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Optical Engineering

High-frequency-modulation spectroscopy: phase noise and refractive index fluctuations in optical multipass cells
Author(s): Peter W. Werle; Bernd Jaenker
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Paper Abstract

Substantial improvements of tunable diode laser absorption spectroscopy (TDLAS) with respect to detection speed and limits were obtained by introducing high-frequency modulation schemes, but the expected quantum-limited performance with optical multipass cells has not been attained yet on a routine basis. This paper is primarily devoted to the question of how refractive index fluctuations generated by a turbulent gas flow through an optical multipass cell affect the phase of a frequency-modulated laser beam and therefore influence the performance of highly sensitive spectroscopic measurements. It has been found that for measured pressure fluctuations of about 80 µbars in such a multipass cell, the expected sensitivity is limited to 7 X 10-7 in terms of optical density, which is more than one order of magnitude above the quantum limit. Further sensitivity improvement by signal averaging is limited by 1/f noise contributions from turbulence for integration times longer than 30 s. The consequences of the pressure fluctuations for absorption measurements with tunable diode laser-based systems are discussed with respect to state-of-the-art technology detection limits.

Paper Details

Date Published: 1 July 1996
PDF: 7 pages
Opt. Eng. 35(7) doi: 10.1117/1.600784
Published in: Optical Engineering Volume 35, Issue 7
Show Author Affiliations
Peter W. Werle, Fraunhofer-Institut fuer Atmosphaerische Umweltforschung (Germany)
Bernd Jaenker, Fraunhofer Inst fuer Atmospharische Umweltforschun (Germany)


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