Abstract

A cavity ring-down spectrometer is built for trace gas detection using telecom distributed feedback (DFB) diode lasers. The longitudinal modes of the ring-down cavity are used as frequency markers without active-locking either the laser or the high-finesse cavity. A control scheme is applied to scan the DFB laser frequency, matching the cavity modes one by one in sequence and resulting in a correct index at each recorded spectral data point, which allows us to calibrate the spectrum with a relative frequency precision of 0.06 MHz. Besides the frequency precision of the spectrometer, a sensitivity (noise-equivalent absorption) of $4\times {10}^{-11}{\mathrm{cm}}^{-1}{\mathrm{Hz}}^{-1/2}$ has also been demonstrated. A minimum detectable absorption coefficient of $5\times {10}^{-12}{\mathrm{cm}}^{-1}$ has been obtained by averaging about 100 spectra recorded in 2  h. The quantitative accuracy is tested by measuring the ${\mathrm{CO}}_2$ concentrations in ${\mathrm{N}}_2$ samples prepared by the gravimetric method, and the relative deviation is less than 0.3%. The trace detection capability is demonstrated by detecting ${\mathrm{CO}}_2$ of ppbv-level concentrations in a high-purity nitrogen gas sample. Simple structure, high sensitivity, and good accuracy make the instrument very suitable for quantitative trace gas analysis.

© 2014 Optical Society of America

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