Assembly-Free-Based Fiber-Optic Micro-Michelson ... - IEEE Xplore

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 28, NO. 6, MARCH 15, 2016

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Assembly-Free-Based Fiber-Optic Micro-Michelson Interferometer for High Temperature Sensing Jinde Yin, Tiegen Liu, Junfeng Jiang, Kun Liu, Shuang Wang, Shengliang Zou, and Fan Wu

Abstract— We demonstrated a novel ultracompact fiber-optic micro-Michelson interferometer (MMI) for high temperature sensing. In the MMI structure, a 45° angled reflector is polished at the tip of a single mode fiber using a fiber-lensing machine. This reflector divides the fiber core into two parts and splits the inputting light into two beams producing Michelson interference fringe. The experimental results indicate that the proposed sensor has average temperature sensitivity of 13.32 pm/°C as the temperature increases from 19 °C to 950 °C. With the all-glass assembly-free-based structure, the sensor has a capability of surviving in the high temperature harsh environment. Index Terms— Fiber optics sensor, microstructure fabrication, interferometry, high temperature.

I. I NTRODUCTION INIATURE in-line fiber-optics interferometric sensors have attracted much interest for high temperature sensing applications owing to their advantages of compact size, excellent measurement performance and survivability in harsh environment [1]–[3]. The typical structures of those sensors are realized by creating micro-interferometers using optical fibers, such as Fabry-Perot, Mach-Zehnder, and Michelson interferometers [4]–[6]. The optical path difference (OPD) of the interferometers is determined by the geometric length and refractive index of interferential optical path, and naturally modulated by temperature through thermal expansion and thermo-optic effect of optical fiber, thus achieving temperature sensing. In recent years, two main structures of interferometric optical fiber sensors categorized into assembly-based and assembly-free-based have been demonstrated for high

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Manuscript received July 5, 2015; revised October 29, 2015; accepted November 19, 2015. Date of publication November 24, 2015; date of current version February 18, 2016. This work was supported in part by the Shenzhen Science and Technology Research Project under Grant JCYJ20120831153904083, in part by the Tianjin Natural Science Foundation under Grant 13JCYBJC16200 and Grant 13JCYBJC16100, in part by the National Instrumentation Program of China under Grant 2013YQ030915, in part by the Science and Technology Key Project through the Chinese Ministry of Education under Grant 313038, in part by the National Basic Research Program of China under Grant 2010CB327802, and in part by the National Natural Science Foundation of China under Grant 61227011, Grant 61378043, 61475114, Grant 11004150, and Grant 61108070. (Corresponding authors: Junfeng Jiang and Kun Liu.) The authors are with the Key Laboratory of Optoelectronics Information Technology, College of Precision Instrument and Optoelectronics Engineering, Tianjin University, Tianjin 300072, China (e-mail: yinjinde@ tju.edu.cn; [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; 13502195560@ 163.com). 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.2015.2503276

temperature sensing. For the assembly-based sensors, multiple components are usually assembled together to form micro-interferometer by using the methods of laser thermal fusion [7] or electric arc fusion splicer [8]. Various configurations of such sensors have been fabricated by splicing a short section of hollow-core fiber (HCF) [9] or photonic crystal fiber (PCF) [10] between two solid-core fibers, or directly splicing a short section of HCF [11], PCF [12], [13], single mode fiber (SMF) [14] to the end of a transmission SMF. However, the assembly-based methods complicate the sensor structure and involve critical difficulty of accurately controlling the OPDs. The mismatch of coefficient of thermal expansion (CTE) between multiple parts deteriorates the thermal stability of the sensors. Moreover, the use of high-cost special fibers (such as HCF and PCF) increases the cost of the sensors. Meanwhile, assemblyfree-based sensors are realized by directly fabricating micro-interferometer on an optical fiber without assembling pieces together, thus achieving more compact structure and higher robustness than the assembly-based ones. Nowadays, femtosecond laser micromachining technology has provided a great opportunity for fabricating assembly-free-based sensors, such as directly cutting a groove on SMF forming Fabry-Perot interferometer [15], removing half of the fiber core and cladding [16] or drilling two microholes on the fiber core [17] forming Mach-Zehnder interferometer. However, the mechanical properties of above structures have been significantly weakened due to directly sculpture on the middle of SMF. As a result, the cross-sensitivities to bending and strain have become a critical issue. Therefore, probe sensors with low cross-sensitivity are proposed and fabricated by drilling a through-hole across the fiber core near the end of SMF forming Fabry-Perot interferometer [18], or cutting a step structure into the fiber core at the tip of SMF composing Michelson interferometer [19]. However, the inwall of laser-sculpture is usually rough, which reduces the optical properties of the interferometer. Moreover, the fabrication of all above-mentioned assembly-free-based sensors requires very expensive high power femtosecond laser and high-precision translation stage, which particularly increases the cost of the sensors. In this letter, we proposed a novel ultra-compact fiber-optic micro-Michelson interferometer (MMI) for high temperature sensing application by directly polishing the end face of a common SMF. The proposed MMI has an assembly-free-based structure with excellent robustness and is fabricated by using a fiber-lensing machine rather than expensive femtosecond

