Humidity sensor using planar optical waveguides with claddings of various oxide materials

A planar optical waveguide, fabricated using an ion-exchange process in soda glass (Na+ exchanged by K+), is studied as a humidity sensor with screen printed claddings of BriTiO(3), SnO2, Al2O3, ZrO2, V2O5 and ZnO as well as sintered ZnO, with cladding thicknesses between 20 and 25 mu m. A He-Ne laser beam (0.1 mm dirt., lambda = 632.8 nm, 0.5 mW) is coupled to the guide using prism film coupling and a silicon photovoltaic detector is used to measure the intensity of transmission. The lengths of the claddings used which give the maximum TM-polarization because of the relative attenuation, are between 3 and 5 mm. The relative humidity (RH) is varied from 3 to 98%. BaTiO3 offers maximum sensitivity as compared to the other oxides used. SnO2 exhibits a response time of 3 s and a recovery time of 10 min, the fastest amongst all. The maximum hysteresis of 1% is observed, except for V2O5 cladding (8%). The sensitivity curves show 3 regions, which are attributed to different possible dominant processes. (C) 1997 Elsevier Science S.A.