Prospects for fibre-optic evanescent-field gas sensors using absorption in the near-infrared

In this paper we investigate the prospects for realization of evanescent-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 interference effects and system degradation through surface contamination. We explain how sensitivity may be improved through sol-gel coatings, how background signal levels may be reduced by use of polarization-maintaining 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 laser 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.