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Abstract
We have proposed and experimentally demonstrated a sapphire-derived fiber (SDF) and silica capillary-based compound Fabry-Pérot interferometer (FPI) for high-pressure and high-temperature sensing. The SDF owns high alumina dopant concentration core, which can generate a mullite crystallization region during an arc discharge process. The crystallization region acts as a reflective interface to form one FPI in the SDF. The other FPI contains an air cavity constructed by the silica capillary and is used for high-pressure sensing. Both gas pressure within a range from 0 MPa to 4 MPa and temperature within a range from 20°C to 700°C are measured. Experimental results show that the wavelength shift of the FPI versus the applied pressure is linear at each tested temperature. The pressure sensitivity is measured to be 5.19 nm/MPa at a high temperature of 700°C, and the linear responses show excellent repeatability with linearity of 0.999. Meanwhile, the proposed FPI can stably function at a high temperature of 700°C with a temperature sensitivity of 0.013 nm/°C. The proposed FPI sensor provides a promising candidate for simultaneous measurement of high pressure and high temperature in extreme conditions.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The measurement of pressure has received extensive research and development in fields of oil and gas exploitation, industrial and environmental monitoring [1]. Especially for the measurement of pressure in internal combustion engines and deep geothermal drilling, etc, a high-temperature pressure sensor has become an urgent need [2]. Over the past few decades, several types of fiber-optic pressure sensors have been widely studied, most of them are based on the Michelson interferometer (MI), the Mach-Zehnder interferometer (MZI) and the Fabry-Pérot interferometer (FPI), etc [3–5]. Compared to other types, the FPI-based sensors have aroused considerable interest of researchers in academia and industry due to their outstanding advantages of compact structure, high accuracy, electromagnetic interference resistance, sensing in harsh environment desirability and reliability. In order to realize the simultaneous measurement of high pressure and high temperature, various methods have been developed to fabricate FPI sensors. These include using fiber Bragg gratings (FBGs) and special optical fibers. For example, in 2020, Zhao et al. proposed a FPI pressure sensor, which was composed by a FBG for temperature sensing and a cascaded air cavity extrinsic Fabry-Pérot interferometer (EFPI) for pressure sensing [6]. This hybrid structure can realize dual-parameter measurements and the FBG can minus the cross-impact on the temperature and the pressure, however, the operation temperature is limited within 200°C and the practicality of such device has been limited by the poor thermal stability of the FBG [7]. This limitation can be overcome by fabricating the sensor by compound FPI hybrid structures using special optical fibers such as solid-core photonic crystal fiber (PCF), hollow-core photonic bandgap fiber (HC-PBF) and hollow-core fiber (HCF). In 2011, Wu et al. proposed a fiber-optic FPI by splicing a solid-core PCF to a standard single-mode fiber (SMF), and the other end of the PCF was collapsed to enhance the reflection coefficient of the reflection mirror. The FPI can work for the measurement of pressure and temperature in a wide range of 0 MPa-40 MPa and 25°C-700°C, respectively [8]. But such device shows relatively low sensitivity of -5.77 pm/MPa. In 2018, Zhang et al. fabricated a dual-cavity FPI by splicing a HC-PBF to a SMF and a HCF [9]. The FPI can withstand 800°C and the measurement of pressure within a range from 0 MPa to 10 MPa. However, the manufacturing process involves HC-PBF and HCF special optical fibers, which increases the difficulty and the cost of the fabrication process. In addition, the sapphire-based pressure sensor has been proposed with a higher maximum detected temperature limitation due to its high melting point of 2040°C. In 2021, Yi fabricated a pressure sensor by bonding two sapphire wafers, whose work range is 25°C to 1000°C and the pressure sensing range is from 0.1 MPa to 2.1 MPa [10]. In 2020, Wang et al. proposed a pressure sensor by using a bonded sapphire wafer and a standard multimode fiber (MMF), which can withstand 1200°C and measure the pressure of 4 MPa at 1200°C [5]. However, a dangerous acid corrosion process or a complex fabrication technique is essential among them, which increases the danger, the cost and the complexity of the fabrication process. Furthermore, these sapphire-based pressure sensors usually involve adhesive or ceramic ferrule to bond the optical fiber or fix the sensor head and other components, respectively. However, the sensors will have a large coefficient of thermal expansion (CTE) mismatch between different materials, which will induce severe stress and crack of the sensor structure, affect the sensor performance when heated [11,12]. Therefore, a simple and cost-effective FPI sensor for simultaneous measurement of high pressure and high temperature has yet to be developed.
