"Rational harmonic mode-locking" in a phase-modulated fiber laser

The repetition rate multiplication was observed in a phase-modulated mode-locking fiber laser when the modulation frequency was detuned by a fraction of the fundamental frequency. This phenomenon is quite similar to the rational harmonic mode-locking in amplitude modulated lasers, which almost everybody believed could not exist when applying phase modulation. The harmonics of the amplified driving signal is the reason behind this phenomenon. Because most RF amplifiers cannot avoid generating high-order harmonics of the amplifying signal, the "rational harmonic mode-locking" can be obtained in most phase-modulation fiber lasers when the driving signal is amplified, even in some cases when using linear amplifiers.