Fiber-Optic Refractive Index Sensor Based on Multi-Tapered SMS ...

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IEEE SENSORS JOURNAL, VOL. 15, NO. 11, NOVEMBER 2015

Fiber-Optic Refractive Index Sensor Based on Multi-Tapered SMS Fiber Structure Yong Zhao, Member, IEEE, Lu Cai, and Hai-Feng Hu

Abstract— A new refractive index (RI) sensor based on multi-tapered single mode–multimode–single mode (SMS) fiber structure is proposed. The sensor is fabricated by tapering several tapers in multimode fiber part of the SMS fiber structure. In the taper region, the evanescent wave is generated and permeates to the surrounding liquid. Thus, the sensor is sensitive to the RI variation and the sensing characteristic is investigated theoretically and experimentally. Through fabricating three tapers, five tapers, and eight tapers, it can be concluded that the more the tapers, the higher the measuring sensitivity. According to the experimental results, when the number of tapers is up to eight, the RI sensitivity of 261.9 nm/RIU in the RI range of 1.3333–1.3737 can be achieved. Since the fabricated process of this all-fiber RI sensor can be completed just using a fusion splicer, the sensor has a series of advantages, such as high sensitivity, low cost, and simple operation. Index Terms— Refractive index, SMS fiber structure, optical fiber sensor, fiber taper, BPM.

I. I NTRODUCTION

F

ROM the last decade, SMS fiber structure has been paid close attention to extensively. In 1997, Slovene scholar Donlagic and Zavrsnik firstly proposed a microbend sensor based on single mode-multimode-single mode fiber structure [1]. Since then SMS fiber structure was demonstrated applying to measuring temperature, refractive index (RI), strain, etc [2]–[4]. The flexible application allowed the single mode-multimode-single mode (SMS) fiber structure to combine with the fiber bragg grating (FBG) to achieve the double-parameter measurement [5], [6], or be inserted into fiber taper in the sensing part [7]. There are many other ways to achieve the RI measurement, such as long period grating (LPG) [8], etched fiber Bragg grating (FBG) [9], surface plasmon resonance (SPR) [10]–[14], Fabry-Perot interferometer (FPI) [15], [16], and multimode interferometer (MMI) [17]–[20]. LPFG and FBG-based sensors exhibited good reliability and potential of

Manuscript received June 9, 2015; revised July 16, 2015; accepted July 17, 2015. Date of publication July 21, 2015; date of current version September 4, 2015. This work was supported in part by the National Natural Science Foundation of China under Grant 61425003 and Grant 61273059, in part by the Fundamental Research Funds for the Central Universities under Grant N140404021, and in part by the State Key Laboratory of Synthetical Automation for Process Industries under Grant 2013ZCX09. The associate editor coordinating the review of this paper and approving it for publication was Dr. Anna G. Mignani. (Corresponding author: Lu Cai.) The authors are with the College of Information Science and Engineering, Northeastern University, Shenyang 110819, China (e-mail: [email protected]; [email protected]; [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSEN.2015.2458893

