High-sensitivity sensor of low relative humidity based on overlay on side-polished fibers

A low relative humidity sensor based on overlay on side-polished-fiber is presented. The evanescent field from a single-mode optical fiber is coupled to a TiO₂ waveguide overlay. The transmission response exhibits sharp resonances whose central wavelengths are linearly shifted with relative humidity. This behaviour is due to the porous columnar nanostructure of the TiO₂ film. The water is adsorbed in the pores of the nanostructure changing the refractive index of the layer and causing a shift of the wavelength resonances. The response of the sensor is determined by the shape and size of the pores. The optical fiber evanescent field sensor developed has a linear response and high sensitivity (0.5 nm/% RH) for low-relative humidity (RH-0%-15%) at 26.1°C ± 0.6°C. The lack of hysteresis in the adsorption-desorption cycle has been checked. Finally, the development of sensor with tailored response is envisaged using techniques to control the porosity of the material.