Femtosecond high-power quantum dot vertical external cavity surface emitting laser

Abstract

We report on the first femtosecond vertical external cavity surface emitting laser (VECSEL) exceeding 1 W of average output power. The VECSEL is optically pumped, based on self-assembled InAs quantum dot (QD) gain layers, cooled efficiently using a thin disk geometry and passively modelocked with a fast quantum dot semiconductor saturable absorber mirror (SESAM). We developed a novel gain structure with a flat group delay dispersion (GDD) of ± 10 fs2 over a range of 30 nm around the designed operation wavelength of 960 nm. This amount of GDD is several orders of magnitude lower compared to standard designs. Furthermore, we used an optimized positioning scheme of 63 QD gain layers to broaden and flatten the spectral gain. For stable and self-starting pulse formation, we have employed a QD-SESAM with a fast absorption recovery time of around 500 fs. We have achieved 1 W of average output power with 784-fs pulse duration at a repetition rate of 5.4 GHz. The QD-SESAM and the QD-VECSEL are operated with similar cavity mode areas, which is beneficial for higher repetition rates and the integration of both elements into a modelocked integrated external-cavity surface emitting laser (MIXSEL).

Experimentally verified pulse formation model for high-power femtosecond VECSELs

“…5b show a strong influence of GDD on the pulse duration. We already realized both QW-and QD-VECSEL structures with a low-dispersion top coating leading to femtosecond operation [22,31]. Up to that point, the dispersion of the SESAM was negligible compared to the one of the VECSEL gain structure.…”

“…To date, the highest average output power from a passively modelocked semiconductor laser has been obtained with such a MIXSEL generating up to 6.4 W with 28-ps pulses at a center wavelength of & 960 nm [21]. In 2011, the first femtosecond VECSEL was demonstrated using QD-VECSEL modelocked by a QD-SESAM [22]. Already in 2012, ultrafast VECSELs generated 5.1 W with 682-fs pulses [23].…”

“…Already in 2012, ultrafast VECSELs generated 5.1 W with 682-fs pulses [23]. In the sub-500-fs regime, 150 mW was obtained [22]. Even shorter pulse durations down to 107 fs with an average output power of 3 mW were demonstrated [24] in fundamental modelocking or down to 60 fs in a multipulsing mode [25], showing the potential of this technology.…”

“…We show that GDD management is extremely important to achieve short pulses of a few 100-fs. Because of this strong influence of the GDD, we designed a quantum well SESAM (QW-SESAM) with fast recovery dynamics and a low-dispersion top coating similar to the ones we used for recent VECSEL designs [22,31]. In combination with the same QD-VECSEL as used in [22] for the generation of 784-fs pulses, we were able to realize an intra-cavity dispersion of about 50 fs 2 which led to 364-fs pulses at an average output power of 70 mW.…”

“…Because of this strong influence of the GDD, we designed a quantum well SESAM (QW-SESAM) with fast recovery dynamics and a low-dispersion top coating similar to the ones we used for recent VECSEL designs [22,31]. In combination with the same QD-VECSEL as used in [22] for the generation of 784-fs pulses, we were able to realize an intra-cavity dispersion of about 50 fs 2 which led to 364-fs pulses at an average output power of 70 mW. This result was obtained with a cavity geometry using the same mode size on the gain and the absorber, usually referred as ''1:1 modelocking'' [2].…”