| The 300 MHz-km fiber bandwidth will cause pulse broadening to occur
more rapidly (with fiber length) than is planned for by the ANSI
standard. The standard allows an exit pulse to have a 5 nanosecond
rise time. Use of fiber with lower bandwidth than 500 MHz-km will
cause this limit to be reached at significantly shorter lengths of
optical fiber.
The model used for the initial design would indicate that you
might expect to use 1.0 km of such lower bandwidth optical fiber,
and still maintain the 5 nanosecond exit pulse rise time.
We have performed NO testing in this matter. The performance that
you achieve may vary from this estimate, so proceed carefully.
Such use of low quality fiber should be avoided if possible. If the local
manufacturer can't produce to the 500 MHz-km bandwidth, then I would
recommend that other fiber parameters should also be examined. The
other parameter than will influence pulse rise time is chromatic
dispersion. This is less easy to measure than modal bandwidth, but
is just as important.
Richard Kirk
Network Products Engineering
DTN 226-7048
Richard Kirk
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