Detection of low chemical concentrations by cavity ring down in an evanescently coupled fused optical fibre taper.

A novel sensor for detecting very low concentrations of chemicals in water and other liquids is presented. A cavity ring-down spectrometer has been developed that can measure chemicals in solution in a harsh environment. The high Q Fabry perot cavity is fabricated in an optical fibre with high reflectivity mirrors on each end. The cavity contains a fused fibre taper, with very low intrinsic loss, for coupling light in the cavity evanescently into a smart surface layer that is bound on to the fibre surface. Small changes in the absorption are detected by changes in the ring-down time of the resonant cavity. The low loss cavity results in ring down times of 1mus for a 2 m cavity, which is equivalent to 100 passes through the smart surface. The ring down time provides a very accurate measure of absorbance because it is independent of source and detector drift and the fibre cavity is unaffected by changes in temperature, vibration or bending. Absorption changes of 5 x 10(-5) dB can be detected with the current configuration and further improvements can be achieved by optimisation leading to detection of atto-molar chemical concentrations.