Multipoint relative humidity measurement by polyvinyl alcohol-coated Fresnel reflection-based optical fiber sensors with an array-waveguide grating.

A simple multipoint humidity measurement by polyvinyl alcohol (PVA)-coated Fresnel reflection-based optical fiber sensors with an Array-Waveguide Grating (AWG) is proposed and demonstrated. Every channel end of the AWG is split as a vertical planar surface, and then is coated with a layer of a PVA whose refractive index is sensitive to moisture. The reflection intensity for each channel will change with its surrounding humidity, since the optical fiber interface's Fresnel reflection is affected strongly by the refractive index difference of the interface two sides. Multiplexing is achieved by the AWG with 16 channels, in which 15 channels can be used as sensing heads when they are coated with a layer of PVA and the left one is used as a reference channel. The experimental setup is simple and easy to handle. Experimental results show that the proposed Fresnel reflection-based optical fiber sensor for multipoint humidity measurement works well and the average sensitivity is 0.135 dB∕% relative humidity (RH) within the measurement range of 30%-80% RH.