Fig. 1. Absorption lines of NO, NO₂, and SO₂ in the 2.6‒4.0 μm range

Fig. 2. Wavelength tunning of PPLN-OPO

Fig. 3. System schematic diagram

Fig. 4. Engineering of pump lasers

Fig. 5. Single-pulse waveform of pump laser

Fig. 6. Optical path and engineering design of dual-wavelength OPO

Fig. 7. OPO structure design. (a) ON-OPO design drawing; (b) ON-OPO physical drawing; (c) OFF-OPO design drawing; (d) OFF-OPO physical drawing

Fig. 8. Signal optical spectra of OPO. (a) Signal optical spectrum diagram of free-running OPO; (b) signal optical spectrum of OPO after seed injection

Fig. 9. Energy curves of ON-OPO

Fig. 10. Comparison between actual mass concentration and detected mass concentration^[17]

Fig. 11. Measurement results of dynamic emissions^［17］