Optical fiber relative humidity sensor based on a hydrogel coated long period grating

Abstract

A hydrogel coated long period grating (LPG) used as relative humidity sensor is investigated theoretically and experimentally. It showed a high degree of sensitivity when relative humidity was above 50%RH. For humidity levels from 50%RH to 72%RH, a wavelength decrease of 15.4 nm is seen, with a sensitivity of 0.66 nm /%RH. For humidity levels from 72%RH to 98%RH, a wavelength increase of 9.5nm is observed, with a sensitivity of 0.36 nm /%RH. For humidity levels greater than 65%RH, the transmission loss at resonance increases rapidly, showing a high degree of sensitivity of 0.53 dB /%RH and offering a high resolution of 0.02%RH. The proposed relative humidity sensor offered good repeatability and low hysteresis errors, with a standard deviation of 0.005dB/%RH and 0.26% hysteresis error while RH levels greater than 65%RH during three cycles of increasing and decreasing humidity. The stability test was also carried out and a standard deviation of 0.14%RH was obtained. In addition, the durability test showed that the humidity sensor can work at least ten months.