We report the generation of deeply-modulated optical pulse trains at high-repetition-rates through modulational instability in optical fibers. The optical pulse train can be generated over the wide repetition-rate-range from sub-THz to a few THz by controlling the anomalous group velocity dispersion of optical fibers. Very-high-repetition-rate of more than 4 THz is attained by the minimization of the absolute value of the anomalous dispersion. The deep modulation of the high-repetition-rate optical pulse train is achieved by inducing modulational instability with pump-probe mixing or external optical feedback. Both inducing methods also enable us to tune the pulse repetition rate precisely. The high repetition rate optical pulse trains seem to go far toward applications such as high-bit-rate optical communication and high-speed optical processing.