A CO 2 sensor based upon a continuous-wave thermoelectrically-cooled quantum cascade laser

Abstract

A CO 2 sensor based upon a continuous-wave thermoelectrically-cooled distributed feedback quantum cascade laser operating between 2305 and 2310 cm −1 and a 54.2 cm long optical cell has been developed. Two approaches for direct absorption spectroscopy have been evaluated and applied for monitoring of the CO 2 concentration in gas lines and ambient laboratory air. In the first approach optical transmittance was derived from the single channel laser intensity, whilst in the second approach a ratio of signal and reference laser intensities (balanced detection) was used. The optimum residual absorption standard deviation was estimated to be 1.9 × 10 −4 for 100 averages of 1 ms duration and 0.1 cm −1 scans over the P(46) CO 2 absorption line of the ν 3 vibrational band at 2306.926 cm −1. A CO 2 detection limit (1 standard deviation) of 36 ppb was estimated for 0.1 s average and balanced detection.