Algorithm study and simulation analysis of double amplitude modulated for stand-off detection of ethanol

Ethanol has a characteristic narrow band absorption peak (2989.6 cm-1) that overlaps heavily with the airborne water vapour spectrum, severely affecting detection accuracy when the ambient humidity varies significantly. To this end it is particularly critical to investigate a method to eliminate background effects and improve the accuracy of ethanol telemetry. Firstly, the second harmonic signal of ethanol concentration under 5000~20000ppm*m water vapour environment was simulated by multi-functional fitting method, and the multivariate linear relationship between the second harmonic amplitude and water vapour concentration and ethanol concentration is obtained. Two sets of signals were obtained in one triangular wave period using double modulated amplitude and linear regression was used to solve the coefficients of the two equations, which in turn inverted the ethanol concentration in other water vapour environments. Simulation results show that the maximum error is less than 2.23ppm*m. The method also eliminates the need for complex reference light paths or expensive wide-range tunable mid-infrared lasers, reduces measurement errors due to variations in ambient humidity, and provides a basis for further development of simple, miniaturised ethanol telemetry systems.