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Encyclopedia of Laser Physics and Technology

© Dr. Rüdiger Paschotta

Last update: 2008-07-11

Definition: electro-optic devices, used for building modulators

A Pockels cell is a device consisting of an electro-optic crystal (with some electrodes attached to it) through which a light beam can propagate. The phase delay in the crystal (→ Pockels effect) can be modulated by applying a variable electric voltage. The Pockels cell thus acts as a voltage-controlled waveplate. Pockels cells are the basic components of electro-optic modulators, used e.g. for Q switching lasers.

Geometries and Materials

Pockels cells can have two different geometries concerning the direction of the applied electric field:

• Longitudinal devices have the electric field in the direction of the light beam, which passes through holes in the electrodes. Large apertures can easily be realized, as the required drive voltage is basically independent of the aperture. The electrodes can be metallic rings (Figure 1, left) or transparent layers on the end faces (right) with metallic contacts.

Figure 1: Pockels cells with longitudinal electric field.

• Transverse devices have the electric field perpendicular to the light beam. The field is applied through electrodes at the sides of the crystal. For small apertures, they can have lower switching voltages.

Figure 2: Pockels cells with transverse electric field. On the left is a bulk modulator and on the right a waveguide modulator.

Common nonlinear crystal materials for Pockels cells are potassium di-deuterium phosphate (KD^*P = DKDP), potassium titanyl phosphate (KTP), β-barium borate (BBO) (the latter for higher average powers and/or higher switching frequencies), lithium niobate (LiNbO₃), lithium tantalate (LiTaO₃), and ammonium dihydrogen phosphate (NH₄H₂PO₄, ADP).

Half-wave Voltage

An important property of a Pockels cell is the half-wave voltage V_π. This is the voltage required for inducing a phase change of π. In an amplitude modulator, the applied voltage has to be changed by this value in order to go from the operation point with minimum transmission to that with maximum transmission.

The half-wave voltage of a Pockels cell with transverse electric field depends on the crystal material, the electrode separation, and the length of the region where the electric field is applied. For larger open apertures, the electrode separation needs to be larger, and hence also the voltages.

For a Pockels cell with longitudinal electric field, the crystal length does not matter, since e.g. a shorter length also increases the electric field strength for a given voltage. Larger apertures are possible without increasing the half-wave voltage.

Typical Pockels cell have half-wave voltages of hundreds or even thousands of volts, so that a high-voltage amplifier is required for large modulation depths. Relatively small half-wave voltages are possible for highly nonlinear crystal materials such as LiNbO₃, and for integrated optical modulators with a small electrode separation, but such devices have a limited power handling capability.

See also: Pockels effect, electro-optic modulators, nonlinear crystal materials, Q switching

Category: photonic devices