Slightly Tapered Optical Fiber With Inner Air-Cavity as a Miniature and ...

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JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 33, NO. 1, JANUARY 1, 2015

Slightly Tapered Optical Fiber With Inner Air-Cavity as a Miniature and Versatile Sensing Device H. F. Chen, D. N. Wang, Senior Member, IEEE, and W. Hong

Abstract—A long-standing goal of optical fiber sensors is the development of a miniature and versatile optical fiber device, which is capable of performing multiple sensing functions, and supporting a simple and efficient system. Here, we demonstrate an elegant way of achieving such a device by use of an inner air-cavity in a slightly tapered optical fiber. Owing to the small size of only a few tens microns and inner cavity structure, a spatially precise “point sensing” with high sensitivity and good robustness can be readily achieved. The refractive index, strain, and temperature sensitivities obtained are ∼1060 nm/RIU (refractive index unit), 22.5 pm/με, and 80 pm/°C, respectively. The inner air-cavity-based device is flexible, ultracompact, versatile, and highly efficient, which provides a promising new way for a wide range of optical fiber sensing applications. Index Terms—Air-cavity, fiber sensor, refractive index sensing, strain sensing, temperature sensing.

I. INTRODUCTION PTICAL fiber sensors have been developed rapidly in the last few decades. Various types of optical fiber sensors have been emerged and been used for a wide range of applications. To improve the optical fiber system simplicity and efficiency as well as reducing its cost, it is always desirable to have a miniature and versatile fiber sensing device that is capable of performing multiple sensing functions. The highly successful optical fiber sensing devices to date utilize fiber grating [1]–[6], photonic crystal fiber (PCF) [7]–[11] and microfiber [12]–[16]. Fiber grating, including fiber Bragg grating (FBG) and long period fiber grating (LPFG), is formed by introducing a peri-

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Manuscript received September 11, 2014; revised October 20, 2014; accepted November 13, 2014. Date of publication November 24, 2014; date of current version December 16, 2014. This work was supported in part by the National Natural Science Foundation of China under Grant 61377094, by the Hong Kong Polytechnic University under grant G-YM19, by the Natural Foundation of Zhejiang Province of China under grant Y1091078 and by the Key program for science and technology development of Zhejiang Province under grant 2010C11068. H. F. Chen is with the College of Optical and Electronic Technology, China Jiliang University, Hangzhou, 310018, China, the Hong Kong Polytechnic University Shenzhen Research Institute, Shenzhen 518057, China, the Department of Electrical Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong, China, and also with the State Key Laboratory of Modern Optical Instrumentations, Zhejiang University, Hangzhou, 310027, China. D. N. Wang is with the Hong Kong Polytechnic University Shenzhen Research Institute, Shenzhen 518057, China, the Department of Electrical Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong, China, the College of Optical and Electronic Technology, China Jiliang University, Hangzhou, 310018, China, and also with the School of Electrical, Electronic and Information Engineering, Hubei Polytechnic University, Huangshi, China (e-mail: [email protected]). W. Hong is with the Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China. 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/JLT.2014.2372057

