Pockels cell
The Pockels cell (after Friedrich Pockels ) is based on the Pockels effect and is mostly used in electro-optical switches for laser beams . In addition to fast switching (within nanoseconds), light and laser radiation can also be continuously modulated in phase, polarization and intensity in this way.
Working principle
The inside of the cell consists of a birefringent crystal in which an electric field can be generated by means of electrodes.
The electro-optical Pockels effect is the generation or change of birefringence in an optical medium by an electric field. It differs from the Kerr effect in the fact that the change in birefringence is linearly proportional to the electric field, while with the Kerr effect it is quadratic. The Pockels effect only occurs in crystals that do not have inversion symmetry, such as lithium niobate , ADP, KDP , RTP, BBO or gallium arsenide .
Pockels cells are therefore voltage-controlled polarization or phase modulators . The electric field can be either transverse or longitudinal to the light beam:
- Transverse Pockels cells: The optical axis of the crystal can be longitudinal. If it is transverse, it consists of two crystals with opposite orientations to compensate for the phase shift due to natural birefringence. This is often not perfect and also depends on the temperature.
- Longitudinal Pockels cells: You need transparent electrodes, often designed as panels. The voltage can be reduced by connecting a chain: electrode-crystal-electrode-crystal-electrode, etc.
Due to the polarization of the laser beam, Pockels cells can be used:
- Pockels cells can be used to modulate a laser beam in its phase. When a sinusoidal electric field is applied to the Pockels cell, the laser beam is phase modulated according to the fluctuations in the E-field.
- A polarization filter is arranged in front of and behind the crystal, the polarization planes of which are rotated by 90 degrees to one another. By changing the applied high voltage , the crystal rotates the plane of polarization of the light passing through, whereby the intensity can be changed between 0 and 100 percent. This means that the arrangement can be used as a fast switch for light.
Due to the high dielectric constant of the crystal and the large electrodes, a Pockels cell behaves like a capacitor. The high operating voltage therefore also requires a large amount of charge. If an optical switch based on a Pockels cell is to switch quickly, a very large current is therefore required to charge or reload the electrical capacitance. Fiber optic Pockels cells can use a continuous electrical wave in a cable-like arrangement - this can reduce the control requirements.
application
Pockels cells have a variety of scientific and technical uses, such as: B. as impulse picker and cavity dumper. Its use as a Q-switch inside laser resonators is particularly well known . Polarization -selective elements such as polarization prisms or thin-film polarizers are used, which prevent the stimulated emission by generating large losses for the light circulating in the resonator when the Q- switch is switched off . During periods of great losses, the active medium is pumped into a highly excited state (high population inversion density ). When the active medium is saturated by pump energy, the Pockels cell is activated by a voltage and the polarization changes to one that is allowed - the light can reach the reflecting mirrors and the stimulated emission (lasing) begins. This causes a laser pulse with a very short rise time and very high intensity .
Web links
- Paper on ultra-fast switching Pockels cell drivers (PDF file; 240 kB)
- Application of Pockels cells in pulse lasers (diploma thesis) (PDF file; 2.58 MB)