The Fiber Optic Association - Tech Topics
MM Fiber: BW Effects Of Sources And Termination
The
actual interaction of
lasers and multimode fiber is more complicated than many people
think. The laser couples light into the fiber in a complicated
interaction of the output power profile of the device and the
geometry of the connection to the fiber. As the light moves down
the fiber, mode mixing (transferral of power from one mode to
another) occurs as a result of stresses in the fiber and connections
between fibers.
In the real world, installed cable plants bend around corners,
may have residual stress from installations, and have connectors
and splices. No two fibers, cables, connectors or splices are
exactly the same, so each end-to-end fiber link is unique. This
makes it very difficult to generalize on what performance to expect
from a link.
One thing is certain: the link performance is not totally determined
by the fiber performance. Using high bandwidth fiber is a condition,
but not the only condition, for a high bandwidth link. Terminations
create mode modification that can greatly change the bandwidth
of the installed fiber link. Fusion splices are the most complicated
to predict, as the melting and solification
To illustrate
this problem, Eric
Pearson and I (Jim Hayes) set up a test using a prototype bandwidth
tester and a demo cable plant that he uses in testing. The cable
plant consisted of two 100 meter segments of FDDI-grade fiber
(62.5/125) with a midspan connection.
The bandwidth tester gives data on the end-to-end bandwidth of
the fiber as well as a profile of the input and output pulses.
Like an OTDR, one can use the bandwidth data for a GO-NO GO test
or examine the actual data to analyze what is happening in the
link.
To evaluate the effects of termination and splicing, we used
connectors,
a mechanical splice and fusion splices to connect the two fibers.
We made multiple terminations and recorded the data. The bandwidth
data is shown in the table below.
Connector | Mechanical Splice | Fusion Splice | |
Bandwidth Range | 1.28-1.34 GHz | 1.09-1.66 GHz | 0.83-1.41 GHz |
Note: Data taken on OM3 50/125 laser-optimized fibers indicates this effect is greatly reduced for more modern fibers and is probably not cause for concern.
(C) 2004-8, The Fiber Optic Association, Inc.
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