The influence of the core diameter and the fiber length on the beam quality of the transmitted beam was investigated theoretically and experimentally for highly multimode step-index fibers with a numerical aperture of 0.22 using a fully monolithic setup. We show that it is possible to maintain a nearly diffraction-limited beam quality (M 2 ≈ 1.3) through 100 m long multimode fibers. For a core diameter of 60 µm and a fiber length of 380 m one can still deliver a beam with an M 2 value of 2.1. The high-power suitability of this approach was shown by transmitting 1 kW of power through a 100 m long fiber with a core diameter of 60 µm without the onset of stimulated Raman scattering while maintaining a nearly diffraction-limited beam quality (M 2 ≈ 1.3). Index Terms-Large mode area (LMA), multimode step-index fiber, nearly diffraction-limited beam, stimulated Raman scattering (SRS). I. INTRODUCTION F OR laser materials processing optical fibers offer a flexible solution for transporting the laser beam from the laser source to the work piece. The fiber delivery of high-brightness solid-state laser radiation is however limited by to the onset of nonlinear effects. The output of industrial high-power cw fiber lasers typically exhibits a broad spectral bandwidth [1], thus the dominant nonlinearity in the delivery fibers is stimulated Raman scattering (SRS) [2]. The threshold power leading to SRS is proportional to the ratio between the effective mode field area and the fiber length [3]. To overcome SRS and enable the fiber-optic transmission of high-power laser beams over long distances the core size must be enlarged, thus making the fiber multimode.

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Step-index fibers (SIFs) with alumina cores were fabricated employing the powder-in-tube technique. The fabricated SIFs have alumina concentrations of up to 32 mol%, which is the highest value reported so far for fibers with core diameters smaller than 25 μm. The mixing mechanisms between alumina and silica during fiber drawing were revealed by energy dispersive X-ray analysis of the neck-down area of the preform. The results of the measurements and simulations indicate that besides diffusion, fluid dynamics between softened silica and alumina powder also play an important role in the resulting alumina and silica concentrations in the fiber. The influence of different drawing parameters on the alumina and silica concentrations of the fibers is also presented.