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Black Diamond

Efficient, compact, high energy lasers (HELs) are potential weapons for a number of important military missions. Current HEL systems and concepts are based on either chemical lasers (such as the Airborne Laser and the Airborne Tactical Laser), or solid state lasers (such as the heat capacity laser (HCL), the diode pumped Joint High Power Solid State Laser (JHPSSL), the High Energy Liquid Laser Air Defense System, and High Power Fiber Lasers). Today's chemical lasers are based on chemical oxygen iodine laser (COIL) technology which uses complex processes and plumbing to mix volatile chemicals to generate the gain medium. COIL technology requires a significant logistical investment in theater, where the chemicals must be prepared. Solid state lasers do not use such volatile and cumbersome materials, but most suffer from poor thermal management issues, requiring either limited duty cycles (as in the HCL), or complex mechanisms for thermal control (as in the JHPSSL). They are also designed for the 100 kW class; it is not clear that they will scale to the megawatt (MW) class.

DARPA's Black Diamond is a new program which will investigate the potential of Raman beam combining for compact, high-efficiency, MW-class HELs. Black Diamond has the advantages of both chemical and solid state lasers while reducing the disadvantages. In Black Diamond, the pumps are all-cryogenic diode-pumped solid state master-oscillator-power-amplifiers (MOPAs). To solve the scaling problem, multiple MOPAs are incoherently combined in a single Raman gain medium - single crystal, chemical vapor deposition (SC-CVD) diamond - to produce a single high power output beam. SC-CVD diamond has excellent thermal and optical properties, which at room temperature are comparable to cryogenic solid state gain media; at cryogenic temperatures, SC-CVD diamond improves by 200 times in power scaling potential. The result is a laser concept which could result in a more efficient, smaller, more rugged and safer HEL system.

The Diamond Laser program will use a high-power, cryogenic Yb:YAG MOPA as the optical pump source. Under a previous DARPA program, a cryogenic-Yb:YAG MOPA was developed at the 2 kW level. However, to achieve the desired Raman optical-to-optical (O-O) efficiencies of better than 70%, and to demonstrate multiple beam pumping of the diamond Raman cell, higher pump powers are needed to increase the Raman gain. Therefore, the 2kW cryogenic-Yb:YAG MOPA will be scaled to the 5 kW level by adding additional power amplifier units. To achieve the desired beam quality, the master oscillator pump modules will be upgraded to fiber-coupled sources. These upgrades will result in 5 kW of total pump power in a near-diffraction-limited beam. The MOPA output will be split into two beams, and combined into the diamond Raman cell to produce better than 2.5 kW Raman output with M2 ≤ 1.1.

Using the understanding obtained from the experimental effort, the Diamond Laser program will use optical, laser, and 3-D modeling, and computational fluid dynamics and finite-element analysis to examine the scaling potential of the concept, first to the 100 kW level, then to the MW level. Of particular interest will be the potential for achieving better than 45% electrical efficiency and 5 kg/kW specific mass at the MW level. This would represent a significant performance breakthrough in solid state HEL technology.