Highly sensitive force sensor based on optical microfiber asymmetrical Fabry-Perot interferometer Yuan Gong,1,* Cai-Bin Yu,1 Ting-Ting Wang,1 Xiu-Ping Liu,1 Yu Wu,1 Yun-Jiang Rao,1,4 Ming-Lei Zhang,1 Hui-Juan Wu,1 Xiao-Xiao Chen,2 and Gang-Ding Peng3 1
Key Laboratory of Optical Fiber Sensing and Communications (Ministry of Education of China), University of Electronic Science and Technology of China, Chengdu, 611731, China National Center of Quality Inspection for Sensors, National Institute of Measurement and Testing Technology, Chengdu, 610021,China 3 School of Electrical Engineering and Telecommunications, University of New South Wales, Sydney, NSW 2052, Australia 4 [email protected] *[email protected] 2
Abstract: An asymmetrical Fabry-Perot interferometric (AFPI) force sensor is fabricated based on a narrowband reflection of low-reflectivity fiber Bragg grating (LR-FBG) and a broadband Fresnel reflection of the cleaved fiber end. The AFPI sensor includes a section of microfiber made by tapering and it achieves a force sensitivity of 0.221pm/μN with a tapered microfiber of 40mm length and 6.1μm waist diameter. Compared with similar AFPI structure in 125μm-diameter single mode fiber, the force sensitivity of the microfiber AFPI structure is greatly enhanced due to its smaller diameter and can be optimized for different force scales by controlling the diameter. The fabrication process of the AFPI sensor is simple and cost-effective. The AFPI sensor has better multiplexing capacity than conventional extrinsic fiber-optic Fabry-Perot sensors, while it also release the requirement on the wavelength matching of the FBG-pair-based FPI. ©2014 Optical Society of America OCIS codes: (060.2280) Fiber design and fabrication; (060.2370) Fiber optics sensors; (060.3735) Fiber Bragg gratings; (230.3990) Micro-optical devices.
References and links 1.
C. Hertlein, L. Helden, A. Gambassi, S. Dietrich, and C. Bechinger, “Direct measurement of critical Casimir forces,” Nature 451(7175), 172–175 (2008). 2. D. G. Grier, “A revolution in optical manipulation,” Nature 424(6950), 810–816 (2003). 3. D. Van Thourhout and J. Roels, “Optomechanical device actuation through the optical gradient force,” Nat. Photonics 4(4), 211–217 (2010). 4. Y. Gong, A. Y. Ye, Y. Wu, Y. J. Rao, Y. Yao, and S. Xiao, “Graded-index fiber tip optical tweezers: Numerical simulation and trapping experiment,” Opt. Express 21(13), 16181–16190 (2013). 5. R. Kumar, H. Kumar, A. Kumar, and V. Kumar, “Long term uncertainty investigations of 1 MN force calibration machine at NPL, India (NPLI),” Meas. Sci. Rev. 12(4), 149–152 (2012). 6. T. Guo, Q. Zhao, H. Zhang, L. Xue, G. Li, B. Dong, B. Liu, W. Zhang, G. Kai, and X. Dong, “Temperatureinsensitive fiber Bragg grating force sensor via a bandwidth modulation and optical-power detection technique,” J. Lightwave Technol. 24(10), 3797–3802 (2006). 7. Y. Q. Liu, K. S. Chiang, and P. L. Chu, “Fiber-Bragg-grating force sensor based on a wavelength-switched selfseeded Fabry-Perot laser diode,” IEEE Photonics Technol. Lett. 17(2), 450–452 (2005). 8. B. Dong, D. P. Zhou, L. Wei, W. K. Liu, and J. W. Y. Lit, “Temperature- and phase-independent lateral force sensor based on a core-offset multi-mode fiber interferometer,” Opt. Express 16(23), 19291–19296 (2008). 9. L. M. Tong, R. R. Gattass, J. B. Ashcom, S. L. He, J. Y. Lou, M. Y. Shen, I. Maxwell, and E. Mazur, “Subwavelength-diameter silica wires for low-loss optical wave guiding,” Nature 426(6968), 816–819 (2003). 10. L. Zhang, J. Y. Lou, and L. M. Tong, “Micro/nanofiber optical sensors,” Photonic Sens. 1(1), 31–42 (2011). 11. F. X. Gu, L. Zhang, X. F. Yin, and L. M. Tong, “Polymer single-nanowire optical sensors,” Nano Lett. 8(9), 2757–2761 (2008).
