High-repetition-rate optical pulse generation using a rational harmonic mode-locked fiber laser

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