All-Sapphire Miniature Optical Fiber Tip Sensor for High Temperature Measurement

High temperature sensor with robust performance, minimum footprint, and fast response is desirable for many applications in extreme environments. This article introduces a miniature high temperature sensor based on fiber Fabry-Perot interferometer. The interferometric cavity is directly fabricated on the tip of a 125 μm single-crystal sapphire fiber through femtosecond laser micromachining and CO
₂
laser welding with the sapphire fiber as a light-guiding element. The sensor has an all-sapphire structure to ensure both physical and chemical stability. The sensor was tested in the lab from room temperature up to 1455 °C and shows an excellent temperature response with 1.32 nm/ °C at room temperature and full-range average resolution of 0.68 °C. The thermal response of the sensor is evaluated via both numerical simulation and experiment. Owing to the small size and fast thermal diffusivity of sapphire, the thermal response time of ~1.25 ms has been experimentally demonstrated.