Relative Humidity Sensor Based on Double Sagnac Interferometers and Vernier Effect

A double Sagnac interferometer (DSI) and vernier effect sensor for relative humidity (RH) is proposed and experimentally implemented, in which two single Sagnac interferometers (SI1 and SI2) cascaded in parallel to construct the structure of the sensor. Owing to the approximate free spectral range (FSR) of the SI1 and SI2, the proposed sensor can produce fundamental and harmonic vernier effect. Theoretical analysis shows that the RH sensitivity of the sensor is only related to the lengths of polarization-maintaining fibers (PMFs). The experimental results show that in the range of 30∼80%RH, the RH sensitivities of the spectral envelope can reach −0.48 nm/%RH, which is 60 times higher than that measured by the single Sagnac interferometer sensor (8 pm/%RH). The experimental results are consistent with the theoretical analysis. The proposed sensor has the superiorities of high sensitivity, good repeatability, simple fabrication, and compact size, and it has potential applications in the areas of biopharmaceuticals, environmental monitoring, food processing, and microbial sensing.