Investigation of Brillouin Effects in Carbon Coating Single-Mode Fiber Using for Inspection of Pipeline Buckling

A conventional SMF-28 was used to conduct localized pipe-wall buckling monitoring in a section of energy pipe with 2,667 mm (105 in) in length and 762 mm (30 in) in diameter by a coherent probe-pump based distributed Brillouin fiber sensor with 15 cm spatial resolution. The locations of pipe-wall buckling have been found by measuring the strain distributions along the outer surface of the pipe. However the sensing fiber (SMF-28) was broken when the bending load increased above 1335 kN (300 kips), which caused the sensing fiber experienced more than the compressive strains of -8,084 με. In order to get strain data after pipeline buckling happens, a high strength sensing fiber with carbon coating instead of conventional acrylate coating should be used. The Brillouin measurement on the carbon coating single-mode fiber has Brillouin frequency shift of vB 12.479 GHz at wavelength of 1320 nm and room temperature. The measured Brillouin bandwidth ΔvB is equal to 66 MHz. The central Brillouin frequency shows a strong dependence on strain with 1.510 GHz shift at 2.5% elongation. The excellent linearity of the central frequency vB on strain is confirmed and the strain coefficient was measured as 16.21 με/MHz. Its strain-stress relation keeps linearity up to 2.5% elongation, which is much bigger than that of SMF-28.