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Bismuth‐doped phosphosilicate fibers have become the most promising gain medium for O‐band amplifiers. Yet scientific challenges on understanding the nature of bismuth active centers (BACs), mechanisms of bismuth cluster formation in the phosphosilicate glass network still exist. It is likely that multiple BACs with different oxidation states in different structural sites all contribute to the broad, nonsymmetric luminescence and gain spectra. Due to the progress in the fundamental understanding of bismuth‐doped phosphosilicate glass, various designs of optical amplifiers with decent performances have been demonstrated.