Fig. 1. System schematic diagram of WM-DAS and CW-CRDS. LC, laser current and temperature controller; FI, fiber isolator; AOM, acousto-optic modulator; APD, avalanche photodiode; PD, photodiode; DDG, digital delay generator; PZT, piezoelectric transducer; RF, radio frequency; DAQ, data acquisition system; WM, wavelength meter; MFC, mass flow controller.

Fig. 2. Laser wavelength calibration and FFT filtering: (a) Etalon signal (black solid line), I_t (blue solid circle), measured (black solid circle) and fitted relative frequency (red solid line), fitting residual (red hollow circle); (b) real part and imaginary part of Fourier coefficients of I_t, and low frequency (0.35, 0.45 kHz) and high frequency (499.7, 514.1 kHz) noise.

Fig. 3. Absorption spectra of CO₂ (red) and CH₄ (blue) measured by WM-DAS in about 1 s at 298 K and 100.9 kPa, and the best fit of Voigt profile. In order to compare with CW-CRDS, the absorptance is converted to absorption coefficient (cm^–1).

Fig. 4. The absorption spectra of CO₂ (red) and CH₄ (blue) measured by CRDS in about 24 min at 298 K and 100.9 kPa: (a) The relationship between the ring down time and the current; (b) the absorption function and the best fits of Voigt profile.

Fig. 5. (a) Comparison of measuring range of CH₄ between the two methods; (b) histograms of two methods at different concentrations of CH₄.

Fig. 6. Time sequence diagram of WM-DAS and CW-CRDS, laser current (black solid line), transmitted light I_t (blue solid line), ring down time (red solid line).

Fig. 7. (a) CO₂ in atmosphere measured by the two methods; (b) linear fitting of the data measured by the two methods.

Fig. 8. (a) CH₄ in atmosphere measured by the two methods; (b) linear fitting of the data measured by the two methods.

Fig. 9. Allan variance measured by the two methods: (a) CO₂; (b) CH₄.