Sobhy Kholaif,1,2 Cesar Jauregui,1 Yiming Tu,1,2 Jens Limpert1,3,2
1Friedrich-Schiller-Univ. Jena (Germany) 2Helmholtz-Institute Jena (Germany) 3Fraunhofer Institute for Applied Optics and Precision Engineering (Germany)
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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.
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