Standard Form 298 (Rev. 8-98)Prescribed by ANSI Std. Z39.18Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing this collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden to Department of Defense, Washington Headquarters Services, Directorate for Information In this paper we analyze the beam combining and atmospheric propagation of high-power lasers for directed-energy (DE) applications. The large linewidths inherent in high-power fiber, and to a lesser extent, slab lasers cause random phase and intensity fluctuations occurring on sub-nanosecond time scales. To coherently combine these high-power lasers would involve instruments capable of precise phase control and operating at rates greater than ~10 GHz. To the best of our knowledge, this technology does not currently exist. This presents a challenging problem 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 lasers; coherent 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 in moderate turbulence and multi-km propagation distances has little advantage over incoherent combining. Additionally, in strong turbulence and multi-km propagation ranges, we find nearly indistinguishable intensity profiles and virtually no difference in the energy on the target between coherently and incoherently combined laser beams. Consequently, we find that coherent beam combining at the transmitter plane is ineffective under typical atmospheric conditions. AbstractIn this paper we analyze the beam combining and atmospheric propagation of high-power lasers for directed-energy (DE) applications. The large linewidths inherent in high-power fiber, and to a lesser extent, slab lasers cause random phase and intensity fluctuations occurring on subnanosecond time scales. To coherently combine these high-power lasers would involve instruments capable of precise phase control and operating at rates greater than ~10 GHz. To the best of our knowledge, this technology does not currently exist. This presents a challenging problem when attempting to phase-lock high-power lasers, which is not encountered when phaselocking 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 lasers; coherent combining is problematic for DE applications. The dephasing ...