The upconverted fluorescence dots that appear under photon-avalanche excitation in an erbium-doped fluorozirconate fiber are quantitatively analyzed with respect to the pump-mode structure into the fiber and the energy spatial diffusion between ions. By computing the mode-propagation constants, we show that the main luminescent structures periods correspond to beating between low-order pump modes. The observed luminescent dots result from these interferences combined with the excitation intensity threshold of avalanche upconversion. We also investigated the influence of energy-transfer spatial diffusion among Er ions by comparing the pump-mode transverse profiles deduced from avalanche and Addition de Photons par Transfert d’Energie (APTE) emissions. At high pump power the modes widths that correspond to APTE experiments are larger than those obtained from avalanche upconversion by 0.8 μm. We attribute this effect to the resonant energy transfer which is negligible in photon avalanches because of strong excited-state absorption. From the experimental broadening we estimate that the one-dimensional diffusion length associated with this transfer is 1.13 μm.
© 1998 Optical Society of America