalized frequency V can be described as [19]: 1
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where λ stands the wavelength. modal Furthermore, profilethe of normalized a graded-index fiber core n(r) is where U denotesfor the dimensionless number,the V index symbolizes frequency, k denotes given by [20]: the free-space wave number. The relative refractive index of the fiber ∆ is calculated as: 1
n rCore . 2 (4) ) /n Core − Clad (6) 1 2 n ∆r = (nnCore . a where nClad denotes the refractive index of the homogeneous cladding. The normalized frequency V can be α described where signifies as the[19]: power coefficient fiber profile. of the graded-index 1 2π 2 2 2 an The amount of guided modes V =propagating · a · invariant nCore − nalong . optical fiber can be determined (5) Clad λ by the normalized frequency V, as the solution of wave-equation describing an electro-magnetic
field The transverse intensitythe distribution diverges strongly along thecore FMF, wheredistribution λ stands for[21,22]. the wavelength. Furthermore, index profile of a graded-index fiber n(r)for is each givenspatial by [20]:mode propagates at a different phase velocity [23–25]. However, to excite a single h r pattern α i 12 higher-order mode in the fiber, a stable transverse beam is required [26,27]. Figure 1 n (r ) = nCore · 1 − 2 · ∆ · . (6) exemplifies the dependency of the normalized group delaya upon the normalized frequency for the αfirst four LP is expressed as [28]: where signifies themodes, power which coefficient of the graded-index fiber profile. The amount of guided modes propagating invariant along an optical fiber can be determined by dbeff the normalized frequency V, as the solution wave-equation bg bof (7) V . describing an electro-magnetic field eff dV diverges strongly along the FMF, for each distribution [21,22]. The transverse intensity distribution spatial mode propagates at a different phase velocity [23–25]. However, to excite a single higher-order In the meantime the corresponding normalized propagation constant beff versus is shown in mode in the fiber, a stable transverse beam pattern is required [26,27]. Figure 1 exemplifies the Figure 2, which regulates the dispersion properties of various fiber modes as attained in the dependency of the normalized group delay bg upon the normalized frequency V for the first four LP following equation [29]: modes, which is expressed as [28]:
beff nCladdbe/f f nCore . bg = · V. bke ff + dV
(8) (7)
Figure 1. Normalized group delay b vs. the normalized frequency V. Figure 1. Normalized group delay g vs. the normalized frequency .
In the meantime the corresponding normalized propagation constant beff versus V is shown in Figure 2, which regulates the dispersion properties of various fiber modes as attained in the following equation [29]: β be f f = − nClad / (nCore · ∆) . (8) k
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Figure Normalizedpropagation propagation constant beff Vvs.for LP V for LP under modesweakly under coupling weakly Figure 2. 2. Normalized constant beff vs. modes coupling approximation. approximation.
1.3.Benefits BenefitsofofSDM SDMSensing SensingSystems Systems 1.3. In addition addition to to the the original original advantages advantages of of distributed distributed and and “smart” “smart” fiber-optic fiber-optic sensors, sensors, which which In include the immunity to electromagnetic interference, the avoidance of electric sparks, as well asthe the include the immunity to electromagnetic interference, the avoidance of electric sparks, as well as resistancetoto harsh hazardous environments, the benefits unique of benefits of implementing SDM resistance harsh andand hazardous environments, the unique implementing SDM techniques techniques in fiber sensors are summarized in the following. in fiber sensors are summarized in the following. 1.3.1. 1.3.1.Measuring MeasuringMore MoreParameters Parameters In fiber-optic sensors, each technique is often applied to the Inconventional conventionalSMF-based SMF-baseddistributed distributed fiber-optic sensors, each technique is often applied to measurement of one single parameter, because each of the parallel sensing signals requires a separate the measurement of one single parameter, because each of the parallel sensing signals requires a channel. instance, temperature sensing (DTS) system based on Raman is separate For channel. Fordistributed instance, distributed temperature sensing (DTS) system basedscattering on Raman only dedicated to determine local temperature, while distributed acoustic sensing (DAS) system based scattering is only dedicated to determine local temperature, while distributed acoustic sensing on Rayleigh scattering mostly provides strainmostly determinations Todeterminations break this bottleneck and further (DAS) system based on Rayleigh scattering provides[30]. strain [30]. To break this extend the functionality distributed sensors, the SDM techniques have been introduced bottleneck and furtherofextend the fiber-optic functionality of distributed fiber-optic sensors, the SDM for the capability responding to afor wide of measurands simultaneously, for each the modes techniques have of been introduced thevariety capability of responding to a wide variety ofofmeasurands or cores within the medium can serve as anwithin orthogonal interrogator or geophone simultaneously, forsensing each of the modes or cores the sensing medium can serveforasone an particular sensing parameter. example, mentioned below, when parameter. minimum two modes orthogonal interrogator or For geophone forasone particular sensing Forspatial example, as are used to separate the strain and temperature variations, the rest of modes can be further utilized to mentioned below, when minimum two spatial modes are used to separate the strain and monitor othervariations, physical changes as pressure, acceleration, [31]. temperature the restsuch of modes can bedisplacement, further utilized to monitoretc. other physical changes such as pressure, displacement, acceleration, etc. [31]. 1.3.2. Multi-Parameter Discriminative Capability
1.3.2.As Multi-Parameter Discriminative Capability for the multi-parameter discrimination issue, SMF similarly has its own limitation. For instance, the most method using a SMF is to measure bothSMF the Brillouin shiftlimitation. (BFS) and the As common for the multi-parameter discrimination issue, similarlyfrequency has its own For Brillouin power level, as Brillouin power is also related to strain and temperature. The different instance, the most common method using a SMF is to measure both the Brillouin frequency shift dependencies of the BFS peaks areascalculated to distinguish between temperature and strain. (BFS) and the Brillouin power level, Brillouin power is also related to strain and temperature. The Nevertheless, the measuring range and resolution of this method are mainly limited by the imprecise different dependencies of the BFS peaks are calculated to distinguish between temperature and Brillouin power measurement [32]. Inrange another approach, bothmethod Ramanare andmainly Brillouin signals strain. Nevertheless, the measuring andSMF resolution of this limited by are the spatially resolved to separate temperature and strain. The magnitude of anti-Stokes Raman signal imprecise Brillouin power measurement [32]. In another SMF approach, both Raman and Brillouin intensity determines temperature, while the strain can then computed from BFS. However, noise signals are spatiallythe resolved to separate temperature andbe strain. The magnitude of anti-Stokes arises mainly the Raman intensity Besides, this approach both direct Raman signalfrom intensity determines the measurement. temperature, while the strain can thenrequires be computed from detection and coherent detection system components that add additional cost and complexity to the BFS. However, noise arises mainly from the Raman intensity measurement. Besides, this approach sensing system [33]. Other groups proposed to utilize large effective-area fiber (LEAF) to achieve requires both direct detection and coherent detection system components that add additional cost simultaneous temperature andsystem strain [33]. sensing, which creates multiple BFSs large within one single fiber and complexity to the sensing Other groups proposed to utilize effective-area fiber core. Nonetheless, this approach leads to poor spatial resolution, limited sensing accuracy and short (LEAF) to achieve simultaneous temperature and strain sensing, which creates multiple BFSs within sensing distance due toNonetheless, large interference betweenleads different wavelengths [34]. Henceforward, the one single fiber core. this approach to poor spatial resolution, limited sensing accuracy and short sensing distance due to large interference between different wavelengths [34]. Henceforward, the above-mentioned problems can be resolved using SDM techniques, because as
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above-mentioned problems can be resolved using SDM techniques, because as explained below, each spatial mode or core possesses a unique Brillouin gain spectrum (BGS) or BFS, with which temperature, strain or other parameters can be accurately separated by solving a set of simultaneous equations. Such exceptional multi-parameter discriminative capability is actually contingent on the correlation in-between spatial modes and/or cores as parallel sensors, with various parameters separated using massive multi-input multi-output digital signal processing (MIMO-DSP) solutions [35]. 1.3.3. Accuracy Improvement Another key advantage of SDM techniques is focused on the accurate detection of the backscattered signal and elimination of noise. The conventional SMF techniques are not effective in reducing the coherent Rayleigh noise (CRN) or fading noise [36]. Since FMF or MCF has a rather short coherence time and coherence length, so the superposition will be incoherent and thus CRN negligible. The noise can be further eliminated by using the frequency shift averaging (FSAV) techniques [37]. Since SDM-based sensors have the ability to be easily multiplexed with digital signal processing to determine the positional variation of the measured field along the interaction length with different groups of modes, two or more different spatial modes can be used to do error correction upon the same channel, thus enabling fiber sensing systems capable of performing much more sophisticated and multifunctional types of measurements with higher spectral resolution and faster time response that previously were only achievable using electronic sensors [38]. For example, with six spatial modes, at most three modes can be applied to measure the temperature, and the other three modes for monitoring the strain. This is analogous to having three independent SMFs determining one physical change, respectively, which will significantly improve the temperature and/or strain measurement accuracy. 1.3.4. Detection Speed Enhancement Detection speed is another important performance aspect for industrial sensing applications, such as oil and gas production monitoring [30,39]. As we know, finding oil leaks does little good if it takes more than several minutes of computer processing to identify them. In SDM measurement systems, the overall number of parallel channels is largely increased with each independent channel on an orthogonal spatial mode, thus enabling higher data rate and real-time sensing for a variety of applications such as well integrity monitoring and down-hole seismic acquisition [40]. For instance, compared to the conventional time-consuming SMF techniques, where several different types of lasers are prepared and alternatively used in order to compare their performances like the hybrid Raman-Brillouin sensing, the SDM approach can simply use two or more spatial modes to simultaneously measure strain and temperature, reducing the process time to about 30 seconds and making the whole process real-time monitoring [39,41]. In this paper, the recent progress in SDM-based sensing systems is reviewed in terms of their operation principles, fabrication methods, experimental design and sensing applications. The outline of the paper is laid out as follows: it starts by introducing the principal components of SDM techniques in Section 2, comprising laser beam shaping, mode conversion, multiplexers, multicore head of sensor elements using LPG and other specific fibers, SDM amplifiers and EDFAs, as well as the detection units of SDM measurement systems. Section 3 describes different examples of SDM-based sensor techniques, including Brillouin optical time-domain reflectometry/Brillouin optical time-domain analysis (BOTDR/BOTDA) using FMF, as well as the MCF-based integrated fiber Bragg grating (FBG) sensors. Section 4 presents the overall summary, comparison and concluding remarks of this paper, dedicated to provide a prospective outlook for the opportunities and challenges of SDM sensing technologies for various markets and applications. 2. Key Components of SDM Technique As discussed in Section 1, the SDM-based optical measurement systems may provide discriminative capability, higher sensitivity and flexibilities, while keeping the fabrication cost at
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a relatively low16,level. Sensors 2016, 1387 Therefore, it is critical to design and fabricate corresponding SDM components 6 of 34 with proper modal properties to support these novel sensing systems. This section illuminates relatively low level. Therefore, it is critical to design and fabricate corresponding SDM components a number of essential SDM components, such as laser beam shaping devices, mode convertors, with proper modal properties to support these novel sensing systems. This section illuminates a multiplexers, amplifiers. It also gives a brief discussion on multicore head of sensor elements, the number of essential SDM components, such as laser beam shaping devices, mode convertors, detection units of SDM measurement systems and the corresponding signal processing algorithms. multiplexers, amplifiers. It also gives a brief discussion on multicore head of sensor elements, the detection units of SDM measurement systems and the corresponding signal processing algorithms.
2.1. Laser Beam Shaping
2.1. Laser Beam Shapingof SDM reply on the rapid development of efficient and automated spatial The key techniques filtering designs, which converts input Gaussian beams into outputs via laser The key techniques of SDMthe reply on the rapid development of desirable efficient and automated spatialbeam filtering designs, which the all-fiber input Gaussian beams into desirableoptical outputsdevice, via laser shaping approaches [42,43].converts With an beam shaper or similar thebeam concave approaches [42,43]. With mode an all-fiber shaper orfibers similarcan optical device, the concave cone shaping tip at the end face of single andbeam multimode be inverse etched, socone that the tip at the end face of single mode and multimode fibers can be inverse etched, so that the light beam light beam can be reshaped with its spatial properties modified [44,45]. These techniques offer can be reshaped its spatial properties modified [44,45]. such Theseastechniques offer extensive extensive coverage of with practical laser beam shaping applications mode converters and spatial coverage of practical laser beam shaping applications such as mode converters and spatial multiplexers/de-multiplexers [46]. The intensity profile distributions of the first six ideal LP spatial multiplexers/de-multiplexers [46]. The intensity profile distributions of the first six ideal LP spatial modes are shown in Figure 3. They are shaped due to slightly dissimilar propagation constants modes are shown in Figure 3. They are shaped due to slightly dissimilar propagation constants between the vector modes, resulting in the cross-sectional intensity pattern rotation between the vector modes, resulting in the cross-sectional intensity pattern rotationofofLP LPmodes modesalong the optical fiber, whereas elliptic specifies the core-cladding interface [47]. along the optical fiber, the whereas theboundary elliptic boundary specifies the core-cladding interface [47].
Figure 3. Intensity profile distributions of ideal LP spatial modes.
Figure 3. Intensity profile distributions of ideal LP spatial modes.
Besides the most commonly used LP modes, the other types of spatial modes applied in SDM
Besides mostthecommonly used LP modes, other types of spatial modes modes, applied in systems the include supermodes, principle modes,thetransverse modes, screw/twisted gallery the modes, as well asprinciple the modes of capillary optical fibers screw/twisted [48]. The so-called SDMwhispering systems include supermodes, modes, transverse modes, modes, supermodes indicate the different transfer between cores in MCF [49];[48]. whileThe to reduce whispering gallery modes, as wellscale as of thepower modes of capillary optical fibers so-called the negative impact modal dispersion in FMF, the principle modes for a[49]; basis of spatial supermodes indicate theofdifferent scale of power transfer between coresstand in MCF while to reduce modes which are free of modal dispersion to the first order in frequency [50,51]. The transverse the negative impact of modal dispersion in FMF, the principle modes stand for a basis of spatial modes, including both the transverse electric (TE) and transverse magnetic (TM) polarization modes which are free of modal dispersion to the first order in frequency [50,51]. The transverse modes, modes, are more fundamental propagation modes with their electric and magnetic field lines including both the transverse electric (TE) and transverse magnetic (TM) polarization modes, are more restricted to directions normal to the direction of modal propagation, whereas complex spatial filters fundamental propagationbymodes with optical their electric andcorresponding magnetic field restricted to directions can be implemented diffractive elements to lines rotationally symmetrical normal to the direction of modal propagation, whereas complex spatial filters can be implemented transverse modes [52]. The screw/twisted modes include the celebrated orbital angular momentum by diffractive optical elements corresponding to rotationally symmetrical transverse modes [52]. (OAM) states or vortex modes, which have a variety of applications from atmospheric turbulence monitoring, lateral motion detecting, and biomedical sensing [53–55]. (OAM) Whispering gallery The screw/twisted modes include the celebrated orbitalimage angular momentum states or vortex modes are shaped by microscopic glass spheres from the micro-cavities of resonant optical sensors, modes, which have a variety of applications from atmospheric turbulence monitoring, lateral motion whichand canbiomedical travel around concave surfaces the applications frequency-comb detecting, image sensing [53–55].forWhispering galleryofmodes are shapedgeneration, by microscopic opto-mechanical cooling as well as chemical species sensing [56,57]. Last but not least, the modes of glass spheres from the micro-cavities of resonant optical sensors, which can travel around concave micro-structured capillary optical fibers include LP modes, TE modes, and TM modes with low surfaces for the applications of frequency-comb generation, opto-mechanical cooling as well as modal confinement losses and group velocity dispersion, shaped by capillaries filled narrowly chemical sensing[58,59]. [56,57]. Last but not least, the modes of micro-structured capillary optical insidespecies round cavities fibers include LP modes, TE modes, and TM modes with low modal confinement losses and group velocity shaped by capillaries filled narrowly inside round cavities [58,59]. 2.2. dispersion, Mode Generation and Conversion The next issue would be spatial mode generation and conversion for SDM systems. To begin with, the LP modes can be converted by imposing spatially varying modulation upon the laser
2.2. Mode Generation and Conversion
The next issue would be spatial mode generation and conversion for SDM systems. To begin with, the LP modes can be converted by imposing spatially varying modulation upon the laser beams
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beams via variable phase/amplitude masks [60,61]. For instance, light modulators is via variable phase/amplitude masks [60,61]. For instance, spatialspatial light modulators (SLM)(SLM) is capable of transforming a fundamental LP01into mode into higher crystal on ofcapable transforming a fundamental LP01 mode higher order order modesmodes usingusing liquidliquid crystal on silicon siliconpanels (LCoS)orpanels or thin phase with prescribed spatial distributions of index refractive (LCoS) thin phase plates withplates prescribed spatial distributions of refractive [62,63]. index [62,63]. Also, LPbemodes couldusing be generated using fiber Bragg fused fiber Also, higher-order LPhigher-order modes could generated fiber Bragg grating, fusedgrating, fiber coupler, as well as well as intermodal four-wave mixing supermodes, [64–66]. Similarly, supermodes, principle modes ascoupler, intermodal four-wave mixing [64–66]. Similarly, principle modes and transverse modes and transverse modes can be rehabilitated via an optically induced long-period grating (LPG) or thin can be rehabilitated via an optically induced long-period grating (LPG) or thin phase plates [67–69]. phase plates [67–69]. The flexible conversion among or multiple vortex by modes can be The flexible conversion among multiple OAM modes vortex OAM modesmodes can beor realized the cylindrical realized by the cylindrical lenses or the helical gratings (HGs) with both transverse and longitudinal lenses or the helical gratings (HGs) with both transverse and longitudinal modulation, or by an optical modulation, or by an optical parametric oscillator based on cavity and anisotropy effects [70,71]. The parametric oscillator based on cavity and anisotropy effects [70,71]. The whispering gallery modes whispering gallery modes can usually be configured through a whispering gallery mode resonator can usually be configured through a whispering gallery mode resonator in a tapered fiber with low in a tapered fiber with low scattering-loss and easy alignment, while the modes of capillary optical scattering-loss and easy alignment, while the modes of capillary optical fibers can be shaped and fibers can be shaped and changed using a capillary tapered mode converter filled inside round changed using a capillary tapered mode converter filled inside round cavities [72,73]. cavities [72,73].
