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Abstract
An all-sapphire fiber-optic extrinsic Fabry-Perot interferometric (EFPI) sensor for the simultaneous measurement of ultra-high temperature and high pressure is proposed and experimentally demonstrated. The sensor is fabricated based on all-sapphire, including a sapphire fiber, a sapphire capillary and a sapphire wafer. A femtosecond (fs) laser is employed to drill a through hole at the side wall of the sapphire capillary to allow gas flow. The sapphire fiber is inserted from one side of the sapphire capillary. The sapphire wafer is fixed at the other side of the sapphire capillary. The first Fabry-Perot (FP) cavity, composed of the end face of the sapphire fiber and the front surface of the sapphire wafer, is used for measuring pressure, while the second FP cavity, composed of the two surfaces of the sapphire wafer, is used for measuring temperature. Experimental results show that the sensor can simultaneously measure ultra-high temperature and gas pressure within the temperature range of 20 - 1400 °C and the pressure range of 0 - 5 MPa. The temperature sensitivity is 0.0033 µm/°C, and the pressure sensitivity decreases as the temperature increases, reaching 1.8016 µm/MPa and 0.3253 µm/MPa at temperatures of 20 °C and 1400 °C, respectively.
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1. Introduction
With the development of the aerospace technology, the simultaneous measurement of temperature and pressure is urgently demanded in ultra-high temperature environments. For example, in-situ monitoring of the temperature and pressure on the surface of the aircraft has the important reference value for evaluating the working state of the aircraft. In addition, monitoring the pressure at the tail of the engine can evaluate combustion efficiency and health state of the engine. However, when the aircraft is flying at high speed, temperatures of the surface and the tail of the engine exceed 1200 °C, which poses a great challenge to measure related physical parameters.
Fiber-optic sensors have characteristics of anti-electromagnetic, intrinsic safety, light weight and high temperature resistance, making them able to measure physical quantities in harsh environments [1–4]. Several types of silica fiber-optic sensors have been developed to simultaneously measure temperature and pressure. A typical scheme is to cascade a fiber Bragg grating (FBG) and an FPI to form FBG/FPI hybrid pressure and temperature sensors [5–7]. Limited by the working temperature of the FBG, the FBG/FPI structure exhibits poor thermal stability when the temperature exceeds 300 °C. To increase the working temperature, another scheme is to cascade two FPIs to form an FPI/FPI structure. Zhang et al. [8] proposed a dual-cavity FP interferometer sensor based on a hollow-core photonic bandgap fiber spliced with a hollow-core fiber, which realized simultaneous measurement of the pressure range of 0 - 10 MPa and the temperature range of 100 - 800 °C. However, limited by the thermal diffusion of doping elements and softening point, silica FPI/FPI structures are difficult to work in environments above 1000 °C [9–12]. In order to simultaneously measure ultra-high temperature and high pressure, a material with stable performance at ultra-high temperature is needed.
With a melting point of 2045 °C and a wide transmission spectral range, the sapphire is an ideal material for developing ultra-high temperature fiber-optic sensors. Currently, sapphire-based fiber-optic sensors have been used to measure physical parameters such as temperature, pressure, and strain at ultra-high temperature [13–15]. For the pressure measurement, sapphire fiber-optic pressure sensors are mainly divided into two types: diaphragm-based type and diaphragm-free type. A sealed sapphire FP cavity is required to measure pressure for the diaphragm-based pressure sensor. Yi et al. [16–18] proposed a FP sensing cavity constructed by combining reactive ion etching with direct wafer bonding, which was verified to measure pressure at 1000 °C. Shao et al. [19,20] proposed an all-sapphire-based EFPI sensor head fabricated by wet etching and sapphire direct bonding process. The surface roughness of the sapphire pressure sensitive diaphragm reached 0.39 nm after etching and the sensor could work at 1200 °C. However, the high pressure may cause the pressure-sensitive diaphragm to crack, limiting the measurement range of the pressure. In addition, the fabrication process is relatively complex and time-consuming. The principle of the diaphragm-free pressure sensor is that the refractive index of the gas changes linearly with the applied pressure. Since there is no pressure difference between the inside and outside of the pressure-sensitive cavity, the high pressure will not destroy the sensor, making the measurement range of the pressure theoretically unrestricted. In 2022, we proposed a diaphragm-free sapphire fiber-optic pressure sensor composed of two sapphire fiber ferrules and a sapphire tube, which can measure gas pressure at 1400 °C [21]. Since the cavity length and the pressure sensitivity of the sensor are both temperature dependent, an external temperature sensor is required to measure ambient temperature. For the simultaneous measurement of temperature and pressure at ultra-high temperature, an all-sapphire sensor head based on dual-cavity is needed.
