Enhanced Relative Humidity Sensing Based on a Tapered Fiber Bragg Grating with Zinc Oxide Nanostructure-Embedded Coatings

A novel concept for an intrinsic relative humidity (RH) sensor that uses a tapered fiber Bragg grating (FBG) with zinc oxide nanostructure-embedded coatings is presented. Tests in a controlled environment indicate that the sensor has a linear and stable response behavior at 55% to 80% RH. The tapered FBG is fabricated using a chemical etching method followed by the coating of zinc oxide (ZnO) nanostructures onto the surface of the taper. Experimental results show that the ZnO coated FBG produced a better sensitivity of 2.51 pm/% in comparison to only 1.36 pm/% for the uncoated FBG in terms of its Bragg wavelength shift. The performance of the humidity sensor in regards to the reflected output power is also presented, showing a significant improvement in sensitivity from 0.0012 dBm/% for bare tapered fiber to 0.089 dBm/% with the ZnO coating. These results indicate the potential of ZnO nanostructures coating in enhancing the performance of the sensor for relative humidity sensing.