We report results from two ~1.5 kW Yb-doped fiber amplifiers with comparable optical to optical efficiencies and linewidths. One amplifier utilized a fiber with a core diameter of 25 μm while the core diameter of the fiber utilized in the other amplifier was 20 μm. Stimulated Brillouin scattering (SBS) suppression in both cases was achieved through pseudo-random bit sequence (PRBS) phase modulation. While the power generated in the larger core fiber was modal instability (MI) limited, no sign of MI was observed in the smaller core fiber. This may allow us to utilize the higher MI threshold fiber to scale further while maintaining sufficiently narrow linewidth for beam combining. Furthermore, in a demonstration of the utility of applying a thermal gradient in conjunction with phase modulation to suppress SBS further, we report on a 1 kW amplifier driven at a PRBS clock rate of 2 GHz. Finally, we compare the coherent beam combining properties of amplifiers seeded with PRBS phase modulated sources to those seeded with white noise sources.
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Iyad Dajani ; Angel Flores ; Roger Holten ; Brian Anderson ; Benjamin Pulford, et al.
Multi-kilowatt power scaling and coherent beam combining of narrow-linewidth fiber lasers
", Proc. SPIE 9728, Fiber Lasers XIII: Technology, Systems, and Applications, 972801 (March 9, 2016); doi:10.1117/12.2218216; http://dx.doi.org/10.1117/12.2218216