Mitigation of transverse mode instability in polarization maintaining, high-power fiber amplifiers

A new passive mitigation strategy for the effect of transverse mode instability is presented in this work. This technique requires the use of a polarization-maintaining fiber in which light is coupled with a polarization state oriented around 45° with respect to the main polarization axes of the fiber. Since the modal beat length in each of the main polarization axes of the fiber is slightly different, the aforementioned coupling condition leads to the modal interference pattern being periodically washed out. Such a situation at the end leads to a weakening of the thermally-induced refractive index grating and to an increase of the threshold for the effect of transverse mode instability that can amount to ~50%.