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POEM 2015 © OSA 2015
Compact temperature sensor by using a specialty fiber of Germania core and silica cladding Xinyong Dong*a, b, c, Jingyi Yangc, Yangzi Zhengc, Perry Ping Shumb, d, and Haibin Sua a
School of Materials Science and Engineering, Nanyang Technological University, Singapore b CINTRA, Research Techno Plaza, 50 Nanyang Drive, Singapore c Institute of Optoelectronic Technology, China Jiliang University, Hangzhou, China d School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore Author e-mail address:
[email protected]
Abstract: A miniature temperature sensor with sensitivity of 87.8 pm/ºC is demonstrated by using a Michelson interferometer formed with a 0.7-mm specialty optical fiber with Germania core and silica cladding spliced to a single mode fiber. OCIS codes: (060.2370) Fiber optics sensors; (060.0060) Fiber optics.
1.
Introduction
In past decades, optical fiber temperature sensors have been intensively studied due to their many intrinsic advantages such as electrically passive operation, long life-time and immunity to electromagnetic interference. Several configurations such as fiber Bragg gratings (FBGs) [1, 2], long period fiber gratings (LPFGs) [3] and fiber interferometers [4-8] have been employed. Among them, FBG-based temperature sensors are widely used in some industrial areas but the sensitivity is relatively low, ~10 pm/ºC [9]. LPFG-based temperature sensors have relatively high sensitivity but they are crossly sensitive to fiber bending and surrounding materials [3]. Optical fiber interferometers including Fabry-Perot (F-P) interferometers [6, 8, 10, 11], Mach-Zenhder interferometers (MZIs), Sagnac fiber loops and Michelson interferometers are good candidates for temperature sensors [5, 7, 12-13]. However, these sensors are all based on silica fibers with normal or micro-structured cross sections [12]. Their sensitivities are usually limited by the relatively low thermal-optical coefficient of silica material. Recently, an optical fiber Michelson interferometer-based temperature sensor was demonstrated with a 4-mm-long dispersion compensation fiber. The sensitivity is up to 68.6 pm/ºC and the maximum temperature is 600ºC. A Mach-Zenhder interferometer (MZI) based on a Germania-doped core optical fiber sandwiched between two single mode fibers was reported which demonstrated high temperature sensitivity of 98 pm/ºC [13]. However, the MZI configuration was operated in transmission mode that is inconvenient in practical applications and the relatively long sensing fiber is not good for point sensing. In this paper, a miniature all-fiber temperature sensor is proposed by using a Michelson interferometer formed by a section of specialty optical fiber with Germania core and silica cladding (Ge-fiber) fusion-spliced to a single mode fiber (SMF). Due to the relatively high differential refractive index compared with that of the conventional stepindex fibers, a reasonable free spectral range (FSR) of 15 nm is obtained with a very short length of 0.7 mm Ge-fiber. The achieved sensitivity is up to 87.8 pm/ºC in the measurement range from room temperature to 400ºC. 2. Sensor Fabrication and Principle A section of Ge-fiber with a cleaved end was properly spliced to a SMF by using a fusion splicer. The optical microscope image of the sensor head is shown in the inset of Fig. 1. The Ge-fiber has a core and cladding with diameters of 2.5 and 125 μm, respectively. The component ratio of GeO2 and SiO2 in the core is 98:2. The cladding is made by pure silica. The refractive indices of core and cladding are 1.5584 and 1.4440 respectively. So the large differential refractive index of Ge-fiber is 0.1144, which can contribute to a reasonable free spectral range (FSR) within a relatively short length. The length of the Ge-fiber configured with the sensor head is only about 0.7 mm. When light propagates from the SMF to the Ge-fiber, cladding mode will be excited due to the large mismatch of core diameter between the two kinds of fibers. Based on the analysis of beam propagation method (BPM), light intensity distribution from the SMF to the Ge-fiber is shown in Fig. 2. It indicates that 25% of injected light can be coupled into the core of Ge-fiber and the rest of the light are coupled into the cladding of the Ge-fiber. Note that the propagation loss for cladding modes is relatively high and some of the cladding modes will radiate during the propagation. When both core and cladding mode are reflected from the cleaved end and recoupled back into the core of the SMF, interference happens, leading to an output spectrum of interference pattern. The FSR depending on the optical path difference between the two modes can be described as [13]: (1) 2 FSR
2nd L
JW3A.50.pdf
Fig. 1 Experimental setup of Ge-fiber based sensor.
