Passively mode-locked 4.6 and 10.5 GHz quantum dot laser diodes around 1.55 μm with large operating regime

Passive mode-locking in two-section InAs/InP quantum dot laser diodes operating at wavelengths around 1.55 µm is reported. For a 4.6-GHz laser, a large operating regime of stable mode-locking, with RF-peak heights of over 40 dB, is found for injection currents of 750 mA up to 1.0 A and for values of the absorber bias voltage of 0 V down to -3 V. Optical output spectra are broad, with a bandwidth of 6--7 nm. However, power exchange between different spectral components of the laser output leads to a relatively large phase jitter, resulting in a total timing jitter of around 35 ps. In a 4-mm-long, 10.5-GHz laser, it is shown that the operating regime of stable mode-locking is limited by the appearance of quantum dot excited state lasing, since higher injection current densities are necessary for these shorter lasers. The output pulses are stretched in time and heavily up-chirped with a value of 16--20 ps/nm. This mode of operation can be compared to Fourier domain mode-locking. The lasers have been realized using a fabrication technology that is compatible with further photonic integration. This makes such lasers promising candidates for, e.g., a coherent multiwavelength source in a complex photonic chip.