odic refractive index (RI) or geometric structure modulation in a small section of fiber length. Since its resonant wavelength is determined by the grating period and the effective RI of the fiber, which can be adjusted by various means such as strain, temperature and RI of the surrounding medium, many sensing functions can be achieved by use of fiber grating [6], [17]–[21]. PCF exhibits a periodic microstructure along the whole fiber length, which enables a different light guiding mechanism compared to the conventional optical fiber and hence brings many new and important sensing functions [22]–[24]. Microfiber has a tiny size but a large evanescent field, especially when the diameter of the microfiber goes to submicron range, in which a large portion of guided light traveling outside the fiber and as a result, light transmission is highly sensitive to the surrounding environment [15], [16], [25]–[29]. However, the above mentioned sensing devices usually exhibit a length of millimeter or even centimeter order, which limits its applications in precise spatial position measurement or “point sensing”. A more compact and versatile sensing device with length of 100 μm order can be obtained by use of fiber in-line interferometer, especially that based on open air-cavity [30]–[32]. Owing to the large RI difference between the fiber core and air, the open air-cavity based interferometer device usually exhibits high sensitivity however, it inherently has poor robustness due to the fact that large part of the fiber material is removed at the cavity position. Recently, by use of inner air-cavity adjacent to the fiber core, a robust Mach-Zehnder interferometer (MZI) with device length of 10 μm order has been demonstrated [33]. However, the isolated inner cavity structure limits its response to environmental variations such as surrounding RI change. Although the MZI based on dual internal mirrors formed by hollow sphere surfaces adjacent to the fiber core can effectively respond to the surrounding RI variation [34], the fabrication of such a device is still complicated and needs in an accurate control. Here we demonstrate an ultra compact sensing device based on optical fiber with inner air-cavity fabricated by femtosecond laser (fs) micromachining, together with fusion splicing technique, and followed by a tapering process. Such a device has an ultra high strain sensitivity of ∼22.5 pm/με and very large temperature sensitivity of ∼80 pm/°C, while being capable of responding to the external environment with a high RI sensitivity of up to ∼1060 nm/RIU (refractive index unit). II. DEVICE FABRICATION The device is fabricated by use of fs laser micromachining, together with fusion splicing technique, and followed by a tapering process. The fs laser pulses (λ = 800 nm) of 120 fs at the

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CHEN et al.: SLIGHTLY TAPERED OPTICAL FIBER WITH INNER AIR-CAVITY AS A MINIATURE AND VERSATILE SENSING DEVICE

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Fig. 2. Schematic diagram of the inner air-cavity based optical fiber modal interferometer.

spectrum analyzer (OSA) to monitor the transmission spectrum in real time. The hollow sphere region was repeatedly scanned with a small hydrogen flame back and forth, with each scanning cycle of 2 s. The fiber was tapered by two translation stages moving slowly and gently to pull the fiber to the opposite directions. By appropriately control the pulling speed of the translation stages, the scanning speed of the flame and the number of scan cycles, slightly tapered optical fiber with inner air-cavity of different diameters could be produced. To facilitate the peak tracking in the sensing tests, the tapering process was stopped when one or two sharp dips emerged. III. PRINCIPLE OF OPERATION Fig. 1. Schematic diagram of fabrication process of the slightly tapered fiber with inner air-cavity. (a) A micro-hole ablated on the SMF tip with fs laser. (b) Fiber tip with micro-hole spliced together with another cleaved SMF. (c) The inner air-cavity created in SMF. (d) Tapering the air cavity region with hydrogen flame.

repetition rate of 1 kHz were focused onto the fiber by a 20 × objective lens with an NA value of 0.5 and a working distance of 2.1 mm. A CCD camera was employed to monitor the fabrication process. A standard single mode fiber (SMF) with effective RI of 1.4682 (at 1550 nm) was mounted on a computer controlled three dimensional translation stage with a 40-nm resolution. The fabrication process includes a number of steps, as illustrated in Fig. 1. (1) The cleaved fiber was fixed vertically on the translation stage. The fs laser was focused onto the center of fiber tip to create a micro-hole with diameter of a few μm, with the energy of ∼3 μJ. (2) The fiber tip with micro-hole was then fusion spliced together with another cleaved SMF tip without microhole to create a hollow sphere centered at the fiber core. The fusion splicer used was ERICSSON FSU975 and the fusing current and fusing duration employed were 16.2 mA and 2.0 s, respectively. (3) Due to the instant high temperature, the fiber material near the fs laser focus point was evaporated, the air in the micro-hole was rapidly expanded and a hollow sphere with rather smooth inner surface was produced in the middle of the SMF as a result of the surface tension of the fused silica. (4) The tapering process was performed by use of flame brushing technique. The SMF with hollow sphere was mounted between two translation stages and connected with a broadband light source (BBS) and an optical

