Sonstige Publikationen

We present a coherently-combined ultrafast fiber laser system consisting of twelve amplifier channels delivering 10.4 kW average power at 80 MHz repetition rate with a pulse duration of 240 fs FWHM and an almost diffraction-limited beam quality of M2 ≤ 1.2. The system incorporates an automated self-adjustment of the beam combination with 3 degrees of freedom per channel. The system today is, to the best of our knowledge, the world's most average-powerful femtosecond laser. Thermographic analysis indicates that power scaling to 100 kW-class average power is feasible.

Applications such as material processing, spectroscopy, particle acceleration, high-harmonic and mid-IR generation can greatly benefit from high repetition rate, high power, ultrafast laser sources emitting around 2 μm wavelength. In this contribution we present a single-channel Tm-doped fiber chirped-pulse amplifier delivering 108 W of average output power at 417 kHz repetition rate with 250 fs pulse duration and 0.73 GW of pulse peak power. To the best of our knowledge, this is the first demonstration of an ultrafast Tm-doped fiber laser with more than 100 W of average power and GW-level peak power.

In this work we present theoretical investigations of the power scaling potential of multicore fibers. In principle it is widely accepted that increasing the number of active cores helps to overcome current challenges such as transversal mode instabilities and non-linear effects. However, in order to do a proper analysis of the average power scaling potential of multicore fibers it is required to pay particular attention to thermal effects arising in such fibers. Therefore, a simulation tool has been developed that is capable of solving the laser rate equations, taking into account the resulting temperature gradient and the distortions in the mode profiles that it causes. In the study several different multicore fibers possessing a rectangular core position layout of 2×2 to 7×7 of active cores have been analyzed. Moreover, we have investigated the influence of the active core size in terms of thermal effects as well as the extractable output power and energy. This includes a study in the maximum achievable coherent combination efficiency of the multicore channels (that is strongly influenced by the distorted mode profile at the fiber end facet), the impact on nonlinear effects, the optical path differences between the cores and the amplification efficiency which are all triggered by thermal effects. Finally the scaling potential as well as the challenges of such fibers will be discussed.

In this work we present a novel way to mitigate the effect of transverse mode instability in high-power fiber amplifiers. In this technique a travelling wave is induced in the modal interference pattern by seeding the amplifier with two modes that have slightly different frequencies. The interference pattern thus formed will travel up-or downstream the fiber (depending on the sign of the frequency difference between the modes) with a certain speed (that depends on the absolute value of the frequency difference). If the travelling speed is chosen properly, the thermally-induced index grating will follow the travelling modal interference pattern creating a constant phase shift between these two elements. Such a constant controllable phase shift allows for a stable energy transfer from the higher-order modes to the fundamental mode or viceversa. Thus, this technique can be adjusted in such a way that, at the output of the fiber almost all the energy is concentrated in the fundamental mode, regardless of the excitation conditions. Moreover, this technique represents one of the first examples of the new family of mitigation strategies acting upon the phase shift between the modal interference pattern and the refractive index grating. Additionally, it even exploits the effect of transverse mode instability for gaining control over the beam profile at the output of the amplifier. Therefore, by adjusting the frequency difference between the seed modes, it is possible to force that the beam at the output acquires the shape of the fundamental mode or that of a higher order mode.

A simplification of segmented-mirror splitters for coherent beam combination based on numerical optimization of coating designs is presented. The simplified designs may facilitate the production of such elements for coherent beam combination while maintaining high combination efficiency. The achievable efficiency and error tolerance, and additional performance characteristics are analyzed in the context of coherently combined multicore fiber laser systems.

In this work we present a multicore fiber design that exploits the Talbot effect to carry out the beam splitting and recombination inside of the fiber. This allows reducing the complexity of coherent combining systems since it makes the splitting and combining subsystems together with the active stabilization redundant. In other words, such a multicore fiber behaves for the user as a single core fiber, since the energy is coupled just in a single core and it is extracted from the same core. This work describes the operating principle of this novel fiber design and analyzes its performance in high power operation using a simulation model based on the supermode theory. This includes a study on the impact on non-linear effects, on the amplification efficiency, on the thermal resilience of this design and on the performance dependence on the pump direction. Moreover, some design guidelines will be provided to tailor the characteristics of the fiber. Finally, it will be discussed how these fibers can be used to increase the TMI threshold of fiber laser systems.