We report the first successful modelocking of a vertical external cavity surface emitting laser (VECSEL) with a quantum dot (QD) gain region. The VECSEL has a total of 35 QD-layers with an emission wavelength of about 1060 nm. In SESAM modelocked operation, we obtain an average output power of 27.4 mW with 18-ps pulses at a repetition rate of 2.57 GHz. This QD-VECSEL is used as-grown on a 450 μm thick substrate, which limits the average output powe

Barbarin, Y.

Golling, M.

Hoffmann, M.

Keller, U.

Kovsh, A.

Krestnikov, I.

Maas, D.

Mikhrin, S.

Südmeyer, T.

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Appl Phys B (2008) 93: 733–736DOI 10.1007/s00340-008-3267-0R A P I D C O M M U N I C AT I O NModelocked quantum dot vertical external cavity surface emittinglaserM. Hoffmann · Y. Barbarin · D.J.H.C. Maas ·M. Golling · I.L. Krestnikov · S.S. Mikhrin ·A.R. Kovsh · T. Südmeyer · U. KellerReceived: 14 October 2008 / Published online: 25 October 2008© The Author(s) 2008. This article is published with open access at Springerlink.comAbstract We report the first successful modelocking of avertical external cavity surface emitting laser (VECSEL)with a quantum dot (QD) gain region. The VECSEL has atotal of 35 QD-layers with an emission wavelength of about1060 nm. In SESAM modelocked operation, we obtain anaverage output power of 27.4 mW with 18-ps pulses at arepetition rate of 2.57 GHz. This QD-VECSEL is used as-grown on a 450 µm thick substrate, which limits the averageoutput power.PACS 42.55.Px · 42.60.Fc1 Introduction and motivationOptically pumped vertical external cavity surface emittinglasers (VECSELs) offer high output power, excellent beamquality and good efficiency [1]. They produced average out-put powers as high as 30 W with an M2 value of 3 [2] or20 W with an M2 value below 1.1 [3]. The output powercan be scaled up simply by increasing the mode area on thegain region and the pump power if the heat flow throughthe layer structure is nearly one-dimensional [4]. Moreover,semiconductor bandgap engineering can be used to obtain alarge variation of emission wavelengths that are not coveredM. Hoffmann () · Y. Barbarin · D.J.H.C. Maas · M. Golling ·T. Südmeyer · U. KellerDepartment of Physics, ETH Zurich, 8093 Zurich, Switzerlande-mail: mh@phys.ethz.chFax: +41-44-6331059I.L. Krestnikov · S.S. Mikhrin · A.R. KovshInnolume GmbH, Konrad-Adenauer-Allee 11, 44263 Dortmund,Germanyby established solid state lasers. In addition, semiconductorlasers are ideally suited for mass production because theyare based on a wafer-scale technology with a high level ofintegration.In combination with a semiconductor saturable absorbermirror (SESAM) [5], VECSELs have been passively mod-elocked, and average output powers as high as 2.1 W [6],pulse repetition rates as high as 50 GHz [7] and pulses asshort as 260 fs [8] were achieved. We recently introduceda new concept of ultrafast semiconductor lasers, where weintegrated the saturable absorber and the active region intoa single semiconductor based device. We refer to this newclass of modelocked VECSELs as modelocked integratedexternal-cavity surface emitting lasers (MIXSEL) [9]. Onekey requirement was the development of quantum dot sat-urable absorbers that support the integration with the samelaser mode size in the absorber and the gain. The MIXSELplatform has a high potential for applications in optical com-munication, optical clocking of multi-core microprocessorsand compact supercontinuum generation for bio-medical ap-plications. However, so far we always used quantum wells(QWs) for the gain section, and the pulse duration has beenlimited to the picosecond regime. Our motivation for usingquantum dot (QD) gain structures within a MIXSEL or amodelocked VECSEL is the larger inhomogeneous broaden-ing. Self-assembled InAs/GaAs QDs produce an inhomoge-nously broadened gain [10], which is wider compared toQWs and is less susceptible to temperature changes [11].Previously, we have demonstrated with diode-pumped solid-state lasers that inhomogenous gain broadening relaxes therequirements on the saturable absorber and supports shorterpulse durations [12, 13]. With inhomogeneous broadening,the lasing bandwidth is increased at continuous wave (cw)operation because spectral hole burning enables many dif-ferent axial modes to lase at the same time. Therefore,734 M. Hoffmann et al.the saturable absorber only needs to provide sufficient self-amplitude modulation to phase-lock these modes withouthaving to transfer the full energy from one