Rational harmonic mode locking takes place in an actively mode-locked fiber laser when the modulation frequency f/sub m/=(n+1/p)f/sub c/, where n and p are both integers and f/sub c/ is the inverse of the cavity round-trip time, the 22nd order of rational harmonic mode locking has been observed when f/sub m//spl ap/1 GHz. An optical pulse train with a repetition rate of 40 GHz has been obtained using a modulation frequency f/sub m/=10 GHz. The theory of rational harmonic mode locking has also been developed. The stability of the mode-locked pulses is improved considerably when a semiconductor optical amplifier is incorporated into the fiber laser cavity. The supermode noise in the RF spectrum of a mode-locked laser is removed for a certain range of current in the semiconductor optical amplifier.