High Performance Continuous-Wave Laser Cavity Enhanced Polarimetry Using Rf-Induced Linewidth Broadening

We present precise optical rotation measurements of gaseous chiral samples using near-IR continuous-wave cavity-enhanced polarimetry. Optical rotation is determined by comparing cavity ring-down signals for two counter-propagating beams of orthogonal polarisation which are subject to polarisation rotation by the presence of both an optically active sample and a magneto-optic crystal. A broadband RF noise source applied to the laser drive current is used to tune the laser linewidth and optimise the polarimeter, and this noise-induced laser linewidth is quantified using self-heterodyne beat-note detection. We demonstrate the optical rotation measurement of gas phase samples of enantiomers of alpha-pinene and limonene with an optimum detection precision of 10 mu deg per cavity pass and an uncertainty in the specific rotation of similar to 0.1 deg dm(-1) (g/ml)(-1) and determine the specific rotation parameters at 730 nm, for (+)- and (-)-alpha-pinene to be 32.10 +/- 0.13 and -32.21 +/- 0.11 deg dm(-1) (g/ml)(-1), respectively. Measurements of both a pure R-(+)-limonene sample and a non-racemic mixture of limonene of unknown enantiomeric excess are also presented, illustrating the utility of the technique. (C) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement