  We give detailed insight into photon counting OTDR (nu-OTDR) operation,
ranging from Geiger mode operation of avalanche photodiodes (APD), analysis of
different APD bias schemes, to the discussion of OTDR perspectives. Our results
demonstrate that an InGaAs/InP APD based nu-OTDR has the potential of
outperforming the dynamic range of a conventional state-of-the-art OTDR by 10
dB as well as the 2-point resolution by a factor of 20. Considering the trace
acquisition speed of nu-OTDRs, we find that a combination of rapid gating for
high photon flux and free running mode for low photon flux is the most
efficient solution. Concerning dead zones, our results are less promising.
Without additional measures, e.g. an optical shutter, the photon counting
approach is not competitive.
