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OPTICS LETTERS / Vol. 37, No. 20 / October 15, 2012
Polymer optical fiber for large strain measurement based on multimode interference Jie Huang,1 Xinwei Lan,1 Hanzheng Wang,1 Lei Yuan,1,2 Tao Wei,3 Zhan Gao,1 and Hai Xiao1,* 1
2
Department of Electrical and Computer Engineering, Missouri University of Science and Technology, Rolla, Missouri 65409, USA Laser Micro/Nano Fabrication Laboratory, School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China 3
Department of Electrical, Computer and Biomedical Engineering, University of Rhode Island, Kingston, Rhode Island 02881, USA *Corresponding author:
[email protected] Received August 14, 2012; revised September 14, 2012; accepted September 14, 2012; posted September 14, 2012 (Doc. ID 174318); published October 11, 2012 This Letter reports a polymer optical fiber (POF) based large strain sensor based on the multimode interference (MMI) theory for the application of structural health monitoring. A section of POFs is sandwiched between two silica single mode fibers to construct a single-mode-multimode-single-mode structure that produces a MMI spectrum. The strain sensing mechanism of the device was investigated and experimentally verified. A large dynamic range of 2 × 104 με (2%) and a detection limit of 33 με have been demonstrated. © 2012 Optical Society of America OCIS codes: 130.5460, 060.2370.
Fiber-optic strain sensors have unique advantages, such as high signal-to-noise ratio, light weight, small size, and insensitivity to ambient electromagnetic fields [1]. Many fiber-optic strain sensors have been reported [2–4], and some of them are commercially available [2]. However, fused silica glass has been used in most reported strain sensors, which has a tensile strain limit of 4000 με (0.4%) [5,6]. As a result, optical fiber strain sensors have limited applications, especially in some highly loaded engineering structures, such as bridges, buildings, pipelines, dams, offshore platforms, etc. A fiber-optic strain sensor with large dynamic range is highly demanded. Recently, polymer optical fibers (POFs) as strainsensing substrates have attracted much attention. Aside from their great optical performance as optical fibers [7], their flexibility and deformability make it possible for them to sustain a large strain load. Similar fiber sensor structures have been successfully implemented using POFs. For example, Liu et al. implemented a Bragg grating structure in a single mode POF with a >28 dB signalto-noise ratio [8]. The Bragg wavelength shifted 52 nm in response to a 3.61% axial strain. Recently, a Bragg grating has also been successfully fabricated on a microstructured POF for large strain measurement [9] with high resolution. In addition, the dynamic characterization of the POF has also been investigated [10]. It was found that the POF based strain sensor has a limited frequency response due to its viscoelastic property. Although great efforts have been made to improve the performance of POF, the commercially available single mode POFs currently are still expensive. Microstructured POFs are even more difficult to fabricate. As such, it is desired to use regular multimode POFs for sensor construction. Nobuo reported a multimode POF based strain sensor by measuring the optical transmission loss [11]. Rawal et al. calibrated the optical time domain reflectometry response of standard multimode POFs and performed a detailed analysis to understand the optical loss mechanism [12]. More recently, a time-of-flight measurement was conducted in order to monitor the strain in an aircraft using multimode POF [13]. So far, most of the existing strain sensors based on multimode POFs use time-domain signal analysis by 0146-9592/12/204308-03$15.00/0
measuring the transmission loss. In general, optical loss based measurement is difficult to achieve with high accuracy. This Letter discusses a new type of multimode POF strain sensor that uses frequency-domain signal interrogation to obtain high resolution. The multimode POF was sandwiched between the two silica-based single mode fibers (SMFs) to form a single-mode-multimodesingle-mode (SMS) structure. The SMS structure has been successfully applied in sensing application based on the multimode interference (MMI) theory [14–18]. The MMI theory can also be understood by the phenomenon of self-imaging [12]. Through self-imaging, an input single mode field profile is reproduced at periodic intervals along the multimode fiber (MMF) as a result of constructive interference. A SMS structure can thus generate interference maxima or minima in its transmission spectrum. The transmission spectrum is dependent on the length and effective refractive index (RI) of the MMF. An axial tensile strain applied to the MMF would produce a shift in the transmission spectrum of the SMS structure as a result of the photoelastic effect. The SMS structure provides a high resolution while the multimode POF offers a large dynamic range. The combination of these two might provide a solution for some challenging issues faced in many applications. The schematic diagram of the experimental setup is shown in Fig. 1. A broadband light source (BBS, Agilent
