High‐frequency‐modulation spectroscopy: phase noise and refractive index fluctuations in optical multipass cells

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 ex- pected 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 perfor- mance of highly sensitive spectroscopic measurements. It has been found that for measured pressure fluctuations of about 80 ubars in such a multipass cell, the expected sensitivity is limited to 7 x10 -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. 0 1996 Society of Photo-Optical Instrumentation Engineers. Subject terms: multipass cells; refractive index fluctuations; quantum limit; diode laser spectroscopy.