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.