Fig. 1. The simulated absorption spectra of CO and H₂O near 4 602.5 nm（T = 293 K，P = 101 325 Pa，L =1 cm），where the CO concentration is 100% and the H₂O concentration is 2%

Fig. 2. Structure and optical field of the HPCW

Fig. 3. Optimization of HPCW plate structure

Fig. 4. Optimization of HPCW lattice constant

Fig. 5. Plot of S versus L，where α_int = 27.5 dB/cm for HPCW

Fig. 6. Structure and optical field of the RWG

Fig. 7. Structure optimization of the panel of ridge waveguide

Fig. 8. Ridge structure optimization of ridge waveguide

Fig. 9. Plot of S versus L，where α_int=3 dB/cm for RWG

Fig. 10. Plots of L_opt and C_LoD versus α_int