Analysis of calibration-free wavelength-scanned wavelength modulation spectroscopy for practical gas sensing using tunable diode lasers

TLDR

Injection‑current‑tuned tunable diode lasers cause simultaneous wavelength and intensity variations that complicate Fourier expansion of WMS signals, and 1f‑normalization strategies are discussed to achieve calibration‑free wavelength‑scanned WMS. The method simulates WMS signals by using the measured laser intensity to model transmitted intensity, then applies digital lock‑in and low‑pass filtering to expand both simulated and measured intensities into harmonics (WMS‑nfm), avoiding analytic intensity models and remaining valid across all optical depths, modulation indices, and scanned wavelengths. A novel calibration‑free strategy for wavelength‑scanned WMS with tunable diode lasers was developed and validated on dilute H₂O in air at 1 atm and 296 K, where extracted WMS‑nfm harmonics (n = 1–6) matched simulations across the entire lineshape.

Abstract

A novel strategy has been developed for analysis of wavelength-scanned, wavelength modulation spectroscopy (WMS) with tunable diode lasers (TDLs). The method simulates WMS signals to compare with measurements to determine gas properties (e.g., temperature, pressure and concentration of the absorbing species). Injection-current-tuned TDLs have simultaneous wavelength and intensity variation, which severely complicates the Fourier expansion of the simulated WMS signal into harmonics of the modulation frequency (fm). The new method differs from previous WMS analysis strategies in two significant ways: (1) the measured laser intensity is used to simulate the transmitted laser intensity and (2) digital lock-in and low-pass filter software is used to expand both simulated and measured transmitted laser intensities into harmonics of the modulation frequency, WMS-nfm (n = 1, 2, 3,... ), avoiding the need for an analytic model of intensity modulation or Fourier expansion of the simulated WMS harmonics. This analysis scheme is valid at any optical depth, modulation index, and at all values of scanned-laser wavelength. The method is demonstrated and validated with WMS of H2O dilute in air (1 atm, 296 K, near 1392 nm). WMS-nfm harmonics for n = 1 to 6 are extracted and the simulation and measurements are found in good agreement for the entire WMS lineshape. The use of 1f-normalization strategies to realize calibration-free wavelength-scanned WMS is also discussed.