Temperature-Independent and Strain-Independent Twist Sensor ...

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 26, NO. 15, AUGUST 1, 2014

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Temperature-Independent and Strain-Independent Twist Sensor Based on Structured PM-CFBG Bin Yin, Haisu Li, Suchun Feng, Yunlong Bai, Zhibo Liu, Wanjing Peng, Shuo Liu, and Shuisheng Jian

Abstract— In this letter, we present a structured polarizationmaintaining chirped fiber Bragg grating (PM-CFBG) for temperature-independent and strain-independent twist measurement. The structured PM-CFBG, which has two transmission peaks, is created by tapering directly on the PM-CFBG. The grating transmissivity of structured PM-CFBG changes linearly with twist while it is insensitive to strain and temperature. The wavelength interval changes proportionally to the temperature, but remains the same as the strain increases. This novel sensor can be used to measure temperature, strain, and twist simultaneously.

Fig. 1. (a) Experimental setup of the sensing system based on structured PM-CFBG. (b) Experimental setup of twist sensor.

Index Terms— Chirped fiber grating, fiber sensor, twist.

I. I NTRODUCTION

M

ONITORING in situ of physical, chemical, engineered and biological parameters is of great importance for process control in manufacturing industries, railway, protection of ecosystems, and so on. Temperature, strain, and twist are the most important parameters in these applications, especially in chemical or manufacturing industries for quality control and in railway and pipeline for monitoring signal. In recent years, fiber-optic sensors have received significant attention for their unique advantages such as immunity to electromagnetic interference, compact size, potential low cost, and the possibility of distributed measurement over a long distance [1]. The previous sensors for measuring the temperature, strain, or twist use a fiber loop mirror based on suspended twin-core fiber [2], Sagnac interferometer with polarization maintaining side-hole fiber [3], high-birefringent Sagnac loop interferometer [4], Polymer micro-fiber Bragg grating [5], phase-shifted chirped grating [6], a longperiod grating combined with a high-birefringence fiber loop mirror [7], fiber Bragg grating (FBG) structures with fused tapers [8], and so on. X. Chen et al. propose 81° tilted Bragg grating to measure twist parameter, but special Bragg Manuscript received April 20, 2014; revised May 12, 2014; accepted June 2, 2014. Date of publication June 5, 2014; date of current version July 15, 2014. This work was supported in part by the Major State Basic Research Development Program of China under Grant 2010CB328206, in part by the National Natural Science Foundation of China under Grant 61107094, Grant 61178008, and Grant 61275092, and in part by the Fundamental Research Funds for the Central Universities under Grant 2013JBM017, China. The authors are with the Key Laboratory of All Optical Network and Advanced Telecommunication Network of EMC, Institute of Lightwave Technology, Beijing Jiaotong University, Beijing 100044, China (e-mail: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2014.2329315

grating is difficult to fabricate [9]. Z. Peng et al. report Photonic-Crystal-Fiber-Based Sagnac Interferometer to realize temperature-insensitive twist sensor, but the fiber is expensive and the structure is complex [10]. Twist sensor based FBG can be made very simple, sensitive to the twist direction and furthermore grating transmissivity is insusceptible to temperature and strain. Y.P. Wang et al. use the Hi-Bi FBG to measure twist parameter [11]. We utilize the structured polarizationmaintaining chirped fiber Bragg grating (PM-CFBG) to realize to measure the temperature, strain and twist simultaneously. Andrea Cusano et al. produce a tapered region in a CFBG by the arc discharge technique [12]. This structured CFBG can be used to be as a sensitive sensing device. It can be applied to the optical fiber sensor, optical fiber laser, optical fiber filter, and so on. In this letter, we report a structured PM-CFBG for temperature-independent and strain-independent twist measurement. The structured PM-CFBG sensor can be made very small, and it should be particularly suitable for smart structure applications. To our best knowledge, it is the first time the structured CFBG inscribed on the PM fiber (structured PMCFBG) is applied for temperature-independent and strainindependent twist measurement. II. E XPERIMENTAL T HEORY AND S ETUP The schematic configuration of temperature-independent and strain-independent twist sensor is shown in Fig. 1 (a). Amplified spontaneous emission (ASE) generated by erbiumdoped optical fiber amplifier (EDFA) becomes linearly polarized light after a polarizer. Polarization controller (PC) modulates the polarization state of the incident light before structured PM-CFBG.A structured PM-CFBG fabricated by the arc discharge technique is used to measure the temperature, strain, and twist change. We used two horizontally separated translation stages to axially strain the structured PM-CFBG while using a temperature chamber placed between the stages

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Fig. 3. (a) The tapered region of structured PM-CFBG under the microscope. (b) The experimental transmission spectrum of structured PM-CFBG filter without tapering (black line) or with tapering (red line).

