Abstract

Resonators are promising platforms for sensing applications. Nevertheless, their practical applications are hindered by the complicated waveguide-resonator coupling configuration, which requires very precise position control of the separation distance between resonator and waveguide to match the coupling condition. Here, we demonstrate a single-sided microsphere resonator based compact fiber-optic sensor for real-time detection of humidity. Instead of coupling light via evanescent waves, the light is injected into the cavity by connecting a single-mode-fiber and a agarose-coated microsphere directly. The light on resonance will be absorbed by microsphere and the rest will be partially reflected back. Since the agarose is hydrophilic, the variation of ambient humidity can change the resonance condition, which shifts the fringe of reflection spectrum. Therefore, the humidity could be demodulated by monitoring the reflection spectrum of the proposed structure at the end of the fiber. The sensor was successfully used to detect ambient humidity over the relative humidity range of 30%–90% under room temperature (25 $^\circ$ C). Thanks to the small size, the sensor exhibits fast dynamic response and recovery time which makes it can monitor human respiration and water evaporation in real time. Due to the compact structure and fast response/recovery time, the sensor is promising for applications in personal health monitoring as well as environmental monitoring.

© 2019 IEEE

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