Temperature and Humidity Sensor Based on PMMA Microsphere Whispering-Gallery-Mode Resonator

A Whispering-Gallery-Mode (WGM) resonator based on polymethylmethacrylate (PMMA) microsphere was proposed for high-sensitivity temperature and humidity sensing. The proposed sensor is fabricated by offsetting a PMMA microsphere onto the end face of a single-mode fiber (SMF). When the light field propagates to the coupling region between the SMF end face and the PMMA microsphere, light satisfying the phase-matching condition couples into the microsphere via the evanescent field. After continuous total internal reflection on the inner wall of the microsphere, the WGM is excited. The diameter and refractive index of the microsphere change with variations in external temperature and humidity, causing drift in the interferometric spectrum of the WGM sensor. Experimental results indicate that within the relative humidity (RH) range of 30% to 90%, the sensor exhibits a humidity sensitivity of 73.8 pm/%RH. Within the temperature range of 25°C to 60°C, its temperature sensitivity reaches -62.7 pm/°C. Additionally, the sensor exhibits excellent repeatability and stability, with a minimum detection limit of 0.39% RH. Operating at a resonance wavelength of 1550.78 nm, it achieves a high quality factor (Q-factor) of 7.38×10³ , making it highly valuable for high-precision temperature and humidity detection applications.