Nonlinear dual-frequency modulation QEPAS using a single laser for resolving overlapping spectral features

Conventional single-frequency wavelength modulation spectroscopy faces inherent limitations in multi-component gas sensing, due to restricted detection dimensionality and unavoidable trade-offs in modulation parameters. To overcome these issues, we propose a Quartz-Enhanced Photoacoustic Spectroscopy sensing technique based on single-laser dual-frequency superimposed modulation. By simultaneously injecting two independent high-frequency modulation signals, matching the fundamental and overtone resonance modes of a Quartz Tuning Fork(QTF), into a single Distributed Feedback laser, dual-channel real time synchronous detection is achieved within a compact single path optical architecture. A comprehensive physical model incorporating both Residual Amplitude Modulation and high order non-linear competition effects has been proposed. We demonstrate that the fourth derivative of the gas absorption line shape plays a dominant role in inducing signal suppression and waveform splitting under strong dual-frequency modulation. A waveform optimization strategy based on dynamically tuning the laser scanning rate allow to regulate the effective filtering behavior of the lock-in amplifier.