Sensitivity-enhanced Michelson interferometric humidity sensor with waist-enlarged fiber bitaper

A fiber-optic humidity sensor composed of a chitosan-coated, waist-enlarged bitaper-based Michelson interferometer (WEBMI) is proposed and experimentally demonstrated. Humidity changes interference pattern of the sensor head through refractive index of the chitosan coating thus relative humidity level can be determined from corresponding wavelength shift. The achieved sensitivity is further enhanced by more than 5 times, from 26 pm/%RH to 135 pm/%RH, by reducing the fiber cladding thickness and oxidizing the chitosan film. Experimentations show that the measurement is fully reversible and repeatable with fast response and recovery time of ∼5 s and 3 s, respectively.