Relative Humidity Sensor Based on S-Taper Fiber Coated With SiO2 Nanoparticles

A simple and compact optical fiber relative humidity (RH) sensor based on SiO₂ nanoparticles has been proposed and experimentally demonstrated in this paper. S-taper fiber is fabricated as the sensitive element using simple fusion spicing and the S-tapered region is coated with a layer of hydrophilic material by direct immersion into the SiO₂ nanoparticle solution. The resonance wavelength as well as peak transmission are both sensitive to environmental humidity due to the change of effective cladding refractive index caused by the strong surface absorption of the porous SiO₂ nanoparticle coating with hydrate activity. Experimental results show that this humidity sensor has a good reversibility from 26.5%RH to 95.2%RH and a good linearity from 83.8%RH to 95.2%RH. The maximum sensitivities of 1.1718 nm/%RH and 0.441 dB/%RH have been achieved for a high humidity range of 83.8%RH to 95.2%RH. The proposed humidity sensor has such distinguished features as compact size, low cost, and ease of fabrication, and it has potential applications for high humidity environments.