A novel probe for a fiber optic humidity sensor

A fiber optic humidity sensor based on the moisture dependence absorption of light by the phenol red doped polymethylmethacrylate (PMMA) film over a small portion of the lower refractive index core of the plastic clad silica (PCS) fiber is reported. The high refractive index of the film increases the interaction between the light guided in the fiber and the dye in the film and hence it increases the sensitivity of the sensor. To increase the sensitivity many fold the probe is designed as U-shaped. The U-shaped probe decreases the angle of incidence of the ray at the core-film interface in the bent region and hence increases the transmission of light from core to the film. The increase in the transmitted light increases its interaction with the dye present in the film and hence increases the sensitivity of the sensor. It has been shown that as the bending radius of the probe decreases the sensitivity of the sensor increases. The sensor operates in the humidity range 20 to 80% RH and its response time is about 5 s. Further a good degree of reproducibility of the results is obtained.