Anisotropic fluorescence emission of a dye-doped fibre ring that is pumped by a ring laser beam

Abstract

The fluorescence characteristics of a dye-doped polymer fibre were studied by exciting the fibre ring with a ring laser beam. An efficient excitation was achieved from the fibre side owing to the high absorbance of the organic dye. The pump-laser power could be raised to over 400 kW (pulse duration: 5 ns), which was unattainable by the conventional fibre-end pumping method because of the poor laser power durability of the dye-doped polymers. The fibre ends, which were polished obliquely (45°) to pick up the circulating fluorescence, were coupled to each other with an index-matching oil. When the pump power density exceeded 0.2 kW mm−2 (total power: 40 kW), the circulating beam exhibited both a nonlinear peak-growth and a spectral narrowing that were caused by the stimulated emission. By contrast, the fluorescence in the radial direction became weaker as the pump power increased, indicating that the stimulated emission in the axial direction suppressed the fluorescence in the radial direction.