axial mode toall other adjacent modes. We believe that the inhomoge-neous broadening of QD-gain will enable sub-picosecondpulse generation of modelocked VECSELs and MIXSELswith fast quantum dot saturable absorbers [14, 15] in thenear future.VECSELs based on QD-gain regions have recently beenreported in cw-operation at an emission wavelength of1034 nm [16] and 1210 nm [17], where output powers of1.4 W and 300 mW, respectively, were achieved using dia-mond heat spreaders.Here, we report on the first passively modelocked QD-gain VECSEL. We employed a standard quantum wellSESAM (QW-SESAM) and obtained self-starting stablepulses of 18-ps duration with an average output power of27.4 mW at a pulse repetition rate of 2.57 GHz. The averageoutput power of 27.4 mW is currently limited by the reducedthermal conductivity of the 450 µm thick GaAs wafer, ontowhich the VECSEL structure has been grown.2 Laser cavity, QD-VECSEL gain structure andSESAMWe use a typical V-shaped laser cavity, which is shown inFig. 1 [18]. The QD-VECSEL is pumped at 808 nm at anangle of 45 degrees. A QW-SESAM and a curved outputcoupling mirror constitute the end-mirrors of the laser cav-ity. The operation wavelength can be tuned by changing theFig. 1 Schematic of the laser setup. A V-shaped laser cavity is usedwith a quantum well SESAM (QW-SESAM) and the output coupler asthe cavity end-mirrors. Inside the cavity we used a quantum dot gainVECSEL (QD-VECSEL) structure and a 20 µm thick etalon to tunethe central wavelength of the laser. The 808-nm pump light is focusedonto the QD-VECSEL structure under an angle of 45 degreesangle of a 20-µm thick etalon. In this configuration the pulsepasses the active region twice per cavity roundtrip.The QD-VECSEL gain structure was grown by Innol-ume GmbH by molecular beam epitaxy (MBE) on a GaAssubstrate of 450-µm thickness. The structure consists of adistributed Bragg reflector (DBR) centered around 1060-nmwavelength and an active QD region. The DBR is madeof 29 pairs of quarter wavelength layers of GaAs andAl0.9Ga0.1As, which results in a high reflectivity at the laserwavelength (calculated 99.99%). The active region con-sists of five stacks of seven QD-layers embedded in GaAs,whereas the GaAs serves as absorbing medium for the pumpradiation. In each stack, the seven QD-layers are separatedfrom each other by 10 nm using GaAs spacers, and the fivestacks are placed in five different antinodes of the standingwave pattern of the electrical field of the laser.The QD stacks were formed by cyclic deposition of InAs(1/4 monolayer thickness) and GaAs (1/3 monolayer thick-ness) followed by short growth interruptions. The substratetemperature was kept as low as 510 to 520◦C, monitored byan infrared pyrometer. No dislocations have been detectedin the grown structure.The SESAM used for modelocking the QD-VECSELcontains a QW saturable absorber in a standard anti-resonantdesign [19]. It was grown by MBE in the FIRST cleanroomfacility at ETH Zurich and consists of a 30-pair AlAs/GaAsDBR centered at a wavelength of 1062 nm and a sin-gle InGaAs QW saturable absorber. The saturable absorp-tion parameters of the SESAM were characterized with ahigh-precision nonlinear reflectivity setup [20]. At 1064-nmcenter wavelength, we determined a saturation fluence of34 µJ/cm2, a modulation depth of 0.6% and nonsaturablelosses of 0.2%.The length of the laser cavity was 58.5 mm, which corre-sponds to a repetition rate of 2.6 GHz. The full folding angleof the cavity was 10 degrees. The dielectric output couplingmirror had 0.28% transmission at the laser wavelength anda radius of curvature of 60 mm. The round pump spot on thegain chip had 90 µm radius and the pump beam had an M2value of 12. The cavity geometry was chosen such that thelaser mode radius on the SESAM was about 50 µm.For our experiment, the QD-VECSEL structure was di-rectly soldered onto a heat sink, and no further steps weretaken towards improved thermal management [4]. Duringmodelocked laser operation, the heat sink temperature wasset to −20◦C.3 Experimental results and discussionFundamental modelocking with an average output power of27.4 mW was achieved for a pump power of 1.56 W (Fig. 2).Pulses of 18-ps duration were measured with a second har-monic generation autocorrelator. We measured the opticalModelocked quantum dot vertical external cavity surface emitting laser 735Fig. 2 The autocorrelation trace (black) and its fit (gray dashed), theemission spectrum with the beam profile as inset and the microwavespectrum are plotted