Simulation and Analysis of Dynamic Regimes Involving Ground and Excited State Transitions in Quantum Dot Passively Mode-Locked Lasers

We present a modified version of the multi-section delayed differential equation (MS-DDE) model, for quantum dot (QD) passively mode-locked (ML) lasers when competition between ground state (GS) and excited state (ES) ML takes place. The model takes into account the difference in the group velocity of GS and ES fields. Sole GS, sole ES, and dual-state lasing and ML have been studied. The results were verified with time domain traveling wave (TDTW) simulations and compared, when possible, with experimental results. These tests confirmed the reliability of the model. We found that, in two-section ML lasers, GS lasing and mode locking are always more easily established. For instance, GS lasing can be either self-starting or induced by the initial lasing from the higher energy ES. On the contrary, GS lasing tends to inhibit, to a certain extent, the onset of ES lasing, especially at low injection current and low reverse voltage. Moreover, ES shows less potential to achieve stable ML than GS. Based on these findings, we propose proper theoretical explanation of the achieved lasing and ML regimes in realized devices. Especially, we demonstrate a novel stable dual-state ML regime with remarkable enhanced pulse peak power and pulse width.