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Theoretical modeling of Watt-level average power Tm-doped fluoride glass fiber amplifiers operating at 1.87 μm is presented. To characterize and optimize these devices a computer model has been developed taking into account the full spectral information of the laser transition as well as all important ionic levels, their decay schemes and important cross-relaxation rates, being capable of modeling steady-state and especially transient characteristics of an optically pumped fiber as is needed for the amplification of short pulses. As a result, optimum fiber lengths and core sizes for maximum output power can be determined. It is shown that the influence of amplified spontaneous emission (ASE) onto amplifier efficiency and gain strongly depends on the fiber length for a given amplifier geometry, thus realistic modeling of the ASE background and its wavelength shift with respect to the fiber length is a key issue for the layout of amplifier fibers. The model is compared with experimental results obtained by amplification of 20-30-ns pulses at repetition rates in the range of 5-60 kHz. A good agreement between experiment and numerical results was reached without a substantial adjustment on the input parameters concerning amplification as well as continuous-wave ASE output power of an unseeded fiber.