Picosecond to sub-picosecond pulse generation from mode-locked VECSELs at 1.55 μm

Mode-locked vertical-extended-cavity-surface emitting lasers (ML-VECSEL) are promising candidates for the generation of stable short pulses at multi-GHz rate. However, the poor thermal behavior of quaternary InP-based semiconductor compounds often limits the performance of ML-VECSELs operating at 1.55 μm. In this work, we report on a specific approach using downward heat sinking to optimize the heat dissipation out of the active region. VECSEL chips with a low thermal resistance are fabricated using a hybrid metal-metamorphic GaAs/AlAs mirror and bonding to a highly thermally conductive host substrate. We show that superior performance can be obtained with a CVD diamond substrate, while electroplated copper host substrate can afford a flexible and low cost alternate approach for moderate (~100 mW) output power. The VECSEL chip assembled with a 1.55μm fast InGaAs(Sb)N/GaAsN semiconductor saturable absorber mirror (SESAM) produces nearly Fourier transform-limited mode-locked pulses at ~ 2 GHz repetition frequency, and the RF linewidth of the free running laser is measured to be less than 1000 Hz. When the resonance and group delay dispersion of the SESAM microcavity are tuned by selective etching of specific top phase layers, the modelocked pulse width is reduced from several picoseconds to less than 1 ps.