Hybrid optical fiber Fabry–Perot interferometer for simultaneous measurement of gas refractive index and temperature

We present a hybrid miniature optical fiber Fabry-Perot interferometer for simultaneous measurement of gas refractive index and temperature. The interferometer is fabricated by cascading two short sections of capillary tubes with different inner diameters. One extrinsic interferometer is based on the air gap cavity formed by the capillary tube with large diameter. Another section of capillary tube with small inner diameter performs as an intrinsic interferometer and also provides a channel enabling gas to enter and leave the extrinsic cavity freely. The experiment shows that the different dips or peaks in fringe exhibit different responses to the changes in gas refractive index and temperature. Owing to this feature, simultaneous measurement of the gas refractive index and temperature can be realized.