  Many protocols for long distance quantum communication require
interferometric phase stability over long distances of optical fibers. In this
paper we investigate the phase noise in long optical fibers both in laboratory
environment and in installed commercial fibers in an urban environment over
short time scales (up to hundreds of us). We show that the phase fluctuations
during the travel time of optical pulses in long fiber loops are small enough
to obtain high visibility first order interference fringes in a Sagnac
interferometer configuration for fiber lengths up to 75 km. We also measure
phase fluctuations in a Mach-Zehnder interferometer in installed fibers with
arm length 36.5 km. We verify that the phase noise respects gaussian
distribution and measure the mean phase change as a function of time
difference. The typical time needed for a mean phase change of 0.1 rad is of
order of 100 us, which provides information about the time scale available for
active phase stabilization. Our results are relevant for future implementations
of quantum repeaters in installed optical fiber networks.
