Tm and Tm–Tb-doped germanate glasses for S-band amplifiers

The mechanism involved in the Tm3+(3F4)→Tb3+(7F0,1,2) energy transfer as a function of the Tb concentration was investigated in Tm:Tb-doped germanate (GLKZ) glass. The experimental transfer rate was determined from the best fit of the 3F4 luminescence decay due to the Tm→Tb energy transfer using the Burshtein model. The result showed that the 1700 nm emission from 3F4 can be completely quenched by 0.8 mol% of Tb3+. As a consequence, the 7F3 state of Tb3+ interacts with the 3H4 upper excited state of Tm3+ slighting decreasing its population. The effective amplification coefficient β(cm−1) that depends on the population density difference Δn=n(3H4)-n(3F4) involved in the optical transition of Tm3+ (S-band) was calculated by solving the rate equations of the system for continuous pumping with laser at 792 nm, using the Runge–Kutta numerical method including terms of fourth order. The population density inversion Δn as a function of Tb3+ concentration was calculated by computational simulation for three pumping intensities, 0.2, 2.2 and 4.4 kWcm−2. These calculations were performed using the experimental Tm→Tb transfer rates and the optical constants of the Tm (0.1 mol%) system. It was demonstrated that 0.2 mol% of Tb3+ propitiates best population density inversion of Tm3+ maximizing the amplification coefficient of Tm-doped (0.1 mol%) GLKZ glass when operating as laser intensity amplification at 1.47 μm.