SENSITIVITY IMPROVEMENT FOR EVANESCENT-WAVE GAS SENSORS

Fibre-optic gas sensors based on evanescent-field absorption principles have advantages of long interaction lengths and distributed sensing capabilities, but suffer from low sensitivity per unit length due to the small percentage of the power carried in the evanescent field. We demonstrate that by proper design of a D-fibre and by using a thin high-index overlay or a porous overcoat to enhance the surface fields, the sensitivity can be significantly improved by up to two orders of magnitude. Evanescent-wave gas sensors should be possible with sensitivities of 1-2% that of direct absorption spectroscopy per equal length.