Hollow-waveguide gas sensing with room-temperature quantum cascade lasers

The application of a room-temperature-operated distributed-feedback quantum cascade laser (DFB-QCL), coupled with a silica hollow waveguide for small-volume gas sensing is demonstrated. An internally silver-coated silica capillary, with a length of 4 m and an inner diameter of 700 μm simultaneously acts as waveguide and microcapillary gas cell. This configuration provides a well defined optical path length, while maintaining a small sample volume. Ethyl chloride gas was mixed with air and detected by attenuation of the emitted laser radiation at 971cm^-1 down to concentration levels of 5 ppm (v/v). With the current experimental setup a limit of detection of 0.5 ppm (v/v) has been achieved. The feasibility study shows that hollow fibres provide facile light guiding for QCLs over several metres and, therefore can be used for remote gas sensing or IR light delivery in medical applications.