Elements for high average power lasers

Faraday isolators (FI) are an integral part of the vast majority of high-power laser systems. Due to the relatively high absorption (~ 10—3 cm—1), the temperature distribution is nonuniform in the magneto-optical elements of the FI, and so passing radiation is subjected to strong thermal self-action, deteriorating the isolation ratio. As the average laser power is steadily growing, the problem of developing and improving FIs remains relevant.
Theoretical and experimental studies of polarization and phase distortions of the transmitted radiation arising from the nonuniform heating of the magneto-optical elements in FIs, as well as ways to reduce these effects are being conducted under the direction of O. V. Palashov. A series of FI schemes for high average power lasers have been developed, which can provide a stable isolation ratio at sub-kilowatt and multi-kilowatt power levels.
The IAP RAS developments include
• FI with compensation for thermally induced distortions, either by replacing one magneto-optical element with two elements separated by a reciprocal optical element, or by adding a compensator outside the magnetic field of the FI;
• FI with an enhanced magnetic field created by magnetic conductors or nonorthogonally magnetized sectorial rings, which allows shortening the magneto-optical element;
• high-vacuum FIs designed for use in high vacuum (~ 10—6 Torr);
• FI with thermal stabilization of the magneto-optical element using water cooling or Peltier elements;
• FI with a magneto-optical element cooled with Peltier elements or liquid nitrogen. Cooling not only significantly improves the thermo-optical properties of the magneto-optical element, but also permits to make it much shorter. A prototype of a cryogenic FI is designed, in which both the magneto-optical element and the magnet system are cooled to liquid nitrogen temperatures. This FI is capable of providing a stable isolation ratio at the multikilowatt power level.

Cryogenic (left) and high-vacuum (right) Faraday isolators

Pockels cells with plasma electrodes are designed to operate in large-aperture (100—400 mm) high-power lasers. The use of large aperture DKDP crystals can significantly reduce, compared with foreign analogs based on KDP crystals, the loss of laser radiation caused by linear absorption and the amplitude of the control voltage (approximately by two times), and thus much improve the weight and dimensions of electronic units.
The research group headed by N. F. Andreev from IAP RAS jointly with the Institute of Laser Phy-sics Research RFNC — VNIIEF (Sarov) have created Pockels cells with plasma electrodes based on the DKDP crystal with an aperture of 100 × 100 mm and 300 × 300 mm for optical beam control in high-power multi-pass laser systems that are being developed within the framework of the laser fusion program.

Pockels cells with plasma electrodes with an aperture
of 100 × 100 mm and 300 × 300 mm