Pockels Cell Q-Switch

The Pockels cell contains an electro-optic crystal in which a refractive index change is produced by an externally applied electric field. Crystals are classified into 32-point groups according to their structure. Only 20-point groups, namely those that lack a center on symmetry, exhibit a nonvanishing electro-optic effect. The index change produced by an externally applied field is described in each case by a 6 × 3 matrix of electro-optic coefficients. The number of coefficients is greatly reduced by the structural characteristics of crystals. The most commonly used electro-optic materials have only a few distinct coefficients. The location of a coefficient in the matrix array interrelates the crystal orientation, the applied field direction, and the polarization and direction of the optical beam. The magnitude of each coefficient determines the strength of the electro-optic effect for each geometry.

The basic requirements for a crystal to be useful as an electro-optic Q-switch are: a good optical quality combined with a high laser damage threshold and a large electro-optic coefficient for light propagating parallel to the optic axis. The latter requirement is important because the two-phase-shifted orthogonal components of the beam travel along the same path only if the direction of the light beam is either parallel or normal to the optical axis of the crystal. For other directions the fast and slow axes of the beam include a small angle. Two crystals that meet these criteria and are widely employed in electro-optical Q-switches are potassium dihydrogen phosphate (KDP) and lithium niobate (LiNbO₃). KDP (KH₂POH) and its isomorph KD^∗P (KD₂POH) are grown at room temperature from a water solution that yields large distortion-free single crystals. The attributes of this family of crystals are their high damage threshold and excellent optical quality combined with a large electro-optic coefficient. Crystals with cross sections up to 100mm have been produced. A disadvantage is the fact that the crystals are fairly soft and hygroscopic and must be protected from the environment by enclosing them in cells that are hermetically sealed or filled with an index matching fluid. In order to avoid walk-off between the fast and slow beam axes, the electric field has to be applied longitudinally in the same direction as the beam propagation axis and the optical axis of the crystal. The electric field is applied to the crystal by means of a pair of electrodes that contain openings for passage of the laser beam.
Lithium niobate is grown from the melt; it is hard and nonhygroscopic. The crystals can be antireflection coated and do not need a special protection from the environment. Also, the quarter and half-wave voltages are about a factor of 2 lower compared to KD^∗P. In LiNbO₃, the electric field is applied perpendicularly to the beam propagation and optical axis of the crystal. As we will see, this has the advantage of reducing the required voltage by the width to length ratio of the crystal. A drawback of LiNbO₃ is that the crystal is only available in relatively small sizes, with cross sections of about 10 mm, and the laser damage threshold is considerably below that of KD^∗P.