Fig. 2 shows the schematic diagram of the optical fiber sensing device. A hollow sphere centered on the fiber axis and located within the fiber tapering region forms an inner air-cavity. When the incident light beam traveling in the fiber core approaches the inner air-cavity, part of light is converted into air-cavity mode and the rest is excited into cladding modes, both of them are recombined in the fiber core at the air-cavity end, thus forming a modal interferometer. It is found that the transmission spectrum of the device varies rapidly and continuously, and sometimes abruptly during the tapering process. Fig. 3 displays the microscope images of the two hollow spheres before and after tapering process, respectively, their transmission spectra for different number of scanning cycles and the corresponding spatial frequency spectra, obtained by use of fast Fourier transform. Here, the scanning cycle is defined as the cycle of flame moving along the fiber and back to its original position. It can be seen from Fig. 3 that the spatial frequency spectrum and hence the nature of the device are dependent on the number of scanning cycles in the tapering process, and for a relatively small number of scanning cycles, a number of frequency peaks appear in the spatial frequency spectrum, whereas for a large number of scanning cycles, only two dominant frequency peaks exist and a rather smooth two mode MZI fringe pattern in the transmission spectrum can be observed. Although the light transmission mode in the fiber is related to many factors besides the number of scanning cycles, such as the cavity diameter, the thickness of the silica wall and the taper shape formed, etc., the regularity related to the number of scanning cycles is relatively easy to be observed in the corresponding spatial frequency spectrum. For a two mode MZI, the interferometer output, I, is expressed as 2πLΔn (1) I = I1 + I2 + 2 I1 I2 cos λ

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Fig. 3. Microscope images of the hollow sphere, its spatial frequency spectra evolution with different scanning cycles, the insets shows the corresponding transmission spectra (a) Sample with initial diameter 80 μm, tapered to 145 μm. (b) Sample with initial diameter 40 μm, tapered to 59 μm.

Fig. 4. Device samples (S1 –S4 ) applied in the sensing tests (a) transmission spectra. (b) Spatial frequency spectra and microscope images (inset).

where I1 and I2 represent the intensity of light beam passing through the inner air-cavity and travelling along silica wall, respectively, λ is the wavelength, L is the cavity length, Δn = nco − ncl denotes the effective RI difference between the core mode and the cladding mode, and nco and ncl are the effective RI of the core mode and the cladding mode, respectively. When the phase term satisfies the condition 2πLΔn/λ = (2m + 1)π, where m is an integer, the intensity dip appears at the wavelength λdip =

2LΔn 2m + 1

(2)

IV. EXPERIMENTAL RESULTS AND DISCUSSION Four fiber device samples (S1 –S4 ) with different diameter values of ∼86, 61, 49 and 40 μm, corresponding to the cladding thicknesses of ∼18, 30, 36 and 40 μm respectively, are used for various types of sensing tests. The transmission spectra of the samples (S1 –S4 ) are displayed in Fig. 4(a), respectively, and their corresponding spatial frequency spectra and the microscope images are shown in Fig. 4(b) and their insets, respectively. The dominant dip (marked by a small circle) in the transmission spectrum of each sample is selected to implement the test. The experimental arrangements used to implement the sensing tests are shown in Fig. 5, where the fiber with inner air-cavity is fixed on two translation stages by use of instant adhesive. Incident light beam from a BBS is launched into the fiber device and the output is sent to an OSA to record its spectrum. During the RI sensing test, each device sample was immersed in a series of RI liquids (from Cargille Laboratories) in the RI range of 1.30–1.44 (@589.3 nm) with an interval of 0.01. As demonstrated in Fig. 4(a), a glass slide was placed slightly below the fiber to allow the RI liquid to immerse the fiber cavity area. Each time after the measurement, the sample was rinsed with

Fig. 5. Experimental set-up for (a) RI sensing test; (b) strain sensing test; (c) temperature sensing test.

methanol carefully until the original spectrum (i.e., the reference spectrum) could be restored and no residue liquid was left on the fiber surface. The evolution of transmission spectra of sample S1 with different RI values around 1500 nm is revealed in Fig. 6(a), where a blue shift can be observed. The dip wavelength shift with RI variation is shown in Fig. 6(b). It can be found from the figure that different samples may have spectral shift in different directions, and when the RI value is approaching 1.44, close to the RI value of the fiber cladding material, a good linear relationship appears and relatively large wavelength shift can be obtained. The highest sensitivity achieved is ∼1060 nm/RIU within the RI region between 1.43 and 1.44, when sample S3 is used. In terms of RI sensitivity, our device is much higher than that could be achieved by LPFG based sensor [35] while exhibiting smaller


