Mode-locking Instabilities for High-Gain Semiconductor Disk Lasers Based on Active Submonolayer Quantum Dots

Submonolayer quantum dots (SML QDs) combine the large gain cross section of quantum wells (QWs) with the potential benefits of a stronger confinement. We demonstrate here an optically pumped vertical external-cavity surface emitting laser (VECSEL) based on active SML QDs. The ultrafast SML QD VECSEL is optimized for passive modelocking with an intracavity semiconductor saturable absorber mirror (SESAM) around 1030 nm. We have achieved the highest cw output power of 11.2 W from a QD-based VECSEL to date. With a birefringent filter inside the VECSEL cavity, we obtain a tuning range of 47 nm centered at 1028 nm. Any attempt to passively modelock the VECSEL with a QW SESAM reveals fundamental limitations. The intrinsically higher linewidth enhancement factor of SML QDs compared to QWs or self-assembled Stranski-Krastanov QDs is further increased at higher pump powers. Our experiments confirm prior theoretical predictions that a strong amplitude-phase coupling can destabilize cw modelocking and introduce chaotic multiple pulse fluctuations.