Gain Filtering for Single-Spatial-Mode Operation of Large-Mode-Area Fiber Amplifiers

Gain filtering of higher order modes in large-mode-area fibers is an extremely robust method for providing diffraction-limited performance regardless of core diameter or input beam quality. Analytic calculations demonstrate that reducing the diameter of the gain dopants compared to the waveguide diameter produces differential gain that is higher for the fundamental mode than all other fiber modes at all saturation levels. Matching the gain dopant to the mode profile is not as beneficial as a simple step profile since the primary mechanism of gain filtering is to deny gain toward the edge of the waveguide where most of the higher order mode power is contained. Numerical simulations of multikilowatt fiber amplifiers with up to 100-mu m-diameter cores show that gain filtering is extremely robust, providing 99% of the output power in the fundamental mode output with only 90% of the seed power in the fundamental mode. Even with poor seed launch with 50% of the power in the fundamental mode, gain filtering can provide up to 90% of the output power in the fundamental mode.