Pulse Propagation in Multimode Dielectric Waveguides

Using coupled power equations to describe the average performance of a multimode waveguide with random coupling, it is shown that a Gaussian input pulse remains approximately Gaussian with a pulse width that increases proportionally to the square root of the length of the waveguide. The proportionality factor is determined for the model of a slab waveguide. Since coupling between guided modes of necessity causes coupling of some of the guided modes to radiation modes, radiation losses are un-avoidable. A desired improvement in pulse distortion that is accomplished by coupling the guided modes intentionally to each other must be paid for by a certain loss penalty. This loss penalty is also evaluated for the special case of the slab waveguide model. Pulse dispersion improvement can be achieved by providing intentional roughness of the core-cladding interface of the dielectric waveguide. The “power spectrum” of the core-cladding interface function must be designed very carefully in order to minimize the radiation loss penalty that accompanies any attempt to reduce pulse dispersion. The dependence of the loss penalty on the shape of the “power spectrum” of the core-cladding interface function is studied in this paper. Design criteria for the improvement of multimode pulse dispersion are given based on the slab waveguide model. The connection between the slab waveguide model and the round optical fiber is pointed out.