Structured-core thulium-doped fibers for 2 μm fiber lasers (Invited Paper)

Thulium-doped fiber lasers working in the 2-μm wavelength range are particularly important because they belong to the group of so-called eye-safe lasers. Although their efficiencies are getting closer to the efficiencies of the matured ytterbium fiber lasers, the record output power of thulium fiber lasers is still orders of magnitude below its potential, looking for technology breakthroughs that would overcome the current limitations. Novel fiber designs, e.g., using structured core of the active fiber, and new ways of mitigating thermal and temperature effects may enable further increase of the output power. In the paper, we will review our proof-of-concept experiment of the pedestal-free thulium doped silica fiber with a large nanostructured core, where the initial preforms of the active medium were made by the nanoparticle-doping and MCVD methods. Next, measurement of temperature-dependent thulium cross sections will be reviewed as well as application of these cross-section spectra for prediction of thulium fiber laser operation using recently derived closed form expressions for the laser threshold and slope efficiency under pumping at 790 nm by the two-for-one process.