We present a compact polarimeter, which can perform sensitive measurements of optical rotation in vapor. The operation of the polarimeter is based a Cavity Ring-Down scheme which employs two signal reversals, which increase sensitivity and reduce noise, allowing the realization of sensitive measurements in the presence of spurious birefringence. We describe the operation of the polarimeter, give the basic equations for the signal analysis and retrieval of optical rotation angle, and present measurements that demonstrate a sensitivity of ~80 μdeg/pass.
An improved optical cavity-based polarimetry method is employed to measure the optical activity of lysozyme in water solution, in the concentration range of 0-2 mg/ml. We employ a signal reversing technique, which gives the absolute optical rotation, without needing to remove the sample for a null measurement. We report an absolute sensitivity limit on the order of 0.1 mdeg, corresponding to a detection limit of <50 μg/ml for a sample volume lower than 50 μL, thus surpassing the sensitivity of existing commercial polarimeters. We discuss how these sensitivity levels can be further improved using existing methods and technologies.
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