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Abstract
A Fabry-Perot interferometer (FPI) based on an alumina ceramic derived fiber (CDF) is proposed and demonstrated for high temperature strain sensing. The strain sensor is constructed by splicing a piece of CDF between two standard single-mode fibers (SMFs). The strain properties of the sensor are investigated from room temperature to 1200 °C. Experimental results show that the wavelength shift of the CDF-FPI presents a linear relationship with the tensile strain at both room temperature and high temperature with up to 1000 °C. The strain sensitivity is calculated to be 1.5 pm/µɛ at 900 °C, and the linear response is repeatable within 0–3000 µɛ. Moreover, for each applied force at 1000 °C, the wavelength shift versus time shows the stability of the developed CDF-FPI sensor within 0–2000 µɛ. The obtained results show that such a CDF-FPI has potential application in various engineering areas, such as aeronautics, metallurgy, and gas boiler.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Fiber-optic strain sensors have attracted much attention in the application of structure health monitoring due to the advantages of small size, high sensitivity, anti-electromagnetic interference capability, and so on. Especially, the fiber strain sensor becomes an imperative requirement in some harsh environments under high temperature, including energy, petroleum, chemical, aerospace and other industrial fields [1–3]. In order to increase the working temperature of strain sensor, researchers have devoted to modify the structure and the materials of the fiber sensors [4–6].
Conventional high temperature strain sensors are fabricated based on standard silica optical fibers, including fiber Bragg gratings (FBGs) [7], long period fiber gratings (LPGs) [8], Mach-Zehnder interferometers (MZIs) [9], Michelson interferometers (MIs) [10], and Fabry-Perot interferometers (FPIs) [11], etc. Among various high temperature strain sensors, FBG structure is the mostly adopted. However, the FBGs could be easily erased at 650 °C due to rapid diffusion of dopants in the core [12]. To increase the temperature stability of FBGs, regenerated FBGs are proposed for higher temperature strain sensing, which can be used to measure temperature exceed 1000 °C [13]. Although regenerated FBGs can effectively improve the high temperature resistance of conventional FBGs, the mechanical properties of regenerated FBGs will be affected when the working temperature exceeds 850 °C because the regenerated grating fibers become very brittle after high temperature annealing [14]. The working principle of LPGs and MZIs depends on mode interference for temperature or strain sensing. However, the cladding mode will be unstable when the optical fiber is placed under high temperature, which will affect the signal transmission. Moreover, LPGs and MZIs are not compact enough because the sensing length is usually in centimeter. MIs prepared based on taper processing [15] or arc discharge [16] or misalignment treatment forms a reflecting surface [17], but these treatments tend to damage the optical fiber. Compared with other sensors, FPIs have many advantages, such as compact structure, diverse preparation methods, and high temperature sensitivity, etc, which is suitable for testing under high temperature environment.
In recent years, specialty fibers are explored to fabricate high temperature FPI-based strain sensors, such as splicing a short section of silica tube [18], a photonic crystal fiber (PCF) [19] or a hollow core fiber (HOF) [20], single crystal sapphire fiber (SCSF) [21], and sapphire derived fiber (SDF) [22] with standard SMFs. Such a hybrid structure effectively utilizes the high temperature resistance of special optical fiber and the high quality transmission characteristics of SMF. In order to improve the performance of strain sensor in the environment above 1000 °C, it is necessary to design a structure with better mechanical strength and seek a special fiber with better high temperature resistance. In 2015, Ferreira et al. designed a special silica fiber for high temperature strain sensing, which is drawn from a preform composed of a hollow core and four small rods [18]. The strain sensing range is from 0 µɛ to 1000 µɛ and its strain sensitivity is about 5 pm/µɛ under maximum operation temperature of 900 °C. In 2007, Liu et al. fabricated a FPI strain sensor based on a HOF [20], which can work at 1000 °C. However, the sensitivity of such a HOF-FPI sensor is only 0.46 pm/µɛ at 21 °C and 520 °C, and the maximum strain range is 1400 µɛ. Meanwhile, the temperature resistance is limited by the silica material since the softening temperature of silica glass is about 1200 °C [23]. SCSF has been introduced for high temperature sensing because of its high melting point (∼2050 °C). However, the surface of unclad SCSF is extremely susceptible to damage. SDF is a new type of high temperature resistant fiber whose core is a co-doped medium of alumina and silica. Studies have shown that the core-doped alumina fiber can work stably at 1200 °C for a long time [24]. However, there is no report on the fabrication of strain sensors based on a SDF. Moreover, SDF is drawn from a single crystal sapphire rod as a core material, so the preparation cost is high.
