High-sensitivity optical fiber temperature sensor of cascaded FSI and MZI based on Vernier effect

A high-sensitivity optical fiber temperature sensor based on the Vernier effect of cascaded fiber Sagnac interferometer (FSI) and Mach-Zehnder interferometer (MZI) is proposed and experimentally demonstrated. The FSI is composed of a 3 dB four-port coupler and a section of polarization maintaining fiber, which has high sensitivity and good stability as the sensing part. As the filtering part, MZI is made up of two three-port couplers by controlling the length of the two arms. The free spectral range (FSR) of these two interferometers is similar but not equal, so the cascade system can effectively amplify the temperature sensing sensitivity based on the vernier effect. The experimental results show that the temperature sensitivity of a single FSI is only -1.68 nm/degrees C, and the cascade system can amplify it to 14.30 nm/degrees C with a gain coefficient of 8.51, which is consistent with the theoretical results.