In this paper, we demonstrate an all-sapphire fiber-optic dual-cavity EFPI sensor for the simultaneous measurement of temperature and pressure. A gas flowing FP cavity for measuring pressure is obtained between the end face of the sapphire fiber and the surface of the sapphire wafer by using a fs laser to drill a through hole at the side wall of the sapphire capillary. The FP cavity for measuring temperature is composed of two surfaces of the sapphire wafer. Due to the all-sapphire structure, the proposed sensor can simultaneously measure temperature and gas pressure within the temperature range of 20 - 1400 °C and the pressure range of 0 - 5 MPa. The measurement range of the pressure is mainly limited by the experimental device. It can be expected that the proposed sensor can measure pressure much higher than 5 MPa at ultra-high temperature. As far as we know, it is the first time to measure the temperature of 1400 °C and the pressure of 5 MPa simultaneously.
2. Sensor fabrication and principle
2.1 Sensor fabrication
The schematic diagram of the proposed sensor is shown in Fig. 1(a). The sensor head is composed of all-sapphire material, ensuring the high temperature resistance performance. The sapphire fiber is produced by MicroMaterials Inc in the United States. The core diameter and the length are 100 µm and 20 cm, respectively. The inner diameter and the outer diameter of the sapphire capillary are 106 µm and 1 mm, respectively, as shown in Fig. 1(b). The aperture of the sapphire capillary is very close to the core diameter of the sapphire fiber, making the end face of the sapphire fiber parallel to the surface of the sapphire wafer to reduce the excitation of high-order modes of the sapphire fiber and improve the quality of the interference signal. The sapphire wafer is double-sided polished with a thickness of 100 µm. Firstly, a fs laser is employed to drill a through hole at the side wall of the sapphire capillary. The center wavelength, the repetition rate and the pulse duration of the fs laser are 800 nm, 1 kHz, and 35 fs, respectively. The laser pulse power is attenuated to 5 mW through a neutral density filter. The sapphire capillary is fixed on a six-dimensional micro-motion platform. The laser beam is focused perpendicularly on the surface of the sapphire capillary by an objective lens with an amplification factor of 20× and a numerical aperture of 0.45. By the line-by-line scanning method (X direction: length 500 µm, Y direction: step size 20 µm, length 300 µm, Z direction: step size 30 µm, length 500 µm), the fs laser scans the surface repeatedly to penetrate the side wall of the sapphire capillary, allowing the gas to flow into the sapphire capillary. During laser scanning, a blower is used to purge the debris. The micrograph of the through hole (Size: 500 µm × 300 µm × 500 µm) is shown in Fig. 1(c). The through hole is located at the center of the sapphire capillary rather than above the gas cavity, which can reduce the contact between reflective surfaces (R1, R2) and the external environment to keep reflective surfaces clean. The outer surface of the sapphire wafer can be wiped clean with the dust-free paper. Secondly, two end faces of the sapphire fiber are polished by the grinding paper. The sapphire fiber is fixed vertically on the fiber grinding machine. The end face is flat after grinding for 20 min. Finally, the sensor is packaged. The sapphire fiber is inserted into the sapphire capillary. The distance between the end face of the sapphire fiber and the surface of the sapphire wafer is adjusted to 600 - 700 µm. The sapphire wafer and the sapphire fiber are fixed by the high temperature resistant adhesive (SINWE-S522, main components: silicate, copper powder, alumina powder, maximum operating temperature: 1730 °C). To solidify the adhesive, the sensor head is placed in a muffle furnace and maintained at 80 °C and 150 °C for 2 h, respectively. The photograph of the sensor is shown in Fig. 1(d). An encapsulation of the sensor is shown in Fig. 1(e). The encapsulation will be designed according to different test interfaces and requirements.
Fig. 1. (a) Schematic diagram of the sensor. (b) Side view micrograph of the sapphire capillary. (c) Micrograph of the through hole. (d) Photograph of the sensor. (e) Encapsulation of the sensor.
