Preserving Nearly Diffraction-Limited Beam Quality Over Several Hundred Meters of Transmission Through Highly Multimode Fibers

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 (M² ≈ 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 M² 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 (M² ≈ 1.3).