Sapphire-derived fiber (SDF) is a high-alumina content all-glass optical fiber, which has been attracted much interest because it offers many outstanding advantages, such as controllable core diameter, confined modes by cladding protection and high temperature resistance [13]. Reheating and cooling the SDF during an arc discharge process, the high alumina dopant concentration core will show a strong compositional change and the mullite crystallization region occurs when the temperature reaches the crystallization temperature [14–16]. Because of the excellent high temperature resistance, the SDF and the mullite crystallization region have attracted a lot of attention and been utilized for designing high temperature sensors [16–24].
In this study, a compound FPI is produced by using a CO2-laser processing system for the simultaneous measurement of gas pressure within a range from 0 MPa to 4 MPa and temperature within a wide range of 20°C to 700°C. The FPI is composed of two cavities, of which a section of SDF with mullite crystallization region acts as cavity 1 for high-temperature sensing and the air cavity constructed by silica capillary acts as cavity 2 for high-pressure sensing. The mullite crystallization region can be easily fabricated in the SDF core by fusion splicing SDF with a SMF. Such a compound FPI sensor exhibits a high pressure sensitivity of 5.19 nm/MPa at high temperature 700°C, and a temperature sensitivity of 0.013 nm/°C. In addition, such FPI sensor owns high-pressure sensitivity and stability in a wide measurement range, high temperature resistance, which is imperative for its potential use in high-pressure and high-temperature measurement.
2. Sensor fabrication
The proposed FPI schematic diagram is illustrated in Fig. 1. In this experiment, a SDF and a SMF with inner and outer diameters of 16/125 µm and 9/125 µm, respectively, and a silica capillary with inner diameter of 140 µm and wall thickness of 80 µm were employed. The fabrication process includes four steps. Firstly, the two fibers were cleaved and spliced together by a fusion splicer (S176, Furukawa). In order to prevent the formation of bubble at the fusion point, the intensity and the duration time of the arc discharge were optimized as standard and 3000 milliseconds, respectively. Due to the Gaussian-like temperature distribution along the axial direction of the SDF induced by the arc discharge [16], there exists gradual mullite crystallization region along the axial direction of the SDF with a longer length than 100 µm. Subsequently, the SDF was cleaved to a length of hundreds of microns with a flat end face to obtain a high reflectivity. Cavity 1 between the mullite crystallization region and the end face of SDF was formed. Then, the SDF with mullite crystallization region and a well cleaved SMF were inset into the silica capillary and welded with the use of a CO2-laser processing system (LZM-100, AFL Fujikura), respectively. The air cavity between two parallel end faces of SDF and SMF was formed as cavity 2. It should be noted that the thermo-optic and the thermal expansion effect of the SDF section could alter the optical path difference (OPD) in cavity 1. Nevertheless, as cavity 1 is insensitive to pressure, it is selected as a temperature sensor. On the contrary, as cavity 2 is formed by an air cavity between two parallel end faces of SDF and SMF, the air cavity is highly sensitive to the deformation of the silica capillary caused by pressure while it is less sensitive to temperature. As a result, cavity 2 can be employed as a pressure sensor. Additionally, the CTE of SMF and silica capillary are similar, which minimizes the problem of temperature cross-sensitivity caused by the occurrence of thermal mismatch of materials at the welding point [6,25–28]. Here, the FPI welding points were fabricated by the CO2-laser processing system. The welding parameters were optimized in our experiment, which would greatly improve the repeatability and long term stability of the FPI at high temperature.