Fig. 2. (a) Region tapering setup of the PM-CFBG; and (b) configuration of the structured PM-CFBG. (c) The typical simulation transmission spectra of the structured PM-CFBG filter without tapering (black line) or with tapering (red line).

to heat the grating independently. Fig. 1(b) shows the experimental setup of the twist measurement. One end of the fiber is fixed by a clamp, and the other end is fixed on a fiber rotator with an engraved dial. The twisted length is about 16 cm. To reduce the influence of any bending effect, the pre-strain is applied to the fiber to keep it straight. The configuration of the structured PM-CFBG is showed in Fig. 2(b). We create a tapered region at the middle of a PMCFBG directly by the arc discharge technique with a fiber fusion splicer, as shown in Fig. 2(a). The transmission spectrum of the structured PM-CFBG has two transmission peaks which are corresponding to X and Y polarization transmission peaks generated by tapering on PM-CFBG. The typical simulation transmission spectra of the structured PM-CFBG filter without tapering (black line) or with tapering (red line) are shown in Fig. 2(c). Because the grating of short tapered region is removed by the arc discharge technique, the tapered region approximates infinitesimal. Thus the structured PM-CFBG can be explained on the basis of phase shift theory [13]–[15]. The tapered region characteristics such as length, waist diameter and surrounding refractive index (SRI) influence the phase delay, when the optical signal go through the tapered region. The phase delay can be defined as 4·π · n e f f (S R I, Dt h ) · L t h (1) = λ B (z t h ) where λ B is the optical wavelength corresponding to the tapered region position z t h · n e f f expresses the effective refractive index variation caused by tapering, which depends on the SRI and the waist diameter Dt h .L t h is the length of the tapered region [16].

From Ref. 17, we know that the difference between the peak wavelengths of the structured PM-CFBG decreases with rising temperature, while other forces applied to the sensor are unchanged. Meanwhile, the difference between the strain sensitivities of the two peak wavelengths is less than 1 × 10−6 nm/με, which is far less than temperature coefficient. So the difference between the two peak wavelengths of the structured PM-CFBG hardly changes with the axial strain. Our experimental results are in good agreement with the above analysis. This sensor system can also allow for simple and straight forward twist measurement (as shown in Fig. 1(b)). The polarizer and PC provide a linearly polarized light at the input of the PM fiber. Polarization-dependent loss (PDL) is defined as the maximum change in the transmitted power when the input state of polarization is varied over all polarization states. In the case of the structured PM-CFBG, the final expression of PDL for transmission is [18]: (2) P DL = 10 log10 (TX /TY ) where TX and TY express the transmitted power of X and Y polarization modes corresponding to the slow and fast axis, respectively. The PDL parameter acts as a birefringence amplifier and thus a weak twist applied to the structured PM-CFBG could produce significant PDL responses. The relationship between twist and the structured PM-CFBGs’ PDL can be analyzed and numerically simulated following coupled mode theory. Based on the analysis, because the strain and temperature have the same effects on both polarization modes, which means there will be no birefringence effects, only the resonance wavelength changes could be observed rather than the PDL parameter. Therefore, the twist sensor shows strain and temperature independent characteristics [11]. The structured PM-CFBG is fabricated as follows. A PMCFBG with a length of 14mm was written in a hydrogen loaded PM fiber through using 248nm KrF excimer laser. After the PM-CFBG was fabricated, A tapered region

YIN et al.: TEMPERATURE-INDEPENDENT AND STRAIN- INDEPENDENT TWIST SENSOR

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Fig. 4. (a) Measured transmission spectra of the sensor at different strain without additional applied twist at fixed temperature 20°C; and (b), (c) relationship between peak wavelength (L 1 , L 2 ), peak wavelength interval (L 2 − L 1 ) and strain.

