Multi-Component Broadband Absorbers Detection via Wavelength Modulation Spectroscopy: First Harmonic Signal Analysis and Processing

Abstract Measuring broadband absorbers remains a challenging task for tunable diode laser absorption spectroscopy (TDLAS), and the simultaneous quantification of multi-component mixtures further complicates such measurements due to overlapping absorption spectra and inter-species interference. In this paper, we propose a new WMS approach based on first harmonic (1f) signal analysis and processing for simultaneous detection of multiple broadband absorbers. By applying a logarithmic transformation to the transmitted intensity, the demodulated 1f component becomes linear in absorbance and thus does not rely on the absorption line shape, which allows for the retrieval of broadband absorbers concentration by measuring 1f signal. A reference gas featuring a narrow absorption line within the absorption band of the broadband absorbers is introduced for wavelength localization, using the peak of its second harmonic (2f) signal as the wavelength marker. Furthermore, we develop a four-point elimination scheme to suppress error amplification in multi-gas measurements. In a proof-of-concept experiment, we selected ethanol (C2H6O) and isobutane (C4H10) as broadband absorbers and ethane (C2H6) as reference gas. Accurate retrieval is achieved under varying C2H6 backgrounds (15, 25, 35, 45, and 55 ppm), with the measured C2H6O and C4H10 concentrations remaining close to their nominal values and a maximum relative error below 2.5%. Continuous measurements yield 1σ precisions of 2.71 ppm (C2H6O) and 2.81 ppm (C4H10), while Allan-deviation analysis provides detection limits of 116 ppb at 6.4 s and 158 ppb at 6.3 s, respectively. This method enables simple, effective, and simultaneous high-precision sensing of multiple broadband absorbers without detailed line-shape modeling, and can be readily extendable to other broadband multi-gas applications.