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Optical Fiber Structures

The principles explained in the preceding section apply to optical fiber with a "step index" (SI) structure. This is the structure used for most POFs, including those manufactured by Mitsubishi Rayon. On the other hand, quartz fiber used for telephone lines uses a "graded index" (GI) structure for increased transmission volume. In GI-POF, the index of refraction progressively increases toward the center of the optical fiber. Therefore, it utilizes the principle of refraction, not reflection as in the previous example. This method is the same as what occurs when light refracts at the surface of water. GI fiber uses this principle to progressively change the track of the light to contain it within the fiber. This type of fiber is suitable for high-speed, high-volume transmission. The only GI-POF on the market today is Eska™ GIGA manufactured by Mitsubishi Rayon.

Multi-step structure fiber uses both of the principles above for transmission. As its name indicates, the structure uses multiple-step indices. Although the basic principle is the same as that of SI-POF, because the index of refraction changes in multiple steps, the focus of the light is shifted toward the center at the same time. This structure was recognized as a simple solution to increasing bandwidth, and in 1999, Mitsubishi Rayon developed and successfully tested Eska™ Miu, the first multi-step index fiber. This fiber is still under final development.

Consumer demand for POF dictates that this fiber remain at reasonable prices. The multi-step index structure can be mass produced much easier than GI-POF. Also, since it can easily be applied to varying bandwidths by changing the number of steps, it has the added benefit of simple conversion to larger capacities in the future.

Mitsubishi Rayon will recommend Eska™ Miu, rather than the higher transmission capacity GI structure Eska™ GIGA, as the backbone for home networks, not only because Eska™ Miu satisfies bandwidth requirements, but also because the economy and superior productivity of the multi-step structure are primary concerns for home networks.