Highly Sensitive Humidity Sensor Based on Ultra-long-Period Fiber Gratings with Asymmetric Refractive Index Modulation

摘要

An Ultra-long-period fiber grating (ULPFG) with asymmetric refractive index modulation is fabricated in standard telecommunication optical fiber by using high-frequency CO2 laser pulses. A highly sensitive humidity sensor based on this grating can be realized due to its high refractive index sensitivity. The ULPFG is coated with a new nano-composite hydrogel material that has a strong water absorption capacity. This kind of hydrogel can absorb water vapor in the air substantially to make its own refractive index change, inducing the resonant peak shift of the ULPFG to realize the humidity measurement. The experimental results show that a resonant peak shift of～30nm corresponding to a variation of the surrounding humidity in the range of 38%~96%, which is three times higher than that of the humidity sensors reported. Such a novel humidity sensor could find important applications in humidity measurement.