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

Ruxton, Keith C. and Chakraborty, Arup Lal and Johnstone, W. and Lengden, Michael and Stewart, G. and Duffin, K. (2010) Tunable diode laser spectroscopy with wavelength modulation : elimination of residual amplitude modulation in a phasor decomposition approach. Sensors and Actuators B: Chemical, 150 (1). pp. 367-375. ISSN 0925-4005

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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.