Shock wave is a kind of compression wave in which the wavefront propagates in the form of a synoptic surface in an elastic medium. Its typical feature is the discontinuous abrupt changes of state parameters of the medium on the abrupt surface, such as pressure, density, and temperature. As the study of shock waves progresses, it has been found that shock wave technology has great civilian value, so the measurement of shock wave signals has become increasingly important. The formation and propagation of shock waves are accompanied by overpressure and rapid changes in pressure. The response speed and reliability of the corresponding pressure sensors have more demanding requirements. Traditional electrical shock wave pressure sensors are susceptible to electromagnetic interference, temperature range tolerance, rise time, and other issues, which limit the application of such sensors. Fiber-optic Fabry-Perot (F-P) pressure sensors, as an important branch of fiber-optic sensors, provide new possibilities for dynamic pressure measurement of shock waves due to their advantages of fast response speed, high sensitivity, small size, and high resistance to electromagnetic interference. To achieve the dynamic pressure measurement of shock waves, a thin-film fiber-optic F-P pressure sensor with a fiber-tip coating is studied.

The basic structure of the thin-film fiber-optic F-P sensor studied in this paper mainly consists of two gold films with different thicknesses, a layer of parylene film serving as the F-P cavity, and a single-mode optical fiber for optical field coupling. When the shock wave pressure was applied to the end surface of the sensor, the parylene film was subjected to pressure, and deformation was produced, causing a change in the F-P cavity length. This change in length then affected the interference of reflected light produced by the two gold films on the front and back surfaces of the F-P cavity. Before the sensor was fabricated, the optical and mechanical aspects of the sensor were simulated using finite element simulation software, and the performance of the sensor under different parameters was calculated by combining theoretical formulas. In addition, the parameters of the sensor were determined. After the sensor was fabricated, the static and dynamic pressure measurement system was designed and constructed, and the experimental results were analyzed.

In the pressure range of 0-60 MPa, a static pressure measurement experiment is conducted on a thin-film fiber-optic F-P pressure sensor using a bench-top oil pressure pump. The reflected spectrum signal of the sensor is obtained and processed to calculate the cavity length of the F-P cavities of different pressure sensors. From the reflectance spectrum curves (Fig. 12) of the wavelength and corresponding light intensity under different pressures, it can be seen that with increasing pressure, the overall reflectance spectrum of the sensor drifts to the left. Based on the wave valley values at different pressures, the length information of the sensor cavity corresponding to the pressure is calculated (Fig. 13), yielding wavelength sensitivity and cavity length sensitivity of the sensor of 0.0809 nm/MPa and 0.3200 nm/MPa, respectively, which are consistent with the simulation results. In the dynamic pressure measurement experiments, the sensor successfully captures the shock wave signal with a peak pressure of 7.47 MPa and a rise time of 75 ns (Fig. 15).

For measuring shock wave signals, we propose a thin-film fiber-optic F-P pressure sensor. The effective structure of the sensor is a three-layer structure consisting of gold film, polymer film, and gold film. By utilizing the change of the peak position of the sensor's reflected spectral wave, the sensor causes a change of spectral intensity, so as to realize the measurement of the signal pressure. In the pressure measurement range of 0-60 MPa, the wavelength sensitivity is 0.0809 nm/MPa, and the cavity length sensitivity is 0.3200 nm/MPa. Within the range of dynamic pressure measurement, the sensor can measure the dynamic signals with a pressure rise time of 75 ns and a pressure rise amplitude of 7.41 MPa. The experimental results show that the sensor has a large range of pressure measurement ability and high sensitivity, and it has a small size, light weight, and anti-electromagnetic interference. Therefore, the sensor has greater application prospects in the field of shock wave pressure measurement.