Open Access
Add to BibSonomy
Abstract
We demonstrated an optical fiber sensor based on a cascaded fiber Fabry-Perot interferometer (FPI)-regenerated fiber Bragg grating (RFBG) for simultaneous measurement of temperature and strain under high temperature environments. The FPI is manufactured from a ∼74 µm long hollow core silica tube (HCST) sandwiched between two single mode fibers (SMFs). The RFBG is inscribed in one of the SMF arms which is embedded inside an alundum tube, making it insensitive to the applied strain on the entire fiber sensor, just in case the temperature and strain recovery process are described using the strain-free RFBG instead of a characteristic due-parameter matrix. This feature is intended for thermal compensation for the FPI structure that is sensitive to both temperature and strain. In the characterization tests, the proposed device has exhibited a temperature sensitivity ∼ 18.01 pm/°C in the range of 100 °C - 1000 °C and excellent linear response to strain in the range of 300 °C - 1000 °C. The measured strain sensitivity is as high as ∼ 2.17 pm/µɛ for a detection range from 0 µɛ to 450 µɛ at 800 °C, which is ∼ 1.5 times that of a FPI-RFBG without the alundum tube.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Strain in equipment at high temperature is a key measurement physical quantity in many high-tech fields such as aerospace, nuclear power, petrochemical, etc [1]. For the effects of temperature on strain measurement in high temperature fields, there is a pressing need for accurate temperature measurement for thermal compensation especially when optical fiber sensors are used [2]. Compared with conventional electronic sensors for strain and temperature monitoring, optical fiber sensors have attracted intensive interest due to their compact size, flexible structure, strong immunity to chemical attack and electromagnetic interference, and robust even under harsh environmental conditions [3–5].
Of all the fiber optic sensors for temperature and strain monitoring, fiber Bragg gratings (FBG), long period fiber grating (LPFG), and in-line fiber intermodal interferometers [6] are the most commonly used. FBG-based sensors, such as regenerated fiber Bragg gratings (RFBGs) or FBGs cascaded [7–11], are predominant in distributed temperature sensing, but the strain sensitivity is small to some extent. The working principle of LPFG-based on temperature or strain sensor depends on core-cladding mode interference. However, the unstable and lossy cladding modes of the fiber at high temperature are the main limitations of this sensor for the applications under extreme temperature conditions.
Meanwhile, FPI-based optical fiber sensors and in-line fiber intermodal interferometers have proven to be reliable and robust for temperature and strain measurements. They offer higher working range, high sensitivities, and good stability [12–24]. Nevertheless, the thermal drift and interdependency of temperature and strain in their physical properties are the major drawbacks and hurdles to wider real-world usage. Typically, a characteristic matrix of dual-parameter is used to demodulate the change of the temperature and strain, but these techniques is based on linear temperature response and strain response at a certain temperature [25]. However, for a strain sensor, its strain sensitivities usually vary with ambient temperature change. This demodulation approach for sensor presents some limitations for estimating of the maximum error. Therefore, developing new measurement methods to avoid the inaccuracy in matrix of dual-parameter of temperature and strain under high-temperature environments is extremely urgent.
In this work, we present an optical fiber sensor by cascading fiber Fabry-Perot air-cavity interferometer (FPI) and a RFBG nested within an alundum tube, for accurately detecting temperature and strain under high temperature environments. For this sensor, the FPI is sensitive to temperature and strain, whereas the RFBG within the alundum tube only responds to temperature, compensating for any temperature variations of the FPI. The experimental results reveal that this proposed device simultaneously exhibits precise temperature and strain at high temperatures. The strain sensitivity of sensor is about 1.5 multiple that of the FPI with no cascade tube structure. Moreover, the temperature and strain recovery process use a temperature compensated by the temperature response of strain-free RFBG in the alundum tube instead of a characteristic matrix of dual-parameter demodulation. This design shows a more directive demodulation of temperature and strain, and at the same time avoids the errors induced by dual-parameter demodulation. Therefore, the proposed sensor is a potential solution for temperature and strain sensing for various applications under extreme condition.
