VECSEL / MIXSEL

We have observed a rapidly developing field of optically and electrically pumped vertical external cavity surface emitting lasers (VECSELs). These lasers, which are also referred to as optically pumped semiconductor lasers (OPSs) or semiconductor disk lasers (SDLs) have gained a strong interest for power scaling. In a VECSEL, the light is emitted perpendicular to the epitaxial layers, unlike edge-emitting lasers, where the beam propagates in the epitaxial layers. In contrast to a VCSEL (i.e. a vertical cavity surface emitting laser) the external cavity of the VECSEL offers additional mode control for excellent transverse beam quality even at highest power levels and enables the integration of elements for nonlinear intracavity frequency conversion, wavelength tuning elements or passive modelocking.

Since the first demonstration in 1997, we have seen tremendous progress in VECSEL research, both in continuous-wave (cw) and in the ultrafast regime. Optically pumped VECSELs can convert fairly low-cost, low-beam-quality optical pump power from high-power diode laser bars into a near-diffraction-limited output beam with good efficiency. This resulted in more than 20 W cw power in a fundamental transverse mode, which is substantially higher than from any other type of semiconductor laser. An important advantage compared to well-established diode-pumped solid-state lasers based in ion-doped dielectric materials is that VECSELs operate in wavelength regions which are not covered by solid-state laser gain materials.

Also in modelocked operation, the performance of ultrafast VECSELs has surpassed other semiconductor lasers. Ultrafast VECSELs generated average power levels of several watts in picosecond and femtosecond pulses. Pulse durations below 100 fs were achieved, and the repetition rate of fundamentally modelocked VECSELs was increased to tens of GHz. Furthermore the SESAM and the VECSEL gain structure were integrated into a single semiconductor structure, which is referred to as a modelocked integrated external-cavity surface emitting laser (MIXSEL). MIXSELs allow for full wafer scale integration, offering the potential for substantially reduced fabrication costs of compact ultrafast lasers.

Such VECSEL performance should be well-suited for many applications such as laser displays, telecommunications, optical clocking, frequency metrology, high resolution nonlinear multiphoton microscopy, and optical coherence tomography.

An overview on the current status regarding pulse duration and average output power is illustrated here.