A Humidity Sensor Based on a Whispering-Gallery-Mode Resonator With an L-shaped Open Microcavity

A whispering-gallery-mode (WGM) resonator with an L-shaped microcavity for humidity sensing is proposed and demonstrated. The L-shaped microcavity is fabricated by using femtosecond laser micromachining, fusion discharge and chemical etching methods. By placing two microspheres in the microcavity, of which one is used as the whispering gallery mode resonant cavity, and the other plays the role of changing the optical path to help activating whispering gallery mode resonance. The device has a linear response to humidity with the linearity of 99.1% and the sensitivity of −0.23 dB/%RH, within the relative humidity range of 30%–70%, under the room temperature of 25 °C. The response time for the humidity increasing is as short as 240 ms, and the recovery time is 453 ms. The device is simple in fabrication, robust and has good operation stability.