We investigate the radio-frequency (RF) up-conversion and waveform generation properties of an optical fiber loop including a frequency shifter and an amplifier. By seeding the loop with a single-frequency continuous-wave laser, one can develop a wide optically carried RF comb, whose spectral extension is governed by the loop net gain. In addition, by choosing the fiber loop length and the RF shifting frequency, arbitrary waveforms can be generated. We present an analytical interference model that includes the time delay, the frequency shift, and the gain. Experiments are conducted with 1.06-μm fiber-optic components. Using a 200-MHz acousto-optic frequency shifter, we find a 19-fold up-conversion up to 3.8 GHz with a typical in-loop gain of 3. Various waveforms including bright and dark pulses, square- or triangle shaped are achieved by properly adjusting the loop length and the frequency shift. A good agreement between experimental and theoretical results is obtained. The fully fibered microwave-photonic source is applied to a laboratory Doppler velocimetry demonstration. The gain in sensitivity obtained with the up-converted signal is readily observed.