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Fig. 1.


Schematic diagram of the proposed fiber-optic MMI.

Fig. 2. (a) Fabricate a 45° angle at the end face of SMF. (b) Polish a 0° angled step at the top of the angled SMF to form MMI structure.

laser. Smooth reflector can be obtained during polishing process, which is helpful for keeping well optical properties of the sensor. The OPD of the MMI is absolutely determined by the radius and refractive index of fiber cladding, which offers convenience for accurate OPDs controlling, thus the performance is predictable. Experimental results indicate that the sensor has average temperature sensitivity of 13.32pm/°C under the temperature range of 19°C to 950°C. II. S ENSOR D ESIGN AND FABRICATION The proposed fiber-optic MMI structure is shown in Fig. 1. The sensor is made by polishing a 45° angled reflector at the tip of SMF. The key concept here is to steer the 45° angled reflector dividing the fiber core into two parts, thus forming two mirrors at the end of fiber core, respectively labeled as mirrors 1 and 2 (M1 and M2). Therefore, the inputting light propagating inside the fiber core splits into two beams. Beam 1 is reflected at M1 and served as the one optical path of MMI. Beam 2 is firstly reflected at M2 based on total internal reflection principle, then reflected back at the fiber sidewall (M3) and coupled into the fiber core again at M2, which served as the other optical path of MMI. The two reflections are denoted as I1 and I2 , respectively, and superimpose in the fiber core to generate an interference fringe. The OPD between the two interfering beams is twice the product of the radius L and refractive index n of the fiber cladding. To demonstrate the concept, the MMI structure was fabricated at the tip of a SMF with core/cladding diameter of 8.3/125μm using a fiber-lensing machine. The sensor fabrication process can be completed in two steps, as shown in Fig. 2. Firstly, we will fabricate a 45° angled fiber end face, as shown in Fig. 2(a). A 45° angled glass ferrule with 126μm through-hole is prepared in advance for holding the SMF. With the ferrule prepared, a bare SMF is inserted and fixed into it with extension of 40∼50μm past the front face of the ferrule. A 1μm diamond film is stuck onto the platen. After the platen

Fig. 3. (a) The 45° angled end face of SMF. (b) Fabricated MMI structure. (c) Top view of the sensor with injecting red laser. (d) Side view of the sensor with injecting red laser, the inset shows the emergent light from the end face of the sensor.

