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.