From Event: SPIE Optical Engineering + Applications, 2016
To achieve the power levels necessary for directed energy applications with fiber or slab lasers, it is necessary to combine multiple lasers into a single beam director. Here we compare the performance of incoherent and coherent beam combining strategies and address three important issues that should be considered before a beam combining architecture is implemented. First, we consider the difficulty in phase locking high-power fiber and slab lasers. The large linewidths of high-power fiber and slab lasers induce random phase fluctuations occurring on sub-nanosecond time scales. To coherently combine these high-power lasers can involve rapid and precise phase control to compensate for these fluctuations. Even with a master oscillator - multiple power amplifier system, the coherence length of the beams to be combined is very short necessitating continuous precise control of optical path lengths. Second, we consider the dephasing effects of atmospheric turbulence. We find that in moderate to strong turbulence conditions and kilometer propagation distances, coherent combining at the transmitter plane has negligible impact on the energy delivered to a target. Finally, we consider the multifaceted task of coherent combining at the target plane. This is effectively an adaptive optics situation in which the distortions caused by atmospheric turbulence are partially compensated for.
William Nelson, Chensheng Wu, Phillip Sprangle, and Christopher C. Davis, "Propagation of combined laser beams through atmospheric turbulence
(Conference Presentation)," Proc. SPIE 9979, Laser Communication and Propagation through the Atmosphere and Oceans V, 99790N (Presented at SPIE Optical Engineering + Applications: August 31, 2016; Published: 7 December 2016); https://doi.org/10.1117/12.2239140.5178875896001.
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