Spot size variation in non-adiabatic single mode fibre tapers

In an adiabatic taper, spot size is determined by the fundamental local mode alone. In non-adiabatic tapers, some of the power in the fundamental mode is coupled into cladding local modes, so that spot size is determined by a superposition of all these modes and can depart significantly from the adiabatic result. Both analytical and numerical solutions are presented