Fibers and Cables Continued

Types of Fibers: Singlemode or Multimode?
In the simplest optical fiber, the relatively large core has uniform optical properties. Termed a step-index multimode fiber, this fiber supports thousands of modes and offers the highest dispersion - and hence the lowest bandwidth.

By varying the optical properties of the core, the graded-index multimode fiber reduces dispersion and increases bandwidth. Grading makes light following longer paths travel slightly faster than light following a shorter path. Put another way, light traveling straight down the core without reflecting travels slowest. The net result is that the light does not spread out nearly as much. Nearly all multimode fibers used in networking and data communications have a graded index.

The structure of the fiber determines how the light propagates through it.

But the ultimate in high-bandwidth, low-loss performance is singlemode fiber. Here the core is so small that only a single mode of light is supported. The bandwidth of a singlemode fiber far surpasses the capabilities of today's network electronics. Indeed, the information-carrying capacity of the fiber is essentially infinite. Not only can the fiber support speeds of tens of gigabits per second, it can carry many gigabit channels simultaneously. This is done by having each channel carried by a different wavelength of light. The wavelengths do not interfere with one another. Singlemode fiber is the preferred medium for long distance telecommunications. It finds use in networks for interbuilding runs and will eventually become popular for high-speed backbones. Applications for singlemode fiber to the desk are not anticipated.

The most popular fiber for networking is the 62.5/125 multimode fiber. The numbers mean that the core diameter is 62.5 micrometer and the cladding is 125 micrometer. Other common sizes recognized by building-cabling standards include 50/125, 100/140, and 200/230 micrometer, although these are declining in use.

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