Pockel cells are voltage-controlled wave plates. The Pockels effect is the basis of Pockels cells operation, Pockels cells may be used to rotate the polarization of a passing beam. See Applications below for uses.
A transverse Pockels cell comprises two crystals in opposite orientation, which give a zero order wave plate when voltage is turned off. This is often not perfect and drifts with temperature. But the mechanical alignment of the crystal axis is not so critical and is often done by hand without screws; while misalignment leads to some energy in the wrong ray (either e or o – for example, horizontal or vertical), in contrast to the longitudinal case, the loss is not amplified through the length of the crystal.
The electric field can be applied to the crystal medium either longitudinally or transversely to the light beam. Longitudinal Pockels cells need transparent or ring electrodes. Transverse voltage requirements can be reduced by lengthening the crystal.
Alignment of the crystal axis with the ray axis is critical. Misalignment leads to birefringence and to a large phase shift across the long crystal. This leads to polarization rotation if the alignment is not exactly parallel or perpendicular to the polarization.
Read more about this topic: Pockels Effect
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