T. C. Newell

Thermal and stress characterization of various thin disk laser configurations at room temperature

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 described. 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.

Extracting fundamental transverse mode operation in broad area quantum cascade lasers

Power scaling in broad area quantum cascade lasers results in the operation of high order transverse modes with a far-field profile consisting of two lobes propagating at large angles relative to the optical axis. We report a method of suppressing the high order transverse modes that can extract the fundamental mode and provide emission along the optical axis. By generating a lateral constriction in the waveguide in the form of short trenches defined by the focused ion beam milling technique, we report broad area devices in which most of the power is contained in a near diffraction-limited beam that provides high brightness.

Yb:YAG thin-disk laser performance at room and cryogenic temperatures

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 (LN2). 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.

Thermal Management Investigations in Ceramic Thin Disk Lasers

Directed energy applications for thin disk lasers demand improvements in materials, efficiency, thermal management, and most importantly beam quality. At the Air Force Research Laboratorys 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.

On-chip unstable resonator cavity 2-μm quantum well lasers

Focused ion beam milling was used to fabricate on-chip unstable resonator cavity quantum well laser devices. A cylindrical mirror was formed at the back facet of the broad area device emitting near 2 μm. Compared to the Fabry-Pérot cavity device, the unstable resonator cavity device exhibits a 2x diffraction limited beam. The preliminary results demonstrate that a much higher brightness can be reached in this class of broad area devices.

On-chip unstable resonator cavity GaSb-based quantum well lasers

The focused ion beam milling tool was used to convert a GaSb-based broad area gain-guided quantum well laser device with a standard Fabry-Perot cavity into one with an unstable resonator cavity. A cylindrical mirror was formed at the back facet of the broad area device emitting near 2 μm. Compared to the Fabry-Perot cavity device, where the coherency of the beam is severely disrupted by filamentation, the unstable resonator cavity device exhibits an ∼2× diffraction limited beam. The relatively small penalty in slope efficiency demonstrates that a much higher brightness can be reached in this class of broad area devices.

Distributed loss method to suppress high order modes in broad area quantum cascade lasers

We describe a method where the standard fabrication of broad area quantum cascade lasers is modified to provide a controlled amount of direct contact of device sidewalls with metal. We demonstrate that this provides sufficient levels of distributed losses to suppress the high order transverse modes in favor of the fundamental or near-fundamental transverse mode operation. We observe that the quantum cascade laser power and slope efficiency are degraded by a small amount, resulting in a large increase in brightness to accompany the power scaling.

Cryogenic Yb: YAG Thin-Disk Laser

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.

High efficiency Yb:YAG thin disk laser at room and cryogenic temperatures

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.