2.2 Principle
As shown in Fig. 1(a), the sensor head consists of three reflective surfaces, including the end face of the sapphire fiber and two surfaces of the sapphire wafer. The interference spectrum formed by three reflected beams can be expressed as:
Due to the thermo-optic effect and the thermal expansion effect, the OCL of FPI2 is temperature-dependent. The temperature sensitivity with the OCL2 is:
The OCL of FPI1 is related to both temperature T and pressure P, which can be expressed as [21]:
The increase of the cavity length can improve the pressure sensitivity of the sensor, thereby improving the resolution of the sensor. However, when the cavity length is too long, the contrast of the interference signal will be reduced due to the loss of light. As mentioned above, after testing, the cavity length in the range of 600 - 700µm is appropriate. According to Eq. (5), the pressure can be expressed as:
3. Experiment and discussion
3.1 Test system
The pressure test system at ultra-high temperature is shown in Fig. 3. The sapphire fiber is fusing spliced to the multi-mode fiber (MMF) with a core diameter of 62.5µm and a cladding diameter of 125 µm and then the interference signal is picked up by the single-mode fiber (SMF) and transmitted to the homemade white-light interference demodulator [22]. The sensor is placed in the constant temperature zone of the muffle furnace (KSL-1700X, temperature fluctuation in the constant temperature zone: ± 1 °C). The ceramic tube with the sensor is connected to the gas chamber through a stainless steel tube. The pressure is controlled by a compressed nitrogen cylinder. A pressure meter with a resolution of 0.01 MPa is used to calibrate the pressure.
3.2 Interference signal demodulation
The schematic diagram of the demodulator is shown in Fig. 4. A tunable wavelength scanning optical fiber laser with a wavelength range of 1510 - 1590 nm is used as the light source. The output wavelength is adjusted by a sawtooth wave generator. The incident light is divided into two beams by coupler 1. One beam is detected by a photodiode (PD1). An etalon and a FBG are used to calibrate the wavelength of the spectrum. The other beam is injected into the sensor via coupler 2, and the interference signal is detected by a photodiode (PD2). Two electrical signals are collected by A/D acquisition card and transmitted to a personal computer (PC).
The interference spectrum of the sensor collected by the homemade white-light interference demodulator with a center wavelength of 1550 nm is shown in Fig. 5(a). The reflected spectrum is formed by the three-beam interference. Due to multimode interference, clear interference fringes appear in the wavelength range of 1517 - 1563 nm. The intercepted spectrum is shown in Fig. 5(b). The fast Fourier transform is performed on the intercepted spectrum, and the frequency spectrum is shown in Fig. 5(c). The first peak corresponds to the frequency of the interference signal formed by FPI2. The second peak corresponds to the frequency of the interference signal formed by FPI1. As mentioned above, the pressure and the temperature are measured by monitoring the OCL of FPI1 and FPI2, respectively. Two bandpass filters are used to extract main frequencies of two FPIs. Interference spectrums can be recovered by the inverse fast Fourier transform, as shown in Fig. 5(d) and Fig. 5(e). Finally, the OCL of two FPIs are interrogated by the peak-to-peak method [22,23], which can be expressed as:
Fig. 5. (a) Interference spectrum of the sensor. (b) Intercepted spectrum. (c) Frequency spectrum (black line) and bandpass filters (red line and blue line). (d) Recovered spectrum of FPI1. (e) Recovered spectrum of FPI2.
3.3 Sensor test
Firstly, the pressure response of the sensor at the room temperature is investigated. The gas pressure is increased from 0 MPa to 5 MPa with a step of 1 MPa. Spectrums at three pressures are shown in Fig. 6. There is no obvious change in fringe contrast. The OCL of FPI1 is recorded after stabilization at each pressure point. The pressure response is shown in Fig. 7. The OCL1 at 0 MPa is 659.6313 µm and increases linearly in the pressure range of 0 - 5 MPa. The pressure sensitivity at the room temperature is 1.8016 µm/MPa. When L = 659.6313 µm, T = 20 °C, P = 0 MPa, the theoretical value of the pressure sensitivity calculated by Eq. (5) is 1.7697 µm/MPa. It can be seen that the actual test result is close to the theoretical value, proving the feasibility of the entire test system.
