High-Repetition-Rate Real-Time Automatic Mode-Locked Fibre Laser Enabled by a Pre-Stretch Technique

To apply the real-time automatic mode-locking technique to mode-locked lasers with high repetition rates, we propose a pre-stretch technique, where a slice of dispersion is used to stretch the output pulses thereby reducing the required sampling rate and the bandwidth of the acquisition system. To illustrate, we demonstrate a real-time automatic mode-locked fibre laser with a repetition rate of 48 MHz, where a real-time feedback circuit with a 400-MSa/s analog-to-digital converter is used as the hardware backend. Given a fixed acquisition system, the correlation between the repetition rate and the optimum amount of dispersion is well discussed through comparing the countable threshold range for mode-locking discrimination. Furthermore, a conclusion based on experiments is drawn that, to apply automatic mode-locking technique, the minimum acquisition bandwidth should be above twice of the fundamental repetition rate, and the minimum sampling rate follows the Nyquist’s law, which is four times of the fundamental repetition rate.