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Abstract
A sapphire fiber high-temperature vibration sensor with an extrinsic Fabry-Perot interferometer (EFPI) structure is proposed and experimentally demonstrated. The vibrating diaphragm of the sensor is a supported beam structure fabricated by etching a single-side polished sapphire wafer using a femtosecond laser. The FP cavity of the sensor is composed of the sapphire fiber end face and the polished surface of the vibrating diaphragm. The interference signal of the sensor is picked up by the sapphire fiber and transmitted to a laser interferometry demodulator through a multimode fiber. Experimental results show that the acceleration response is linear in the range of 0-10 g along with an acceleration sensitivity of 20.91 nm/g. The resonance frequency of the sensor is 2700 Hz, which is consistent with the ANSYS simulation results. The sensor can also work in the temperature range from room temperature to 1500 ℃, providing a feasible method for vibration measurements in high-temperature environments.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
With the development of aerospace technology, the ability to perform vibration measurements in high-temperature environments is urgent. In particular, the vibration measurement of an aircraft engine is related to the engine’s performance and lifetime. Since the vibration generated by the engine is in a high-temperature environment, neither ordinary electrical sensors nor silicon-based optical sensors can be used to precisely measure the actual internal vibrations [1–5]. Optical fiber sensors with light weight, small size, high sensitivity, large dynamic range, and anti-electromagnetic interference are widely used in vibration measurement [6–9]. The existing optical fiber vibration sensors include intensity modulation, wavelength modulation, and phase modulation types. The intensity modulation type utilizes a vibration signal to modulate the transmission light coupling efficiency and thus realizes light intensity modulation to achieve vibration measurement. However, the output light intensity is easily affected by external conditions such as light source fluctuations and transmission fiber perturbation [10–12]. The wavelength scheme changes the reflective wavelength of a fiber Bragg grating (FBG) through fiber deformation, which then reflects the vibration signal. The sensitivity of the wavelength-modulated fiber-optic vibration sensor depends on the linewidth of the fiber grating, so high sensitivity and small size cannot coexist [13–15]. The phase modulation type is mainly the extrinsic Fabry-Perot interferometer (EFPI). Vibration causes a change in the FP cavity length, which in turn makes the interference signal contain vibration features [16–18]. EFPI sensors have received extensive attention and research in the field of vibration measurement due to their simple structure, easy production, and high resolution. A typical optical fiber FP vibration sensor is composed of silica fiber combined with a silicon-based microstructure. However, due to the low melting point of silica, the sensor cannot work in a high-temperature environment above 1000 °C. In addition, the mechanical properties of silicon begin to deteriorate at 600 °C, and silicon is prone to plastic deformation. Therefore, EFPI sensors made of silicon microstructures have difficulties working in temperatures above 600 °C [19–22]. As a high-temperature resistant material, sapphire mainly comprises alumina with a melting point of 2045 °C, and the light transmission range is 0.5 μm - 5.5 μm. It has suitable mechanical properties and has become a good choice for high-temperature sensing applications [23]. The sapphire optical fiber sensor can be adapted to the measurement of varieties of physical parameters in a high-temperature environment, such as temperature [24], pressure [25], strain [26], and vibration [27]. Currently, sapphire temperature sensors and pressure sensors have been investigated, whereas vibration sensors are rarely reported. Y. Huang et al. proposed a 6H-SiC sapphire fiber vibration sensor and conducted a test with a temperature range from room temperature to 1200 °C and an acceleration range of 0-5 g. The vibration acceleration sensitivity was 17.86 mV/g at 800 °C [27].
In this paper, a high-temperature sapphire optical fiber vibration sensor is proposed and experimentally demonstrated. The sensor is an EFPI composed of a sapphire vibrating diaphragm and a sapphire fiber. The vibrating diaphragm is a simply supported beam structure fabricated by a femtosecond laser. The vibration causes the length of the FP cavity to change. The interference signal is collected by the sapphire fiber and transmitted to a laser interferometric demodulator via a multimode fiber. A three-wavelength symmetrical demodulation algorithm is used to recover vibration signals. The sensor was tested from ambient temperature (25 °C) to ultrahigh temperature (1500 °C).