#202791 - $15.00 USD Received 9 Dec 2013; revised 25 Jan 2014; accepted 29 Jan 2014; published 6 Feb 2014 (C) 2014 OSA 10 February 2014 | Vol. 22, No. 3 | DOI:10.1364/OE.22.003578 | OPTICS EXPRESS 3578
12. Y. Wu, Y. J. Rao, Y. H. Chen, and Y. Gong, “Miniature fiber-optic temperature sensors based on silica/polymer microfiber knot resonators,” Opt. Express 17(20), 18142–18147 (2009). 13. K. M. Chung, Z. Liu, C. Lu, and H. Y. Tam, “Highly sensitive compact force sensor based on microfiber Bragg grating,” IEEE Photonics Technol. Lett. 24(8), 700–702 (2012). 14. W. Luo, J. L. Kou, Y. Chen, F. Xu, and Y. Q. Lu, “Ultra-highly sensitive surface-corrugated microfiber Bragg grating force sensor,” Appl. Phys. Lett. 101(13), 133502 (2012). 15. T. Wieduwilt, S. Bruckner, and H. Bartelt, “High force measurement sensitivity with fiber Bragg gratings fabricated in uniform-waist fiber tapers,” Meas. Sci. Technol. 22(7), 075201 (2011). 16. Y. Liu, C. Meng, A. P. Zhang, Y. Xiao, H. Yu, and L. M. Tong, “Compact microfiber Bragg gratings with highindex contrast,” Opt. Lett. 36(16), 3115–3117 (2011). 17. Y. Ran, Y. N. Tan, L. P. Sun, S. Gao, J. Li, L. Jin, and B. O. Guan, “193 nm excimer laser inscribed Bragg gratings in microfibers for refractive index sensing,” Opt. Express 19(19), 18577–18583 (2011). 18. S. Gao, L. Jin, Y. Ran, L. P. Sun, J. Li, and B. O. Guan, “Temperature compensated microfiber Bragg gratings,” Opt. Express 20(16), 18281–18286 (2012). 19. X. Fang, C. R. Liao, and D. N. Wang, “Femtosecond laser fabricated fiber Bragg grating in microfiber for refractive index sensing,” Opt. Lett. 35(7), 1007–1009 (2010). 20. J. Li, X. Shen, L. P. Sun, and B. O. Guan, “Characteristics of microfiber Fabry-Perot resonators fabricated by UV exposure,” Opt. Express 21(10), 12111–12121 (2013). 21. J. Zhang, Q. Sun, R. Liang, J. Wo, D. Liu, and P. Shum, “Microfiber Fabry-Perot interferometer fabricated by taper-drawing technique and its application as a radio frequency interrogated refractive index sensor,” Opt. Lett. 37(14), 2925–2927 (2012). 22. Y. Wang, H. Bartelt, M. Becker, S. Brueckner, J. Bergmann, J. Kobelke, and M. Rothhardt, “Fiber Bragg grating inscription in pure-silica and Ge-doped photonic crystal fibers,” Appl. Opt. 48(11), 1963–1968 (2009). 23. Y. Wang, H. Bartelt, W. Ecke, R. Willsch, J. Kobelke, M. Kautz, S. Brueckner, and M. Rothhardt, “Sensing properties of fiber Bragg gratings in small-core Ge-doped photonic crystal fibers,” Opt. Commun. 282(6), 1129– 1134 (2009). 24. Y. Fujii, “Method of generating and measuring static small force using down-slope component of gravity,” Rev. Sci. Instrum. 78(6), 066104 (2007). 25. K. Miyamoto, T. Jomori, K. Sugano, O. Tabata, and T. Tsuchiya, “Mechanical calibration of MEMS springs with sub-micro-Newton force resolution,” Sens. Actuators A Phys. 143(1), 136–142 (2008). 26. O. Frazão, R. M. Silva, J. Kobelke, and K. Schuster, “Temperature- and strain-independent torsion sensor using a fiber loop mirror based on suspended twin-core fiber,” Opt. Lett. 35(16), 2777–2779 (2010). 27. Y. J. Rao, X. K. Zeng, Y. Zhu, Y. Wang, T. Zhu, Z. Ran, L. Zhang, and I. Bennion, “Temperature-strain discrimination sensor using a WDM chirped in-fibre Bragg grating and an extrinsic Fabry-Perot,” Chin. Phys. Lett. 18(5), 643–645 (2011). 28. Y. Yu, H. Tam, W. Chung, and M. S. Demokan, “Fiber Bragg grating sensor for simultaneous measurement of displacement and temperature,” Opt. Lett. 25(16), 1141–1143 (2000). 29. J. Villatoro, V. P. Minkovich, and D. Monzón-Hernández, “Temperature-independent strain sensor made from tapered holey optical fiber,” Opt. Lett. 31(3), 305–307 (2006). 30. X. Y. Dong, Y. Liu, Z. Liu, and X. Y. Dong, “Simultaneous displacement and temperature measurement with cantilever-based fiber Bragg grating sensor,” Opt. Commun. 192(3–6), 213–217 (2001). 31. T. Guo, A. Ivanov, Ch. Chen, and J. Albert, “Temperature-independent tilted fiber grating vibration sensor based on cladding-core recoupling,” Opt. Lett. 33(9), 1004–1006 (2008). 32. Y. J. Rao, “Recent progress in fiber-optic extrinsic Fabry-Perot interferometric sensors,” Opt. Fiber Technol. 12(3), 227–237 (2006). 33. M. Han, Y. Zhang, F. Shen, G. R. Pickrell, and A. Wang, “Signal-processing algorithm for white-light optical fiber extrinsic Fabry-Perot interferometric sensors,” Opt. Lett. 29(15), 1736–1738 (2004). 34. T. Wei, Y. Han, Y. Li, H. L. Tsai, and H. Xiao, “Temperature-insensitive miniaturized fiber inline Fabry-Perot interferometer for highly sensitive refractive index measurement,” Opt. Express 16(8), 5764–5769 (2008). 35. X. Wan and H. F. Taylor, “Intrinsic fiber Fabry-Perot temperature sensor with fiber Bragg grating mirrors,” Opt. Lett. 27(16), 1388–1390 (2002). 36. W. Liang, Y. Huang, Y. Xu, R. K. Lee, and A. Yariv, “Highly sensitive fiber Bragg grating refractive index sensors,” Appl. Phys. Lett. 86(15), 151122 (2005). 37. Y. O. Barmenkov, D. Zalvidea, S. Torres-Peiró, J. L. Cruz, and M. V. Andrés, “Effective length of short FabryPerot cavity formed by uniform fiber Bragg gratings,” Opt. Express 14(14), 6394–6399 (2006).