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where δλ = δλdip , and next is the RI of the surrounding medium. Since ∂∂nnecxot ≈ 0, we have ∂nco ∂ncl 2L − 1− 2m + 1 ∂λ ∂λ ∂ncl 2L (5) δnext δλdip = − 2m + 1 ∂next and δλdip δnext

∂ncl 2L 2m + 1 ∂next = − ∂nco ∂ncl 2L − 1− 2m + 1 ∂λ ∂λ ∂ncl 2L 2m + 1 ∂next = − ∂Δn 2L 1− 2m + 1 ∂λ

(6)

thus

Fig. 6. (a) The transmission spectra of S1 at different external RI values. (b) Wavelength shift with external RI variation for different device samples.

size and when compared with open air-cavity interferometer sensor, greatly improved robustness is achieved. As can be seen from Fig. 6(b), different types of dip wavelength shift are obtained for different device samples. This can be explained as follows. 1) The interference between the core mode and the lower order cladding mode (which indicates a negligible dispersion effect) produces a blue shift. When external RI is increased, the cladding effective RI is also increased, while the core mode effective RI is hardly affected. The dip wavelength shift, δλdip , with the RI change can be derived from (2) as δλdip =

−2Lδncl 2m + 1

(3)

where δncl denotes the change in effective RI of the cladding mode. Thus, the external environment RI variation induces a blue shift of dip wavelength. 2) The interference between the core mode and the higher order cladding mode (which indicates a relatively large dispersion) produces either a red shift or a blue shift. The shift of dip wavelength in this case not only depends on the change of effective RI of core mode and cladding mode due to the variation of surrounding RI but also on dispersion. From (2), it follows that ∂nco ∂ncl 2L − δλ δλdip = 2m + 1 ∂λ ∂λ ∂nco ∂ncl 2L − (4) δnext + 2m + 1 ∂next ∂next

2m + 1 ∂Δn > , ∂λ 2L ∂Δn 2m + 1 < , ∂λ 2L

for a red shift; for a blue shift.

(7)

In the axial strain measurement as illustrated in Fig. 5(b), the device sample to be tested was fixed on two translation stages using instant adhesive. The stages were adjusted by screwing and the total displacement was ΔL. The axial strain applied would change not only the length of the interferometer device, but also the effective RI of the fiber core mode and cladding modes, thus shifting the dip wavelength. Fig. 7(a) shows the spectral behavior of sample S1 around 1500 nm when axial strain is applied, and a red shift appears. The variation of dip wavelength with axial strain in the range between 0 and 2000 με is demonstrated in Fig. 7(b) where it can be found that the average sensitivities obtained within the range from 0 to 800 με are between −18.5 and 22.5 pm/με, with the largest sensitivity value achieved by S1 . Such a sensitivity value is more than 5 times that of fiber MZI [36]–[37], ∼20 times that of FBG [38]–[39] and ∼10 times that of conventional LPFG [40], in addition, with a largely reduced device size. As can be observed from Fig. 7(b), different device samples have different types of dip wavelength shift, which can be explained as follows. The dip wavelength shift due to the change of axial strain can be derived from Eq. (2) as δλdip = ≈

2LΔn 2(L + δL)(Δn + δΔn)] − 2m + 1 2m + 1 2(ΔnδL + Lδnco − Lδncl ) 2(ΔnδL + LδΔn) = 2m + 1 2m + 1 (8)

where δL is the change of cavity length, δnco and δncl denote the change of the effective RI of the core mode and the cladding mode respectively, δΔn represents the change of effective RI

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Fig. 7. (a) The transmission spectra of S1 at different axial strain. (b) Wavelength shift with axial strain variation for different device samples.

difference between the core mode and the cladding mode, induced by the increased axial strain. Thus, the dip wavelength would take blue shift when

