Wavelength modulation absorption spectrometry -: an extensive scrutiny of the generation of signals

The signal generation process in the wavelength modulation absorption spectrometry (WMAS) technique, mainly applied to diode lasers, is scrutinized in detail. The basic foundations of the technique, including its partly unique nomenclature and its relation to conventional absorption spectrometry, are reviewed. Most of the description of the signal generation process is made in terms of a newly developed formalism based on Fourier series. It is shown that the nth harmonic output of the lock-in amplifier in a WMAS instrument is given by the nth Fourier coefficient of the detector signal. This implies that many of the intricate characteristics of the WMAS technique can be derived from various properties of Fourier analysis. Analytical expressions for an arbitrary harmonics of the WMAS signals from Lorentzian, Gaussian and Voigt absorption profiles are given. It is shown how an associated laser-power modulation affects the WMAS signals and how multiple reflections, so-called etalons, give rise to background signals that often limits the applicability of the technique. It is furthermore shown that the traditional description of the WMAS technique, applicable only when small frequency-modulation amplitudes are used and often referred to as derivative spectroscopy, is a subset of the new formalism. Additional features covered are: WMAS signals from multiline transitions; the temperature dependence of the signal; WMAS under optically thick conditions (including the concept of an extended dynamic range); the advantages of multi-harmonic detection; WMAS background signals from frequency-doubled diode laser light; and double modulation techniques. It is also demonstrated that the new formalism can be used to predict the shift of zero crossings of odd harmonics, which is a feature often used for frequency-locking of lasers. Although some of these topics have been discussed previously in the literature, this work presents much new information. It also constitutes the first review of the WMAS technique based upon the new Fourier series-based formalism. (C) 2001 Elsevier Science B.V. All rights reserved.