Simultaneous Detection of Mixed Gases Based on Overlapped Spectra Separation With SLIDT

This letter investigates a simultaneous detection method of mixed gases concentration based on overlapped spectra separation with software lock-In demodulation technique (SLIDT). Based on the additivity of the gases absorption lines, the spectra separation method of mixed gases was proposed theoretically and experimentally. The SLIDT instead of the hardware lock-in amplifier technique is introduced into the system, with structure simplification and cost reduction. The overlapped spectra of CO and CO₂ mixed gases near 1582 nm were obtained based on SLIDT with the optimized modulation parameters. Using the sum of first, second, and third derivatives of Lorentzian function with the optimized fitting coefficients, the respective spectra of mixed gases with 4% CO and 6% CO₂ were separated successfully with fitting errors of 2.401 and 2.470 mV. The relationship between different CO and CO₂ concentrations and amplitudes of the separated second harmonics is linear with the linear correlation coefficients of 0.99828 and 0.99715.