Pockels effect
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The Pockels effect, or Pockels electro-optic effect, is the production of birefringence in an optical medium induced by a constant or varying electric field. It is distinguished from the Kerr effect by the fact that the birefringence is proportional to the electric field, whereas in the Kerr effect it is quadratic in the field. The Pockels effect occurs only in crystals that lack inversion symmetry, such as lithium niobate or gallium arsenide.
Friedrich Carl Alwin Pockels studied the effect, which bears his name, in 1893.
Pockels cells
The Pockels effect is used to make Pockels cells, which are voltage-controlled wave plates. The electric field can be applied to the crystal medium either longitudinally or transversely to the light beam.
The electric field can be transversal to the light ray. Voltage requirements can be reduced by lengthening the crystal. The electric field can also be longitudinal. These pockels cells need transparent or ring electrodes.
The crystal axis can be longitudinal. This cell has to be quite big, as the crystals are somehow inefficient in this mode, even lithium niobate. Alignment of the crystal axis with the ray axis is critcal as misalignment leads to birefringence and to a large phase shift cross the long crystal. This leads to rotation of polarization if the misalignment is not exactly parallel or perpendicular to the polarization. The crystal axis can be transversal. These pockels cells consists of two crystal in opposite orientation, which give a zero order wave plate when voltage is turned off. This is often not perfect and drifts with temperature. But the mechanical alignment of the crystal axis is not so critical and is often done by hand without screws, because misalignment leads to some energy in the wrong ray ( either e or o ), but in contrast to misalignment in the longitudinal case, this is not amplified through the length of the crystal.
Pockels cells may be used to modulate the phase of a passing beam. If the Pockels cell is rotated 45° with respect to the polarization of the light, this leads to rotation of the polarization of a light beam by 90°. In combination with a polarizer, this can be used to create a fast shutter which can open and close in nanoseconds.
Because of the high dielectric constant of the crystal, Pockels cells behave like a capacitor. When switching these to high voltage a high charge is needed. When switching quickly, a very large current is needed. Pockels cells for fibre optics may employ a travelling wave design to reduce current requirements.
Applications of Pockels cells
Pockels cells are used in a variety of scientific and technical applications:
- Preventing the feedback of a laser cavity by using a polarizing prism. This prevents optical amplification by directing light of a certain polarization out of the cavity. Because of this, the gain medium is pumped to a highly excited state. When the medium has become saturated by energy, the Pockels cell is switched, and the intracavity light is allowed to feedback into . This creates a very fast, high intensity pulse. Q-switching, chirped pulse amplification, and cavity dumping use this technique.
- Pockels cells can be used for quantum key distribution by polarizing photons.
- pulse picking
See also
- Electro-optic modulator
- Acousto-optic modulator
- Pockels readout optical modulator
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