Realization of intentionally introduced mode-coupling in optical fibers by downward axial vapor deposition

Step-index optical fibers have been fabricated with a bandwidth increase over conventional ones of more than one order-of-magnitude. The improved performance value of approximately 600 MHz.km was achieved by mode-coupling due to intentionally introduced refractive index variations along the length of the preform. The preforms were fabricated by a soot deposition process called Downward Axial Vapor Deposition (DAVD), which allows for the perturbation of the soot stream, and hence the introduction of small index changes as a soot boule is being grown. The index perturbations which are spaced on the order of 1 μm in the preform and are a few percent of the total index difference, lead to a coupling length in the fiber of a few meters, as evidenced by far-field numerical aperture measurements. The invariance of the output mode pattern to input launch conditions also makes these fibers potentially useful as jumper and connector cables. This represents the first reported achievement of reduction in multimode pulse dispersion in optical fibers by mode-coupling introduced in the fabrication process, an idea awaiting implementation almost since fiber transmission became a practical reality.