Finished Core Specifications

A complete fiber- optic core is characterized by the diameter of its core, its cladding, its polymer coating, and the various mechanical and optical tolerances associated with those items (Figure 11.3). Of all the various parameters that define a fiber core, the key parameter that differentiates fiber in the marketplace is the core diameter. Fiber is segmented into different grades of electrical performance according to its core diameter.

Figure 11.3. Construction of typical coated 62.5 m m multimode fiber core.

As illustrated in Figure 11.4, large cores are classified as multimode fiber (MMF), while small cores as classified as single-mode fiber (SMF). See Section 11.5.1, "Multimode signal propagation," and Section 11.6.1, " Single-Mode Signal Propagation," for definitions of these terms. The distinction between MMF and SMF has to do with the relation between the size of the core and the wavelength of the light flowing through it.

Figure 11.4. Segmentation of the fiber marketplace according to core diameter.

The diameter of the core strongly affects both the cost and signal transmission bandwidth of a finished fiber. Enlarging the core renders a finished system less expensive but reduces the bandwidth. Larger cores reduce the overall system cost because they can accept light from less mechanically precise and less costly packages and connectors. Unfortunately, larger cores also reduce the bandwidth because they allow the light to bounce around more inside the fiber, dispersing the received optical energy over time. The smallest cores (about 10 m m diameter) deliver the greatest signal transmission bandwidth at the greatest finished system cost.

The largest cores are made from plastic, an inexpensive and easy-to-handle material. Plastic fibers have the same general optical characteristics as glass fibers; however, the optical attenuation of plastic is much higher, and the bandwidths are much lower. Plastic fibers are relegated mostly to relatively low-bandwidth, short-distance applications.

High-volume desktop LAN applications use multimode glass fiber. This fiber is produced in standard core diameters of 50, 62.5, 85, 100, and 140 m m. The most popular fibers for LAN applications are 50 and 62.5 m m.

Single-mode systems with a 10- m m core are found in long-distance telecommunications applications and LAN backbone applications where electrical performance, not cost, is the leading criteria.

For any TIA/EIA-568-B-compliant or ISO/IEC 11801-compliant core size smaller than 85 m m, the nominal cladding diameter is 125 m m and the nominal polymer coating diameter is 250 m m. This use of standard cladding and coating diameters simplifies the standardization of the connectors into which all fibers must fit.

POINT TO REMEMBER

• The key parameter that differentiates fiber in the marketplace is core diameter.