Power Distribution and Radiation Losses in Multimode Dielectric Slab Waveguides

correlation of power. in which the guide. dependent Light communications systems using optical fibers as the guidance medium are presently being planned for two different modes of operation. High-capacity systems are likely to be used with a laser as the light source and should be operated in the fundamental HE,, mode in order to minimize delay distortion that accompanies multimode operation. For less ambitious, low-capacity systems excitation of the fiber with a light emitting diode appears more economical. However, the output of light emitting diodes cannot be used to excite a single fiber mode with high efficiency. A low-capacity fiber to be used with a light emitting diode must thus be designed to operate with many modes. Multimode optical fibers are not as easily characterized as single-mode 429 430 T H E BELL SYSTEM TECHNICAL JOURNAL, FEBRUARY 1972 fibers. The power loss of such a fiber is usually not simply expressed by an exponential decay law but depends in a complicated way 011 the distribution of the power over the many modes. The present study is an attempt to describe the loss behavior of multimode optical fibers. We use the TE modes of the simplified model of a slab waveguide with the added requirement that there is no field variation or change in the slab geometry in the y direction of the coordinate system. This model makes it possible to describe the multimode waveguide rather simply. Even though it cannot directly be used to predict the loss behavior of round multimode optical fibers, it provides insight into the operating principles of multimode waveguides that can be used to obtain an understanding of the properties of multimode fibers of different shape.