Abstract: An optical fibre has a core and a plurality of rings surrounding the core. In one arrangement the core and subsequent alternate rings are light transmitting, while the intermediate rings are of cladding material. In an alternative arrangement the core and alternate rings are of cladding material and intermediate rings are light transmitting. The light transmitting rings can be of varying refractive index or of constant refractive index.

Abstract: An optical fibre has a plurality of light conducting cores surrounded by a cladding layer of lower refractive index than the cores. A light conducting layer is on the cladding layer with a refractive index higher than that of the cladding layer, and a second cladding layer surrounds the light conducting layer and has a refractive index lower than that of the light conducting layer. Data signals can be propagated in the cores and a monitoring signal in the light conducting layer. Any attempt to tap the fibre to abstract data information results in aberration of the monitoring signal. Thus there is provided a secure optical fibre cable.