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High-frequency-modulation spectroscopy. Phase noise and refractive index fluctuations in opical multipass cells

: Werle, P.; Jänker, B.


Optical engineering 37 (1996), Nr.7, S.2051-2057 : Ill., Lit.
ISSN: 0091-3286
ISSN: 0036-1860
Fraunhofer IFU; 2002 in Helmholtz-Gesellschaft integriert
Diode Laser Spectroscopy; multipass cell; quantum limit; refractive index fluctuation

Substantial improvements of tunable diode laser absorption spectroscopy (TIDLAS) 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 my bars in such a multipass cell, the expected sensitivity is limited to 7 x 10(exp -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.