1. Introduction There have been great demands for force measurements ranging from femto-Newtons (fN) to nano-Newtons (nN) in the field of the nanoparticle interaction and cell mechanics [1–4]. And for more engineering applications it can be extended up to mega-Newtons (MN) [5]. Calibration of the force sensor at all the scales is necessary for practical applications. Conventional fiber-optic force sensors have been developed based on fiber Bragg grating (FBG) in single mode fiber (SMF) [6, 7] or a fiber modal interferometer [8]. However, due to
#202791 - $15.00 USD Received 9 Dec 2013; revised 25 Jan 2014; accepted 29 Jan 2014; published 6 Feb 2014 (C) 2014 OSA 10 February 2014 | Vol. 22, No. 3 | DOI:10.1364/OE.22.003578 | OPTICS EXPRESS 3579
the relatively-large diameter of 125μm and the elastic structure design, they aim to measure forces at the Newton level. Since its first development [9], the optical micro/nanofiber has been used as a platform for studying fiber-optic sensing, providing very high sensitivity to various parameters [10–12] due to its quite small diameter. Milli-Newton-level force sensing is demonstrated via a 30-μm diameter microfiber Bragg grating (MFBG) [13]. The force sensitivity is further enhanced by using MFBGs with diameters down to 2-3 μm [14, 15]. Various micromachining technologies have been used to fabricate such a MFBG, including focused ion beam milling [14, 16], 193nm ArF excimer laser inscription [17, 18], and femtosecond laser micromachining [19]. Fiber-optic Fabry-Perot interferometers are also formed by symmetrical MFBGs [20]. In prior to the fabrication process of MFBGs, the pre-fabricated optical microfiber is easy to be broken and the precise alignment of the microfiber and the laser beam for micromachining is essential, making the fabrication slow and cost-ineffective. It is much more efficient to fabricate a FBG in SMF firstly and then form a microfiber interferometer by the taperdrawing method [21]. In this paper, we report an optical microfiber force sensor based on an asymmetrical Fabry-Perot interferometric (AFPI) sensor, formed by a low-reflectivity (LR-) FBG and a cleaved fiber end. Besides the structure is spatially asymmetrical, the narrowband FBG and the broadband Fresnel reflection can also be considered asymmetrical in the wavelength domain. The fabrication process is simple and low cost. Compared with similar structure in 125μm-diameter SMF, the force sensitivity of the microfiber AFPI sensor is greatly enhanced due to its smaller diameter. 2. Fabrication of AFPI structure
Fig. 1. Schematic diagram of experimental setup for (a) the fabrication of the asymmetrical Fabry-Perot interferometric (AFPI) structure and (b) force sensing.
The experimental setup for the fabrication of the AFPI structure and the force sensing are shown in Figs. 1(a) and 1(b), respectively. First, a LR-FBG is inscribed in a hydrogen-loaded SMF using a 248nm KrF excimer laser (BraggStar Industrial, Coherent) and a phase mask with a period of 1076.7nm (Lasiris), which is more frequently-used than the Talbot interferometer arrangement [22, 23]. The energy and the repetition rate of the laser are set to be 6.5mJ/pulse and 100Hz, respectively. A total 750 pulses are used to fabricate a 3mm-long FBG with a transmission dip of around 0.2dB, which corresponds to a reflectivity of about 4.5%. The reflective and transmitted spectra of a typical LR-FBG in our experiment are measured by a wavelength-swept-laser-based optical spectrum analyzer (OSA, Agilent) and shown in Fig. 2. The spectral resolution of the OSA can be set to as high as 0.1pm. The Bragg
#202791 - $15.00 USD Received 9 Dec 2013; revised 25 Jan 2014; accepted 29 Jan 2014; published 6 Feb 2014 (C) 2014 OSA 10 February 2014 | Vol. 22, No. 3 | DOI:10.1364/OE.22.003578 | OPTICS EXPRESS 3580
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wavelength is about 1558.7nm. The sideband rejection is about 13.3dB with no apodization. Second, one end of the LR-FBG is tapered down to 6 −7 μm. The pigtails of the LR-FBG are fixed on a three-dimensional (3-D) translation stage and a one-dimensional (1-D) motorized translation stage via two fiber holders. One end of the LR-FBG is heated by a hydrogen flame and at the same time the motorized translation stage is controlled to stretch the pigtail into a microfiber. By a flow controller, the hydrogen flame can be controlled to have a diameter of about 2mm. After an optimization on the movement parameters, the acceleration and the velocity are set to be 1mm/s2 and 3mm/s, respectively. In our experiment the taper length is controlled between 20mm and 40mm by setting the displacement of the 1-D motorized translation stage. Then the far end of the LR-FBG is cleaved to obtain the Fresnel reflection. The AFPI structure is shown in the inset of Fig. 1(a). The waist diameter of the microfiber is 7.3μm and 6.1μm, measured by a scanning electronic microscope (SEM) and as shown in Fig. 3, when the taper length is set to be 20 and 40mm, respectively.
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Wavelength (nm) Fig. 2. Reflective (blue) and transmitted spectra (red) of a typical FBG for the fabrication of the AFPI structure.
Fig. 3. SEM images for the waist part of the AFPI sensor with taper lengths of (a) 20mm and (b) 40mm. The waist diameters are 7.3μm and 6.1μm, respectively.
