Information Technology Reference
In-Depth Information
it is not as great. Consider that the transition from a large 50 µm fiber core to the
tiny 4 to 8 µm single-mode core obviously wastes most of the incident light, unless
you are fortunate enough to be using a laser source that has inadvertently positioned
the beam in the center of the multimode core.
Mode converters typically put a pair of wide-bandwidth transceivers back to
back in a box. One set of optics is single-mode and the other is multimode. In
between the interfaces is the proper electronics to repeat the signal to the opposite
interface. For this reason, you can expect fiber mode converters to cost several times
more than a simple fiber to copper converter.
This section will briefly describe some of the installation requirements for fiber-optic
cable. Optical fibers vary widely in construction, unlike their copper counterparts.
This is due in part to the fact that a complete fiber circuit requires only two
strands—one pair (although not twisted, we hope). Thus, the simplest fiber link
could be two fiber strands with as little as the primary coating protecting the fiber.
Conversely, a fiber link could be the same two fiber strands, but with tight buffer,
strength members, thick jacketing, and even armor or imbedded steel cable. The first
cable type would need several types of protective routing devices, while the second
type would need virtually none.
Fiber Cable Protection
The degree of physical protection a fiber link requires depends on two factors: con-
struction of the fiber cable and location of the cable run. In a fiber distribution
panel, very little is needed in the way of physical protection for the fibers. What is
needed is some type of fiber management and fiber storage. On the other hand,
fibers that are used for horizontal or backbone runs do need more protection. These
fiber runs will require the use of moderate jacketing, trays, and raceways to provide
the appropriate amount of additional physical protection.
Blown-in fiber places relatively unprotected strands inside plastic tubes that
have been preinstalled between the TR and the work area. The tubes are technically
raceways
. They must meet all the flammability and smoke requirements of any
plenum-rated device, if necessary. Likewise, the enclosed fibers must be appropri-
ately rated for the installation location.
Another type of protective fiber raceway is called
innerduct
. Innerduct is used
in applications where several fiber cables must be protected (Fig. 11.10). The race-
way is generally a ribbed construction to allow it to easily bend at a fairly large
