Dynamic pressure measurements under high-temperature and other harsh environments,such as the pressure monitoring in aerospace engines[
Acta Photonica Sinica, Volume. 51, Issue 6, 0606005(2022)
Silica-MEMS-based Fiber-optic Fabry-Perot Pressure Sensor for High-temperature Applications
Dynamic pressure measurements under high-temperature and other harsh environments, such as the pressure monitoring in aerospace engines, on-line health monitoring and control of molten salt reactors and gas-cooled reactors in nuclear applications, in-cylinder pressure monitoring in the automotive internal combustion engines, have a wide range of application requirements. The sensors used for dynamic pressure measurement include electronic pressure sensors and fiber-optics sensors. Among them, electronic sensors are highly dependent on temperature or close proximity electronics, limiting their high-temperature capabilities. In order to effectively protect electronic pressure sensors used in harsh environments, the engineering solutions, such as impulse lines, have been used to isolate sensors sensitive to heat and corrosion. But the impulse lines can also dampen the pressure signals, which will make it difficult to achieve in situ dynamic pressure monitoring, and increase the likelihood of blockages or bubbles impacting pressure measurements. Compared with electronic pressure sensors, fiber-optic pressure sensors have attracted widespread attention due to their high-temperature resistance, high sensitivity, anti-electromagnetic interference, corrosion resistance, simple structure, and small size. At present, most of reported fiber-optic pressure sensors are used for static pressure measurement and various types of fiber-optic FP pressure sensors have been fabricated using the MEMS, chemical corrosion, arc-discharge, laser processing techniques. Among them, the pressure sensors fabricated by chemical corrosion, arc discharge and laser processing technology are usually produced in a single piece, which results in poor consistency between sensors. By contrast, the MEMS technique can be applied in mass production and the materials used to fabricate fiber-optic pressure sensors by MEMS technology mainly include silicon, Pyrex glass, sapphire. Due to the limitation of temperature resistance of the material itself, the pressure sensors made by silicon-glass bonding will have a lower operating temperature. Pressure sensors made of sapphire can withstand high temperatures, but adhesives are usually used to connect the sensitive head and the signal transmission fiber. And if different materials are used to fabricate the fiber-optic sensor or use adhesive to realize the connection between the sensor head and optical fiber, the mismatch of the Coefficients of Thermal Expansion (CTE) between different materials will easily reduce the sensor stability and result in large temperature cross-sensitivity in the high temperature environment. In this paper, we propose a MEMS-based all-silica fiber-optic Fabry-Perot dynamic pressure sensor used the silica wafer with ultralow CTE and softening point as high as about 1 750 ℃. The sensor heads are batch-fabricated with silica wafers using MEMS technique and three-layer silica direct bonding technology, which ensures consistency in the sensor heads and cost effectiveness and have the desired pressure measurement range and sensitivity by flexibly designing the related parameters. The all-silica adhesive-free integration between the sensor head, hollow silica tube and the optical fiber is achieved using CO2 laser fusion. The sensor exhibits an ultralow thermal drift (about 0.069 nm/℃) and good thermal stability owing to the low CTE of silica and the all-silica adhesive-free design, which can effectively avoid the sensor damage induced by the CTE mismatch of different materials at high temperatures and increase the lifetime of the sensor in high temperature environments. To investigate the high-temperature static pressure performance of the all-silica pressure sensor, a static test system was set up and the system includes a high temperature and pressure testing platform, a demodulator, and a personal computer. High-temperature static pressure experimental results show that the proposed all-silica fiber-optic pressure sensor can function under the temperature range from 23 to 800 ℃ with the nonlinearity of approximately 1.13% at 800 ℃ and exhibited a good linear response to pressure at high temperatures, and the pressure sensitivity at room temperature and 800 ℃ was 810.84 nm/MPa and 755.52 nm/MPa, respectively. At the same time, a dynamic test system was set up and the system includes the standard piezoelectric sensor, the sinusoidal pressure generator, and a personal computer. Room-temperature dynamic pressure experimental results show that the proposed all-silica fiber-optic pressure sensor can function under the 2 kHz dynamic pressure environment and exhibited good dynamic pressure response characteristics. Furthermore, the frequency response of the all-silica fiber-optic pressure sensor is in good agreement with the standard piezoelectric sensor. We believe that the proposed all-silica fiber-optic FP dynamic pressure sensor will find broader and more promising applications in dynamic pressure measurement fields at a high temperature and extreme environments due to its low cost, small size, batch-production, and ultralow temperature coefficient.