distributed monitoring but the fabrication of them is complex and expensive. FPI has the advantage of high sensitivity, good mechanical strength and stability but shows low sensitivity in the RI sensing applications and difficulty process in controlling the cavity length and parallel end. It is widely shared that SPR technology is welcomed by the scholars due to its ultrahigh RI sensitivity and good mechanical strength. Moreover, the SPR-based fiber RI sensor can be combined with different kind of sensitive film like reference [12] to increase the RI sensitivity but the fabricated process is complex and expensive too. It should be noticed that the MMI devices exhibit more advantages like low cost, robustness, all-fiber system, easy to be fabricated and simple structure and among them, SMS fiber structure is more sensitive to surrounding RI than multimode-single modemultimode (MSM) fiber structure [17], taper-offset fiber structure [18] and double-tapered fiber structure [19], [20]. As we know the diameter of common multimode fiber (MMF) is from 50μm to 105μm. Light propagates in the fiber core and then permeates out in the form of evanescent wave and as a consequence to be sensitive to the RI change. Therefore, a feasible and operable method to leak the light out of fiber core and then be permeated into surrounding media is particularly crucial. Chemical etching as common method is adopted widely by researchers [4]–[6] and the etching process needs hydrofluoric acid and concentrated sulfuric acid so it is complex and dangerous. Thank to the thinned fiber core, in some works thin-core fiber is used as MMF approximately [21], [22]. In recent years, fiber taper is frequently appears in the modal interferometers [23]–[26] especially in the RI sensors. It is an efficient approach to leak out the light of core mode into fiber cladding. There are some methods to fabricate fiber tapers such as chemical etching [23], CO2 laser tapering [24], flame-heated drawing [25] and fusion splicer drawing [26], etc. Here, a commercial fusion splicer is chose to fabricate tapers due to its unique advantages of convenience, safety, good repeatability and easy to control, which compared to the other methods. Multimode fiber with a large core/cladding diameter ratio of 105μm/125μm is utilized to avoid the useless interference between core and cladding modes as that in thin core fiber and the price is much lower. In this paper, a multi-tapered SMS fiber structure is proposed. The multi-tapered fiber structure has been studied for RI measure [14], [22] but this is the first to introduce multiple tapers to the SMS fiber structure and investigate how the taper number influences on the RI sensitivity. Several tapers are fabricated in the MMF part of SMS fiber structure.

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ZHAO et al.: FIBER-OPTIC RI SENSOR BASED ON MULTI-TAPERED SMS FIBER STRUCTURE

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difference between two modes at the second splicing joint can be write as: n ϕ = (βm − βn )L = 2π L(n m e f f − n e f f )/λ

Fig. 1.

The multiple fiber tapers in MMF section play the role of leaking the light in the fiber core into the external media as form of evanescent wave. As known, the evanescent wave could build the internal relationship between external media and the guiding mode. As the external RI changes, the effective RI of core modes would be changed accordingly. From Eq. 4 it can be seen that the phase difference would change following the external RI variation. The transmission spectrum can be calculated as [29]:

Diagram of experimental setup and fiber sensor structure.

The power of higher order modes are easy to be leaked out into surrounding RI as a form of evanescent wave through the taper region. Beam propagating method (BPM) is employed to simulate the RI characteristic. SMS fiber structures with different number of tapers are respectively investigated in theory and experiment. The consistent conclusion is that the more the tapers are, the higher the RI sensitivity can be. The highest sensitivity is 261.9nm/RIU when the number of tapers is eight. This sensitivity value is little higher than the value in reference [22] in which the number of tapers are twenty. So it could be seen that the easier method and simpler process are employed in the experiment to have achieved a better capability with lower cost than that of reference [22]. II. S ENGING P RINCIPLE AND S IMULATION A NALYSIS The fiber structure is shown in the lower portion of the Fig. 1. The light propagates in the SMF and then injects into the multi-tapered MMF. Due to the fiber core mismatch, multiple modes are excited in the multimode fiber core. In the first splicing point, the transverse field distribution can be expressed as [27], [28]: E(r, 0) =

M

cm ψm (r )

(1)

m=1

where cm is the excitation coefficient of each eigenmode, and it can be expressed by the overlap integral between E(r, 0) and ψm (r ) as follows: ∞ cm =

E(r, 0)ψm (r )r dr

0 ∞

(2) ψm (r )ψm (r )r dr

0

When the light propagates in the multimode fiber core, the field profile at a distance z can then be written as a superposition of all the guided mode field distributions: E(r, z) =

M

cm ψm (r ) exp(iβm z)

(4)

(3)

m=1

where β = kn e f f = 2πn e f f /λ is the propagating constant and the M is the whole number of core modes and the phase

