High order modes suppression in large mode area active fibers by controlling the radial distribution of the rare earth dopant

摘要

Large mode area fibers that are necessary in high power applications frequently support multiple high order modes. Even in the case where a relatively low number of high order modes (2 to 4) are supported, the beam quality of the output beam can be significantly degraded. One way to reduce the power in the higher order modes is to radially control the distribution of the rare earth dopant concentration. In this paper, we have used Liekki Application Designer v3.1 to simulate the propagation of all supported modes of the doped fiber in a seeded amplifier and have obtained the output power distribution among the modes and the near-field spatial profile of the output beam. We have shown that a dopant confinement reduces significantly the output power in higher order modes. The confinement requires longer fiber length and/or higher dopant concentration. Parabolic doping may provide a good compromise, providing relatively good high order mode suppression for reasonable fiber length. Further tuning the doping profile and concentration can optimize the mode suppression.