Application notes on circular dichroism and linear dichroism typically show no optical components between the sample and the detector. In actual experiments, however, there may be windows, lenses, mirrors, etc. with birefringence and/or diattenuation effects. These effects can cause artifacts in the detected measurement.
By adding a second PEM immediately after the sample, the artifacts referred to above can be eliminated from the measurement.
The second PEM must operate at a different frequency than the first PEM and must be set to a retardation A0 = 2.405 radians (.383 waves) where the Bessel function J0(A0) = 0.
J.C. Cheng, Polarization scrambling using a photoelastic modulator: Application to linear dichroism measurement, Rev. Sci. Instrum. 47, 6, p 0004, June, 1976.
Contact us for more information about using PEMs in a polarization scrambling set-up.