Status & Perspectives in Science & Education

107 106 Principal Scientist Profiles Jens Limpert Jens Limpert Principal Scientist Profiles PROFESSOR FOR NOVEL SOLID-STATE LASER CONCEPTS AT INSTITUTE OF APPLIED PHYSICS Jens Limpert leads the Laser Development Group at the Institute of Applied Physics. Furthermore, he is a scientific member of the Helmholtz Institute Jena and a member of the directorate of the Fraunhofer Institute for Applied Optics and Precision Engineering, Jena. He was distinguished with a European Reseach Council (ERC) Starting Grant in 2009, a Consolidator Award in 2014 and an Advanced Grant in 2019. JENS LIMPERT RESEARCH AREAS In his research, Prof. Limpert investigates novel laser source concepts. Research thrusts include: • Innovative laser sources, high power lasers • Novel fiber structures • Propagation effects of short laser pulses • Light matter interaction at high intensities • Nonlinear optics, parametric amplifiers • Ultrafast lasers • Coherent combination of lasers • Cavity enhancement of femtosecond pulses • High harmonic generation (HHG) and attosecond pulse generation • THz generation TEACHING FIELDS Prof. Limpert teaches courses in the fundamentals of laser physics both for the M.Sc. Photonics and for the M.Sc. Physics. RESEARCH METHODS The laboratories run by Prof. Limpert offer a wide range of complex setups and characterization methods to establish and study novel optical components: • Fiber optics and fiber technology • Numerous laser sources and pump diodes • Setups for spatial and temporal shaping of ultrashort pulses • Ultrashort optical pulse characterization devices RECENT RESEARCH RESULTS The Fiber and Waveguide Laser Group has demonstrated a significant performance scaling of fiber-based laser systems in recent years. Based on a fundamental knowledge of waveguide optics and laser physics, novel fiber designs such as the rod-type large pitch photonic crystal fiber have been invented. This fiber design is based on a novel mechanism, the delocalization of higher order transverse modes, and allows for single-mode extraction from a core size of ytterbium-doped fibers as large as 135 µm, 135 times larger than the guided wavelengths. This record mode area has enabled an enormous performance increase in ultrafast fiber laser systems. Gigawatt peak power, in combination with several 100 W of average power, constitutes unique laser parameters [1, 2]. To extract a performance which is beyond the capabilities of a single aperture emission, the approaches of spatially separated amplification, followed by the coherent addition of amplified femtosecond pulses, are pursued. These concepts are based on the idea of distributing the load or challenges, respectively, to more than just one amplifier channel. In this regard, an amplifying interferometer is constructed. Besides producing a careful numerical analysis, the group has been able to extract parameters beyond the capabilities of a single channel emission [3], demonstrating a new and promising scaling concept for ultrafast lasers. Based on this work, fiber based laser systems are now considered potential drivers for laser wake-field particle accelerators. Besides performance scaling fundamental effect in amplifying fibers are investigated. Among them thermally induced modal instabilities. This new affect is a serious issue for high average power fiber laser system. Over the recent years the group has contributed to the understanding of that effect and proposed most efficient mitigation strategies [4]. Transferring the performance of high-repetition rate fiber lasers to new wavelength regions would enable a number of novel applications. We have revealed an unprecedented potential of Thulium-doped fiber lasers most recently. The favorable scaling when shifting the emission wavelength to 2 µm of mode area and nonlinear effects are the basis for a push in obtainable peak power, whereas the higher thermal robustness of long-wavelength fiber lasers hold the promise for high average powers. In addition, the provided gain bandwidth of thuliumdoped silica would support pulses as short as 60 fs. In terms of optical cycles that would correspond to a 25 fs emission at 800 nm. The group has demonstrated multi-GW peak power and kW average power ultrafast thulium-based fiber laser systems [5,6]. Therefore, 2 µm fiber lasers might be considered as the long-wavelength counterpart of Titanium:Sapphire lasers in the future, but in an average-power scalable platform, which is most beneficial for an inexhaustible number of applications. GENERATION OF FEMTOSECOND PULSES WITH MORE THAN 10 KW AVERAGE POWER In lasers, waste heat is generated in the process of light emission. Laser geometries with a large surface-to-volume ratio, such as fibers, can dissipate this heat very well. Thus, an average power of about 1 kilowatt is obtained from today’s high-power lasers. Beyond this power, the heat load degrades the beam quality and poses a limit. To circumvent this limitation, the we have created a new laser that externally combines the output of 12 laser amplifiers. They showed that the laser can produce 10.4 kW average power without degradation of the beam quality [7]. Thermographic imaging of the final beam combiner revealed a marginal heating. Thus, power scaling to the 100-kW level could be accomplished by adding even more amplifier channels. In the future, high-power combined lasers not only will accelerate industrial processing, but also enable formerly visionary applications such as laser-driven particle acceleration and space debris removal. [1] Stutzki et al., Optica Vol. 1, 233 (2014). [2] Limpert et al., Light: Science & Applications 1 (e8), 1 (2012). [3] Klenke et al., Opt. Lett. 39, 6875 (2014). [4] Jauregui et al., Nature Photon. 7, 861 (2013). [5] Gaida et al., Opt. Lett. 41, 4130 (2016). [6] Gaida et al., Opt. Lett. 43, 5853 (2018). [7] Müller et al., OpT. Lett. 43, 3083 (2020). Contact: Phone: + 49 3641 9-47811 Email: jens.limpert@uni-jena.de

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