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Abstract
In this paper, we proposed an all-sapphire-based extrinsic Fabry-Perot interferometer (EFPI) sensor based on wet etching and the direct bonding process. Temperature measured by the EFPI is used to calibrate pressure measurement. The problem of repeatable measurement of dynamic pressure in a harsh environment is solved. The EFPI sensor can be applied in the temperature range of 25°C to 800°C and the pressure range environment of 0MPa to 5MPa. The pressure sensitivity of 355.8nm/MPa and the temperature sensitivity of 1.64nm/°C are obtained by a cross-correlation function (CCF) algorithm to interrogate the optical sensing system. Therefore, the proposed sensor has a great potential for pressure monitoring, such as jet engines, industrial gas turbine, and so on due to its 8×8mm size and compact structure.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
High temperature and pressure sensors are of great significance for the monitoring of the dynamic pressures of jet engines, coal gasifier furnaces [1], industrial gas turbine [2], turbine engine [3,4] and nuclear reactor [5]. This kind sensing is a key component of process control and optimization in harsh environment [1]. Extrinsic Fabry-Perot interferometer (EFPI) sensors have attracted considerable attention. It is a preferred scheme of high-temperature optical fiber pressure sensor due to high temperature resistance and low temperature cross-sensitivity, etc [6].
The optical material of the sensor, the way of EFPI fabrication and the demodulation method have a great impact on large range pressure measurement at high temperature environments. They are always the research difficulties and hotspots of EFPI applied for harsh environments. Qingchao Zhao et al. [7] reported that they used two section single mode fibers and a silver film coated quartz capillary bonded an EFPI structure, achieved that the pressure measurement range is 0MPa – 42MPa, the temperature tolerance range is only 26°C – 150°C. Xue Wang et al. [8] reported that they used SOI wafer and Pyrex glass wafer, bonding with Au-Au thermal-compression technique, to fabricate a sealed F-P cavity structure. They can obtain the pressure measurement range of 3KPa – 283KPa, and temperature measurement range is just from -20°C to 70°C. Wangwang Li et al. [9] reported that an all-sapphire EFPI fabricated by direct bonding in a temperature of 1000°C with a compress of 4MPa for 2h, the pressure and temperature measurement ranges of 20KPa – 700KPa and 25°C – 800°C can be obtained, respectively. Due to the amorphous structure of quartz capillary, the good ductility of gold and the insufficient sapphire chips bonding time, they are difficult to maintain the stable EFPI structure in high temperature and pressure environment for a long time. Sapphire is an extremely hard, corrosion resistant crystal with a melting point of over 2000°C [1,10]. The combination of high hardness and optical properties is ideal for scratch resistant windows and precision mechanical elements that undergo high wear [4]. Appropriate bonding temperature and time will make all-sapphire EFPI show good mechanical and optical properties in high pressure environment over 500°C for a long time.
In this paper, we present an all-sapphire-based EFPI sensor head fabricated by wet etching and sapphire direct bonding process. Experimental results show that the EFPI sensor has stable performance at the range from 25°C to 800°C and 0MPa to 5MPa. Meanwhile, it can measure temperature and pressure in real time and has an excellent repeatability. The pressure and temperature sensitivity are 355.8nm/MPa and 1.64nm/°C respectively. Therefore, the all-sapphire EFPI sensor is expected to be used in the harsh temperature environment for engineering applications.
2. Design and principle of operation
Figure 1 shows that schematic diagram of multi cavity EFPI sensor structure with fiber optic connector. The sensor structure contains sapphire diaphragm, vacuum cavity and basal cavity. Fiber optic connector is a sapphire cylinder with a 2.52mm thorough-hole. The vacuum cavity is obtained by wet etching process. The EFPI structure is formed by multiple parallel optical reflecting surfaces. The vacuum cavity and basal cavity are sensitive to external pressure and temperature, respectively.
When a uniform pressure is imposed on the sensor, the front diaphragm of the sensor will be deformed accordingly and will cause the length of the vacuum cavity to change. The thickness variation of the vacuum cavity, ΔLv, in the axial center is linearly related to the change of the imposed pressure, and can be expressed as,
where v is the Poisson’s ratio, Δp is the change of the imposed pressure, E is the elastic modulus, R and d1 are the effective radius and thickness of the diaphragm, respectively. Based on the linear relationship between ΔLv and Δp, the external pressure can be measured by detecting the change of d2.The vacuum cavity we designed is formed in high temperature and vacuum environment. It is difficult for a few gas molecules in the vacuum cavity to transfer the external pressure to the basal cavity surface. The crosstalk of external pressure on the basal cavity is minor and negligible [10], thus the basal cavity can only applied to environmental temperature measurement. When the environmental temperature is changed, the optical path difference (OPD) of the basal cavity and the sapphire diaphragm will be changed because of the thermal expansion effect and thermo-optic effect. The OPD of these two cavities increase when the temperature increases, can be expressed as,
where Ls is the length of basal cavity, ns is the refractive index of sapphire, αT is the coefficient of thermal expansion, dns/dT is the coefficient of thermo-optic and ΔT is the value of the temperature variation.3. All-sapphire EFPI fabrication
C-plane (0001) sapphire wafer as the raw material for the EFPI sensor head, it has one direction along the optical axis in which birefringence does not occur [11]. Meanwhile, C-plane (0001) sapphire wafer is easy to be etched into a flat optical plane which meets Fresnel reflection.
