A humidity fiber sensor based on both end-sides of a fiber Bragg grating coated with polyimide

A humidity fiber sensor based on both end-sides of a fiber Bragg grating (FBG) coated with polyimide has been proposed and demonstrated. Polyimide is coated on both end-sides of FBG, and a thermal curing treatment is performed to form a dense film of a certain shape. The reflection centre wavelength of the FBG is shifted 0.5 nm by bring prestress to bear on the FBG, and then the two ends of this FBG are fixed on a base with ultraviolet (UV) glue. This structure will not damage the grating part of FBG and ensure the bandwidth and reflectivity of the FBG keeping stable. In addition, this sensor avoided the curing shrinkage of the moisture sensitive film, chirp effect caused by uneven coating or the warpage of the film caused by thermal curing. The experimental results showed that the sensor has a linearity of 99%, a sensitivity of 3–4 pm/%RH and an accuracy of 5% over a wide RH range from 20 to 95%. Furthermore, this sensor has a small hysteresis, better long-term stability and simple structure and can realize multi-point quasi-distributed sensing, which has broad application prospects.