The Investigation of All-Sapphire Fabry–Perot Fiber Acoustic Sensor Operating Up to 800 °C

Acoustic sensors for ultrahigh temperature environments are urgently demanded in the aerospace field. An all-sapphire Fabry–Perot (F-P) cavity acoustic sensor has been proposed, which acquires acoustic signals by detecting changes in the refractive index of the air. Based on high-temperature direct bonding technology, the sensor with a rigid structure is fabricated in three layers on a whole 4-in wafer. A 3/4-period length gradient index optical fiber (GIOF) is employed as a mode-field adapter, reducing heterogeneous fiber fusion losses between sapphire and silica fiber. It is clearly demonstrated that the sensor has superior spectrum quality at a wide range of temperatures (25 °C–1000 °C). The fringe visibility of the sensor is 40.68% and 24.58% at 25 °C and 1000 °C, respectively. At 800 °C, the sensor achieves a frequency response flatness of less than ±2 dB, a sensitivity of 0.15 mV/Pa, and a maximum measurable sound pressure level of 171.16 dB. The proposed F-P fiber acoustic sensor has the advantages of a compact structure and tolerance to high temperature. This work provides a feasible strategy for achieving the acoustic performance of F-P fiber acoustic sensors in ultrahigh temperature and high sound pressure level environments.