I (λ) =

M i=1

Ii +

M−1 M−1

2 Ii I j cos(ϕ)

(5)

i=1 j =i+1

It can be known that when the external RI changes, the transmission spectrum would shift in wavelength. Thus the RI change can be measured by calculating calibrated curve and monitoring the wavelength shift of transmission spectrum. In order to investigate sensing mechanism, BPM is employed to calculate the amplitude of optical fields along the direction of light propagation and transmission spectrum of fiber sensor in the media with different RI as Fig. 2 and Fig. 3. The RI values of core and cladding used in simulation for SMF and MMF are respectively 1.4681/1.4628 and 1.4447/1.4271. The interference patterns and amplitude of optical fields with different number of tapers are shown in Fig. 2. From the patterns it can be seen that the light is significantly leaked out in the tapering position. The light in the fundamental mode of MMF is feeble gradually along z direction because more and more light permeates to the external media through fiber tapers. The larger the number of tapers is, the weaker the output light can be. So we infer that the higher RI sensitivity can be obtained with more tapers. To verify this inference, transmission spectra for three tapers and five tapers are simulated in different RI of media as Fig. 3. The wavelength shifts with the external RI changing from 1.33 to 1.37 are 6.8 nm and 7.6 nm for three tapers and five tapers. It is truly that the RI sensitivities are increasing with more tapers. However, in consideration of the limit of the computer configuration, the structure with more tapers has not been simulated. It needs further confirmation in experimental part. III. E XPERIMENTAL R ESULTS AND D ISCUSSION In the experiments, the diameters of core and cladding for SMF and MMF are respectively 8.3μm/125μm and 105μm/125μm. The image of multi-taper structure under microscope is given in the bottom right of Fig. 1. The length of MMF section L ≈ 4cm and the taper length l ≈ 550 μm. The minimum diameters of tapers are around 52 μm. The tapers could be regarded as equally spacing in the MMF section. Before RI experiment, to evaluate the fabrication repeatability and performance repeatability of individual taper, single taper has been fabricated for 10 times on the same single mode fiber patch cord. The averages of these 10 values

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Fig. 2. Electric field distribution along z axis. (a) Three tapers. (b) Five tapers.

are 551.65μm, 52.06μm and −1.452dB for taper length, waist diameter and transmission loss. And the standard deviations for them are ±14.23μm, ±1.50μm and ±0.189dB. The relative standard deviations are 2.6%, 2.9% and 5.6%, respectively. So it can be proof that the individual taper has good fabrication and performance repeatability using this tapering method. The fiber sensor is linked to the amplified spontaneous emission (ASE) source and optical spectrum analyzer (OSA). The sensing part is placed on a polymethyl-methacrylate (PMMA) groove and immersed in different RI of NaCl solutions from 1.3333 to 1.3737. The fiber is fixed by two fiber holders to ensure the fiber straight and tight. The two cylinders play the role of prestretching to protect the fiber. The light propagates through the MMF core and the evanescent wave is generated at the taper region. Thanks to this part of light leaking out to the surrounding liquid, the effective RI of the eigenmodes would be changed accordingly. The interference spectra for different number of tapers are given in Fig. 4 (a). Similar to the simulated spectrum, the profile of the spectrum is not uniform like Mach-Zehnder interferometer since there is a superposition of several modes interference. It can be seen that the interference spectral envelope has some transformation after fabricating different number of tapers because of the coupling between internal modes and the generation of new modes as well as the power redistribution among modes. The fast Fourier transform (FFT) is applied to the interference spectra in Fig. 4 (a) and the spatial frequency spectra are


Fig. 3. Simulated transmission spectra shift with different external RI. (a) Three tapers. (b) Five tapers.

shown in Fig. 4 (b). With the increasing of the number of tapers, the dominant mode participated in interference, which is indicated with the frequency peak around 0.1 in dotted ellipse, moves to larger frequency value. This is because the effective RI of this mode decreases accompany with the further leaking out of the light with more tapers as Fig. 2 (a) and (b). The larger the frequency is, the smaller the effective RI can be for eigenmode. When the sensor is immersed in solutions of different RI values, the shifted spectra are shown as Fig. 5. The dips around 1560nm for three-taper and five-taper fiber structures and the dips around 1530nm for eight-taper fiber structure are monitored. Fig. 5 (a), (b) and (c) show the results. The interference spectra have red shift as the external RI increasing. This is in consistence with the simulation results in Fig. 3. The wavelength shifts within RI value of 1.3333 to 1.3737 are respectively 7.1nm, 8.6nm and 10.4nm for three, five and eight tapers. And the simulation results exhibit smaller values of 6.8nm and 7.6nm mentioned above within RI of 1.33 to 1.37 for three and five tapers. It can be seen that the experimental results are in the same order of magnitudes with the simulated results but a little higher. The one reason is that the RI range in experiment is a little wider than that in simulation. Another is that the measured RI range in experiment is slightly moved to higher range in which the RI sensitivity is generally higher because of the stronger evanescent wave. The RI sensitivities for three, five and eight tapers are respectively 173.3nm/RIU, 209.7nm/RIU and 261.9nm/RIU as shown in Fig. 6. It is


