In this work, deuterium loaded Yb-doped fiber has been proposed to mitigate mode instability in laser oscillator. Experimental results reveal that mode instability threshold power rises from ~459W to ~533W and ~622W at the condition of pristine fiber and fiber loaded with deuterium for 2 weeks and 4 weeks respectively. Mode instability threshold power is raised by more than 16% and 35% after 2 and 4 weeks deuterium loading compared to pristine fiber respectively, and laser slope efficiency is not affected by deuterium loading. The experimental results indicate that deuterium loading is effective in mode instability mitigation and showing potentials in further power scaling of high power fiber lasers.
The laser efficiency and temperature distribution in distributed side-coupled cladding-pumped (DSCCP) fiber lasers are investigated, using a precise and complete analytic model. We show that high-order mode pump handling and (N + 1) (N ≥ 2) DSCCP fibers can achieve a high laser efficiency when the fibers are separated. In terms of multistage pump schemes, a high laser efficiency requires an appropriate pump-node number and arrangement. According to our calculations, the temperature evolution along the signal fiber could give rise to a fluctuation in the case of strong coupling, which could decrease the mode instability threshold. The fluctuation period is usually determined by the DSCCP structure including the pump fiber number, the fiber diameter, and the fiber spacing. It is suggested that the mode instability threshold can be depressed through excellent DSCCP structure design and the selection on an appropriate pump scheme.