Frequency-domain measurement of cavity ring-down spectroscopy

We have recorded the transmission spectrum of an optical cavity absorption cell filled with methane gas using a tunable continuous-wave light source in the mid-infrared region, To determine the absorption strength of the gas, we have measured the linewidth of the cavity fringe. This method corresponds to frequency-domain measurements of cavity ring-down spectroscopy (CRDS) because the linewidth is the Fourier transform of the photon lifetime from which the absorption strength is derived in CRDS. The absorption strengths determined from the present linewidth measurements indicate excellent agreements with both those derived from the transmittance measurements and those calculated from line parameters listed in the HITRAN 2004 database.