- Optical Time Domain Reflectometers
- (OTDRs)
The OTDR Trace
Information in the OTDR Trace
They say a picture is
worth a thousand words, and the OTDR picture (or "trace"
as they are called) takes a lot of words to describe all the
information in it! Consider the trace in Figure 4.
Figure 4. OTDR Trace Information
The slope of the fibre trace
shows the attenuation coefficient of the fibre and is calibrated
in dB/km by the OTDR. In order to measure fibre attenuation,
you need a fairly long length of fibre with no distortions, or
non linearities, caused by the OTDR initial pulse, along its
length or at the launch cable connection at the near end or at
the far end caused by "ghosts" or highly reflective
events. If the fibre looks nonlinear at either end, especially
near a reflective event like a connector, avoid that section
when measuring loss.
Connectors and splices are called "events" in OTDR
jargon. Both should show a loss, but connectors and mechanical
splices will also show a reflective peak. The height of that
peak will indicate the amount of reflection at the event, unless
it is so large that it saturates the OTDR receiver. Then the
peak will have a flat top and tail on the far end, indicating
the receiver was overloaded.
Sometimes, the loss of a good fusion splice will be too small
to be seen by the OTDR. That's good for the system but can be
confusing to the operator. It is very important to know the lengths
of all fibres in the network, so you know where to look for events
and won't get confused when unusual events show up (like ghosts,
which we'll describe below.)
Reflective pulses can show you the resolution of the OTDR. You
cannot see two events closer than is allowed by the pulse width.
Generally longer pulse widths are used to be able to see farther
along the cable plant and narrower pulses are used when high
resolution is needed, although it limits the distance the OTDR
can see.
