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The fiber laser geometry:  An attractive laser geometry for continuous and pulsed high-power lasers

Andreas Tünnermann, Director, Fraunhofer-Institute for Applied Optics and Precision Engineering, Chair, Institute of Applied Physics, Friedrich-Schiller University Jena

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Abstract

 Solid-state lasers are attractive sources of coherent radiation for various scientific and industrial applications. in the past 60 years different laser geometries have been developed to overcome challenges in power scaling conserving the beam quality. Rare-earth-doped fiber lasers have emerged as most attractive and power scalable solid-state laser concept due to the outstanding thermo-optical properties of an actively doped fiber. The large ratio of surface to active volume of such a fiber ensures excellent heat dissipation, furthermore the beam quality is defined by the refractive index profile of the active core and is therefore independent on the pump power. Fiber lasers and amplifiers offer a very high single-pass gain and therefore low laser thresholds and efficient diode-pumped operation. Using advanced fiber designs, in continuous-wave and pulse operation output powers exceeding the 10 kW-level with diffraction-limited beam quality have been demonstrated.

However, power and energy scaling of cw and pulsed single-mode fiber lasers and amplifiers is restricted due to nonlinear pulse distortions, which are enforced by the large product of intensity and interaction length inside the fiber core. In addition, transverse mode instabilities are observed which degrade the beam quality emitted by high-power fiber laser systems once that a certain average power threshold has been reached. This sudden degradation of the output beam quality is accompanied by temporal fluctuations of the beam profile. Most recently, strategies have been developed to mitigate or even, ideally, to overcome these limitations. Coherent combination of different fiber amplifier channels enables a further scaling of these parameters.

This contribution presents the physical and technical basics of fiber lasers and their power scalability. The state of the art of science and technology and prospects for future developments in fiber lasers and amplifiers are reviewed.

Biography

Andreas Tünnermann is Director of the Fraunhofer-Institute for Applied Optics and Precision Engineering and Chair for the Institute of Applied Physics at Friedrich-Schiller-University Jena. His main research interests include scientific and technical aspects associated with the tailoring of light. Research topics are the design and manufacturing of novel micro- and nano-optical photonic devices using high-end microlithography and its application for generation, amplification, steering and switching of light. In particular, his work on high power diode pumped fiber and waveguide lasers is widely recognized. Since recently he has also been dealing with challenges in the application of phenomena of quantum physics. His special interests here lie in the identification of added value in imaging, communication and computing.

Andreas Tünnermann is member of the German Physical Society, European Physical Society and acatech, fellow of OSA and SPIE. His research activities on applied quantum electronics have been awarded with the Röntgen-Award, WLT-Award, Otto-Schott-Award, Leibinger Innovation Award and the Gottfried-Wilhelm-Leibniz-Award. 2015 he received the ERC-Advanced Grand of the EU. In 2024 he received the Order of Merit of the Federal Republic of Germany.