Abstract
The real remoteness of a distributed optical fiber sensor based on Brillouin optical time-domain
analysis is considerably extended in this paper using seeded second-order Raman amplification and optical pulse
coding. The presented analysis and the experimental results demonstrate that a proper optimization of both methods
combined with a well-equalized two-sideband probe wave provide a suitable solution to enhance the signal-to-noise
ratio of the measurements when an ultra-long sensing fiber is used. In particular, the implemented system is based on
an extended optical fiber length, in which half of the fiber is used for sensing purposes, and the other half is used
to carry the optical signals to the most distant sensing point, providing also a long fiber for distributed Raman
amplification. Power levels of all signals launched into the fiber are properly optimized in order to avoid nonlinear
effects, pump depletion, and especially any power imbalance between the two sidebands of the probe wave. This last
issue turns out to be extremely important in ultra-long Brillouin sensing to provide strong robustness of the system
against pump depletion. This way, by employing a 240 km-long optical fiber-loop, sensing from the interrogation
unit up to a 120 km remote position (i.e., corresponding to the real sensing distance away from the sensor unit)
is experimentally demonstrated with a spatial resolution of 5 m. Furthermore, this implementation requires no
powered element in the whole 240 km fiber loop, providing considerable advantages in situations where the sensing
cable crosses large unmanned areas.
© 2013 IEEE
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