for a pump power of 1.56 W on the QD-VECSEL.The deconvoluted pulse has 18 ps FWHM duration; the width of theoptical spectrum is 0.22 nm, measured with a resolution bandwidth of0.1 nm. The microwave spectrum shows a clear peak at 2.57 GHz, thespectral span was 10 MHz with a resolution bandwidth of 100 kHzspectrum with an optical spectrum analyzer that had a reso-lution bandwidth of 0.1 nm and obtained an FWHM width of0.22 nm. A simple deconvolution would result in an FWHMwidth of 0.2 nm and a time bandwidth product of 1 (i.e.about 3 times the transform limit of a sech2 pulse). Themicrowave spectrum is measured over a span of 10 MHzwith a resolution bandwidth of 100 kHz. We see a clearpeak at 2.57 GHz, and additional measurements also clearlyshow the peaks for the higher harmonics of this frequency.In Fig. 2, also the beam profile of the laser beam is shown asan inset within the emission spectrum of the laser.For pump powers higher than 1.56 W, a thermal roll-over of the average power occurred. This roll-over is causedby the low thermal conductivity of the VECSEL structure.We did not optimize thermal management because this ex-periment was a proof-of-principle for modelocking a QD-VECSEL. Thus the 450 µm thick GaAs substrate has notbeen removed, and the average power is limited by thermalheat sinking [4]. The advantage of this structure is that al-most no additional processing is required before the devicecan be used in the laser cavity.We numerically simulated the temperature increasewithin our structure using a finite-element method for apump spot of 90 µm and a pump power of 1.5 W. We ob-tained a difference in temperature between the active regionand the heat sink of more than 100 K. Lasing experimentswith cw-output of the VECSEL in a straight cavity setupalso confirmed that the output power level is limited bythe absolute temperature in the active region of the VEC-SEL. These numerical simulations furthermore show thatthe temperature difference between the heat sink and the ac-tive region can be reduced to 25 K for equally high pumppowers if the 450 µm thick GaAs substrate is removed fromthe VECSEL and the grown structure is soldered directlyonto the heat sink. We expect that such a QD-VECSELwill be power-scalable to several watts in analogy to QW-VECSELs. We would expect that a faster QD-SESAM andbetter dispersion management [21, 22] will ultimately resultin shorter pulse durations.4 Conclusion and outlookWe have demonstrated the first modelocked QD-VECSEL.Pulses of 18-ps duration with a repetition rate of 2.57 GHzwere generated at a laser wavelength of 1059 nm. We couldachieve an average output power of 27.4 mW. The averagepower performance of the laser can be further improved byoptimizing the thermal properties of the VECSEL structureby substrate removal. The ongoing development of fasterSESAMs using quantum dots and improved cavity disper-sion management should allow the generation of femtosec-ond pulses in the near future.Acknowledgements This work was supported in part by the Euro-pean Community’s Seventh Framework Programme FAST-DOT undergrant agreement 224338, the Intel Corporation through a universitysponsored research agreement, and ETH Zurich with the FIRST clean-room facility. We thank Oleg Okhotnikov from the Tampere Universityof Technology and Edik Rafailov from the University of Dundee forfruitful discussions.Open Access This article is distributed under the terms of the Cre-ative Commons Attribution Noncommercial License which permitsany noncommercial use, distribution, and reproduction in any medium,provided the original author(s) and source are credited.736 M. Hoffmann et al.References1. M. Kuznetsov, F. Hakimi, R. Sprague, A. Mooradian, High-power(>0.5-W CW) diode-pumped vertical-external-cavity surface-emitting semiconductor lasers with circular TEM00 beams. IEEEPhotonics Technol. Lett. 9, 1063–1065 (1997)2. J. Chilla, S. Butterworth, A. Zeitschel, J. Charles, A. Caprara,M. Reed, L. Spinelli, High power optically pumped semiconductorlasers, in Photonics West 2004, Solid State Lasers XIII: Technol-ogy and Devices, in Proc. SPIE, vol. 5332, pp. 143–150 (2004)3. B. Rudin, D.J.H.C. Maas, A.-R. Bellancourt, M. Hoffmann,E. Gini, T. Südmeyer, U. 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Modelocked quantum dot vertical external cavity surface emitting laser

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Modelocked quantum dot vertical external cavity surface emitting laser

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