Study on the dopant concentration ratio in thulium-holmium doped silica fibers for lasing at 2.1µm.

We present the fabrication and laser performance of thulium-holmium co-doped silica fibers when cladding pumped at ∼790 nm. By using the hybrid gas phase-solution doping process in conjunction with the MCVD preform fabrication technique, the doping concentration and the Tm:Ho ratio were varied to study the energy transfer efficiency from Tm3+ to Ho3+. Our study indicates that for a thulium concentration that has resulted in an efficient two-for-one cross- relaxation process with 790 nm pumping, and while maintaining a Tm:Ho concentration ratio in the range ∼ 10 to 20, the energy transfer efficiency has reached above 75%. In a free-running laser cavity, the pump power limited laser output of 38W with a slope efficiency of 56% at 2.1 microns is demonstrated.

Highly efficient thulium-doped high-power laser fibers fabricated by MCVD.

We report a hybrid process by combining both vapor-phase and solution-doping techniques of rare-earth doped preform fabrication in conjunction with the MCVD technique, in order to fabricate highly efficient Tm-doped laser fibers. The proposed fabrication route takes advantage of co-doping silica with high alumina content through the vapor-phase doping process, which is otherwise difficult to achieve using conventional solution doping technique. In addition, by employing the solution doping method, high-purity thulium halide precursors that have low vapor pressures up to several hundred degree Celsius. These high-purity thulium halide precursors can be used to dope the fiber core region with a high thulium concentration that is optimized for an efficient two-for-one cross-relaxation process for 79xnm diode pumped thulium-doped fiber laser. Fibers fabricated using the hybrid approach show more homogeneous and flat-top dopant profiles, compared with the conventional approach, where both aluminum and thulium are incorporated in the core through solution doping. This will ensure that more doped region will take part in the cross-relaxation process. Superior laser performance with a slope efficiency of >70% in the two-micron band has been demonstrated when diode pumped at ~790nm.