External-Cavity Mode-Locked Quantum-Dot Laser Diodes for Low Repetition Rate, Sub-Picosecond Pulse Generation

This paper presents an investigation of the mode-locking performance of a two-section external-cavity mode-locked InGaAs quantum-dot laser diode, focusing on repetition rate, pulse duration and pulse energy. The lowest repetition rate to-date of any passively mode-locked semiconductor laser diode is demonstrated (310 MHz) and a restriction on the pulse energy (at 0.4 pJ) for the shortest pulse durations is identified. Fundamental mode-locking from 310 MHz to 1.1 GHz was investigated, and harmonic mode-locking was achieved up to a repetition rate of 4.4 GHz. Fourier transform limited subpicosecond pulse generation was realized through implementation of an intra-cavity glass etalon, and pulse durations from 930 fs to 8.3 ps were demonstrated for a repetition rate of 1 GHz. For all investigations, mode-locking with the shortest pulse durations yielded constant pulse energies of ~0.4 pJ, revealing an independence of the pulse energy on all the mode-locking parameters investigated (cavity configuration, driving conditions, pulse duration, repetition rate, and output power).