A bend-tolerant BOTDR distributed fiber sensor

A standard single mode input fiber combined with a graded index multimode sensing fiber (GI-MMF) with large numerical aperture is used as a distributed sensing fiber to realize bend-tolerant and low-cost structural safety and health monitoring of large infrastructures. In the designed distributed sensing fiber, the single mode fiber at the near end of the GI-MMF is used to excite the fundamental mode as much as possible in the GI-MMF to achieve good bending resistance, and filter out the high-order modes in the backscattering signal that may degrade the bending resistance and deteriorate the sensing performance, so as to realize the bend-tolerant distributed sensing. The principle of bend loss is analyzed, and the bending resistance of different sensing structures are discussed on the basis of theoretical and experimental comparisons. By measuring the Brillouin frequency shift and scattering spectrum width of the sensors, a stable and reliable optical coupling method is determined for the single mode fiber and GI-MMF combination, and a frequency-shifted local heterodyne Brillouin optical time domain reflectometry system is constructed for temperature measurements to evaluate the bending resistance of the sensors. The experimental results show that the sensor with the single mode fiber alignment fusion to a 5 km long and 62.5 μm diameter GI-MMF has the best bending resistance, and the achieved minimum bend radius, temperature coefficient and measurement accuracy are 1.75 mm, 0.933 MHz/°C and 1.4 °C, respectively, which indicates that the proposed sensing structure is capable of realizing bend-tolerant sensing and has huge potential in the field of high-accuracy structural safety and health monitoring of large infrastructures.