Waveplates (retardation plates or phase shifters) are made from materials which exhibit birefringence. The velocities of the extraordinary and ordinary rays through the birefringent material varies inversely with their refractive indices. This difference in velocities gives rise to a phase difference when the two beams recombine.
Half Waveplate
A half waveplate rotates linearly polarized light to any desired orientation. The rotation angle is twice the angle between the incident polarized light and the optical axis. Therefore, the half waveplate can be used as a continuously adjustable polarization rotator. The half waveplate is used to rotate the plane of polarization, electro-optic modulation and as a variable ratio beamsplitter when used in conjunction with a polarization cube.
Quarter Waveplate
When linearly polarized light is input at 45deg to the axis of a quarter waveplate, the output is circularly polarized, similarly, input circularly polarized light is transformed into linearly polarized light. The quarter waveplate is used in creating circular polarization from linear or linear polarization from circular, ellipsometry, optical pumping, suppressing unwanted reflection and optical isolation.
