Recent advances in holmium doped fibers for high-energy lasers (Conference Presentation)

Holmium doped fiber lasers (HoDFL) are attractive candidates for high energy lasers used in directed energy applications because they operate at wavelengths that are safer to the eye. The common solution-doping technique for making HoDFs can result in the incorporation of hydroxyl (OH) impurity in the active fiber core. The HoDFL operational wavelength of ~2.0 µm is near the 2.2 µm combination absorption band of the OH fundamental mode and the SiO4 tetrahedron vibration, so the OH concentration must be <1 ppm to prevent degradation of the laser performance. We have routinely fabricated HoDF with [OH] < 0.5 ppm. We have developed an ultralow OH processing technique based on both atmospheric exclusion from the silica core soot preform and careful, extensive drying. We report a resonantly-pumped solution doped Ho3+ fiber laser with a slope efficiency of 74%, and an output power of 96W. We are also investigating nanoparticle (NP) doping, where the holmium ions are encaged in a nanoparticle host selected for properties such as low phonon energy, where they are shielded from the Silica lattice. By optimizing variables such as precursor concentrations, NP ripening times, and surfactant selection during synthesis we have been able to increase the Ho NP concentration levels in Silica fiber cores. This has also allowed us to increase concentrations of otherwise incompatible low phonon energy host materials into the cores of the fibers. Cores comprising Ho doped LaF3 and Lu2O3 nanoparticles exhibited slope efficiencies as high as 85% at 2.06 µm in a MOPA configuration.