We present a novel high-energy, single-mode, all-fiber-based master-oscillator-power-amplifier (MOPA) laser system operating in the C-band with 3.3-ns pulses and a very widely tunable repetition rate, ranging from 30 kHz to 50 MHz. The laser with a maximum pulse energy of 25 \muJ and a repetition rate of 30 kHz is obtained at a wavelength of 1548 nm by using a double-clad, single-mode, Er:Yb co-doped fiber power amplifier.