Pockels effect

From Wikipedia, the free encyclopedia

The Pockels effect, or Pockels electro-optic effect, produces 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.

[edit] 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 longitudinal or transverse to the light ray. Longitudinal Pockels cells need transparent or ring electrodes. Transverse voltage requirements can be reduced by lengthening the crystal.

The crystal axis can be longitudinal or transverse. A longitudinal cell has to be quite big, as the crystals are somehow inefficient in this mode. Alignment of the crystal axis with the ray axis is critical as misalignment leads to birefringence and to a large phase shift across the long crystal. This leads to polarization rotation if the alignment is not exactly parallel or perpendicular to the polarization. A transverse cell consists of two crystals 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; while misalignment leads to some energy in the wrong ray (either e or o), in contrast to the longitudinal case, this is not amplified through the length of the crystal.

Pockels cells may be used to rotate the polarization of a passing beam. See Applications below for uses.

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; consequently, fast switching requires large currents. Pockels cells for fibre optics may employ a travelling wave design to reduce current requirements.

[edit] Applications of Pockels cells

Pockels cells are used in a variety of scientific and technical applications:

A Pockels cell, combined with a polarizer, can be used for a variety of applications. Switching between no optical rotation and 90° rotation creates a fast shutter capable of "opening" and "closing" in nanoseconds. The same technique can be used to impress information on the beam by modulating the rotation between 0° and 90°; the exiting beam's intensity, when viewed through the polarizer, contains an amplitude-modulated signal.
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 exit. 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.