Pockels Cell

A Pockels Cell is a device which contains a photo refractive electro-optic crystal. When a voltage is applied to this crystal it can change the polarisation or phase of the light beam.
Depending on the orientation of the crystal either the polarisation can be altered or a phase change can be introduced.

Polarisation & Intensity Modulation

A Pockel Cell may be considered to act as a variable wave retarder plate. When a voltage is applied the state of polarisation of the input laser beam is changed. With the addition of a polariser at the output intensity modulation is achieved.

Electro-optic crystals have an inherent residual birefringence which can be compensated for by the application of a bias voltage.

varying polarisation by application of voltage

The voltage required to make the device act as a halfwave plate is called the half-wave voltage. This half-wave voltage is wavelength dependant but can be calculated using the following formula:

Half-wave voltage formula

Phase Modulation

The phase change introduced to the laser beam is directly proportional to the applied voltage.

Phase Modulation Formula

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Subjects: Optics