Low quantum-defect pumped 5 kW fiber amplifier

Combinable high average power fiber amplifiers with diffraction-limited beam that is suitable for High Energy Laser (HEL) applications is currently primarily limited in output power by TMI and SBS. SBS is better understood and numerous methods have been shown to be effective for scaling power to several kilowatt levels. Currently, TMI limits output power to ~3-kW for approximately 20-µm core conventional step-index dual-clad fiber amplifiers. We report on the use of low-quantum defect pumping scheme to increase the TMI threshold. We have demonstrated 5kW and further power-scaling is underway and the latest results will be presented.