2.3. Multiplexers and De-Multiplexers 2.3. Multiplexers and De-Multiplexers A mode multiplexer (M-MUX) can launch all the spatial modes into the FMFs or MCFs, A mode multiplexer (M-MUX) can launch all the spatial modes into the FMFs or MCFs, whereas whereasthe thespatial spatiallight lightmodulators modulators(SLM) (SLM)based basedon onliquid liquidcrystal crystalon onsilicon silicon(LCoS) (LCoS)can canconvert convert the fundamental mode in the SMF into arbitrary desirabledesirable spatial modes [74–76]. typicalThe configuration the fundamental mode in the SMF into arbitrary spatial modesThe [74–76]. typical ofconfiguration an M-MUX isofpresented in Figure 4, which uses three programmable SLMs to generate specific an M-MUX is presented in Figure 4, which uses three programmable SLMs to LP modes [77]. The relationship generatedbetween spatial modes in the fiber andmodes their corresponding generate specific LP modes between [77]. The the relationship the generated spatial in the fiber SLM patterns is provided in Figure In Figurein4b, SLM3 for launching and phase their corresponding SLM phase patterns4a. is provided Figure 4a. is In set Figure 4b, SLM3 isthe set pump for power into athe spatial SLM2 are configured for the detection of back-scattered light launching pumpmode, powerSLM1 into aand spatial mode, SLM1 and SLM2 are configured for the detection of in separable modes, while two half-wave-plates (HWPs) are placed after SLM1 and SLM3 to switch each back-scattered light in separable modes, while two half-wave-plates (HWPs) are placed after SLM1 andofSLM3 to switch statemay of also polarization. may also serve asif athemode state polarization. Thiseach M-MUX serve as a This modeM-MUX de-multiplexer (M-DMUX) beam is de-multiplexer if the beam is launched thru the opposite direction. Furthermore, in launched thru the(M-DMUX) opposite direction. Furthermore, in order to further reduce the passive multiplexing order to further reduce the passive multiplexing loss, the photonic lantern (PL) is purposed loss, the photonic lantern (PL) is purposed to transfer the transverse field into super-modestoof a transfer the transverse into super-modes a three-coupled-core and then intoembodiment the FMF three-coupled-core fiber,field and then into the FMFof mode profiles using anfiber, integrated-optics profiles using an integrated-optics embodiment of the spot coupler [78,79]. ofmode the spot coupler [78,79].
Figure 4. (a) Intensity distributions of the first five spatial modes in FMF and their corresponding SLM Figure 4. (a) Intensity distributions of the first five spatial modes in FMF and their corresponding phase patterns; (b) Schematic diagram of the free-space SLM-based M-MUX. CL1~CL3: collimating SLM phase patterns; (b) Schematic diagram of the free-space SLM-based M-MUX. CL1~CL3: lens, M1/M2: turning mirrors, HWP1/HWP2: half wave-plates, BS: beam-splitter, PBS: polarizing collimating lens, M1/M2: turning mirrors, HWP1/HWP2: half wave-plates, BS: beam-splitter, PBS: beam-splitter [77]. polarizing beam-splitter [77].
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2.4. Multicore Elements Using Special Fibers The SDM sensing technologies can be further advanced via multi-core approaches to achieve better system scalability and flexibility along with combination of multi-level modulation. This subsection mainly describes the multi-core elements for addressing individual cores in optical sensors. For specialty sensing MCFs with enhanced spatial-channel densities, by using adhesive and heating up the fiber bundle in the capillary, a more compact configuration with fiber cores fixed in the capillary can be achieved compared to just a bundle of SMFs [80]. When the light is introduced into the cores of MCFs by separate light sources, the alignment of tapered MCF and SMF can be used to control the division of light power among the non-uniformly distributed fiber cores in the cross-section of MCF [81]. Since there is no overlapping between modes propagating in the discrete cores of MCF, each with identical transverse-mode profiles, the MCF-based SDM measurement systems are assumed hypothetically to be with ignorable loss at multiplexers/de-multiplexers, thus provide substantial improvements in the signal-to-noise ratio of backscattered sensing signals [82]. One possible drawback could be the additional complexity and cost of transmitting and detecting signals from different cores to accomplish simultaneous measurements. Thanks to the stack and draw procedure generally used for the PCF fabrication, the multi-core elements can be manufactured at a relatively acceptable price. 2.5. Multicore Head of Sensor Elements Using LPG and Other Specific Fibers As a crucial component of MCF-based SDM sensing systems, the multicore head of sensing elements can be fabricated based on the amorphous wire magneto-impedance elements in combination with a complementary metal-oxide-semiconductor (CMOS) pulse sensor circuit [83]. Such design can be used to develop a highly sensitive magneto-impedance sensor with low noise and stable pico-tesla resolution. Another way to design such multicore head of sensing elements is to UV-inscribe a long period grating (LPG) into MCF, which is fusion spliced into SMFs at both ends [84]. Such configuration leads to a taper transition between MCF and SMF, and creates a non-adiabatic mode evolution, whose spectral characteristics can be used for highly sensitive curvature sensing applications [85]. The MCF-based SDM sensing systems can be employed for a variety of sensing applications. For example, the bending radius as well as the orientation of bending plane can be measured. The bending direction and the amount of deflection can be detected by the outer cores, while the center core provides the reference level for it has the lowest bending loss in the middle of the cross-section of MCF [86]. Besides bend/shape sensing, another significant sensing application of MCF-based SDM measurement systems is simultaneous multi-parameter sensing by utilizing one single optical fiber, since each core can be designed sensitive to different external factor such as strain, temperature or pressure. For instance, the phase shift of far-field interferometric grid-pattern can be generated as a function of curvature, twisting angle and temperature gradient by a four-beam interferometer using MCF for prospective applications in smart structural health monitoring [87]. 2.6. Single-Core Multimode Elements as Asymmetrical Coupler Moreover, single-core multimode elements as asymmetrical coupler serve as a significant optical component in SDM-based measuring systems, because the efficiency of the asymmetrical coupler determines the power budget and the quality of optical signal passed to the detection unit, thus affecting the overall sensitivity of detection block [88]. Such asymmetrical coupler can be fabricated via a fusion-tapering technique by stripping off the polyethylene jacket and gluing upon a glass substrate so as to allow maximum multi-mode signal extraction [89]. The constructional parameters of thus-designed asymmetrical coupler include the length/depth of the coupling area as well as the curvature radius, whereas the sensor head is mounted on a mini-lift and formed by the end of a large-core polymer optical fiber [90].
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2.7. SDM EDFAs 2.7. SDM Amplifiers Amplifiers and and EDFAs Inline amplifiers Erbium-doped fiber or Raman Raman amplifiers amplifiers are are sometimes sometimes Inline amplifiers as as Erbium-doped fiber amplifiers amplifiers (EDFA) (EDFA) or used in measurement systems where the signals are weak [91,92]. If these measurement systems are used in measurement systems where the signals are weak [91,92]. If these measurement systems extended to the spatial domain, thethe overall system performance of are extended to the spatial domain, overall system performancecould couldbebehindered hinderedby by the the use use of conventional amplifiers, because each spatial mode experiences a different value of optical gain conventional amplifiers, because each spatial mode experiences a different value of optical gain due due to distinctive profile configurations [93]. Therefore, to extend sensing reach and achieve to distinctive field field profile configurations [93]. Therefore, to extend sensing reach and achieve stable stable performance of SDM-based measuring systems, the design of SDM amplifiers would be performance of SDM-based measuring systems, the design of SDM amplifiers would be hypothetically hypothetically essential to the process. essential to the process. Up to to now, now, various various multimode multimode amplification amplification approaches approaches have have been been purposed purposed towards towards the the Up improvement of mode-dependent gain [94]. Though most of them were still mainly intended for improvement of mode-dependent gain [94]. Though most of them were still mainly intended transmission purposes, these amplifiers have great potential in various sensing and network for transmission purposes, these amplifiers have great potential in various sensing and network application, for instance, to to achieve achieve ultra-long ultra-long distance distance sensing sensing systems systems [95]. [95]. Firstly, few-mode application, for instance, Firstly, few-mode erbium-doped fiber amplifier (FM-EDFA) has been purposed to equally and efficiently amplify both erbium-doped fiber amplifier (FM-EDFA) has been purposed to equally and efficiently amplify both modes and wavelengths over the target band as a balance between differential group delay, noise modes and wavelengths over the target band as a balance between differential group delay, noise figure, crosstalk crosstalk and and cost cost efficiency efficiency [96]. [96]. It It has has been been experimentally experimentally demonstrated demonstrated that that by by adjusting adjusting figure, pump mode orientation, higher modal gain and favorable gain equalization can be realized for pump mode orientation, higher modal gain and favorable gain equalization can be realized for FM-EDFA, while while its its major major challenges challenges include include the the signal signal power power fluctuations fluctuations due due to to random random mode mode FM-EDFA, coupling (RMC) [97]. On the other hand, to tune the modal dependent gain over a dynamic range, coupling (RMC) [97]. On the other hand, to tune the modal dependent gain over a dynamic range, few-mode Raman favorable alternative alternative for for SDM SDM systems systems in in comparison comparison with with few-mode Raman amplifier amplifier serves serves as as aa favorable FM-EDFAs [98]. [98]. The The few-mode few-mode Raman Raman amplifier amplifier may may accomplish accomplish aa substantial substantial improvement improvement in in the the FM-EDFAs noise performance, for the distributed nature of Raman amplification allows a lower input signal noise performance, for the distributed nature of Raman amplification allows a lower input signal power [99]. [99].The Thetypical typical configuration of few-mode Raman amplifier is schematized in5Figure power configuration of few-mode Raman amplifier is schematized in Figure whereas5 whereas theand LPLP 11o/e and LP21o/e tributaries are also shown in the inset. The signal and pump lead the LP11o/e 21o/e tributaries are also shown in the inset. The signal and pump lead through the through the SLMs convert to higher-order while the modalofdependence of figure gain oratnoise SLMs to convert to to higher-order modes, whilemodes, the modal dependence gain or noise each figure at each output port of M-DMUX could be measured by an optical spectrum analyzer (OSA) [100]. output port of M-DMUX could be measured by an optical spectrum analyzer (OSA) [100].
Figure 5. Schematic of the few-mode distributed Raman amplifier. Figure 5. Schematic of the few-mode distributed Raman amplifier.
Moreover, have also been designed and and constructed for amplifying SDM Moreover,multicore multicorefiber fiberamplifiers amplifiers have also been designed constructed for amplifying signals, in order to minimize the noise figure while achieving large gain and broad band width, which SDM signals, in order to minimize the noise figure while achieving large gain and broad band is dependent the overlapon integral of the integral cladding-guided pump field andpump the doped [101,102]. width, whichon is dependent the overlap of the cladding-guided field cores and the doped Hypothetically, a FM-EDFA should be slightly more cost-efficient than a multi-core EDFA thanks to its cores [101,102]. Hypothetically, a FM-EDFA should be slightly more cost-efficient than a multi-core denser spatial packing of multiple channels [103]. EDFA thanks to its denser spatial packing of multiple channels [103]. 2.8. Opto-Electronic Sources and Detection Units of Sensing Systems 2.8. Opto-Electronic Sources and Detection Units of Sensing Systems This subsection describes a number of detectors and opto-electronic light sources for SDM This subsection describes a number of detectors and opto-electronic light sources for SDM measurement systems. The opto-electronic sources mainly include distributed feedback laser diodes measurement systems. The opto-electronic sources mainly include distributed feedback laser diodes (DFB lasers), vertical-cavity surface-emitting lasers (VCSEL), Fabry-Perot laser diodes (FP lasers), (DFB lasers), vertical-cavity surface-emitting lasers (VCSEL), Fabry-Perot laser diodes (FP lasers), Nd:YAG lasers, and quantum cascade lasers (QCLs) [104,105]. The development of SDM-based
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sensing pushing the boundaries of high-speed multi-wavelength opto-electronic sources Nd:YAGsystems lasers, is and quantum cascade lasers (QCLs) [104,105]. The development of SDM-based and modules, making other kinds of low-cost light sources possible for SDM. For instance, such sensing systems is pushing the boundaries of high-speed multi-wavelength opto-electronic sources opto-electronic sources other can bekinds integrated on the silicon platform on GaAs, and InP, etc. [106,107]. and modules, making of low-cost light sources possible for Si SDM. For instance, such The detection units can be divided into two groups, direct detectors and coherent detectors. opto-electronic sources can be integrated on the silicon platform on GaAs, Si and InP, etc. [106,107]. High-efficient directunits detection be achieved with avalanche-photodiode-array (APD-array) by The detection can becan divided into two groups, direct detectors and coherent detectors. adopting appropriate modulation [108]. Compared with non-coherent High-efficient direct detection canand be multiplexing achieved withtechniques avalanche-photodiode-array (APD-array) by direct-detection receivers, coherent receivers have plentiful advantages including remarkably adopting appropriate modulation and multiplexing techniques [108]. Compared with non-coherent improved selectivity and sensitivity, the costhave of higher computational In direct-detection receivers, coherent at receivers plentiful advantagescomplexity including[109–111]. remarkably digital coherent detection, in-phase and quadrature components of optical signals from improved selectivity and both sensitivity, at (I) the cost of higher(Q) computational complexity [109–111]. different mode channels are coherently and synchronously digitized using a carrier phase reference In digital coherent detection, both in-phase (I) and quadrature (Q) components of optical signals generated at the receiver, and then processed using digital signal processing (DSP), for the phase mode from different mode channels are coherently and synchronously digitized using a carrier coupling in the fiber is sensitive to the phase of the signals [112,113]. The coherent detection can be reference generated at the receiver, and then processed using digital signal processing (DSP), for the implemented using either homodyne detection or heterodyne detection. As a general rule, the mode coupling in the fiber is sensitive to the phase of the signals [112,113]. The coherent detection homodyne approachusing involves bandwidthdetection on the level of the symbol rateAs with two balanced can be implemented eithera homodyne or heterodyne detection. a general rule, the receivers; meanwhile heterodyne needs one balanced optical receiver with twice the electrical homodyne approach involves a bandwidth on the level of the symbol rate with two balanced receivers; bandwidth meanwhile [114]. heterodyne needs one balanced optical receiver with twice the electrical bandwidth [114]. In SDM sensing sensing systems, systems,since sincemode modecoupling couplingmight mightoccur occur between spatial modes induced In SDM between thethe spatial modes induced by by M-MUX/M-DMUX and/or FMF, multiple-input-multiple-output (MIMO) DSP is usually M-MUX/M-DMUX and/or FMF, multiple-input-multiple-output (MIMO) DSP is usually prerequisite prerequisite to de-multiplex signalsmodes on different modes andcompensate dynamically compensate much to de-multiplex the signals onthe different and dynamically much impairment in impairment in the electric domain [115–117]. Figure 6 illustrates the coherent receiver structure for a the electric domain [115–117]. Figure 6 illustrates the coherent receiver structure for a single carrier single with 6scheme, × 6 MIMO scheme, whereas the coefficient could be achieved systemcarrier with 6system × 6 MIMO whereas the coefficient adaptationadaptation could be achieved with the with the decision-directed (LMS) algorithm Suchtechniques multiplexing decision-directed least mean least squaremean (LMS)square algorithm [118,119]. Such[118,119]. multiplexing can techniques can achieve better signal-to-noise ratio (SNR) compareddecoding, with complementary achieve better signal-to-noise ratio (SNR) while compared withwhile complementary thus suitable decoding, thus sensing suitableapplications for distributed sensing applications [120,121]. for distributed [120,121].
Figure 6. 6. Typical Typical DSP DSP architecture architecture for for the the MIMO MIMO equalization. equalization. ADC: ADC: analog-to-digital analog-to-digital converter; converter; CD: CD: Figure chromatic dispersion. chromatic dispersion.
Examplesof ofSDM SDMBased BasedSensing SensingSystems Systems 3. Examples As described thethe general maturing of SDM leads to the potential developing describedearlier, earlier, general maturing of components SDM components leads to theofpotential of a highly sensitive andsensitive stable optical sensing system for multi-parameter sensing with discrimination developing a highly and stable optical sensing system for multi-parameter sensing with capability, which is suited which to structural health (SHM) monitoring systems in harsh discrimination capability, is suited to monitoring structural health (SHM)environment systems in applications, such as applications, temperature and/or sensingand/or for thepressure petroleum industry In this harsh environment such aspressure temperature sensing for[122,123]. the petroleum section, numerous SDM-based sensing techniques are explored, such as distributed sensors industry [122,123]. types In thisofsection, numerous types of SDM-based sensing techniques are explored, usingas FMFs and/or sensors MCFs, and discrete sensorsMCFs, based and on Fiber Bragg grating (FBG). the such distributed using FMFs and/or discrete sensors based on Besides, Fiber Bragg whispering gallery modesthe forwhispering fiber profiling and chemical species screw/twisted grating (FBG). Besides, gallery modes for fibermeasurements, profiling andthe chemical species
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modes for examining water quality, and the optical beam shaping to improve cantilever deflection measurements arethe discussed as well. measurements, screw/twisted modes for examining water quality, and the optical beam shaping to improve cantilever deflection measurements are discussed as well.
3.1. Distributed Sensors Based on Mode-Division Multiplexing (MDM)
3.1. Sensors Based on Mode-Division Multiplexingover (MDM) ToDistributed achieve advanced flexibilities and sensitivity conventional SMF-based approaches,
in recentTotimes there has been an emergent in over developing FMF-based optical sensors, in whilst achieve advanced flexibilities and interest sensitivity conventional SMF-based approaches, recent times has beencost an emergent interest in developing FMF-based optical sensors, few-mode whilst maintaining thethere fabrication at a comparatively low level [124]. In this subsection, maintaining the are fabrication cost a comparatively level [124]. In this approaches, subsection, few-mode fiber-optic sensors evaluated in at terms of operationlow principle, fabrication experimental fiber-optic sensors are evaluated in terms of operation principle, fabrication approaches, design and sensing applications. experimental design and sensing applications.
3.1.1. Operation Principle 3.1.1. Operation Principle
There are a number of fiber-optic distributed sensing techniques that rely on three different There are a number of fiber-optic distributed sensing techniques that rely on three different scattering mechanisms including Raman, Brillouin and Rayleigh scattering, amongst which Raman scattering mechanisms including Raman, Brillouin and Rayleigh scattering, amongst which Raman and Rayleigh could not fully provide information of temperature and/or strain distribution, because and Rayleigh could not fully provide information of temperature and/or strain distribution, because Raman scattering is only related to temperature, while Rayleigh scattering has no Stokes/anti-Stokes Raman scattering is only related to temperature, while Rayleigh scattering has no Stokes/anti-Stokes waves [125]. In In contrast, servesas asaauseful usefultool tool distributed temperature waves [125]. contrast,Brillouin Brillouinscattering scattering serves forfor thethe distributed temperature and/or strain measurements, which has been intensively studied for SMFs in the past few decades and/or strain measurements, which has been intensively studied for SMFs in the past few[126]. Since numerous are spatial involved in FMFs, the stimulated Brillouin scatteringBrillouin (SBS) could decades [126].spatial Since modes numerous modes are involved in FMFs, the stimulated occur not only(SBS) within theoccur samenot fundamental but between different as well.different scattering could only withinmode, the same fundamental mode,modes but between modes as well. of a few-mode optical sensing system is presented in Figure 7a with υB and υo The schematic The schematic of a few-mode optical sensing system is presented in Figure 7a with the and as the altered and reference Brillouin frequency shifts (BFS) correspondingly, whereas Brillouin as the altered and reference Brillouin frequency shifts (BFS) correspondingly, whereas the Brillouin scattered light is propagating in the opposite direction and shifted by BFS, which is caused by the scattered light is propagating in the opposite direction and shifted by BFS, whichphonons. is caused by nonlinear interaction between the incident light and thermally excited acoustic Thethe scalar nonlinear interaction between the incident light and thermally excited acoustic phonons. The scalar wave equation of the optical field can be described by [127]: wave equation of the optical field can be described by [127]:
d2 f od 2 f 1 1d f odf 2 2 2 o k o 2· n o2 (r ) − n 2 + o · + oe f f k n r n f o ·f o0.= 0. 2 o o o eff dr dr 2 r rdrdr
(9)
(9)
where f o signifies thetheoptical as aafunction functionofof radial position r; the subscript o signifies opticalfield field distribution distribution as radial position ; the subscript where indicates the optical field; n r symbolizes the optical refractive index for the fundamental mode; ( ) indicates the optical field; o ( ) symbolizes optical refractive index for the fundamental mode; represents effective refractive index of the optical guided modes, while denotes no e f f represents the the effective refractive index of the optical guided modes, while k o denotes thethe optical optical waverelated number to thewavelength optical wavelength by 2 / . wave number torelated the optical λ by 2π/λ.