2. Fabrication and operating principle
2.1 Optimum design of the FPI
A section of hollow core silica tube (HCST, inner / outer diameters ∼ 19 µm / 125 µm) with both ends being flat cleaved. The two ends of the HCST are spliced to two segments of single mode fibers (SMF) by a commercial fusion splicer (Fujikura 80S), which formed a FPI. Thermal treatment for the FPI can concentrate stress in the fiber structure and thus improve its thermal repeatability [26]. Accordingly, we placed the fabricated FPIs ware in a glass tube furnace with a temperature range from room temperature to 1200 °C with accuracy of ±1 °C to complete heat treatment. The length of heating zone is ∼ 200 mm. The heat treat temperature is linearly increased from room temperature to 900 °C with increment rate of 10 °C/min. After held a constant temperature at 900 °C for 180 min, then slowly cooled down to room temperature. As described by this approach, five thermal treated FPIs with different cavity lengths of ∼ 83.8 µm, ∼ 123.6 µm, ∼ 139.7 µm, ∼ 171.5 µm, and ∼ 250.4 µm were manufactured respectively. Figure 1 depicts the interference spectrum of the thermal treated FPI structures at room temperature. It is commonly known that the free spectral range (FSR) of the interference spectrum is a function of the FPI cavity length L as FSR=λ1λ2/2L, where λ1 and λ2 are the adjacent operating wavelength. This feature provides the guidance of controlling and managing the number of fringes in a given observable spectral range for easy analysis and measurement.
Fig. 1. Reflection spectra of the fiver bare FPI devices with different cavities lengths: ∼ 83.8 µm, ∼ 123.6 µm, ∼ 139.7 µm, ∼ 171.5 µm, and ∼ 250.4 µm.
It should be noted that the strain sensitivity ($\mathrm{\beta }1$) of the FPI structure varies with the FPI cavity length ($\textrm{L}$FPI) as well as the temperature (T). As it is a complex system and hardly find a simple formula to describe the relationship of the strain sensitivity with FPI cavity length and temperature, here we give a fitted curve polynomial (Eq. (1)) according to the experimental data (shown in Fig. 2) as a guide.
2.2 Fabrication of the proposed sensor
The proposed sensor was manufactured from an FPI cascaded with a RFBG embedded in an alundum tube based on the fabrication steps illustrated in Fig. 3. First, a section of HCST with a flat cleaved end was spliced to one SMF by using a commercial fusion splicer (Fig. 3(a)). After that, the other end of the HCST segment was flat-cleaved (see Fig. 3(c)). This segment will be used for forming the FPI cavity. A 1 cm long Type-I seed grating (SG) was inscribed in a SMF using an argon ion SHG UV laser (Fig. 3(b)). Similar to the procedure in Fig. 3(c), one side of the SG was flat-cleaved at a distance of 2 cm (see Fig. 3(d)) before it was fusion spliced with the fiber structure prepared in Fig. 3(c) to form a FPI/SG sensor as illustrated in Fig. 3(e). The arc discharge power and duration time were optimized to achieve good mechanical strength and a low-loss silica–air interface. Lastly, SG was encapsulated within an alundum tube with inner / outer diameters of 300 µm / 600 µm and fixated inside the tube by infilling some high temperature resistant ceramic adhesive (HTA) in the gap between the fiber and inner wall of the tube. The liquid infill inside the tube was kept to a length of 8 mm and it was ∼ 6 mm away from the FPI cavity (HCST) and SG as depicted in Fig. 3(f).
The HTA was solidified through a thermal curing treatment at a temperature > 800 °C for a minimum of 30 mins. After that, the bonding can hold in the temperature range of 300 ∼ 1800 °C. The thermal curing process was executed concurrently with the thermal regeneration process for the SG and with the heat treatment for the FPI to obtain a wide working temperature range. The final FPI / SG structure together with an alundum tube was carefully inserted into a glass furnace under a strain-free condition, where the glass tube furnace is used to provide a stable temperature environment (Fig. 3(g)). After that, the furnace temperature is linearly increased from room temperature to 900 °C at an increment rate of 10 °C/min and held constant at 900 °C for 180 min to complete the thermal regeneration process. Subsequently, the annealing temperature is slowly cooled down to room temperature to complete the formation of the final RFBG and the heat treatment for the FPI. The optical fiber sensor by cascading an FPI and a RFBG nested within an alundum tube is from Fig. 3(h). Throughout the study of the sensor spectrum was analyzed by using an FBG Interrogator (SM125, Micron Optics Inc., wavelength resolution ∼ 1 pm). The Bragg wavelength of the RFBG can be observed at ∼ 1535 nm whereas the periodic interference spectrum is the output of the FPI structure with a cavity length of ∼ 74 µm.