In this paper, a novel special fiber FPI is proposed and demonstrated for high temperature strain sensing. The FPI is fabricated in a ceramic derived fiber (CDF) which is drawn by rod-in-tube method. A high purity alumina ceramic rod and a silica glass tube are used, and the drawn CDF presents high concentration alumina composition in the all-glass core. With arc-discharge processing, mullite particles are precipitated, which exhibits local high refractive index (RI) modulation and resistance to high temperature [25]. Utilizing the mullite crystalline composite zone as reflective mirrors, the FPI can survive up to 1200 °C. It presents good repeatability for strain testing at high temperature up to 1000 °C. The strain sensitivity is ∼1.5 pm/µɛ within the sensing range of 0 µɛ to 3000 µɛ at 900 °C, which is approximately three times the strain measurement range that have been reported [18,26,27]. Such a CDF-FPI sensor has the advantages of high temperature resistance, large strain range at high temperature, high linearity, repeatability, and low cost, etc. Therefore, it shows potential application in many harsh engineering areas, such as aeronautics, metallurgy, gas boiler, etc.
2. Sensing principle
The FPI based on the CDF is depicted schematically in Fig. 1. The two crystallization zones serve as FPI mirrors. When light is coupled into the crystallization of CDF-FPI, strong interference spectrum can be achieved attributed to the high refractive index difference of the crystalline mirrors.
The optical path difference (OPD) within the FPI is determined by the cavity length L, $OPD = 2 \cdot n \cdot L$, where n is the effective refractive index of the CDF cavity. When the length of the cavity is defined, the reflection will present a periodic interference spectrum. The free spectral range (FSR) of the FPI is calculated with,
where $\lambda$ is the light wavelength, $\Delta \lambda $ is the FSR, referring to the wavelength difference between adjacent interference dips.When the sensor is subjected to an external pulling stress axially, the cavity length is changed. Meanwhile, the strain induces refractive index change due to the elasto-optic effect [28], which can be expressed as,
According to the interference principle of the FPI, the change of the OPD is proportional to the FSR and accordingly leads to shifting of the dips wavelength ${\lambda _d}$, $\frac{{\Delta OPD}}{{OPD}} = \frac{{\Delta {\lambda _d}}}{{{\lambda _d}}}$, where $\Delta {\lambda _d}$ represents the drift value of ${\lambda _d}$ varying with $OPD$. Therefore, we can obtain the relationship between the strain and the dips wavelength as,
Obviously, the change of the dips wavelength is linearly related to the strain.3. Fabrication of the CDF-FPI
The CDF was fabricated by using the rod-in-tube technique [29]. The rod is made from high purity (≥ 99%) alumina ceramic with the diameter of 1 mm, and the tube is made from silica with the outer and inner diameter of 10 mm and 1.1 mm, respectively. The rod-in-tube preform is drawn at the temperature of 2200 °C which is higher than the melting point of high purity alumina ceramic rod (2050 °C). During the fiber drawing process, the ceramic rod is melted and the silica tube is in a melting state, then the alumina will diffuse with the silica tube each other, resulting a graded index all-glass core. A few hundred meters long CDF is drawn with uniform core-cladding structure. The cross section of the drawn CDF was observed by an optical microscope (BX53M, OLYMPUS). The diameters of the core and the cladding are 16 µm and 100 µm, respectively. The targeted core diameter of CDF is intended to match with that of standard SMF. In this work, the silica tube for drawing CDF is collapsed by a modified chemical vapor deposition (MCVD) lathe. Such home-made silica tube is fabricated imperfectly with less thickness and larger inner diameter than the targeted values, resulting in a larger core diameter of the CDF than that of SMF. The alumina concentration in the core is ∼44 mol% according to the scanning electron microscope (EDS) analysis. The transmission loss of the CDF is approximate 0.27 dB/cm at wavelength of 1550 nm by a cut-back method. The loss of CDF is mainly due to the impurities in the alumina ceramic rod, for example, a small amount of iron oxide. The less impurities the rod incorporates, the higher transmittance of the CDF can be achieved [30]. In addition, the cleanliness of the surface of the ceramic rod also affects the loss. Therefore, it is expected to reduce the loss by choosing higher purity alumina rods and clearing the contaminant on the surface of the alumina rod before fiber drawing.
