Mode-Controlled Tapered Lasers Based on Quantum Dots

780 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 15, NO. 3, MAY/JUNE 2009

Mode-Controlled Tapered Lasers Basedon Quantum Dots

Pia Weinmann, Student Member, IEEE, Christian Zimmermann, Thomas Walter Schlereth, Christian Schneider,Sven Hofling, Member, IEEE, Martin Kamp, and Alfred Forchel, Member, IEEE

Abstract—Wavelength control of tapered quantum dot (QD)lasers was achieved by adding a distributed Bragg reflector aswavelength-selective element to the devices. The Bragg wavelengthof around 920 nm is matched to the gain of the single layer ofInGaAs QDs that is used as active material. Devices with 1.4-mm-long tapers have reached output powers of over 1 W, and over2 W were obtained from lasers with 3-mm-long tapers. The emis-sion of the devices is restricted to a very narrow spectral range,with side-mode suppression ratios of over 40 dB.

Index Terms—Continuous-wave (CW) lasers, distributed Braggreflector (DBR) lasers, quantum dots (QDs), semiconductor lasers.

I. INTRODUCTION

AN increasing number of applications require high-brightness lasers that emit a single wavelength or that emit

in a very narrow wavelength interval. Examples are frequencydoubling of infrared lasers into the visible spectral range orpumping of solid-state and fiber lasers, where the emission ofthe pump has to be matched with the absorption of the gainmedium. The standard way to achieve single-wavelength emis-sion is the use of feedback gratings that are either placed at theend of the laser resonator in order to realize a distributed Braggreflector (DBR) or along the laser cavity [distributed feedbacklasers (DFB)] [1]. The combination of these gratings with nar-row lateral waveguides gives devices that emit light with a veryhigh spectral purity and output power levels of a few hundredmilliwatts [2], [3]. The maximum output power of such lasersis limited by a number of effects, e.g., the high thermal load,spectral hole burning, and the power density on the facets. In-creasing the width of the lateral waveguide to mitigate theseissues is not an option since it will lead to multimode behav-ior and a deterioration of the spectral purity and beam quality.One possible solution is the use of tapered laser diodes. Thesedevices have emerged as very promising high-power sourceswith simultaneous excellent beam quality [4]. No frequencyselection is provided in standard tapered lasers, but a number

Manuscript received November 1, 2008; revised November 21, 2008.First published May 2, 2009; current version published June 5, 2009. Thiswork was supported by the European Union through the Integrated Projectwww.brighter.eu under Contract IST-2005-035266.

P. Weinmann, T. W. Schlereth, C. Schneider, S. Hofling, M. Kamp,and A. Forchel are with the Technische Physik, Universitat Wurzburg, D-97074 Wurzburg, Germany (e-mail: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected]).

C. Zimmermann is with Nanoplus Nanosystems & Technologies GmbH,D-97218 Gerbrunn, Germany. He is also with the Universitat Wurzburg,D-97074 Wurzburg, Germany (e-mail: [email protected]).

Digital Object Identifier 10.1109/JSTQE.2008.2010874

Fig. 1. Tapered laser with DBR grating. The device consists of three sections:a DBR grating for wavelength control, an RWG to provide a single lateral mode,and a gain-guided taper.

of approaches for the realization of single-mode behavior havebeen reported. Among these are the use of an external Bragggrating for wavelength-selective feedback [5] or a master os-cillator power amplifier (MOPA) concept [6]. In both cases,additional optics is required that can be avoided by integratingthe frequency-selective element directly into the tapered laser.From a fabrication point of view, a process that requires no re-growth (as opposed to the standard technology for DFB/DBRfabrication) is preferable. In addition to a reduced complexityof the process, problems related to the formation of defects atthe regrowth interface or the oxidation of aluminum containinglayers can be avoided. For low-power DFB lasers, the conceptof lateral feedback gratings has been employed [7], [8], and ta-pered lasers based on lateral feedback gratings have also beenreported [9]. However, the use of large optical cavities to reducethe beam divergence and facet load leads to a drastic reductionof the coupling coefficient to the lateral grating structure. A bet-ter choice seems to be the use of a DBR grating at the rear endof the ridge waveguide (RWG), as shown in Fig. 1.

