301 W Narrow-Linewidth In-Band Pumped Er:Yb Co-Doped Fiber Amplifier at 1585 Nm and Related Modeling for Dynamics Study and Optimization

To overcome Yb lasing, a kilowatt-level 1535 nm fiber laser is utilized to in-band pump an Er:Yb co-doped fiber（EYDF） amplifier. The output power of a 301 W narrow-linewidth EYDF amplifier operating at 1585 nm, with 3 dB bandwidth of 150 pm and M 2 < 1.4, is experimentally demonstrated. To the best of our knowledge, it is the highest output power achieved in L-band narrow-linewidth fiber amplifiers with good beam quality. Theoretically, a new ion transition behavior among energy levels for in-band pumping EYDF is uncovered, and a spatial-mode-resolved nonlinearityassisted theoretical model is developed to understand its internal dynamics. Numerical simulations reveal that the reduction in slope efficiency is significantly related to excited-state absorption（ESA）. ESA has a nonlinear hindering effect on power scaling. It can drastically lower the pump absorption and slope efficiency with increasing pump power for in-band pumped EYDF amplifiers. Meanwhile, optimized approaches are proposed to improve its power to the kilowatt level via in-band pumping.