Figure Operational principle principle of optical using FMF; (b) (b) Schematic of the Figure 7. 7.(a)(a)Operational optical sensing sensingsystems systems using FMF; Schematic of the Brillouin frequency shifts; (c)Brillouin 3D Brillouin gain spectrum with the temperature strain Brillouin frequency shifts; (c) 3D gain spectrum with the temperature and/orand/or strain variations; variations; (d) Experimental Brillouin spectrum LP01/11 modes. (d) Experimental Brillouin spectrum example forexample LP01/11for modes.
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12 of 35 in Figure 7b, the back-scattering spectrum after Lorentzian fitting is symmetrical around the incident frequency, and the newly generated peaks are equally spaced by the BFS, which is proportional to both temperature and strain variations. Since it’s theoretically impossible to As displayed in Figure 7b, the back-scattering spectrum after Lorentzian fitting is symmetrical separate these two effects by only measuring one BFS, lots of methods have been proposed to around the incident frequency, and the newly generated peaks are equally spaced by the BFS, achieve multi-parameter sensing with discrimination capability [128]. Nonetheless, most early which is proportional to both temperature and strain variations. Since it’s theoretically impossible to approaches using SMFs either led to poor sensing accuracy or added extra noise and complexity to separate these two effects by only measuring one BFS, lots of methods have been proposed to achieve the system [129]. For the meantime, FMF-based sensors serve as a promising candidate to resolve multi-parameter sensing with discrimination capability [128]. Nonetheless, most early approaches this issue, because each spatial mode in FMF may have different Brillouin properties, so more than using SMFs either led to poor sensing accuracy or added extra noise and complexity to the system [129]. one BFS can be provided to simultaneously discriminate the alterations occurred in temperature For the meantime, FMF-based sensors serve as a promising candidate to resolve this issue, because and/or strain applied to the optical fiber. To compare the principle of FMF-based sensors with each spatial mode in FMF may have different Brillouin properties, so more than one BFS can be SMF-based counterparts, the BFS of spatial mode one and mode two, Δ and Δ , are provided to simultaneously discriminate the alterations occurred in temperature and/or strain applied associated with the temperature change ∆ and the strain variation ∆ by the following to the optical fiber. To compare the principle of FMF-based sensors with SMF-based counterparts, equations [130]: the BFS of spatial mode one and mode two, ∆νB Mode 1 and ∆νB Mode 2 , are associated with the Mode 1 temperature change ∆T and the strain ∆ε 1by the [130]: C T Mode equations C following T B Mode1 variation . (10) ! Mode Mode Mode 2 Mode 2 ! ! B 1 2 C ∆νB Cν TT Mode 1 C Cνε Mode 1 ∆T = · . (10) ∆νB Mode 2 ∆ε Cν T Mode 2 Cν ε Mode 2 Thus the temperature change ∆T can be expressed as: Mode 2 be expressed Thus the temperature change can C ∆T Mode1 Cas:Mode 1
T
B
Mode 2
Mode 1
Mode 1
Mode 2 B Mode 2 T B Mode 2 ∆ν
Mode 2 C Mode 1 C Mode 1 C CC · ∆νBT − Cνε · ν ε
.
∆T = . Cν ε Mode 2 · Cν T Mode 1 − Cν ε Mode 1 · Cν T Mode 2 Meanwhile the strain variation ∆ε is of the form: 1 Meanwhile the strain variation ∆ε is2 of the form: C Mode Mode C
T
B
T
Mode 1
B Mode 2
. 2 2 Mode Mode11 Mode 1 Mode 2 2 C Mode Mode C CνCTMode · ∆νB − Cν TT Mode 1 C · ∆ν B T ∆ε = . Cν T Mode 2 · Cν ε Mode 1 − Cν T Mode 1 · Cν ε Mode 2
(11) (11)
(12) (12)
Hence, the strain and temperature effects can be discriminated by solving the simultaneous equations, attains the sensing information along the FMF. In addition, the Brillouin Hence,and thehereafter strain and temperature effects can be discriminated by solving the simultaneous gain spectrum (BGS) is shown in Figure 7c, while the experimental Brillouin backscatter spectrum is equations, and hereafter attains the sensing information along the FMF. In addition, the Brillouin presented in Figure gain spectrum (BGS)7d. is shown in Figure 7c, while the experimental Brillouin backscatter spectrum is The BGS in a FMF presented in Figure 7d. that supports LP01 and LP11 mode are shown in Figure 8, whereas ΔPB denotes of 01Brillouin caused by 8, strain or temperature Thethe BGSpower-level in a FMF thatdifference supports LP and LP11scattered mode arelight shown in Figure whereas ∆PB denotes applied to FMF. difference of Brillouin scattered light caused by strain or temperature applied to FMF. the power-level
Figure 8. 8. Brillouin Brillouin Gain Gain Spectra Spectra for for LP LP01/11 01/11 modes Figure modesininFMF. FMF.
3.1.2. 3.1.2. Fabrication Fabrication Methods Methods The The fabrication fabrication and and characterization characterization of of aa few-mode few-mode Brillouin Brillouin sensing sensing system system is is illustrated illustrated in in Figure 9. When the incident light propagates through a FMF, the thermally excited mechanical Figure 9. When the incident light propagates through a FMF, the thermally excited mechanical vibrations canpropagate propagate as guided acoustic in while the fiber, while Brillouin scattering vibrations can as guided acoustic modes modes in the fiber, Brillouin scattering spontaneously spontaneously yields either the frequency down-shifted (Stokes) or(anti-Stokes) up-shifted (anti-Stokes) photons, yields either the frequency down-shifted (Stokes) or up-shifted photons, due to the
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due to the16,interaction between the acoustic modes on the optical modes. The corresponding BFS Sensors 2016, 1387 13 of 35 is
due to the interaction described as [131]: between the acoustic modes on the optical modes. The corresponding BFS is described as [131]: 2 nthe optical modes. no effcorresponding BFS νB is described VClad 2The interaction between the acoustic modes on B 2 no effo eff Veff VClad 2 no eff . (13) as [131]: Veff na eff . B (13) 2n 2no ef f n o ef f V · Ve f f = Clad · a eff . νB = (13) λ λ n a e fisf given by: and the effective refractive index of the acoustic guided modes and the effective refractive index of the acoustic guided modes is given by: and the effective refractive index of the acoustic guided VCladmodes is given by: . naeff VClad (14) naeff VClad Veff . (14) n a e f f = Veff . (14) Ve f f where denotes the effective longitudinal velocity, and signifies the longitudinal acoustic where denotes the effective longitudinal velocity, and signifies the longitudinal acoustic velocity fiber cladding. where Ve f in the effective longitudinal velocity, and VClad signifies the longitudinal acoustic f denotes velocity in fiber cladding. velocity in fiber cladding.
Figure Fabricationand and Characterization a few-mode Brillouin sensing system. Figure 9.9.Fabrication Characterization ofof a few-mode Brillouin sensing system. Figure 9. Fabrication and Characterization of a few-mode Brillouin sensing system.
As mentioned above, the intensity of Brillouin scattering depends on the strong correlation As mentioned above, the intensity of Brillouin scattering depends on the strong correlation mentioned Brillouin strong correlation between the longitudinal acoustic and optical modes. The normalized modal overlap integral between the longitudinal acoustic andand optical modes. The normalized modal overlap integral between between the longitudinal acoustic optical modes. The normalized modal overlap integral between optical and acoustic fields can be defined as [132]: optical and acoustic Iu can be defined asdefined [132]: as [132]: between optical andfields acoustic fields can be
2
R Eo E*∗o* *∗u * r drd22 dr dθ ( ) EEoo Eoo ρuu rrdrd u Iu I= . . R I u R ( EEo EEo ∗*)222rrdrdrd . ∗ r* rdrdrd dθ · ρ ρ dθ o *o *
(15) (15) (15)
E E r drd r drd o
o
Here θ with Herethe theintegral integralbrackets bracketsdenote denotethe theintegration integrationover overthe thepolar polarcoordinates coordinatesr and and withthe the Here the integral brackets denote the integration over the polar coordinates and with the electric field distribution of optical modes E and acoustic density variation ρ for the acoustic mode electric field distribution of optical modes and acoustic density variation for the acoustic o electric field distribution optical modes and acousticofdensity variation forare the of order of u. order In the meantime, the intensity of optical/acoustic modes in a modes FMF mode . In the of meantime, theprofiles intensity profiles optical/acoustic inillustrated aacoustic FMF are mode of order . In the meantime, the intensity profiles of optical/acoustic modes in a FMF are inillustrated Figure 10,in whereas thewhereas optical and acousticand profiles match wellmatch for LPwell mode, while the overlap Figure 10, the optical acoustic profiles for LP 01 mode, while the 01 illustrated in Figure 10, whereas the optical and acoustic profiles match well for LP 01 mode, while the integral of optical/acoustic profiles for LP mode is apparently much smaller. This might elucidate overlap integral of optical/acoustic profiles 11 for LP11 mode is apparently much smaller. This might overlap of optical/acoustic profiles for LP 11 mode is apparently much smaller. This might why eachintegral spatial mode has slightly Brillouin property in FMF, and henceforth thishenceforth overlap elucidate why each spatial mode different has slightly different Brillouin property in FMF, and elucidate why each spatial mode has slightly different Brillouin property in FMF, and henceforth integral can be controlled thru the acoustic velocity profile design as well as the fiber refractive index this overlap integral can be controlled thru the acoustic velocity profile design as well as the fiber this overlap integral can be controlled thru the acoustic velocity profile design as well as the fiber profile design. refractive index profile design. refractive index profile design.
(a) (a)
(b) (b)
Figure The intensity profiles of optical/acoustic modes inaaFMF. FMF. Figure 10.10. The intensity profiles of optical/acoustic modes forfor LPLP01/11in Figure 10. The intensity profiles of optical/acoustic modes for LP01/11 01/11 in a FMF.
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3.1.3. 3.1.3. Experimental Experimental Design Design The experimentalsetup setup a few-mode BOTDR for simultaneous temperature strain The experimental of aoffew-mode BOTDR for simultaneous temperature and strainand sensing is sensing in A Figure 11. A 1550 nm distributed feedback laser is diode is used as a depictedisindepicted Figure 11. 1550 nm distributed feedback (DFB) laser (DFB) diode (LD) used(LD) as a light source, light source, output of which divided into two arms by aThe 50:50 coupler. pump wave is the output of the which is divided intoistwo arms by a 50:50 coupler. pump wave The is modulated by an modulated by an electro-optical modulator (EOM) driven with 30 ns Gaussian pulse to achieve high electro-optical modulator (EOM) driven with 30 ns Gaussian pulse to achieve high pump power in the pump powerwhich in theis upper which dividedtowith a 1 pump × 2 coupler provide pump upper path, furtherpath, divided withisafurther 1 × 2 coupler provide powertofor two different power for two different spatial modes. The lower path is amplified by an EDFA, and then divided spatial modes. The lower path is amplified by an EDFA, and then divided again by a 1 × 2 coupler again by a two 1 × 2carriers coupler two carriers as the the upcoming local oscillators (LO) coherent for the upcoming to generate as to thegenerate local oscillators (LO) for heterodyne detection. heterodyne coherent controllers detection. The polarization controllers (FPC) ensure that they are The fiber polarization (FPC)fiber ensure that they are co-polarized with the pumps. For the co-polarized with the pumps. For the upper two paths, another two FPCs and EDFAs are used to upper two paths, another two FPCs and EDFAs are used to control the power and the polarization control theoptical polarization state (OC) of theare pumps. Two optical (OC) inserted state of the the power pumps.and Two circulators inserted before thecirculators M-MUX in the are pump path, before the M-MUX in the pump path, and the mode converter (MC) using phase plate makes sure and the mode converter (MC) using phase plate makes sure the pump is launched into any desired the pump is launched into any desired higher order LP modes. Then two different spatial modes higher order LP modes. Then two different spatial modes are mode multiplexed and launched into are the mode multiplexed andA launched into the step-index fiber under testis(FUT). km with circular-core step-index fiber under test (FUT). 4 km circular-core FMF used asAa 4FUT, a reflective end (RE) FMF is used as other a FUT,side. with a reflective end (RE) attached to the other side. attached to the
Figure 11. Configuration Figure 11. Configuration of of BOTDR BOTDR using using FMF FMF for for simultaneous simultaneous temperature temperature and and strain strain sensing. sensing. DFB-LD: distributed distributed feedback feedback laser laser diode; diode; EOM: EOM: electro-optic electro-optic modulator; modulator; LO: LO: local local oscillators; oscillators; FPC, FPC, fiber polarization controller; OC: optical circulator; FUT: fiber under test; RE: reflective end; OCR-FE: circulator; FUT: test; RE: reflective end; OCR-FE: optical coherent receiver front end; PD: photo-detector; photo-detector; TDS: TDS: time-domain time-domain sampling sampling scope. scope.
The counter-propagating probes are mode de-multiplexed to the original two spatial modes by The counter-propagating probes are mode de-multiplexed to the original two spatial modes by the same MC component in the pump path. Through two OCs, the optical signal in each mode is the same MC component in the pump path. Through two OCs, the optical signal in each mode is re-amplified by EDFAs, and then directed to the optical coherent receiver front end (OCR-FE), which re-amplified by EDFAs, and then directed to the optical coherent receiver front end (OCR-FE), which is coupled with a 1550 nm LO from the same light source, whose state of polarization (SOP) is also is coupled with a 1550 nm LO from the same light source, whose state of polarization (SOP) is also maintained through FPCs. The coherent receiver includes the optical hybrid and real-time maintained through FPCs. The coherent receiver includes the optical hybrid and real-time oscilloscope, oscilloscope, followed by low-pass filtering, photo-detectors (PDs), ADCs and DSP blocks. The followed by low-pass filtering, photo-detectors (PDs), ADCs and DSP blocks. The electrical signals of electrical signals of both modes are sampled by a time-domain sampling scope (TDS). both modes are sampled by a time-domain sampling scope (TDS). Furthermore, the maximum errors for temperature and strain measurements using FM-BOTDR Furthermore, the maximum errors for temperature and strain measurements using FM-BOTDR can be determined in the form of [133]: can be determined in the form of [133]: C Mode 2 Mode1 C Mode1 Mode 2 Mode 1 B Mode 2 B 2 Mode Mode 1 + Cν ε · δνB · δνB . T C ν ε (16) δT = C Mode 2 C T Mode1 C Mode1 C T Mode 2 . (16) Cν ε Mode 2 · Cν T Mode 1 − Cν ε Mode 1 · Cν T Mode 2
C T Mode 2 B Mode1 C T Mode1 B Mode 2 C T Mode 2 C Mode1 C T Mode1 C Mode 2
.
(17)
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Cν T Mode 2 · δνB Mode 1 + Cν T Mode 1 · δνB Mode 2 . δε = Cν T Mode 2 · Cν ε Mode 1 − Cν T Mode 1 · Cν ε Mode 2
(17)
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The BFS dependence of LP 01 and LP 11 modes on temperature isisshown shown in Figure 12, when the TheBFS BFSdependence dependenceof ofLP LP01 andLP LP11 modeson ontemperature temperatureis shownin inFigure Figure12, 12,when whenthe the The 01and 11modes strain is fixed at 0 με, in the inset of which the proportionality coefficients are calibrated, about 1.3 strain is fixed at 0 µε, in the inset of which the proportionality coefficients are calibrated, about 1.3 MHz strain is at 0 με, in the inset of which the proportionality coefficients are calibrated, about 1.3 MHz per Celsius Celsius degree, via aa squares least squares squares fitting of linear linear regression. per Celsius degree, via a via least fittingfitting of linear regression. MHz per degree, least of regression.
Figure12. 12.Calibration Calibrationofoftemperature temperaturecoefficients coefficientsfor fordifferent differentmodes modesin inFMF. FMF. Figure Figure 12. Calibration of temperature coefficients for different modes in FMF.
Likewise, the calibrations of of the thestrain strainproportionality proportionalitycoefficients coefficients for different modes in FMF FMF Likewise, the the calibrations forfor different modes in FMF are Likewise, the strain proportionality coefficients different modes in ◦ are illustrated in Figure 13, when the temperature is set as 25 °C. By linear regression, the illustrated in Figure 13, when temperature is set as 25 C. By regression, theregression, proportionality are illustrated in Figure 13, the when the temperature is set aslinear 25 °C. By linear the proportionality coefficient is calculated calculated to be around around 58 strain. KHz per per micro micro strain. strain. coefficient is calculated to is be around 58to KHz per micro proportionality coefficient be 58 KHz
Figure 13. 13. Calibration of of strain coefficients coefficients for different different modes in in FMF. Figure Figure 13.Calibration Calibration ofstrain strain coefficientsfor for differentmodes modes inFMF. FMF.
Furthermore, Figure Figure 14 14 illustrates illustrates the the signal-to-noise signal-to-noise ratio ratio (SNR) (SNR) distribution distribution of of the the few-mode few-mode Furthermore, Furthermore, Figure 14 illustrates the signal-to-noise ratio (SNR) distribution of the few-mode BOTDR system system after after 20 20 times times averaging, averaging, which which indicates indicates the the LP LP01 01 mode experiences a bit higher gain BOTDR mode experiences a bit higher gain BOTDR system after 20 times averaging, which indicates the LP mode experiences a bit higher gain 01 over the the higher higher order order mode, mode, owing owing to to their their dissimilar dissimilar optical/acoustic optical/acoustic correlation correlation profiles. profiles. The The SNR SNR over over the higher order mode, owing to their dissimilar optical/acoustic correlation profiles. The SNR is expressed expressed as as the the ratio ratio of of maximum maximum and and minimum minimum of of Lorentzian Lorentzian fitting fitting curve curve for for all all the the amplitude amplitudeis is expressed the ratio of maximum andthe minimum of Lorentzian fitting curve for all the data at aa as fixed frequency, whereas amplitude distribution variability leads to amplitude the SNR SNR data at fixed frequency, whereas the amplitude distribution variability leads to the data at a fixed frequency, whereas the amplitude distribution variability leads to the SNR fluctuations; fluctuations; in the intervening time heterodyne detection has been implemented to increase the fluctuations; in the intervening time heterodyne detection has been implemented to increase the in the intervening time heterodyne detection has been implemented to increase the system sensitivity, system sensitivity, while averaging is performed to enhance the SNR. system sensitivity, while averaging is performed to enhance the SNR. while averaging is performed to enhance the SNR. In contrast to the proportionality coefficients in standard silica SMF at 1550 nm [128], which are averagely 1.08 MHz/◦ C and 43 kHz/µε respectively, both the strain and temperature coefficients (f -ε and f -T) in FMF are slightly larger, as shown in Table 1, which is caused by the difference of structural deformation in FMF. The LP01 mode has slightly larger coefficients, because its intensity profile has stronger correlation between optical and acoustic modes.
