Fig. 1. Quartz enhanced spectrophone with different structures. (a) On-beam structure; (b) off-beam structure

Fig. 2. Absorption spectra of CH₄ and H₂O at standard atmospheric pressure and 1 cm optical path length. (a) Absorption spectra of CH₄ (volume fraction of 5×10^-4) in the range of 1620-1680 nm; (b) absorption spectra of H₂O (volume fraction of 1%) and CH₄ (volume fraction of 5×10^-4) in the range of 1653-1654 nm

Fig. 3. CH₄ detection system and miniature gas chamber. (a) Diagram of CH₄ detection system; (b) miniature gas chamber; (c) mechanical design model of miniature gas chamber

Fig. 4. Variation of normalized 2f signal amplitude and modulation depth with modulation voltage signal amplitude

Fig. 5. Measured 2f signal waveforms at different CH₄ concentration levels

Fig. 6. Measured 2f signal amplitude under different CH₄ concentration levels and relationship curve between CH₄ concentration and averaged 2f signal amplitude. (a) 2f signal amplitude; (b) relationship curve between CH₄ concentration and averaged 2f signal amplitude

Fig. 7. Measurement system noise in N₂ environment

Fig. 8. Variation curve of CH₄ concentration measured in N₂ environment with average time and variation curve of Allan variance with average time. (a) Variation curve of CH₄ concentration with average time; (b) Allan variance with average time