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Fig. 4. Spectra for different number of tapers. (a) Interference spectra. (b) Spatial spectra.

very near to the average RI sensitivities of 170nm/RIU and 190nm/RIU for three and five structure in Fig. 3. In addition, the quantity of wavelength shift depends on the number of fiber tapers. There is a positive correlation between them as shown in Fig. 6. The RI sensitivity has been improved from 173.3nm/RIU to 261.9nm/RIU with the tapers being added from three to eight. The significant progress in capability can be owed much to the large amount of evanescent wave generated by multi-tapers region. And there is not a linear relationship between taper number and RI sensitivity. As the light propagating through the multi-tapered region, the higher order modes are gradually leak out to the surrounding liquid as the form of evanescent and the lower order one would couple to the higher order and then leak out consequently. In view of the much more power distributed in the lower order modes, the evanescent wave generated by each taper is increased along the propagating direction. The more the taper number is, the stronger the evanescent wave can be. Thus the RI sensitivity would increase faster and faster with the increasing taper number. The accurate quantitative relationship require further detailed studies and beyond the scope of this article. The tapers can be much more and thinner to further improve the RI sensitivity providing that the package is robust enough. The measurement errors are respectively 0.00114 RIU, 0.00154 RIU and 0.00152 RIU for the three sensors with 3 tapers, 5 tapers and 8 tapers and the corresponding linear dependent coefficients R2 are 0.9974, 0.9961 and 0.9951. Thus it can be seen that the sensors have the good capability of linear response and high precision.

Fig. 5. Measured transmission spectra with different external RI for different number of tapers. (a) Three tapers. (b) Five tapers. (c) Eight tapers.

Fig. 6. tapers.

Wavelength shift with external RI change for different number of

It can be seen that not all the dips perform linear red shift in Fig. 5. The reason can be described following. The large core diameter of multimode fiber could accommodate a host

