Active fibre design with flattened fundamental mode profile

Single spatial mode operation of a fiber laser leads to perfect output beam quality, which is important for industry. Increasing of the effective mode area allows for achieving higher laser power at the wave field intensity lower than the limit imposed by non-linear effects. However, the large mode area (LMA) fiber lasers are open to higher order mode (HOM) appearance. This paper considers a novel approach to realize the flattened fundamental mode in an all-glass design of an Yb-doped fiber laser by special profiling of the refractive index. In particular, large-diameter core rimmed by circular chain of round waveguides was simulated numerically to find optimal geometry for stable single-mode operation. Paraxial and scalar approximations are used for wave field modelling in combination with Rigrod´s approximation for the gain.