A Sensitive Relative Humidity Sensor Based on a Tapered Fiber Mach-Zehnder Interferometer Coated with Hydrogel

Relative humidity (RH) sensing plays a crucial role in various applications, ranging from environmental monitoring to industrial processes. This paper presents a highly sensitive and compact relative humidity sensor based on a tapered fiber Mach-Zehnder interferometer coated with hydrogel. The fabrication process involves core-offset splicing and fused tapering techniques, and the tapering technique is applied to enhance the effect between the light wave and the external environment. A hydrogel mixture comprising Poly (ethylene glycol) diacrylate and 2-Hydroxy-2-methylpropiophenone is applied to the surface of the RH sensor. The proposed sensor takes advantage of the evanescent field interaction between the hydrogel layer and the guided light within the tapered MZI structure, resulting in a remarkably responsive RH measurement. The proposed RH sensor has a humidity sensitivity of 0.466 nm/%RH within humidity range of 80–98 %RH. The experimental results indicate that sensors with different tapered lengths exhibit varying humidity sensitivities. Furthermore, the stability, repeatability, and temperature response characteristics of the RH sensor are thoroughly investigated, revealing satisfactory stability and repeatability performance.