PROSPECTS FOR FIBEROPTIC EVANESCENT-FIELD GAS SENSORS USING ABSORPTION IN THE NEAR-INFRARED

In this paper we investigate the prospects for realization of evanesce nt-wave gas sensors using gas absorption Lines within the transmission window of silica fibres. Our work focuses on methane and uses D-fibre as the evanescent-field sensor. We discuss the three major obstacles, namely, low sensitivity, high background signal levels from interfere nce effects and system degradation through surface contamination. We e xplain how sensitivity may be improved through sol-gel coatings, how b ackground signal levels may be reduced bg use of polarization-maintain ing fibre and suggest a way of compensating for surface contamination by continuous monitoring of the birefringence of the D-fibre, Improved performance can be obtained through use of a distributed-feedback las er instead of a light-emitting diode, but at much greater cost. In the long term, the likely application for the evanescent-field sensor is in distributed measurement systems.