Cavity ringdown spectroscopy using mid-infrared quantum-cascade lasers

Cavity ringdown spectra of ammonia at 10 parts in 10(9) by volume (ppbv) and higher concentrations were recorded by use of a 16-mW continuous-wave quantum-casacde distributed-feedback laser at 8.5 mu m whose wavelength was continuously temperature tuned over 15 nm. A sensitivity (noise-equivalent absorbance) of 3.4 x 10(-9) cm(-1) Hz(-1/2) was achieved for ammonia in nitrogen at standard temperature and pressure, which corresponds to a detection limit of 0.25 ppbv. (C) 2000 Optical Society of America.