2. Sensor design and manufacture
The structure of the sapphire fiber vibration sensor is shown in Fig. 1. A 300 μm thick single-side polished sapphire wafer is cleaved into a simply supported beam structure by a femtosecond laser, acting as the vibrating diaphragm. First, the sapphire diaphragm is placed on the processed ceramic base, and make sure the center position of the vibrating diaphragm aligns to the center position of the ceramic base. Then, high temperature resistant inorganic glue is used to fix the two ends on the ceramic piece, as shown in Fig. 1(c). At the same time, the ceramic ferrule (outer diameter 2.5 mm) with the sapphire fiber (core diameter 75 μm) is fixed on the square ceramic plate (15 × 15 mm), and the high temperature resistant inorganic glue is used to fix it, as shown in Fig. 1(d). The main component of the high temperature resistant inorganic glue (SINWE S522) is inorganic aluminosilicate and solid-liquid mixing method for bonding with heat resistance temperature of 1730 ℃. The coefficient of linear expansion is similar to that of ceramics (8.0×10−6 /℃). The high temperature resistant inorganic glue was cured at room temperature for 12 hours, at 100 ℃ for 2 hours, and at 150 ℃ for 2 hours, and then cooled naturally. Finally, the ceramic plate is installed on the ceramic base and fixed by ceramic screws, as shown in Fig. 1(e). The sapphire fiber end face and the upper surface of the sapphire diaphragm form an FP cavity. When the incident light is injected into the sapphire fiber, the first reflection occurs on the end face of the sapphire fiber. Then the transmitted light passes through the air cavity and the second reflection occurs on the upper surface of the sapphire diaphragm. The two reflected light beams form double-beam interference. The interference signal is collected by the sapphire fiber and transmitted to the laser interferometric demodulator through the multimode fiber.
Fig. 1. Sapphire optical fiber vibration sensor (a) configuration, (b) physical layout, (c) ceramic base, (d) ceramic plate, (e) ceramic assembly
The cavity length of the EFPI was investigated. The interference spectrum was recorded by a white light interferometric demodulator with a wavelength ranging from 1525 to 1570 nm as shown in Fig. 2. The features of the interference signal exhibit not smooth, low contrast, and uneven amplitude, which are affected by the intermodal interference and mode coupling in the sapphire fiber. First, the high-frequency noise is filtered through low-pass filtering. Then, the frequency spectrum of this interference signal is obtained through the Fourier transform, the main frequency is filtered out with a filter, and the interference spectrum with better quality is obtained through the inverse Fourier transform. Finally, the interference order method [28] was applied to resolve the cavity length of the EFPI, and the result of this sensor is 135 μm.
The structure of the sapphire diaphragm is shown in Fig. 3 (a) and (b). The sensor adopts a simply supported beam structure manufactured by a femtosecond laser. The total length of the simply supported beam is 16 mm, and the effective length is 14 mm. The square with a middle side length of 4 mm is the inertial mass, and the length and width of the beams at both ends are 4 mm and 2 mm respectively. There is an arc-shaped transition zone between the beams and the square. The finite element analysis method is used to simulate on ANSYS software with the square grid size of 0.1 mm. An acceleration load of 100 m/s2 is applied to the center of the diaphragm. In the case of no damping, results of the ANSYS simulation of the diaphragm are shown in Fig. 3 (c) and (d).
Fig. 3. Sapphire diaphragm structure (a) 3D view, (b) physical diagram, ANSYS simulation results of the vibrating diaphragm (c) vibration mode simulation, and (d) frequency response
The simulation model neglects both ends of the original structure because semicircular structures with a radius of 1 mm at both ends are used as the fixed points of the simply supported beam, so the effective beam length of the vibrating diaphragm is 14 mm. Figure 3(c) demonstrates that, when a vibration is applied perpendicularly to the surface, the displacement gradually increases along the beam from the two fulcrums. The maximum displacement is in the middle square area of the diaphragm, and the displacement in the square area is basically the same. The reflecting surface remains stable during the vibration process, and it can significantly improve the quality of the interference signal. Figure 3(d) illustrates that the resonance frequency of the vibrating diaphragm is 2720 Hz, and the vibration response is stable in the range of 0 - 2000Hz. The sensitivity of the sensor is 23.578 nm/g.
3. Experiment
The demodulation system is shown in Fig. 4. The light generated by three lasers of different wavelengths is injected into the sensor through a fiber coupler and a fiber circulator. The reflected interference signal is picked up by the sapphire fiber and transmitted to a three-channel wavelength division multiplexer (WDM). The three-channel interference signals are transformed into electrical signals by photodiodes (PDs) and collected by an analog to digital converter (ADC). The vibration demodulation method used in this article is a three-wavelength symmetrical demodulation algorithm demonstrated in Ref. [29].
The vibration measurement system is shown in Fig. 5. A sinusoidal signal is generated by a signal generator and amplified by a power amplifier. Then, the amplified signal is transmitted to a vibration generator to generate sinusoidal vibration, through which the electrical sensor and the sapphire fiber vibration sensor are forced to vibrate. A vibration measuring instrument obtains the vibration signal of the electrical sensor, and the vibration signal of the sapphire optical fiber vibration sensor is obtained by the laser interference demodulator. In this system, an electrical sensor is used to calibrate the acceleration value of the optical fiber sensor. When measuring high-temperature acceleration, the vibration generator is placed horizontally and the sensor is inserted into the muffle furnace. High-temperature acceleration measurements in the range of 0-10 g were carried out at room temperature, 300 °C, and 600 °C, respectively.
