Distributed acoustic/vibration sensing schemes based on phase-OTDR are naturally sensitive to environmental perturbation. Nevertheless, further sensitivity enhancement is possible by using specialty fibers. In this paper, a nitrogen doped single-mode fiber with increased Rayleigh scattering properties is tested alongside a standard telecommunications single-mode fiber (SMF) for comparative phase-OTDR measurements. The high Rayleigh scattering fiber (HRF) does not only benefit from a higher numerical aperture, but also from a higher non-homogeneity of material density resulting in an enhanced scattering coefficient. For perturbations caused by shaker-induced vibration applied on a fiber section or by an acoustic signal emitted from a loudspeaker, the ability of localizing the perturbation and determining the frequency is studied simultaneously for the HRF and the SMF, using a direct detection phase-OTDR setup. Vibration frequencies in the range 100-7000 Hz with accelerations of up to 0.1g and acoustic signals in the frequency range 100-10000 Hz at sound pressure levels up to 115 dBC are tested. The signal-to-noise ratios (SNRs) for the differential phase-OTDR traces are calculated as the maximum difference signal level in the perturbation zone and a noise reference level outside the zone. Moving average methods are also employed for further enhancement. As expected, the HRF has superior performance for the localization and the frequency characterization, and it allows detection of signal levels that are undetectable with an SMF without using of denoising methods. On average, a 7 dB and a 3 dB improvement can be achieved for vibration detection and acoustic detection, respectively.
Johan Jason, Sergei M. Popov, Oleg V. Butov, Yuri K. Chamorovskiy, Konstantin M. Golant, Andrei A. Fotiadi, and Marc Wuilpart, "Sensitivity of high Rayleigh scattering fiber in acoustic/vibration sensing using phase-OTDR," Proc. SPIE 10680, Optical Sensing and Detection V, 106801B (Presented at SPIE Photonics Europe: April 26, 2018; Published: 9 May 2018); https://doi.org/10.1117/12.2307569.
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