According to the two-beam interference theory, the reflective spectrum of the AFPI sensor can be expressed as 2π OPD I ( λ ) = I FBG ( λ ) + I Fresnel ( λ ) + 2 I FBG ( λ ) I Fresnel ( λ ) cos . λ
(1)
#202791 - $15.00 USD Received 9 Dec 2013; revised 25 Jan 2014; accepted 29 Jan 2014; published 6 Feb 2014 (C) 2014 OSA 10 February 2014 | Vol. 22, No. 3 | DOI:10.1364/OE.22.003578 | OPTICS EXPRESS 3581
Here I FBG ( λ ) is the reflective spectrum of the LR-FBG. I Fresnel ( λ ) is determined by both the Fresnel reflection and the transmission losses along the microfiber, both of which are only slightly dependent on the incident wavelength. Thus I Fresnel ( λ ) can be approximately considered to be constant in the FBG reflective bandwidth. Maximum interference fringe contrast can be obtained when the intensity of the two beams are equal. OPD =2nl is the optical path difference, with n the effective refractive index and l the effective length between the two reflective beams. By measuring the strain-induced OPD changes or wavelength shift of the interference fringes, the external force changes can be determined. 3. Force sensing experiment and discussion In the force sensing experiment, the AFPI structure is vertically adhered to a glass substrate, as shown in Fig. 1(b). Both ends of the FBG are fixed to a glass substrate via UV glues in order to eliminate the force-induced Bragg wavelength shift. A mass with uniform density in the longitudinal direction is employed to calibrate the force response of the AFPI sensor. It is adhered to the 125μm-SMF section between the microfiber and the Fresnel surface. This will influence neither the transmission loss of the microfiber nor the Fresnel reflection of the fiber end. Each time the force is changed slightly by cutting a small section of the long-shaped mass and then the reduced weight is measured with an accuracy of 0.0001g by an analytical balance. When the waist diameter of the microfiber is 7.3μm, obtained by controlling the taper length to be 20mm, the interference spectrum of the AFPI sensor is shown in Fig. 4(a). The maximum fringe contrast is obtained at the Bragg wavelength according to the two-beam interference theory, as the reflection at the Bragg wavelength is approximately equal to that of the Fresnel reflection. The wavelength swept rate and the spectral resolution of the OSA are 50nm/s and 0.8pm, respectively. Each spectra is averaged 8 times in order to eliminate the random variations. The spectrum in one free spectral range (FSR) is enlarged and shown as a function of the external force changes in Fig. 4(b). A fringe contrast of higher than 7dB is easily obtained and can be further enhanced greatly if the intensities of two reflected beams are closer and higher spectral resolution of the OSA is set.
Fig. 4. (a) Interference spectra in the LR-FBG reflective bandwidth and (b) enlarged spectra in one free spectral range of the microfiber AFPI sensor as a function of the external force changes.
The wavelength shift as a function of the external force change is shown in Fig. 5(a) with solid circles for the AFPI sensor with a microfiber waist diameter of 7.3μm. The force sensing experiment is repeated three times and the force sensitivity is statistically determined to be 0.120 ± 0.004 pm/μN. The linearity is about 0.998 ± 0.001. The range of the force changes is controlled within several hundreds of micro-Newtons. There are many applications for the
#202791 - $15.00 USD Received 9 Dec 2013; revised 25 Jan 2014; accepted 29 Jan 2014; published 6 Feb 2014 (C) 2014 OSA 10 February 2014 | Vol. 22, No. 3 | DOI:10.1364/OE.22.003578 | OPTICS EXPRESS 3582
force sensor in this scale [24]. One of the typical applications is the stiffness calibration of microelectromechanical-system (MEMS) springs [25].
Fig. 5. Wavelength shift of the interference spectra as a function of the external force change. (a) Microfiber AFPI sensor with a waist diameter of 7.3μm (solid circles) and similar structure in 125μm-diameter SMF (void squares); (b) Microfiber AFPI sensors with different waist diameters.
Further we investigate the influence of microfiber waist diameter on the force sensitivity. In principal, the sensor measures the force by sensing the axial strain. The force induced strain depends strongly on the diameter of the fiber. Here, the waist diameter of the tapered fiber is controlled by the tapered length, i.e., the displacement of the motorized translation stage. The wavelength shift as a function of external force change is shown in Fig. 5(b) for microfiber AFPI sensors with waist diameters of 7.3μm, 6.7μm and 6.1μm, corresponding to taper lengths of 20mm, 30mm, and 40mm, respectively. The force sensitivities are determined to be 0.122 ± 0.003 pm/μN, 0.171 ± 0.002 pm/μN, and 0.221 ± 0.005 pm/μN, respectively. The linearities are all above 0.997. It is indicated by the experimental results that the force sensitivity can be enhanced with a smaller microfiber waist diameter. This kind of sensor will suffer from the temperature-force cross-sensitive problem and should be used in a constant-temperature environment for the precise force calibration. In our experiment the force responses of the sensor were tested in a constant-temperature environment. The good linearity shown in Fig. 5 indicates that the ambient temperature variation during the experiment can be neglected. Subsequent effort will be made to compensate the temperature effect on the force sensing. There have already been many methods proposed to reduce the cross sensitivity between force/strain and temperature, or to simultaneously measure dual parameters [26–31]. In our experiment we employed an