To make use of this effect, light must propagate through the crystal normal to a direction in which the refractive index change can be produced. If the incident polarisation direction is at 45° to the principle refractive index values, then the light will exit the crystal as two orthogonal components with a phase difference between the ordinary and the extra-ordinary ray which is linearly proportional to the applied electric field. These components combine in space to form generally elliptical polarisation states (linear and circular polarisation being special forms of elliptical polarisation).
The Pockels cell designer must choose an appropriate material which produces a large induced birefringence and then maximize its effect by a sensible choice of crystal cut, light path and applied electric field directions. When the electric field is applied along the light path then the modulator is referred to as longitudinal and if the field is across the light path, then the modulator is transverse.
The most frequently used materials for longitudinal mode Pockels cells are KDP and more commonly, the deuterated form KD*P. This has a modest half wave voltage (the voltage which must be applied to produce a pi radian phase shift, equal to a polarisation rotation of 90°) of around 6kV at 1064nm and has extremely high resistance to laser damage as well as being available in excellent quality crystals of large sizes. For these reasons, all our longitudinal mode Pockels cells are manufactured from >95% deuterated KD*P.