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Yb₂O₃-doped yttrium aluminium garnet (YAG) nano-crystals within the silicate glass-based optical fiber preforms were obtained through the conventional-modified chemical vapour deposition (MCVD) and solution doping technique. Nano-crystals were developed with soaking of the porous phospho-silica or pure silica core layer in a solution containing the ytterbium, yttrium and other co-dopants, including fluorine using 10–15% fluorosilicic acid, and through post-annealing of the preform at a temperature of around 1450°C. The size, shape and nature of Yb₂O₃-doped phase-separated nano-crystallites were evaluated from HRTEM images along with the electron diffraction pattern based on the doping levels of phosphorous and fluorine. The size of nano-crystallites was maintained within 6–10nm when doped with 0.25mole% of fluorine. X-ray analyses EDX data reveals that the nano-particles are rich in Yb:YAG …