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Pockels Cells

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Definition: electro-optic devices, used for building modulators

German: Pockelszelle

Category: photonic devices

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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:

Pockels cells with longitudinal electric field

Figure 1: Pockels cells with longitudinal electric field. The electrodes are either rings on the end faces (left side) or on the outer face (right side).

Pockels cells with transverse electric field

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

Pockels cell

Figure 3: A Pockels cell based on KD*P, which can be used for Q switching of solid-state lasers. The photograph has been kindly provided by EKSPLA.

An important property of a Pockels cell is the half-wave voltage Vπ. 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
and other articles in the category photonic devices

In the RP Photonics Buyer's Guide, 30 suppliers for Pockels cells are listed.

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