Figure 14. 14. Signal-to-noise Signal-to-noise ratio ratio (SNR) (SNR) comparison comparison for for FM-BOTDR FM-BOTDR system system between between LP LP01 01 and LP11 Figure and LP11 mode along the sensing fiber. mode along the sensing fiber.
over the higher order mode, owing to their dissimilar optical/acoustic correlation profiles. The SNR is expressed as the ratio of maximum and minimum of Lorentzian fitting curve for all the amplitude data at a fixed frequency, whereas the amplitude distribution variability leads to the SNR fluctuations; in the intervening time heterodyne detection has been implemented to increase the Sensors 2016, 16, 1387 while averaging is performed to enhance the SNR. 16 of 35 system sensitivity,
Figure 14. 14. Signal-to-noise Signal-to-noise ratio ratio (SNR) (SNR) comparison comparison for for FM-BOTDR FM-BOTDR system system between between LP and LP LP11 01 and 11 Figure LP01 mode along the sensing fiber. mode along the sensing fiber. Table 1. Comparison of f -T and f -ε coefficients in FMF. Mode
CT (MHz/◦ C)
Cε (kHz/µε)
LP01 LP11
1.29 1.25
58.5 57.6
3.1.4. Sensing Applications The distributed MDM sensing systems serve as a novel technique to make simultaneous measurements of both the temporal and spatial behavior utilizing the special properties of FMF as a non-intrusive and dielectric sensing medium. One flexible FMF embedded within the smart structure might substitute thousands of closely attached expensive traditional electronic point sensors, making the distributed sensing system cost efficient [134]. Another key advantage of this technique concentrates on the accurate detection of the backscattered signal as well as the elimination of noise. The conventional SMF techniques are not effective in reducing the coherent Rayleigh noise (CRN) or fading noise. Since FMF has a relatively short coherence length, so the superposition will be incoherent and thus CRN becomes negligible. Coherent detection is necessary for detecting light which propagates with lower and higher order modes, and noise can be further eliminated by using the frequency shift averaging (FSAV) techniques [135]. Thus FMF-based distributed sensors have attracted considerable attention due to their discriminative capability to measure strain and/or temperature, and thus can be applied in a variety of civil and geotechnical structure health monitory (SHM), such as the deformation monitoring and health diagnosis of tunnels, bridges, dams, pipelines, dikes and buildings. Additionally, the MDM sensing systems play an extremely significant role in operation safety for a variety of applications in energy industry, such as well-integrity monitoring and downhole seismic acquisition. 3.2. Distributed Sensors Based on Core Multiplexing An alternative solution to the ever-increasing demand of SDM sensors is based on multicore fibers (MCF). In recent years, novel sensors utilizing MCF have been proposed and demonstrated experimentally for distributed sensing purposes subjected to harsh environments, based on the interference effects in-between the central core and outer cores with longitudinal strain or heat applied to the MCF segment [136]. 3.2.1. Operation Principle With a broadband light-source at the transmitter side and an OSA at the receiver side, the MCF interference pattern spectrum can be monitored, which would be shifted either by applied strain in keeping with the refractive index changes, or by temperature changes due to the thermo-optic coefficient. The experimental example dips in the transmission spectra of the etched MCF device is
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shown in Figure 15, whereas different colors of the curves are corresponding to altered magnitudes of applied strain onto the testing MCF [137]. The corresponding Young’s modulus E can be expressed as: E=
σ F·L = . ε ∆L · A
(18)
where σ signifies stress, ε denotes applied strain, F is force, L and ∆L are the fiber length and its change due to the applied Sensors 2016, 16, 1387 strain, and A stands for the cross-sectional area. 17 of 34
Figure15. 15.Force-induced Force-induced wavelength wavelength shift Figure shift of ofthe theMCF MCFsensing sensingdevice device[137]. [137].
As mentioned above, the MCF structure has linear responses to both strain and As mentioned above, the MCF structure has linear responses to both strain ε and temperature T, temperature , while the wavelength shift Δ can be described as: while the wavelength shift ∆λ MCF can be described as: MCF C CT T . (19) ∆λ MCF = Cε · ∆ε + CT · ∆T. (19) where and denote the strain and temperature coefficients derived from elasto-optical coefficient thermalthe expansion of the fiber respectively. Additionally, in order to where Cε andand CT denote strain andcoefficient temperature coefficients derived from elasto-optical coefficient discriminate the cross sensitivities of two heterogeneous cores in MCF, this correlation can be further and thermal expansion coefficient of the fiber respectively. Additionally, in order to discriminate the expressed in the matrix form as [138]: cross sensitivities of two heterogeneous cores in MCF, this correlation can be further expressed in the Core 2 matrix form as C Core1 CT Core1 Core1 CT Core1 Core1 [138]: 1 CT . ! (20) Core 2 Core 2 Core 2 Core1 ! ! T ! CT ! Cε Core1 2 Core 2 det H CCCore2 −CC Core1 Core C CT Core1 ∆λ ∆ε ∆λ T T Core1 Core1 . = = det1( H ) (20) Cε Core2 CT Core2 ∆T −Cε Core2 Cε Core1 for the ∆λ ∆λCore2 Core2 and denote the strain and temperature coefficients heterogeneous where core in MCF, and det ( ) represents the determinant of the coefficient matrix connecting Core i i where C CT Core the strain coefficients for the heterogeneous core i ε temperatureandand straindenote responses withand twotemperature spatial channels. in MCF, and det ( H ) represents the determinant of the coefficient matrix connecting temperature T 3.2.2. Fabrication Methods and strain ε responses with two spatial channels. The fabricated cross-section of sensing MCF with seven germanium-doped coupled cores inside 3.2.2. Fabrication Methods is shown in Figure 16a, while the configuration of the MCF sensing structure made of one MCF The fabricated cross-section of sensing in MCF with16b seven coupled and coresforce inside spliced between two SMFs is presented Figure forgermanium-doped simultaneous temperature purposes is sensing shown in Figure[138]. 16a, while the configuration of the MCF sensing structure made of one MCF spliced between two SMFs is presented in Figure 16b for simultaneous temperature and force sensing purposes [138]. This etched MCF sensing device was fabricated in-house in 6:1 buffered oxide etch (BOE) at a rate of 0.24 µm/min, with a numerical aperture (NA) of 0.13, a pitch of 12.1 µm, an insertion loss of less than 0.05 dB, with core diameters of 10.6 µm as well as an outer diameter of 125 µm. Besides, the sensing sensitivity of the MCF sensors can be improved by decreasing the fiber outer diameter, for a smaller cross-sectional area leads to a higher average applied force per area in accordance with the 16. (a) Microscope image of a 7-core optical sensing fiber; (b) Schematic diagram of Young’sFigure modulus. MCF-based sensing device structure [138].
This etched MCF sensing device was fabricated in-house in 6:1 buffered oxide etch (BOE) at a rate of 0.24 μm/min, with a numerical aperture (NA) of 0.13, a pitch of 12.1 μm, an insertion loss of less than 0.05 dB, with core diameters of 10.6 μm as well as an outer diameter of 125 μm. Besides, the
3.2.2. Fabrication Methods The fabricated cross-section of sensing MCF with seven germanium-doped coupled cores inside is shown in Figure 16a, while the configuration of the MCF sensing structure made of one MCF spliced between two SMFs is presented in Figure 16b for simultaneous temperature and force Sensors 2016, 16, 1387 18 of 35 sensing purposes [138].
Figure (a) Microscope image of a 7-core (b) Schematic diagram of Figure 16. 16. (a) Microscope image of a 7-core opticaloptical sensingsensing fiber; (b)fiber; Schematic diagram of MCF-based MCF-based sensing device structure [138]. sensing device structure [138]. Sensors 2016, 16, 1387 18 of 34
This etched MCF sensing device was fabricated in-house in 6:1 buffered oxide etch (BOE) at a 3.2.3. Experimental Design 3.2.3. Experimental Design rate of 0.24 μm/min, with a numerical aperture (NA) of 0.13, a pitch of 12.1 μm, an insertion loss of The spectral responses to temperature thecentral andthe the outercore core in MCF MCF The spectral responses temperature ofofthe outer in when there less than 0.05 dB, with coretodiameters of 10.6 μm ascentral well core ascore anand outer diameter of 125 μm. Besides, the is ◦C ◦ is no applied strain are presented in Figure 17a,b, whereas as temperature rises from 20.0 °C nosensing applied strain are presented in Figure 17a,b, whereas as temperature rises from 20.0 to 80.0 sensitivity of the MCF sensors can be improved by decreasing the fiber outer diameter, forto a C, 80.0 °C,cross-sectional thedips spectrum dips would to aaverage longer with wavelength corresponding temperature thesmaller spectrum wouldarea shiftleads to a longer wavelength corresponding temperature sensitivities to shift a higher applied forcewith per area in accordance with the of 2 ◦ ◦ 2 sensitivities of 47.37 pm/°C and 53.20 pm/°C respectively based on a linear fitting of R all 47.37 pm/ modulus. C and 53.20 pm/ C respectively based on a linear fitting of R values all abovevalues 0.998 [139]. Young’s above 0.998 [139]. Furthermore, the SDM multi-parameter measurement coefficients with Furthermore, the SDM multi-parameter measurement coefficients with discrimination using MCF are discrimination using2.MCF are summarized in Table 2. summarized in Table
Figure Transmissionspectrum spectrum shift shift of of the the central Figure 17.17.(a)(a)Transmission central core core and andthe theouter outercore corewith withdifferent different temperature; (b) Transmission spectrum response as a function of temperature [139]. temperature; (b) Transmission spectrum response as a function of temperature [139]. Table 2. Measured value λ1, λ2, determined temperature, and strain using MCF [139]. Table 2. Measured value λ1 , λ2 , determined temperature, and strain using MCF [139]. (
)
( ) λ2 (nm) 1528.244 1524.864 1528.244 1527.560 1524.864 1524.356 1527.560 1524.356
λ1 (nm)
T (°C) T (◦ C) 33.5 33.5 34.3 34.3
S (με) 1210.5
S (µε)
1868.4 1210.5 1868.4
3.2.4. Sensing Applications 3.2.4. Sensing Applications Likewise, the distributed fiber sensors based on core multiplexing can be deployed in multiple Likewise, the distributed sensors based on core multiplexing can be deployed multiple industrial segments, such asfiber oil and gas production, power cable monitoring, leakage in detection industrial such as oil and production, power cablecarriers monitoring, leakage detection at dikessegments, and dams, integrity of gas liquid natural gas (LNG) and terminals, thanks at to dikes its superiorofsensitivity, light-weight, remote-access, ease of superior being andlow-cost, dams, integrity liquid natural gas (LNG)electrical-safety, carriers and terminals, thanksand to itsthe low-cost, multiplexed [140]. Whenelectrical-safety, the minimum two spatial channels arethe used to separate applied strain sensitivity, light-weight, remote-access, and ease of beingthe multiplexed [140]. andthe temperature, of cores can beare further to monitor other strain physical suchthe When minimumthe tworest spatial channels usedutilized to separate the applied andmeasurand temperature, or acceleration. restasofpressure, cores candisplacement, be further utilized to monitor other physical measurand such as pressure, displacement, or acceleration. 3.3. Fiber Bragg Grating (FBG) Sensors Based on Core Multiplexing A novel SDM sensor based on co-located multicore FBGs is introduced in this subsection in terms of operation principle, fabrication methods, experimental design and sensing applications, which provides an estimation of fiber shaping and bending thru measuring distributed fiber curvature for potential applications such as submarine towed-instrument tracking and
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3.3. Fiber Bragg Grating (FBG) Sensors Based on Core Multiplexing A novel SDM sensor based on co-located multicore FBGs is introduced in this subsection in terms of operation principle, fabrication methods, experimental design and sensing applications, which provides an estimation of fiber shaping and bending thru measuring distributed fiber curvature for potential applications such as submarine towed-instrument tracking and morphing-wing shape monitoring [141]. Sensors 2016, 16, 1387
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3.3.1. Operation Principle
3.3.1. Operation Principle
Although thethe SMF FBG resonantdip dip in the transmission spectrum, FBGsonbased Although SMF FBGonly onlyhas has one one resonant in the transmission spectrum, FBGs based on MCF might have more than one resonant dips. By analyzing the changing spectra of the MCF might have more than one resonant dips. By analyzing the changing spectra of the dips, the dips, the changes stressingorortemperature temperature fluctuations be distinguished. Different changesinduced induced by by bending, bending, stressing fluctuations cancan be distinguished. Different different sensitivities bendingfluctuations, fluctuations, attributable to to thethe difference in structural dips dips havehave different sensitivities ininbending attributable difference in structural deformation when strain is applied to MCF. the MCF. similar reasons, shapes of multiple dips deformation when strain is applied to the For For similar reasons, thethe shapes of multiple dips would would be impacted by temperature variations in different behaviors [142]. Besides, one of major be impacted by temperature variations in different behaviors [142]. Besides, one of major advantages advantages of grating-based fiber-optic sensors theymultiplexed. can be simply of grating-based fiber-optic sensors is that they can is bethat simply Asmultiplexed. each gratingAs is each inscribed grating is inscribed at different locations on the sensing fiber with different grating periods, the at different locations on the sensing fiber with different grating periods, the signals coming from each signals coming from each core are encoded at different positions in the wavelength domain. The core are encoded at different positions in the wavelength domain. The FBG resonant wavelength FBG resonant wavelength depends on the effective index of refraction of the core and the periodicity depends ongrating, the effective refraction of thewavelength ∆ core and the periodicity of theand grating, so the shift of the so the index shift inofMCF FBG center owing to strain temperature in MCF FBG center wavelength ∆λ B owing to strain and temperature variations ∆ε and ∆T can be variations ∆ and ∆ can be written as [143]: written as [143]: ppee) · (αΛ +αnn) ·∆T T +CC .] . (21) (21) ∆λBB=λBB[(11− ∆ε + denotes the effective strain-optic constant,αΛ signifies signifiesthe thethermal thermal expansion expansion coefficient wherewhere pe denotes the effective strain-optic constant, coefficient for for the fiber, while represents the thermo-optic coefficient. Last of all, the constant proportional the fiber, while αn represents the thermo-optic coefficient. Last of all, the proportional C stands constant stands for thecaused FBG wavelength shift causedsuch by other parameters such concentration as pressure, or for the FBG wavelength shift by other parameters as pressure, chemical chemical concentration or PH values, etc. Thus, multiple physical quantities can be easily and PH values, etc. Thus, multiple physical quantities can be easily and simultaneously measured by the simultaneously measured by the spectral peak shift in the wavelength range, thru multi-core FBG spectral peak shift in the wavelength range, thru multi-core FBG sensing along the fiber. sensing along the fiber.
3.3.2. Fabrication Methods
3.3.2. Fabrication Methods
The short segment fabrication of four-core shape-sensing Figure 18, 18,with with one The short segment fabrication of four-core shape-sensingFBGs FBGsisisdisplayed displayed in in Figure nominally on-axis central core as a reference and three extensively displaced outer cores in azimuthal one nominally on-axis central core as a reference and three extensively displaced outer cores in angles for explicit bending sensing purposes, whereas fourwhereas interrelated temperature or twist-induced azimuthal angles for explicit bending sensing purposes, four interrelated temperature or signals can be detected by meansrosettes of four-FBG rosettes at all axial aligned at axial for straintwist-induced signals can strain be detected by means of four-FBG all aligned coordinate coordinate fordiscrimination multi-parameter[144]. discrimination [144]. multi-parameter
Figure 18.18. Configuration twist-biased FBGs [144]. Figure Configurationof of four-core four-core twist-biased FBGs [144].
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3.3.3. Experimental Design
3.3.3. Experimental Design
The twist-to-strain response and robust shape prediction within 4-core fiber tethers are tested The twist-to-strain response and robust shape prediction within 4-core fiber tethers are tested under low reflectivity using rosette solution algorithms by incorporating twist measurements into under low reflectivity using rosette solution algorithms by incorporating twist measurements into the shape elucidation, as as shown whereasthe thetwist twistcoefficients coefficients range the shape elucidation, shownininFigure Figure 19, 19, whereas range fromfrom 4.8 to4.8 to 8.9 nε/degree-m in various FBG cores, and the twist accuracy is approximately 50 degrees/m due to 8.9 /degree-m in various FBG cores, and the twist accuracy is approximately 50 degrees/m due to imperfections in some fiber cores [145]. imperfections in some fiber cores [145].
Figure Twist-to-strainresponse response of rosette [145]. Figure 19.19. Twist-to-strain of MCF-FBGs MCF-FBGsininone one rosette [145].
