We present the nearly diffraction-limited propagation of 800 W of cw laser power through 100 m of delivery fiber
having a core diameter of 30 μm. The laser source was a single-mode MOPA consisting of a fiber oscillator and two
amplifier stages and was matched to the delivery fiber through a 500 mm long taper. At a maximum power of 800 W, a
M<sup>2</sup> value of 1.35 was measured after 100 m of passive fiber. A minor Stokes-shifted spectral content was observed above
A passive few-mode multicore fiber consisting of 7 coupled cores is investigated. The fiber is compared to a largemode-
area step-index fiber, with the same number of modes and a similar mode field area. Based on the 7-core
fiber results a single transverse mode multicore fiber, with a mode field area of 465 μm<sup>2</sup> at 1050 nm, delivering
virtually diffraction limited output beam quality is demonstrated. Stimulated Raman threshold measurements
are presented and a fundamental mode high-power beam transport with more than 350 W is shown.
In the recent years high-power laser beams with radial polarization have attracted an increasing interest because of their
interesting properties in material processing. We present an overview of the current activities and different techniques to
generate such beams in CO<sub>2</sub> and solid-state lasers. With a polarising end-mirror which comprises a resonant grating on a
dielectric multilayer Bragg-structure we recently demonstrated a 3-kW radially polarised CO<sub>2</sub> laser. Current
investigations are also focused especially on the application of this technology to thin-disk lasers. The specific
requirements and the whole development from the design and fabrication to the characterization and test are illustrated
with the example of a multilayer polarizing grating mirror developed to generate a radially polarised beam in an
Yb:YAG thin-disc laser resonator. The potential of this kind of beams are discussed with a number of first application
results, which largely confirm the predictions presented by Niziev et al1.
The objective of the present work is to develop a fiber suitable for high-power fundamental-mode beam delivery over a
useful length for material processing (~100 m). The investigated design is based on evanescent-field coupled
waveguides, also called multi-core fibers (MCFs) -. The investigated MCF consists of a hexagonal array of 7 cores
in which the fundamental mode, the so-called in-phase supermode, has an effective mode area (A<sub>eff</sub>) of 348 μm<sup>2</sup> and a
numerical aperture (N.A.) of 0.035.
We show how the bending-induced losses and the mode-mixing depend on the bending radius and on the structure of the
waveguide. The experimental results on the behavior of the near-field and the far-field for different bending radii will
also be reported. Additionally, we will show another method to reproduce experimental results with multi-mode fibers
supporting few modes where the conventional approach leads to irreproducible results due to the mode-mixing effect.
Beams with radial polarization have attracted an increasing interest during the past few years because of their attractive
properties. An overview of the different intra- or extra-cavity techniques to generate such beams is given in the present
paper. The design, fabrication and characterization of a multilayer polarizing grating mirror developed for an Yb:YAG
thin-disc laser resonator are reported. The potential of the proposed solution is discussed together with the first
demonstration of a radially polarized Yb:YAG thin-disc laser.
With the help of a newly implemented circular perfectly matched layer for complex coordinate stretching a fast
and accurate calculation of radiation losses of optical waveguides is reported in the present contribution. We
will show the results of a fully vectorial finite-element calculation used for the design of special fibers for highpower
high-brilliance beam delivery. In particular, we have investigated the propagation losses in the so-called
hollow-core and solid-core Bragg-type fibers. These optical fibers have claddings consisting of alternating high
and low index layers and offer an asymptotically single-mode behavior even for large core sizes. In the case of the
hollow-core Bragg fibers, the preferred mode is a non-degenerated azimuthally polarized doughnut mode (TE01)
because it experiences the lowest losses, whereas for the solid-core Bragg fiber a two fold degenerated linearly
polarized mode (LP01) experiences the lowest losses. We will describe how to design Bragg fibers for minimal
propagation losses and how to reduce the bending sensitivity of these structures. Combining high mode-field
diameter with low losses and a low bending sensitivity makes these fibers suitable for high-power single-mode
beam delivery systems.