A cavity ring-down spectrometer (CRDS) was built based on telecom distributed feedback (DFB) diode lasers.The ring down cavity was formed by mirrors of 99.997% reflectivity with a separation of 130 cm,with an empty ring down time of about 150 μs.A minimum detectable absorption coefficient of 5×10-12 cm-1 was obtained by averaging about 1 000 recorded spectra.The ring down cavity is thermo-isolated,and used as an interferometer to calibrate the recorded spectrum.A feedback control scheme was applied to step scan the laser frequency,successively matching each of the longitudinal modes of the cavity.By measuring the CO contents in a standard gas sample,the quantitative capability of the CRDS instrument was demonstrated,and a CO detection limit of 4 ppb was achieved.The instrument was applied to monitor the CO concentration in ambient air.