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Bright Blue Semiconductor Laser Arrays for Military Applications


OUSD (R&E) CRITICAL TECHNOLOGY AREA(S): Integrated Network System of Systems, Directed Energy OBJECTIVE: Develop compact chip-scale blue laser systems with high beam quality useful for machining and propagation. Advances based upon the coherent beam combining of diode lasers of high brightness are sought. DESCRIPTION: Laser systems in the infrared have a long history of development for both DoD and commercial applications. Blue laser diode systems have been developed with improved performance over the past 2 decades; however, their brightness and power levels are much less than the best infrared systems. Of particular interest is GaN based blue laser diodes which have superior brightness and power scaling potential over the current state-of-the-art. Blue light at wavelengths around 450 nm is of particular interest due to the increased absorption in many materials, particularly metals. The laser energy can thus be transmitted into the material more quickly for more precise machining with less power. The Army would like to develop superior blue laser systems to assess applications in machining and directed energy where more compact and high performing systems may be possible. Diode systems are of interest due to their compact size and GaN is known as a high thermal conductivity material so may be amenable to significant power scaling if coherent combining architectures can be developed. Finally, high beam quality and brightness are of interest for the applications and may require consideration of the laser diode architecture itself, and not just the beam combining architecture. However, the desired metrics for this topic allow for flexibility in the device approach. PHASE I: Pursue chip-scale directed energy beam combining techniques using high efficiency diode lasers exceeding 30% wall-plug efficiency each with 0.4-0.46 micron wavelengths. Design coherent beam combining architecture for either surface emitting arrays or in-plane laser beam combining. Use of monolithic cavities or chip-scale solutions should be pursued both to demonstrate minimal footprint and show a path toward combining larger numbers of lasers. Additional design considerations should be investigated for the incorporation of effective liquid cooling of arrays to explore maximum power levels. Brightness levels of 200 MW/cm2*sr should be shown to be feasible along with power scaling to > 100 W power levels/cm2 – without coherent combining, but to show thermal heat dissipation design considerations. A demonstration of high-brightness, single mode, Watt-level single emitters should be made along with designs for coherent combining of arrays to reach at least 15 W. PHASE II: Continue implementation of coherent beam combining designs. Pursue 15 - 100 W peak power, uncooled coherently combined arrays and designs for higher power, cooled arrays. Brightness levels of 1000 MW/ cm2*sr should be demonstrated that achieve combining efficiencies of 70% or more for the chip-scale architecture. Optimization of the arrays and studies on minimal spacing between individual lasers for the nominal power target level and within the beam combining architecture should continue along with needed studies to explore power scaling with larger arrays. Demonstration of chip-scale DE systems that achieve > 15 W peak power with designs that can scale to over 100 W and potential to achieve kWs. An assessment of cooling for the array to achieve continuous wave operation should be made toward phase III demonstrations. Eventually, cooled arrays of 100 W or more per square centimeter average power are desired. PHASE III DUAL USE APPLICATIONS: Pursue further optimization of array cooling and power scaling with refined chip-scale designs. In addition, multi-stage architectures should be pursued to combine lower power arrays to achieve kW power level output. Monolithic cavities should be pursued for at least the first stage of combining with secondary combining by either external cavities or secondary monolithic cavities. Other consideration to utilize techniques to create lower power arrays (still multi-Watt) for additive manufacturing, under-water laser communications, and beam scanning and surveillance lidar should be made. Particular consideration for phased arrays should be considered for beam steering and adaptive optical beam control to mitigate atmospheric turbulence to achieve maximum power on target. REFERENCES: 1. J.A. Davis, “HEL Wavelengths & Platform Locations! What are the Impacts?” 2020 DEPS Systems Symposium, November 2020. 2. M. S. Zediker, "Blue laser technology for defense applications," Proc. SPIE 12092, Laser Technology for Defense and Security XVII, 1209207 (30 May 2022). 3. Inoue, T., Yoshida, M., Gelleta, J. et al. General recipe to realize photonic-crystal surface-emitting lasers with 100-W-to-1-kW single-mode operation. Nature Communications 13, 3262 (2022). 4. M. Ali et al., "Recent advances in high power blue laser diodes," 2017 IEEE High Power Diode Lasers and Systems Conference (HPD), 2017, pp. 47-48, doi: 10.1109/HPD.2017.8261094. 5. R. Liu, Y. Liu, Y. Braiman, “Coherent beam combining of high power broad area laser diode array with a closed-V-shape external Talbot cavity,” Optics Express, Vol. 18, No. 7, 29 March 2010. 6. D. Zhou, J.-F. Seurin, G. Xu, P. Zhao, B. Xu, et al, “Progress on high-power high brightness VCSELs and applications,” Proc. SPIE Vol. 9381, Vertical-Cavity Surface-Emitting Lasers XIX,9 3810B (4 March 2015); doi: 10.1117/12/2080145. KEYWORDS: blue laser diodes, additive manufacturing, brightness, gallium nitride, directed energy, coherent beam combining
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