Metal Film Surface Plasmon Resonance (MFSPR) Fibre Couplers

摘要

The project goal was to develop a mode selective coupler, based on long-rangesurface-plasmons (LRSP's), that provides tunable tapping of individual modes or small groups of modes from a multi-mode fibre carrying several communications channels simultaneously in different areas of its mode spectrum. When a thin metal layer, deposited on a side-polished fibre, supports a LRSP that phase-matches to a fibre mode, resonant tunneling of light from the core into the metal layer can occur; this may then be tapped off through a high index prism (or a second identical fibre) placed against the fibre face, the intervening space being filled with a layer of variable index. Mode selectivity might then be achieved by tuning the index of this layer. In the project, the fibre side-polishing apparatus was designed and built; the polishing procedure perfected; a fully automated computer-driven prism coupler and monochromator set up; and reliable film deposition achieved. The theory of the LRSP modes was fully developed, and a separate field matrix analysis carried out for modelling the behaviour of the entire device both for excitation from the prism and the fibre modes. The general conclusion is that the modal selectivity is fundamentally limited by the unavoidable fact that higher order modes (those close to cut-off) couple much more strongly than do lower order modes. The result is that lower order modes cannot be coupled out without a substantial (of order 50% or worse) amount of cross-talk from higher order modes. These observations were confirmed in a number of experiments. Optical fibres/surface, Plasmon-polaritons/mode, Selective couplers.