Modeling of a two-color, two-stage, ultrafast Yb-doped fiber amplifier

►We model a two-stage, two-color Yb:fiber for amplifying ultrafast laser pulses. ►Preamplifier and amplifier are single-clad and double-clad Yb:fibers respectively. ►Modeled output spectra from the preamplifier and amplifier agree with experiment. ►Adding preamplifier dramatically decreases amplified spontaneous emission.