2.3 Roles of the alundum tube
As it is described above, during the manufacture of the sensor, we introduced an alundum tube glued around the SMF next to the FPI. The roles of this alundum tube are as follows.
2.3.1 Increasing the sensitivity of the strain sensor
The relationship between the strain coefficient (β1(T)) of the FPI and the cross-sectional area of the alundum tube (SAT), can be written as
Fig. 4. At room temperature, the relationship of β1(T) and β1,FPI(T) with LFPI for the conditions of (a) without and (b) with alundum tube.
2.3.2 Closed the temperature sensor excluding the influence of mechanical strain
The encapsulation of RFBG in an alundum tube can effectively isolate the RFBG from the influence of axial strain and single out its strain sensitivity. Furthermore, the tube provides an excellent protection to the fragile RFBG while maintaining its temperature detection capability and sensitivity. The following provides the mathematically expressions on how the temperature crosstalk in FPI is compensated with the measurement from RFBG [18]:
Let λ1 and λ2 be the measured resonant wavelength of the FPI and RFBG. λ01 and λ02 are their initial resonant wavelengths. The relationships between their wavelength shifts with the temperature change, ΔT and induced axial strain ɛ are given by
3. Results and discussions
3.1 Temperature response
In the thermal characterization test, the annealing temperature was increased from 100 °C to 1000 °C with a step size of 100 °C. A dwelling time of 10 min was applied for each incremental step before the optical spectrum was acquired to ensure the measurements were consistent. The output spectra of the FPI-RFBG sensor at different temperatures are presented in Fig. 5(a). As expected, both resonant wavelength of the FPI and Bragg wavelength of RFBG red-shift with increasing temperature. The relationship between the resonant wavelength of FPI, the Bragg wavelength of RFBG and temperature are shown in Fig. 5(b). The FPI has a temperature sensitivity of 0.768 pm/°C whereas RFBG has a higher temperature sensitivity of 18.01 pm/°C in the temperature range from 100 °C to 1000 °C.
Fig. 5. Temperature responses of the FPI-RFBG (a) reflection spectra. (b)The relationships between the FPI-RFBG wavelengths and different temperatures.
3.2 Strain response
In the strain characterization tests. The SMF to the left of HCST (Fig. 3(h)) and the alundum tube of the sensor are extended outside of a glass furnace, in which one of them is mounted a vertical post whereas the other one is mounted on a micro-displacement linear stage with a 0.01 mm displacement resolution. The induced axial strain on the sensor structure is induced by controlling the elongation on the fiber structure with the linear stage. The proposed sensor was subject to an axial strain from 0 µɛ to 450 µɛ with an incremental step size of 50 µɛ. The characterization was repeated for different temperatures from 300 °C to 1000 °C with an interval of 100°C. The axial strain was increased from 0 µɛ to 450 µɛ with an incremental step size of 50 µɛ. A dwell time of ∼2 mins was applied before the spectrum was recorded for every incremental step. Figure 6(a) shows the reflection spectra of proposed sensor under different strain at 800 °C. These results show that the wavelength of the FPI red-shifts with the increase of strain, whereas the RFBG is in a strain-free condition but subjects to the same ambient temperature as the FPI to provide thermal compensation for the measurement (as shown in inset figure). Figure 6(b) shows the strain response at 800 °C, where each data point represents the mean sensitivities of FPI and RFBG acquired from three measurement test results. From the linear fit of the FPI strain data, the slope is found to be 2.17 pm/µɛ with a linear fitting of R2 > 0.99, while the Bragg wavelength of the RFBG remains almost idle throughout the strain test. This indicates that the RFBG is well-isolated inside the tube and unaffected by the external strain except the temperature.
