Examples Of Traces
Examples of How to Use the OTDR and Avoid Errors
Here are some actual examples
of how to use an OTDR properly (and improperly) and some tricks
to overcome some of the problems you encounter. The most adamant
advice we will give is this: Do not blindly accept the data without
interpretation. Most of today's OTDRs have "automatic modes"
where they will find splices and connectors and calculate all
the losses, if the data follows preprogrammed guidelines.
In our experience, these numbers are not to be used without human
interpretation of the results, as the OTDR is basically a PC,
isn't too smart, and the software has limitations if the cable
plant has high reflections or short cable runs.
What Does a Good OTDR Trace Look Like?
Setting the markers slightly away from the event won't affect the loss measurement by adding significant fiber loss. Remember fiber loss is only 0.01 dB per 10 meters (33 feet) at 1300 nm! By going further from the event, you reduce the possibility that the measurement is in error because of instrument limitations.
If the trace never shows a straight
line between events, like Figure 5, it means either the distance
between events is too short for the OTDR resolution or reflections
are too high for the recovery time needed before the next event.
In this case, you cannot get good data on either fiber loss or
This "Trick" Can Help
There is a trick you can try if you are desperate. The reflection at an event that causes overloading the OTDR receiver can be tamed by using "index matching fluid" to reduce the effect that causes the reflection. The reflection is caused by an air gap between the ends of the fibers. Connectors are particularly bad at having this air gap and resulting high reflections. An index matching fluid replaces the air with a fluid or gel that closely matches the optical characteristics of the glass, causing the reflections to be greatly diminished.
Figure 11 two traces of the same fiber. It is only 700 feet or 200 meters long, which is normally considered quite short for OTDR analysis. The intial trace shows the connector on the OTDR end has high reflections, saturating the OTDR and causing the baseline of the trace to decay slowly. Even the second pulse, from the connector at the far end of the cable has very high reflections. Both reflections caused ghosts also.
Since the baseline between the two ends never becomes flat or straight, there is no reliable reference point for making a loss measurement.
Index matching fluid can be purchased from a specialty fiber optic distributor or from your local supermarket or druggist. The special fluids work very well but are expensive. Mineral oil or plain petroleum jelly work almost as well! If you can get silicone "high vacuum grease", it works very well too.
Using this technique gives us
visibility we would never have otherwise, but the technique requires
care. The index matching fluid or gel must be thoroughly cleaned
from the connectors after use with the OTDR. The mating adapter
used must not be one in the network hardware, but a separate
item that can be used and discarded or thoroughly cleaned in
solvent afterwards. Index matching fluid or gel will act like
a magnet for dirt if it gets on any hardware or stays on the
connectors, as its sticky texture will grab and hold lots of
airborne dirt to attenuate the signal in the fiber link.
Testing fiber optic cable plants are easy if you have the right instruments and follow industry testing standards. When diagnosing problems, you must be creative to develop techniques that help reveal problems that show up on standard tests. It is most important to know your tools operation and limitations, and how to work around them. Get to know the applications support staff at your instrument vendor so you can call with questions. Finally, it is most helpful to have good cable plant documentation, since knowing what you are looking at will make it much easier to find problems.
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