Influence of key operational parameters on harmonic signals in WMS-TDLAS: a simulation study.

Abstract

Tunable diode laser absorption spectroscopy (TDLAS) combined with wavelength modulation spectroscopy (WMS) is widely used for trace-gas sensing. This study presents a reproducible numerical analysis of the effects of gas concentration, pressure, temperature, lock-in reference phase, and modulation index on first- and second-harmonic signals. The model combines the Beer-Lambert law, the complete HITRAN temperature-dependent line-strength expression, pressure- and temperature-dependent Lorentzian linewidths, and time-domain dual-phase lock-in demodulation. Simulations were performed for a specified methane transition and baseline parameter set under weak-absorption conditions. The signed first-harmonic quadrature extremum and phase-independent second-harmonic magnitude increase linearly with concentration over the investigated range. The temperature response is transition-specific and results from the combined effects of line strength, molecular number density, linewidth, and modulation conditions. Pressure effects are evaluated separately for fixed frequency-modulation amplitude and fixed modulation index to distinguish linewidth-induced changes from changes in modulation depth. The signed in-phase and quadrature components vary with the reference phase, whereas the dual-phase harmonic magnitude is phase independent. The 1f and 2f responses show different dependences on the modulation index, with the 2f response reaching a maximum near m=2.2 under the present conditions. The numerical implementation was independently benchmarked against direct Fourier coefficients and assessed through sampling- and filter-parameter convergence tests. These results provide a validated numerical reference for interpreting parameter-dependent WMS-TDLAS responses within the stated model assumptions.

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