0 Introduction
Dynamic pressure measurements under high-temperature and other harsh environments,such as the pressure monitoring in aerospace engines[
Compared with electronic pressure sensors,fiber-optic pressure sensors have attracted widespread attention due to their high-temperature resistance,high sensitivity,anti-electromagnetic interference,corrosion resistance,simple structure,and small size[
In this paper,we propose a MEMS-based all-silica fiber-optic Fabry-Perot dynamic pressure sensor used the silica wafer with ultralow CTE and softening point as high as about 1 750 °C. The sensor heads are batch-fabricated with silica wafers using MEMS technique and three-layer silica direct bonding technology,which ensures consistency in the sensor heads and cost effectiveness and have the desired pressure measurement range and sensitivity by flexibly designing the related parameters. The all-silica adhesive-free integration between the sensor head and the optical fiber is achieved using CO2 laser fusion. The sensor exhibits an ultralow thermal drift and good thermal stability owing to the low CTE of silica and the all-silica adhesive-free design,which can effectively avoid the sensor damage induced by the CTE mismatch of different materials at high temperatures and increase the lifetime of the sensor in high temperature environments. Moreover,the proposed sensor was subjected to a static pressure measurement at a high temperature of 800 °C and a 2 kHz dynamic pressure test at a normal temperature to verify the performance of the sensor.
1 Sensor working principle
The proposed all-silica pressure sensor comprises a sensor head,Hollow Silica Tube(HST),and gold-coated Multimode Fiber(MMF),as shown in
Figure 1.All-silica fiber-optic FP pressure sensor
where
The silica diaphragm is used as the sensing element in the all-silica pressure sensor. Under the effect of external pressure,the round silica diaphragm undergoes deformation,leading to a change in the FP cavity length. According to the elastic deformation theory for round diaphragms,the center deflection of the diaphragm can be expressed as[
where
The frequency response of the pressure sensitive diaphragm is an important issue in dynamic pressure measurement. The diaphragm is defined as a free vibrating circular plate clamped rigidly at the edge then its natural frequency can be expressed as[
where
where
The sensitivity,pressure measurement range,and diaphragm natural frequency of the all-silica pressure sensor are determined by the effective radius and thickness of the silica diaphragm. The sensitivity of the sensor increases as increasing the effective radius and decreasing the thickness of the diaphragm. However,the large effective radius and small thickness of diaphragm could result in smaller pressure measurement range and frequency response of the sensor. It is,therefore,necessary to comprehensively consider the overall size,sensitivity,pressure measurement range,and frequency response when designing the sensor. We can obtain the desired sensor size,pressure measurement range,sensitivity,and frequency response of the all-silica dynamic pressure sensor by flexibly designing the diaphragm radius and thickness of diaphragm. The diaphragm effective diameter,thickness,theoretical pressure measurement range,sensitivity and frequency response of the proposed all-silica dynamic pressure sensor are 2.55 mm,200 μm,72 MPa,829.6 nm/MPa,and 337 kHz,respectively.
2 Sensor processing technology
The MEMS fabrication process of the FP cavity,as illustrated in
Figure 2.FP cavity MEMS etching process flow chart
Further,the silica wafer was put into the NLD-570 etching machine for batch etching of the FP cavity. Since there was no accurate silica etching rate,the cavity depth was measured with a step profiler after 10 minutes of etching,and it was found that the etching rate of silica was about 0.5 μm/min. Therefore,the etching time was set to 30 minutes,the diameter of the etching FP cavity was about 2.55 mm,and the depth was about 15 μm,as shown in
The all-silica fiber-optic FP dynamic pressure sensor comprises a silica sensor head,an HST,and a gold-coated MMF. Among them,the sensor head is composed of a silica sensitive diaphragm,a silica cavity,and a silica pedestal. The proposed sensor heads are batch-fabricated via the MEMS technique and the three-layer silica direct bonding technology. As illustrated in
Figure 3.Fabrication process of the all-silica pressure sensor based on the silica wafer
During the fabrication process,a double-side polished 2-inch silica wafer with a thickness of 2 mm and 0.2 mm are employed for the pedestal and diaphragm,respectively. First,the 2-inch silica wafer employed for the pedestal with through-hole arrays fabricated by the Computer Numerical Control(CNC)technology,the 2-inch silica wafer with the microcavity arrays etched by MEMS technique,the 2-inch silica wafer employed for the diaphragm were directly bonded by three-layer silica direct bonding technology(the bonding temperature was about 850°C,and the bonding pressure was about 6.5 MPa),as shown in
Finally,the sensor head is assembled with the HST and MMF by using the CO2 laser. The diameter and height of the all-silica sensor unit are 4.6 mm and 2.4 mm,respectively. The end of the gold-coated MMF was cleaved flat and inserted into the HST. The gold-coated MMF and HST were fused using CO2 laser welding technology. Similarly,the protruding structure and the HST were fused using the CO2 laser fusion splicer. Thus,the fabrication of the all-silica fiber-optic FP dynamic pressure sensor was completed,as shown in
3 Experimental results and discussions
To investigate the high-temperature static pressure performance of the all-silica pressure sensor,a test system was set up,as shown in
Figure 4.Experimental setup of high-temperature static pressure test
The all-silica FP pressure sensor was evaluated under temperatures ranging from room temperature(23 ℃)to 800 ℃,with increments of 100 ℃. At each temperature step,the all-silica pressure sensor was tested from approximately 0 MPa to 1 MPa,with increments of 200 kPa. When performing the pressure test,the pressure was kept constant for 2 min at each pressure point in order to record the cavity length more accurately.