3.3.4. Sensing Applications
3.3.4. Sensing Applications
Thanks to their distinctive filtering properties and adaptability as in-fiber devices, FBGs have
Thanks to their distinctive properties andpast adaptability devices,simple, FBGs have been under much attentionfiltering and reporting for the decades as forin-fiber being reliable, and been underwell-suited much attention and reporting for past decades for being reliable, simple, and well-suited for many applications. Thethe multicore FBG sensing systems have the ability to respond to for manya applications. multicore FBG sensing systems have the ability to to a wide variety wide variety ofThe measurand, resistance to harsh environments, avoidance of respond electric sparks, as well as the ease resistance of integration into large-scale fiber networking and thus of measurand, to harsh environments, avoidance of communication electric sparks,systems as well[146], as the ease of makinginto themlarge-scale suitable forfiber a variety of applications, including SHM of dams, highways, bridges,them integration networking and communication systems [146], thus making railways, as well as spacecraft fuel tanks. suitable for a aircraft varietywings, of applications, including SHM of dams, highways, bridges, railways, aircraft wings, as well as spacecraft fuel tanks. 3.4. Other Examples of SDM Sensors
3.4. OtherLast Examples of SDM Sensors but not least, other prospective cases of SDM sensing systems include the whispering gallery modes for fiber profiling and chemical species measurements, the screw/twisted modes for
Last but not least, other prospective cases of SDM sensing systems include the whispering gallery examining water quality, and the optical beam shaping to improve cantilever deflection measurements. modes for fiber profiling and chemical species measurements, the screw/twisted modes for examining water3.4.1. quality, and theGallery opticalModes beamfor shaping to improve cantilever deflection measurements. Whispering Chemical Species Measurements As mentioned above, the whispering-gallery modes (WGMs) are confined by quasi-total 3.4.1. Whispering Gallery Modes for Chemical Species Measurements
internal reflection along the material interface with virtually cropping incidence patterns, and
As mentioned above, microsphere the whispering-gallery modesoptical (WGMs) are confined byrefractive quasi-total internal generated in dielectric with high-pitched resonances in lower index medium [147], Figureinterface 20 showswith the intensity profile of the incidence WGMs [148]. As WGMs with small in reflection along thewhile material virtually cropping patterns, and generated mode-volume and strong confinement may orbit resonances for many times before escaping theindex resonator, these[147], dielectric microsphere with high-pitched optical in lower refractive medium modes have been confirmed to provide greatly enhanced detection sensitivity with regard to the while Figure 20 shows the intensity profile of the WGMs [148]. As WGMs with small mode-volume refractive index variations of the sensing environment compared with the conventional planar and strong confinement may orbit for many times before escaping the resonator, these modes surface-based approaches, with enhanced spontaneous emission threshold-less lasing [149]. Besides have been confirmed to provide greatly enhanced detection sensitivity with regard to the refractive other applications in telecommunications, photonics and quantum electrodynamics, such as indexhigh-efficiency variations of the sensing environment compared conventional optical frequency combs, WGMs havewith alsothe been applied inplanar varioussurface-based sensing approaches, withincluding enhanced spontaneous emission threshold-less lasing [149]. Besides applications, temperature, pressure and force sensors, etc. [150]. In particular, recentlyother applications in telecommunications, and to quantum electrodynamics, as high-efficiency WGMs have attracted considerablephotonics attention due their applications in speciessuch concentration and optical frequencysensing combs,byWGMs havesharp also been applied in various sensing applications, including biochemical exploiting photonic resonances, including label-free detection of macromolecules such as proteins and DNA, as well as bacteria and animal cells with accurate temperature, pressure and force sensors, etc. [150]. In particular, recently WGMs have attracted permittivity and dielectric [9,151]. Such SDMin sensors areconcentration realized using tapered fibers or prism considerable attention due toloss their applications species and biochemical sensing by exploiting sharp photonic resonances, including label-free detection of macromolecules such as proteins and DNA, as well as bacteria and animal cells with accurate permittivity and dielectric
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loss [9,151]. Such SDM sensors are realized using tapered fibers or prism couplers via coating a zeolite film on the external surface an optical with surface target attached on target the with spheretarget couplers via a zeolite filmfilm onmicrosphere the surface of anbiomolecules optical microsphere with couplers via coating coating a ofzeolite on external the external of an optical microsphere surface, attributable to the on sensitivity of surface, their evanescent field to the refractive index changes of nearby Sensors 2016, 16, 1387 21 of 34 biomolecules attached the sphere attributable to the sensitivity of their evanescent field biomolecules attached on the sphere surface, attributable to the sensitivity of their evanescent field to the refractive index changes of nearby entities [152–154]. entities [152–154]. to the refractive index changes of nearby entities [152–154]. couplers via coating a zeolite film on the external surface of an optical microsphere with target Sensors 2016, 16, 1387 21 of 34 biomolecules attached on the sphere surface, attributable to the sensitivity of their evanescent field Sensors 2016, 16, 1387 21 of 34 to the refractive index changes of nearby entities [152–154]. Sensors 2016, 16, 1387 couplers via coating a zeolite film on the external surface of an optical microsphere with 21 of 34 target couplers via coating a zeolite film on the external surface of an optical microsphere with target biomolecules attached on the sphere surface, attributable to the sensitivity of their evanescent field Sensors 2016, 16, 1387 21 of 34 couplers via coating a zeolite film on the external surface of an optical microsphere with target biomolecules attached on the sphere surface, attributable to the sensitivity of their evanescent field to the refractive index changes of nearby entities [152–154]. biomolecules attached on the sphere surface, attributable to the sensitivity of their evanescent field to the refractive index changes of nearby entities [152–154]. couplers via coating a zeolite film on the external surface of an optical microsphere with target to the refractive index changes of nearby entities [152–154]. biomolecules attached on the sphere surface, attributable to the sensitivity of their evanescent field to the refractive index changes of nearby entities [152–154]. Figure 20. (Left) The whispering gallery under a dome of St. Paul’s cathedral and (Right) the sound sound Figure 20. (Left) The whispering gallery under a dome of St. Paul’s cathedral and (Right) the intensity profile showing the whispering gallery mode (WGM) [148]. intensity profile showing the whispering gallery mode (WGM) [148]. Figure 20. (Left) The whispering gallery under a dome of St. Paul’s cathedral and (Right) the sound
3.4.2.intensity profile showing the whispering gallery mode (WGM) [148]. Screw/Twisted Modes for Examining Water Quality
3.4.2. Screw/Twisted Modes for Examining Water Quality The screwed or twisted modes, i.e., the OAM modes, are defined as a phase structure in light Figure 20. (Left) The whispering gallery under a dome of St. Paul’s cathedral and (Right) the sound 3.4.2. Screw/Twisted Modes for Examining Water Quality The screwed twisted OAMvector, modes,which are defined as a phasethru structure in light beams with a or local skew modes, angle of i.e., the the Poynting can be converted nonlinear intensity profile showing the whispering gallery mode (WGM) [148]. [155,156]. The screwed or twisted modes, i.e., the OAM modes, are defined as a phase structure in light processes as second harmonic (SHG)vector, or parametric (PDC) beams with asuch local skew angle of generation the Poynting whichdown-conversion can be converted thru nonlinear thru Such Figure 20. (Left) The whispering gallery under a dome of St. Paul’s cathedral and (Right) the sound optical with helical phase-fronts can be observed using interference which can nonlinear beams with a local skew angle of the Poynting vector, which can be fringes, converted processes such asvortex second harmonic generation (SHG) or parametric down-conversion (PDC) [155,156]. 3.4.2. Screw/Twisted Modes for Examining Water Quality be applied for the translational motion detection of various surfaces and fluids [157]. Specifically, the can Figure 20. (Left) The whispering gallery under a dome of St. Paul’s cathedral and (Right) the sound processes such as second harmonic generation (SHG) or parametric down‐conversion (PDC) [155,156]. intensity profile showing the whispering gallery mode (WGM) [148]. Such optical vortex with helical phase-fronts can be observed using interference fringes, which The screwed or twisted modes, i.e., the OAM modes, are defined as a phase structure in light screwed modes can be used for examining water quality thru the laser spectroscopic approaches in Figure 20. (Left) The whispering gallery under a dome of St. Paul’s cathedral and (Right) the sound intensity profile showing the whispering gallery mode (WGM) [148]. Such optical vortex with helical phase‐fronts can be observed using interference fringes, which can be applied for the translational motion detection of various surfaces and fluids [157]. Specifically, the bulk regions or at the heterogeneous interface of liquid water droplets [158]. When a highly beams with a local skew angle of the Poynting vector, which can be converted thru nonlinear intensity profile showing the whispering gallery mode (WGM) [148]. 3.4.2. Screw/Twisted Modes for Examining Water Quality be applied for the translational motion detection of various surfaces and fluids [157]. Specifically, the the screwed modes can be used for examining water quality thru the laser spectroscopic approaches energetic laser pulse is shooting at the target samples as the excitation source to produce the Figure 20. (Left) The whispering gallery under a dome of St. Paul’s cathedral and (Right) the sound processes such as second harmonic generation (SHG) or parametric down‐conversion (PDC) [155,156]. 3.4.2. Screw/Twisted Modes for Examining Water Quality in thescrewed modes can be used for examining water quality thru the laser spectroscopic approaches in bulk regions or at the interface of liquid droplets [158]. due When highly The screwed or twisted modes, i.e., the OAM modes, are defined as a phase structure in light absorption spectrum, the heterogeneous dielectric micro-particles would be water rotated and trapped to athe intensity profile showing the whispering gallery mode (WGM) [148]. 3.4.2. Screw/Twisted Modes for Examining Water Quality Such optical vortex with helical phase‐fronts can be observed using interference fringes, which can the bulk regions or at the heterogeneous interface of liquid water droplets [158]. When a highly energetic laser pulse is shooting at the target samples as the excitation source to produce the absorption The screwed or twisted modes, i.e., the OAM modes, are defined as a phase structure in light scattering based on the intermolecular interaction between thewhich OH radical andconverted water molecule, beams with a local skew angle of the Poynting vector, can be thru nonlinear be applied for the translational motion detection of various surfaces and fluids [157]. Specifically, the energetic laser pulse is shooting at the target samples as the excitation source to produce The screwed or twisted modes, i.e., the OAM modes, are defined as a phase structure in light spectrum, the dielectric micro-particles would be rotated and due the scattering based on the beams with a local skew angle of the Poynting vector, can tobe converted thru nonlinear whereas the OAM modes would be partially quenched due to trapped thewhich corresponding water asymmetric processes such as second harmonic generation (SHG) or parametric down‐conversion (PDC) [155,156]. 3.4.2. Screw/Twisted Modes for Examining Water Quality screwed modes can be used for examining water quality thru the laser spectroscopic approaches in absorption the dielectric micro‐particles would be rotated and sample trapped due to the beams a interaction local skew angle of Poynting vector, which can be converted thru nonlinear the intermolecular between thethe OH radical water molecule, the OAM modes stretch with and spectrum, OH radical stretch, depending on the and quality and purity ofwhereas water [159]. processes such as second harmonic generation (SHG) or parametric down‐conversion (PDC) [155,156]. Such optical vortex with helical phase‐fronts can be observed using interference fringes, which can the bulk regions or at the heterogeneous interface of liquid water droplets [158]. When a highly The screwed or twisted modes, i.e., the OAM modes, are defined as a phase structure in light scattering based on the intermolecular interaction between the OH radical and water molecule, Likewise, such measurement method works on the gaseous environment of the atmosphere or ice processes such as second harmonic generation (SHG) or parametric down‐conversion (PDC) [155,156]. would be partially quenched due to the corresponding water asymmetric stretch and OH radical Such optical vortex with helical phase‐fronts can be observed using interference fringes, which can be applied for the translational motion detection of various surfaces and fluids [157]. Specifically, the energetic laser pulse is shooting at the Poynting target samples as the excitation source systems to produce the beams with a The local the vector, which can be converted thru nonlinear crystals [160]. benefits ofangle using screwed the laser spectroscopic sensing whereas the OAM modes would be partially quenched due to the corresponding water asymmetric Such optical vortex with helical phase‐fronts can be observed using interference fringes, which can stretch, depending on theskew quality andof purity ofmodes water in sample [159]. Likewise, such measurement be applied for the translational motion detection of various surfaces and fluids [157]. Specifically, the screwed modes can be used for examining water quality thru the laser spectroscopic approaches in absorption spectrum, the stretch, dielectric micro‐particles would be rotated and trapped due to the compared with the gaseous conventional approach have been summarized Table 3purity below. processes such as second harmonic generation (SHG) or parametric down‐conversion (PDC) [155,156]. stretch and radical depending on the quality and of [158]. water sample [159]. be applied for the translational motion detection of various surfaces and fluids [157]. Specifically, the method works onOH the environment of the atmosphere orinice crystals [160]. The When benefits screwed modes can be used for examining water quality thru the laser spectroscopic approaches in the bulk regions or at the heterogeneous interface of liquid water droplets a of highly scattering based on the intermolecular interaction between the OH radical and water molecule, Such optical vortex with helical phase‐fronts can be observed using interference fringes, which can Likewise, such measurement method works on the gaseous environment of the atmosphere or ice screwed modes can be used for examining water quality thru the laser spectroscopic approaches in usingthe screwed modes or in at the laser spectroscopic sensing systems compared with the conventional bulk the heterogeneous interface liquid water droplets [158]. When a highly energetic laser is at the target as the excitation to produce the Table regions 3. The pulse benefits ofshooting using screwed modes in samples the of laser spectroscopic sensing source systems in whereas the OAM modes would be partially quenched due to the corresponding water asymmetric be applied for the translational motion detection of various surfaces and fluids [157]. Specifically, the crystals [160]. The benefits of using screwed modes in the laser spectroscopic sensing systems the bulk regions or at the heterogeneous interface of liquid water droplets [158]. When a highly approach have been summarized in Table 3 below. energetic laser pulse is shooting at the target samples as the excitation source to produce the absorption the stretch, dielectric micro‐particles rotated trapped due to comparison withradical the conventional approach. stretch and spectrum, OH depending on the would quality be and purity and of water sample [159]. screwed modes can be used for examining water quality thru the laser spectroscopic approaches in compared with the conventional approach have been summarized in Table 3 below. energetic laser pulse is shooting at the target samples as the excitation source to produce absorption spectrum, the dielectric micro‐particles would the be rotated and and trapped due to the scattering based on the intermolecular interaction between OH radical water molecule, Advanced Laser Spectroscopic Sensing Likewise, such measurement method works on the gaseous environment of the atmosphere or ice the bulk regions or at the heterogeneous interface of liquid water droplets [158]. When a highly Table 3. The benefits of using screwed modes in the laser spectroscopic sensing systems in comparison Conventional Laser Spectroscopic Approach absorption spectrum, the dielectric micro‐particles would be Using rotated and Modes trapped due to the scattering based on the intermolecular interaction between the OH Screwed radical and water molecule, whereas the OAM modes would be partially quenched due to the corresponding water asymmetric crystals [160]. The benefits using screwed modes in System the the laser spectroscopic sensing systems energetic laser pulse is intermolecular shooting at the target samples as excitation source to produce Table 3. The benefits of of using screwed modes in the laser spectroscopic sensing systems in the with the conventional approach. scattering based on the interaction between the OH radical and water molecule, whereas the OAM modes would be partially quenched due to the corresponding water asymmetric quality Increasesand the overall number of parallel stretch and OH radical stretch, depending on the purity of water sample [159]. compared with the conventional approach have been summarized in Table 3 below. comparison with the conventional approach. absorption the stretch, dielectric micro‐particles be rotated and trapped due to the whereas the OAM modes would be partially quenched due to the corresponding water asymmetric channels, each as an individual sensorsample stretch and spectrum, OH radical depending on the would quality and purity of water [159]. Likewise, such measurement method works on the gaseous environment of the atmosphere or ice Advanced Laser Spectroscopic Sensing scattering based on the intermolecular interaction between the OH radical and water molecule, Appropriate for the detection of stretch and OH radical depending on the quality and purity of water sample [159]. Conventional Laserstretch, Spectroscopic Approach Likewise, such measurement method works on the gaseous environment of the atmosphere or ice Advanced Laser Spectroscopic Sensing crystals [160]. The benefits using screwed modes in laser the laser spectroscopic sensing systems Table The benefits of of using screwed modes in the spectroscopic sensing System Using Screwed Modes •3. Conventional Laser Spectroscopic Approach Non-intrusive remote sensing broadband multiple absorption linessystems in whereas the OAM modes would be partially quenched due to the corresponding water asymmetric Likewise, such measurement method works on the gaseous environment of the atmosphere or ice System Using Screwed Modes crystals [160]. The benefits of using screwed modes in the laser spectroscopic sensing systems compared with the conventional approach have been summarized in Table 3 below. comparison with the conventional approach. • Monitor concentration in gas phase Higher sensitivity and selectivity stretch OH radical stretch, depending on the quality and purity of water sample [159]. Pros and crystals [160]. The benefits of affordable using screwed in the laser spectroscopic systems Increases the overall number of parallel Increases the overall of compared with the conventional approach have been summarized in Table 3 below. • Compact, robust and in harsh modes Better spectral efficiency and number reachsensing Likewise, such measurement method works on the gaseous environment of the atmosphere or ice Advanced Laser Spectroscopic Sensing parallel channels, each as an channels, each as an individual sensor compared with the conventional approach have been summarized in Table 3 below. operating environments More modes → error correction → systems in Table 3. Conventional Laser Spectroscopic Approach The benefits of using screwed modes in the laser spectroscopic sensing System Using Screwed Modes [160]. crystals The benefits benefits using screwed modes in the laser spectroscopic sensing individual sensor sensing Appropriate for the detection of noise reduction Table 3. The of of using screwed modes in the laser spectroscopic systems systems in comparison with the conventional approach. Increases the overall number of parallel Appropriate for the detection of Improves signal contrast by Encoding, • The Non‐intrusive remote sensing broadband multiple absorption lines compared with the conventional approach have been summarized in Table 3 below. Table 3. benefits of using screwed modes in the laser spectroscopic sensing systems in comparison with the conventional approach. such as code modulation broadband multiple channels, each as an individual sensor • remote sensing •Non-intrusive Monitor concentration in gas phase Advanced Laser Spectroscopic Sensing Higher sensitivity and selectivity comparison with the conventional approach. Conventional Laser Spectroscopic Approach Pros • Only one spatial component of the field Advanced Laser Spectroscopic Sensing System Using Screwed Modes absorption lines • 3. concentration in gas phase modes in the Appropriate for the detection of •Monitor Compact, robust and affordable in harsh Better spectral efficiency and reach Table The benefits of using screwed laser spectroscopic sensing systems in Needs intensive signal processing Conventional Laser Spectroscopic Approach Pros • vectors captured System Using Screwed Modes Advanced Laser Spectroscopic Sensing Increases the overall number of parallel Higher sensitivity and selectivity robust and affordable in harsh •Compact, Non‐intrusive remote sensing broadband multiple absorption lines operating environments More modes → error correction → Complexity of OAM measurement comparison with the conventional approach. Conventional Laser Spectroscopic Approach • operating Relies on environments small change in power System Using Screwed Modes Increases the overall number of parallel Better spectral efficiency channels, each as an individual sensor • Monitor concentration in gas phase noise reduction Cons (That’s Higher sensitivity and selectivity why we need MIMO DSP)and reach Pros • Trade-off between sensitivity and selectivity Advanced Laser Spectroscopic Sensing Increases the overall number of parallel More modes → error correction → channels, each as an individual sensor Appropriate for the detection of • Conventional Laser Spectroscopic Approach Compact, robust and affordable in harsh Lack ofBetter spectral efficiency and reach theory for OAM features in Improves signal contrast by Encoding, due to limited wavelength/mode(s) System Using Screwed Modes noise reduction specific laser spectroscopic system. channels, each as an individual sensor Appropriate for the detection of • operating environments Non‐intrusive remote sensing broadband multiple absorption lines More modes → error correction → such as code modulation • Sensitivity deteriorated by noise Improves Increases the overall number of parallel signal contrast by Appropriate for the detection of •• Non‐intrusive remote sensing broadband multiple absorption lines Monitor concentration in gas phase Higher sensitivity and selectivity noise reduction Only one spatial component of the field Pros Needs intensive signal processing Encoding, such as code modulation channels, each as an individual sensor Non‐intrusive remote sensing broadband multiple absorption lines Monitor concentration in gas phase Higher sensitivity and selectivity • Compact, robust and affordable in harsh Better spectral efficiency and reach Improves signal contrast by Encoding, vectors captured Pros Complexity of OAM measurement Appropriate for the detection of Monitor concentration in gas phase Higher sensitivity and selectivity Compact, robust and affordable in harsh Better spectral efficiency and reach More modes → error correction → such as code modulation •• operating environments Relies on small change in power Pros Cons (That’s why we need MIMO DSP) •• Trade‐off between sensitivity and selectivity Non‐intrusive remote sensing broadband multiple absorption lines Compact, robust and affordable in harsh Better spectral efficiency and reach operating environments More modes → error correction → noise reduction Only one spatial component of the field Needs intensive signal processing • due to limited wavelength/mode(s) Monitor concentration in gas phase Lack of theory for OAM features in Higher sensitivity and selectivity operating environments More modes → error correction → noise reduction Improves signal contrast by Encoding, vectors captured Pros specific laser spectroscopic system. Compact, robust and affordable in harsh Complexity of OAM measurement Better spectral efficiency and reach noise reduction Improves signal contrast by Encoding, such as code modulation Relies on small change in power •• Sensitivity deteriorated by noise Cons (That’s why we need MIMO DSP) operating environments More modes → error correction → such as code modulation Improves signal contrast by Encoding, Only one spatial component of the field • Trade‐off between sensitivity and selectivity Lack of theory for OAM features in Needs intensive signal processing noise reduction such as code modulation • due to limited wavelength/mode(s) Only one spatial component of the field vectors captured Needs intensive signal processing Complexity of OAM measurement specific laser spectroscopic system. Improves signal contrast by Encoding, Only one spatial component of the field vectors captured Relies on small change in power • Sensitivity deteriorated by noise Needs intensive signal processing (That’s why we need MIMO DSP) Complexity of OAM measurement Cons
the bulk regions or at the heterogeneous interface of liquid the water droplets When a highly absorption spectrum, the dielectric micro‐particles would be rotated and [158]. trapped due to the scattering based on the intermolecular interaction between OH radical and water molecule, energetic laser pulse is shooting at the target samples as the excitation source to produce the scattering based on the intermolecular interaction between the OH radical and water molecule, whereas the OAM modes would be partially quenched due to the corresponding water asymmetric absorption spectrum, the dielectric micro‐particles would be rotated and trapped due to the whereas the OAM modes would be partially quenched due to the corresponding water asymmetric stretch and OH radical stretch, depending on the quality and purity of water sample [159]. scattering based the intermolecular interaction between OH radical water molecule, stretch and OH on radical stretch, depending on the quality the and purity of and water sample [159]. Likewise, such measurement method works on the gaseous environment of the atmosphere or ice whereas the OAM modes would be partially quenched due to the corresponding water asymmetric Likewise, such measurement method works on the gaseous environment of the atmosphere or ice [160]. The benefits of using screwed modes in the laser spectroscopic sensing systems Sensorscrystals 2016, 16, 1387 22 of 35 stretch and OH radical stretch, depending on the quality and purity of water sample [159]. crystals [160]. The benefits of using screwed modes in the laser spectroscopic sensing systems compared with the conventional approach have been summarized in Table 3 below. Likewise, such measurement method works on the gaseous environment of the atmosphere or ice compared with the conventional approach have been summarized in Table 3 below. Table 3. Cont. crystals [160]. The benefits benefits using screwed modes in laser the laser spectroscopic sensing Table 3. The of of using screwed modes in the spectroscopic sensing systems systems in compared with the conventional approach have been summarized in Table 3 below. Table 3. The benefits of using screwed modes in the laser spectroscopic sensing systems in comparison with the conventional approach.
