A Pockels cell is an electro-optical device used for switching the direction of the polarization of light beams. It is the primary component of various optical devices, such as electro-optical modulators, and Q-switches for lasers. The device was named after Friedrich Carl Alwin Pockels who first studied the Pockels effect in 1893, based on which the Pockels cell operates.
Pockels cells have two types of geometries with respect to the direction of application of an electric field. Longitudinal devices have an electric field in the direction of light beam that passes via the holes in the electrodes. They are provided with large apertures.
Transverse devices have an electric field that is applied via electrodes, and at a right angle to the light beam. These devices have small apertures and lower switching voltages.
Working Principle of Pockels Cell
Pockels cells work on the principle of Pockels effect, which describes the linear changes in the refractive index of optical materials with the application of an external electric field. A typical Pockels cell consists of an electro-optical crystal, through which a light beam can propagate.
When a constant or variable voltage is applied to the crystal, it produces linear changes in the birefringence of the crystal. A Pockels cell behaves as a voltage-controlled wave plate when applied with a constant voltage. The application of a variable voltage to the cell allows modulating the phase delay in the crystal.
The Pockels effect takes place only on crystals that do not have inversion symmetry, such as gallium arsenide and lithium niobate crystals, and in other non-centrosymmetric media, such as glasses or electric-field poled polymers.
Applications of Pockels Cell
Pockels cells are widely used in the following applications:
- Light intensity modulation
- Q-switching of laser cavities
- Quantum key distribution
- Coupling light within and outside regenerative amplifiers