Axial and radial fluorescence of dye-doped polymer fiber

Fluorescence spectra of perylene-doped polymer fiber were measured in both radial and axial directions, i.e., for leaky and guided beams, by exciting the fiber with a pulsed laser of 460-540-nm wavelength. Green (540-550 nm) and yellow (580 nm) peaks were observed in the radial fluorescence spectrum. Measuring fluorescence change at positions closer to the input end and progressing forward to the output end, we observed that green fluorescence decreased rapidly corresponding to an attenuation constant at incident laser wavelength. In contrast, yellow fluorescence decreased gradually, because it was induced by the absorption of green fluorescence, which attenuated more gradually than incident laser. Therefore, in axial output beams, green fluorescence was much weaker than yellow. As incident laser power increased, intensities of axial green fluorescence and radial fluorescence (both green and yellow) saturated to a certain level. In contrast, axial yellow fluorescence increased nonlinearly with increase in laser power. The output light intensity became strongest at a fiber length of 20-30 mm. These phenomena were analyzed theoretically taking self-absorption and stimulated emission by fluorescent dyes into consideration.