Nanoparticle and Solution Doping for Efficient Holmium Fiber Lasers

Efficient holmium fiber lasers have been studied as attractive laser sources operating around 2.1 μm. We report on holmium-doped silica fibers prepared by the modified chemical vapor deposition in combination with either a solution-doping method or a nanoparticle-doping method. A set of 15 fibers with various compositions was characterized and compared with respect to their fluorescence lifetime, laser slope efficiency and laser threshold. This set of fibers in wide concentration ranges allowed us to assess reliably the influence of material composition and the influence of doping method. The best-performance fibers exhibited slope efficiency 83.1%, laser threshold 155 mW and a record value of upper laser level lifetime of 1.35 ms. These results were achieved in fibers with holmium concentration lower than 800 molar ppm and Al/Ho molar ratio greater than 70. Significant differences between fibers prepared by solution doping and nanoparticle doping were not observed. The behavior of Al2O3 nanoparticles during fiber preparation is discussed in details.