We further fabricated crystallization induced FPI by using a fusion splicer (FSM-30P, Fujikura). The arc discharge intensity and duration are about 300 bits and 3000 ms, respectively. In order to achieve a good crystallization effect, the spacing between the fibers and the splice pushing distance are set to 15 µm and 12 µm, respectively. First, the CDF is spliced with a SMF. As demonstrated by [22], with excessive treatment by the arc discharge, a crystallization effect occurs at the position approximate a few hundred micrometers off the discharge center. The splice loss is about 2 dB. Then, the CDF is cut away with keeping the crystallization zone and a certain length untreated CDF. Finally, the left CDF is spliced with another SMF. Another crystallization zone is generated. As a result, the crystallization-based FPI is constructed. The microscopic image of the fabricated CDF-FPI is shown in Fig. 2(a). The cavity length between the two crystallization zones is approximate 384 µm.
Fig. 2. The microscopic images of (a) the CDF FPI sensor (cross section of a CDF) and (b) refractive index profile of a CDF (black) and the CDF FPI at typical locations with 1-z:0 µm (red), 2-z:278 µm (blue), 3-z:310 µm (green), 4-z:502 µm (pink).
The reflecting property of the CDF-FPI mainly depends on the reflection of the crystalline mirrors and the refractive index of the CDF. Therefore, the refractive index profile of the CDF-FPI sensor is characterized by using a RI profiler (SHR-1602) based on the 3D holographic method. To investigate the RI modulation profile near the crystalized region of CDF, we test the CDF starting from the fusion point to the center of the two crystallization zones. As shown in Fig. 2(b), the RI difference between the cladding and the core of the CDF is about 0.065, while the RI difference in the crystalline zone is about 0.078. Therefore, crystallization causes the RI difference of approximate 0.013.
Figure 3(a) records the reflection spectrum of the developed CDF-FPI. The reflection spectrum shows the FSR of 2.08 nm and the extinction ratio (ER) of approximate 15 dB. The cavity length is calculated to be about 388 µm by Eq. (1), which is in accord with the interval of two crystalline zones. The upper and the lower envelopes of the reflection spectrum are not flat because the crystallization zone is not a single plane but a short section. This short section also introduces a small cavity, resulting in a floating interference spectrum [31].
4. Characterization of the CDF-FPI strain sensor
Figure 4 shows the schematic diagram of a high-temperature strain measurement system for the CDF-FPI sensor. A swept laser source (resolution ± 0.005 nm) scans the wavelength ranging from 1510 nm to 1590 nm. A photodiode detects and records the reflected spectrum. Note that the end of the SMF at the upper clamp needs to be cut flat with a fiber cleaver and inserted into the matching liquid to reduce the reflection noise. By using a universal tensile strength testing machine (SUNS UTM6000 series), the CDF-FPI is characterized for strain sensing at high temperature condition. A tube furnace (NS-1400-18) is equipped on the testing machine to heat the sensor, and the heating zone is 5 cm.
In order to confirm the high temperature resistance of crystallization mirrors, the reflection spectrum of the CDF-FPI is tested from room temperature to 1200 °C with a step of 100 °C. The FPI sensor with SMF pigtails passes through the tube furnace. The duration time of 10 min is kept at each temperature to stabilize inside the furnace. As shown in Fig. 5(a), the spectrum shifts toward longer wavelengths with increasing the temperature. Meanwhile, benefiting from the high temperature resistance of crystalline mullite mirrors as well as low diffusion rate of the CDF with solid state at 1200 °C, the CDF-FPI sensor can work stably up to 1200 °C. Figure 5(b) shows an approximate linearity in the temperature response. The sensitivity of 15.6 pm/°C is achieved.
