Pockels Effect is the linear electro-optic effect in which the refractive index of the medium is modified with respect to an applied electric signal or voltage. This effect was discovered by the German physicist Friedrich Pockels in 1893. It occurs only in non-centrosymmetric crystals, i.e., crystals with inversion symmetry. Lithium niobate (LiNbO₃), lithium tantalate, potassium dihydrogen phosphate (KDP), etc are examples of such materials. These crystals that work on the Pockels effect are called Pockels cells. They are transparent and have a high electro-optic coefficient, which means that they exhibit a large change in refractive index in the presence of an electric field.

Figure 1: Pockels Effect

Applying a voltage across the Pockels cell causes the plane-polarized light passing through the crystal to split into two perpendicular vectors. The difference in phase or the magnitude of polarization between these vectors is directly proportional to the strength of the electric field. Intensity modulation of the output laser beam is analyzed by crossed polarizers.

The change in refractive index is proportional to the square of the electric field strength and the electro-optic coefficient of the crystal. The electro-optic coefficient is a material property that describes the extent to which the refractive index changes in response to an electric field.

Pockels Cell

Pockels cells are optical devices that use an electric field to modulate the polarization state of light. They are voltage-controlled waveplates. Pockels cells operate based on the Pockels effect. The Pockels cell consists of a crystal made of an electro-optic material sandwiched between two electrodes. They may be used to rotate the polarization of a beam that passes through it. Pockels cells are commonly used in a variety of applications, including telecommunications, optical switching, and laser technology.

Working of Pockels Cell

Figure 2: Working of Pockels cell

When an electric field or voltage is applied to the Pockels cell, the refractive index of the crystal changes. This change in the refractive index leads to a change in the polarization of light passing through the crystal. The polarization of the input beam should be aligned with one of the optical axes of the crystal. The Pockels cell modifies the polarization state of the incident light beam, which is then converted into a change in the transmitted optical power and amplitude. The degree of polarization change is proportional to the strength of the electric field.

Pockels cell for Q-switching

Pockels cells can be used for Q-switching, which is a technique for producing short, high-energy laser pulses. In Q-switching, a laser is first pumped to a high-energy state, and then the laser output is blocked by a shutter or a Pockels cell. When the shutter or Pockels cell is opened, the stored energy is released in the form of a short, high-energy pulse.

Figure 3: Q-switching using Pockels cell

Before switching, on its first pass through the cell, the light transforms into circular polarization. On the next pass, the light becomes horizontally polarized and is then rejected by the polarizer. After switching, the light is unaffected by the Pockels’ cell and hence is passed by the polarizer.

Applications of Pockels cell

Pockels cells have applications in science and technology. When combined with a polarizer, a Pockel cell can serve various purposes. Switching between no optical rotation and 90˚ rotation creates a faster shutter capable of “opening” and “closing” in nanoseconds. 

They are used in telecommunications, particularly in fiber-optic communication systems to modulate the polarization of light, which is then used to transmit information. Pockels cells can also be used in laser technology to control the polarization of laser light, which is important in applications such as laser machining and laser spectroscopy.

Pockels cells find application in Q-switching, chirped pulse amplification, and cavity dumping. Polarizing photons with Pockels cells enables their use in quantum key distribution. Also, Pockel cells can be combined with other electro-optic elements to create electro-optic probes. 

They are also used in many applications in optics, including electro-optic modulators, optical switches, and optical phase shifters.