Mitigation of transverse mode instability by modal birefringence in polarization-maintaining fibers.

The effect of transverse mode instability (TMI) poses a fundamental obstacle for a further scaling of diffraction-limited, high-power fiber laser systems. In this work we present a theoretical and experimental study on the mitigation of TMI by modal birefringence in a polarization maintaining (PM) fiber. With the help of comprehensive simulations, we show that the thermally-induced refractive index grating responsible for TMI can be modified and washed out when light is coupled with a polarization input angle detuned from the main axes of the fiber. To confirm the theoretical predictions, we have designed and manufactured an Yb-doped large-mode-area PM fiber. Using this fiber, we have systematically investigated the dependence of the TMI threshold on the polarization input angle of the seed laser. We experimentally demonstrate that when the polarization input angle of the seed is aligned at 50° with respect to the slow-axis, the TMI threshold increases by a factor of 2, verifying the theory and the numerical simulations. A high speed polarization mode-resolved analysis of the output beam is presented, which reveals that at the onset of TMI both polarization axes fluctuates simultaneously.