AFPI structure based on a weak FBG and a cleaved fiber end. Its advantages can be revealed by comparing it with two symmetrical interferometric structures. One is based on two Fresnel reflections, known as extrinsic fiberoptic Fabry-Perot interferometer (EFPI) [32–34]. The other is FBG-pair-based fiber-optic Fabry-Perot interferometer (FBG-FPI) [35–37]. In the EFPI, two broadband reflective beam interfere and the fringes cover a wide range in the wavelength domain (> 100 nm). The wavelength division multiplexing (WDM) becomes invalid. Therefore, spatial frequency multiplexing (SFM) is often employed by using EFPIs with different cavity lengths [32]. However, the number of sensors multiplexed by the SFM method is limited. As the cavity length increases, the propagation loss of light in the cavity increases, making the fringe contrast degrade and indicating a maximum cavity length. For the fabrication of a fiber-optic Fabry-Perot interferometer based on FBG pairs, it is requested that the narrow reflective band, generally with a 3dB bandwidth of 0.2 - 0.3 nm, of the two FBGs should be exactly overlapped [35–37]. Further the Bragg wavelengths of the two FBGs should change synchronously during the sensing process. If either of two above demands is not met, the fringes will degrade or even disappear. #202791 - $15.00 USD Received 9 Dec 2013; revised 25 Jan 2014; accepted 29 Jan 2014; published 6 Feb 2014 (C) 2014 OSA 10 February 2014 | Vol. 22, No. 3 | DOI:10.1364/OE.22.003578 | OPTICS EXPRESS 3583
Therefore, the AFPI structure proposed in this paper has two advantages. Compared with the EFPI, the use of one FBG in the structure restricts the spectral range of each sensor. This makes WDM available and thus greatly enhance the multiplexing capacity of this kind of sensor. Compared with the FBG-FPI, the replacement of one FBG with a Fresnel reflection of the fiber end releases the strict requirement of the wavelength matching of the FBG pairs. In order to evaluate the sensitivity enhancement of the microfiber AFPI sensor, the force sensing experiment is repeated with a similar structure in 125μm-diameter SMF, also composed of a LR-FBG and the Fresnel reflection from a cleaved fiber end, except for the fiber tapering. The structure and the reflective spectrum are shown in Figs. 6(a) and 6(b), respectively. The interference fringes in the FBG reflection band and in one FSR are shown in Figs. 6(c) and 6(d), respectively. The wavelength shift as a function of the external force change is shown in Fig. 5(a) with void squares. It is clear that it is not sensitive to the external force changes.
Fig. 6. (a) AFPI structure in 125μm-diameter SMF and (b) its interference spectrum. Enlarged spectra (c) in the Bragg reflection band and (d) in one free spectral range as a function of external force changes.
4. Conclusion An optical microfiber asymmetrical Fabry-Perot interferometric force sensor has been fabricated by a low-reflectivity fiber Bragg grating and a Fresnel reflection of a cleaved fiber end. The force sensing characteristics have been explored in detail. Compared with similar structure in single mode fiber, the force sensitivity has been greatly enhanced. A force sensitivity of 0.221pm/μN has been obtained with a microfiber of 40mm length and 6.1μm waist diameter. It also has the advantages of high multiplexing capacity over conventional EFPI and no wavelength matching, like in the FBG-pair-based FPI, between the two reflective beams is required. Acknowledgments This work is supported by National Natural Science Foundation of China (61107073, 61107072 and 61290312), Fundamental Research Funds for the Central Universities (ZYGX2011J002), Research Fund for the Doctoral Program of Higher Education of China (20110185120020), Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT, IRT1218), and the 111 Project (B14039).
#202791 - $15.00 USD Received 9 Dec 2013; revised 25 Jan 2014; accepted 29 Jan 2014; published 6 Feb 2014 (C) 2014 OSA 10 February 2014 | Vol. 22, No. 3 | DOI:10.1364/OE.22.003578 | OPTICS EXPRESS 3584
Key Laboratory of Optical Fiber Sensing and Communications (Ministry of Education of China), University of Electronic Science and Technology of China, Chengdu, 611731, China National Center of Quality Inspection for Sensors, National Institute of Measurement and Testing Technology, Chengdu, 610021,China 3 School of Electrical Engineering and Telecommunications, University of New South Wales, Sydney, NSW 2052, Australia 4 [email protected] *[email protected] 2
Abstract: An asymmetrical Fabry-Perot interferometric (AFPI) force sensor is fabricated based on a narrowband reflection of low-reflectivity fiber Bragg grating (LR-FBG) and a broadband Fresnel reflection of the cleaved fiber end. The AFPI sensor includes a section of microfiber made by tapering and it achieves a force sensitivity of 0.221pm/μN with a tapered microfiber of 40mm length and 6.1μm waist diameter. Compared with similar AFPI structure in 125μm-diameter single mode fiber, the force sensitivity of the microfiber AFPI structure is greatly enhanced due to its smaller diameter and can be optimized for different force scales by controlling the diameter. The fabrication process of the AFPI sensor is simple and cost-effective. The AFPI sensor has better multiplexing capacity than conventional extrinsic fiber-optic Fabry-Perot sensors, while it also release the requirement on the wavelength matching of the FBG-pair-based FPI. ©2014 Optical Society of America OCIS codes: (060.2280) Fiber design and fabrication; (060.2370) Fiber optics sensors; (060.3735) Fiber Bragg gratings; (230.3990) Micro-optical devices.
References and links 1.