Figure 5.High-temperature static pressure experimental results of the proposed all-silica fiber-optic pressure sensor
Meanwhile,it can be seen from
To investigate repeatability and stability,the all-silica fiber-optic pressure sensor was evaluated from approximately 0 MPa to 1 MPa at increments of 200 kPa with the temperature of 23 ℃,400 ℃,and 800 ℃,as shown in
Figure 6.Repeatability and stability experimental results of the proposed all-silica fiber-optic pressure sensor
To measure the dynamic characteristics of the all-silica fiber-optic FP dynamic pressure sensor,the room-temperature dynamic pressure test was carried out on the all-silica fiber-optic FP dynamic pressure sensor and the standard piezoelectric sensor(CYG401F,Kunshan Shuangqiao Sensor Measurement and Control Technology Co.,Ltd,China)at the same time on the sinusoidal pressure generator(LAISEN LS-ZX7M,Suzhou Dina Precision Equipment Co.,Ltd,China). The all-silica fiber-optic dynamic pressure sensor and the standard piezoelectric sensor were installed opposite to each other on the sinusoidal pressure generator,so that they had the same dynamic pressure environment,and the output dynamic pressure frequency of the sinusoidal pressure generator was set to be about 2 kHz. The dynamic pressure test result of standard piezoelectric sensor is shown in the blue waveform in
Figure 7.Room-temperature dynamic pressure experimental results of the proposed all-silica fiber-optic pressure sensor
4 Conclusion
In this paper,we have realized the high-consistency and batch-production of the all-silica fiber-optic FP dynamic pressure sensor heads with ultralow temperature coefficient using the MEMS technique and the three-layer silica direct bonding technology,which significantly reduced the variations in the sensor heads and the processing costs. The all-silica adhesive-free integration of sensor head and gold-coated MMF was realized by the CO2 laser,which improves the stability of the sensor and allows the sensor to have an ultralow temperature coefficient(about 0.069 nm/℃)in high temperature environments. High-temperature static pressure experimental results show that the proposed all-silica fiber-optic pressure sensor can function under the temperature range from 23 ℃ to 800 ℃ with the nonlinearity of approximately 1.13% at 800 ℃ and exhibited good linear response to pressure at high temperatures. Room-temperature dynamic pressure experimental results show that the proposed all-silica fiber-optic pressure sensor can function under the 2 kHz dynamic pressure environment and exhibited good dynamic pressure response characteristics. Furthermore,the frequency response of the all-silica fiber-optic pressure sensor is in good agreement with the standard piezoelectric sensor. We believe that the proposed all-silica fiber-optic FP dynamic pressure sensor will find broader and more promising applications in dynamic pressure measurement fields at high temperature and extreme environments due to its low cost,small size,batch-production,and ultralow temperature coefficient.
[26] D GIOVANNI. Flat and corrugated diaphragm design handbook(1982).
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Jiashun LI, Pinggang JIA, Jun WANG, Jia LIU, Guowen AN, Jijun XIONG. Silica-MEMS-based Fiber-optic Fabry-Perot Pressure Sensor for High-temperature Applications[J]. Acta Photonica Sinica, 2022, 51(6): 0606005
Category: Fiber Optics and Optical Communications
Received: Mar. 3, 2022
Accepted: Apr. 18, 2022
Published Online: Sep. 23, 2022
The Author Email: XIONG Jijun (xiongjijun@nuc.edu.cn)