Overview

Principle of Waveplate

Waveplates  (retardation plates or phase shifters) are made from materials which  exhibit birefringence. The velocities of the extraordinary and ordinary  rays through the birefringent materials vary inversely with their  refractive indices. The difference in velocities gives rise to a phase  difference when the two beams recombine. In the case of an incident  linearly polarized beam this is given by a=2p*d (ne-no)/l (a-phase  difference; d-thickness of waveplate; ne, no-refractive indices of  extraordinary and ordinary rays respectively; l-wavelength). At any  specific wavelength the phase difference is governed by the thickness of  the retarder.

Half Waveplate

The thickness of a  half waveplate is such that the phase difference is 1/2-wavelength  (true-zero order) or some multiple of 1/2-wavelength [(2n+1) L/2multiple  order].
A linearly polarized beam incident on a half waveplate emerges as a  linearly polarized beam but rotates such that its angle to the optical  axis is twice that of the incident beam. Therefore, half waveplates can  be used as continuously adjustable polarization rotators. Half  waveplates are used in rotating the plane of polarization, electro-optic  modulation and as a variable ratio beamsplitter when used in  conjunction with a polarization cube.

Quarter Waveplate

The thickness of  the quarter waveplate is such that the phase difference is 1/4  wavelength (L/4, true-zero order) or some multiple of 1/4 wavelength  [(2n+1)L/2,multiple order].
If the angle q (between the electric field vector of the incident  linearly polarized beam and the retarder principal plane) of the quarter  waveplate is 45, the emergent beam is circularly polarized. When a  quarter waveplate is double passed, i.e. by mirror reflection, it acts  as a half waveplate and rotates the plane of polarization to a certain  angle. Quarter waveplates are used in creating circular polarization  from linear or linear polarization from circular, ellipsometry, optical  pumping, suppressing unwanted reflection and optical isolation.