Planar optical waveguide with PbCl2 cladding: A chlorine sensor

Single-mode optical waveguides of 1 mm width are fabricated by thermal indiffusion so that Na+ is replaced by K+ in a simple soda-lime glass substrate. PbCl2 is selectively vacuum evaporated on to the waveguide surface as a sensitive layer (cladding). The thickness of the cladding is varied from 170 nm to 480 nm, with clad length from 2 mm to 6 mm. The prism-film coupling method with an He-Ne laser (lambda=632.8 nm) is used for characterization. The output light intensity of a TM-mode is detected by a silicon photovoltaic detector. The waveguides are tested for different gases (O-2, H-2, CO2, N-2, Cl-2, H2O), in an airtight glass cham ber. The presence of only Cl-2 in the surrounding air ambient reduces the output, even at a few p.p.m.. The Cl-2 gas concentration is varied from 3 p.p.m. to 1.5%. There are slight changes in sensitivity with variation in the cladding thickness and length. A drastic change in sensitivity for different concentration ranges is observed as three distinct regions. The response time over a few p.p.m. to hundreds of p.p.m. is around 5 s and beyond 500 p.p.m, is around 2 s.