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of modes propagating in the fiber core which are interference with each other in a complex fashion. As the RI increasing, the phase differences between interference modes are changed and then the wavelength dips are moved accordingly. If one dip contains several components of interference and the intensity radio of each component is fixed, the dip would reveal a linear red shift because the linear superposition. However, the power distribution among modes may refresh when the surrounding RI changes according to the new boundary conditions. Thus the superposition with different surrounding RI shows different linear coefficient and then performs nonlinear red shift. This is why some dips shift nonlinearly. When the dip only contains or a pair of two-mode interference occupies dominant position, it would reveal almost linear shift. These dips can be used to achieve RI measurement as an indicator. When the spectrum range is wide enough to be several times of the free spectral range (FSR), there will be one or more linear dips. It is worth noting that the multi-tapered structure has unique advantage over the single taper structure [30] because its RI sensitivity can be improved by fabricating more tapers and optimizing the taper region including waist diameter, taper length and tapering profile. When the RI sensitivities are near, the tapers in multi-tapered structure are much thicker than the single taper structure like reference [30] so it could be of good mechanical strength. IV. C ONCLUSION In conclusion, a RI sensor based on multi-tapered SMS fiber structure has been proposed and demonstrated in this paper. The sensor is sensitive to the external RI variation and the sensing characteristic is investigated in theory and experiment. Through fabricating three tapers, five tapers and eight tapers, it can be concluded that the more the tapers, the higher the measuring sensitivity. According to the experimental results, when the number of tapers is up to eight, the RI sensitivity of 261.9nm/RIU in the RI range of 1.3333 to 1.3737 can be achieved and this value is higher than that in the reference [7], [8], [17]–[20]. R EFERENCES [1] D. Donlagi´c and M. Završnik, “Fiber-optic microbend sensor structure,” Opt. Lett., vol. 22, no. 11, pp. 837–839, 1997. [2] R. X. Gao, Q. Wang, F. Zhao, B. Meng, and S. L. Qu, “Optimal design and fabrication of SMS fiber temperature sensor for liquid,” Opt. Commun., vol. 283, no. 16, pp. 3149–3152, Aug. 2010. [3] E. Li, “Sensitivity-enhanced fiber-optic strain sensor based on interference of higher order modes in circular fibers,” IEEE Photon. Technol. Lett., vol. 19, no. 16, pp. 1266–1268, Aug. 15, 2007. [4] Q. Wu, Y. Semenova, P. Wang, and G. Farrell, “High sensitivity SMS fiber structure based refractometer—Analysis and experiment,” Opt. Exp., vol. 19, no. 9, pp. 7937–7944, 2011. [5] Q. Z. Rong et al., “Temperature-calibrated fiber-optic refractometer based on a compact FBG-SMS structure,” Chin. Opt. Lett., vol. 10, no. 3, pp. 30604–30606, 2012. [6] Y. Zhao, L. Cai, X.-G. Li, and F.-C. Meng, “Liquid concentration measurement based on SMS fiber sensor with temperature compensation using an FBG,” Sens. Actuators B, Chem., vol. 196, pp. 518–524, Jun. 2014. [7] Y. Zhang, W. Zou, X. Li, J. Mao, W. Jiang, and J. Chen, “Modal interferometer based on tapering single-mode-multimode-single-mode fiber structure by hydrogen flame,” Chin. Opt. Lett., vol. 10, no. 7, pp. 070609–070611, 2012.

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Yong Zhao received the M.A. and Ph.D. degrees in precision instrument and automatic measurement with laser and fiber-optic techniques from the Harbin Institute of Technology, China, in 1998 and 2001, respectively. In 2006, he was a Visiting Scholar with the University of Illinois in Urbana and Champagne, USA. He received the first prize scholarship from the China Instrument and Control Society in 2000, and the Sintered Metal Corporation Scholarship in Japan. He was a scholarship holder in Japan. He was a PostDoctoral Fellow with the Department of Electronic Engineering, Tsinghua University, from 2001 to 2003, and then an Associate Professor with the Department of Automation, Tsinghua University. He is currently with Northeastern University as a Full Professor. As the Academic Leader and Director of his research institute, his current research interests are the development of fiber-optic sensors and device, fiber Bragg grating sensors, novel sensor materials and principles, slow light and sensor technology, and optical measurement technologies. He has authored or co-authored over 200 scientific papers and conference presentations, 17 patents, and five books. He received the New Century Excellent Talents in University by the Ministry of Education of China in 2008, and the Liaoning Bai-Qian-Wan Talents by Liaoning Province in 2009. In 2011, he was awarded by the Royal Academy of Engineering as an Academic Research Fellow of City University London. In 2014, he was awarded by the National Science Foundation for Distinguished Young Scholars of China. He is a member of the Editorial Boards of the international journals of Sensor Letters, Instrumentation Science & Technology, the Journal of Sensor Technology, and Advances in Optical Technologies.

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Lu Cai was born in Liaoning, China, in 1990. She received the B.A. degree from the College of Information Science and Engineering, Northeastern University, China, in 2013, where she is currently pursuing the Ph.D. degree. Her research interests are fiber optical sensors, modal interference sensors, in-fiber interferometer, and its sensing applications. She has co-authored four scientific papers.

Hai-Feng Hu was born in Liaoning, China, in 1984. He received the Ph.D. degree from the Institute of Semiconductors, Chinese Academy of Sciences, China, in 2013. He is currently with the College of Information and Engineering, Northeastern University. He has authored or co-authored over 20 scientific papers, two patents, and five conference presentations. His research interests are nanooptics, plasmonics, fiber-optic sensors, and their applications in biosensing.