The effect of changes in the local phonon modes and electron-phonon coupling strengths on the 1.31 mu m fluorescence efficiency of Dy3+-doped sulfide glass was investigated. An addition of 10 mole% CsBr resulted in a sharp increase in the 1.31 mu m emission intensity at the expense of the 1.75 mu m emission. The lifetime of the 1.31 mu m emission level (F-6(11/2)-H-6(9/2)) increased to 1.58 ms, which was more than 40 times larger than that of the Ge-Ga-S host. Quantum efficiency increased from 17% to approximately 100%. These changes in the optical characteristics resulted from the large changes in the local phonon modes and electron-coupling strengths through the addition of CsBr to Ge25Ga10S65 glass.