We report on a Holmium-doped femtosecond polarization-maintaining (PM) all-fiber amplifier system at a central wavelength of 2050nm that is designed as a seed source for a Holmium:YLF-based amplifier. The amplifier system employs a three-stage amplification process for power scaling while reducing the repetition rate. The first two Holmium-fiber amplifiers are core-pumped at a wavelength of 1940 nm. The final amplification stage is based on a double-clad Thulium-fiber, whose gain spectrum allows for the simultaneous amplification of the remaining pump light from the second amplifier stage and the Holmium-signal. Temporal pulse-stretching to a pulse duration of 190 ps using a chirped fiber Bragg grating (CFBG) prevents nonlinearities. We obtained pulse energies up to 300 nJ at 50 kHz.