Cabling the Fiber

Coated glass fibers are manufactured by a small handful of large, high-tech corporations. [90] The reeled fibers are then shipped to any of a much larger number of cable assembly plants located around the world where buffering and jacketing operations take place. The cable assembly plants then sell their fiber cables to distributors , who sell them to you.

[90] Lucent, Corning, and Siemans are prime examples.

If you need technical information about the optical properties of a fiber core, don't look to the distributor. Don't even look to the cable manufacturer. Find out who makes the core that interests you, and call directly to the core manufacturer. The optical properties of the fiber are determined almost entirely by the coated glass core . [91]

[91] Some optical properties are modified slightly by the heating and stretching that takes place during the buffer encapsulation process.

The mechanical properties of the cable, such as durability, weather-proofing, plenum-rating, pull strength, and bend radius are determined almost entirely by the buffer and jacket construction. This is what people at the cable assembly plant understand. If you cut open a lot of cables, you can learn about it too. When a cable is cut, you can count the fibers, you can see if the individual fibers are buffered, and you can see the various strength-enhancing features, such as nylon wraps, thick plastic jacketing, or metal sheathing (Figure 11.5). These cable features are all designed to protect the delicate nature of the fiber.

Figure 11.5. Cross section of all-dielectric, self-supporting, cable with 42 fibers.

When you think about it, a fiber core deserves quite a lot of protection. It's made of glass. It's 1/100 of an inch thick. An unprotected fiber stretched by as little as 1 part in 1,000 will break. A fiber subject to a series of small kinks, or microbends , suffers greatly increased attenuation.

A cable must endure the enormous forces exerted on the cable jacket when the cable is pulled, hung, or buried, without transmitting those forces to the internal fiber cores and without causing the fiber to kink.

There are two methods used to protect, or buffer, a fiber core [92] . The methods are the tight buffer and the loose buffer (Figure 11.6). The tight buffer method surrounds each coated fiber with a tight-fitting protective sleeve having a diameter (for 50 to 62.5 m m glass fiber) of 900 m m. A tight-buffer cable is easy to manufacture and works well for most horizontal in-building applications.

Figure 11.6. Various options for tight or loose buffering.

The loose buffer method surrounds each coated fiber with a loose-fitting tube or slot within which the fiber is free to move. The fibers inside a loose-buffered cable generally lie in some sort of helical spiral. A loose-buffered cable tends not to transmit external stretching and bending forces directly to the fibers and is therefore a good choice for tall vertical runs in high-rise buildings , or out-of-doors applications subject to large amounts of extrinsic stress. The interstitial spaces within a loose-buffered cable may be filled with a gooey gel to prevent the intrusion of water should the cable jacket be breached by animals, erosion , or mechanical forces.

Most desktop LAN cables incorporate an individual tight buffer on each fiber. Individual buffering enlarges the fibers to a workable size and provides a convenient way to color -code each fiber. Cables with individually buffered fibers are significantly easier to terminate in the field than are loose-buffered cables.. Unfortunately, individual buffering also enlarges the overall cable size, limiting the number of fibers that may be drawn through a conduit of fixed dimensions. In short-haul applications within a single campus, where ease of installation is the primary consideration, buffering of each individual fiber makes a lot of sense. In long-haul applications, where the density of fibers within a given conduit is the primary consideration, the fibers are jacketed in loose-buffered bundles.

All fiber core types are available in either tight-buffered or loose-buffered versions and with and without plenum-rated insulation. [92] As a digital system designer, you don't need to worry about what style of cable jacket is used in the field. Just specify the optical properties of the fiber core and let your installation people worry about the cable jacket.

[92] See Section 7.7, "Plenum-Rated Cables"

POINTS TO REMEMBER

• The optical properties of the fiber are determined almost entirely by the coated glass core.
• The mechanical properties of the cable are determined almost entirely by the buffer and jacket construction.