Polarization Dependence of Transverse mode Instability in Yb-Doped LMA PM Fibers

Power scaling of fiber lasers is hindered by nonlinear and thermal limitations. The most dramatic limiting effect is transverse mode instability (TMI). It manifests as fluctuations between the fundamental mode (FM) of the fiber and a high order mode (HOM). So far there has not been much research of TMI in polarization maintaining (PM) fibers. Simulations in [1] predicted that different input polarization angles can lead to suppression of TMI. Experimentally, it was reported in [2] that rotating the fiber at the splicing point of a PM fiber led to changes on the TMI threshold. Here in this work, we present systematic measurements of the TMI threshold in a PM large mode area (LMA) fiber in dependence on the input linear polarization angle. We demonstrate that TMI can be strongly influenced by detuning the linear polarization of the seed with respect to the slow-axis of the fiber. The experimental setup consisted of a fiber amplifier used in counter-propagating pump configuration, Fig 1. A linearly polarized seed laser centered at 1 030nm was amplified in a LMA PM in-house fiber. The fiber had a signal core diameter of 35μm, a pump core diameter of 200μm, and a low NA of 0.036 (without correction of stress). A fiber length of 1.2 m was coiled with a diameter of 27 cm. The complete fiber was water cooled, twists along the fiber were avoided, and the stress rods were aligned horizontally. The fiber was few-mode for both the slow and the fast-axis, thus the fiber supports the propagation of the LP01 and LP11 modes. The pump power was provided by a wavelength stabilized diode at 976nm with a maximum output power of 800W. We characterized the TMI threshold in dependence on the polarization input angle by rotating the polarization input angle with a half-waveplate (WPl, Fig 1).