Ultrasensitive Fiber Humidity Sensor Based on Mode Transition Sensitized LPFG With Low Refractive Index Coating

Dual-layer polymer-coated long-period fiber gratings in which the refractive index of the inner layer is lower than that of quartz can effectively enhance the sensitivity of LPFGs operating in the mode transition region. When coated with a high refractive index film, the inner film serves as a barrier to mode transition, resulting in a higher mode transition threshold and an improvement in performance as the mode transition region becomes narrower. This research demonstrates that the sensitivity improves with increasing thickness of the low refractive index inner layer. It has been demonstrated that the humidity sensitivity of the humidity sensor proposed in this paper increases with the thickness of the inner layer. The experimental results show that the humidity sensitivity can be enhanced to 7473 pm/% RH with the PDMS near 1 μm. This work uses dual-layer polymer film for tuning the mode- transition sensitivity of LPFGs, which is promising for application to the various types of ultra-high sensitivity sensors.