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Thulium-doped fibers pumped with 79xnm high power diodes enable a cross-relaxation process to achieve two excited ions for one pump photon, reaching a quantum efficiency up to 200% in the two-micron band. For high power operation at longer wavelengths <2100nm, where the Thulium-doped fiber laser (TDFL) efficiency drops considerable, holmiumdoped fiber lasers (HDFLs) are usually the preferred choice. However, as the Ho3+ ion has no absorption bands where high power diodes are available, TDFLs are traditionally used as a pump source. Therefore, the overall optical to optical conversion efficiency is dependent on the performance of TDFL. This approach also brings additional challenges to the fiber fabrication process as conventional low-index polymers that serve as a pump cladding in double clad fibers cannot be used due to the high absorption of polymer at two microns band and thus, there is a need for an all-glass fiber structure. In this work, we report on the fabrication and laser characterization of a Tm:Ho co-doped fiber in an aluminosilicate host fabricated by using a hybrid gas phase-solution doping technique combined with the MCVD process, where aluminum was introduced into the silica matrix through vapor phase deposition and the rare-earths by solution doping process. The proposed fabrication technique allows more uniform dopant distribution within the fiber core region that helps to achieve a good laser performance. We have demonstrated a free running laser, operating at 2105nm with an output power of <37W and a laser efficiency of ~56% with respect to the absorbed pump power when cladding pumped by a 793nm laser diode. As evident from the experimental results that the donor-acceptor energy transfer from Tm3+ to Ho3+ ions is working in our Tm:Ho co-doped fiber.
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