Information Technology Reference
In-Depth Information
If a strand is bent too far, it may form micro-fractures and the resultant
increased loss may cause the link to be marginal or to fail. It is rare for a fiber to be
bent so far that it fractures completely, but it is possible. That is why we are very
careful to keep individual strands protected within fiber outlet boxes or fiber termi-
nation boxes.
The best way to find micro-fracture faults is with a fiber-optic test set. If your
installation has good records, you should have the link loss measurements of all of
your fiber strands. An increased loss reading indicates either a connector failure or
a fiber fracture. By testing a strand with a time-domain reflectometer (TDR), you
will be able to determine exactly where the severe bend or break has occurred. If
you have the same failure at about the same place in multiple strands, you can be
sure that there is a mechanical problem with the cable at that point.
It is also possible to simply cut through a fiber-optic cable. Cable cuts usually
occur during construction or remodeling. However, cuts can also happen when
other unrelated systems are being maintained. In any event, you can use the TDR
method to optically measure where the cut has occurred, and then physically locate
the problem.
Fiber-optic cables may be fusion spliced, if there is sufficient slack in the cable.
In other cases, a jumper-splice can be used. For most applications, a fusion splice
will function almost at the same level as the original strand. But be careful, as new
rules recognize the additional loss and misalignment that can occur with fusion
splices.
Fiber-Optic Mode and Core Mismatches.
After the initial installation, the most
common problem with fiber-optic cable is mismatching user and patch cords with
the installed fiber-optic cable strands. Remember that optical fiber comes in three
main types, 50/125 µm multimode, 62.5/125 µm multimode, and 10 µm single-
mode fiber. In addition, some multimode fiber is “laser optimized” so it can handle
gigabit and 10 Gigabit Ethernet.
The most serious problems are mode mismatches. If a multimode transmitter
and receiver are inadvertently patched into a single-mode horizontal or backbone
cable, the losses can be excessive. The optical power that should have been spread
over a 50 or 60 micron core is faced with a tiny 10 micron core. In addition, only
one mode of light from the transmitter source will propagate down the single-mode
fiber. The result is generally no useable connection, even though the user cords
(sometimes called fiber jumpers) look just fine.
For multimode mismatches, the problem is less serious, but trying to couple
62.5 micron fibers or optics into a 50 micron fiber creates a significant signal loss,
almost 40% of the signal cross-section. Fortunately, going the other way is not as
