A theoretical model in the 1900 nm waveband for amplified spontaneous emission (ASE) of thulium-doped silica fiber is presented. The ASE spectral power as functions of the fiber length and the pump power is investigated by solving the rate and propagation equations. By calculation, when the concentration of thulium in fiber is 2.25×1025 m-3, the fiber core diameter is 2.6 μm, and the pump power is 200 mW at 808 nm, the optimal fiber length is 8.1 m, and the output power of ASE can reach 60 mW in the range of 1800-1950 nm with this fiber length. The optical-to-optical conversion is 30.3%. The optimal pump power also can be obtained by this model.
