Light Scattering Characteristics of Interface Micro-Defects in Factory Joints of High-Voltage Submarine Cables

Abstract

High-voltage submarine cable joint is fabricated by extrusion molding of crosslinked polyethylene materials, in which micrometer-scaled defects cannot be completely eliminated at cone-shaped interface (stress cone) between cable body and recovery insulation. Characterizing interface defects in stress cone area is of great significance to improve submarine cable manufacturing technology. This paper presents a new light-scattering scheme of scanning and detecting interface micro-defects of cable joints, which is suggested to be implemented in joint fabricating process just after factory insulation recovery and before outer shield-layer preparation. According to Mie light scattering theory, we calculate analytically the angle-resolved scattering characteristics of micro-void, water-bead, and antioxidant or carbide micro-particle in polyethylene medium for incident 632.8nm wavelength laser. Since micron-scaled particles scatter light waves into oscillatory characteristics of intensity amplitude due to optical interference, the integral or averaged intensity of the scattered light on a specific angular range should be taken as test signal with a high resolution for distinguishing size or optical properties of micro-defects in polyethylene material. Various micro-defects represent a high intensity in forward scattering, and the scattered intensity integration in 10~30° range increase monotonously with micro-defect enlargement. Especially, lateral scattering strength in 70~100° is remarkably sensitive to refractive index of micro-defects, as manifested by one-magnitude of differences in the averaged scattering intensity between micro-void and micro-antioxidant. It is well suggested that the ratio of scattering intensity integrals in 70~100° to 10~30° ranges can be exploited as a valid criterion for discriminating the species of micro-defects, which is expected to develop 632.8nm Helium-Neon laser scanning test systems of detecting micro-defects in submarine cable factory joints. We also calculate the angle-resolved spectra scattered by micro-defects from light sources of 450~550nm continuous waveband, which are smooth with the amplitude oscillation being significantly alleviated by intensity summation of incoherent waveband components. We suggest exploiting wavelength-averaged angle-resolved spectrum of incident waveband to form a high resolution contrast of scattering strengths, which promises competent applications for laser scanning microscopy or photonic dark-field imaging.