Optical Fiber Structures

The principles explained in 'The Principles of Optical Fiber' apply to optical fiber with a "step index" (SI) structure. This is the structure used for most POFs. 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.

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.

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.