A scheme for the generation of ultra-short optical pulse, based on optical frequency comb (OFC), is proposed and demonstrated experimentally. An OFC with 29 comb lines within 1.5 dB spectral power variation is obtained by cascading intensity and phase modulators modulating a continuous wave laser. The OFC is dispersed by a segment of single mode fiber (SMF). Ultra-short optical pulses with 10 GHz repetition rate and 2.68 ps pulse width are demonstrated. The pulses are used for sampling 1~4 GHz radio frequency signals. The signal-to-noise ratio of this scheme is 33.83 dB, equivalent to effective bit number of 5.33 bit.