Fig. 6. Strain responses of the FPI-RFBG sensor at 800 °C (a) reflection spectra. (b)The relationships between the FPI-RFBG wavelengths and different strain.
Figure 7(a) shows the output responses of the proposed sensor to increasing axial strain from 0 to 450 µɛ. For every strain test, the proposed sensor had shown an excellent linear response with an R2 >0.99, or every temperature condition starting from 300 to 1000 °C with an interval of 100°C. The test cycle was repeated for three times and the average strain sensitivities at different temperatures were determined (See Fig. 7(b)). The purple dots represent the mean strain coefficient at each temperature. The blue error bars represent the strain coefficient measurement errors at setting temperature. It can be seen that the strain measurement in the temperature range of 300 °C to 900 °C has small measurement errors. The measurement error mildly increases at 1000 °C, which indicates such high temperature has associated with a certain level of induced plasticity. In the temperature range of 300 - 900 °C, the strain sensitivity is quite consistent, the fluctuation is limited to a small range between 2.06 pm/µɛ and 2.37 pm/µɛ, and linear curve fitting is applied to the strain sensitivities of the FPI in Fig. 5 from 300 °C to 900 °C
Fig. 7. (a) Strain responses of the proposed sensor in the range of 300-1000 °C. (b) The relationship of the strain sensitivity with temperature.
The high strain sensitivity at 1000 °C suggests that the glass material of the fiber is at the transition temperature, that was where the glass viscosity and elasticity begin to rapidly change from this point forward [11].
3.3 Demodulation principle of sensor
For the proposed sensor, the temperature and strain recovery process are described using stress-free RFBG. When a certain stress is applied in axis of the sensor at some temperature environment, considering of the temperature response of embedded RFBG in the alundum tube (as shown in Fig. 5(b)), α2=0.01801nm/°C, thus, the surrounding temperature, T, can be described as
Combining the temperature response of FPI (as shown in Fig. 5(b)), α1=7.68e-4 nm/°C, and the relationship of the strain sensitivity and temperature (Eq. (9) and Eq. (10)), the applied strain, ɛ, can be described as3.4 Comparative analysis
To show the roles of the alundum tube in the proposal sensor. A similar sensor without alundum tube is produced for comparing, and we call this sensor as comparative sensor in the following description. As shown in Fig. 8, compared with the proposed sensor (as shown in Fig. 7(b)), the strain sensitivity of the comparative sensor is reduced by a factor of ∼1.5, this is in agreement with the estimated enhancement by theoretical analysis (see section 2.3.1). The trends of the relationship between the strain sensitivity of FPI and temperature are different for the optical fiber sensors with and without alundum tube because of the HTA’s character.
Fig. 8. The results of the comparative sensor. (a)Temperature responses. (b) Strain responses of the FPI. (c) Strain responses of the RFBG. (d) The relationship between strain sensitivity with temperature.
For the comparative sensor, the temperature (ΔT) and strain (Δɛ) recovery process is usually according to a characteristic matrix of dual-parameter
4. Conclusion
In summary, we proposed and demonstrated an optical fiber sensor based on an FPI-RFBG cascaded structure for simultaneous temperature and strain measurement under high temperature condition. The embedded RFBG in the alundum tube is deliberately made to be solely sensitive to temperature, making it a reliable device for thermal compensation for the FPI structure, just in case the temperature and strain recovery process are described using stress-free RFBG instead of a characteristic matrix of dual-parameter to provide a more reliable the temperature and strain recovery process. The characterization tests show that the proposed device has an excellent operating range up to 1000 °C, with a maximum temperature sensitivity of ∼18.01 pm/°C. Besides, it presents a high linear response for strain testing at 300 °C-1000 °C, and the strain sensitivity is as high as ∼2.17 pm/µɛ for the detection range of 0 µɛ to 450 µɛ at 800 °C. This sensitivity is ∼1.5 times that of a FPI-RFBG without the alundum tube.