The DBR section can be fabricated without any regrowthstep, either by etching deep slots into the RWG section [10]or by placing a shallow grating at the end of the RWG, as dis-cussed in this paper. The latter approach is less demanding as faras the aspect ratio of the grating is concerned. The drawback isthat the grating is not pumped, which leads to additional losses.For long devices, the reflectivity curve of the grating extendsover several cavity modes, so the emission is not restrictedto a single longitudinal mode as in a classical DFB or DBRlaser. However, the emission spectrum will still be very narrow(<0.1 nm), which is sufficient for most of the applications men-tioned earlier. Moreover, the shift of the emission wavelengthwith temperature will be reduced by a factor of about 3 in

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WEINMANN et al.: MODE-CONTROLLED TAPERED LASERS BASED ON QUANTUM DOTS 781

Fig. 2. SEM images of uncapped Ga0 .57 In0 .43 As/GaAs/Al0 .18Ga0 .82 As QDs. A larger area of the same sample is shown in the inset.

comparison to devices without DBR gratings since the wave-length shift caused by the temperature dependence of the re-fractive index is smaller than the shift of the gain curve withtemperature. A good temperature stability would enable passivecooling of pump lasers, which allows for lower power consump-tion, cost reduction, and more efficient laser modules.

II. DEVICE FABRICATION

The laser material was grown by solid source molecular beamepitaxy on a (0 0 1) n-doped GaAs substrate. The structure con-sists of a highly p-doped 250-nm-thick GaAs contact layer and1.5-µm-thick Al0.4Ga0.6As cladding layers with p-type (Be)and n-type (Si) doping, respectively. The active region is formedby a single layer of quantum dots (QDs) symmetrically embed-ded in an AlGaAs waveguide with graded index profile (GRIN-SCH) that is realized by a short-period superlattice. Herein,the aluminum content is varying from 40% to 18%. The QDsare grown in the Stranski–Krastanow mode by deposition of6.5 monolayers of In0.43Ga0.57As and are surrounded by 3-nmGaAs barriers. Fig. 2 shows SEM images of the equivalent un-capped QDs. The emission wavelength of the QDs is spectrallypositioned around 920 nm.

QDs have several potential advantages over quantum wells,e.g., reduced threshold current densities, a suppression of car-rier diffusion, and less filamentation. An additional benefit, es-pecially for lasers with wavelength-selective elements, is theirreduced shift of the gain curve with temperature. This allowsto match the emission wavelength of the laser with the peakgain of the active region over a larger temperature range than inquantum well devices. Detailed information about the materialgrowth can be found in [11]. Before the processing of taperedlasers, broad-area devices with different lengths were fabricatedto obtain the intrinsic material parameters. The characterizationshowed a low threshold current density of 178 A/cm2 for laserswith a width of 100 µm and a length of 1 mm. Measurements oflasers with different lengths gave values of 0.83 for the internalquantum efficiency and 2.2 cm−1 for the internal absorption.

The fabrication of the tapered lasers starts with the defini-tion of alignment marks and beam spoiling elements that aredeeply etched through the complete laser layer. Next, the RWGwith a width of 6 µm and a length of 1 mm is defined by op-

Fig. 3. SEM image of the shallow DBR grating positioned at the rear end ofthe RWG. The inset shows the zoomed image of the first-order grating with aperiod of 140 nm.

tical lithography. The ridge is subsequently transferred into thesemiconductor by an electron cyclotron resonance reactive ionetching (ECR-RIE) process. Then, first- or second-order grat-ings with a length of 600 µm and periods of 140 and 280 nmare defined by high-resolution electron beam lithography at therear end of the RWG. A chromium etch mask for the followingECR etch step is evaporated. The etch depth of the gratings isaround 150–200 nm, resulting in an index modulation of about5 × 10−3 . An SEM image of the grating is presented in Fig. 3.

