Ultrashort soliton source, with very high repetition rate, is a necessity for next generation optical fiber communication. A very simple passively mode-locked fiber ring laser centered at 1566 nm, which is based on the modulation instability (MI) theory, is proposed. Using high Erbiumdoped (6470ppm) Bismuth fiber, a soliton pulse train, with 660 GHz repetition rate and pulse width of 420 fs, has been observed in a 11.5m short cavity for the first time.