High power operation of a CW 2 μm thulium-doped fibre laser

Silica fibres doped with thulium ions have provided efficient, tunable sources around 2 μm, a wavelength of interest in the medical field and in laser radar. To date these have been low power devices, e.g., 2.7 mW in the first reported result. The authors used a germano-silica-based fibre doped with approximately 840 ppm of Tm³⁺ . Initial results were obtained with a core diameter of 8.6 μm. This gave a cut-off wavelength of 1.7 μm and ensured monomode operation at 2 μm while allowing a good launch efficiency for the 1.064 μm pump, which was launched into the fibre with a 10× microscope objective. A resonator was constructed by butting cleaved fibre ends against dielectric mirrors. The output mirror had a nominal transmission of <5% in the range 2.0-2.4 μm and the input mirror was >99.5% reflecting over the same range. Laser action was observed at 1.986 μm, with a maximum power output of 209 mW for 1.3 W absorbed