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Mitigation of transverse mode instability by heat load modulation in high-power fiber laser amplifiers

Several studies have reported mitigation strategies of transverse mode instability. These are usually based either on reducing the strength of the thermally induced refractive index grating (RIG) and/or the modal interference pattern (MIP) along the fiber amplifier or on manipulating the phase shift between the RIG and the MIP. Here we report that a heat load modulation can be achieved by pulsing the seed and/or the pump in bursts, which induces a positive phase shift. In this way, a beam cleaning is achieved, which improves the performance of the amplifier system by allowing for operation with a diffraction-limited beam profile at an average output power significantly higher than the TMI threshold. The heat load modulation technique delivers a stable beam at an average output power higher than 1.5 times the TMI threshold with an instantaneous intra-burst power of around four times higher than the TMI threshold.