Information Technology Reference
In-Depth Information
It is often much more expensive to replace an entire run of fiber than simply to
find the location of the fault and repair it by splicing the cable, particularly if out-
side plant cable is involved. The problem is how to find the fault.
An optical version of the TDR, called an
optical time-domain reflectometer
(OTDR), is the solution. This device operates in the same manner as the conven-
tional TDR, but uses a pulse of light, rather than an electrical pulse. Modern
OTDRs display a graphic that shows the signal return of the fiber along its length.
By moving a cursor, you can pinpoint the distance from the source to any anomaly.
Then, the area of the fault can be located by direct measurement. If the fault is obvi-
ous from the outside of the cable, the bad fiber may simply be cut and spliced, via
fusion or coupling. Otherwise, it may be necessary to replace a section of cable, sim-
ilarly spliced.
OTDRs may provide built-in connections for both single-mode and multimode
fiber. Because the TIA-568-C standard recognizes both fiber types, both will need to
be tested in large systems. The standard allows several types of fiber-optic connec-
tors, including the new SFF types as well as traditional types such as ST and the
568SC, for use with both fiber types. Single-mode fiber requires special laser-type
sources to couple light effectively into the fiber. Because part of the measurement
involves optical loss, it is essential that the coupling be to the proper type of source.
Adaptation to different styles of fiber-optic connectors may be required.
Basic Fiber-Optic Testers
Optical fiber may also be effectively tested with simpler equipment. Very basic fiber
test equipment couples light into one end of a fiber link and detects it at the other
end. Some of these testers use visible light, so you can theoretically see the “light at
the end of the tunnel.” However, some higher-power links may have enough
infrared light output that it could be dangerous to the eye. Because the infrared light
is invisible, you should never look directly into a fiber from any close distance. A
good practice with one of these testers is to use a detector or to observe the light, if
visible, on another surface, such as a sheet of paper.
Optical loss measurement may also be used to test optical fiber. This equip-
ment measures simple optical loss over a fiber link by placing a calibrated light
source at one end of the cable and a sensitive optical power meter at the other end.
