Recent findings and approaches for the suppression of fluctuations and background drifts in tunable diode laser spectroscopy

Abstract

Substantial improvements of tunable diode laser absorption spectroscopy (TDLAS) with respect to detection speed and detection 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. TDLAS measurements are usually limited by signal instabilities due to fluctuations, which can be divided generally into slow and fast fluctuations according to their time scale. While slow fluctuation or drift effects can be attributed to thermal effects, the question is now, what are the short term fluctuations that limit sensitivity. Therefore, in this paper we will identify phase noise and refractive index fluctuations caused by a turbulent gas flow in optical multipass cells as one limiting factor for the achievement of quantum limited sensitivities. Additionally some new results for a laser sample double modulation experiment based on the Stark-effect in molecular spectra will be presented and we will show that a significant suppression of background fluctuations caused by drift effects can be obtained with this technique.