Southwest Sciences Inc

Envelope Functions for Modulation Spectroscopy

U. S. Patent 6,940,599, issued September 6, 2005

U. S. Patent No. 7,230,711, issued June 12, 2007

Wavelength modulation, frequency modulation and tone burst spectroscopy techniques provide high sensitivity for detecting absorption features of gases, allowing for detection of very small quantities of these gases. The instrument function for a modulated laser spectrometer differs from that of a direct absorption spectrometer, in ways that are well-understood (see for example Silver, Appl. Opt. 31, 707-717, 1992, or Kluczynski et al., Appl. Opt. 38 5803-5815 1999). Modulating with a sine wave leads to an instrument function that is not well matched to the spectral features of the gases. This results in unwanted sensitivity to etalon fringes. By introducing a second sine wave modulation, Cassidy and Reid showed that this unwanted sensitivity could be suppressed for a small range of unwanted fringes (Applied Physics B: Lasers and Optics, 29 279-285, 1982).

These patents describe a method for matching the instrument function of a modulated spectrometer to the target absorption feauture. Sensitivity to etalons is suppressed over a wide range of etalon periods. This is achieved by multiplying the sine wave by an envelope function that is allowed to vary. The figure shows the sensitivity of the instrument to etalons as a function of the etalon period. Unlike conventional modulation spectrometer, for which the response oscillates, the envelope-modulated spectrometer is sensitive to only a small range of etalons. The remaining etalons have spectral signatures similar to those of the target absorption feature, so minimizing the response to those etalons would not improve the sensitivity of the spectrometer.


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Southwest Sciences, Inc.
1570 Pacheco St., Suite E-11, Santa Fe, NM 87505
tel. (505) 984-1322/ fax (505) 988-9230



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