Delivery of 800 W of nearly diffraction-limited laser power through a 100 m long multi-mode fiber

We present the efficient propagation of a nearly diffraction-limited laser beam with a continuous wave power of 800 W through a multi-mode step-index delivery fiber with a core diameter of 30 µm and a numerical aperture of 0.056. The M2-value was measured to be 1.35 after 100 m of this passive fiber. This is an important advance as the delivery fiber length for high-brightness beams in the kilowatt range is usually limited to a few meters by the onset of nonlinear effects. For this demonstration a single-mode MOPA system was set-up consisting of a fiber oscillator and two amplifier stages. This source was coupled into the delivery fiber through a 500 mm long mode field adapter.