Highly sensitive in-line fiber-optic structure for refractometric measurements

A refractometric structure consisting of a side-polished single-mode optical fiber covered with a thin film of hydrogenated amorphous silicon (a-Si:H) has been studied. a-Si:H films have been deposited by plasma-enhanced chemical vapour decomposition of silane. The sensors elements working with zero and first order optical modes of the amorphous silicon waveguide have been demonstrated. A high sensitivity of 5300 nm per unit refractive index has been obtained for the low values of superstrate refractive indices in the range of 1.33-1.38 by using a-Si:H film with a thickness of about 14 nm, i.e. the interaction with TE 0 mode of the PWG. Despite of the very low thickness of the a-Si:H film there is no need of any additional layer protecting the structure from the analyzed medium. The mechanical and optical stability of the film and its very good adhesion to fused silica eliminate the possibility of the film damaging and makes the structure reliable in use.