An ultra-long adiabatic tapered single-mode fiber is proposed to enhance the received scattered light in the fiber without additional loss. By controlling the radius and refractive index distribution on the length of the fiber, the backscattering coefficient of the scattered light is increased, and the signal-to-noise ratio of the scattered light is improved while the long distance sensing length is maintained. By analyzing and optimizing the length, radius and refractive index distribution of tapered optical fibers, scattering enhancement of 7dB can be achieved at 45km. Ultra-long adiabatic tapered single-mode fiber can realize Rayleigh backscattered light enhancement of equal signal-to-noise ratio at each position, which can be used to improve the signal-to-noise ratio and measurement range of fiber sensing system based on Rayleigh backscattered, and further promote the application of fiber sensing technology for ultra-long distance sensing.
Residual stress in a part is a key factor in its fatigue life, structural stability, corrosion resistance and other properties. Therefore, accurate measurement of residual stress is essential. The mechanical release method is widely used in residual stress measurement for its high accuracy and mature technology. However, the method assumes that the material is homogeneous, and its accuracy is greatly affected in non-uniform materials. Aiming at the problem of inaccurate residual stress measurement in non-uniform materials, a fiber-optic ring array residual stress measurement method based on high spatial resolution distributed optical fiber measurement technology is proposed. The drilling method is taken as an example. In this method, we arrange the fiber-optic ring array around the hole in the same center to measure the residual stress on the circumference with high spatial resolution. The finite element simulation is used to analyze the influence of the spatial resolution of the fiber sensor, the non-uniform size, position and Young's modulus on the strain distribution and residual stress measurement. It is proved that the method can accurately measure the residual stress and obtain the information of the non-uniform state of the material.
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