An evanescent-wave optical fiber relative humidity sensor with enhanced sensitivity

We report a fiber optic relative humidity (RH) sensor based on the evanescent wave absorption spectroscopy using a single U-bend plastic-clad silica fiber with high dynamic range and high sensitivity. The sensor is fabricated using a CoCl2 doped thin polymer film coated on the bare fiber core. A comprehensive study of the sensor was made in terms of performance optimization. Sensor response was investigated in terms of the chemically synthesized cladding thickness over the centrally de-cladded U-bent probe. The effect of fiber core diameter on the sensitivity was also studied and the fiber with smaller core diameter was observed to be more sensitive unlike the previously reported results. In addition, we observed that the sensor was having a very fast response to the relative humidity, and was fully reversible, repeatable with a large dynamic range.