Information Technology Reference
In-Depth Information
equipment interface are thus paired, and are usually marked Tx and Rx (or T and
R) to indicate transmit and receive, respectively. Taken together, the transmit and
receive optics are called an
optical transceiver
.
Some optical connector pairs are arranged with an exact spacing, so that a pair
of optical plugs can attach simultaneously. Fibers are terminated into two separate
plugs, and then snapped into a duplex adapter. Both the 568-SC connector and the
so-called SFF connectors specified in TIA-568-C are duplexed this way. Paired fiber
cords, called duplex cords or zip cords, are normally used for fiber-optic jumpers.
The cords should be terminated with the appropriate type of fiber-optic connector
for the application.
For transmission over the fiber, the light from the optical source may be colli-
mated into a narrow beam to more efficiently couple its energy into the fiber. In the
case of the laser sources, the light beam is already quite collimated and must simply
be aimed accurately at the fiber core. As a matter of fact, the coupling of the opti-
cal source and the interconnection of terminated fibers are critical parts of fiber-
optic transmission technology. If the termination is properly done, the connector
hardware will normally ensure that the fiber-to-fiber or fiber-to-source coupling is
maximally effective, resulting in an absolute minimum coupling loss.
Fiber Types
Optical fiber is available in both glass and plastic compositions. Glass is the most
prevalent in LAN uses, because of its inherently greater bandwidth. Several uses for
plastic fiber exist, and this is an area that has had more development over the past
several years.
3
However, at this point in time, the higher bandwidth and longer dis-
tance requirements of modern networking still demand the use of silica-based fiber
in most applications.
The fiber's
core
actually transmits the light beam, while the
cladding
provides
strength, flexibility, and a controlled stop in refractive index. All are necessary for
proper fiber-optic operation. Do not mistake the cladding for the plastic
buffer
and
the
primary coating
that coat the fiber strand in a tight-buffer cable construction.
The buffer and the primary coating are removed by a stripping tool in preparation
for the termination of a strand. The cladding, on the other hand, is an integral part
of the fiber strand and cannot be removed.
Optical fiber is also available in two modal compositions, single-mode and
multimode, described by the light modes that each will propagate (see Fig. 11.2).
3
Graduated-index plastic optical fiber (GIPOF), for example, can be used to transmit moderately fast
LAN signals over shorter distances.