Fig. 5. (a) Reflection spectrum of the FPI sensor with length at 32 °C (black), 400 °C (red), 800 °C (blue) and 1200 °C (green); (b) temperature response of the FPI sensor.
After demonstrating the high temperature resistance of the CDF-FPI, we investigated the strain sensing characteristic at room temperature firstly. The two SMF pigtails are wound two turns on the upper and the lower capstans with diameter of 50 mm and then clamped firmly, which can avoid breaking and sliding of the SMF pigtails. Before each strain testing, the fiber is kept straight. The displacement controlling mode is selected. The tested length of the fiber between the two capstans is approximate 50 cm. The stretching speed was set to 0.1 mm/min. When the travelling distance reaches 1.5 mm, the return trip is carried out immediately with 0.1 mm/min. During the tensile strength testing process, the real-time tensile force is recorded by using an equipped strain gauge. The corresponding reflection spectrums are recorded at the tensile force changing step of 0.1 N for a round-trip tensile strength testing. The experimental results are shown in Figs. 6(a) and 6(b), the spectrum shifts toward longer wavelengths with increasing the force. It can be seen that the force response of the CDF-FPI from 0 N to 2.1 N presents linear behavior, and the sensitivity is approximate 2.2 nm/N.
Fig. 6. (a) Reflection spectrum of the FPI sensor with length of 384 µm at the force of 0 N (green and red solid lines) and 2.1 N (blue and black dash lines); (b) force response of the FPI sensor. Forth (black solid square) and back (red hollow circle) represent increasing and decreasing force, respectively.
We further tested the force sensing property of the CDF-FPI from room temperature to 1200 °C. The sensor is heated by using the tube furnace with a step of 100 °C, and stays for 10 min at each temperature to stabilize. As shown in Fig. 7(a), the force sensitivity decreases firstly and then increases as temperature increases. The reason is due to the elasticity coefficient changing with temperature. According to Eqs. (2) and (6), the force sensitivity of the sensor is related to the elasticity coefficient (p12). Previous reports have shown that the elasticity coefficient of silica fiber increases firstly and then decreases with temperature [32]. Such temperature-dependent elasticity coefficient could lead to a nonlinear trend of force sensitivity, which has also been verified in the reported high temperature strain sensors [18]. According to the results in Fig. 7(b), the wavelength cannot return to the original value as the force drops to zero. It is attributed to the plastic deformation effect. When the force exceeds the elastic limit of the material, the deformation is irreversible [33,34]. Therefore, the force sensitivity slightly decreases for the return trip.
Fig. 7. (a) Dependence of the force sensitivity of increasing force and decreasing force at different temperatures; (b) strain response at 1000 °C, the inset shows forth and back paths of the tension machine during tensile test at 1000 °C. Forth (black solid square) and back (red hollow circle) represent increasing and decreasing force, respectively.
The insert in Fig. 7(b) shows forth and back paths of the tension machine during tensile test at 1000 °C. Since the fibers are not ideally straightened in the initial state, the forth and back paths do not coincide. According to Eq. (3), we can get the strain dynamic range and strain sensitivity of the CDF-FPI sensor. The Young's modulus can be estimated to be about 51.8 GPa by the relationship of the linear region of between the tensile force and the displacement in Fig. 7(b). Thus, the strain dynamic range is from 0 µɛ to 3000 µɛ, and the strain sensitivity is about 1.4 pm/µɛ at 1000 °C. According to the relevant parameters in Eq. (6) provided by [35], the theoretical sensitivity of the sensor is calculated to be about 1.47 pm/µɛ, where the refractive index of the CDF is set to be 1.527 at 1550 nm. The theoretical sensitivity is approximately equal to the test sensitivity.
In order to find the operating temperature limitation for the stain range of 3000 µɛ, the CDF-FPI sensor was stretched back and forth at 900 °C for three times, as shown in Fig. 8(a). The experimental results show that CDF-FPI sensor can work stably with a strain range of 3000 µɛ at 900 °C, and the strain sensitivity is about 1.5 pm/µɛ. Furthermore, it can be seen from Fig. 7(b) that the CDF-FPI sensor has a good linearity at 1000 °C, but the spectrum cannot return to the original value during the back trip, which means that a slight plastic deformation is introduced under strain of the 3000 µɛ. Thus, we reduce the strain range to test the repeatability of the sensor at 1000 °C. As shown in Fig. 8(b), when the strain range is 2000 µɛ, the sensor will not introduce deformation at 1000 °C, indicating a stabilization after few cycles.