3. Theoretical analysis of the FPI sensor
The theory of the compound high-temperature pressure FPI sensor can be described as follows. As shown in Fig. 1, there are three reflection mirrors in the FPI, labeled as reflection mirror M1, M2 and M3. The reflection coefficients from the reflection mirror M1, M2 and M3 are ${R_1}$, ${R_2}$ and ${R_3}$, respectively. M1 and M2 form cavity 1 with a cavity length of ${L_1}$. M2 and M3 form cavity 2 with a cavity length of ${L_2}$. The reflected lights at three reflection mirrors are coupled back into the lead-in optical fiber, resulting in a compound interference reflection spectrum [9]. Due to the low reflection coefficient of each reflection mirror, the multiple reflections can be neglected [29]. The normalized reflection spectrum ${I_R}$ can be expressed by
Since the cavity 1 is formed by the mullite crystallization region and the SDF`s end face, the RI of the cavity 1 (n1) mainly depends on the RIs of them. Therefore, a RI profiler (SHR-1602) based on 3D holographic method is employed to characterize the RI profiles of the mullite crystallization region and the SDF [16,24]. As shown in Fig. 2(b), the RI difference between the cladding and the core of the SDF (black curve) is about 0.066 and the RI difference of the mullite crystallization region (blue curve) is about 0.076. Therefore, mullite crystallization region causes a RI difference of about 0.010. In addition, the RI difference of the SDF near the splicing point is 0.016 (red curve). According to the Fresnel formula, the reflection coefficient is only 0.014%, which hence can be neglected.
Fig. 2. (a) Microscopic image; (b) the corresponding RI profiles of the SDF (black curve), the mullite crystallization region (blue curve) and the splicing point (red curve), respectively.
When the FPI is subjected to pressure, the silica capillary is squeezed to produce deformation, the fibers slide in the silica capillary, therefore the pressure will change the length of the cavity 2. The change will cause variation of the reflection spectrum, and the applied pressure can be achieved by demodulating spectrum changes [6]. The relative length change $\Delta L$ of the cavity 2 caused by the applied pressure $P$ can be derived as [26]
The relationship between the operating wavelength and the applied pressure can be determined as
It can be found that the relative operating wavelength shift demonstrates a proportional relationship with the applied pressure of cavity 2.
Similarly, when the cavity 1 is subjected to temperature variation, the effective RI and ${L_1}$ will change, the relationship between the operating wavelength and temperature can be given by
4. Experimental results and discussion
In the experiments, we firstly measured the reflection spectra of the fabricated FPI at room temperature and atmospheric pressure when the lead-in optical fiber is the SDF with mullite crystallization region and the SMF, respectively. Figure 3(a) displays the microscopic image of the FPI with cavity 1 and cavity 2 length of 897.9 µm and 104.6 µm, respectively. The inset shows the SDF mullite crystallization region morphology. Figures 3(b) and 3(c) display the corresponding reflection spectra and spatial frequency spectra. Here an optical sensing interrogator (sm125, Micron Optics) with a wavelength range of 1510 nm∼1590 nm and a resolution of 5 pm is employed to monitor the FPI reflection spectrum. It is shown that when the lead-in optical fiber is SDF with mullite crystallization region, the interference spectrum related to cavity 1 can be distinguished evidently. Two main frequency points with substantially different OPDs can be clearly identified, corresponding to the cavity 1 and cavity 2, respectively. On the contrary, when the lead-in optical fiber is SMF, the interference spectrum and its spatial frequency peak related to cavity 1 become quite weak. This is due to the fact that the mullite crystallization region and the SDF section within cavity 1 cause relatively larger transmission loss compared with air cavity 2. It will lead to a significant imbalance in the reflected powers from the SDF`s end face and the crystallization region, which deteriorates the contrast of the interference spectrum. As a result, we choose the SDF with mullite crystallization region as lead-in optical fiber for the following high-pressure and high-temperature measurements. According to the spatial frequency spectrum, the filtering method is applied to the FPI reflection spectrum for cavity 1 and cavity 2, which are separated and displayed in Figs. 3(d) and 3(e), respectively.
Fig. 3. (a) Microscopic image of the fabricated FPI. The inset shows the SDF mullite crystallization region morphology; (b) and (c) corresponding reflection spectra and spatial frequency spectra when the lead-in optical fiber is SDF with mullite crystallization region and SMF, respectively; (d) the spectra of cavity 1 and (e) cavity 2 after filtering out.
