Fig. 1. Schematic diagram of direct absorption spectroscopy

Fig. 2. Flow velocity measurement optical path layout diagram. (a) Single-channel laser reflection method; (b) dual-optical path flow velocity measurement method

Fig. 3. Schematic diagram of wavelength modulation spectroscopy

Fig. 4. Four light-path configurations with protecting structure of TDLAS on 2DSM-B^[39]

Fig. 5. Experimental layout for water vapor absorption measurement in the LENS I wind tunnel^[22]. (a) Plan view of the water vapor absorption measurement experimental device in the LENS I wind tunnel; (b) schematic layout of the electronic and optical components in the probe and the remote laser control system

Fig. 6. Experimental layout of nitric oxide absorption measurement in LENS I wind tunnel^[22]. (a) Physical diagram of temperature and concentration measurement; (b) physical diagram of velocity measurement

Fig. 7. Top-view schematic diagram of the experimental optical workbench setup^[19]

Fig. 8. Schematic of T5 facility measurement scheme^[62]

Fig. 9. TDLAS schematic diagram of T-ADFA wind tunnel^[64-65]

Fig. 10. Layout of TDLAS experiments inside and outside the wind tunnel^[71-72]

Fig. 11. 3-beam experiment with attenuation tube^[73]. (a) Physical picture; (b) optical path diagram

Fig. 12. Layout and the result of the shear layer shielding experiments^[74]. (a) Physical picture; (b) experimental result picture

Fig. 13. TDLAS optical path design^[74]. (a) Single perpendicular beam; (b) single-angled beam; (c) shear layer shield; (d) double-beam velocity