Ultra-Compact Humidity Sensor Based on Twin-Core Fiber Tip Integrated Dual Microcavities

While conventional optical fiber humidity sensors offer high sensitivity and stability, their size limits their application in narrow spaces. To overcome the limitation, this paper presents an ultra-compact fiber humidity sensor fabricated via 3D printing. It integrates a monolithic dual Fabry–Perot (F–P) microcavity structure at the tip of a twin-core fiber (TCF). The sensor utilizes a low humidity-sensitive photopolymer to form the structural frame and the reference cavity(A) and utilizes a humidity-sensitive hydrogel to act as the sensing layer in the sensing cavity(B). Humidity changes cause significant deformation of the hydrogel, resulting in a much greater change in the cavity length of cavity B compared to cavity A. By precisely controlling the cavity lengths to 43 μm (Cavity A) and 39.5 μm (Cavity B), the sensor can harness the Vernier effect to achieve a significant sensitivity enhancement. Experimental results demonstrate a high sensitivity of -4.056 nm/%RH over a relative humidity (RH) range of 32.8% to 85.1%. Furthermore, the sensor also exhibits a temperature sensitivity of -0.35 nm/°C, with response times of 508 ms and 583 ms during humidification and dehumidification processes, respectively. This work provides a practical solution for high-sensitivity gaseous humidity detection in narrow and confined spaces.