- Optical Time Domain Reflectometers
- (OTDRs)
Examples on Use
Index Matching Gel
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?
Take a look at Figure 10. You can see the end of the "pulse
suppressor" cable, the first fibre out from the OTDR, allows
time for the OTDR to recover from the initial pulse. Note the
fibres are all straight lines between "events", as
splices and connectors are called in OTDR jargon. Markers for
loss measurements should always be set far enough on either side
of an event to be on the straight part of the fibre trace. If
there is any curve, it is likely caused by the resolution of
the OTDR or recovery from an overload.
Figure 10. Good OTDR trace for analysis
Setting the markers slightly
away from the event won't affect the loss measurement by adding
significant fibre loss. Remember fibre loss is only 0.01 dB per
10 metres (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 fibre loss or event loss.
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 fibres. 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 fibre. It is only 700 feet or
200 metres long, which is normally considered quite short for
OTDR analysis. The initial 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.
But we can reduce the reflections by adding index matching fluid.
We just add a drop of gel or fluid between the two connectors
that matches the index of reflection of the glass in the fibre,
reducing the reflections. Once we reduce the reflections, causing
the receiver overload to go away, the OTDR will give us a good
fibre trace just beyond the connector and a flat fibre trace
which we can use for measuring the actual loss! We even got rid
of the confusing ghosts!
Figure 11. Improving OTDR Traces with Index Matching Gel
Index matching fluid can be purchased
from Tech Optics - click here for
details. It is available in a convenient 6cc syringe. 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 splice bushing 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 fibre link.
Summary
Testing fibre 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. For information on our Micro OTDR Click
Here.
