Abstract

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 $\mu$ m. 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 $\mu$ m, corresponding to a mode field area of 2800 $\mu$ m², 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 ( $\text{r}_{\text{bend}}$ $\geq$ 25 cm) with a total length of 10 m and a core diameter of 80 $\mu$ m.

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