Optically pumped vertical external cavity surface emitting lasers (OP-VECSELs) evolved to high-power laser sources offering excellent beam-quality, wavelength flexibility and low-noise properties in a compact and simple cavity. Passively modelocked with a semiconductor saturable absorber mirror (SESAM), VECSELs demonstrated fs-pulses with multi-Watt average output powers at gigahertz repetition rates. Electrical pumping (EP) is an obvious step to make these semiconductor lasers even more compact and suitable for chip integration, potentially enabling access to applications such as data communication or optical clocking. With SESAMmodelocked EP-VECSELs, 57-ps pulses with an average output power of 40 mW and 9.5-ps pulses with 7.6 mW have been obtained. However, due to the intrinsic trade-off between electrical and optical properties in the design of EPVECSELs, short pulses at high average output power are difficult to achieve. This challenge was previously addressed in our theoretical guidelines for power scaling and modelocking optimization and later experimentally verified. Here, we report on the successful implementation of an improved design and fabrication scheme for EP-VECSELs, grown and fabricated at ETH Zurich. These lasers enabled a further decrease in pulse duration to 7.3 ps while increasing the average output power to 13.1 mW at 1.46-GHz repetition rate. The shortest pulse duration measured was 6.3 ps with an average power of 6.2 mW. In addition to the modelocking experiments, we present a thorough cw-characterization of our EP-VECSELs of different sizes and in different cavity configurations, pointing out the inevitable trade-off between high-power multi-mode and low-power single-mode operation limiting the modelocking performance.
One of the main advantages of using VECSEL lasers for mode-locked operation is their power scalability. Best
performance data available for mode-locked semiconductor lasers have been achieved with optically pumped
VECSELs, reaching pulses in the femtosecond regime and average powers in the watt regime.<sup>1</sup> This advantage
is challenging for electrically pumped VECSELs, where a homogeneous carrier injection into the center must be
provided in order to maintain a single-mode operation for large diameter devices.
In this paper we investigate the current injection from the bottom contact of a VECSEL design, and estimate
the leakage of the hole current. Then we introduce two designs that can reduce the leakage current and enhance
the injection into the center of the device, thereby increasing the simulated output power by more than 20% in
CW-mode while maintaining an optimal gain profile suitable for single-mode operation.
Fundamental mode operation along with high output power is a major challenge for an electrically pumped VECSEL
(EP-VECSEL) suitable for passive modelocking. We present an experimental study on the influence of the intermediate
DBR reflectivity on the beam quality and the output power of EP-VECSELs. For designs with reflectivities of 90%, 82%
and 71% the highest possible power for the best achievable beam quality was 15.1 mW (M<sup>2</sup> = 1.1, 82% device). We can
demonstrate that a correctly chosen intermediate DBR reflectivity is necessary for both good beam quality and high
power, and that a trade-off in power has to be accepted.
Vertical External Cavity Surface Emitting Lasers (VECSELs) feature scaling to large active areas and combine
surface emission with high optical power output. In principle, they can be designed with electrical pumping
operating in continuous wave or passively mode-locked operation. In this paper, our design and modeling
activities towards a high power passively mode-locked VECSEL are described. In particular, design towards
single mode high-power CW operation is discussed as prerequisite for passive mode-locking.