CROSS-SECTIONAL TRANSFORMERS FOR THE SIGNAL ENHANCEMENT OF INTENSITY-BASED EVANESCENT-FIELD SENSORS

A cross-sectional transformer (CST) that allows efficient interconnect ion between a conventional multimode optical fibre and a broad shallow waveguide, designed for strong interaction of guided modes with sensi ng materials in the evanescent field, is described. A ray approach is used to determine a simple theoretical relationship between the geomet rical configuration of the transformer and the enhancement in sensitiv ity to absorption. Thinly clad rectangular optical fibres have been dr awn from a composite preform, and a CST has been fabricated using a su itably formed bundle of these fibres. An enhancement of 3.9+/-0.6 dB o f the power collected from a broad slab waveguide compared with that c ollected directly by a multimode fibre is measured. The attenuation of the rectangular fibres that comprise the CST is 0.1+/-10.07 dB cm(-1) at 550 nm.