A master-oscillator fiber power amplifier (MOPA) system is presented, which consists of a single mode laser as the master oscillator and an Yb^{3+}$-doped large-mode-area double-clad fiber as the power amplifier. The system emits up to 6 W of amplified radiation at a wavelength of 1064 nm. The slope efficiency and extracted pulse energy as a function of pulse repetition rate are analyzed.