Next, the sample is planarized by spin-coating an insulat-ing layer of bisbenzocyclobutene (BCB). The gain-guided tapersection is formed by a removal of the highly conductive GaAscontact cap layer outside the taper region by wet chemical etch-ing. The gain-guided taper has an opening angle of 6◦ and alength of 1.4 mm, leading to a facet width of 150 µm. For ad-ditional isolation, a layer of MgO is deposited on the surfaceoutside the taper area. Standard p-contacts (Cr/Pt/Au) and n-contacts (AuGe/Ni/Au) are evaporated on the ridge and tapersection. No contact is made to the DBR section that is buriedunder the BCB layer, and therefore, unpumped. A dielectric an-tireflection coating (Al2O3) is applied to the front facet. Dueto the DBR feedback, there is no need for a high-reflectivitycoating of the back facet. The devices are mounted p-side downon a copper submount with indium solder for testing.

III. CHARACTERIZATION

A. Light Output Power

The continuous-wave (CW) output power characteristic of atapered laser at room temperature is shown in Fig. 4. A max-imum output power of 1 W is reached at a drive current of2 A. The wall plug efficiency reaches a maximum value of27% for drive currents between 1.6 and 2.0 A. When comparingthese values with wall plug efficiencies of multimode broad-arealasers, additional geometrical losses due to the taper section [12]need to be taken into account as well as losses caused by thefeedback grating. The threshold is reached at a current of 380mA, which is comparatively low for a 3-mm-long device. Thiscorresponds to a threshold current density of about 350 A/cm2 .The slope efficiency is determined to be 0.61 W/A; it is nearly


Fig. 4. Light output characteristics of the tapered device at RT. The L–I curve(solid line) shows a maximum output power of over 1 W at a drive current of2 A.

Fig. 5. Emission spectrum of the tapered DBR laser. Stable single-mode be-havior at an emission wavelength of 922 nm with a high SMSR of more than40 dB is achieved.

constant till the maximum output power is reached, which islimited by thermal rollover.

B. Spectral Properties

The emission spectrum was measured under CW operationat 1 W optical output power and is plotted in Fig. 5. The devicehas a narrow emission line at 922 nm, with a spectral linewidthclose to the resolution limit of the optical spectrum analyzer(0.05 nm).

The side-mode supression ratio (SMSR) is more than 40 dB.The inset of Fig. 5 shows the superimposed spectra of four emit-ters with the same grating period. The spectra are very similarin terms of SMSR and emission wavelength. The wavelengthdeviation between the four devices is less than 0.1 nm, which isan indication of an excellent process uniformity.

C. Temperature Stability

When talking about the spectral properties of diode lasers,one important issue is the temperature stability of the emissionwavelength. As already mentioned before, a low-temperature

Fig. 6. Plot of emission wavelength of the tapered DBR device versus heatsink temperature. The wavelength shift is 4.4 nm for the range of 15 ◦C–80 ◦C.The temperature coefficient is determined to be 0.07 nm/K and is fixed by theindex change in the grating.

coefficient ∆λ/∆T is one of the key properties required forthe realization of low-cost pump modules. To efficiently pumpan Yb-doped fiber, it is important to keep the wavelength ofthe pump laser in the maximum absorption peak of the spe-cific material. Especially for the 920-nm range where the Yb-absorption spectrum is rather broad, the high-temperature sta-bility could enable coolerless operation. By controlling the lasertemperature by a Peltier element, we therefore also investigatedthe temperature-dependent properties of the tapered device. InFig. 6, the emission wavelength is plotted in dependence ofthe heat sink temperature. For the temperature range between15 ◦C and 80 ◦C, the temperature-induced wavelength shiftis 4.4 nm. This corresponds to a temperature coefficient of0.07 nm/K, which is set by the temperature dependence of theindex of refraction within the DBR grating.

