BBO Pockels Cells
The Pockels cell is an electro-optic device that is used to rotate the polarization of light beams. To accomplish something, their method of operation is the Pockels effect, which changes the refractive index of non-centrosymmetric crystalline materials by means of an applied electric field. Pockels cells offer a faster response time than acousto-optic devices because their switching behavior is strongly affected by the drive electronics.
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The price, performance, and reliability advantages of Pockels cells fueled their recent rise to prominence in modern optics. The device is composed of an electro-optic crystal that changes its refractive index linearly when voltage is applied to the attached electrodes. Linearly polarized light that travels in the direction of the optical axis can’t be altered by the crystal. But the crystal becomes birefringent when exposed to an external electrical field, which causes a phase shift in the propagating light wave. Because of this, an electric signal can modulate a part of the light beam with accurate and quick control.
Pockels cells are frequently used in numerous everyday applications, including:
- Laser cavity Q-switching
- Using regenerative amplifiers that couple light in and out of the circuit
- Modulation of light intensity, when combined with a pair of polarizers
- Pulse Picking
Pockels cells feature a unique property that comes in handy when trying to calculate the half-wave voltage: Uπ (also known as Uλ/2 or Vλ/2). Equivalent to half an optical wavelength, this voltage is required to induce a phase change, or change in state, of π. In an amplitude modulator, the applied voltage must be changed by that amount in order to change the device’s transmission characteristics from the operating point with the least amount of transmission to the operating point with the greatest amount of transmission.
When using a Pockels cell with transverse electric field, the voltage is affected by the type of crystal, the spacing between the electrodes, and the length of the electric field applied region. Other things can be done to decrease its size, for example, using a longer crystal. The voltages needed for larger open apertures are larger, which means that the electrode separation should be increased.
The crystal length has no impact on the Pockels cell’s longitudinal electric field, which, for example, also increases as the crystal length decreases. Increasing the half-wave voltage of a larger aperture is not required. Pockels cells require high-voltage amplifiers because they output half-wave voltages in the hundreds or even thousands of volts. Nonlinear crystal materials such as LiNbO3 permit relatively small half-wave voltages, and for integrated optical modulators with a small electrode separation, power handling capability is limited.
For example, let’s consider a Pockels cell-based intensity modulator where the input beam is polarized at an angle of 45° with respect to the crystal’s optical axis. When there is no applied electric field, we assume that the crystal has no birefringence, and that it has a specific half-wave voltage of Uπ. Polarizers are mounted behind the crystal, and then aligned so that 100% transmission occurs without any applied voltage. This in-phase component can be regarded as a superposition of two amplitude-equivalent in-phase components. Those field components acquire a phase difference of Δφ=πU/Uπ with an applied electric field. We can now calculate the total transmitted amplitude as follows: Total transmitted amplitude = 0.5 × (1 + exp [iΔφ])
The polarizer must be rotated so that there is zero voltage and zero transmission, and in doing so, we instead get sin instead of cos. It is demonstrated in the calculation that in order to change the transmission of an identity modulator from zero to 100%, only one-half of a voltage needs to be changed. In practice, the voltage is usually held at zero volts and allowed to vary from zero to two times the half-wave voltage. However, one may also hold the voltage at −Uπ / 2 volts and allow it to vary from this value to +Uπ / 2 volts.
Pockels cells use practically no current at all to maintain a certain voltage, because the crystal material is a dielectric, which is defined as an electric insulator. However, because of the Pockels cell’s significant electric capacitance, charging or discharging something in order to change the applied voltage is usually required. To make a significant voltage change quickly, substantial currents will be required. Take into consideration the inductance of the connecting wires, which influence voltage drop but do not modify the required current.