Propagation Modes

Fiber-optic cable has two propagation modes: multimode and single mode. They perform differently with respect to both attenuation and time dispersion. The single-mode fiber-optic cable provides much better performance with lower attenuation. To understand the difference between these types, you must understand what is meant by "mode of propagation."

Light has a dual nature and can be viewed as either a wave phenomenon or a particle phenomenon that includes photons and solitons. Solitons are special localized waves that exhibit particle-like behavior. For this discussion, let's consider the wave mechanics of light. When the light wave is guided down a fiber-optic cable, it exhibits certain modes. These are variations in the intensity of the light, both over the cable cross section and down the cable length. These modes are actually numbered from lowest to highest. In a very simple sense, each of these modes can be thought of as a ray of light. For a given fiber-optic cable, the number of modes that exist depends on the dimensions of the cable and the variation of the indices of refraction of both core and cladding across the cross section. The various modes include multimode step index, single-mode step index, single-mode dual-step index, and multimode graded index.

September 4, 2011

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