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.
Successful power scaling of the Er-doped fiber laser heavily depends on Er dopant concentration. In order to scale the power up to a kW class, core absorption should be well over the limits defined by current commercial doping techniques. Recently developed nanoparticle (NP) doping technique of fabrication of erbium-doped fibers allows the desired dopant concentration increases while mitigating both Er upconversion and clustering effects. Here we present the latest resonantly-pumped Er fiber laser power scaling results enabled by further development of the NP doping technique of Er-doped fiber fabrication. Using resonantly cladding-pumped (at 1530 nm) large mode area 20/125 µm fiber with the Er-NP-doped core we achieved pump-limited CW power of over 30 W at ~1605 nm with the slope efficiency versus absorbed pump power of ~63%. These are, to the best of our knowledge, the highest power and efficiency demonstrated so far for from the Er-NP-doped fiber. Further considerations on fiber design optimization are presented as well.
Erbium doped fiber lasers are attractive candidates for high energy lasers (HELs) used in directed energy applications because they operate at wavelengths that are both safer to the eye and in a high atmospheric transmission window. A significant issue for erbium doped fibers is detrimental clustering effects such as upconversion and quenching. We have investigated the use of ytterbium-erbium co-doping in both solution and nanoparticle form, where Yb ions are used to help disperse and separate the Er ions in order to avoid these effects. Both solution doping and nanoparticle doping were investigated and optimal concentrations for both were determined. By optimizing variables such as Yb/Er ratio and Al precursor concentrations during synthesis we have been able to increase erbium concentration levels in Silica fiber cores while minimizing detrimental clustering effects. In-band pumping of Er ions at 1475 nm in a single-mode master oscillator power amplifier set-up was used to investigate lasing efficiency, and therefore the Yb ions do not absorb and are optically passive. This ensures that the fibers are operating in the eye-safer regime. We have achieved optical to optical slope efficiencies of 50% for Yb-Er NP and solution doping in a single mode fiber with Er concentrations that are much greater than are achievable with conventional solution doping. Results indicate improved potential for power scaling.
A significant issue for holmium-doped fiber lasers (HoDFLs) operating near 2 μm is multiphonon quenching due to the high phonon energy ~1100 cm<sup>-1</sup> of the silica host, which complicates power scaling due to reduced lifetimes and increased heating. Nanoparticle (NP) doping is a new technique where the structure surrounding the Ho ions is developed chemically prior to doping into the silica core. We have incorporated Ho<sup>3+</sup> ions into various NPs, such as LaF<sub>3</sub>, Al<sub>2</sub>O<sub>3</sub> and Lu<sub>2</sub>O<sub>3</sub>, to shield them from the silica glass matrix. Results indicate slightly longer lifetimes with Ho:LaF<sub>3</sub> NPs and the possibility of further improvement with oxide NPs. We report the first of lasing in a Ho:Lu<sub>2</sub>O<sub>3</sub> NP-doped fiber pumped at 1.95 μm and operating at 2.09 μm with a record slope efficiency of 85.2%.