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High Efficiency 808 nm Laser Pump Diode Arrays with Excellent Beam Quality


OBJECTIVE: To significantly improve the overall efficiency of 808 nm, kilowatt class laser pump diode arrays through 1) electrical to optical efficiency improvement and 2) combined beam quality improvements to most effectively use the optical energy in the pumped, lasing medium. DESCRIPTION: Today's commercially available kilowatt class, 808 nanometer (nm) laser diode arrays and stacks are approaching 60% electrical to optical conversion efficiency. However, due to the low combined beam quality, high temperature coefficient and line width, the generated optical pump poser cannot be used efficiently. The DARPA Super-High-Efficiency Diode Sources (SHEDS) program made great strides in the last decade, particularly in the 975 nm range. At that time though, the 808 nm efficiency was advanced to approximately 45%. To reduce the size, weight and power draw of current, near-term, and future Navy laser sensors, particularly Nd:YAG based laser transmitters, further improvements in 808 nm diode array efficiencies, approaching or exceeding 85%, must be made. This SBIR topic focuses on several aspects of high power laser diode efficiency. The first is electrical to optical conversion efficiency, sometimes called wall plug efficiency. This high efficiency goal includes cooling of the laser diode array. If cryogenic techniques are used, this requires additional electrical power. Therefore room temperature operation with standard water or convection cooling techniques are desired. The next aspect is combined beam quality. While Gaussian beams with M2 (a popular measure of beam quality) values of 1 with low beam divergence are desired, it is difficult to achieve. However, the beam quality is very important for maximizing the optical power density and uniformity in the laser media pumped volume. This might be considered optical to optical efficiency where the maximum laser diode pump power is converted to the end product laser power, 1064 nm power in the Nd:YAG case. A third aspect that contributes to efficacy is the stability and line width of the laser diode array. The pumped laser media tends to have peaked absorption spectra so a low temperature coefficient and a narrow line width output of the laser pump diode array will contribute to the optical power conversion efficiency in the overall laser system. In summary, Key performance objectives that are to be optimized are: 1) maximizing diode array, efficiency for kilo-watt class stacks, 2) maximizing beam combining efficiency and beam quality and 3) minimizing the temperature coefficient and laser line width. PHASE I: Demonstrate the feasibility of the technical approach. Perform preliminary bench-top testing to verify performance of components or design. PHASE II: Develop and demonstrate a working bench-top design. Sufficiently harden bench-top design for testing and demonstration in dynamic environment. Design and develop working proto-type based on results of bench-top device. PHASE III: Complete prototype development and document the design. Units procured under this phase may be tested/demonstrated in Navy systems. PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: In the laboratory environment efficiency is not required, however, the cost per KiloWatt will be reduced and therefore highly desirable in a wide range of commercial laser systems.
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