Fig. 1. (Color online) Schematic diagram of the experimental setup for measuring large strain of POF-based SMS fiber structure. © 2012 Optical Society of America
October 15, 2012 / Vol. 37, No. 20 / OPTICS LETTERS
83437A) was fed into a section of silica based SMF. A section of multimode POF (Paradigm Optics, MMPOF250) made of polymethyl methacrylate (PMMA, RI ∼1.481 @ 1550 nm) was spliced to the silica based SMFs on both ends to form a SMS structure. An optical spectrum analyzer (OSA) (Yokogawa, AQ 6319) with a resolution of 10 pm was used to monitor the transmission signal. The POF has a core of 240 μm and a cladding of 10 μm in diameter. A section of POF without buffer layer with a length of 160 mm was vertically cleaved on both ends by a commercial preheated blade. A piece of silica SMF with the end face cleaved was spatially aligned to one end face of the POF with a separation of several micrometers using a motorized 3D translation stage. A drop of optical adhesive [Norland optical adhesive (NOA) 85, RI ∼1.46 @ 1550 nm] was used to fill the gap to reduce the transmission loss. By exposing the optical adhesive to ultraviolet light for 30 s, the NOA was solidified and the SMF was attached to the POF. The same method was used to attach the other end face of the POF to a second SMF to complete the SMS structure. Figure 2 shows the transmission spectrum of the SMS structure. Multiple peaks were observed within the spectrum range. The strongest fringe had a visibility over 10 dB. In a typical SMS structure, multiple modes were excited in the MMF and participated in the interference. As a result, the interference fringes were distorted. Although any of these peaks (or valleys) can be used for sensing, the least distorted fringe is typically chosen to minimize the measurement error. The relatively high transmission loss is due to the asymmetric splicing, imperfect end faces of the POF and optical absorption of POF in the infrared region. It is expected that the loss can be further reduced by operating the sensor in the visible or near infrared range where the optical absorption is small. The applied strain in the POF will induce a change in the length (ΔL), core radius (Δa), and RI of the core (Δnco ), which can be expressed as [15], ΔL Ls ε; Δnco −
When an axial force is applied to the POF of the SMS structure, a wavelength shift will be introduced because of the changes in fiber dimension and RI. The strain induced wavelength shift ΔλS can be expressed as follows (based on Eq. 1): Ls Δnco Δa ΔL 2 Lt nco a Ls Ls λ − P eff 2v 1ελ; Lt
ΔλS −
(1)
where Ls is the sensing length of the POF, ε is the applied strain, a is the core radius, v is the Poisson ratio of the POF, nco is the RI of the core, p11 and p12 are the Pockel’s coefficients of the stress-optic tensor, and P eff is the effective strain-optic coefficient.
(2)
where λ is the interrogated wavelength and Lt is the total length of POF. The typical values of P eff and v for PMMA based POF are 0.099 (0.0009) and 0.35–0.45, respectively. The negative sign indicates that the axial strain to the POF will cause a blueshift in the transmission spectrum. A quick calculation based on Eq. 2 revealed that the anticipated slope of the strain response at the wavelength of 1570 nm is −2.82 pm=με (assume Ls =Lt 1, v 0.35). The strain-temperature crosstalk is always a concern for strain sensors. The wavelength shift ΔλT caused by temperature variations is dn ΔT; ΔλT λ αn dT
(3)
where α (∼3.3 × 10−5 =°C for PMMA) is the coefficient of thermo expansion, dn=dT (∼1 × 10−5 =°C for PMMA) is thermo-optic coefficient of the POF, n is the RI of the core in the POF, and ΔT is the change in temperature. An increase in temperature will cause a redshift in the transmission spectrum, and the slope is determined by the characteristics of the POF. Assuming ΔλS ΔλT , the temperature-strain crosstalk of the sensor is given by dn × 106 Lt αn dT με : − Ls P eff 2v 1 ΔT
Δa −vaε;
n3co p − vp11 p12 ε −nco P eff ε; 2 12
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(4)
Using typical parameters of PMMA POF, the crosstalk με=ΔT is calculated to be about 33 με=°C (assume Lt =Ls 1). As a result, the temperature-strain crosstalk is an issue for the POF sensor and it may introduce a measurement error in a temperature-uncontrolled environment. -24
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Fig. 3. (Color online) Shift in transmission spectra as the axial strain increased.
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Fig. 4. (Color online) Wavelength shift as a function of axial strain for POF-based SMS structure.
