Polarisation change of light during a lightning strike: isotropic zones of an anisotropic optical fibre

A numerical experiment for the light passing through a section of an optical cable in a ground wire during a lightning strike is performed, and the ‘isotropy effect’ of the sections of time profiles of the rotation speed of the state of polarization is discovered and explained. The shapes of the time profiles for anisotropic and isotropic fibres virtually coincide in short sections at the beginning and at the end of the time profile of the change in polarization, but differ significantly in the middle part. This behaviour of the time profiles is determined by the smallness of the length of the interaction of the light with the region of a rapid change in the magnetic field at the beginning and at the end of the lightning front, while the effect of the magnetic field on short sections of an anisotropic fibre almost coincides with that on sections of isotropic fibre of the same length. Using the model of light propagation in fibre with a random distribution of linear birefringence, located in the magnetic field of the lightning, a formula is obtained for calculating the matrix rotation speed of the polarization state for an extended fibre.