Information Technology Reference
In-Depth Information
local tester. The two units may be interchangeable, or at least have dual displays, so
the installer at the remote end knows what is going on. Many of the tester pairs have
mike/headphones and an optical “order wire” so the two installers can talk when
connected to the same cable.
Testing installed fiber cable is a matter of measuring the link loss. In general,
if the link loss is within budget for the standard (and within stricter link budgets for
certain applications), your cable is fine. You should really test at the appropriate
wavelength for the application, and the tester should use time-domain reflectometry
(TDR) to ensure that the cable length is within prescribed lengths. Some more
sophisticated testers may ensure that return losses (a measure of reflections) are
within limits, but if you use appropriate cable and connectors, this should not be a
problem. A really sophisticated test would be to measure the link's bandwidth-
length product, and it would catch cables that had insufficient bandwidth for
gigaspeed operation at extended distances.
Most fiber-optic testers use the “overfilled launch” testing method, where the
entire fiber core diameter is flooded with light, usually from an LED source. This
method is effective in dealing with 50 and 62.5 µm core diameters. In testers that use a
VCSEL diode source, a mode dispersion technique is used to avoid the mode blockage
that can occur in some multimode fibers. You should be aware that this testing tech-
nique can mask some problems that may be experienced when you actually hook up
the cable run to a piece of equipment. Be aware of the specifications and limitations of
your particular tester. That takes care of the basic pass/fail testing of a fiber-optic link.
Troubleshooting a marginal or failed link is a different matter. One of the prob-
lems you encounter with a failed link is determining where the failure is located,
with respect to the cable ends. This issue is a problem whether you are troubleshoot-
ing newly installed cable or cable that has been installed and operating for some
time. Your tester may be able to function as a field TDR, in addition to the basic
loss measurements. If so, it is simple to determine the location of a fiber break or a
botched connector installation. Knowing which connector is bad, or where the fiber
is damaged, saves a lot of repair time. Fiber breaks are permitted to be spliced
because splices have such inherently low loss. Bad connectors should simply be
replaced, so be thankful that extra fiber was left at the point of termination.
Fiber-optic technology has safety aspects that are quite different from those of cop-
per cabling. As long as the proper procedures are followed, fiber-optic installations
should be as safe as any type of cabling. Eye protection in the form of safety glasses
should be used for all fiber-optic assembly work.
