Over 10-GHz Peak-Equalized Pulse Train Source With Pulsewidth Shortening

Peak-flattened and width-shortened pulses have been successfully generated in an improved rational harmonic mode-locking (RHML) fiber laser in which an intensity modulator is driven by square waveform signals at gigahertz (GHz) repetition rates rather than typically used sinusoidal radio frequency (RF) signals. The repetition frequency of a fifth-order RHML under a driving signal at 3.125 GHz has been up to 15.7 GHz in our system. The equalized peaks of pulses generated reached to a minimum standard deviation of 0.2%, which is the best performance to our knowledge. The pulsewidth is shortened to 17.8 ps in the same RHML fiber laser driven by the square-wave signals compared with 30.3 ps driven by traditional sine-wave signals. The 3-dB spectral width of the RHML pulses was increased almost two times and the spectrum has a clear comb-like structure which can be developed into a frequency comb with GHz frequency spacing.