If linearly polarized light passes through a chiral media, for example a solution of chiral molecules, the polarization plane of the incident light will be rotated. This is called optical rotation (or natural optical rotation in order to distinguish this from Faraday rotation in a magnetic field).
Any phase difference (retardance) between the two circular components produces a rotation of the polarization plane.
Optical rotation has been measured using this setup at Hinds. High sensitivity for optical rotation (0.001 degree) has been achieved. The signal is detected at 2f.
The optical rotation r is proportional to V2f/VDC. If the second polarizer is in a precision rotator, the system may be calibrated by removing the sample, varying the rotator angular position and recording V2f/VDC vs. the rotator position.
Chiral analysis in chemistry, pharmaceutical and biological industries
Signaloc™ 2100, dual-phase, analog lock-in amplifier
J. C. Kemp, Basic Laboratory setup for various measurements possible with the photoelastic modulator, Application note, Hinds Instruments, Inc. (1975)
T. C. Oakberg, Linear Birefringence and Optical Rotation, Application note, Hinds Instruments, Inc. (1992).
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