  A coherent, short-length reference arm reflectometer, which utilizes a 76-MHz
repetition rate mode-locked fiber laser, was investigated experimentally for
long fiber links (> 10 km). The reflectometer combines the advantages of
optical time-domain and frequency-domain reflectometry without the need for
high-speed photodetectors and electronics to achieve high spatial resolution of
2.5 mm at 10 km. To our knowledge, this is the highest resolution reported for
fiber reflectometry at this distance for reflective type event using
non-photon-counting detection. Phase and polarization diversity detection,
combined with spectral compression using frequency-chirp, are proposed to
improve sensitivity and discrimination against the fiber Rayleigh
backscattering background.
