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- Sameh Mahmoud
January 10, 2011, 5:04 am
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Re: OTDR Test
Near end, or dead zone, varies with equipment. For that matter, so does
far end, though that also varies with how bad the fiber/connections are,
as it's power limited - so a few bad connections (or a lot of good
connections) will mean you can't see nearly as far down the fiber as you
would with a good connection and a lot of fiber in good shape. Proper
wavelength and power for the fiber under test is also needed (if the
fiber can run signal 120km, the properly set-up ODTR can look at
60-120km of fiber.) It's a combination of power in and detector
sensitivity and varies from unit to unit as they are built to work on
specific (or multiple specific) wavelengths and power ranges.
In multimode fiber you'll also run into dispersion limiting your
resolution at long distances, as I understand it, but I'm doing
singlemode and have not paid too much attention to that - and multimode
doesn't do too well for truly long distances anyway.
Anritsu makes a nice box I can't afford that has a really short dead
zone (less than a meter) and considerably more fiber built into the unit
than that dead zone, so it has effectively no dead zone in practical
use. I've seen it at a show and in catalogs. Long sections of coiled up
fiber are typically used to get past dead zone for older machines with
long dead zones - they are generally large enough that they have to be
carried externally, but the effect is the same - by using that to
connect to the fiber of interest, dead zone is eliminated in practice.
Good clean connections on those test fibers are required to get anything
useful from the ODTR, of course.
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