Advanced Laser Spectroscopic Sensing comparison with the conventional approach. Conventional Laser Spectroscopic Approach System Using Screwed Modes Advanced Laser Spectroscopic Sensing Table 3. Conventional Laser Spectroscopic Approach The benefits of using screwed modes in the laser spectroscopic sensing systems in Advanced Laser Spectroscopic Sensing System Using Screwed Modes comparison with the conventional approach. Conventional Laser Spectroscopic Approach Only one spatial component of the field System Using Screwed Modes • Needs Increases the overall number of parallel intensive signal processing vectors captured Advanced Laser Spectroscopic Sensing Increases the overall number of parallel Complexity of OAM measurement channels, each as an individual sensor Conventional Laser Spectroscopic Approach • Relies on small change in power System Using Screwed Modes (That’s why we need MIMO DSP) channels, each as an individual sensor Appropriate for the detection of Cons • Trade-off between sensitivity and selectivity Lack Increases the overall number of parallel of theory for OAM features in Appropriate for the detection of •dueNon‐intrusive remote sensing broadband multiple absorption lines to limited wavelength/mode(s) laser spectroscopic system. channels, each as an individual sensor •Sensitivity Non‐intrusive remote sensing broadband multiple absorption lines Monitor concentration in gas phase specific Higher sensitivity and selectivity • deteriorated by noise Pros Appropriate for the detection of Monitor concentration in gas phase Higher sensitivity and selectivity • Compact, robust and affordable in harsh Better spectral efficiency and reach Sensors 2016, 16, 1387 22 of 34 Pros Non‐intrusive remote sensing broadband multiple absorption lines • operating environments Compact, robust and affordable in harsh Better spectral efficiency and reach More modes → error correction → • operating environments Monitor concentration in gas phase Higher sensitivity and selectivity More modes → error correction → noise reduction 3.4.3. Shaping Cantilever Deflection Deflection Measurements Pros Beam 3.4.3. Optical Optical Beam Shaping for for Improving Improving Cantilever • Compact, robust and affordable in harsh Measurements Better spectral efficiency and reach noise reduction Improves signal contrast by Encoding, Last all, the the operating environments optical beam beam profiles profiles can can be be modified modified easily easily by spatial light light phase phase modulator modulator More modes → error correction → by Improves signal contrast by Encoding, such as code modulation Last of of all, optical aa spatial noise reduction such as code modulation • examples Only one spatial component of the field (SLPM), while of at SLPM are presented in in (SLPM), while the the examples of the the observed observed beam beam profile profile reflected reflected at the the SLPM are presented Needs intensive signal processing Improves signal contrast by Encoding, • asvectors captured Only one spatial component of the field Figure 21, with N the number of the rotated micro-mirror in series [161]. The micro-cantilevers Figure 21, with N as the number of the rotated micro-mirror in series [161]. The micro-cantilevers in in Needs intensive signal processing Complexity of OAM measurement such as code modulation vectors captured • Relies on small change in power atomic force microscopes (AFM) could be employed as ultrasensitive measure (That’s why we need MIMO DSP) Complexity of OAM measurement Cons microscopes (AFM) could be employed as ultrasensitive atomic force sensors to sensors measure to biochemical Only one spatial component of the field Relies on small change in power well as temperature fluctuations [162,163]. Such • Trade‐off between sensitivity and selectivity biochemical reactions via surface stress imaging as Needs intensive signal processing Cons (That’s why we need MIMO DSP) Lack of theory for OAM features in reactions via surface stress imaging as well as temperature fluctuations [162,163]. Such detection vectors captured • due to limited wavelength/mode(s) Trade‐off between sensitivity and selectivity Complexity of OAM measurement detection system can be tailored thru optical beam shaping techniques to further boost accuracy to Lack of theory for OAM features in specific laser spectroscopic system. system can be tailored thru optical beam shaping techniques further boost the the accuracy of Relies on small change in power • due to limited wavelength/mode(s) Sensitivity deteriorated by noise Cons (That’s why we need MIMO DSP) specific laser spectroscopic system. of cantilever deflection measurements,while whilethe therelationship relationshipbetween betweenthe thecantilever cantilever deflection deflection and and cantilever deflection measurements, Trade‐off between sensitivity and selectivity • Sensitivity deteriorated by noise linearized Lack of theory for OAM features in the photo-sensitive detector (PSD) measurement can be simply by means of geometric due to limited wavelength/mode(s) the photo-sensitive detector (PSD) measurement can be simply linearized by means of geometric optics specific laser spectroscopic system. optics arrangement and standard analysis of the optical beam/cantilever [10,164]. Sensitivity deteriorated by noise arrangement and• standard vector vector analysis of the optical beam/cantilever [10,164].
Figure 21. Examples of of the the observed observed beam beam profile profile reflected reflected at at aa spatial Figure 21. Examples spatial light light phase phase modulator modulator (SLPM) [161]. (SLPM) [161].
4. Prospective ProspectiveOutlook Outlook In this section, a prospective outlook for the summary, challenges and further opportunities of SDM optical including various markets andand applications for optical sensing sensingtechnologies technologieshas hasbeen beenprovided, provided, including various markets applications the SDM technologies, multiplexing merits in sensing cost comparison for the SDM technologies, multiplexing merits in system sensingdesigns, systemcomponent designs, component cost for SDM measurement systems, as well as the effects of noise and nonlinearity upon comparison for SDM measurement systems, as well as the effects of noise and nonlinearity upon the overall performance.
4.1. Summary and Comparison In this subsection, various markets and applications of SDM-based sensing systems are explored first. On the other hand, how these complex mode multiplexing techniques can improve the already working fiber-optic sensor techniques is discussed as well. 4.1.1. SDM Sensing Systems for Various Markets and Applications
Sensors 2016, 16, 1387
23 of 35
4.1. Summary and Comparison In this subsection, various markets and applications of SDM-based sensing systems are explored first. On the other hand, how these complex mode multiplexing techniques can improve the already working fiber-optic sensor techniques is discussed as well. 4.1.1. SDM Sensing Systems for Various Markets and Applications The wide range of applications for SDM-based measurement systems are covered in this subsection. The distributed optical sensors using FMFs are quite useful in civil and geotechnical structure health monitory, safety for tunnels, bridges, dams, pipelines, dikes and buildings, fire detection, well-integrity monitoring as well as downhole seismic acquisition. The core multiplexing based systems are popular in the fields of oil and gas production, power cable monitoring, leakage detection at dikes and dams, integrity of liquid natural gas (LNG) carriers and terminals, railway safety monitoring. FBG sensors based on multiplexing are suitable for structure health monitoring of dams, highways, bridges, railways, aircraft wings, spacecraft fuel tanks, and pressure, displacement, acceleration monitoring. Whispering gallery modes are particularly advantageous for label-free detection of macromolecules such as proteins and DNA, as well as bacteria and animal cells, while the screw or twisted modes are for examining water quality, gaseous environment of the atmosphere, ice crystals, as well as motion detection of various surfaces and fluids. Last but not least, optical beam shaping can be used for measuring biochemical reactions through surface stress imaging, and improving cantilever deflection measurements of atomic force microscopes (AFM). A comparison table covering the examples of SDM sensors, their measured parameters, as well as the corresponding sensor applications is shown in Table 4. Table 4. SDM sensing systems for various markets and applications. Example of SDM-Based Sensing Systems
Corresponding Markets and Applications
Measured Parameters
Reference
Distributed sensors based on mode-division multiplexing
Civil and geotechnical structure health monitory, safety for tunnels, bridges, dams, pipelines, dikes and buildings, fire detection, well-integrity monitoring and downhole seismic acquisition
Temperature; Strain; Pressure; Stress; Force; Acoustic; Vibration; Bending; Refractive index
[14,28,38,127]
Distributed sensors based on core multiplexing
Oil and gas production, power cable monitoring, leakage detection at dikes and dams, integrity of liquid natural gas (LNG) carriers and terminals, railway safety monitoring
Temperature; Strain; Pressure; Stress; Vibration; Bending; Shape; Displacement
[49,80,136–140]
Fiber Bragg grating sensors based on multiplexing
Structure health monitoring of dams, highways, bridges, railways, aircraft wings, as well as spacecraft fuel tanks; pressure, displacement, or acceleration monitoring
Temperature; Strain; Pressure; Bending; Shape; Displacement; Acceleration
[84,141–145]
Whispering gallery modes for chemical species measurements
Label-free detection of macromolecules such as proteins and DNA, as well as bacteria and animal cells; temperature, pressure sensors
Temperature, Pressure, Force, Refractive Index, Species Concentration, Biochemical Compounds
[9,147–152]
Screw/twisted modes for examining water quality
Examining water quality, gaseous environment of the atmosphere, ice crystals; atmospheric turbulence monitoring, motion detection of various surfaces and fluids.
Species Concentration, Biochemical Compounds, Shape; Displacement; Acceleration
[53–55,70,71,155–159]
a zeolite film on the external surface of an optical microsphere with target Sensors 2016, 16, 1387 couplers via coating a zeolite film on the external surface of an optical microsphere with 21 of 34 target 21 of 34 a on the sphere surface, attributable to the sensitivity of their evanescent field zeolite film on the external surface of an optical microsphere with target Sensors 2016, 16, 1387 21 of 34 biomolecules attached on the sphere surface, attributable to the sensitivity of their evanescent field 21 of 34 on the sphere surface, attributable to the sensitivity of their evanescent field Sensors 2016, 16, 1387 couplers via coating a zeolite film on the external surface of an optical microsphere with 21 of 34 target changes of nearby entities [152–154]. to the refractive index changes of nearby entities [152–154]. a changes of nearby entities [152–154]. zeolite film on the external surface of an optical microsphere with target couplers via coating a zeolite film on the external surface of an optical microsphere with target biomolecules attached on the sphere surface, attributable to the sensitivity of their evanescent field a on the sphere surface, attributable to the sensitivity of their evanescent field zeolite film on the external surface of an optical microsphere with target couplers via coating a zeolite film on the external surface of an optical microsphere with target biomolecules attached on the sphere surface, attributable to the sensitivity of their evanescent field to the refractive index changes of nearby entities [152–154]. Sensors 2016, 16, 1387 24 of 35 on the sphere surface, attributable to the sensitivity of their evanescent field changes of nearby entities [152–154]. biomolecules attached on the sphere surface, attributable to the sensitivity of their evanescent field to the refractive index changes of nearby entities [152–154]. Sensors 2016, 16, 1387 21 of 34 changes of nearby entities [152–154]. to the refractive index changes of nearby entities [152–154]. 21 of 34
Table Cont. Sensors 2016, 16, 1387 21 of 34 couplers via coating a zeolite film on the 4.external surface of an optical microsphere with 21 of 34 target 21 of 34 a zeolite film on the external surface of an optical microsphere with target biomolecules attached on the sphere surface, attributable to the sensitivity of their evanescent field of SDM-Based Markets Sensors 2016, 16, 1387 21 of 34 a on the sphere surface, attributable to the sensitivity of their evanescent field zeolite film on the Example external couplers surface via coating of an optical a Corresponding zeolite microsphere film on the with external target surface Parameters of an optical microsphere target Measured Reference with 21 of 34 to the refractive index changes of nearby entities [152–154]. Sensing Systems and Applications a on the sphere surface, attributable to the sensitivity of their evanescent field zeolite film on the external surface of an optical microsphere with target biomolecules attached on the sphere surface, attributable to the sensitivity of their evanescent field changes of nearby entities [152–154]. deflection Sensors 2016, 16, 1387 a on the sphere surface, attributable to the sensitivity of their evanescent field zeolite film on the external couplers surface via coating of an Cantilever optical a zeolite microsphere film on the with external target surface of an optical microsphere with 21 of 34 target changes of nearby entities [152–154]. to the refractive index changes of nearby entities [152–154]. measurements, atomic force Temperature; Species Optical beam shaping for on the sphere surface, attributable to the sensitivity of their evanescent field biomolecules attached on the sphere surface, attributable to the sensitivity of their evanescent field changes of nearby entities [152–154]. microscopes (AFM), measuring Concentration, Biochemical improving cantilever couplers via coating a zeolite reactions film on external surface of an optical [10,162–164] microsphere with target biochemical viathe surface Compounds; changes of nearby entities [152–154]. to the refractive index changes of nearby entities [152–154]. deflection measurements e whispering gallery under a dome of St. Paul’s cathedral and (Right) the sound Figure 20. (Left) The whispering gallery under a dome of St. Paul’s cathedral and (Right) the sound stress imaging and Refractive Index biomolecules attached on the sphere surface, attributable to the sensitivity of their evanescent field e whispering gallery under a dome of St. Paul’s cathedral and (Right) the sound owing the whispering gallery mode (WGM) [148]. temperature fluctuations intensity profile showing the whispering gallery mode (WGM) [148]. to the refractive index changes of nearby entities [152–154].
