Fig. 1. Off-axis integral cavity measurement system

Fig. 2. The simulated CH₄ absorption spectrum and the relationship between the laser injection current and the wavenumber

Fig. 3. Absorption spectral signal changes of CH₄ under different pressures. (a) Without RF; (b) with RF

Fig. 4. The fitted integrated area of CH₄ under different pressures. (a) Without RF; (b) with RF

Fig. 5. Absorption signal of CH₄ with and without RF noise at different concentrations. (a) 0.4 μmol/mol; (b) 0.6 μmol/mol; (c) 0.8 μmol/mol; (d) 1.0 μmol/mol; (e) 1.2 μmol/mol; (f) 1.4 μmol/mol

Fig. 6. Voigt fitting of 0.4 μmol/mol CH₄. (a) The fitting signal without RF; (b) the fitting standard deviation without RF;(c) the fitting signal with RF; (d) the fitting standard deviation with RF