3.1 Wet etching process
The square vacuum cavity is fabricated by wet etching in this section. Figure 2 shows the schematic diagram of process flow for sapphire wafer fabrication by wet etching. A double-sided polished 2 inch sapphire wafer with thickness of 210µm and root-mean-square roughness of ∼0.33nm is employed. Firstly, 20µm SiO2 and 4µm photoresist are deposited on sapphire wafer in turn. Secondly, a 2×2mm square UV light spot is designed by a mask and ultraviolet light and used to evaporate a 2×2mm square photoresist and 5% hydrofluoric acid is used to corrode SiO2, exposing 2×2 mm sapphire surface. Then, the part of the sapphire wafer layer is etched by concentrated sulfuric acid (H2SO4, 98%) and phosphoric acid (H3PO4, 97%) solution with the concentration ratio of 3:1 at 280°C for 30 minutes. After that, the excess photoresist and SiO2 removed from the etched sapphire wafer surface, we get a sapphire wafer with many etched square holes. The etched square hole with a depth of 100µm and root-mean-square roughness of ∼0.39 nm is obtained.
Figure 3(a) shows that a part of sapphire wafer layer after wet etching process. There are no obvious defects on the etched sapphire wafer and the etched cavity is very smooth. The wet etching process can avoid the risk that the vacuum cavity surfaces with visible defects will damaged under high temperature and pressure environment, provides a guarantee for direct bonding process.
Figure 3(b) shows that the cross sectional scanning electron microscope (SEM) image of an etched cavity. There is no obvious micro hole on the etched surface and cross section because gas bubbles produced by chemical reaction leaving the surface in time. The etched intersecting line is approximately a straight line, which indicates the proper ratio of etching solution is beneficial to the fabrication of low roughness etched sapphire surface and the rate of chemical reaction is uniform.
3.2 Direct bonding process
The etched sapphire wafer is cut into 8×8mm square chips by laser in order to make vacuum cavity and sapphire diaphragm of EFPI. Firstly, a sapphire column, a 8×8mm etched sapphire chip and a 8×8mm sapphire chip with 280µm thickness, are immersed in H3PO4 (85%) solution and H2SO4 solution (1mol/L) for 10 minutes in turn, and washed with deionized water for 3 h. Secondly, the three sapphire parts are axially symmetrical and placed in the bonding machine in 1250°C with vacuum of 1×10−3Pa and compressed under a 10Kg for 50 h. Figure 4 shows the EFPI cross-section of direct bonding interface SEM image. The direct bonding interfaces form a strong and compact adhesion due to atomic diffusion [12]. A large number of hydroxyl (-OH) groups on the sapphire surfaces are break, and Al-O bonds of sapphire surfaces are recombined. With the compress of 10Kg, bonding surfaces contact more fully, and all-sapphire EFPI with a sealed vacuum cavity is formed.
4. Experiments and results
In order to research the properties of the all-sapphire EFPI sensor, temperature measurement experiment, pressure measurement at gradual change temperature experiment are conducted. Figure 5 shows the schematic of the high-temperature pressure sensing system. This optical system is composed of broad band source (BBS) with the central wavelength of 1550nm and 40-nm-bandwidth, optical circulator, micro-optical spectrum analyzer (micro-OSA, Ocean Optics, NanoQuest) with 1.25nm maximum resolution, high temperature tube furnace with the maximum working temperature of 1700°C and the precision of ±15°C, metal tube, modular precision pressure controller (Mensor, CPC 6000) with the resolution of ±1Pa, nitrogen gas cylinder and personal computer (PC). The EFPI sensor head is sealed in the metal tube by brazing technology, the modular precision pressure controller can deliver stable and accurate controlled pressure for metal tube, the nitrogen cylinders is used for supply pressure and high temperature tube furnace provides different temperature variables for the EFPI sensor head.
The broadband light emitted from the BBS, then incident into port 1 of the fiber circulator and coupled into the EFPI multi cavity. After the multi-beam reflected from the sensor head and the lights interfere, they are transmitted to the micro-OSA from 3# of the fiber circulator and the spectral data is transmitted to the PC through the USB interface for data analysis to realize the demodulation of the pressure and temperature.