owing the whispering gallery mode (WGM) [148]. Figure 20. (Left) The whispering gallery under a dome of St. Paul’s cathedral and (Right) the sound e whispering gallery under a dome of St. Paul’s cathedral and (Right) the sound Modes for Examining Water Quality Figure 20. (Left) The whispering gallery under a dome of St. Paul’s cathedral and (Right) the sound intensity profile showing the whispering gallery mode (WGM) [148]. 3.4.2. Screw/Twisted Modes for Examining Water Quality 4.1.2. Multiplexing Merits in Sensing System Designs e whispering gallery under a dome of St. Paul’s cathedral and (Right) the sound owing the whispering gallery mode (WGM) [148]. Modes for Examining Water Quality Figure 20. (Left) The whispering gallery under a dome of St. Paul’s cathedral and (Right) the sound intensity profile showing the whispering gallery mode (WGM) [148]. wisted modes, i.e., the OAM modes, are defined as a phase structure in light The screwed or twisted modes, i.e., the OAM modes, are defined as a phase structure in light owing the whispering gallery mode (WGM) [148]. The subsection concentrates on explaining how these complex mode/core multiplexing techniques intensity profile showing the whispering gallery mode (WGM) [148]. 3.4.2. Screw/Twisted Modes for Examining Water Quality
wisted modes, i.e., the OAM modes, are defined as a phase structure in light kew angle of the could Poynting vector, which be angle converted thru nonlinear Modes for Examining Water Quality beams with a local can skew of the Poynting vector, which can be converted nonlinear improve the existing fiber-optic sensor techniques such as distributed temperature sensing (DTS) thru 3.4.2. Screw/Twisted Modes for Examining Water Quality kew angle of the Poynting vector, which can be converted thru nonlinear The screwed or twisted modes, i.e., the OAM modes, are defined as a phase structure in light nd harmonic generation (SHG) or parametric down‐conversion (PDC) [155,156]. Modes for Examining Water Quality processes such as second harmonic generation (SHG) or parametric down‐conversion (PDC) [155,156]. and distributed acoustic sensing (DAS). As mentioned earlier, in conventional SMF-based sensing 3.4.2. Screw/Twisted Modes for Examining Water Quality Figure 20. (Left) The whispering gallery under a dome of St. Paul’s cathedral and (Right) the sound wisted modes, i.e., the OAM modes, are defined as a phase structure in light The screwed or twisted modes, i.e., the OAM modes, are defined as a phase structure in light nd harmonic generation (SHG) or parametric down‐conversion (PDC) [155,156]. beams with a local toskew angle only of the vector, which be converted nonlinear th helical phase‐fronts can be observed using interference fringes, which can thru Such optical vortex with helical phase‐fronts can be observed using interference fringes, which can systems, DTS is dedicated determine thePoynting local temperature based can on Raman scattering, while e whispering gallery under a dome of St. Paul’s cathedral and (Right) the sound wisted modes, i.e., the OAM modes, are defined as a phase structure in light intensity profile showing the whispering gallery mode (WGM) [148]. kew angle of the Poynting vector, which can be angle converted thru nonlinear The screwed or twisted modes, i.e., the OAM modes, are defined as a phase structure in light beams with a local skew of the Poynting vector, which can be converted thru nonlinear th helical phase‐fronts can be observed using interference fringes, which can processes such as second harmonic generation (SHG) or parametric down‐conversion (PDC) [155,156]. slational motion detection of various surfaces and fluids [157]. Specifically, the be applied for the translational motion detection of various surfaces and fluids [157]. Specifically, the typically provides strain determinations via Rayleigh scattering [41]. For SDM-based systems e whispering gallery under a dome of St. Paul’s cathedral and (Right) the sound Figure 20. (Left) The whispering gallery under a dome of St. Paul’s cathedral and (Right) the sound owing the whispering gallery mode (WGM) [148]. kew angle of the DAS Poynting vector, be converted thru nonlinear nd harmonic generation (SHG) or parametric down‐conversion (PDC) [155,156]. beams with which a local can skew angle of the Poynting vector, which can be converted thru nonlinear processes such as second harmonic generation (SHG) or parametric down‐conversion (PDC) [155,156]. slational motion detection of various surfaces and fluids [157]. Specifically, the Such optical vortex with helical phase‐fronts can be observed using interference fringes, which can used for examining water quality thru the laser spectroscopic approaches in screwed modes can be used for examining water quality thru the laser spectroscopic approaches in e whispering gallery under a dome of St. Paul’s cathedral and (Right) the sound using FMF or MCF, each of the modes or cores within the sensing medium can serve as an orthogonal owing the whispering gallery mode (WGM) [148]. intensity profile showing the whispering gallery mode (WGM) [148]. 3.4.2. Screw/Twisted Modes for Examining Water Quality nd harmonic generation (SHG) or parametric down‐conversion (PDC) [155,156]. th helical phase‐fronts can be observed using interference fringes, which can processes such as second harmonic generation (SHG) or parametric down‐conversion (PDC) [155,156]. Such optical vortex with helical phase‐fronts can be observed using interference fringes, which can used for examining water quality thru the laser spectroscopic approaches in be applied for the translational motion detection of various surfaces and fluids [157]. Specifically, the the heterogeneous interface of geophone liquid water [158]. When a highly e whispering gallery under a dome of St. Paul’s cathedral and (Right) the sound Figure 20. (Left) The whispering gallery under a dome of St. Paul’s cathedral and (Right) the sound the bulk regions or for at droplets the specific heterogeneous interface of thus liquid water droplets [158]. When a highly owing the whispering gallery mode (WGM) [148]. interrogator or one sensing parameter, responding to an extensive variety Modes for Examining Water Quality th helical phase‐fronts can be observed using interference fringes, which can slational motion detection of various surfaces and fluids [157]. Specifically, the Such optical vortex with helical phase‐fronts can be observed using interference fringes, which can be applied for the translational motion detection of various surfaces and fluids [157]. Specifically, the the heterogeneous interface of liquid water droplets [158]. When a highly The screwed or twisted modes, i.e., the OAM modes, are defined as a phase structure in light screwed modes can be used for examining water quality thru the laser spectroscopic approaches in is shooting at the target samples as the excitation source to produce the owing the whispering gallery mode (WGM) [148]. intensity profile showing the whispering gallery mode (WGM) [148]. Modes for Examining Water Quality 3.4.2. Screw/Twisted Modes for Examining Water Quality energetic laser pulse is shooting at the target samples as the excitation source to produce the of measurands simultaneously. For example, as shown in Table 5 below, since MMF has a higher slational motion detection of various surfaces and fluids [157]. Specifically, the used for examining water quality thru the laser spectroscopic approaches in be applied for the translational motion detection of various surfaces and fluids [157]. Specifically, the wisted modes, i.e., the OAM modes, are defined as a phase structure in light screwed modes can be used for examining water quality thru the laser spectroscopic approaches in is shooting at the target samples as the excitation source to produce the beams with a local skew angle of the Poynting vector, which can droplets be converted thru nonlinear Figure 20. (Left) The whispering gallery under a dome of St. Paul’s cathedral and (Right) the sound the bulk regions or at the heterogeneous interface of liquid water [158]. When a highly the dielectric micro‐particles would be rotated and trapped due to Modes for Examining Water Quality absorption spectrum, the dielectric micro‐particles would be rotated and trapped due to backscattering coefficient than SMF, the DTS systems with multiplexing can avoid the usage of the used for examining water quality thru the laser spectroscopic approaches in the heterogeneous interface of liquid water droplets [158]. When a highly wisted modes, i.e., the OAM modes, are defined as a phase structure in light The screwed or twisted modes, i.e., the OAM modes, are defined as a phase structure in light screwed modes can be used for examining water quality thru the laser spectroscopic approaches in kew angle of the Poynting vector, which can be converted thru nonlinear the bulk regions or at the heterogeneous interface of liquid water droplets [158]. When a highly the dielectric micro‐particles would be rotated and trapped due to the processes such as second harmonic generation (SHG) or parametric down‐conversion (PDC) [155,156]. intensity profile showing the whispering gallery mode (WGM) [148]. Modes for Examining Water Quality 3.4.2. Screw/Twisted Modes for Examining Water Quality energetic laser pulse is shooting at the target samples as the excitation source to produce the he intermolecular interaction between the OH radical and water molecule, scattering based on intermolecular interaction between the OH radical and water molecule, high-peak power pulses forthe input, while providing spatial resolution [142]. Meanwhile, the heterogeneous interface of liquid water droplets [158]. When a enhanced highly wisted modes, i.e., the OAM modes, are defined as a phase structure in light is shooting at the the target samples as the source to produce the kew angle of Poynting beams vector, with which a local can skew be angle converted of the thru Poynting nonlinear vector, which can droplets be converted thru nonlinear the bulk regions or excitation at the heterogeneous interface of would liquid water [158]. When a highly nd harmonic generation (SHG) or parametric down‐conversion (PDC) [155,156]. energetic laser pulse is shooting at the target samples as the excitation source to produce the he intermolecular interaction between the OH radical and water molecule, Such optical vortex with helical phase‐fronts can be observed using interference fringes, which can absorption spectrum, the dielectric micro‐particles be rotated and trapped due to des would be partially quenched due to the corresponding water asymmetric due towhereas the OAM modes would be partially quenched due to the corresponding water asymmetric modal dispersion and nonlinearity accumulations, DTS systems with multiplexing are wisted modes, i.e., the OAM modes, are defined as a phase structure in light The screwed or twisted modes, i.e., the OAM modes, are defined as a phase structure in light is shooting at the the target samples as the excitation source to produce the the kew angle of Poynting vector, which can be converted thru nonlinear the dielectric micro‐particles would be rotated and trapped due to nd harmonic generation (SHG) or parametric down‐conversion (PDC) [155,156]. processes such as second harmonic generation (SHG) or parametric down‐conversion (PDC) [155,156]. 3.4.2. Screw/Twisted Modes for Examining Water Quality energetic laser pulse is shooting at the target samples as the excitation source to produce the th helical phase‐fronts can be observed using interference fringes, which can absorption spectrum, the dielectric micro‐particles would be rotated and trapped due to des would be partially quenched due to the corresponding water asymmetric be applied for the translational motion detection of various surfaces and fluids [157]. Specifically, the scattering based on the intermolecular interaction between the OH radical water molecule, cal stretch, depending on the and quality and purity of sensing water sample [159]. quality stretch OH radical stretch, depending on the and purity of water sample [159]. more intended for short-to-medium distance, while SMF-based systems areand more suitable kew angle of the Poynting beams vector, with which a local can skew be angle converted of the thru Poynting nonlinear vector, which can be converted thru nonlinear the dielectric micro‐particles would be rotated and trapped due to the nd harmonic generation (SHG) or parametric down‐conversion (PDC) [155,156]. he intermolecular interaction between the OH radical and water molecule, th helical phase‐fronts can be observed using interference fringes, which can Such optical vortex with helical phase‐fronts can be observed using interference fringes, which can absorption spectrum, the dielectric micro‐particles would be rotated and trapped due to the slational motion detection of various surfaces and fluids [157]. Specifically, the scattering based and on the intermolecular interaction between the OH radical and water molecule, cal stretch, depending screwed modes can be used for examining water quality thru the laser spectroscopic approaches in on the quality purity of water sample [159]. whereas the OAM modes would be partially quenched due to the corresponding water asymmetric rement method works on the gaseous environment of the atmosphere or ice The screwed or twisted modes, i.e., the OAM modes, are defined as a phase structure in light Likewise, such measurement method works on the gaseous environment of the atmosphere or ice for long/ultra-long distance. As for DAS, since Rayleigh scattering depends on a random collection nd harmonic generation (SHG) or parametric down‐conversion (PDC) [155,156]. processes such as second harmonic generation (SHG) or parametric down‐conversion (PDC) [155,156]. he intermolecular interaction between the OH radical and water molecule, th helical phase‐fronts can be observed using interference fringes, which can des would be partially quenched due to the corresponding water asymmetric slational motion detection of various surfaces and fluids [157]. Specifically, the be applied for the translational motion detection of various surfaces and fluids [157]. Specifically, the scattering based on the intermolecular interaction between the OH radical and water molecule, used for examining water quality thru the laser spectroscopic approaches in whereas the OAM modes would be partially quenched due to the corresponding water asymmetric rement method works on the gaseous environment of the atmosphere or ice the bulk regions or at the heterogeneous interface of liquid water droplets [158]. When a highly stretch and OH radical stretch, depending on the quality and purity of sample [159]. nefits of using screwed modes in the laser spectroscopic sensing systems beams with a The local skew angle of the Poynting vector, which can be converted thru nonlinear crystals [160]. benefits of using screwed modes in (ASE) the laser spectroscopic sensing systems of phases, mode coupling and amplified spontaneous emission noise would bewater added to each th helical phase‐fronts can be observed using interference fringes, which can Such optical vortex with helical phase‐fronts can be observed using interference fringes, which can des would be partially quenched due to the corresponding water asymmetric slational motion detection of various surfaces and fluids [157]. Specifically, the cal stretch, depending on the quality and purity of water sample [159]. used for examining water quality thru the laser spectroscopic approaches in screwed modes can be used for examining water quality thru the laser spectroscopic approaches in whereas the OAM modes would be partially quenched due to the corresponding water asymmetric the heterogeneous interface of liquid water droplets [158]. When a highly stretch and OH radical stretch, depending on the quality and purity of water sample [159]. nefits of using screwed modes in the laser spectroscopic sensing systems energetic laser pulse is shooting at the target samples as the excitation source to produce the Likewise, such measurement method works on the gaseous environment of the atmosphere or ice nventional approach have been summarized in Table 3 below. processes such as second harmonic generation (SHG) or parametric down‐conversion (PDC) [155,156]. compared with the conventional approach have been summarized in Table 3 below. channel, making it a bitwater difficult toheterogeneous improve sensitivity. However, this water issue can be easily resolved via slational motion detection of various surfaces and fluids [157]. Specifically, the be applied for the translational motion detection of various surfaces and fluids [157]. Specifically, the cal stretch, depending on the quality and purity of water sample [159]. used for examining water quality thru the laser spectroscopic approaches in rement method works on the gaseous environment of the atmosphere or ice the bulk regions the heterogeneous interface of liquid or at droplets the [158]. When interface a highly of liquid droplets [158]. When a highly stretch and OH radical stretch, depending on quality and purity of water sample [159]. is shooting at the target samples as the excitation source to produce the Likewise, such measurement method works on the gaseous environment of the atmosphere or ice nventional approach have been summarized in Table 3 below. absorption spectrum, the dielectric micro‐particles would rotated and trapped due systems to the crystals [160]. The benefits of using screwed modes in the be laser spectroscopic sensing Such optical vortex with helical phase‐fronts can be observed using interference fringes, which can code modulation using MIMO DSP or precoding schemes [35,116]. used for examining water quality thru the laser spectroscopic approaches in screwed modes can be used for examining water quality thru the laser spectroscopic approaches in rement method works on the gaseous environment of the atmosphere or ice the heterogeneous interface of liquid water droplets [158]. When a highly nefits of using screwed modes in the laser spectroscopic sensing systems is shooting at the target energetic samples laser as the pulse excitation is shooting source at to the produce target samples the as the excitation source to produce the Likewise, such measurement method works on the gaseous environment of the atmosphere or ice the of dielectric micro‐particles be rotated and trapped due to efits using screwed modes in would the laser spectroscopic sensing systems in crystals [160]. The benefits of using screwed modes in laser the the laser spectroscopic systems scattering based on the intermolecular interaction between OH radical and sensing water molecule, compared with the conventional approach have been summarized in Table 3 below. Table 3. The benefits of using screwed modes in the spectroscopic sensing systems in be applied for the translational motion detection of various surfaces and fluids [157]. Specifically, the the bulk regions the heterogeneous interface of liquid water or at droplets the heterogeneous [158]. When interface a highly of liquid water droplets [158]. When a highly nefits of using screwed modes in the laser spectroscopic sensing systems is shooting at the target samples as the excitation source to produce the nventional approach have been summarized in Table 3 below. the dielectric micro‐particles absorption would spectrum, be rotated the dielectric and trapped micro‐particles due to would rotated and trapped due systems to the efits of using screwed modes in the laser spectroscopic sensing systems in crystals [160]. The benefits of using screwed modes in the be laser spectroscopic sensing he intermolecular interaction between the OH radical and water molecule, e conventional approach. compared with the conventional approach have been summarized in Table 3 below. whereas the OAM modes would be partially quenched due to the corresponding water asymmetric comparison with the conventional approach. Table 5.pulse Multiplexing comparison in DTS and DAS sensing system designs.source to produce the screwed modes can be used for examining water quality thru the laser spectroscopic approaches in is shooting at the target energetic samples laser as the excitation is shooting source at to the produce target samples the as the excitation nventional approach have been summarized in Table 3 below. the dielectric micro‐particles would be rotated and trapped due to he intermolecular interaction scattering between based the on the intermolecular interaction between the OH radical and water molecule, OH radical and water molecule, e conventional approach. compared with the conventional approach have been summarized in Table 3 below. des would be partially quenched due to the corresponding water asymmetric Table 3. OH The radical benefits stretch, of using depending screwed modes in the laser and spectroscopic sensing systems stretch and on the quality purity of water When sample [159]. the bulk regions or at the the heterogeneous interface of would liquid water droplets [158]. a in highly Advanced Laser Spectroscopic Sensing the dielectric micro‐particles absorption would spectrum, be rotated dielectric and trapped micro‐particles due to the be rotated and trapped due to the Advanced Laser Spectroscopic Sensing efits of using depending screwed modes in the laser spectroscopic sensing systems in he intermolecular interaction between the OH radical and water molecule, des would be partially quenched due to the corresponding water asymmetric whereas the OAM modes would be partially quenched due to the corresponding water asymmetric onal Laser Spectroscopic Approach Table 3. The benefits of using screwed modes in the laser spectroscopic sensing systems in cal stretch, on the quality and purity of water sample [159]. comparison with the conventional approach. Likewise, such measurement method works on the gaseous environment of the atmosphere or ice Conventional Laser Spectroscopic Approach DTS DAS source to produce the Advanced Laser Spectroscopic Sensing System Using Screwed Modes energetic laser pulse is radical shooting at water the target samples as the excitation System Using Screwed Modes between efits of using depending screwed modes in the laser spectroscopic sensing systems in he intermolecular interaction scattering based the on the intermolecular interaction between the OH and molecule, OH radical and water molecule, e conventional approach. des would be partially quenched due to the corresponding water asymmetric onal Laser Spectroscopic Approach Table 3. The benefits of of using screwed modes in the laser spectroscopic sensing systems in cal stretch, stretch on the and quality OH and radical purity stretch, of water depending sample on [159]. the quality and purity of water sample [159]. comparison with the conventional approach. rement method works on the gaseous environment of the atmosphere or ice crystals [160]. The benefits using screwed modes in the laser spectroscopic sensing systems System Using Screwed Modes Increases the overall number of parallel Have higher backscattering coefficients absorption spectrum, the dielectric micro‐particles would be rotated and trapped due to the Advanced Laser Spectroscopic Sensing Increases the overall number of parallel Mode coupling and ASE noise in e conventional approach. des would be partially quenched due to the corresponding water asymmetric whereas the OAM modes would be partially quenched due to the corresponding water asymmetric cal stretch, depending on the quality and purity of water sample [159]. comparison with the conventional approach. rement method works on the gaseous environment of the atmosphere or ice Likewise, such measurement method works on the gaseous environment of the atmosphere or ice Conventional Laser Spectroscopic Approach nefits of using screwed modes in the laser spectroscopic sensing systems compared with the conventional approach have been summarized in Table 3 below. Advanced Laser Spectroscopic Sensing Increases the overall number of parallel Avoids the usage of high peak power channels, each as an individual sensor Advanced Laser Spectroscopic Sensing System Using Screwed Modes and each mode added toand the system scattering based on the intermolecular interaction between the OH radical water molecule, channels, each as an individual sensor onal Laser Spectroscopic Approach cal stretch, depending stretch on the quality OH and radical purity stretch, of using water depending sample on [159]. the quality and purity of water sample [159]. rement method works on the gaseous environment of the atmosphere or ice Conventional Laser Spectroscopic Approach crystals [160]. nefits of using screwed modes in the The laser benefits spectroscopic of sensing screwed systems modes in the laser spectroscopic sensing systems nventional approach have been summarized in Table 3 below. Advanced Laser Spectroscopic Sensing System Using Screwed Modes pulses for input channels, each as an individual sensor Appropriate for the detection of Advanced Laser Spectroscopic Sensing System Using Screwed Modes Increases the overall number of parallel Difficult to align all modes correctly Appropriate for the detection of whereas the OAM modes would be partially quenched due to the corresponding water asymmetric onal Laser Spectroscopic Approach rement method works on the gaseous environment of the atmosphere or ice Likewise, such measurement method works on the gaseous environment of the atmosphere or ice Conventional Laser Spectroscopic Approach nefits of using screwed modes in 3. the laser spectroscopic sensing modes systems With nventional approach have been summarized in Table 3 below. compared with the conventional approach have been summarized in Table 3 below. System Using Screwed Modes Increases the overall number of parallel Provides better Table The benefits of spatial using resolution screwed in the laser spectroscopic sensing systems in Appropriate for the detection of usive remote sensing broadband multiple absorption lines System Using Screwed Modes and Increases the overall number of parallel Challenging to improve sensitivity channels, each as an individual sensor •OH Non‐intrusive remote sensing broadband multiple absorption lines stretch radical stretch, depending on the quality and purity of water sample [159]. Multiplexing crystals [160]. nefits of using screwed modes in the The laser benefits spectroscopic of using sensing screwed systems modes in the laser spectroscopic sensing systems nventional approach have been summarized in Table 3 below. Increases the overall number of parallel Modal dispersion slightly degrade spatial efits of using screwed modes in the laser spectroscopic sensing systems in channels, each as an individual sensor usive remote sensing broadband multiple absorption lines concentration in gas phase Higher sensitivity and selectivity comparison with the conventional approach. Signals Increases the overall number of parallel in different modes channels, each as an individual sensor Appropriate for the detection of • Monitor concentration in gas phase Higher sensitivity and selectivity Likewise, such measurement method works on the gaseous environment of the atmosphere or ice Pros nventional approach have been summarized in Table 3 below. compared with the conventional approach have been summarized in Table 3 below. resolution mainly for efits of using screwed modes Table in the 3. laser The spectroscopic benefits of using sensing screwed systems modes in in the laser spectroscopic sensing systems channels, each as an individual sensor Appropriate for the detection of concentration in gas phase Higher sensitivity and selectivity t, robust and affordable in harsh Better spectral efficiency and reach e conventional approach. propagate in different speeds, only in channels, each as an individual sensor Appropriate for the detection of Non‐intrusive remote sensing broadband multiple absorption lines • Compact, robust and affordable in harsh Better spectral efficiency and reach Advanced Laser Spectroscopic Sensing crystals [160]. The benefits of using screwed modes in the laser spectroscopic sensing systems short-to-medium distance efits of using screwed modes in the laser spectroscopic sensing systems in Appropriate for the detection of usive remote sensing broadband multiple absorption lines t, robust and affordable in harsh Better spectral efficiency and reach g environments More modes → error correction → e conventional approach. comparison with the conventional approach. short-to-medium distance Appropriate for the detection of • Conventional Laser Spectroscopic Approach Non‐intrusive remote sensing broadband multiple absorption lines Monitor concentration in gas phase for Higher sensitivity and selectivity operating environments More modes → error correction → Advanced Laser Spectroscopic Sensing System Using Screwed Modes Pros compared with the conventional approach have been summarized in Table 3 below. efits of using screwed modes Table in the 3. laser The spectroscopic benefits of using sensing screwed systems modes in in the laser spectroscopic sensing systems in usive remote sensing broadband multiple absorption lines concentration in gas phase Higher sensitivity and selectivity g environments More modes → error correction → e conventional approach. noise reduction onal Laser Spectroscopic Approach • Non‐intrusive remote sensing broadband multiple absorption lines Monitor concentration in gas phase Higher sensitivity and selectivity
