Highly Sensitive Fiber Relative Humidity Sensor Based on Tip Michelson Interferometer With Agarose Gel Coating

A tip Michelson interferometer (MI) is proposed and experimentally demonstrated for relative humidity (RH) sensing. The device consists of a short capillary tube with a small inner diameter that is spliced to a single-mode fiber (SMF) and coated with agarose gel on its end face, and the solid capillary wall and gel act as the sensing arm of the MI. When the agarose gel absorbs water modules, its refractive index (RI) and volume vary, resulting in the shift of the reflection spectrum of the MI. The relationship between the structural parameters and the performance of the device is revealed after performing theoretical analysis and experimental verification, which provides guidance for optimizing the device. The maximum sensitivity of 0.752 nm/% RH with a linear correlation coefficient of (Adjusted ${R}^{{2}}) 0.9927$ within the range of 25%–90% RH is obtained. Moreover, the proposed sensor exhibits good performances of robustness, compact size, high repeatability, fast response (< 100 ms), and small measurement uncertainty (< 0.21% RH), which makes the sensor promising for accurately static or dynamic moisture measuring.