1 July 1996 High-frequency-modulation spectroscopy: phase noise and refractive index fluctuations in optical multipass cells
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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.
Peter W. Werle, Peter W. Werle, Bernd Jaenker, Bernd Jaenker, } "High-frequency-modulation spectroscopy: phase noise and refractive index fluctuations in optical multipass cells," Optical Engineering 35(7), (1 July 1996). https://doi.org/10.1117/1.600784 . Submission:
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