Gelatin-Coated Long-Period Fiber Grating Humidity Sensor Based on a Microtaper Structure

A long-period fiber grating (LPFG) based on a microtaper structure with gelatin coating for relative humidity (RH) measurement is proposed and demonstrated. Periodic microtapers are formed by periodic arc discharge on a single-mode fiber (SMF), thereby periodically modulating the refractive index of the fiber and forming an LPFG with a microtaper structure. The entire sensor is coated with gelatin to detect changes in RH. The experimental results show that the humidity sensitivity can be improved by reducing the diameter of the waist taper. When the diameter of the waist taper is $85~\mu $ m, the humidity sensitivity reaches a maximum of −0.1282 nm/% RH in the range from 20% RH to 80% RH with the response time of 0.048 s.