Cryogenic solid-state laser materials offer many improvements in thermal, optical, structural, and lasing properties over
their room temperature counterparts. As the temperature of Yb:YAG decreases from room to 80K it transitions from
quasi-three-level lasing to a 4-level laser. In this study, we compare Yb:YAG thin-disk laser performance at room 293K
and 80K. To achieve this direct comparison we have built two cooling systems based on R134A refrigerant and also on
liquid nitrogen (LN<sub>2</sub>). We have made an analytical calculation of the small signal laser gain that takes into account the
spurious amplified spontaneous emission and photon re-absorption. The cold thin-disk laser clearly outperforms room
temperature operation, and the theoretical results shows room temperature gain flattening.
A ceramic ytterbium:yttrium aluminum garnet (Yb:YAG) thin-disk laser is investigated at 15°C (288 K) and also at 80 K, where it behaves as a four-level laser. We introduce a new two-phase spray cooling method to cool the Yb:YAG. One system relies on R134a refrigerant while the other uses liquid nitrogen (LN2). The use of two systems allows the same disk to be tested at the two temperatures. When the Yb:YAG is cooled from room to cryogenic temperatures, the lasing threshold drops from 155 W to near 10 W, while the slope efficiency increases from 54% to a 63%. A 277 W laser with 520 W of pump is demonstrated. We also model the thermal and structural properties at these two temperatures and estimate the beam quality.
At cryogenic temperatures, Yb:YAG behaves as a 4-level laser. Its absorption and emission cross-sections increase, and
its thermal conductivity improves. Yb:YAG thin disk laser performance at room and cryogenic (80°K) temperatures will
be presented. The Yb:YAG gain media is cooled using either a pressurized R134A refrigerant system or by a two-phase
liquid nitrogen spray boiler. Interchangeable mounting caps allow the same Yb:YAG media to be switched between the
two systems. This allows direct comparison of lasing, amplified spontaneous emission, and temperature performance
between 20°C and -200°C.
Yb:YAG thin-disk laser performance at room and cryogenic (80K) temperatures is presented. The Yb:YAG gain media,
which is Indium soldered to specialized CuW mounting caps, is cooled using either a pressurized R134A refrigerant
system or by a two-phase liquid nitrogen spray boiler. At cryogenic temperatures spontaneous emission measurements
reveal sharper transition lines and a decrease in the fluorescence lifetime. Lasing reflects that a true four-level laser.
Interchangeable mounting caps allow the same Yb:YAG media to be switched between the two systems. This allows
direct comparison of lasing, amplified spontaneous emission, and temperature performance at 15 °C and at -200 oC.
Operational performance of kilowatt-class thin-disk ceramic and single crystal Yb:Yag lasers is presented. High pump
power is applied to various thin-disk assemblies on two different test beds. The assemblies are composed of ASE caps,
200μm gain media, and heat sinks made of SiC, sapphire, or diamond. A novel mounting and cooling process is
FEA modeling of the assemblies is performed using COMSOL stress and thermal computations to understand and
quantify thermal and stress effects on beam quality and laser output power. Under increased pump power, the thin-disk
can deform 5-10 μm in the center, destroying cavity stability. This is observed experimentally. The results of this work
indicate that a single thin-disk laser could simultaneously produce high beam quality and high power if novel thermal
management techniques are employed.
Directed energy applications for thin disk lasers demand improvements in materials, efficiency, thermal management,
and most importantly beam quality. At the Air Force Research Laboratory's Directed Energy Directorate ceramic
Yb:YAG materials are being investigated along with various cooling techniques. 10-14mm diameter 0.2mm thick disks
are mounted on silicon carbide (SiC), sapphire, and diamond submounts. From a larger platform, more than 6kW power
is obtained from unmounted and sub-mounted 35mm diameter disks. In conjunction with thermal modeling, we project
a path towards high performance high power lasers.