Passively modelocked VECSELs

Ursula Keller

     ETH Zurich, Switzerland

A number of applications like optical clocking and telecommunications have created a demand for very-high-repetition-rate sources (tens of gigahertz) with high average powers (up to the watt range).  Passively modelocked vertical-external-cavity surface-emitting semiconductor lasers (VECSELs) have the potential for meeting such specifications, and we believe that their further development should lead to commercially very attractive devices that can be produced with cost-effective wafer-scale technologies.

The optically pumped VECSEL is a laser source that has sparked widespread interest in the past few years due to its capability of producing high average output powers in a diffraction-limited beam.  Furthermore, the semiconductor gain medium is well suited for being passively modelocked with a semiconductor saturable-absorber mirror (SESAM), and its large gain cross-section allows stable modelocking at multi-GHz repetition rates without Q-switching instabilities.  After the first passively modelocked VECSEL was demonstrated in the year 2000, the milestone of nearly 1 W average output power was achieved in 2002.  The pulses in the early experiments were often strongly chirped, but modelocking dynamics in VECSELs revealed that a soliton-like pulse shaping mechanism in the positive dispersion regime can help to generate short pulses with low chirp.  With the aid of intracavity dispersion control, it then became possible to obtain nearly transform-limited picosecond pulses with record-high output powers of 2.1 W at 4 GHz and 1.4 W at 10 GHz.

One of the most application-relevant milestones that remain to be achieved in the field of passively modelocked VECSELs is the integration of the semiconductor absorber into the gain structure, enabling the realization of ultra-compact high-repetition-rate laser devices suitable for wafer-scale integration.  We have recently succeeded in fabricating the key element in this concept, a quantum-dot-based SESAM with a very low saturation fluence, which supports stable modelocking in a simple, low-divergence cavity with identical mode areas on VECSEL gain and SESAM.  With this approach we have achieved a repetition rate of 50 GHz in 3.3-ps pulses and with 100 mW average output power at a center wavelength of 960 nm.