Fig. 5. (a) Measured transmission spectra of the sensor at different temperature without additional applied strain and twist; and (b), (c) relationship between peak wavelength (L 1 , L 2 ), peak wavelength interval (L 2 − L 1 ) and temperature.

(approximately 0.265 mm long, waist diameter 115 ± 2μm) at the middle of the PM-CFBG was created by a fusing-andpulling treatment with a fiber fusion splicer (FSU975). The tapered region of structured PM-CFBG under the microscopes is shown in Fig. 3(a), and the measured transmission spectrum of the experimental structured PM-CFBG using an optical spectrum analyzer (OSA) (Ando AQ6317, 0.01nm resolution) was shown in Fig. 3(b).

coefficient of the two peak wavelengths is 9.7×10−4nm/με by calculating experimental data and using the linear regression. While temperature (0 ∼ 50°C) changes without additional applied strain and twist, the transmission spectra of the sensor are shown in Fig. 5(a). We find that peak wavelength shifts (L 1 , L 2 ) and peak wavelength interval (L 2 − L 1 ) change, as shown in Fig.5 (b) and (c), but the grating transmissivity of the structured PM-CFBG don’t change. The temperature coefficients of the two peak wavelengths (L 1 , L 2 ) are calculated to be 8.66 × 10−3nm/°C and 8.23 × 10−3nm/°C. At the same time, the temperature coefficient of peak wavelength interval (L 2 − L 1 ) is 4.4 × 10−4 nm/°C. When twisted angle in the unit length of the structured PMCFBG (−6.25°/cm ∼ 6.25°/cm) changes without additional applied strain at fixed temperature 20°C, the transmission spectra of the sensor are shown in Fig. 6(a). We find that only the grating transmissivity of the structured PM-CFBG and PDL of two polarization states (P) change, as shown in Fig. 6(b) and (c), but peak wavelength shifts (L 1 , L 2 ) and peak wavelength interval (L 2 − L 1 ) are almost the same. Temperature and strain variations can be negligible for change

III. E XPERIMENTAL R ESULT We make the transmission spectrum of the structured PM-CFBG have two polarization states through adjusting PC. The responses of the sensor to temperature, strain, and twist are measured separately. While the temperature is fixed at 20°C, we change axial strain (0 ∼ 1200με) on the sensor with adjustable strain equipment, its transmission spectra at different strain are shown in Fig. 4(a) without additional applied twist. Wavelength shifts (L 1 , L 2 ) show linear relationship with strain, but the peak wavelength interval (L 2 − L 1 ) is unchanged, as shown in Fig. 4(b) and (c). The strain

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can be used to measure temperature, strain and twist simultaneously. R EFERENCES

Fig. 6. (a) Measured transmission spectra of the sensor at different twisted angle in unit length without additional applied strain at fixed temperature 20°C; and (b), (c) relationship between the grating transmissivity of the PMCFBG (X-polar or Y-polar), PDL of two polarization states (P) and twist.

of the measured angle based on the results shown in Ref. 11 and this experiment. The twist sensitivity of the structured PM-CFBG is calculated to be 0.256d B (°/cm). IV. C ONCLUSION In conclusion, we have reported a structured PM-CFBG for temperature-independent and strain-independent twist measurement. The tapered region is created by tapering directly on the PM-CFBG. The tapered region approximates 0.265 mm long at the middle of the PM-CFBG and its waist diameter is 115 ± 2μm. The twist change is measured independently by the grating transmissivity of structured PM-CFBG. The wavelength interval changes proportionally to the temperature, but remains the same as the strain increases. This novel sensor