The experimental setup similar to that in [5] is used, where a pressure regulator (QY-C1-10-Y, resolution ±0.02 MPa) and a digital pressure meter (700G08, Fluke, resolution of 0.0001 MPa) are employed to deliver and calibrate the pressure, respectively. A tube furnace (NS-1400-18) with a high-temperature resistance alumina ceramic tube and 5 cm heating zone is used as gas chamber and to control the temperature. To research the high-pressure and high-temperature sensitivity of the sensing structure, the sensor is placed into the alumina ceramic tube, sealed by strong glue to the pigtail fiber outside the tube for real-time measurement. Meanwhile, the response of the reflection spectrum corresponding to the FPI is monitored simultaneously by using the optical sensing interrogator [24].
Firstly, the stability of the prepared FPI was experimentally examined at 20°C. The pressure was gradually increased to 4 MPa and kept for 60 mins. The reflection spectrum for the FPI was recorded in every 1 min. Figure 4 shows the wavelength shift of the fabricated FPI cavity 2 at 4 MPa for 60 mins, obtained by filtering each recorded FPI reflection spectrum. It can be seen that the relative shift of wavelength $\Delta \lambda$ of cavity 2 is within 0.003 nm. The results show that the FPI has a good sealing state.
Fig. 4. Relative shift of wavelength $\Delta \lambda$ of the fabricated FPI cavity 2 at 4 MPa for 60 mins.
The performance of the FPI in high temperature has been investigated. The tube furnace temperature was programmed to linearly increase from 20°C to 700°C at the ramp rate of 10°C/min. Each measurement point was maintained for 10 mins to keep the conditions inside the furnace stable. Early work showed that due to rapid cooling down process in the fabrication of the SDF, strong internal residual stress was built due to the high RI difference between the SDF core and the cladding [18]. Therefore, before demonstrating the feasibility as a high temperature sensor, an annealing process was firstly conducted by repeating the heating and cooling cycle one time to relax the internal residual stress of the SDF. Next, the test processes of heating up and cooling down were repeated to evaluate the repeatability of the FPI temperature sensor. The relationships between the relative shifts of wavelength $\Delta \lambda$ of cavity 1, cavity 2 and varying temperatures in heating and cooling processes, obtained by filtering each recorded reflection spectrum, is plotted in Figs. 5(a) and 5(b), respectively. The reflection spectrum of cavity 1 shifts monotonically towards the longer wavelength direction with the increased temperature while to the shorter wavelength direction with the decreased temperature, indicating a good linear temperature response with a sensitivity of 0.013 nm/°C. This shift is attributed to the thermo-optic effect and the thermal expansion effect in SDF core [18,31,32]. Figure 5(b) displays the relative shift of wavelength of cavity 2 with increasing temperature, which was deduced by the wavelength shift difference between the compound cavity (including the cavities 1 and 2) and the cavity 1. It can be found that the wavelength presents very slight shift less than 0.57 nm, which is mainly induced by the thermal expansion of the silica capillary section of the cavity 2.
Fig. 5. (a) and (b) Temperature responses of the fabricated FPI cavity 1 and cavity 2 from 20°C to 700°C, respectively.
After verifying the high temperature resistance of the fabricated FPI, we firstly experimentally investigated the pressure sensing characteristic at 20°C. The pressure was increased stepwise over a range from 0 MPa to 4 MP at interval of 0.4 MPa. At each determined pressure, the reflection spectrum for the FPI was recorded by the optical sensing interrogator. For verifying the stability and the repeatability of the FPI, three cycle pressure tests were repeated for each measurement point. The relative shift of wavelength $\Delta \lambda$ of the cavity 1 and cavity 2 were filtered out as described previously. The pressure responses are displayed in Figs. 6(a) and 6(b), respectively. As expected, the relative shift of wavelength for cavity 1 is hardly dependent on the pressure change, and the maximum wavelength shift is less than 0.03 nm. Therefore, the cavity 1 is insensitive to pressure change. Meanwhile, the resonant wavelength of cavity 2 shift linearly toward a shorter wavelength with the increased pressure. The pressure sensitivity is estimated to be 5.57 nm/MPa. Therefore, our fabricated FPI could be developed as a promising high-pressure and high-temperature sensor.
