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Strathprints serves world leading Open Access research by the University of Strathclyde, including research by the Strathclyde Institute of Pharmacy and Biomedical Sciences (SIPBS), where research centres such as the Industrial Biotechnology Innovation Centre (IBioIC), the Cancer Research UK Formulation Unit, SeaBioTech and the Centre for Biophotonics are based.

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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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Abstract

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.