Tunable diode laser spectroscopy with wavelength modulation: elimination of residual amplitude modulation in a phasor decomposition approach

Recovery and analysis of the first harmonic signals in tunable diode laser spectroscopy (TDLS) with wavelength modulation (WM) are limited by the presence of a high background signal upon which the small gas signals are superimposed. This high background signal is a result of direct modulation of the source laser power and is referred to as the residual amplitude modulation (RAM) signal. This paper presents further details of a recently reported technique to optically remove the RAMand an analytical model that enables the use of the phasor decomposition (PD) method with it to extract the absolute gas absorption line-shape from the recovered first harmonic signals. The PD method is important as it provides a calibration-free technique for gas concentrationmeasurements. A major benefit of RAM nulling is that signal amplification can be increased without equipment saturation due to the background RAM, resulting in improved signal resolution and system sensitivity. A comparison of experimental measurements of the 1650.96 nm absorption line of methane (CH4) with line-shapes derived from HITRAN data illustrates and validates the use of the PD method with the new RAMnulling procedure. This advancement is useful for industrial applications where stand-alone and calibration-free instrumentation is required.