A quantum cascade laser based room temperature spectrometer for sensitive detection of ammonia and ethylene

We investigated the use of a pulsed, distributed feedback (DFB) quantum cascade (QC) laser centered at 970 cm(-1) in combination with an astigmatic Herriot cell with 150 in path length for the detection of ammonia and ethylene. This spectrometer utilizes the intra-pulse method, where a linear frequency down-chirp, that is induced when, a top-hat current pulse is applied to the laser, is used for sweeping across the absorption line. This provides a real time display of the spectral fingerprint of molecular gases which can be a few wave numbers wide. A 200 ns long pulse was used for these measurements which resulted in a spectral window of similar to 1.74 cm(-1). A room temperature mercury-cadmium-telluride detector was used, resulting in a completely cryogen free spectrometer. We demonstrated detection limits of similar to 3 ppb for ammonia and similar to 5 ppb for ethylene with less than 10 s averaging time.