High-Sensitivity and Fast-Response Humidity Sensor Based on a Simple Fiber-Tip Interferometer With Thin Agarose Gel Coating

A high-sensitivity and fast-response relative humidity (RH) sensor based on a fiber-tip interferometer coated with agarose gel is proposed and demonstrated. To fabricate the interferometer, focusing the femtosecond laser on the fiber core near the end face of a well cleaved single-mode fiber (SMF) for less than 0.1 second to create a tiny refractive index-modulated dot is enough, then a thin film of hydrophilic material (agarose gel) is deposited on the end facet of the SMF. Owing to a high RI sensitivity of the interferometer and a humidity-induced refractive index (RI) change in the agarose gel, the fiber-tip sensor is qualified for RH sensing. The experimental results show that the sensor is extremely sensitive to high RH, with a maximum sensitivity of −2.04 dB/%RH and 0.673 nm/%RH at 90%RH. Furthermore, it exhibits low hysteresis as demonstrated by a breath monitoring of about 60 ms ultrafast response time and 160 ms recovery time. Additionally, it also shows a good long-term stability with a small standard deviation. The prominent advantages of high sensitivity, fast response, easy fabrication and compact size (total length of about 50 μm) imply that the proposed sensor has many potential applications for static or dynamic RH sensing.