Abstract
We describe the use of a phase-sensitive optical time domain reflectometer ( $\phi {\bf OTDR}$ ) over an ultra-long Raman fiber laser cavity allowing fully distributed detection of vibrations over 125 km. Compared to a first-order Raman-assisted $\phi {\bf OTDR}$ , this scheme shows an enhanced signal-to-noise ratio (SNR). This is due to the fact that the relative intensity noise introduced by the Raman amplification is mostly transferred to a lower frequency range, where the balanced detection implemented in the setup provides better suppression of the common-mode noise. The sensor was able to measure vibrations of up to 380 Hz (limit set by the time of flight of light pulses) in a distance of 125 km with a resolution of 10 m and an average SNR of 8 dB with no postprocessing. This implies a >3 dB improvement in SNR over a first-order Raman-assisted setup with similar characteristics.
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