Then, the performance of the sensor at high temperatures is investigated. The temperature is increased from 20 °C to 1400 °C with a step of 200 °C. Spectrums at high temperatures are shown in Fig. 8. The fringe visibility decreases slightly with the increase of temperature, which should be caused by the increase of the transmission loss of the optical fiber at high temperature.
During the heating process, the OCLs of FPI1 and FPI2 are recorded in real time. After each temperature point is kept for 30 min, the pressure response of FPI1 is measured. The cavity length of FPI1 (L1(T)) increases with the increasing temperature, and the temperature response is shown in Fig. 9(a). The temperature sensitivity of FPI1 is about 0.0115 µm/°C. The temperature cross-sensitivity of FPI1 is computed to be 6.38 kPa/°C. The pressure response within the temperature range of 20 - 1400 °C is shown in Fig. 9(b). The initial OCL is normalized. All fitting curves exhibit high linearity (R2>0.999). The pressure sensitivity decreases with increasing temperature. Figure 9(c) shows that measured pressure sensitivities are close to theoretical values calculated by the Eq. (5). As expected, the OCL of the gas cavity is affected by both temperature and pressure, and the temperature cross-sensitivity increases with the increase of temperature. Temperature compensation is required when measuring pressure.
Fig. 9. (a) Temperature response of the gas cavity. (b) Pressure response of the gas cavity. (c) Relationship between the pressure sensitivity and temperature. (d) Temperature response of the sapphire wafer.
The temperature response of the sapphire wafer is shown in Fig. 9(d). The measured temperature sensitivity is 0.0033 µm/°C. When ns = 1.7505, αn = 7.5 × 10−6 /°C [24], αd = 8.1 × 10−6 /°C [25], according to Eq. (2), the theoretical value of the temperature sensitivity is 0.0022 µm/°C. The measured sensitivity is slightly larger, which may be caused by the increase of the thermo-optic coefficient and the thermal expansion coefficient at high temperatures. The quadratic fitting has a higher fitting degree. Then, the temperature can be calculated by:
After obtaining the temperature, △OCL1(T) can be calculated by the temperature response of FPI1. When the temperature and the pressure are changed, △OCL1(P, T) is monitored by the demodulator, and the pressure can be calculated by Eq. (6).
Based on the above calibration, the sensor is tested within the temperature range of 20 - 1400 °C and the pressure range of 0 - 5 MPa. The comparison between the measured temperature calculated by Eq. (9) and the applied temperature is shown in Fig. 10. The measured temperature is in excellent agreement with the applied temperature. In the whole temperature range, the maximum error is 12 °C (Applied temperature: 1400°C, Measured temperature: 1388 °C). In the process of temperature increase and temperature decrease, the maximum deviation of test points is 11 °C. The pressure test and zero drift within the temperature of 20 - 1400 °C are shown in Fig. 11(a) and Fig. 11(b), respectively. The influence of the temperature on the pressure measurement is effectively removed, and the maximum error is 0.17 MPa (At 1200 °C: Applied pressure: 4.06 MPa, Measured pressure: 3.89 MPa). At 1400 °C, the maximum deviation of test points is 0.09 MPa during pressure increase and pressure decrease.
Finally, the stability of the sensor is tested. When the applied temperature and pressure are 1400 °C and 3.05 MPa, respectively, measurement results of the sensor are shown in Fig. 12. The fluctuation of measured temperatures is caused by the temperature fluctuation in the constant temperature zone of the muffle furnace. Due to the slight leakage of the pressure test system, the measured pressure decreases slightly with time. The sensor can stably measure temperature and pressure at high temperatures.
4. Conclusion
In conclusion, an all-sapphire fiber-optic sensor for the simultaneous measurement of ultra-high temperature and high pressure is demonstrated. The sensor adopts the structure of cascaded FPIs. By employing the fs laser to drill a through hole at the side wall of the sapphire capillary, a gas flowing FPI is fabricated for measuring pressure. Another FPI composed of two surfaces of the sapphire wafer is used for measuring temperature. The sensor realizes the temperature measurement in the range of 20 - 1400 °C and the pressure measurement in the range of 0 - 5 MPa. Maximum measurement errors are 12 °C and 0.17 MPa, respectively. Based on the all-sapphire structure, the sensor is suitable for the simultaneous measurement of temperature and pressure in harsh environments.
Funding
National Natural Science Foundation of China (U20B2057).
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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