A Mach-Zehnder interferometric humidity sensor based on waist-enlarged tapers

A novel humidity sensor based on an in-fiber Mach-Zehnder interferometer (MZI) is proposed and demonstrated. The sensor head is formed by a single-mode-multimode-single-mode (SM-MM-SM) fiber structure through arc fusion splicing. The intermodal interference is achieved by two waist-enlarged fiber tapers at the coupling points of the multimode fiber and single-mode fibers. The sensor has a linear response to humidity with enhanced sensitivity of −0.119 dB/%RH in the range of 35-90%RH. Additionally, the sensor exhibited excellent temperature immunity in the temperature test. Such easily fabricated, cost-effective and temperature-immune fiber interferometer could be used for high sensitivity humidity sensing applications.