Extension of wavelength-modulation spectroscopy to large modulation depth for diode laser absorption measurements in high-pressure gases

TLDR

High‑pressure diode laser absorption spectroscopy relies on large modulation depths to resolve collisional‑broadened or densely spaced rovibrational lines, but large wavelength excursions induce strong nonlinear intensity modulation in the lasers. The authors extend the WMS model by measuring the intensity modulation and its phase shift, incorporating these nonideal laser parameters, and using lock‑in detection of the 2f signal across pressures to isolate the modulation effect. Experimental data agree well with the improved model, showing that laser nonidealities become significant away from line center and grow with the large modulation depths required at elevated pressures.

Abstract

Tunable diode laser absorption measurements at high pressures by use of wavelength-modulation spectroscopy (WMS) require large modulation depths for optimum detection of molecular absorption spectra blended by collisional broadening or dense spacing of the rovibrational transitions. Diode lasers have a large and nonlinear intensity modulation when the wavelength is modulated over a large range by injection-current tuning. In addition to this intensity modulation, other laser performance parameters are measured, including the phase shift between the frequency modulation and the intensity modulation. Following published theory, these parameters are incorporated into an improved model of the WMS signal. The influence of these nonideal laser effects is investigated by means of wavelength-scanned WMS measurements as a function of bath gas pressure on rovibrational transitions of water vapor near 1388 nm. Lock-in detection of the magnitude of the 2f signal is performed to remove the dependence on detection phase. We find good agreement between measurements and the improved model developed for the 2f component of the WMS signal. The effects of the nonideal performance parameters of commercial diode lasers are especially important away from the line center of discrete spectra, and these contributions become more pronounced for 2f signals with the large modulation depths needed for WMS at elevated pressures.