Abstract Emission characteristics and energy transfer in chalcohalide glasses doped with Tm 3+ and Tb 3+ were investigated and their potential as S-band fiber-optic amplifiers was evaluated. Upon the addition of CsBr into sulfide glass, intensity of the 1.48 μm emission ( 3H 4 → 3F 4 transition) from Tm 3+ increased. At the same time, the lifetime of the 3H 4 level increased to 1.2 ms, approximately eight times longer than the value obtained from sulfide glass. Quantum efficiency of the transition also increased to approximately 90%. Cross relaxation between the 3H 4 and 3F 4 levels was significant when Tm 3+ ion concentration exceeded 0.2 mol%. Population density of the 3F 4 level in Tm 3+ decreased upon the addition of Tb 3+ due to the Tm 3+: 3F 4 → Tb 3+: 7F 0,1,2 energy transfer. Population inversion became possible when Tb 3+ concentration reached 0.11 mol%.