Upconversion luminescence in bismuth-doped germano-silicate glass optical fiber

We report, for the first time, a broadband upconversion emission (800-1000 nm) in Bi-doped germano-silicate glass optical fibers with excitation in the range of 1515-1585 nm, which can explain the low laser efficiency at wavelengths longer than 1500 nm. The mechanism responsible for the emission is two-step upconversion process. The origin of this upconversion luminescence is discussed and new energy transition schemes are proposed.