Recent advances in electrically pumped VECSELs for modelocking

Abstract

Vertical external cavity surface emitting lasers (VECSELs) are excellent high power semiconductor lasers with diffraction-limited circular output beam and outstanding modelocking performance even at tens of GHz repetition rate. The output power can be scaled up by simply increasing the mode area on the gain region. It makes them very attractive for numerous applications such as RGB displays, biomedical imaging or optical clocking of multi-core processors. Passively modelocked optically-pumped VECSELs, using a semiconductor saturable absorber mirror (SESAM), have generated shorter pulses and higher average powers than any other modelocked semiconductor laser (135-fs pulses at 35-mW average power and 2.1-W in 4.7-ps pulses). Electrical pumping (EP) of modelocked VECSELs is the obvious next step towards compact high-power ultrafast laser sources. In 2003, Novalux Corporation reported a continuous wave (cw) output power of nearly one Watt from their proprietary EP-VECSEL (NECSEL). The modelocking of a NECSEL has been demonstrated with 40 mW of average power in 57-ps pulses. Since then, very few EP-VECSEL results have been reported. Recently, we started to develop EP-VECSELs designed for modelocking, which require an optimized balance between electrical resistance, optical losses, dispersion and beam quality. We discuss our design approach and present initial EP-VECSEL devices generating >100-mW cw power. Homogeneous current injection is achieved even for large devices, showing very good agreement with our numerical simulations. Sufficient power in a diffraction-limited beam and a carefully designed SESAM are required to modelock an EP-VECSEL.