A single-mode fiber of increased core size consists of a "few-mode" fiber,
of core size sufficient for guiding up to three high-order modes in
addition to the fundamental mode, by interposing perturbations of such
spacing along the fiber, as to selectively couple any such high-order
modes to (unguided) cladding modes, thereby rejecting all but the
fundamental mode. Unwanted coupling of the fundamental mode, leading to
added fiber loss, is minimized by appropriate refractive index profile
design.