runs for a minute or so with a slow lap speed of 50rpm, we gradually lower the SMF until contacting the lapping surface. As the glass material is removed, we increase the lap speed to 100rpm. After grinding the fiber end face with a slight pressure for about 50 seconds, a coarse 45° angled fiber end face is completed. In order to keep high optical properties, the polish obtained from the 1μm diamond film is insufficient. So we change the lapping film to a finer 0.3μm diamond film. Polishing for about 20 seconds again, a smooth 45° angled fiber end face is finally obtained, as shown in Fig. 3(a). Next, we will polish a 0° angled step at the top of the angled fiber, as shown in Fig. 2(b). Another ferrule with flat end is prepared and the polish angle is adjusted to 0°. We lower down the ferrule and keep about 10μm distance between the end face of ferrule and the lapping surface. The lap speed is slowed down to 50rpm and a 0.3μm diamond film is chose. Then we gradually inset the angled fiber into ferrule and lower down to contact the lapping surface with a slight pressure. Keeping polish for about 30 seconds, the flat step is completed, as shown in Fig. 3(b). It is worth to note that the key process of sensor fabrication is to control the polish depth of the top step to make the fiber core divided into two parts. Unfortunately, the polish speed depends on the factors of the lap speed, roughness of lapping film, and loaded pressure. The errors of polish control parameters lead to a critical problem that the practical polish speed of the step is indeterminate. In order to accurately control the polishing depth, we must timely take out the fiber at every 5 seconds and observe the top view of the polished step by using an optical microscope, and a 650nm red laser beam is injected into the fiber core during observation. Fig. 3(c) shows the top view of the polished step. We can clear see that the intersection line of the step (M1) and the 45° angled end face (M2) splits the laser beam into two parts. From Fig. 3(d), we can observe two light spots, which are reflected at the M1 and fiber sidewall (M3), respectively. The two split beams serve as the two optical path of MMI. The inset of Fig. 3(d) shows two perpendicular emergent beams from the end face of sensor, which indicates the successful

YIN et al.: ASSEMBLY-FREE-BASED FIBER-OPTIC MMI FOR HIGH TEMPERATURE SENSING

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Fig. 4. (a) Schematic diagram of experiment setup for high-temperature sensing. (b) Measured reflection spectra of MMI.

fabrication of the MMI structure. In general, by using this method, if we promise the area ratio of M1 and M2 approximately to be 1:1, according to the geometrical optical theory, the tolerance of polish angle deviation can be approximatively calculated as 45° ± 1.03° in order to promise the beam 2 coupling to the fiber core again after reflected from the fiber wall (M3). III. E XPERIMENTS R ESULTS AND D ISCUSSION In order to investigate the performance of the proposed sensor, we have carried out temperature experiments. The schematic diagram of experiment setup is shown in Fig. 4(a). The broadband light from a super luminescent diode (SLD) with central wavelength of ∼1532nm is launched into the sensor via an optical circulator. The reflected interference fringe from MMI can be modeled by using two-beam optical interference, and is received by optical spectrum analyzer (OSA, YOKOGAWA AQ6370), as shown in Fig. 4(b). With the fiber cladding radius of 62.5μm and refractive index of 1.46, the initial OPD 2n L of the MMI is determined as 182.5μm. The free spectral rang (FSR) at the wavelength λ of 1532nm can be described as FSR = λ2 /2n L and theoretically calculated as 12.86nm, which is very close to the experimental value of 12.75nm. The 15dB fringe visibility exhibits high-quality interference signal of the sensor. With the variation of ambient temperature of the sensor, both the radius L and refractive index n of the fiber cladding will change due to the thermal expansion and the thermo-optic effect of the optical fiber, respectively. Therefore, the change in OPD of the interference fringe can be expressed as function of temperature T , OPD ≈ 2n L(σTO + αCET )T , where, σTO and αCET are the thermos-optics coefficient and thermal expansion coefficient of silica fiber. Based on the equation of 2n L = mλ0 , the spectral shift of the interference fringe is a function of temperature, λ0 ≈ λ0 (σTO + αCET )T , where λ0 is the wavelength of a characteristic spectral position (peaks or valleys), m is the corresponding interference order, and λ0 is the wavelength shift at λ0 . With the typical values of σTO = 8.3 × 10−6 °C−1 and αCET = 0.55 × 10−6 °C−1 [20], the theoretical temperature sensitivity of the sensor at the fringe valley of 1532.70nm is calculated to be 13.56pm/°C. To test survivability in high temperature, the sensor was firstly packaged inside a sealed section of quartz glass capillary with inner diameter of 250μm and length of ∼5cm by using high temperature resistant ceramic adhesive for protection. After that, the sensor was horizontally placed in a quartz tube furnace heated from 19°C to 950°C with an interval

Fig. 5. (a) Measured reflection spectrum shift of the MMI as temperature changes. (b) Wavelength shift of interference fringe valley at 1532.70nm of the temperature up-down cycle experiments. (c) Relationships between the sensitivity and tempeature. (d) and (e) are the measured temperature error under temperature up and down process, respectively.

