Fig. 1. Schematic diagram of detection system. (a) Laser drive circuit board; (b) optical systems; (c) gas environment simulation system; (d) signal modulation and demodulation boards; (e) data processing module

Fig. 2. Absorption spectra of HCN and H₂O based on HITRN database

Fig. 3. WMS-2f of HCN at 0.01% calibration experiment

Fig. 4. Changes in the second harmonic signal of HCN. (a) Measuring the amplitude of the second harmonic signal of HCN in the range of 0.001%-0.01%; (b) the amplitude of the second harmonic signal varies with the concentration of HCN

Fig. 5. Analysis of Allan variance for the detection system. (a) Raw data of untreated HCN concentration; (b) Allan variance of HCN concentration; (c) histogram of HCN concentrations distribution

Fig. 6. Measurement of hydrogen cyanide during cigarette combustion. (a) HCN concentration of flue gas at different flow rates;(b) HCN concentrations of flue gas in different oxygen environments