Fig. 1. Simulation diagram of V-shaped cavity

Fig. 2. Spot distributions on M₁ and M₂ surfaces at different angles

Fig. 3. Spot distributions on lens M₂ surface at different off-axis incidence angles

Fig. 4. Intensity distributions of light emitting from surface of cavity mirror for V-OA-ICOS and OA-ICOS. (a) V-OA-ICOS; (b) OA-ICOS

Fig. 5. Structure design of V-shaped cavity

Fig. 6. V-OA-ICOS experimental measuring device

Fig. 7. Cavity mode signals at different off-axis incidence angles.(a) γ₁; (b) γ₂; (c) γ₃

Fig. 8. Measurement data of V-OA-ICOS device. (a) CO₂ absorption spectrum; (b) residual of Voigt fitting

Fig. 9. Measurement results of standard CO₂ gas with volume fraction of 400×10^-6 during 50 min. (a) Stability test; (b) Allan variance

Fig. 10. Simulated absorption spectra of CO₂, H₂O, and NH₃ near 4987 cm^-1

Fig. 11. NH₃ absorption spectrum and Savitzky-Golay smoothing results at different window widths. (a) Raw absorption spectrum; spectral data obtained after smoothing by Savitzky-Golay algorithm when window width is (b) 30, (c) 40, (d) 50, (e) 60, and (f) 70

Fig. 12. NH₃ absorption spectrum and Voigt fitting residual of V-OA-ICOS device. (a) Before smoothing by Savitzky-Golay algorithm; (b) after smoothing by Savitzky-Golay algorithm