Analysis of Fundamental-Mode Beam Transport in Highly Multimode Fibers

In view of the growing demand for fiber beam delivery in manufacturing systems with close to diffraction-limited beam quality, we present an analysis of the propagation of a nearly diffraction-limited laser beam through 5 to 10 m long multimode step-index fibers with a numerical aperture of 0.111 and core diameters of up to 80 μm. The influence of the launching conditions are investigated theoretically and experimentally by varying the beam size of the input beam. At a core diameter as high as 80 μm, corresponding to a mode field area of 2800 μm², and an optimum launching condition, the transmitted M² was measured to be around 1.3. It is shown that bending of the fibers down to radii as small as 2 cm barely influences the beam quality factor of the transmitted beam for all the tested fibers as the M² was always found to be smaller than 1.5. The threshold of stimulated Raman scattering was measured to be around 60 kW for a fiber loosely placed on the table (r_bend ≥ 25 cm) with a total length of 10 m and a core diameter of 80 μm.