(color online) (a) signal of the target CO₂ absorption, the modulation depth a = 0.051 cm⁻¹, peak to valley separation , the inferred absorption linewidth was 0.0802 cm⁻¹; (b) signal of the target CO₂ absorption, the modulation depth a = 0.084 cm⁻¹, zero-crossing separation , the inferred absorption linewidth was 0.0755 cm⁻¹.