Pulse Spreading in Multimode, Planar, Optical Fibers

This introduction gives first a brief review of the general concepts of pulse transmission in multimode waveguides, 1 - 2 and subsequently considers the case of planar structures that ensure transverse confinement of the optical beams. The most important parameters of optical fibers for communication are loss (perhaps a few decibels per kilometer) and pulse spreading (perhaps a few tens of nanoseconds per kilometer). Given these two 1599 parameters, the maximum repeater spacing and the transmission capacity of the fiber are pretty much determined, considering the limitations t h a t presently exist in source power (L.E.D. or injection lasers) and detector sensitivity (avalanche photodiodes). If the loss is the limiting factor, a reduction in bandwidth allows an increase in repeater spacing because of the increased receiver sensitivity, but only by a modest distance. Inversely, baseband equalization allows the transmission capacity to be increased at the expense of optical power, but not by a very large factor. In this paper, we consider only the problem of pulse spreading. Consider first a single-mode waveguide; for instance, a rectangular waveguide whose width is less than a wavelength. The wave number /3 may be a rapidly varying function of a>, particularly near cut-off. The transit time of a pulse of radiation is equal to the ratio Ldp/dto of the path length L and the group velocity dco/d/3. Because a pulse of small duration has a broad frequency spectrum, some components arrive ahead of the others if d(3/dw varies with w; t h a t is, if d2p/du2 0.