Poincare sphere interpretation of Kerr-effect measurements using a photoelastic modulator

The author describes a highly visual application of the Poincare sphere to magnetooptic detection with a photoelastic modulator (PEM) that clarifies how such a large modulation can yield a measurement that is sensitive to small polarization changes in ellipticity and rotation. By viewing the PEM action on the Poincare sphere, one can directly see not only the modulation of the polarization, but also the measured output intensity with time. The mechanism by which changes in ellipticity and rotation can be independently detected then becomes apparent. Through proper use of the Poincare sphere, experimental set-ups for measuring Kerr rotation and ellipticity become much easier to interpret