In this work we optimize the design of coherently-combined multicore fiber amplifiers. It has been shown that increasing the number of cores in such fibers helps to increase the combinable output power. However, in counter-pumped multicore fibers, thermal effects will finally lead to strong non-uniform mode-shrinking in each core. This, in turn, will result in a significant reduction of the combining efficiency. In this study we will examine the power and energy scaling potential for different pumping schemes and different fiber designs. To this purpose, a simulation tool is used that solves the laser rate equations taking into account the resulting temperature gradient and the transverse mode distortions caused by it. In the simulation co- and counter pumped multicore fibers with a square core arrangement and a core number ranging from 2x2 up to 10x10 will be considered. Moreover, we investigate the influence of the active core size in terms of thermal effects as well as the extractable output power and energy. Particular attention is paid to the mitigation of non-uniform mode-shrinking at the fiber end-facet. By comparing the co- and counter-pumped cases, we will show that a combinable output power of 26 kW (co-pump) instead of 14 kW (counter-pump) with a 10x10 MCF and 30 μm cores should be achievable.
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.
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.
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.
Pump-limited kW-class operation in a multimode fiber amplifier using adaptive mode control was achieved. A photonic lantern front end was used to inject an arbitrary superposition of modes on the input to a kW-class fiber amplifier to achieve a nearly diffraction-limited output. We report on the adaptive spatial mode control architecture which allows for compensating transverse-mode disturbances at high power. We also describe the advantages of adaptive spatial mode control for optical phased array systems. In particular, we show that the additional degrees of freedom allow for broader steering and improved atmospheric turbulence compensation relative to piston-only optical phased arrays.