One of the most promising directions in the development of high-power lasers is connected with a cryogenically cooled disk-shaped active medium. In these works the ytterbium ion is widely used as an active ion as it is characterized by a small quantum defect (9%), long lifetime of inversion and absence of excited-state absorption. Active elements shaped as a thin disk permit thermal distortions to be reduced significantly due to front-end heat removal at which the temperature gradient is oriented along rather than across the direction of light propagation. In addition, due to the small length of the active element, powerful short pulses can be amplified without any risk of self-focusing.

Schematic diagram of a 4V-pass two-cascade amplifier (left) and general view of the cryostat

Another method for suppressing thermal effects is to cool the active element to liquid nitrogen temperatures. This increases the thermal conductivity, decreases the thermal expansion coefficient and the temperature gradient of refractive index changes dn/dT, increases the absorption cross sections of the pump transition and the laser transition, and almost completely depletes the lower lasing level (the medium becomes a four-level one). Modern technologies allow fabricating active elements of optical ceramics with unique properties. These include large aperture (like in glass) and high thermal conductivity (like in a single crystal), as well as the ability to produce ceramics from materials that are not grown as high-quality single crystals, but yet possess an optimum combination of properties (Y₂O₃, TAG).
The research team headed by O. V. Palashov has developed a cryogenic disk Yb:YAG laser operating by the following scheme: master oscillator — multipass (12V-pass) preamplifier — 4V-pass two-cascade amplifier. A pulse energy of 0.23 J with nanosecond pulse duration with a repetition rate of 140 Hz has been achieved. A model for calculating stored energy in the disk-shaped active elements, which takes into account the amplified spontaneous emission, and a technology for fabricating composite active elements by thermal diffusion welding of Yb:YAG and YAG crystals have been developed. The use of composite active elements will significantly reduce the parasitic effects of amplified spontaneous emission and double the stored energy. With this scheme it is intended to achieve the kilowatt-level average power. A microwave heating-based technology for fabrication of Yb:(YLa)2O3 ceramics with properties similar to the Yb:Y₂O₃ ceramics is being developed at IAP RAS in collaboration with the Institute of Chemistry of High-Purity Substances RAS.

Composite YAG/Yb:YAG active element	Active element made of Yb:(YLa)2O3 ceramics