In addition to the shift of the emission wavelength, one stillhas to consider the larger shift of the gain peak with temperature.The different temperature coefficients will lead to a mismatchof the emission wavelength and the material gain at high tem-peratures, and a decrease of the laser performance. Hence, asmall temperature coefficient of the gain is desirable in orderto achieve stable performance over a wide temperature range.QD active layers usually have a much lower shift of the peakgain with temperature than quantum wells. For quantum well(QW) material, the temperature-induced shift of the gain is typ-ically around 0.3 nm/K. For QDs values down to 0.09 nm/Ktogether with high characteristic temperatures are reported forhigh-power material at 980 nm [13].

D. Beam Quality

To determine the beam quality of the laser diode, the beamdivergence of the laser was measured. The resulting far field ofthe slow and the fast axis are presented in Fig. 7. In the fast axis,the far field is Gaussian with a divergence angle of 48.8◦.

Regarding the slow axis, a low divergence by an angle of 8.2◦

at 1/e2 is observed. The deviation from a diffraction-limitedbeam is given by the beam quality factor M 2 [14]. To extractthis factor, we investigated the caustic of the refocused beam inthe slow axis. The measurement was done for different pump

WEINMANN et al.: MODE-CONTROLLED TAPERED LASERS BASED ON QUANTUM DOTS 783

Fig. 7. Far field measured at 2 A drive current and 1 W of output power. Thedashed line corresponds to the far field in the fast axis with a divergence angle of48.8◦ at 1/e2 . The solid curve is the far field for the slow axis with a divergenceangle of 8.3◦ at 1/e2 .

Fig. 8. Beam caustic of the tapered laser. The beam radius is measured at1/e2 over a definite distance for different currents.

currents. Fig. 8 shows the beam radius versus distance for fourdifferent drive currents.

This data allow the calculation of the M 2 factor by the fol-lowing dependence of the beam waist w (measured at a powerlevel of 1/e2) on the propagation distance

w(x) =

√w2

0 + M 4 λ2

π2w20

(x − x0)2 (1)

where w0 is the minimum beam waist, which is obtained atposition x0 . For a current of 2 A, corresponding to an outputpower of 1 W, an M 2 value of 1.9 is obtained. This meansthe beam quality of the device is constantly high up to themaximum output power. Fig. 9 shows the development of thebeam quality with increasing output power by plotting M 2 overthe applied drive current. The device features an almost constantbeam quality up to the maximum output power.

Fig. 9. Beam quality factor M 2 versus drive current.

Fig. 10. Output power characteristic of two tapered DBR lasers with differenttaper length. By stretching the taper from 1.4 to 3 mm, the output power isdoubled from 1 to 2 W.

E. Devices With Longer Tapers

In order to increase the output power of the lasers, deviceswith longer taper sections were fabricated on the same epitax-ial material. In comparison to the device discussed earlier, thelength of the taper was roughly doubled from 1.4 to 3 mm. Allother device dimensions were kept constant. The output powercharacteristic of this device is shown in Fig. 10, together withthe characteristic of a device with a 1.4-mm-long taper.

The maximum output power of the longer devices exceeds2 W. Due to the increased area, the threshold increases to1.9 A. The emission spectrum and its shift with temperatureare similar to that of the device with the shorter taper, showinga very narrow peak with an SMSR of over 40 dB.

IV. CONCLUSION

To summarize, we have realized high-brightness lasers witha tapered design and a DBR grating for frequency selection.The DBR approach turns out to warrantee wavelength controland stabilization. The temperature-induced wavelength shift isdetermined to be 0.07 nm/K. Together with the high-temperaturestability of the QD gain, this means that the laser performance is


substantially stable over a wide temperature range. The InGaAsQD-based device is operating around 920 nm with a maximumoutput power of 2 W. The tapered lasers also feature good beamquality with a low divergence angle, leading to a high brightnessof the laser.

ACKNOWLEDGMENT

The authors would like to thank S. Handel and A. Wolf forexcellent technical assistance in device processing.

REFERENCES

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[2] H. Wenzel, J. Fricke, A. Klehr, A. Knauer, and G. Erbert, “High-power980-nm DFB RW lasers with a narrow vertical far field,” IEEE Photon.Technol. Lett., vol. 18, no. 6, pp. 737–739, Mar. 2006.

