Investigations on high-reflective Fiber-Bragg-Gratings in multimode fibers

Due to the large core diameter and high number of guided modes in fibers with large core diameters, increasing the reflectivity of Fiber-Bragg-Gratings (FBG) in these multimode (MM) fibers is technically challenging. To increase the homogeneity and reflectivity of FBG in MM fibers, we analyze the impact of laser inscription parameters onto the FBG dimensions. Limitations of the processable core volume due to the cylindrical fiber geometry are observed. A new inscription strategy is developed, utilizing the rotational symmetry of the fiber to make a larger core volume accessible. In combination with a chirp of the FBG’s grating period a maximum reflectivity of 61% is archived in a 105 µm core diameter fiber.