We theoretically investigated the effect of the spatial distribution of the active-ion concentration in multimode step-index waveguides on transverse-mode selection for continuous-wave laser operation. We found that uniform doping of a central portion of as much as 60% of the full waveguide core width is highly effective for the selection of fundamental-mode operation, even under highly saturated, high-power conditions. Profiling the dopant distribution to match that of the particular mode desired was also found to be effective, especially if it is the saturated inversion profile that is matched to the shape of the mode.
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