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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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