Power transmitted by different modes in an optical-fibre waveguide and behaviour as cut-off is approached

Traces the flow of power in both the core and the cladding of a step-index fibre. Particular attention is focused on behaviour as cut-off is approached, where the power in the cladding rises very rapidly, extending outwards over the whole cross-section of the fibre to the surface of the cladding. Cut-off must be accompanied by collapse of the field of the mode concerned and release of the energy stored in that field, to give radiation and/or conversion to another guided wave.