Multi-kW Fiber Lasers Based on Novel Light Guide Combiners

ABSTRACT: The efficient coherent beam combination of a master oscillator power amplifier (MOPA) system requires a reduction in non-linearities of the power amplifier sections. Fiber non-linearities, such as stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS), provide an upper limit on the operating power of kW-class fiber laser systems. These non-linear effects can be reduced with the use of short fiber lengths, which requires high brightness pump sources and efficient pump/signal combination methods. Wavelength stabilized 976 nm pump sources allow for maximum pump absorption and reduced quantum defect, improving performance and stable operation over a wide range of temperatures. Such advances are required to enable field-able directed energy weapons. To achieve these goals, nLIGHT proposes the development of a novel"Light Guide"combiner that results in minimal pump brightness loss and minimal power loss, enabling high power CW fiber lasers for DE applications. A key design aspect for these kW-class fiber laser amplifiers for directed energy applications is the reliability, robustness, and manufacturability of the pump/signal combiner as well as the fiber amplifier system. This SBIR proposal addresses this need by providing a pump/signal combiner that optimizes the brightness preservation of the pump lasers. A unique feature of this combiner is that it enables the use of robust glass-clad fiber, thereby addressing the most common source of fiber failure in high-power fiber laser systems. BENEFIT: nLIGHT is actively pursuing kW class CW fiber lasers, which will also be based on its advanced fiber technologies. This direction of research will enable improvements in nLIGHT"s understanding of handling high average powers in fiber lasers, which will ultimately tie to power scaling as well. Commercial roadmaps also call for further improvements in the output power and beam quality of nLIGHT"s commercial products, pushing towards multi-kW CW fiber lasers. The advanced fiber combiners developed under this effort will enable CW fiber lasers to be co-propagation and counter-propagation pumping of CW fiber lasers, improving the output power and beam quality achievable before the onset of nonlinear effects such as SRS. Finally, fiber architectures and knowledge gained in the pursuit of this effort will inform the ability to design similar fibers based on dopants other than Yb. Such fibers, based on Er or Tm dopants will allow CW fiber laser systems at wavelengths in the eye-safe regime, which are of potentially great commercial utility as well as having clear applicability in ranging and other applications of interest to the Air Force.