The multi-core fiber laser is a promising fiber laser concept since it combines good beam quality and a high mode field
diameter to reduce nonlinear effects especially for pulsed laser operation. Therefore this concept is a good candidate for
high power fiber laser operation with a good beam quality. In the present paper we report on the characterization and the
laser operation of a fiber laser with a hexagonal array of 19-cores. The near and the far field intensity distribution of the
emitted beam as well as the bending-induced transversal mode selection have been investigated. The obtained
experimental and simulation results and show a good agreement.
To qualify passive fibers for (high power) laser beam delivery, different experimental approaches (interferometric,
heterodyn, <i>M</i><sup>2</sup>, ...for beam characterization at fiber output are under test in the community. Measurement of
the individual strength of different components (eigenmodes) contained in the superposition at the fiber output
in dependence for example on bending radius seems to be very promising. This can be done by means of optical
correlation filters based on DOEs. For a standard telecommunication fiber SMF-28, operated at 633 nm, this
could be demonstrated earlier<sup>1</sup> by us. Here we present experimental results for quantitative proof of LP modes
in LMA fibers as well as in SMF-28 fibers by means of such correlation filters, and demonstrate potential and
limitations of this approach.
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.
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.