C. Hertlein, L. Helden, A. Gambassi, S. Dietrich, and C. Bechinger, “Direct measurement of critical Casimir forces,” Nature 451(7175), 172–175 (2008). 2. D. G. Grier, “A revolution in optical manipulation,” Nature 424(6950), 810–816 (2003). 3. D. Van Thourhout and J. Roels, “Optomechanical device actuation through the optical gradient force,” Nat. Photonics 4(4), 211–217 (2010). 4. Y. Gong, A. Y. Ye, Y. Wu, Y. J. Rao, Y. Yao, and S. Xiao, “Graded-index fiber tip optical tweezers: Numerical simulation and trapping experiment,” Opt. Express 21(13), 16181–16190 (2013). 5. R. Kumar, H. Kumar, A. Kumar, and V. Kumar, “Long term uncertainty investigations of 1 MN force calibration machine at NPL, India (NPLI),” Meas. Sci. Rev. 12(4), 149–152 (2012). 6. T. Guo, Q. Zhao, H. Zhang, L. Xue, G. Li, B. Dong, B. Liu, W. Zhang, G. Kai, and X. Dong, “Temperatureinsensitive fiber Bragg grating force sensor via a bandwidth modulation and optical-power detection technique,” J. Lightwave Technol. 24(10), 3797–3802 (2006). 7. Y. Q. Liu, K. S. Chiang, and P. L. Chu, “Fiber-Bragg-grating force sensor based on a wavelength-switched selfseeded Fabry-Perot laser diode,” IEEE Photonics Technol. Lett. 17(2), 450–452 (2005). 8. B. Dong, D. P. Zhou, L. Wei, W. K. Liu, and J. W. Y. Lit, “Temperature- and phase-independent lateral force sensor based on a core-offset multi-mode fiber interferometer,” Opt. Express 16(23), 19291–19296 (2008). 9. L. M. Tong, R. R. Gattass, J. B. Ashcom, S. L. He, J. Y. Lou, M. Y. Shen, I. Maxwell, and E. Mazur, “Subwavelength-diameter silica wires for low-loss optical wave guiding,” Nature 426(6968), 816–819 (2003). 10. L. Zhang, J. Y. Lou, and L. M. Tong, “Micro/nanofiber optical sensors,” Photonic Sens. 1(1), 31–42 (2011). 11. F. X. Gu, L. Zhang, X. F. Yin, and L. M. Tong, “Polymer single-nanowire optical sensors,” Nano Lett. 8(9), 2757–2761 (2008).
#202791 - $15.00 USD Received 9 Dec 2013; revised 25 Jan 2014; accepted 29 Jan 2014; published 6 Feb 2014 (C) 2014 OSA 10 February 2014 | Vol. 22, No. 3 | DOI:10.1364/OE.22.003578 | OPTICS EXPRESS 3578
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1. Introduction There have been great demands for force measurements ranging from femto-Newtons (fN) to nano-Newtons (nN) in the field of the nanoparticle interaction and cell mechanics [1–4]. And for more engineering applications it can be extended up to mega-Newtons (MN) [5]. Calibration of the force sensor at all the scales is necessary for practical applications. Conventional fiber-optic force sensors have been developed based on fiber Bragg grating (FBG) in single mode fiber (SMF) [6, 7] or a fiber modal interferometer [8]. However, due to
#202791 - $15.00 USD Received 9 Dec 2013; revised 25 Jan 2014; accepted 29 Jan 2014; published 6 Feb 2014 (C) 2014 OSA 10 February 2014 | Vol. 22, No. 3 | DOI:10.1364/OE.22.003578 | OPTICS EXPRESS 3579
the relatively-large diameter of 125μm and the elastic structure design, they aim to measure forces at the Newton level. Since its first development [9], the optical micro/nanofiber has been used as a platform for studying fiber-optic sensing, providing very high sensitivity to various parameters [10–12] due to its quite small diameter. Milli-Newton-level force sensing is demonstrated via a 30-μm diameter microfiber Bragg grating (MFBG) [13]. The force sensitivity is further enhanced by using MFBGs with diameters down to 2-3 μm [14, 15]. Various micromachining technologies have been used to fabricate such a MFBG, including focused ion beam milling [14, 16], 193nm ArF excimer laser inscription [17, 18], and femtosecond laser micromachining [19]. Fiber-optic Fabry-Perot interferometers are also formed by symmetrical MFBGs [20]. In prior to the fabrication process of MFBGs, the pre-fabricated optical microfiber is easy to be broken and the precise alignment of the microfiber and the laser beam for micromachining is essential, making the fabrication slow and cost-ineffective. It is much more efficient to fabricate a FBG in SMF firstly and then form a microfiber interferometer by the taperdrawing method [21]. In this paper, we report an optical microfiber force sensor based on an asymmetrical Fabry-Perot interferometric (AFPI) sensor, formed by a low-reflectivity (LR-) FBG and a cleaved fiber end. Besides the structure is spatially asymmetrical, the narrowband FBG and the broadband Fresnel reflection can also be considered asymmetrical in the wavelength domain. The fabrication process is simple and low cost. Compared with similar structure in 125μm-diameter SMF, the force sensitivity of the microfiber AFPI sensor is greatly enhanced due to its smaller diameter. 2. Fabrication of AFPI structure
Fig. 1. Schematic diagram of experimental setup for (a) the fabrication of the asymmetrical Fabry-Perot interferometric (AFPI) structure and (b) force sensing.
