Fig. 1. The schematic diagram for second harmonic signal

Fig. 2. The theoretical scatter plot of Peak width D_x versus modulation depth m

Fig. 3. The theoretical scatter plot of peak height H_x versus modulation depth m

Fig. 4. The theoretical scatter plot of peak height H_x versus Peak width D_x

Fig. 5. Harmonic peak width at different concentrations

Fig. 6. （a）Absorption lines of oxygen near 760 nm，（b）simulation absorption of oxygen at 760.885 nm

Fig. 7. The schematic diagram for the overall simulation system

Fig. 8. The scatter plot of valley spacing D_x versus modulation depth m in simulations

Fig. 9. The scatter plot of peak height H_x versus modulation depth m in simulations

Fig. 10. The scatter plot of peak height H_x versus Peak width D_x in simulations

Fig. 11. The normalized peak height distribution before and after peak width correction

Fig. 12. The peak height distribution before and after peak width correction

Fig. 13. The on-site physical picture of the online testing system

Fig. 14. The structure diagram of online detection system

Fig. 15. The actual 2f signal monitored on an oscilloscope

Fig. 16. Raw data of the 2f signal in actual environment

Fig. 17. Fitting diagram of data points between actual valleys and normalized peak heights

Fig. 18. The peak fluctuation of the harmonic signal

Fig. 19. Correct recognition rate of glass medicine vials before and after calibration under different SNR