As shown in Fig. 1, the POF was tightly attached to two translation stages at two points using all-purpose glue. The length between two attaching points, considered as the sensing length, was precisely controlled to 100 mm. The spectra of the SMS structure were recorded as the distance between the two points was increased step by step. Figure 3 shows the transmission spectral shift as the tensile strain increased at a step of 10 μm (corresponding to 100 με for the total length of 100 mm). The black line represents the original spectrum. The increasing strain did not incur any significant loss in the transmission spectra. The increasing axial strain induced a blueshift as predicted by Eq. 2. By applying the fourth order polynomial curve-fitting and monitoring the first notch of the transmission spectra, the wavelength shift was plotted as a function of the axial strain. Figure 4 shows the wavelength shift in the total applied strain of 2 × 104 με (2% without break) at an increasing step of 1000 με. The inset in Fig. 4 represents the strain-wavelength response with an increasing step of 100 με. The strain-wavelength response was quite linear, indicating that it can be used as a strain sensor after proper calibration. The total shift in interference spectra was ∼35 nm in response to the 2% strain. The slope of the strain-wavelength was −1.72 pm=με interrogated at the wavelength of 1570 nm, which agreed with the theoretic predicting of −1.76 pm=με (Ls =Lt 1=1.6), according to Eq. 2. The sensitivity of the SMS sensor was similar to that of the POF-based Bragg gratings [8,9]. Figure 5 plots the repeatability test of the axial strain under the elastic deformation limit of the POF. An axial strain of 500 με was applied to and removed from the sensor at every minute at room temperature. The spectra were recorded at every 6 s. Figure 5 shows the temporal response of the sensor for the duration of 522 s. The fluctuation of the spectra was around 18 pm, corresponding to 10 με, indicating a good repeatability of the sensor for axial strain sensing. The experiment was performed in a relative short period of time in which the temperature fluctuation of the environment was small. Considering the large temperature-strain crosstalk (33 με=°C) as predicted in Eq. 4, the strain sensor needs to be temperature compensated before it can be used in practical applications. To summarize, a multimode POF based sensor was reported for large strain measurement. A section of
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Fig. 5. Repeatability test of axial strain under the range of elastic deformation of POF-based SMS structure.
multimode POF was sandwiched between two silica SMFs. The resulting SMS structure generated interference patterns governed by the self-imaging and MMI theory. The sensing mechanism was investigated and experimentally validated. By tracking the spectral shift, a linear strain-wavelength response was obtained with a strain sensitivity of −1.72 pm=με in a large dynamic range of 2 × 104 με. The temperature-strain crosstalk is about 33 με=°C. With good linearity, decent sensitivity and a large dynamic range, the POF based SMS sensor may find many applications in structural health monitoring after proper calibration and temperature compensation. This work is supported by National Science Foundation under the contract CMMI-1200787. Reference 1. J. M. López-Higuera, Handbook of Optical Fibre Sensing Technology (Wiley, 2002). 2. A. D. Kersey, T. A. Berkoff, and W. W. Morey, Opt. Lett. 18, 1370 (1993). 3. K. A. Murphy, M. F. Gunther, A. M. Vengsarkar, and R. O. Claus, Opt. Lett. 16, 273 (1991). 4. Y.-P. Wang, L. Xiao, D. N. Wang, and W. Jin, Opt. Lett. 31, 3414 (2006). 5. V. Bhatia and A. M. Vengsarkar, Opt. Lett. 21, 692 (1996). 6. Y.-J. Rao, Y.-P. Wang, Z.-L. Ran, and T. Zhu, J. Lightwave Technol. 21, 1320 (2003). 7. P. Kara, Smart Mat. Struct. 20, 013002 (2011). 8. H. Y. Liu, G. D. Peng, and P. L. Chu, Photon. Technol. Lett. 14, 935 (2002). 9. A. Stefani, Y. Wu, C. Markos, and O. Bang, IEEE Photon. Technol. Lett. 23, 660 (2011). 10. A. Stefani, S. Andresen, W. Yuan, and O. Bang, IEEE Sens. J. 12, 3047 (2012). 11. T. Nobuo, Int. J. Fatigue 24, 281 (2002). 12. H. Irwan Rawal, N. Kentaro, and U. Sadayuki, Meas. Sci. Technol. 15, 1553 (2004). 13. G. Durana, M. Kirchhof, M. Luber, I. S. de Ocariz, H. Poisel, J. Zubia, and C. Vazquez, IEEE Sens. J. 9, 1219 (2009). 14. Q. Wang, G. Farrell, and W. Yan, J. Lightwave Technol. 26, 512 (2008). 15. Q. Wu, Y. Semenova, P. Wang, and G. Farrell, Opt. Express 19, 7937 (2011). 16. A. Mehta, W. Mohammed, and E. G. Johnson, IEEE Photon. Technol. Lett. 15, 1129 (2003). 17. E. Li, Opt. Lett. 32, 2064 (2007). 18. X. Lan, J. Huang, Q. Han, T. Wei, Z. Gao, H. Jiang, J. Dong, and H. Xiao, Opt. Lett. 37, 1998 (2012).
