Impact of Gain Saturation on Passive Mode Locking Regimes in Quantum Dot Lasers with Straight and Tapered Waveguides

We report on the simulation of passive mode locking (ML) in quantum dot (QD) two-section lasers via a multi-section delayed differential equation model. The influence of the laser structural parameters on the achieved ML regimes is investigated. We show that, consistently with experimental findings, increasing the saturable absorber (SA) to gain section length ratio from 17% to 25%, a significant pulse shortening can be achieved. A physical explanation for this behavior is obtained via a detailed study of the gain and absorption saturation dynamics leading to the self consistent ML solution, in particular the competition between spectral hole burning non-linearity and total carrier density depletion responsible for the gain saturation in the gain section, balancing the absorption bleaching in the SA, is shown to play a central role in determining the observed ML regimes. In addition, the impact of introducing a tapered gain section in QD ML lasers are investigated, the possibility to achieve high power subpicosecond pulses from such devices is theoretically demonstrated and attributed to the significantly increased gain saturation energy in the tapered gain section.