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

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 V_pp 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.