POEM 2015 © OSA 2015
Fig. 2 (a) Light intensity distribution from the SMF to the Gefiber. (b) The normalized intensity ratio from SMF to Ge-fiber.
where nd is the differential refractive index between the core mode and the cladding mode of Ge-fiber, L is the length of the Ge-fiber, and λ is the wavelength. When temperature changed, the dip wavelengths of reflection spectrum are shifting entirely due to different thermo-optic coefficients between the core and the cladding mode of Ge-fiber. The wavelength shift with temperature can be expressed as [13]: dnd dL (2)
T LdT nd dT
where dL/LdT is the thermal expansion ratio and dnd /nd dT is differential refractive index of the interference modes induced by temperature.
3. Experimental Results and Discussions The experimental setup for temperature measurement is shown in Fig. 1. Light from a broadband source (BBS) was launched into the proposed sensor through an optical circulator with wavelength range from 1545 to 1605 nm. The reflected light was guided into an optical spectrum analyzer (OSA, Yokogawa, AQ6370C) with the resolution of 0.02 nm for measurement. The temperature measurement was conducted in an oven with accuracy of 1ºC. The maximum testing temperature is up to 400ºC, limited by the oven capability. The sensor head, free from any bending and vibration, was fixed in a platform in the oven. When temperature was changed from room temperature to 400ºC, reflection pattern of sensor head shifted towards longer wavelength. Several of the measured spectra are shown in Fig. 3. Meanwhile, intensity of the reflection spectrum was increased gradually with temperature. It may be caused by the radial expansion of the Ge-fiber core, which increases the coupling ratio of the light to the fiber core and finally increases the power level of the output signal. Fig. 4 shows the wavelength shift against temperature for the four chosen fringe dips (A, B, C and D). The calculated temperature sensitivities are 77.2, 78.4, 86.8 and 87.8 pm/ºC for dip A to D, respectively. So longer the dip wavelength is, higher the temperature sensitivity is. The temperature sensitivity is dependent on the value of wavelength, which agrees well to the prediction by theoretical analysis in Eq. (2). The stability test was carried out in a custom-made chamber with temperature accuracy of 0.1ºC, which is much higher than the previously used oven. The chamber temperature was fixed at 166ºC in time duration of 100 minutes and the reflection spectrum was recoded in every 5 minutes. Then temperature was changed to 174ºC and the testing was repeated. The experimental results are shown in Fig. 5. The maximum fluctuation is only ±0.08 nm during the test, which corresponds to a maximum measurement error of ±1ºC, if we take the maximum sensitivity of 87.8 pm/ºC into account. The temperature measurement resolution of the proposed sensor is 0.25ºC, based on the wavelength resolution, 0.02 nm, of the OSA. Since the sensitivity is mainly determined by thermo-optic coefficients of the Ge-fiber material, length of Ge-fiber will have no impact on temperature sensitivity, which was verified by our experimental studies. However, length of the sensing fiber affects FSR of the interference pattern seriously and the latter affects the sensor performance significantly. For example, a too large FSR will lead to large wavelength reading error and a too small one will result in overlap between neighboring interference maxima or minima. For optical fiber sensors based on this particular Michelson interferometer configuration, normally long sensing fibers are required to reduce FSR to a reasonable value because differential refractive indices of the normally used sensing fibers are usually much lower than that of the Ge-fiber we used. Here we successfully reduced the sensing fiber to less than 1mm that will increase the measurement accuracy greatly in point sensing applications.
