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

Mark C. Farries; Andrew M. Shaw; Jonty Fisk; Liam Garvey

doi:10.1117/12.578549

29 December 2004 Detection of low chemical concentrations by cavity ring-down in an evanescently coupled fused optical fibre taper

Mark C. Farries, Andrew M. Shaw, Jonty Fisk, Liam Garvey

Author Affiliations +

Proceedings Volume 5617, Optically Based Biological and Chemical Sensing for Defence; (2004) https://doi.org/10.1117/12.578549
Event: European Symposium on Optics and Photonics for Defence and Security, 2004, London, United Kingdom

Abstract

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 1μs 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 5x10^-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.

Citation Download Citation

Mark C. Farries, Andrew M. Shaw, Jonty Fisk, and Liam Garvey "Detection of low chemical concentrations by cavity ring-down in an evanescently coupled fused optical fibre taper", Proc. SPIE 5617, Optically Based Biological and Chemical Sensing for Defence, (29 December 2004); https://doi.org/10.1117/12.578549

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