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Applied Optics

Applied Optics


  • Vol. 40, Iss. 6 — Feb. 20, 2001
  • pp: 794–805

Background signals in wavelength-modulation spectrometry by use of frequency-doubled diode-laser light. II. Experiment

Pawel Kluczynski, Åsa M. Lindberg, and Ove Axner  »View Author Affiliations

Applied Optics, Vol. 40, Issue 6, pp. 794-805 (2001)

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In an accompanying paper [Appl. Opt. 40, 783–793 (2001)], we predict the existence of background signals from a frequency-doubled wavelength-modulated diode-laser system. We now demonstrate and characterize various nf harmonics of such background signals from a system producing light in the 422-nm region by use of a single-pass KNbO3 crystal with respect to the modulation amplitude, the laser center frequency, and the crystal temperature. It is demonstrated that 2f detection is plagued by considerably larger amounts of background signal than is detection at other higher, even harmonics. This result implies that 4f or 6f detection is often to be preferred in comparison with 2f detection when frequency-doubled wavelength-modulation spectrometry (WMS) is to be used. This preference is illustrated by the detection of Ca in an acetylene–air flame. It is also shown that the background signals have a much stronger dependence on the modulation amplitude than do the analytical signals. This difference implies that the optimum detectability for frequency-doubled WMS is often reached for modulation amplitudes lower than those normally used. An analysis of the effect of a finite temperature stability of the doubling crystal on the drift of the background signals as well as on the detectability is included. The results verify the theoretical description given in our accompanying paper.

© 2001 Optical Society of America

OCIS Codes
(190.2620) Nonlinear optics : Harmonic generation and mixing
(300.1030) Spectroscopy : Absorption
(300.6260) Spectroscopy : Spectroscopy, diode lasers
(300.6380) Spectroscopy : Spectroscopy, modulation

Original Manuscript: May 30, 2000
Revised Manuscript: September 14, 2000
Published: February 20, 2001

Pawel Kluczynski, Åsa M. Lindberg, and Ove Axner, "Background signals in wavelength-modulation spectrometry by use of frequency-doubled diode-laser light. II. Experiment," Appl. Opt. 40, 794-805 (2001)

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  18. Although the Doppler broadening, which gives rise to a Gaussian line broadening, in many instances can rival (and even dominate) the collision–lifetime broadening, which gives rise to a Lorentzian line profile, in this study, we chose to describe the analytical signal as a pure Lorentzian profile. The main reason for our choice is that the use of a Lorentzian profile simplifies the SBR analysis in this study considerably. There are several reasons for this simplification: First, the previously developed theoretical formulation makes use of various Fourier components of the line-shape function. There exist convenient analytical expressions for a Lorentzian profile, whereas no comparable expressions have yet been found for the Gaussian or the Voigt functions. Second, there is only a small difference in the WM signal shape from the Lorentzian- and the Gaussian-shaped profiles. Third, the main aim of the SBR analysis is to investigate the behavior of the various nf harmonics of the background signal under various conditions. For the purpose of a comparison of various types of background signal to an analytical signal, we are of the opinion that a Lorentzian description of the analyte therefore serves the purpose of this study sufficiently well.
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