Optimization of the slope efficiency of a core-pumped thulium-doped fiber laser by the thermally diffused expanded-core technique

Abstract

We report on the mode field adaption of an active thulium-doped fiber by using the thermally-diffused expanded-core technique. The fiber core diffusion is analyzed by splice transmission measurements and visually from side view images. The obtained heating parameters are used to build a thulium-doped fiber laser emitting at 2036nm that is core-pumped by an erbium:ytterbium fiber laser. By allowing the fiber cores to diffuse, the mode fields of the active and passive fibers are adapted for both the signal and pump wavelength. The adaptation of the mode fields increases the slope efficiency from 66.1% to 75.0%. The obtained slope efficiency is close to the stoke efficiency of 77.0%. By comparing the results with a fiber laser simulation, the slope efficiency of 75.0% is verified to be the maximum slope efficiency taking the active fiber length into account.