The measurement consists of two parts. The first part is the frequency response measurement. When the acceleration is 1 g, a sinusoidal signal with a frequency range of 10 - 4200 Hz is generated by the vibration generator, and the vibration amplitude is obtained with a uniform or exponential frequency interval. The second part is the acceleration response measurement. At a fixed frequency of 640 Hz, the vibration generator generates a sinusoidal signal in the range of 0 - 10 g with an acceleration interval of 1 g. The vibration amplitude is recorded at room temperature, 300 °C, and 600 °C, respectively.
4. Results and discussion
The demodulated signal and the frequency spectrum are shown in Fig. 6 (a) and (b), respectively, when the input signals of 160 Hz and an acceleration of 1 g were applied to the vibration generator. The demodulated signal and the frequency spectrum are shown in Fig. 6 (c) and (d) when the input signals changed to 640 Hz and the acceleration was 1 g. Frequency spectra include the vibration frequency component with signal-noise ratio (SNR) 35 dB/√Hz, the second-harmonic signal, and the higher-order frequency signal.
Fig. 6. (a) The demodulated signal and (b) the frequency spectrum at a vibration frequency of 160 Hz and acceleration of 1 g, and (c) the demodulated signal and (d) the frequency spectrum at a vibration frequency of 640 Hz and acceleration of 1 g.
The frequency response of the sapphire optical fiber vibration sensor is shown in Fig. 7. At an acceleration of 1 g, the frequency response is relatively flat in the range of 0 - 2000Hz, and the amplitude is between 18.47 and 21.3 nm with a small fluctuation. Above 2000Hz, the amplitude sharply increases, and the resonance frequency is approximately 2700 Hz, which is similar to the results of the ANSYS simulation (2720 Hz). There are two reasons for the difference between the simulation result and the actual measurement result. First, when the sapphire diaphragm is bonded to the ceramic base with high-temperature-resistant inorganic glue, the two arms of the diaphragm cannot be guaranteed to be completely symmetrical. Second, after the sapphire diaphragm is bonded, the fixed points at both ends are not completely fixed, and there will be slight disturbances.
Figure 8 shows that the acceleration response of the sensor during the acceleration increase (black) and decrease (blue) process was measured at a frequency of 640 Hz. At different temperatures, the sensor changes linearly within the acceleration range of 0-10 g. The fitting equation at each temperature is as follows:
The vibration sensor is calibrated according to the above linear fitting curve at room temperature. Then, an acceleration measurement is carried out according to the previous measurement method, and the results of this measurement are shown in Fig. 9. The measured value is basically the same as the applied value. After linear fitting, the R2 value is as high as 0.9997, and the error does not exceed 1% of the full scale. The experimental results show that the sensitivity of the sensor is consistent with the sensitivity of the acceleration during the process of decreasing acceleration. When the acceleration is reduced to 0, the sensor can also return to the initial state, the hysteresis is not obvious, so it can be used repeatedly.
After the measurement of the acceleration response and frequency response for the vibration sensor, the temperature response is required. The sensor was placed in a muffle furnace, and the temperature was raised from 25 ℃ to 1500 ℃ at 100 ℃ intervals. Figure 10 shows that the sensor has a quadratic dependence of cavity length on temperature between 25 ℃ and 1500 ℃:
where L is the optical cavity length of the sensor, and T is the temperature. The change of the sensor FP cavity length is the same during the heating and cooling process. And after returning to room temperature, the FP cavity length of the sensor can return to the initial state, demonstrating the repeatability of the sensor performance.5. Conclusion
In conclusion, a sapphire optical fiber high-temperature vibration sensor is proposed. The vibration diaphragm of the sensor is a simple beam structure etched by a femtosecond laser on a single polished sapphire wafer. A ceramic ferrule with a polished sapphire fiber is mounted on the ceramic base, which is aligned with the sapphire diaphragm and forms EFPI. The incident light is reflected by the end face of the sapphire fiber and the upper surface of the diaphragm, thereby forming double-beam FP interference. The interference signal is collected by the sapphire fiber and transmitted to a laser interferometric demodulator by a multimode fiber. A three-wavelength symmetrical demodulation algorithm is used for signal demodulation. Experimental results show that the resonance frequency of this sensor is 2700 Hz with an SNR of 35 dB/√Hz. The amplitude fluctuates between 18.47 nm-21.3 nm within the linear frequency range of 10-2000Hz. The sensor varies linearly in the acceleration range of 0-10 g with a sensitivity of 20.91 nm/g. As the temperature increases, the sensitivity of the sensor decreases. At 600 ℃, the sensitivity of the sensor is reduced to 15.6 nm/g. After calibration, the actual measurement results are consistent with the calibration results, and the error is less than 1% of the full scale. The temperature response of this sensor between room temperature and 1500 ℃ is fitted to a quadratic curve. The proposed sapphire optical fiber vibration sensor for high-temperature vibration measurement may exhibit advantages in aerospace industry applications.
Funding
National Natural Science Foundation of China (61775020, 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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