There is significant interest in developing high pulse energy (Joules) and high average power diode-pumped solid state lasers for applications where a low energy laser pulse is amplified by one or more amplifiers to achieve high pulse energy. The active medium in such amplifiers is a solid material which is optically excited or pumped by light from a laser diode array or other source of light, whereby energy is stored and gain is produced. A portion of the pump energy is locally converted to heat within the material, thereby limiting average power of these amplifiers as a result of the onset of thermal effects due to thermal gradients within the amplifier material, such as thermal lensing, thermal birefringence or, in some materials, a reduction of gain from excessive heating.
A number of amplifier geometries have been developed for efficiently cooling these amplifiers and mitigate the above effects including: the zig-zag slab geometry in which thermal aberrations are somewhat cancelled, the active mirror geometry in which the lasing medium is cooled through one face resulting in high cooling capacity and non-aberration causing longitudinal thermal gradients, and the thin disk approach in which the laser material is made to be very thin allowing highly efficient heat removal.
Operation at cryogenic temperatures is known to significantly improve the thermal characteristics of some laser materials including titanium sapphire and Yb:YAG. The thermal conductivity of Yb:YAG, to which all average power-limiting thermal effects scale inversely, increases approximately a factor of 7-10 when cooled to the boiling temperature of liquid nitrogen, 77 K. Additionally, the thermo-optic and expansion coefficients are both significantly decreased at low temperatures, and the stimulated emission cross section, which partially determines the gain of the material, is increased at cryogenic temperature, thereby allowing efficient energy extraction at non-damaging laser intensities.
Cryogenic cooling of laser materials is typically accomplished either by heat transfer through direct contact with boiling liquid nitrogen, or through conduction to a cryogenically-cooled heat sink. In the latter, the laser material is either soldered or otherwise placed in thermal contact with a conductive heat sink, typically made of copper, that is cooled to cryogenic temperature by liquid nitrogen boiling or by a closed-cycle cryostat. For large area disks required for high-energy amplifiers, the mismatch in thermal expansion between the laser material and heat sink may result in large deformations in the laser material when cooled to cryogenic temperatures in a similar manner to that for bimetallic materials. Such deformations cause deterioration of beam quality, which is unacceptable for most applications. Additionally, the use of any material between the laser material and the cooling source results in an increase in temperature due to conduction within the heat sink.
Direct, boiling heat transfer through contact with a cryogenic fluid, usually liquid nitrogen, can avoid the problems of laser material deformations from soldering to a copper heat sink, and temperature increase due to conduction within the heat sink. However, the maximum heat power flux that can be dissipated through this technique is limited by the transition from the nucleate boiling regime to the film boiling regime. At low and moderate thermal fluxes, thermal power is transferred to the boiling liquid through nucleate boiling characterized by the formation of small bubbles which rapidly leave the liquid-solid interface. As the thermal flux is increased, a thin layer of vapor may form over the interface effectively shielding the solid surface from the liquid coolant, and significantly reducing heat transfer efficiency. This effect is known as film boiling, and the thermal flux at the onset of film boiling is referred to as the critical heat flux. The critical heat flux of liquid nitrogen at atmospheric pressure is about 20 W/cm2, which limits the average power of such laser amplifiers. At heat fluxes above this value, the temperature of the cooling surface is substantially raised.