A fiber amplifier system for generating a high power output at a stable polarization, comprising: a master oscillator for generating a signal beam; a polarization actuator for receiving the signal beam from the master oscillator and adding a polarization dither in accordance with a feedback signal before outputting the signal beam to the fiber amplifier; a multiple mode fiber amplifier for receiving the signal beam and outputting an amplified beam with a power in the order of kW; a beam sampler for receiving an output of the fiber amplifier; a polarization filter, coupled to the beam sampler, for transmitting only power contained within a desired output state of polarization SOP; and a controller for generating the feedback signal, the controller further comprising: a dither signal generator for generating a dither waveform; a processor, coupled to the polarization filter and the dither signal generator for generating an error signal; and a summer coupled to the dither signal generator and the processor for generating the feedback signal; characterised in that parameters of said dither waveform generated by the dither signal generator are selected based on the thermal diffusion time across a core of the fiber amplifier so that the dither waveform does not trigger polarization-induced high order mode instability PI-HOMI, wherein the thermal diffusion time is between 0.3 and 3 ms, and wherein the dither waveform either has a period that is much less than the thermal diffusion time or uses a dither slew rate, defined in degrees of rotation on the Poincaré sphere per second, that is much less than 6 degrees of rotation on the Poincaré sphere during the thermal diffusion time. a master oscillator for generating a signal beam; a polarization actuator for receiving the signal beam from the master oscillator and adding a polarization dither in accordance with a feedback signal before outputting the signal beam to the fiber amplifier; a multiple mode fiber amplifier for receiving the signal beam and outputting an amplified beam with a power in the order of kW; a beam sampler for receiving an output of the fiber amplifier; a polarization filter, coupled to the beam sampler, for transmitting only power contained within a desired output state of polarization SOP; and a controller for generating the feedback signal, the controller further comprising: a dither signal generator for generating a dither waveform; a processor, coupled to the polarization filter and the dither signal generator for generating an error signal; and a summer coupled to the dither signal generator and the processor for generating the feedback signal; characterised in that parameters of said dither waveform generated by the dither signal generator are selected based on the thermal diffusion time across a core of the fiber amplifier so that the dither waveform does not trigger polarization-induced high order mode instability PI-HOMI, wherein the thermal diffusion time is between 0.3 and 3 ms, and wherein the dither waveform either has a period that is much less than the thermal diffusion time or uses a dither slew rate, defined in degrees of rotation on the Poincaré sphere per second, that is much less than 6 degrees of rotation on the Poincaré sphere during the thermal diffusion time. a dither signal generator for generating a dither waveform; a processor, coupled to the polarization filter and the dither signal generator for generating an error signal; and a summer coupled to the dither signal generator and the processor for generating the feedback signal; characterised in that parameters of said dither waveform generated by the dither signal generator are selected based on the thermal diffusion time across a core of the fiber amplifier so that the dither waveform does not trigger polarization-induced high order mode instability PI-HOMI, wherein the thermal diffusion time is between 0.3 and 3 ms, and wherein the dither waveform either has a period that is much less than the thermal diffusion time or uses a dither slew rate, defined in degrees of rotation on the Poincaré sphere per second, that is much less than 6 degrees of rotation on the Poincaré sphere during the thermal diffusion time.