Fig. 1. The phase of the vector sum of two beat note signals with a frequency of ω_m两个1倍频拍频信号的矢量和信号的相位

Fig. 2. Laser wavelength and output power as a function of drive current at 34.5 °C

Fig. 3. Schematic diagram of CH₄ detection system based on dual-sideband beat-suppressed HPSDS

Fig. 4. (a) The measured output power of EOM as a function of bias voltages; (b) measured dispersion phase signals of 10% CH₄ with or without dual-sideband beat suppression at the modulation frequency of 1200 MHz (T = 298 K, P = 1 atm, L = 20 cm); (c) the frequency spectrum of the beat note signal with dual-sideband beat suppression; (d) the frequency spectrum of the beat note signal without dual-sideband beat suppression

Fig. 5. (a) Measured dispersion phase signals at different CH₄ concentrations; (b) peak-to-peak values of HPSDS signals as a function of CH₄ concentration

Fig. 6. (a) HPSDS peak-to-peak values, WMS-2f and WMS-2f/1f signals at different CH₄ concentrations; (b) linear relationship between WMS-2f, WMS-2f/1f signals and concentration within the range of 0.1% to 0.8% CH₄, respectively

Fig. 7. (a) Diagram of concentration distribution results for continuous measurement of 10% CH₄ for 15 minutes; (b) frequency distribution of the measured concentration and the Gaussian profile fitting; (c) Allan deviation analysis of HPSDS, WMS-2f and WMS-2f/1f signals

Fig. 8. Continuous monitoring results of CH₄ in the gas flow

Fig. 9. (a) Dispersion phase signals at different laser powers; (b) variation of HPSDS peak-to-peak values, peak values of WMS-2f and WMS-2f/1f signals at different laser powers