A CO2 sensor based upon a continuous-wave thermoelectrically-cooled quantum cascade laser

A CO2 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 CO2 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) CO2 absorption line of the ν3 vibrational band at 2306.926 cm−1. A CO2 detection limit (1 standard deviation) of 36 ppb was estimated for 0.1 s average and balanced detection.