Recovery of Absorption Line Shapes With Correction for the Wavelength Modulation Characteristics of DFB Lasers

Abstract

Tunable diode laser spectroscopy combined with wavelength modulation spectroscopy (WMS) is an important technique for noninvasive measurements of gas parameters such as pressure, concentration, and temperature in high-noise harsh environments. A variety of laser types are used for these applications, and the modulation characteristics can have significant effects on line shape recovery. Here, we identify important characteristics of distributed feedback (DFB) lasers that need to be taken into account in the context of WMS and illustrate the effects with a 2- $\mu\textrm{m}$ wavelength multiquantum-well DFB laser used for CO2 detection. The modulation response of the laser is measured, and we demonstrate how the phasor decomposition method (PDM) may be used to obtain accurate line shapes from the first harmonic WMS signals by correcting for phase variation across the laser's low-frequency current sweep. We also demonstrate how the PDM approach can be improved by removing the need to preset the orientation of the lock-in axis to isolate the residual amplitude modulation component, making it more suitable for field applications.