Limitations of Transmission Distances over Multimode Fiber

This paper details the limitations in transmission distances suffered by laser-based Gigabit networks operating near a wavelength of 1300nm over multimode fibers. The discussion focuses on the relationship between the actual fiber modal bandwidth and various launch conditions. Fundamental Concepts of Multimode Fibers Multimode fiber simply refers to the fact that numerous modes or light rays are carried simultaneously through the waveguide. One key characteristic of a multimode fiber is its modal bandwidth. It represents the capacity of a fiber to transmit a certain amount of information over a certain distance and is expressed in MHz*km. An intuitive model is to consider that the signal consists in a number of delayed lines, each of which corresponds to a particular mode. In this scheme, and due to the index profile properties of a graded-index multimode fiber, the “high-order modes” propagating near the edges of the core travel faster for a longer distance, thereby transiting the fiber in approximately the same time as the “low-order modes” or rays traveling more slowly near the center of the core (see Figure 1). However, due to fiber imperfections, these rays cannot arrive simultaneously and the delay between the fastest and slowest modes is known as modal dispersion. Another aspect to consider is that the intermediate modes carry most of the power. Therefore, the power from “high-order modes” traveling near the edges arrives late relative to most of the power, and the power from “low-order modes” arrives early. As a consequence, the bandwidth is reduced and the information-carrying capacity is limited. This model proves that the bandwidth depends on the way modes are excited in the fiber, and therefore that the transmission capacity depends on the launch conditions. Figure 1. Impulse Response and Modal Dispersion of a Multimode Graded-Index Fiber