Highly multimode step-index fibers are very commonly used as laser beam delivery systems in material processing with solid state lasers. Since the Fresnel absorption at the workpiece, and therefore the process efficiency, strongly depends on the local polarization state of the laser beam, an analysis of the polarization maintaining effects of beam delivery fibers is a direct requirement in order to tailor the polarization and to maximize the absorption on the workpiece.

Multimode fibers do, in general, not maintain the input state of polarization, but fibers typically used in material processing are short enough to maintain it to a certain extend.

To study the polarization state used in laser material processing with standard multimode transport fibers, a setup for a complete, spatially resolved Stokes parameter analysis was built up and applied for different beam delivery fibers.

Polarization ellipses were analyzed after the transport of the laser beam through the fiber over the whole fiber core. Measurements of the degree of polarization and the orientation of its ellipses show clear dependencies on different fiber lengths, bending radii and coupling conditions. Even for fibers with a length of 90 meters, the degree of polarization could be measured to 30 %. By adapting the input properties of the laser, the degree of polarization of the coupled out beam could even be increased to more than 60 %.

In the last part of this work, the polarization state after a beam delivery fiber was measured directly in a laser cutting machine equipped with a fiber-coupled disk laser. For the first time, spatially resolved polarization ellipses at the workpiece position could be measured. To determine the impact of the polarization state on laser cutting, the degree of polarization and the orientation of its ellipses were analyzed at laser powers up to 8 kW.