Recovery of absorption line shapes with correction for the wavelength modulation characteristics of DFB lasers

Tunable diode laser spectroscopy combined with wavelength modulation spectroscopy (WMS) is an important technique for non-invasive 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 2μm wavelength, multi quantum 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 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 pre-set the orientation of the lock-in axis, to isolate the residual amplitude modulation(RAM)component, making it more suitable for field applications.