Pockels cells are voltage-controlled wave plates. The basis of the operation is the Pockels effect which describes the change or generation of birefringence in an optical medium induced by a linear electric field. Pockels cells are commonly used in science and industry to rotate the polarisation of a beam that passes through.
The electric field can be applied to the crystal medium either in longitudinal or transverse configuration with respect to the light beam. Typical longitudinal Pockels cells possess no natural birefringence and thus achieve high extinction ratios. Their halfwave voltage is high but independent (in first order) of the crystal geometry which is useful for high power applications. Transverse arrangements allow geometry dependent halfwave voltage adjustments but suffer from imperfect natural birefringence compensations which limits the extinction ratio and temperature stability.
Alignment of the crystal axis with respect to the ray axis is critical. Misalignment leads to unwanted birefringence which reduces the extinction ratio.