Adaptive Dual-Harmonic Switching WMS for CO₂ Measurement Over a Wide Dynamic Range

A dual-harmonic switching wavelength modulation spectroscopy (DHS-WMS) technique has been proposed for gas sensing. The optimal signal-to-noise ratio (SNR) is achieved by adaptively switching harmonic signals based on gas concentrations. To implement the DHS-WMS technique, a gas sensing system with a specific absorption transition at 1572.33654 nm for CO2 detection, and an effective optical absorption length of 3.3 m has been established. Typically, atmospheric greenhouse gas CO2, with concentrations ranging from 5% to 60%, was selected as the target analyte to evaluate the performance of DHS-WMS. Subsequently, the baseline fluctuation and SNR of the 2f and 4f signals were analyzed in detail. In cases of wide concentration variations, the strategy of switching between different harmonic detection techniques proved superior to using a single harmonic detection method. The results indicate that the DHS-WMS technique, dynamically adjusting harmonic orders, outperforms traditional wavelength modulation spectroscopy (WMS) with fixed harmonics, especially across a wide concentration range. The minimum detection limit (MDL) for CO2 has been determined to be 26.7 ppm at an integration time of 52 s, based on the Allan variance analysis. DHS-WMS, with its capacity for high sensitivity and extensive measurement ranges, holds tremendous potential for applications in urban air quality monitoring and industrial emission control.