ΔnδL + Lδnco − Lδncl < 0

red shift when

ΔnδL + Lδnco − Lδncl > 0.

and

The temperature sensing test was implemented by use of an electric oven as depicted in Fig. 5(c). The fiber device was heated from room temperature to 100 °C with a step of 10 °C and stayed for 5 min at each step to allow thermal equilibrium be established. The spectral evolution of sample S1 around 1500 nm is displayed in Fig. 8(a), when temperature is varied. The dip wavelength shift with the temperature variation is demonstrated in Fig. 8(b). It can be observed from the figure that for all the samples, the dip wavelength experiences a red shift with the increase of temperature. The temperature sensitivities obtained are ranging from ∼44 to ∼80 pm/°C, and the largest sensitivity is achieved by S4 , which is six times that of FBG based sensor [3] and much larger than that of MZI with an open air-cavity [31], while exhibiting a reduced device size or a highly improved robustness. According to (2), the dip wavelength shift due to temperature increase can be expressed as δλdip = ≈

2LΔn 2(L + δLT )(Δn + δΔnT ) − 2m + 1 2m + 1 2(ΔnδLT + LδΔnT ) 2m + 1

Fig. 8. (a) The transmission spectra of S1 at different temperature. (b) Wavelength shift with temperature variation for different device samples.

where δLT is the change of inner cavity length induced by material thermal-expansion and δΔnT denotes the change of effective RI difference between the core mode and the cladding mode, due to thermal-optical effect. Since the thermo-optic coefficient is 7.8 × 10−6 in silica and thermal expansion coefficient is 4.1 × 10−7 , ΔnδLT + LδΔnT > 0, a red wavelength shift is always obtained. The insertion loss of the device is mainly due to the fact that only a few of the excited cladding modes become the transmission modes, and its value is affected by the cavity shape, the thickness and length of the silica wall, etc. The insertion loss of the device samples used is ranging from 16 to 20 dB, with the lowest insertion loss achieved by S4 , which exhibits the smallest diameter or air-cavity length of the four samples. The low insertion loss helps in cascade connection of two or more inner air-cavities and hence enhancing the device function. The sensor head of our device has a very small length of only a few tens of microns. Such a small size makes it possible to measure a tiny sample in an accurate spatial location, or enables a “point sensing” capability, which has high potential in bio-sensing such as single molecular detection. Moreover, as the transmission spectrum of the modal interferometer device exhibits a number of wavelength dips, a simultaneous multiple parameter measurement can be achieved or the temperature cross-sensitivity can be avoid by monitoring the shift of two or more dip wavelengths. V. SIMULATION RESULTS

(9)

The numerical simulation results are obtained by using COMSOL Multiphysics. The parameters used in the simulations are:


Fig. 9. Effective index of core mode and cladding mode of different orders. The corresponding mode field distribution is given in (a)–(d), respectively.

SMF with core radius of 4.1 μm and cladding radius of 62.5 μm, RI of core of 1.4682 and cladding of 1.46291448 (0.36% lower than that of the core), and surrounded by air; the sensing region with the inner air-cavity radius of 20 μm, cladding thickness of 40 μm and RI of 1.46291448, and surrounded by medium with RI of 1.4. The results obtained in Fig. 9 clearly show that ∂∂nλc l is negative and decreases for higher order modes. ∂n ∂ nc l 2L co > 1 may be satisfied if the cladding mode − 2m +1 ∂λ ∂λ involved in the interference is of an appropriate order and in this case the dip wavelength is in a red shift. VI. CONCLUSION By fabricating an inner air-cavity in a slightly tapered optical fiber, a tiny optical fiber interferometer device is created. Such a device exhibits an ultra compact size down to a few tens micrometer, good robustness due to its inner cavity structure and high sensitivity to a range of parameters, while being extremely suitable to spatially precise measurement, thus providing wide spreading opportunities for versatile optical fiber sensing applications. ACKNOWLEDGMENT The authors would like to thank the assistance of Dr. Y. Wang and Mr. T. Y. Hu in this work and Prof. Z. Y. Wen from the Dept. of mathematics, Tsinghua University, China, for helpful advice. REFERENCES [1] K. O. Hill, Y. Fujii, D. C. Johnson, and B. S. Kawasaki, “Photosensitivity in optical fiber waveguides: application to reflection filter fabrication,” Appl. Phys. Lett., vol. 32, pp. 647–649, May 1978. [2] G. Meltz, W. W. Morey, and W. H. Glenn, “Formation of Bragg gratings in optical fibers by a transverse holographic method,” Opt. Lett., vol. 14, pp. 823–825, Aug. 1989. [3] A. Othonos and K. Kalli, Fiber Bragg Gratings: Fundamentals and Applications in Telecommunications and Sensing. Boston, MA, USA: Artech, 1999. [4] K. O. Hill and G. Meltz, “Fiber Bragg grating technology fundamentals and overview,” J. Lightw. Technol., vol. 15, no. 8, pp. 1263–1276, Aug. 1997.