The experimental setup for the fabrication of the AFPI structure and the force sensing are shown in Figs. 1(a) and 1(b), respectively. First, a LR-FBG is inscribed in a hydrogen-loaded SMF using a 248nm KrF excimer laser (BraggStar Industrial, Coherent) and a phase mask with a period of 1076.7nm (Lasiris), which is more frequently-used than the Talbot interferometer arrangement [22, 23]. The energy and the repetition rate of the laser are set to be 6.5mJ/pulse and 100Hz, respectively. A total 750 pulses are used to fabricate a 3mm-long FBG with a transmission dip of around 0.2dB, which corresponds to a reflectivity of about 4.5%. The reflective and transmitted spectra of a typical LR-FBG in our experiment are measured by a wavelength-swept-laser-based optical spectrum analyzer (OSA, Agilent) and shown in Fig. 2. The spectral resolution of the OSA can be set to as high as 0.1pm. The Bragg
#202791 - $15.00 USD Received 9 Dec 2013; revised 25 Jan 2014; accepted 29 Jan 2014; published 6 Feb 2014 (C) 2014 OSA 10 February 2014 | Vol. 22, No. 3 | DOI:10.1364/OE.22.003578 | OPTICS EXPRESS 3580
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wavelength is about 1558.7nm. The sideband rejection is about 13.3dB with no apodization. Second, one end of the LR-FBG is tapered down to 6 −7 μm. The pigtails of the LR-FBG are fixed on a three-dimensional (3-D) translation stage and a one-dimensional (1-D) motorized translation stage via two fiber holders. One end of the LR-FBG is heated by a hydrogen flame and at the same time the motorized translation stage is controlled to stretch the pigtail into a microfiber. By a flow controller, the hydrogen flame can be controlled to have a diameter of about 2mm. After an optimization on the movement parameters, the acceleration and the velocity are set to be 1mm/s2 and 3mm/s, respectively. In our experiment the taper length is controlled between 20mm and 40mm by setting the displacement of the 1-D motorized translation stage. Then the far end of the LR-FBG is cleaved to obtain the Fresnel reflection. The AFPI structure is shown in the inset of Fig. 1(a). The waist diameter of the microfiber is 7.3μm and 6.1μm, measured by a scanning electronic microscope (SEM) and as shown in Fig. 3, when the taper length is set to be 20 and 40mm, respectively.
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Wavelength (nm) Fig. 2. Reflective (blue) and transmitted spectra (red) of a typical FBG for the fabrication of the AFPI structure.
Fig. 3. SEM images for the waist part of the AFPI sensor with taper lengths of (a) 20mm and (b) 40mm. The waist diameters are 7.3μm and 6.1μm, respectively.
According to the two-beam interference theory, the reflective spectrum of the AFPI sensor can be expressed as 2π OPD I ( λ ) = I FBG ( λ ) + I Fresnel ( λ ) + 2 I FBG ( λ ) I Fresnel ( λ ) cos . λ
(1)
#202791 - $15.00 USD Received 9 Dec 2013; revised 25 Jan 2014; accepted 29 Jan 2014; published 6 Feb 2014 (C) 2014 OSA 10 February 2014 | Vol. 22, No. 3 | DOI:10.1364/OE.22.003578 | OPTICS EXPRESS 3581
Here I FBG ( λ ) is the reflective spectrum of the LR-FBG. I Fresnel ( λ ) is determined by both the Fresnel reflection and the transmission losses along the microfiber, both of which are only slightly dependent on the incident wavelength. Thus I Fresnel ( λ ) can be approximately considered to be constant in the FBG reflective bandwidth. Maximum interference fringe contrast can be obtained when the intensity of the two beams are equal. OPD =2nl is the optical path difference, with n the effective refractive index and l the effective length between the two reflective beams. By measuring the strain-induced OPD changes or wavelength shift of the interference fringes, the external force changes can be determined. 3. Force sensing experiment and discussion In the force sensing experiment, the AFPI structure is vertically adhered to a glass substrate, as shown in Fig. 1(b). Both ends of the FBG are fixed to a glass substrate via UV glues in order to eliminate the force-induced Bragg wavelength shift. A mass with uniform density in the longitudinal direction is employed to calibrate the force response of the AFPI sensor. It is adhered to the 125μm-SMF section between the microfiber and the Fresnel surface. This will influence neither the transmission loss of the microfiber nor the Fresnel reflection of the fiber end. Each time the force is changed slightly by cutting a small section of the long-shaped mass and then the reduced weight is measured with an accuracy of 0.0001g by an analytical balance. When the waist diameter of the microfiber is 7.3μm, obtained by controlling the taper length to be 20mm, the interference spectrum of the AFPI sensor is shown in Fig. 4(a). The maximum fringe contrast is obtained at the Bragg wavelength according to the two-beam interference theory, as the reflection at the Bragg wavelength is approximately equal to that of the Fresnel reflection. The wavelength swept rate and the spectral resolution of the OSA are 50nm/s and 0.8pm, respectively. Each spectra is averaged 8 times in order to eliminate the random variations. The spectrum in one free spectral range (FSR) is enlarged and shown as a function of the external force changes in Fig. 4(b). A fringe contrast of higher than 7dB is easily obtained and can be further enhanced greatly if the intensities of two reflected beams are closer and higher spectral resolution of the OSA is set.
Fig. 4. (a) Interference spectra in the LR-FBG reflective bandwidth and (b) enlarged spectra in one free spectral range of the microfiber AFPI sensor as a function of the external force changes.
The wavelength shift as a function of the external force change is shown in Fig. 5(a) with solid circles for the AFPI sensor with a microfiber waist diameter of 7.3μm. The force sensing experiment is repeated three times and the force sensitivity is statistically determined to be 0.120 ± 0.004 pm/μN. The linearity is about 0.998 ± 0.001. The range of the force changes is controlled within several hundreds of micro-Newtons. There are many applications for the
#202791 - $15.00 USD Received 9 Dec 2013; revised 25 Jan 2014; accepted 29 Jan 2014; published 6 Feb 2014 (C) 2014 OSA 10 February 2014 | Vol. 22, No. 3 | DOI:10.1364/OE.22.003578 | OPTICS EXPRESS 3582
force sensor in this scale [24]. One of the typical applications is the stiffness calibration of microelectromechanical-system (MEMS) springs [25].
