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.