Fig. 8. Strain response at (a) 900 °C and (b) 1000 °C; (c) wavelength shift versus time for each applied force at 1000 °C. Forth (solid) and back (hollow) represent increasing and decreasing force, respectively.
In order to further study the stable tensile property of the CDF-FPI sensor at 1000 °C, the corresponding wavelength shift for each applied force from 0 N to 1.64 N with step of 0.11 N at 1000 °C is measured for 5 minutes, as shown in Fig. 8(c). The result shows that the wavelength shift is very small under each force, and the maximum shift is about 0.1 nm, which proves the stability of CDF-FPI sensor at 1000 °C.
In order to study the working limit of the CDF-FPI sensor, strain response of the CDF-FPI is also tested at 1100 °C and 1200 °C, as shown in Figs. 9(a) and 9(b). When the temperature rises to 1100 °C, the delay effect becomes worse and the sensitivity further decreases for the return trip. Besides, force response curves at 1100 °C with increasing and decreasing force are not coincident. Considering that the temperature of 1100 °C is close to the softening temperature of silica (1200 °C), the viscosities of silica fibers suddenly decrease, which causes the fiber structure to relax and introduces plastic deformation. It can be clearly seen from the inset in Fig. 9(a), the force drops rapidly during the return trip. This means that a certain shaping deformation is introduced at 1100 °C. Thus, force response curves at 1100 °C with increasing and decreasing force are not coincident.
Fig. 9. (a) Strain response of the CDF-FPI sensor at 1100 °C, the inset shows forth (black solid square) and back (red hollow circle) paths of the tension machine during tensile test at 1100 °C; (b) reflection spectrum of at 20 °C (red and black), 1200 °C (green and blue). Up (dash lines) and down (solid lines) represent increasing and decreasing temperature, respectively.
When the temperature reaches 1200 °C, the CDF-FPI sensor cannot perform strain measurement due to the creep effect but can survive. As shown in Fig. 9(b), it is found that the spectra are almost undistorted, and the wavelength drift caused by temperature from 20 °C to 1200 °C is equal to that caused by annealing from 1200 °C to 20 °C of optical fibers. It indicates that the force sensitivity of the sensor at 1100 °C is changed by the Young's modulus and plastic deformation of the fiber, but it is not enough to affect the temperature characteristic of the CDF-FPI sensor at 1200 °C.
5. Conclusion
We have demonstrated the FPI sensor based on the alumina ceramic derived fiber, which can be used for strain measurement up to 1000 °C. Due to high content alumina, the CDF can be crystalized by arc discharge. Crystallization causes local refractive index increase of approximate 0.013, so crystalized zone can serve as the mirrors of the F-P sensor. First, we test the high-temperature response of a CDF-FPI sensor. The results show the CDF-FPI sensor could stand for 1200 °C, and show a temperature sensitivity of 15.6 pm/°C. Subsequently, we test the strain sensitivities of CDF-FPI sensor from room temperature to 1200 °C. The experiment results show a linear response to strain with sensitivity of ∼1.5 pm/µɛ at 900 °C, and the strain dynamic range is from 0 µɛ to 3000 µɛ. Besides, the CDF-FPI sensor has good repeatability at 1000 °C with strain range of 2000 µɛ. Due to the weakening of viscosities and plastic deformation, force sensitivities at 1100 °C with increasing and decreasing force are no longer consistent. Finally, comparing the spectrum of heating and annealing after tension, we find that the introduction of plastic deformation does not affect the temperature response of the sensor. Hence, such a CDF-FPI sensor could be developed for ultra-high temperature strain sensor in harsh environments, such as the fields of the aero engine, boiler, steel furnace, and so on.
Funding
National Natural Science Foundation of China (61735009, 61635006, 61975108, 61605108); Shanghai Young Oriental Scholar (QD2016025).
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