A theoretical description of the wavelength-modulation (WM) spectrometry technique is given. The formalism is based on Fourier analysis and can therefore correctly handle arbitrary large frequency-modulation amplitudes. It can also deal with associated intensity modulations as well as wavelength-dependent transmission effects. It elucidates clearly how various Fourier components of these entities combine with those of the line-shape function to yield separately the final analytical and background <i>nf</i> WM signals. Explicit expressions are given for the <i>f</i> and the <i>f</i> signals. It is shown, among other things, that the <i>f</i> technique in general gives rise to smaller background signals (and therefore larger signal-to-background ratios) than does the <i>f</i> technique when the background is dominated by etalon effects from short cavities and that a finite intensity modulation necessarily leads to an out-of-phase <i>nf</i> WM signal. The formalism is also able to elucidate clearly that a linear intensity modulation is not sufficient to cause any <i>f</i> background residual–amplitude–modulation signals (as was the general consensus until recently in the literature) but that <i>f</i> background signals instead can exist only in systems with either wavelength-dependent transmission or a laser with nonlinear intensity modulation.
© 1999 Optical Society of America
Pawel Kluczynski and Ove Axner, "Theoretical Description Based on Fourier Analysis of Wavelength-Modulation Spectrometry in Terms of Analytical and Background Signals," Appl. Opt. 38, 5803-5815 (1999)