JW3A.50.pdf
POEM 2015 © OSA 2015
Fig. 3 Measured spectra of the Ge-fiber based temperature sensor under different temperatures.
Fig. 4 Wavelengths of Dip A-D against temperature.
Fig. 5 Stability testing results at various temperatures
4. Conclusion A miniature all-fiber temperature sensor has been experimentally demonstrated by using a Michelson interferometer formed by a 0.7-mm-long specialty optical fiber with Germania core and silica cladding fusion-spliced to a SMF. The achieved sensitivity is up to 87.8 pm/ºC in the measurement range from room temperature to 400ºC. It may have good potential applications in a wide range of temperature measurement, especially when point sensing is required. Acknowledgements This work was supported partially by Singapore A*STAR “Advanced Optics in Engineering” Program under Grant No. 1223600004, National Natural Science Foundation of China under Grant No. 61475147 and National Natural Science Foundation of Zhejiang Province, China under Grant No. Z13F050003. References [1] N. Hirayama and Y. Sano, "Fiber Bragg grating temperature sensor for practical use," ISA transactions 39, 169-173 (2000). [2] T. L. Lowder, K. H. Smith, B. L. Ipson, A. R. Hawkins, R. H. Selfridge, and S. M. Schultz, "High-temperature sensing using surface relief fiber Bragg gratings," Photonics Technology Letters, IEEE 17, 1926-1928 (2005). [3] V. Bhatia and A. M. Vengsarkar, "Optical fiber long-period grating sensors," Optics letters 21, 692-694 (1996). [4] C. Wu, H. Fu, K. K. Qureshi, B.-O. Guan, and H.-Y. Tam, "High-pressure and high-temperature characteristics of a Fabry–Perot interferometer based on photonic crystal fiber," Optics letters 36, 412-414 (2011). [5] T.-H. Xia, A. P. Zhang, B. Gu, and J.-J. Zhu, "Fiber-optic refractive-index sensors based on transmissive and reflective thin-core fiber modal interferometers," Optics Communications 283, 2136-2139 (2010). [6] Y. Zhang, L. Yuan, X. Lan, A. Kaur, J. Huang, and H. Xiao, "High-temperature fiber-optic Fabry–Perot interferometric pressure sensor fabricated by femtosecond laser," Optics letters 38, 4609-4612 (2013). [7] J.-J. Zhu, A. Zhang, T.-H. Xia, S. He, and W. Xue, "Fiber-optic high-temperature sensor based on thin-core fiber modal interferometer," Sensors Journal, IEEE 10, 1415-1418 (2010). [8] T. Zhu, T. Ke, Y. Rao, and K. S. Chiang, "Fabry–Perot optical fiber tip sensor for high temperature measurement," Optics Communications 283, 3683-3685 (2010). [9] J. Yang, X. Dong, Y. Zheng, K. Ni, J. Chan, and P. Shum, "Magnetic field sensing with reflectivity ratio measurement of fiber Bragg grating," (2014). [10] T. Chen, R. Chen, C. Jewart, B. Zhang, K. Cook, J. Canning, and K. P. Chen, "Regenerated gratings in air-hole microstructured fibers for high-temperature pressure sensing," Optics letters 36, 3542-3544 (2011). [11] H. Y. Choi, K. S. Park, S. J. Park, U.-C. Paek, B. H. Lee, and E. S. Choi, "Miniature fiber-optic high temperature sensor based on a hybrid structured Fabry–Perot interferometer," Optics letters 33, 2455-2457 (2008). [12] F. C. Favero, R. Spittel, F. Just, J. Kobelke, M. Rothhardt, and H. Bartelt, "A miniature temperature high germanium doped PCF interferometer sensor," Optics express 21, 30266-30274 (2013). [13] T. Huang, X. Shao, Z. Wu, Y. Sun, J. Zhang, H. Q. Lam, J. Hu, and P. P. Shum, "A sensitivity enhanced temperature sensor based on highly Germania-doped few-mode fiber," Optics Communications 324, 53-57 (2014).