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[34] T. Y. Hu and D. N. Wang, “Optical fiber in-line Mach-Zehnder interferometer based on dual internal mirrors formed by a hollow sphere pair,” Opt. Lett., vol. 38, pp. 3036–3039, 2013. [35] X. W. Shu, L. Zhang, and I. Bennion, “Sensitivity characteristics of longperiod fiber gratings,” J. Lightw. Technol., vol. 20, no. 2, pp. 255–266, Feb. 2002. [36] J. Villatoro, V. Finazzi, V. P. Minkovich, V. Pruneri, and G. Badenes, “Temperature-insensitive photonic crystal fiber interferometer for absolute strain sensing,” Appl. Phys. Lett., vol. 91, p. 091109, 2007. [37] L. M. Hu, C. C. Chan, X. Y. Dong, Y. P. Wang, P. Zu, W. C. Wong, W. W. Chan, and T. Li, “Photonic crystal fiber strain sensor based on modified Mach-Zehnder interferometer,” IEEE Photon. J., vol. 4, no. 1, pp. 114–118, Jan. 2012. [38] C. Chen, A. Laronche, G. Bouwmans, L. Bigot, Y. Quiquempois, and J. Albert, “Sensitivity of photonic crystal fiber modes to temperature, strain and external refractive index,” Opt. Exp., vol. 16, pp. 9645–9653, 2008. [39] N. Liu, Y. Li, Y. Wang, H. Wang, W. Liang, and P. Lu, “Bending insensitive sensors for strain and temperature measurements with Bragg gratings in Bragg fibers,” Opt. Exp., vol. 19, pp. 13880–13891, 2011. [40] V. Bhatia and A. M. Vengsarkar, “Optical fiber long-period grating sensors,” Opt. Lett., vol. 21, pp. 692–694, 1996. H. F. Chen received the B.Sc. degree in optical electronics information engineering, and the M.Sc. degree in optical engineering, both from Zhejiang University, Hangzhou, China, in 2001 and 2004, respectively, where she is currently working toward the Ph.D. degree. Her main research interests include femtosecond laser micromachining, optical fiber communications, and optical fiber sensors.


D. N. Wang (SM’12) received the B.Sc. degree in telecommunications from the Beijing University of Posts and Telecommunications, Beijing, China, in 1982, the MBA degree from the University of Ulster, Derry, U.K., in 1989, and Ph.D. degree from City University, London, U.K., in 1995, respectively. Since 1998, he has been with the Department of Electrical Engineering, the Hong Kong Polytechnic University, Hung Hom, Hong Kong. His main research interests include ultrafast optics, femtosecond laser micromachining, fiber laser, optical fiber communications, and optical fiber sensors. He has more than 150 international journal publications.

W. Hong received the M.S. degree in science from the Department of Physics and the Ph.D. degree in physical electronics from the Department of optoelectronic engineering, Huazhong University of Science and Technology (HUST), Hubei, China, in 1999 and 2003, respectively. She is currently an Associate Professor at the Wuhan National Laboratory for Optoelectronics, HUST. Her main research interests include the design of optoelectronic devices and plasmonic waveguide devices. She has more than 30 international journal publications.