Highly Efficient Holmium-Doped All-Fiber ∼2.07-μm Laser Pumped by Ytterbium-Doped Fiber Laser at ∼1.13 μm

We report a 2.07-μm Holmium-doped all-fiber laser (HDFL) pumped by a 1.13-μm Ytterbium-doped fiber laser (YDFL). Home-made alumino-germano-silicate holmium-doped fiber (HDF) served here as an active medium, optimized in terms of chemical composition and co-dopants' concentrations. Laser action at 2.07 μm was assessed in simple Fabry-Perot cavity, formed by a couple of home-made fiber Bragg gratings (FBGs), inscribed directly in the HDF; this allowed notable diminishing of intracavity loss of the 2.07-μm laser. HDF was in-core pumped by the 1.13-μm double-clad YDFL with a power of ~12.5 W, in turn pumped in-clad by a laser diode (LD) operated at 0.97 μm with ~24.5-W output. Using optimal length (~5.0...5.5 m) of the HDF and employing FBG couplers with reflections of ~99% and ~33%, the HDFL provided ~5.0-W output at 2.07 μm. At these conditions, maximal absolute (slope) efficiencies at 2.07 μm of 39% (42%) and 20% (22%) were measured with respect to 1.13-μm (YDFL) and 0.97-μm (LD) pumps, respectively. Moreover, the record slope efficiency (48%) was obtained for the powers ratio of 2.07-μm output (HDFL) to launched 1.13-μm input (YDFL), which is only slightly less than theoretical quantum efficiency limit (53%) for this kind of pump schemes.