Funding
Northwest University (Postgraduate Innovative Talents Training Project (YZZ17094), Young Scientist Support Program (NJ00089)); Natural Science Foundation of Shaanxi Province (2017JM6112); Equipment Pre-Research Project (JZX5Y20190220003501); Science Technology Plan of ShaanXi (2020GY-187).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
References
1. J. Nan, D. Zhang, X. Wen, M. Li, H. Lv, and K. S. J. I. S. Journal, “Elimination of Thermal Strain Interference in Mechanical Strain Measurement at High Temperature Using an EFPI-RFBG Hybrid Sensor with Unlimited Cavity Length,” IEEE Sensors J. 20(10), 5270–5276 (2020). [CrossRef]
2. M. Ghorat, G. B. Gharehpetian, H. Latifi, M. A. Hejazi, and M. B. J. Sensors, “High-Resolution FBG-Based Fiber-Optic Sensor with Temperature Compensation for PD Monitoring,” Sensors 19(23), 5285 (2019). [CrossRef]
3. Z. Wang, H. Liu, Z. Ma, Z. Chen, and F. P. J. O. Express, “High temperature strain sensing with alumina ceramic derived fiber based Fabry-Perot interferometer,” Opt. Express 27(20), 27691 (2019). [CrossRef]
4. D. N. Wang and J. D. J. J. o. L. Technology, “Ultra-Sensitive Strain Sensor Based on Femtosecond Laser Inscribed In-Fiber Reflection Mirrors and Vernier Effect,” J. Lightwave Technol. 37(19), 5149–5157 (2019). [CrossRef]
5. J. Tian, Z. Li, Y. Sun, Y. Yao, and J. J. o. L. Technology, “High-sensitivity fiber-optic strain sensor based on the Vernier effect and separated Fabry–Perot interferometers,” J. Lightwave Technol. 37(21), 5609–5618 (2019). [CrossRef]
6. H. Su, Y. Zhang, K. Ma, Y. Zhao, and J. W. J. O. Express, “High-temperature sensor based on suspended-core microstructured optical fiber,” Opt. Express 27(15), 20156 (2019). [CrossRef]
7. Tao Geng, Hongyi Zeng, Libo Yuan, Jian Li, Wenlei Yang, and F. J. I. P. T. Letters, “Combining Two Types of Gratings for Simultaneous Strain and Temperature Measurement,” IEEE Photonics Technol. Lett. 28(4), 477–480 (2016). [CrossRef]
8. Z. Pan, H. Yang, Y. Wang, L. Hui, L. Kok-Sing, D. S. Gunawardena, and A. J. A. O. Harith, “Strain measurement at temperatures up to 800°C using regenerated gratings produced in the highGe-doped and B/Ge co-doped fibers,” Appl. Opt. 56(22), 6410 (2017). [CrossRef]
9. Q. Yan, W. Liu, S. Duan, C. Sun, S. Zhang, Z. Han, X. Jin, L. Zhao, T. Geng, and W. J. O. F. T. Sun, “A cascade structure made by two types of gratings for simultaneous measurement of temperature and strain,” Opt. Fiber Technol. 42, 105–108 (2018). [CrossRef]
10. X.-G. Qiao, W.-Y. Chong, H.-Z. Yang, M. R. Islam, M.-J. Lim, K.-S. Lim, N. M. Ali, and H. A. J. I. s. journal, “1.3 and 1.55 µm Thermally Regenerated Gratings in Hydrogenated Boron/Germanium Co-Doped Photosensitivity Fiber,” IEEE Sensors J. 14(5), 1352–1356 (2014). [CrossRef]
11. Wang Yupeng, Qiao Xueguang, Yang Hangzhou, Su Dan, Ling Li, and L. J. Sensors, “Sensitivity-Improved Strain Sensor over a Large Range of Temperatures Using an Etched and Regenerated Fiber Bragg Grating,” Sensors 14(10), 18575–18582 (2014). [CrossRef]
12. Y. Pan, T. Liu, J. Jiang, K. Liu, S. Wang, J. Yin, P. He, and J. J. I. P. J. Yan, “Simultaneous Measurement of Temperature and Strain Using Spheroidal-Cavity-Overlapped FBG,” IEEE Photonics J. 7(6), 1–6 (2015). [CrossRef]
