SECTION 3: TRACE ANALYSIS
14
Some highly reflective events are so reflective that they “over-drive” the OTDR
circuitry, and cause the OTDR graph to appear as if the trace completely drops
out.
This issue is called “saturation inversion”, and only appears when overly-
reflective light causes the internal OTDR circuitry to become saturated.
Every OTDR suffers from saturation issues, and many high-end OTDRs
include additional circuitry that tries to quench the saturation as it occurs.
However, irregardless of how well an OTDR can quench the saturation, the fact
remains that the reflective event requires immediate attention.
TRACE ANALYSIS
The sample trace shown here shows what typical saturation inversion looks
like. Four key points have been noted on the trace to explain what is occurring
in the OTDR:
1)
The red line at the top of the reflective peak shows that the
OTDR has been saturated (please note that when the peak
has a red line, the reflectance value is not being reported
properly);
2)
The flat line at the bottom of the trace shows that the OTDR has been “over-driven”, and has inverted the trace;
3)
The OTDR has begun its recovery from saturation;
4)
The OTDR has fully recovered from saturation, and has returned to its normal backscatter line.
In this example, the reflective event is caused by the interconnection that occurs between two connectors at a patch panel. The
connectors on either side of the patch panel are PC (physical contact).
There are several ways to address highly reflective events like this:
a)
Thoroughly clean and inspect both connectors for debris or damage;
b)
If the connectors appear damaged after thorough cleaning, try re-polishing the connector endfaces;
c)
Re-terminate using connectors with better reflectance values (typical values shown here);
PC (physical contact)
-30 to -35 dB
UPC (ultra physical contact)
-45 to -50 dB
APC (angled physical contact)
-55 to -60 dB (or less)
d)
Replace the mating sleeve in the patch panel.
Once the inverted event has been fixed, re-test the fiber to ensure the event is no longer overly reflective.
ANOTHER HELPFUL TIP
Another helpful tip for mitigating reflectance comes from the Fiber Optic Association (FOA) website:
http://www.jimhayes.com/OTDR/otdrs_d.htm
found under the section titled
This "Trick" Can Help
. This tip refers to the use of index-matching fluid between connectors to help
eliminate the air gap causing the overly reflective event.
SATURATION INVERSION
4786m
6126m
24718m
RFL dB
-30.26
1340
dB
0.52
7.43
L m
Fiber #
CURSOR
LOCK
ZOOM
TEST
ENTER
AUTO
1
2
3
4
