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We describe fundamental measurements of the properties of thulium (Tm)-doped silica and power scaling studies of fiber lasers based on the material. Data on the high-lying Tm:silica energy levels, the first taken to our knowledge, indicate that pumping at 790 nm is unlikely to lead to fiber darkening via multiphoton excitation. Measurement of the cross-relaxation dynamics produces an estimate that, at the doping levels used, as much as 80% of the decay of the Tm level pumped is due to cross relaxation. Using a fiber having a 25-mum-diameter, 0.08 numerical aperture (NA) core, we observed fiber laser efficiencies as high as 64.5% and output powers of 300 W (around 2040 nm) for 500 W of launched pump power, with a nearly diffraction-limited beam. At these efficiencies, the cross-relaxation process was producing 1.8 laser photons per pump photon. We generated 885 W from a multimode laser using a 35-mum, 0.2-NA core fiber and set a new record for Tm-doped fiber laser continuous-wave power.