[1] P. Lu, L. Men, K. Sooley, and Q. Chen, “Tapered fiber Mach–Zehnder interferometer for simultaneous measurement of refractive index and temperature,” Appl. Phys. Lett., vol. 94, no. 13, pp. 131110-1–131110-3, 2009. [2] O. Frazão, R. M. Silva, J. Kobelke, and K. Schuster, “Temperatureand strain-independent torsion sensor using a fiber loop mirror based on suspended twin-core fiber,” Opt. Lett., vol. 35, no. 16, pp. 2777–2779, 2010. [3] O. Frazão et al., “Simultaneous measurement of multiparameters using a Sagnac interferometer with polarization maintaining side-hole fiber,” Appl. Opt., vol. 47, no. 27, pp. 4841–4848, 2008. [4] R. M. Silva, M. S. Ferreira, and O. Frazão, “Temperature independent torsion sensor using a high-birefringent Sagnac loop interferometer,” Opt. Commun., vol. 285, no. 6, pp. 1167–1170, 2012. [5] G. Rajan, M. Y. Mohd Noor, N. H. Lovell, E. Ambikaizrajah, G. Farrell, and G.-D. Peng, “Polymer micro-fiber Bragg grating,” Opt. Lett., vol. 38, no. 17, pp. 3359–3362, 2013. [6] S. E. Lima et al., “Fiber laser sensor based on a phase-shifted chirped grating for acoustic sensing of partial discharges,” Photon. Sensors, vol. 3, no. 1, pp. 44–51, 2013. [7] O. Frazao, L. M. Marques, S. Santos, J. M. Baptista, and J. L. Santos, “Simultaneous measurement for strain and temperature based on a long-period grating combined with a high-birefringence fiber loop mirror,” IEEE Photon. Technol. Lett., vol. 18, no. 22, pp. 2407–2409, Nov. 15, 2006. [8] S. F. O. Silva, L. A. Ferreira, F. M. Araújo, J. L. Santos, and O. Frazão, “Fiber Bragg grating structures with fused tapers,” Fiber Integr. Opt., vol. 30, no. 1, pp. 9–28, 2011. [9] X. Chen, K. Zhou, L. Zhang, and I. Bennion, “In-fiber twist sensor based on a fiber Bragg grating with 81 tilted structure,” IEEE Photon. Technol. Lett., vol. 18, no. 24, pp. 2596–2598, Dec. 15, 2006. [10] Z. Peng et al., “A temperature-insensitive twist sensor by using lowbirefringence photonic-crystal-fiber-based Sagnac interferometer,” IEEE Photon. Technol. Lett., vol. 23, no. 13, pp. 920–922, Jun. 1, 2011. [11] Y. P. Wang, X. Q. Huang, and M. Wang, “Temperature- and strainindependent torsion sensor utilising polarisation-dependent loss of Hi-Bi FBGs,” Electron. Lett., vol. 49, no. 13, pp. 840–841, Jun. 2013. [12] A. Cusano, A. Iadicicco, D. Paladino, S. Campopiano, and A. Cutolo, “Photonic band-gap engineering in UV fiber gratings by the arc discharge technique,” Opt. Exp., vol. 16, no. 20, pp. 15332–15342, 2008. [13] A. Cusano, A. Iadicicco, D. Paladino, S. Campopiano, A. Cutolo, and M. Giordano, “Micro-structured fiber Bragg gratings. Part I: Spectral characteristics,” Opt. Fiber Technol., vol. 13, no. 4, pp. 281–290, 2007. [14] T. Erdogan, “Fiber grating spectra,” J. Lightw. Technol., vol. 15, no. 8, pp. 1277–1294, Aug. 1997. [15] J. D. Love, W. M. Henry, W. J. Stewart, R. J. Black, S. Lacroix, and F. Gonthier, “Tapered single-mode fibres and devices. I. Adiabaticity criteria, IEE Proc. J. Optoelectron., vol. 138, no. 5, pp. 343–354, Oct. 1991. [16] M. Pisco, A. Iadicicco, S. Campopiano, A. Cutolo, and A. Cusano, “Structured chirped fiber Bragg gratings,” J. Lightw. Technol., vol. 26, no. 12, pp. 1613–1625, Jan. 15, 2008. [17] C. Guanghui, L. Liying, J. Hongzhi, Y. Jimin, X. Lei, and W. Wencheng, “Simultaneous strain and temperature measurements with fiber Bragg grating written in novel Hi-Bi optical fiber,” IEEE Photon. Technol. Lett., vol. 16, no. 1, pp. 221–223, Jan. 2004. [18] Y. Wang, M. Wang, and X. Huang, “Spectral characterization of polarization dependent loss of locally pressed fiber Bragg grating,” Opt. Exp., vol. 19, no. 25, pp. 25535–25544, 2011.