 The PEM is adjusted to cause an alternating plus and minus 1/4 wavelength shift of one of the two orthogonal components of the ordinary beam.This modulation converts the linearly polarized light into circularly polarized light at the peaks of the modulation cycle.Linearly polarized light can be decomposed into two circular components with intensity represented as formula_2

The PEM will delay one component of linearly polarized light with a time dependence that advances the other component by 1/4 λ (hence, quarter-wave shift).