Theoretical description based on Fourier analysis of wavelength-modulationspectrometry in terms of analytical and background signals

A theoretical description of the wavelength-modulation (WM) spectrometry te chnique is given. The formalism is based on Fourier analysis and can theref ore correctly handle arbitrary large frequency-modulation amplitudes. It ca n also deal with associated intensity modulations as well as wavelength-dep endent transmission effects. It elucidates clearly how various Fourier comp onents of these entities combine with those of the line-shape function to y ield separately the final analytical and background nf WM signals. Explicit expressions are given for the 2f and the 4f signals. It is shown, among ot her things, that the 4f technique in general gives rise to smaller backgrou nd signals (and therefore larger signal-to-background ratios) than does the 2f technique when the background is dominated by etalon effects from short cavities and that a finite intensity modulation necessarily leads to an ou t-of-phase nf WM signal. The formalism is also able to elucidate clearly th at a linear intensity modulation is not sufficient to cause any 2f backgrou nd residual-amplitude-modulation signals (as was the general consensus unti l recently in the literature) but that 2f background signals instead can ex ist only in systems with either wavelength-dependent transmission or a lase r with nonlinear intensity modulation. (C) 1999 Optical Society of America OCIS codes: 300.6260, 020.3690, 300.1030.