Mid-infrared heterodyne phase-sensitive dispersion spectroscopy for simultaneous CO2 and N2O measurement over broad concentration ranges.

A mid-infrared heterodyne phase-sensitive dispersion spectroscopy (HPSDS) system was developed for simultaneous measurement of CO2 and N2O in the 4.47μm region. The system employed a single interband cascade laser (ICL) and a White multi-pass cell with an effective optical path length of 76 m. The effects of modulation frequency and modulation depth on the dispersion signal amplitude, signal-to-noise ratio, lineshape, and linear response were experimentally investigated. Considering the IM/FM coupling of the directly modulated ICL, a modulation frequency of 150 MHz and a modulation voltage of 1.2 Vpp were selected as the operating parameters. Under these conditions, simultaneous quantitative measurements of CO2 and N2O were achieved. Allan deviation analysis yielded minimum 1σ Allan deviations of 0.561 ppm for CO2 at an averaging time of 2.39 s and 0.564 ppb for N2O at 2.78 s. Concentration measurements showed stable responses up to approximately 1600 ppm for CO2 and 3000 ppb for N2O under the same system configuration. Continuous ambient-air measurements further demonstrated the capability of the system to measure both gases near atmospheric background levels. These results demonstrate simultaneous CO2/N2O measurement with good measurement precision and broad concentration coverage using a single mid-infrared laser, showing potential for ambient monitoring and measurements of moderately elevated greenhouse-gas concentrations.