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Modal energy transfer by thermally induced refractive index gratings in Yb-doped fibers

Light(Science & Applications) 2018年第1期7卷 483-494页

作者： christoph stihler Cesar Jauregui Andreas Tünnermann Jens Limpert Institute of Applied Physics Abbe Center of PhotonicsFriedrich-Schiller-Universität JenaAlbert-Einstein-Str.1507745 JenaGermany Helmholtz-Institute Jena Fröbelstieg 307743 JenaGermany Fraunhofer Institute for Applied Optics and Precision Engineering Albert-Einstein-Str.707745 JenaGermany

Thermally induced refractive index gratings in Yb-doped fibers lead to transverse mode instability(TMI)above an average power threshold,which represents a severe problem for many *** obtain a deeper understanding of TMI,the evolution of the strength of the thermally induced refractive index grating with the average output power in a fiber amplifier is experimentally investigated for the first *** investigation is performed by introducing a phase shift between the refractive index grating and modal interference pattern,which is obtained by applying a pump power variation to the fiber *** is demonstrated that the refractive index grating is sufficiently strong to enable modal energy coupling at powers that are significantly below the TMI threshold if the induced phase shift is sufficiently *** experiments indicate that at higher powers,the refractive index grating becomes more sensitive to such phase shifts,which will ultimately trigger ***,the experimental results demonstrate beam cleaning above the TMI threshold via the introduction of a positive phase *** finding paves the way for the development of a new class of mitigation strategies for TMI that are based on controlling the phase shift between the thermally induced refractive index grating and modal interference pattern.

关键词： amplifier pump refractive

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

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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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