Projekte

Pockels cells are often used as optical switches in various types of laser systems. For example, they are used as a pulse picker or as a coupling/decoupling element in regenerative amplifiers. A pulsed high voltage is applied to a birefringent optical crystal, which allows the plane of polarization of individual laser pulses to be rotated in a targeted manner. With the help of a polarization filter, an optical switch will be realized that directs the laser pulses into different optical paths.

The crystals used, such as beta barium borate (BBO), have piezoelectric properties, which means that the crystals expand or contract when the voltage applied changes. This results in the formation of sound waves in the crystal. Depending on the material properties and dimensions of the crystal, it has one or more acoustic resonance frequencies, typically in the range of a few 10 or 100 kHz. These resonances can be excited by the electrical switching processes. Possible consequences are, for example, a reduction in the modulation contrast (due to the piezoelectrically induced electrical voltage in the crystal and the associated undesirable birefringence) or even the destruction of the crystal due to the resulting mechanical stresses.

As part of this project, a control method was developed for use in regenerative amplifiers and pulse pickers, which reliably prevents the excitation of resonances by generating anti-noise in the crystal. The module, implemented in an FPGA, allows flexible programming of the Pockels cell switching windows and the damping of one or more adjustable resonance frequencies.