  A novel ultra-long distributed vibration sensing (DVS) system using forward
transmission and coherent detection is proposed and experimentally
demonstrated. In the proposed scheme, a pair of multi-span optical fibers are
deployed for sensing, and a loop-back configuration is used by connecting the
two fibers at the far end. The homodyne coherent detection is used to retrieve
the phase and state-of-polarization (SOP) fluctuations caused by a vibration
while the localization of the vibration is realized by tracking the phase
changes along the two fibers. The proposed scheme has the advantage of high
signal-to-noise ratio (SNR) and ultra-long sensing range due to the nature of
forward transmission and coherent detection. In addition, using forward rather
than backward scattering allows detection of high frequency vibration signal
over a long sensing range. More than 50dB sensing SNR can be obtained after
long-haul transmission. Meanwhile, localization of 400 Hz, 1 kHz and 10 kHz
vibrations has been experimentally demonstrated with a spatial resolution of
less than 50 m over a total of 1008 km sensing fiber. The sensing length can be
further extended to even trans-oceanic distances using more fiber spans and
erbium-doped fiber amplifiers (EDFAs), making it a promising candidate for
proactive fault detection and localization in long-haul and ultra-long-haul
fiber links.
