Relative humidity measurement using tapered plastic fiber coated with HEC/PVDF

A simple tapered plastic optical fiber (POF) sensor was proposed and demonstrated as a humidity sensor based on the intensity modulation technique utilizing a tapered POF probe coated with a polymer blend of hydroxyethylcellulose/polyvinylidenefluoride (HEC/PVDF) composite which acts as the humidity sensitive cladding. The sensor was fabricated using an etching method and has a waist diameter of 0.45 mm and tapering length of 10 mm. As the relative humidity (RH) varies from 50 to 85 %, the output voltage of the sensor increases linearly from 0.32 mV to 1.25 mV and a sensitivity of 0.023 mV/% with a slope linearity of more than 99.65% were obtained. The HEC/PVDF composite-coated sensor increases the sensitivity of the proposed sensor due its ability to swell in a humid atmosphere resulting in a drop in its refractive index below that of the core and thus allowing more light to be transmitted from the tapered fiber.