Pockels cells, also known as electro-optic modulators, are devices used in building modulators. They fall under photonic devices and comprise electro-optic crystals with some attached electrodes. You can propagate light beams through these, creating a phase delay in the crystal, which goes by the name Pockels Effect. You can modulate this effect by varying the electric voltage. For this reason, the cell acts as a voltage-controlled waveplate and is a building component of electro-optic modulators. An excellent example of Pockels Cells applications would be in Q switching lasers.

Workings of Pockels Cells

The operation of the cell comes down to the direction of the applied electric devices. In this case, we have two options.

Longitudinal

These devices face the electric field in line with the light beam, which propagates through the electrode holes. This direction makes it easy to realize a large aperture (opening of the lens) because the drive voltage remains independent of the aperture. As for the electrodes, these can either be of metallic rings towards the end faces or could have transparent layers towards their outer surfaces. Losses are visible through the crystal length.

Transverse

This other option has the electric field positioned perpendicularly to the light beam. The crystals lie in opposite directions such that when the voltage goes off, a zero-order wave plate is seen. However, this alignment is subject to temperature variations. It helps to note that crystal mechanical alignment does not play such a significant role in the cell's working. As such, you can even do the alignment by hand, devoid of the need to work with screws. Any loss will not be seen throughout the length of the crystal.

You can apply an electric field to the crystal using either longitudinal or transverse alignment, as shown. In the case of a transverse alignment, you can reduce the voltage requirements by cutting the length of the crystal short. Please note that the alignment of the ray axis to the crystal axis plays a vital role in the operation's outcome. Where misalignment is in play, birefringence shows (where the material has a refractive index dependent on polarization and propagation of light).

Types of Pockels Cells

There are many variations of these cells in the market. Let's consider a few options:

KD*P

When you use birefringence-inducing voltage in a crystal such as KD*P and BBO, the cell changes the polarization state of the light moving through it. When you add polarizers to the mix, you can use the cells as laser Q switches, shortening the output pulse and enhancing the peak's intensity. These handle wavelengths from UV up to 1.1 µm, making them practical for longer wavelengths.

BBO

These cells are useful at wavelengths from the UV up to 2 µm. They are better at handling high powers than KD*P though their electro-optic coefficient is much smaller. For this reason, their voltages are usually high.

LiNb03

These cells can work at 1.064 µm as well as higher wavelengths. They can also be configured transversely to operate at a much lower voltage than KD*P by altering the thickness and length of the crystal. These cells also work for infrared wavelengths as long as 4 µm.

EKSMA Optics is a leading provider of Pockels Cells and produces KTP, KD*P, and BBO Pockels Cells. These are useful in pulse picking, laser Q switching, coupling laser pulses from and into regenerative amplifiers, and laser cavity dumping. You can apply these with drivers, power supplies, and mounting stages alike. The company also provides a wide array of electro-optics such as Pockels Cells drivers, HV power supplies, pulse picking systems, and Q-switching kits. For more information, you can visit their website.