13. Z. Wang, H. Liu, Z. Ma, Z. Chen, and F. P. J. O. Express, “High temperature strain sensing with alumina ceramic derived fiber based Fabry-Perot interferometer,” Opt. Express 27(20), 27691 (2019). [CrossRef]
14. J. Tian, Y. Jiao, Q. Fu, S. Ji, Z. Li, M. Quan, and Y. Yao, “A Fabry–Perot Interferometer Strain Sensor Based on Concave-Core Photonic Crystal Fiber,” J. Lightwave Technol. 36(10), 1952–1958 (2018). [CrossRef]
15. T. Yang, X. He, Z. Ran, Z. Xie, Y. Rao, X. Qiao, Z. He, and P. J. M. He, “Highly Integrated All-Fiber FP/FBG Sensor for Accurate Measurement of Strain under High Temperature,” Materials (Basel) 11(10), 1867 (2018). [CrossRef]
16. H. Liu, H. Z. Yang, X. Qiao, M. Hu, Z. Feng, R. Wang, Q. Rong, D. S. Gunawardena, K.-S. Lim, and H. Ahmad, “Strain measurement at high temperature environment based on Fabry-Perot interferometer cascaded fiber regeneration grating,” Sens. Actuators, A 248, 199–205 (2016). [CrossRef]
17. Y. Jiang, D. Yang, Y. Yuan, J. Xu, and J. J. A. O. Zhao, “Strain and high-temperature discrimination using a Type II fiber Bragg grating and a miniature fiber Fabry–Perot interferometer,” Appl. Opt. 55(23), 6341 (2016). [CrossRef]
18. T. Yang, Z. Ran, X. He, Z. Li, Z. Xie, Y. Wang, Y. Rao, X. Qiao, Z. He, and P. J. J. o. L. T. He, “Temperature-compensated multifunctional all-fiber sensors for precise strain/high-pressure measurement,” J. Lightwave Technol. 37(18), 4634–4642 (2019). [CrossRef]
19. K. Naeem, Y. Chung, and B. H. J. I. S. J. Kim, “Cascaded Two-Core PCFs-Based In-Line Fiber Interferometer for Simultaneous Measurement of Strain and Temperature,” IEEE Sensors J. 19(9), 3322–3327 (2019). [CrossRef]
20. Y. Wu, Y. Zhang, J. Wu, and P. Yuan, “Fiber-Optic Hybrid-Structured Fabry–Perot Interferometer Based On Large Lateral Offset Splicing for Simultaneous Measurement of Strain and Temperature,” J. Lightwave Technol. 35(19), 4311–4315 (2017). [CrossRef]
21. C. S. Monteiro, J. Kobelke, K. Schuster, J. Bierlich, O. F. J. Microwave, and O. T. Letters, “High sensitivity strain sensor based on twin hollow microspheres,” Microw. Opt. Technol. Lett. 61(2), 454–458 (2019). [CrossRef]
22. A. Zhou, B. Qin, Z. Zhu, Y. Zhang, Z. Liu, J. Yang, and L. J. O. L. Yuan, “Hybrid structured fiber-optic Fabry–Perot interferometer for simultaneous measurement of strain and temperature,” Opt. Lett. 39(18), 5267–5270 (2014). [CrossRef]
23. M. S. Ferreira, P. Roriz, J. Bierlich, J. Kobelke, K. Wondraczek, C. Aichele, K. Schuster, J. L. Santos, and O. J. O. E. FrazaO, “Fabry-Perot cavity based on silica tube for strain sensing at high temperatures,” Opt. Express 23(12), 16063–16070 (2015). [CrossRef]
24. H. F. Chen, D. N. Wang, and Y. J. A. Wang, “Simultaneous strain and temperature sensing using a slightly tapered optical fiber with an inner cavity,” Analyst 140(6), 1859–1862 (2015). [CrossRef]
25. W. Jin, W. C. Michie, G. Thursby, M. Konstantaki, and B. J. O. Culshaw, “Simultaneous measurement of strain and temperature: error analysis,” Opt. Eng. 36(2), 598–609 (1997). [CrossRef]
26. Q. Tian, H. Z. Yang, K. S. Lim, Y. D. He, H. Ahmad, and X. C. Liu, “Temperature and strain response of in-fiber air-cavity Fabry-Perot Interferometer under extreme temperature condition,” Optik 220, 165034 (2020). [CrossRef]
