Long period gratings in multimode fiber fabricated with high-energy ion implantation

Long period gratings have been written in highly multimoded optical fiber using 2.4 MeV H+ ions (protons). Implantation of ions with this energy produces a permanent index change of up to 1% in the center of the fiber. The presence of a grating with a period of 1.1 mm produces coupling from the fundamental mode into higher-order modes. Noticeable mode scrambling was observed to occur when only three grating periods were written into the fiber, and the amount of coupling between modes increases as the number of periods was increased to six.