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 $8.5 \mathit{\mu }\mathrm{m}$ whose wavelength was continuously temperature tuned over 15 nm. A sensitivity (noise-equivalent absorbance) of $3.4\times {10}^{-9} {\mathrm{cm}}^{-1}$ ${\mathrm{Hz}}^{-1/2}$ was achieved for ammonia in nitrogen at standard temperature and pressure, which corresponds to a detection limit of 0.25 ppbv.