High-power lasers for directed-energy applications

Abstract

In this article, we review and discuss the research programs at the Naval Research Laboratory (NRL) on high-power lasers for directed-energy (DE) applications in the atmosphere. Physical processes affecting propagation include absorption/scattering, turbulence, and thermal blooming. The power levels needed for DE applications require combining a number of lasers. In atmospheric turbulence, there is a maximum intensity that can be placed on a target that is independent of the initial beam spot size and laser beam quality. By combining a number of kW-class fiber lasers, scientists at the NRL have successfully demonstrated high-power laser propagation in a turbulent atmosphere and wireless recharging. In the NRL experiments, four incoherently combined fiber lasers having a total power of 5 kW were propagated to a target 3.2 km away. These successful high-power experiments in a realistic atmosphere formed the basis of the Navy's Laser Weapon System. We compare the propagation characteristics of coherently and incoherently combined beams without adaptive optics. There is little difference in the energy on target between coherently and incoherently combined laser beams for multi-km propagation ranges and moderate to high levels of turbulence. Unlike incoherent combining, coherent combining places severe constraints on the individual lasers. These include the requirement of narrow power spectral linewidths in order to have long coherence times as well as polarization alignment of all the lasers. These requirements are extremely difficult for high-power lasers.

Investigation of Coherent and Incoherent Laser Beams Propagation Through Turbulent Atmosphere

“…where 0 r is the transverse coherence length (Fried parameter) which is a measure of the quality of optical transmission through the atmosphere due to random inhomogeneous in the atmosphere's refractive index and is given by [10]:…”

“…By using Equations (7) and into Equation (8), initial laser intensity and setting Focal L L = . that gives minimum laser spot size at the particular range, the peak intensity on target at range L is given by [10]:…”

Influence of beam shape on piston and tilt error in coherent combined laser array

“…High average power lasers have found their application in materials processing, 1 defense systems, 2,3 and fundamental research. 4 The challenge is not only to obtain multi-kilowatt output power but also to maintain near diffraction-limited beam quality.…”

Understanding laser beam brightness: A review and new prospective in material processing

“…Both combined leads to a high brightness beam which is important for applications such as laser spot welding [77]. A beam combining technique was used to improve the brightness of a fibre laser by Sprangle [78]. This used an array of three fibres focused at a 45°mirror which was then focused at the target.…”

“…It improved the efficiency, and produced a compact, robust, low maintenance, high energy and long lasting laser system. Results showed propagation efficiency of 90% at a range of 1.2 km, at 3 kW laser power in moderate turbulence within the atmosphere [78]. Ishizuka et al [79] stated that the brightness of field-emission tips (electron microbeams) could be substantially improved by using a magnetic field instead of the electrostatic lens to suppress the divergence of such microbeams which inherently enhanced the actual brightness.…”