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Intensity noise as a driver for transverse mode instability in fiber amplifiers

PhotoniX 2020年第1期1卷 245-261页

作者： Christoph Stihler Cesar Jauregui Sobhy E.Kholaif Jens Limpert Institute of Applied Physics Abbe Center of PhotonicsFriedrich-Schiller-Universität JenaAlbert-Einstein-Str.1507745 JenaGermany Fraunhofer Institute for Applied Optics and Precision Engineering Albert-Einstein-Str.707745 JenaGermany Helmholtz-Institute Jena Fröbelstieg 307743 JenaGermany.

The effect of transverse mode instability (TMI) is currently the main limitation for thefurther average-power scaling of fiber laser systems with diffraction-limited beamquality. In this work a main driving force for TMI in fiber amplifiers is identified. Ourexperiments and simulations illustrate that the performance of fiber laser systems interms of their diffraction-limited output power can be significantly reduced whenthe pump or seed radiation exhibit intensity noise. This finding emphasizes the factthat the TMI threshold is not only determined by the active fiber but, rather, by thewhole system. In the experiment an artificially applied pump intensity-noise of 2.9%led to a reduction of the TMI threshold of 63%, whereas a similar seed intensitynoisedecreased it by just 13%. Thus, even though both noise sources have animpact on the TMI threshold, the pump intensity-noise can be considered as themain driver for TMI in saturated fiber amplifiers. Additionally, the work unveils thatthe physical origin of this behavior is linked to the noise transfer function insaturated fiber amplifiers. With the gained knowledge and the experimental andtheoretical results, it can be concluded that a suppression of pump-noise frequenciesbelow 20 kHz could strongly increase the TMI threshold in high-power fiber lasersystems.

关键词： Transverse mode instability Fiber lasers Thermal effects Intensity noise Modal interference refractive index grating Mode coupling Phase shift

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