of 40∼50°C, then cooled down to the room temperature. At every temperature test point, we recorded the reflection spectral of the MMI after the temperature had been steady for about 15 min. Fig. 4(b) shows the measured spectrum at 19°C. One of the interference fringe valleys with wavelength of 1532.70nm was chosen to exhibit the spectrum shift induced by the temperature change, as shown in Fig. 5(a). We can clear see that the fringe valley evenly shifts toward the long wavelength region as temperature creases. The spectrum shift as temperature up-down cycle variation is plotted in Fig. 5(b). Two groups of data are mutually consistent, which shows the good repeatability of the proposed sensor. The response curve of wavelength shift versus temperature was well fitted by using a third-order polynomial. The slope of the fit curve was used to calculate the temperature sensitivity, which is found as a temperature-dependent value rather than a constant, as shown in Fig. 5(c). The measured average temperature sensitivity was estimated to be 13.32pm/°C, which is close to the theoretical value of 13.56pm/°C. This indicates that the experiment results are consistent well with theoretical analysis. In order to analyze the reliability, we carried out 5 times experiments with temperature up-down cycle change. The measured temperature error at each temperature test point are recorded and exhibited in Fig. 5(d) and Fig. 5(e). For up and down process, the average measurement errors are less than ±5.09°C and ±6.26°C, and the corresponding standard deviations (SD) are calculated as 1.87°C and 2.10°C, respectively. At every test point of up and down process, the SDs of 5 times measured errors maintain within 0.89∼2.09°C and 0.98∼2.24°C, respectively. Therefore, the full scale measurement accuracy under the range of 19∼950°C is calculated to be 0.67%F.S. These test results demonstrate the outstanding performance of good reliability and high accuracy of the proposed sensors.

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The measurable temperature upper limit of the proposed sensor is determined by the vitrification of silica fiber and thermal diffusion of dopants in the fiber core. In our experiment, when we increase the temperature up to 1000°C for about 5 min, the reflection spectral begins to produce distortion. Such distortion is probably induced by silica fiber vitrification or the dopants diffusion from core to cladding, which all change the waveguide structure of SMF. In order to further improve the measurable temperature, it is necessary to use a special fiber with high vitrification temperature and pure materials like sapphire fiber. We can predict that if we polish a similar micro-interferometer structure at the tip of a sapphire fiber by using the same fabrication method, the measurable temperature upper limit could be as high as 1600°C [3]. IV. C ONCLUSION A new ultra-compact assembly-free-based optic-fiber high temperature sensor based on MMI is demonstrated. Such sensors are realized by directly fabricating a MMI structure at the tip of a common SMF using low-cost fiber-lensing machine, which is much more affordable than the femtosecond micromachining system. By employing the polishing fabrication method, excellent optical properties of the sensor is promised, and exceeding 15dB interference fringe visibility is achieved. All-glass assembly-free-based structure insures the robustness and survivability in high temperature harsh environment. We have experimentally demonstrated that an average temperature sensitivity of 13.32pm/°C is measured under the range of 19°C∼ 950°C. R EFERENCES [1] L. V. Nguyen, D. Hwang, S. Moon, D. S. Moon, and Y. Chung, “High temperature fiber sensor with high sensitivity based on core diameter mismatch,” Opt. Exp., vol. 16, no. 15, pp. 11369–11375, Jul. 2008. [2] J.-L. Kou, J. Feng, L. Ye, F. Xu, and Y.-Q. Lu, “Miniaturized fiber taper reflective interferometer for high temperature measurement,” Opt. Exp., vol. 18, no. 13, pp. 14245–14250, Jun. 2010. [3] Y. Zhu, Z. Huang, F. Shen, and A. Wang, “Sapphire-fiber-based white-light interferometric sensor for high-temperature measurements,” Opt. Lett., vol. 30, no. 7, pp. 711–713, Apr. 2005. [4] A. Zhou et al., “Hybrid structured fiber-optic Fabry–Perot interferometer for simultaneous measurement of strain and temperature,” Opt. Lett., vol. 39, no. 18, pp. 5267–5270, Sep. 2014.

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