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Atmospheric Propagation and Combining of High-Power Lasers

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Technical Report,01 Aug 2014,31 Aug 2015

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Naval Research Laboratory Washington United States

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In this paper we analyze the beam combining and atmospheric propagation of high-power lasers for directed-energy DE applications. Thelarge linewidths inherent in high-power fiber, and to a lesser extent, slab lasers cause random phase and intensity fluctuations occurring onsub-nanosecond time scales. To coherently combine these high-power lasers would involve instruments capable of precise phase control andoperating at rates greater than 10 GHz. To the best of our knowledge, this technology does not currently exist. This presents a challengingproblem when attempting to phase-lock high-power lasers, which is not encountered when phase-locking low-power lasers, for example mW power levels. Regardless, we demonstrate that even if instruments are developed that can precisely control the phase of high-power laserscoherent combining is problematic for DE applications. The dephasing effects of atmospheric turbulence typically encountered in DE applications will degrade the coherent properties of the beam before it reaches the target. Through simulations, we find that coherent beam combining inmoderate turbulence and multi-km propagation distances has little advantage over incoherent combining. Additionally, in strong turbulence andmulti-km propagation ranges, we find nearly indistinguishable intensity profiles and virtually no difference in the energy on the target betweencoherently and incoherently combined laser beams. Consequently, we find that coherent beam combining at the transmitter plane is ineffectiveunder typical atmospheric conditions.

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