  We demonstrate optical time domain reflectometry over 200 km of optical fiber
using low-noise NbTiN superconducting single-photon detectors integrated with
Si3N4 waveguides. Our small detector footprint enables high timing resolution
of 50ps and a dark count rate of 3 Hz with unshielded fibers, allowing for
identification of defects along the fiber over a dynamic range of 37.4 dB.
Photons scattered and reflected back from the fiber under test can be detected
in free-running mode without showing dead zones or other impairments often
encountered in semiconductor photon-counting optical time domain
reflectometers.