Figure 6 shows the multi cavity EFPI sensor interference spectrum at room temperature and atmospheric pressure. When the light is injected into the EFPI from single mode fiber tail, each surface of EFPI produce reflected light intensity as I1, I2, I3 and I4 shown in Fig. 1. Therefore, the spectrum information collected by the spectrometer is multi-beam interference spectrum that can be expressed as,
The reflectivity of sapphire surface is close to 7%, reflected lights generated from the EFPI form multiple low coherence interference superposition. In order to obtain pressure and temperature information from interference spectrum, the cross-correlation function (CCF) can demodulate any two interference optical signals from whole optical spectrum [13], the principle of the CCF is to construct a large number of interference signals corresponding to different cavity lengths to match the collected interference spectrum from micro-OSA, and search out the target cavity length values. Figure 7 shows the CCF calculated of the return spectrum at around 25°C. We calculated two wide ranges of OPDs, revealing three cavities of interest as a peak in the CCF. Figure 6(a) relates to the vacuum cavity peak and the center OPD of the vacuum cavity is 103µm. However, the side walls of the vacuum cavity are not perpendicular to the F-P cavity surfaces, leading to the two fake cavities with similar peak value and profile are formed near the vacuum cavity. As for the CCF, this situation is difficult to locate the OPD of vacuum cavity. Figure 6(b) relates to the basal cavity and the compound cavity, and the compound cavity is formed by the vacuum and basal cavity. The center OPD of basal cavity and compound cavity are 498.08µm and 600.35µm respectively. The contrast of the basal cavity and the compound cavity are much higher, the CCF values of those two cavities are easier to collect than those of the vacuum cavity. Therefore, the OPD peak of basal cavity and compound cavity are reliable and precise. The particular sensor element that is built and tested has a sapphire diaphragm thickness d1 of 107µm, a vacuum cavity thickness d2 of 103µm, a sapphire basal cavity thickness d3 of 283µm.
As for the temperature experiment, the EFPI sensor head be put into the high temperature tube furnace and heated from 25°C to 800°C with a step of 100°C and every temperature step keeps for 60 minutes. Figure 8 shows the relationship between OPD of basal cavity, sapphire diaphragm cavity, compound cavity and temperature under atmospheric pressure. As the temperature increasing, the OPD of basal cavity and sapphire diaphragm showed a good linear relationship with the increase of temperature. The experiment result shows temperature sensitivity of sapphire diaphragm and basal cavity are 0.583 nm/°C and 1.64 nm/°C respectively, which is close to the theoretical value of 0.62 nm/°C and 1.86 nm/°C.
Fig. 8. The relationship between OPD of basal cavity, sapphire diaphragm cavity, compound cavity and temperature under atmospheric pressure.
Besides, the temperature sensitivity of the basal cavity is almost 3.5 times of that of the sapphire diaphragm, and the basal cavity can only be used for temperature measurement and not affected by external pressure. The OPD of compound cavity showed a quadratic curve with the increasing of temperature. The quadratic curve indicated that the vacuum cavity still contains a small amount of air after direct bonding process, and the effective refractive index of the vacuum cavity is not constant in the wide range of fluctuating temperature environment.
As for the EFPI pressure measurement, Fig. 9(a) shows the relationship between the OPD of compound cavity and pressure with temperature increasing from 25°C to 800°C. The temperature rises and falls in the environment of 25°C – 800°C with a step of 100°C and each temperature step keeps for 60 minutes. In each temperature step, the pressure increases from 0 MPa – 5 MPa with a step of 0.5 MPa and each pressure step keeps for 1minute. With the increase of pressure, the sapphire diaphragm gradually deforms to the EFPI inside, and the length of vacuum cavity decreases gradually. In the same temperature, the relationship between pressure and compound cavity OPD is linear. Meanwhile, with the increase of temperature, the distance between each Pressure-OPD line increases gradually because of the quadratic function relationship between vacuum cavity OPD and temperature. Figure 9(b) shows the relationship between the OPD of compound cavity and pressure with the temperature increasing and decreasing. As the temperature increases, the sensitivity of pressure increases gradually due to the decrease of Young’s modulus. Those fitting lines are completely coincided at the same temperature, the all-sapphire EFPI sensor shows a stable sealed vacuum cavity and an excellent pressure measurement repeatability. The mean pressure sensitivity is 355.8 nm/MPa according to Fig. 9(b), which is close to the mean theoretical value of 380.3 nm/MPa. This sensor for pressure measurement can be calibrated by measured temperature. That is, when the temperature and compound cavity OPD are known, the pressure value is also determined.
Fig. 9. The relationship between the OPD of compound cavity and pressure with the temperature (a) increasing from 25°C to 800°C, (b) increasing and decreasing between 25°C and 800°C.
5. Conclusion
In conclusion, the proposed all-sapphire EFPI sensor has a significant advantage of excellent pressure and temperature measurement repeatability. The EFPI sensor head with an initial sapphire diaphragm length of 107µm, a vacuum cavity length of 103µm and basal cavity length of 283µm is tested. The experiments results show the temperature sensitivity is 1.64nm/°C, the mean pressure sensitivity is 355.8nm/MPa under 0MPa – 5MPa and 25°C – 800°C environment. Another merit of the proposed sensor is its compact size of merely 8×8mm. As a consequence, the EFPI sensor head with is expected to have applications in high temperature harsh environments including land-based gas turbines, aero-engines, oil and gas exploration and process industries.
Funding
National High-tech Research and Development Program.
Disclosures
The authors declare no conflicts of interest.
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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