• Advanced Laser Spectroscopic Sensing Compact, robust and affordable in harsh Advanced Laser Spectroscopic Sensing Better spectral efficiency and reach noise reduction System Using Screwed Modes No Increases the overall number of parallel mode coupling, Need to reduce concentration in gas phase Pros Higher sensitivity and selectivity t, robust and affordable in harsh Better spectral efficiency and reach e conventional approach. comparison with the conventional approach. noise reduction Improves signal contrast by Encoding, onal Laser Spectroscopic Approach Monitor concentration in gas phase Higher sensitivity and selectivity •Conventional Laser Spectroscopic Approach Compact, robust and affordable in harsh Better spectral efficiency and reach operating environments More modes → error correction → Improves signal contrast by Encoding, Advanced Laser Spectroscopic Sensing System Using Screwed Modes System Using Screwed Modes Increases the overall number of parallel Have lower backscattering coefficients ASE noise in fundamental channels, each as an individual sensor Pros Table 3. The benefits of using screwed modes in the laser spectroscopic sensing mode systems in t, robust and affordable in harsh Better spectral efficiency and reach g environments More modes → error correction → Improves signal contrast by Encoding, such as code modulation onal Laser Spectroscopic Approach •Advanced Laser Spectroscopic Sensing Compact, robust and affordable in harsh Advanced Laser Spectroscopic Sensing Better spectral efficiency and reach operating environments More modes → error correction → noise reduction such as code modulation System Using Screwed Modes Increases the overall number of parallel Increases the overall number of parallel Requires high peak power pulses Obtain trace by launching a channels, each as an individual sensor Appropriate for the detection of g environments More modes → error correction → comparison with the conventional approach. noise reduction such as code modulation e spatial component of the field onal Laser Spectroscopic Approach operating environments More modes → error correction → noise reduction Improves signal contrast by Encoding, •Conventional Laser Spectroscopic Approach Only one spatial component of the field Needs intensive signal processing System Using Screwed Modes System Using Screwed Modes Increases the overall number of parallel worse spatial resolution pulse channels, each as an individual sensor channels, each as an individual sensor Appropriate for the detection of • Non‐intrusive remote sensing broadband multiple absorption lines single Needs intensive signal processing Without noise reduction Provides Improves signal contrast by Encoding, e spatial component of the field aptured noise reduction Improves signal contrast by Encoding, such as code modulation vectors captured Advanced Laser Spectroscopic Sensing Needs intensive signal processing Complexity of OAM measurement Increases the overall number of parallel Increases the overall number of parallel More suitable for long/ultra-long May provide higher channels, each as an individual sensor Appropriate for the detection of Appropriate for the detection of usive remote sensing Multiplexing broadband multiple absorption lines • Monitor concentration in gas phase Higher sensitivity and selectivity Complexity of OAM measurement Conventional Laser Spectroscopic Approach Improves signal contrast by Encoding, such as code modulation aptured n small change in power Pros such as code modulation Improves signal contrast by Encoding, Only one spatial component of the field • Relies on small change in power System Using Screwed Modes Complexity of OAM measurement (That’s why we need MIMO DSP) distance due to lack of modal spatial resolution channels, each as an individual sensor channels, each as an individual sensor Appropriate for the detection of usive remote sensing Non‐intrusive remote sensing broadband multiple absorption lines broadband multiple absorption lines concentration in gas phase Higher sensitivity and selectivity • Compact, robust and affordable in harsh Better spectral efficiency and reach Needs intensive signal processing Cons (That’s why we need MIMO DSP) such as code modulation e spatial component of the field n small change in power f between sensitivity and selectivity such as code modulation Only one spatial component of the field vectors captured •• dispersion Trade‐off between sensitivity and selectivity Needs intensive signal processing (That’s why we need MIMO DSP) Lack of theory for OAM features in More Increases the overall number of parallel accumulation suitable for long Appropriate for the detection of Appropriate for the detection of usive remote sensing broadband multiple absorption lines concentration in gas phase Monitor concentration in gas phase Higher sensitivity and selectivity Higher sensitivity and selectivity t, robust and affordable in harsh Better spectral efficiency and reach operating environments More modes → error correction → Needs intensive signal processing Complexity of OAM measurement Lack of theory for OAM features in e spatial component of the field aptured f between sensitivity and selectivity mited wavelength/mode(s) Pros Only one spatial component of the field vectors captured • due to limited wavelength/mode(s) Relies on small change in power Needs intensive signal processing Complexity of OAM measurement Lack of theory for OAM features in specific laser spectroscopic system. distance due to lack of modal channels, each as an individual sensor usive remote sensing Non‐intrusive remote sensing broadband multiple absorption lines broadband multiple absorption lines concentration in gas phase Higher sensitivity and selectivity t, robust and affordable in harsh • Compact, robust and affordable in harsh Better spectral efficiency and reach Better spectral efficiency and reach g environments More modes → error correction → noise reduction Needs intensive signal processing Complexity of OAM measurement Cons (That’s why we need MIMO DSP) specific laser spectroscopic system. aptured n small change in power mited wavelength/mode(s) ty deteriorated by noise vectors captured Relies on small change in power Trade‐off between sensitivity and selectivity Complexity of OAM measurement (That’s why we need MIMO DSP) specific laser spectroscopic system. accumulation Appropriate for the detection of concentration in gas phase Cons •• Sensitivity deteriorated by noise Monitor concentration in gas phase Higher sensitivity and selectivity dispersion Higher sensitivity and selectivity t, robust and affordable in harsh Better spectral efficiency and reach g environments operating environments More modes → error correction → More modes → error correction → noise reduction Improves signal contrast by Encoding, Complexity of OAM measurement (That’s why we need MIMO DSP) Lack of theory for OAM features in n small change in power f between sensitivity and selectivity ty deteriorated by noise Pros Relies on small change in power •• due to limited wavelength/mode(s) Trade‐off between sensitivity and selectivity (That’s why we need MIMO DSP) Lack of theory for OAM features in Non‐intrusive remote sensing broadband multiple absorption lines t, robust and affordable in harsh Compact, robust and affordable in harsh Better spectral efficiency and reach Better spectral efficiency and reach g environments More modes → error correction → noise reduction noise reduction Improves signal contrast by Encoding, such as code modulation Cons (That’s why we need MIMO DSP) Lack of theory for OAM features in specific laser spectroscopic system. f between sensitivity and selectivity • due to limited wavelength/mode(s) mited wavelength/mode(s) Trade‐off between sensitivity and selectivity Sensitivity deteriorated by noise Lack of theory for OAM features in specific laser spectroscopic system. • Monitor concentration in gas phase Lack of theory for OAM features in Higher sensitivity and selectivity g environments More modes → error correction → More modes → error correction → noise reduction operating environments Improves signal contrast by Encoding, Improves signal contrast by Encoding, such as code modulation Only one spatial component of the field specific laser spectroscopic system. mited wavelength/mode(s) ty deteriorated by noise Moreover, types of mode multiplexing, including LP modes, supermodes, principle Pros for• different due to limited wavelength/mode(s) Sensitivity deteriorated by noise specific laser spectroscopic system. • Compact, robust and affordable in harsh Needs intensive signal processing Better spectral efficiency and reach noise reduction noise reduction Improves signal contrast by Encoding, such as code modulation such as code modulation e spatial component of the field vectors captured specific laser spectroscopic system. ty deteriorated by noise modes, transverse•modes, screw/twisted modes, whispering gallery modes, as well as the modes Needs intensive signal processing Complexity of OAM measurement Sensitivity deteriorated by noise More modes → error correction → Improves signal contrast by Encoding, Improves signal contrast by Encoding, such as code modulation e spatial component of the field Only one spatial component of the field aptured • operating environments Relies on small change in power Needs intensive signal processing (That’s why we need MIMO DSP) Needs intensive signal processing Complexity of OAM measurement Cons of capillary fibers, their sensing parameters, mode conversion techniques and operation mechanism noise reduction such as code modulation such as code modulation e spatial component of the field aptured vectors captured n small change in power • Trade‐off between sensitivity and selectivity Needs intensive signal processing (That’s why we need MIMO DSP) Complexity of OAM measurement Complexity of OAM measurement Lack of theory for OAM features in Improves signal contrast by Encoding, e spatial component of the field Only one spatial component of the field aptured n small change in power • due to limited wavelength/mode(s) Relies on small change in power f between sensitivity and selectivity Needs intensive signal processing (That’s why we need MIMO DSP) Needs intensive signal processing Complexity of OAM measurement Cons (That’s why we need MIMO DSP) Lack of theory for OAM features in specific laser spectroscopic system. such as code modulation aptured vectors captured n small change in power f between sensitivity and selectivity Trade‐off between sensitivity and selectivity mited wavelength/mode(s) • Sensitivity deteriorated by noise Complexity of OAM measurement Complexity of OAM measurement (That’s why we need MIMO DSP) Lack of theory for OAM features in Lack of theory for OAM features in specific laser spectroscopic system. Only one spatial component of the field n small change in power • due to limited wavelength/mode(s) Relies on small change in power f between sensitivity and selectivity mited wavelength/mode(s) ty deteriorated by noise (That’s why we need MIMO DSP) Needs intensive signal processing Cons (That’s why we need MIMO DSP) Lack of theory for OAM features in specific laser spectroscopic system. specific laser spectroscopic system. vectors captured f between sensitivity and selectivity • Sensitivity deteriorated by noise Trade‐off between sensitivity and selectivity mited wavelength/mode(s) ty deteriorated by noise Complexity of OAM measurement Lack of theory for OAM features in Lack of theory for OAM features in specific laser spectroscopic system. • due to limited wavelength/mode(s) Relies on small change in power mited wavelength/mode(s) ty deteriorated by noise Cons (That’s why we need MIMO DSP) specific laser spectroscopic system. specific laser spectroscopic system. Trade‐off between sensitivity and selectivity ty deteriorated by noise • Sensitivity deteriorated by noise Lack of theory for OAM features in
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are summarized in Table 6, while their corresponding benefits and key components are described in Table 7 below. Table 6. Comparison of sensing parameters, mode conversion techniques and operation mechanism for different types of mode multiplexing. Corresponding Modes
Sensing Parameters
Mode Conversion Techniques
Sensing Mechanism
LP modes
Temperature; Strain; Pressure; Acoustic; Vibration; Bending; Refractive index; Humidity
Phase plates; FBG; LCoS; fused fiber coupler; FWM
Brillouin/Raman/Rayleigh scattering or spectral shift from FBG
Supermodes
Temperature; Strain; Acoustic; Curvature; Bending; Refractive index
Phase plates; FBG; LCoS
Using either mode/core correlation or spectral shift for sensor interrogation
Principle modes
Temperature; Strain; Bending
Phase plates; LPG; Spatial light modulator
Spatial modes without modal dispersion to first-order in frequency
Transverse modes
Temperature; Strain; Pressure; Acoustic; Bending;
Phase plates; LPG; LCoS;
Brillouin/Raman/Rayleigh scattering or spectral shift from FBG
Screw/twisted modes
Atmospheric turbulence monitoring; lateral motion detecting; biomedical imaging
Cylindrical lenses; Helical gratings; parametric oscillator
OAM states partially quenched due to inter-molecular interaction
Whispering gallery modes
Temperature, refractive index, biochemical species
Whispering gallery mode resonator in a tapered fiber
Travel around concave surfaces with low loss due to quantum tunneling
Modes of capillary optical fibers
Temperature; Strain; Flow rate, pulling force, fiber geometry, biochemical species
Capillary tapered mode converter
Multiple modes excited/interfered to form fringes collected by lead-out SMF
Table 7. Comparison of key components and multiplexing benefits using different types of modes. Corresponding Modes
Measurement Components
Benefits
LP modes
Using FMF, MMF, MCF itself as the sensing medium with direct/coherent detection
References
Simple; Compact; low loss; high sensitivity; good repeatability
[14,24,64,77,122]
Supermodes
A few-millimeter-long piece of seven-core fiber spliced between two single-mode fibers
Compact; low loss; high sensitivity; good repeatability
[49,67,69]
Principle modes
A multimode waveguide system in the vicinity of the phase-matching frequency
High speed; high sensitivity; low modal dispersion
[48,50,51]
Transverse modes
Using FMF, MMF, MCF itself as the sensing medium with direct/coherent detection
Compact; low loss; high sensitivity; good repeatability
[28,52,130]
Screw/twisted modes
Laser spectroscopic devices; atomic force microscopes; photo-sensitive detector
Higher sensitivity and selectivity; Better spectral efficiency
[53–55,70,71,153–157]
Whispering gallery modes
A microscopic glass sphere from micro-cavities of optical fiber resonator
High sensitivity to refractive index; useful in biochemical sensing
[56,57,72,73,147–152]
Modes of capillary optical fibers
A fused-silica capillary and FBG sandwiched by single-mode fibers
High sensitivity to refractive index; useful in biochemical sensing
[58,59]
4.2. Challenges for SDM Measurement Systems This subsection focuses on the possible challenges for SDM-based sensing systems, including the cost efficiency issue, as well as the impact of loss and nonlinearity on system performance.
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4.2.1. Component Cost for SDM Sensing Systems The cost efficiency issue is probably the key element that will lead to or not to the use of SDM techniques in practical fiber-optic sensing systems, which is highly dependent on the development of cost-effective components like light sources and detection units. Technically, lasers with a narrow linewidth (high degree of mono-chromaticity) are desired for fiber-optic sensors as light sources to ensure better resolution. The comparison of wavelength region, output power, linewidth and average cost for different types of lasers in SDM-based measurement systems is summarized in Table 8, from which the laser linewidth from stabilized advanced lasers can be very narrow and reach down to even less than 1 kHz. Table 8. Comparison of linewidth and cost for different types of lasers in SDM-based systems. Type of Light Sources
Wavelength Region
Output Power
Linewidth
Cost
Distributed feedback laser diodes (DFB lasers)
1000 nm–1500 nm
tens of mW
several MHz
$300.00–$3,500.00
Distributed Bragg reflector lasers (DBR lasers)
1000 nm–1500 nm
tens of mW
several MHz
$500.00–$3,950.00
Fabry-Perot Laser Diodes (FP lasers)
400 nm–1550 nm
10–300 mW
1–2 MHz
$1,475.00–$4,000.00
distributed feedback fiber lasers (DFB + FBG)
980 nm–1550 nm
20–150 mW
a few kHz
About $5,000.00
InGaAsP/InP distributed feedback laser
1064 nm–1560 nm
25 mW–300 mW
10 kHz
About $6,000.00
Nd:YAG laser
1064 nm–1550 nm
100 mW–3 W
10 kHz
About $10,000.00
Diode-pumped solid-state bulk lasers
1064 nm–1550 nm
100 mW–1 W
a few kHz
About $14,900.00
Distributed Feedback Quantum Cascade Lasers (QCLs)
760 nm–1600 nm
100 mW–5 W
a few hundred Hz
$6,200.00–$15,000.00
Moreover, the component cost scale comparison for SDM sensing systems, including light sources, mode converters/multiplexers, multicore elements, amplifiers and detection units, has been presented in Table 9 above, with each star symbol representing roughly $1,000.00–$3,000.00 depending on the specific applications. The development of SDM-based sensing systems is pushing the boundaries of high-speed multi-wavelength opto-electronic devices and modules, making low-cost optical components possible for SDM implementation. For instance, the opto-electronic sources can be integrated on the silicon platform. Thus the cost of commercialized SDM-based measurement systems is expected to become more compatible with that of standard approaches using SMF and PCF in a broad range of applications in the near future. Table 9. Comparison regarding proposed sensor component costs for SDM sensing systems. Component Type Light source
Components Distributed feedback laser diodes (DFB lasers) Fabry-Perot laser diodes (FP lasers) Quantum cascade lasers (QCLs) Nd:YAG lasers Long-period grating (LPG) based converter Liquid crystal on silicon (LCoS) panels Thin phase plates
Cost
F FF FFF FFFF F FF FF
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Table 9. Cont. Component Type Mode converter
Multiplexer
Multicore elements
SDM amplifiers
Detection units
Components Helical gratings (HGs) Cylindrical lenses Optical parametric oscillator Whispering gallery mode resonator Capillary tapered mode converter Spatial light modulators (SLM) via LCoS Photonic lantern (PL) LPG based multicore elements Asymmetrical coupler based multicore elements Special fiber based multicore elements Few-mode Raman amplifiers Few-mode erbium-doped fiber amplifiers (FM-EDFA) Multi-core EDFAs Direct detection Homodyne detection Heterodyne detection
Cost
FF FFF FFF FFFF FFFF FF FFF FF FFF FFFF FF FFF FFFF FF FFFF FFFF
F representing roughly $1,000.00–$3,000.00 depending on the specific applications.
4.2.2. The Effects of Noise and Nonlinearity on SDM Sensing Systems In this subsection, some potential performance limitations of SDM-based fiber-optic sensors are further discussed. Since this is a nascent field of research, there is still much unexplored area, involving the effects of noise and nonlinearity. One of the main concerns could be the mode coupling effects, either induced by the index perturbation along the fiber (between non-degenerate modes), or due to the deviations on the transverse index profile (between degenerate modes) [165]. The most common coupling between non-degenerate modes in a FMF is schematically plotted in Figure 22, whereas multiple parallel straight lines symbolize the non-interacting trajectories of two spatial modes. As mode coupling is due to random longitudinal index fluctuation induced by manufacturing process and micro-bending in the cable, the coupling location and strength are random distributed 27 along Sensors 2016, 16, 1387 of 34 the fiber [166]. Mode coupling usually leads to mode group delay (MGD) and crosstalk between mode might degrade the performance of the SDM systems [167]. Other noise major channels, noise or which nonlinearity might include ASE, self-phase modulation, as well as major intermodal or nonlinearity might include ASE, self-phase modulation, as well as intermodal four-wave mixing, four-wave mixing, whereas further studies are warranted to resolve these effects [168]. whereas further studies are warranted to resolve these effects [168].
Figure 22. Schematic of spatial modes propagating in a FMF. Figure 22. Schematic of spatial modes propagating in a FMF.
5. Concluding Remarks 5. Concluding Remarks This paper presents a comprehensive and systematic overview of spatial-division multiplexing This paper presents a comprehensive and systematic overview of spatial-division multiplexing (SDM) based fiber-optic sensors concerning a number of aspects in terms of operation principle, (SDM) based fiber-optic sensors concerning a number of aspects in terms of operation principle, fabrication methods, experimental design, and sensing applications. The examples of SDM-based fabrication methods, experimental design, and sensing applications. The examples of SDM-based sensing systems include mode-division multiplexing (MDM) using few-mode fiber (FMF), core sensing systems include mode-division multiplexing (MDM) using few-mode fiber (FMF), core multiplexing using multicore fiber (MCF) or fiber Bragg grating (FBG), whispering gallery modes for multiplexing using multicore fiber (MCF) or fiber Bragg grating (FBG), whispering gallery modes fiber profiling and chemical species measurements, the twisted modes for examining water quality, for fiber profiling and chemical species measurements, the twisted modes for examining water quality, as well as optical beam shaping to enhance cantilever deflection measurements. Since this is a nascent field of research, there might still be much unexplored area, involving the effects of noise and nonlinearity. As for the cost efficiency issue, which is probably the key element that will lead to or not to the use of SDM in real fiber-optic sensing systems, such systems have the potential to significantly reduce the cost and complexity of parallel systems, which is dependent on the
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as well as optical beam shaping to enhance cantilever deflection measurements. Since this is a nascent field of research, there might still be much unexplored area, involving the effects of noise and nonlinearity. As for the cost efficiency issue, which is probably the key element that will lead to or not to the use of SDM in real fiber-optic sensing systems, such systems have the potential to significantly reduce the cost and complexity of parallel systems, which is dependent on the development of highly-integrated and cost-effective components just round the corner. Thus the cost of commercialized SDM-based measurement systems is expected to become more compatible with that of standard approaches using single-mode fibers (SMF) and photonic crystal fibers (PCF), in a broad range of applications including temperature, refractive index, pressure, acoustic/seismic waves and strain sensing in the near future. Acknowledgments: The authors wish to thank the anonymous reviewers for their valuable suggestions. Conflicts of Interest: The authors declare no conflict of interest.
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