Fig. 5. Wavelength shift of the interference spectra as a function of the external force change. (a) Microfiber AFPI sensor with a waist diameter of 7.3μm (solid circles) and similar structure in 125μm-diameter SMF (void squares); (b) Microfiber AFPI sensors with different waist diameters.
Further we investigate the influence of microfiber waist diameter on the force sensitivity. In principal, the sensor measures the force by sensing the axial strain. The force induced strain depends strongly on the diameter of the fiber. Here, the waist diameter of the tapered fiber is controlled by the tapered length, i.e., the displacement of the motorized translation stage. The wavelength shift as a function of external force change is shown in Fig. 5(b) for microfiber AFPI sensors with waist diameters of 7.3μm, 6.7μm and 6.1μm, corresponding to taper lengths of 20mm, 30mm, and 40mm, respectively. The force sensitivities are determined to be 0.122 ± 0.003 pm/μN, 0.171 ± 0.002 pm/μN, and 0.221 ± 0.005 pm/μN, respectively. The linearities are all above 0.997. It is indicated by the experimental results that the force sensitivity can be enhanced with a smaller microfiber waist diameter. This kind of sensor will suffer from the temperature-force cross-sensitive problem and should be used in a constant-temperature environment for the precise force calibration. In our experiment the force responses of the sensor were tested in a constant-temperature environment. The good linearity shown in Fig. 5 indicates that the ambient temperature variation during the experiment can be neglected. Subsequent effort will be made to compensate the temperature effect on the force sensing. There have already been many methods proposed to reduce the cross sensitivity between force/strain and temperature, or to simultaneously measure dual parameters [26–31]. In our experiment we employed an AFPI structure based on a weak FBG and a cleaved fiber end. Its advantages can be revealed by comparing it with two symmetrical interferometric structures. One is based on two Fresnel reflections, known as extrinsic fiberoptic Fabry-Perot interferometer (EFPI) [32–34]. The other is FBG-pair-based fiber-optic Fabry-Perot interferometer (FBG-FPI) [35–37]. In the EFPI, two broadband reflective beam interfere and the fringes cover a wide range in the wavelength domain (> 100 nm). The wavelength division multiplexing (WDM) becomes invalid. Therefore, spatial frequency multiplexing (SFM) is often employed by using EFPIs with different cavity lengths [32]. However, the number of sensors multiplexed by the SFM method is limited. As the cavity length increases, the propagation loss of light in the cavity increases, making the fringe contrast degrade and indicating a maximum cavity length. For the fabrication of a fiber-optic Fabry-Perot interferometer based on FBG pairs, it is requested that the narrow reflective band, generally with a 3dB bandwidth of 0.2 - 0.3 nm, of the two FBGs should be exactly overlapped [35–37]. Further the Bragg wavelengths of the two FBGs should change synchronously during the sensing process. If either of two above demands is not met, the fringes will degrade or even disappear. #202791 - $15.00 USD Received 9 Dec 2013; revised 25 Jan 2014; accepted 29 Jan 2014; published 6 Feb 2014 (C) 2014 OSA 10 February 2014 | Vol. 22, No. 3 | DOI:10.1364/OE.22.003578 | OPTICS EXPRESS 3583
Therefore, the AFPI structure proposed in this paper has two advantages. Compared with the EFPI, the use of one FBG in the structure restricts the spectral range of each sensor. This makes WDM available and thus greatly enhance the multiplexing capacity of this kind of sensor. Compared with the FBG-FPI, the replacement of one FBG with a Fresnel reflection of the fiber end releases the strict requirement of the wavelength matching of the FBG pairs. In order to evaluate the sensitivity enhancement of the microfiber AFPI sensor, the force sensing experiment is repeated with a similar structure in 125μm-diameter SMF, also composed of a LR-FBG and the Fresnel reflection from a cleaved fiber end, except for the fiber tapering. The structure and the reflective spectrum are shown in Figs. 6(a) and 6(b), respectively. The interference fringes in the FBG reflection band and in one FSR are shown in Figs. 6(c) and 6(d), respectively. The wavelength shift as a function of the external force change is shown in Fig. 5(a) with void squares. It is clear that it is not sensitive to the external force changes.
Fig. 6. (a) AFPI structure in 125μm-diameter SMF and (b) its interference spectrum. Enlarged spectra (c) in the Bragg reflection band and (d) in one free spectral range as a function of external force changes.
4. Conclusion An optical microfiber asymmetrical Fabry-Perot interferometric force sensor has been fabricated by a low-reflectivity fiber Bragg grating and a Fresnel reflection of a cleaved fiber end. The force sensing characteristics have been explored in detail. Compared with similar structure in single mode fiber, the force sensitivity has been greatly enhanced. A force sensitivity of 0.221pm/μN has been obtained with a microfiber of 40mm length and 6.1μm waist diameter. It also has the advantages of high multiplexing capacity over conventional EFPI and no wavelength matching, like in the FBG-pair-based FPI, between the two reflective beams is required. Acknowledgments This work is supported by National Natural Science Foundation of China (61107073, 61107072 and 61290312), Fundamental Research Funds for the Central Universities (ZYGX2011J002), Research Fund for the Doctoral Program of Higher Education of China (20110185120020), Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT, IRT1218), and the 111 Project (B14039).
#202791 - $15.00 USD Received 9 Dec 2013; revised 25 Jan 2014; accepted 29 Jan 2014; published 6 Feb 2014 (C) 2014 OSA 10 February 2014 | Vol. 22, No. 3 | DOI:10.1364/OE.22.003578 | OPTICS EXPRESS 3584
