an optical fiber for use in optical energy transmission system has improved mode coupling in one embodiment thereof and both improved mode coupling and selective attenuation of unwanted modes in another embodiment . the optical fiber includes a plurality of particles of refractive index differing from that of the fiber core , in which the particles are distributed , and from that of the fiber cladding in which , in one embodiment , particles are distributed . in fabricating the fiber , the particles are introduced into the preform from which the fiber is drawn , and , in a co - extrusion process , they are also introduced into the polymer prior to its being extruded with the core material .