The properties of the plasma behind the front of a damage wave generated by laser radiation in an optical fibre are considered. A plasma with a low degree of ionisation but a relatively high electron density is shown to emerge. However, the high absorption coefficient of laser radiation at a temperature of the order of 2000 K cannot be attributed to the presence of bremsstrahlung. The production of a chain of uniformly spaced caverns during the laser damage of the optical fibre is qualitatively explained. It is shown that this effect cannot be explained by the capillary Rayleigh instability because of the high viscosity of the glass. It is found that the fibre core deformation by a high pressure leads to an increase of the fibre volume sufficient to account for the emergence of the caverns after cooling. It is assumed that the periodicity of caverns is caused by the instability of a new type. A high-density double electrical-charge layer is produced at the plasma—liquid interface. Due to the repulsion of similar charges, the surface tends to increase, resulting in the instability development responsible for the production of the chain of caverns.