Fig. 1. Traditional White gas absorption cell^[10]

Fig. 2. Structure of Doussin gas absorption cell (a) and its light pattern (b)^[14]

Fig. 3. “T”-shaped White absorption cell^[15]

Fig. 4. Structure of Chernin type gas absorption cell (a) and its light pattern (b)^[11]

Fig. 5. Structural diagram of Herriott type gas absorption cell

Fig. 6. Structure of astigmatic type gas absorption cell (a) and its light patterns with optical path of 100 m (b) and 36 m (c)^[23]

Fig. 7. Structure of Cylindrical mirror gas absorption cell (a) and its light patterns with curvature radius of 77.3 cm (b) and 262 cm (c)^[24]

Fig. 8. Structure of Robert gas absorption cell (a) and its light patterns (b)^[25]

Fig. 9. Structure (a) and spot pattern (b) of the gas multi-pass cell developed by Liu et al.^[27]

Fig. 10. Dense spot patterns obtained by spherical mirrors (a)^[28] and aspherical mirrors (b)^[29]

Fig. 11. Spot patterns of the dense absorption cell (a)^[21] and the dual-path Herriott-type gas absorption cell (b)^[30]

Fig. 12. Dense spot patterns with optical path of 24.06 m (a) and with optical path of 46.11 m (b)

Fig. 13. Circular multipass cell structure designed by Chernin et al.^[13]

Fig. 14. Calculated (a) and actual (b) beam patterns of the circular multipass cell designed by Manninen et al.^[31]

Fig. 15. Double circular design of circular multipass gas absorption cell^[32]

Fig. 16. Structure diagram of spiral gas absorption cell (a) and its 3D tracing diagram (b), (c)^[33,34]

Fig. 17. Three dimensional view of spherical mirror reflection in double ring gas absorption cell^[35]

Fig. 18. "Hot window" type high temperature gas absorption cell^[46]

Fig. 19. "Cold window" type high temperature gas absorption cell^[50]

Fig. 20. "Windowless" high temperature gas absorption cell^[55]

Fig. 21. Single-pass cryogenic gas absorption cell system designed by Willey et al.^[58] (a) and Gao et al.^[59] (b)

Fig. 22. Herriott type cryogenic gas absorption cell designed by Mondelain et al.^[74]