Polarization scrambling using a photoelastic modulator: Application to circular dichroism measurement

In circular dichroism (CD) measurement using conventional modulation-spectroscopy techniques, spurious signals arise when birefringence and polarization sensitivity are simultaneously present along the optical path. We show that when such spurious signals originate in components subsequent to the sample, they can be removed by a second modulator placed immediately after the sample. This second modulator, which must have a different frequency from the first and must be adjusted to a specific retardation level, scrambles the polarization of the light so that the time average of the signal is zero. The true CD signal of the sample, on the other hand, is unaffected. An experimental demonstration of this result is presented. Spurious signals due to imperfections in components before the sample or in the sample itself cannot be nulled in the same manner.