Ultracompact Fabry-Perot interferometer based on femtosecond laser-assisted wet etching for high-temperature sensing.

We experimentally demonstrate an ultracompact high temperature sensor based on intrinsic Fabry-Perot interferometer (FPI) in a single mode fiber (SMF). The sensor consists of a thin air cavity and the fiber end face. The cavity length of the FPI is 500 µm. The compact structure without pigtail makes the sensor free from strain crosstalk and allows the sensor to be positioned closer to the heat source, enhancing the response time and measurement accuracy. The thin air cavity was fabricated by combining femtosecond laser direct writing and wet etching. Benefiting from wet etching, smooth and parallel reflective surfaces were produced. Therefore, the signal noise ratio (SNR) and fringe visibility of the FP interferometric spectrum were improved, which are crucial for increasing the demodulation accuracy. A wavelength sensitivity of 15.21 pm/°C and a cavity length sensitivity of 493.3 pm/°C as well as good linearity were obtained in the temperature range of 300 - 900 °C. The proposed sensor shows potentials for high-temperature applications in aerospace, nuclear energy, and metallurgy.