The invention is a ribbon of an optical material with a plurality of cores that run along its length. The plurality of cores includes lasing impurity doped cores in an alternating spaced arrangement with index-modifying impurity doped cores. The ribbon comprises an index of refraction that is substantially equal to or greater than the indices of refraction of said array of lasing impurity doped cores. Index-increasing impurity doped cores promote antiguiding and leaky modes which provide more robust single "supermode" operation.
The United States Government has rights in this invention pursuant to Contract No. W-7405-ENG-48 between the United States Department of Energy and the University of California for the operation of Lawrence Livermore National Laboratory.