Fig. 6. (a) and (b) Relative shifts of wavelength $\Delta \lambda$ for cavity 1 and cavity 2 in the fabricated FPI as a function of pressure at 20°C, respectively.
Following that, the pressure sensing characteristic of the FPI at high temperature 300°C, 500°C, 600°C and 700°C were further investigated, respectively. As shown in Figs. 7(a)–7(d), linear fitting curves are added to show the linearity of the measurement results of cavity 2 at each tested temperature. The slope of the linear approximation curve obtained from data fitting shows the corresponding measured pressure sensitivity and the linearity R2 for the data. The relative shifts of wavelength responses of the tests have great linearity of R2 > 0.99 and they are highly repeatable. Furthermore, it can be observed that the pressure sensitivity decreases with increasing temperature from 20°C to 700°C. According to Eq. (5), the pressure sensitivity is inversely proportional to the Young’s modulus of the silica capillary. As displayed in Fig. 7(e), the Young’s modulus of the silica capillary increases and the pressure sensitivity decreases with the increased temperature from 20°C to 700°C. However, the pressure sensitivity decreases with an apparent fluctuation with non-perfect linearity. The reason mainly attributes to the temperature-dependent Young’s modulus of the silica capillary, which has been reported that the Young’s modulus of the silica capillary will increase non-linearly as the temperature increases up to 700°C [33–35]. The non-linear decrease of the pressure sensitivity caused by the increased temperature would lead to the increase of the testing error, which will limit its upper operation temperature [36].
Fig. 7. Relative shifts of wavelength $\Delta \lambda$ for cavity 2 in the fabricated FPI for three cycles at (a) 300°C, (b) 500°C, (c) 600°C and (d) 700°C, respectively; (e) the pressure sensitivity of the fabricated FPI and Young’s modulus of the silica capillary at different temperatures.
From the aforementioned results, simultaneous measurement of high-pressure and high-temperature can be realized by measuring the FPI sensor relative shifts of wavelength. Therefore the cross-sensitivity between pressure and temperature can be constructed as [7]
Since cavity 1 and cavity 2 are almost insensitive to pressure and temperature changes, respectively, the corresponding pressure coefficient ${K_{1P}}$ and temperature coefficient ${K_{2T}}$ are approximately 0. Taking the experimental results of the FPI sensor at 700°C as an example, the Eq. (7) can be updated with the known pressure and temperature sensitivities as
In addition, for higher pressure measurement accuracy, the pressure measurement sensitivity of the FPI cavity 2 can be enhanced by use of the increased length between the two welding points, reducing the thickness ring cladding of the silica capillary or using pure silica capillary with less impurities, so as to reduce the temperature-pressure cross-impact.
5. Conclusion
A compound FPI has been proposed and experimentally fabricated from SDF and silica capillary by using a CO2-laser processing system. The SDF we used owns high alumina dopant concentration core and can form a mullite crystallization region during an arc discharge process. The FPI is composed of two cavities, of which the section of the SDF acts as cavity 1 for high-temperature sensing and the air cavity constructed by silica capillary acts as cavity 2 for high-pressure sensing. Pressures over a range of 0 MPa-4 MPa and temperatures over a range of 20°C to 700°C are determined simultaneously. Experimental results show that the wavelength shift of the FPI versus the applied pressure is linear at each tested temperature. The proposed FPI sensor has a high pressure sensitivity of 5.19 nm/MPa at high temperature 700°C, and the linear response shows excellent repeatability with linearity of 0.999. Meanwhile, the proposed FPI can stably work at temperature as high as 700°C with a temperature sensitivity of 0.013 nm/°C. In addition, such FPI pressure sensor owns high sensitivity, stability in a wide measurement range and high temperature resistance, which could be a potential sensor for simultaneous measurement of high pressure and high temperature in extreme conditions.
Funding
National Natural Science Foundation of China (61735009, 61635006, 61975108).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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