[3] R. M. Lammert, J. E. Ungar, S. W. Oh, H. Qi, and J. S. Chen, “High-powerInGaAs–GaAs–AlGaAs distributed feedback lasers with nonabsorbingmirrors,” Electron. Lett., vol. 34, no. 9, pp. 886–888, 1998.

[4] J. N. Walpole, “Semiconductor amplifiers and lasers with tapered gainregions,” Opt. Quantum Electron., vol. 28, pp. 623–645, 1996.

[5] M. J. Chi, O. B. Jensen, J. Holm, C. Pedersen, P. E. Andersen, G. Erbert,B. Sumpf, and P. M. Petersen, “Tunable high-power narrow-linewidthsemiconductor laser based on an external-cavity tapered amplifier,” Opt.Exp., vol. 13, no. 26, pp. 10589–10596, 2005.

[6] S. Schwertfeger, A. Klehr, G. Erbert, and G. Trankle, “Compact hybridmaster oscillator power amplifier with 3.1-W CW output power at wave-lengths around 1061 nm,” IEEE Photon. Technol. Lett., vol. 16, no. 5,pp. 1268–1270, May 2004.

[7] M. Kamp, J. Hofmann, F. Schafer, M. Reinhard, M. Fischer, T. Bleuel, J.P. Reithmaier, and A. Forchel, “Lateral coupling—A material independentway to complex coupled DFB lasers,” Opt. Mater., vol. 17, pp. 19–25,2001.

[8] J. Seufert, M. Fischer, M. Legge, J. Koeth, R. Werner, M. Kamp, andA. Forchel, “DFB laser diodes in the wavelength range from 760 nm to2.5 µm,” Spectrochim. Acta A, vol. 60, pp. 3243–3247, 2004.

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Pia Weinmann (M’08) was born in Schweinfurt, Germany, in 1982. She re-ceived the Diploma in nanotechnology in 2007 from the Universitat Wurzburg,Wurzburg, Germany, where she is currently working toward the Ph.D. degreeat the Nanophotonics Group, Technische Physik.

She was an Intern at Infineon Technologies AG, Munich, Germany, whereshe was engaged in the field of plasmonic nanostructures. During an annual stayat the University of California (UC), Berkeley, she worked on semiconductorlasers. Her current research interests include the field of high-power lasers, witha focus on mode-controlled quantum dot lasers.

Ms. Weinmann is a member of the IEEE Laser and Electro-Optics Societyand the German Physical Society (DPG).

Christian Zimmermann was born in Erlenbach am Main, Germany, in 1984.He received the Diploma in technology of nanostructures in 2008 from the Uni-versitat Wurzburg, Wurzburg, Germany, where he is currently working towardthe Ph.D. degree.

He was first engaged in the field of high-power laser diodes, which con-centrates on fabrication of GaAs-based single-mode tapered laser diodes. He iscurrently with the Nanoplus Nanosystems & Technologies GmbH, Gerbrunn,Germany. His current research interests include the development of single-modelaser diodes for applications in metrology, sensing, and telecommunication.

Thomas Walter Schlereth was born in Wurzburg, Germany, in 1979. He re-ceived the M.Sc. degree in physics from Rutgers, The State University of NewJersey, New Brunswick, in 2004. He is currently working toward the Ph.D.degree at the Technische Physik, Universitat Wurzburg, Wurzburg, Germany.

He studied physics at the Universitat Wurzburg. He worked for one yearat the Laboratory for Surface Modification, Rutgers, The State University ofNew Jersey, where he was involved in the research on adsorption processes onmetal oxide surfaces. For one year, he was with OSRAM Opto SemiconductorsGmbH, Regensburg, Germany, where he worked on semiconductor device pro-cessing. His current research interests include the growth and characterizationof semiconductor quantum dots and their application in improved quantum dotlasers.

Christian Schneider was born in Wurzburg, Germany, in 1981. He received theDipl. Ing. in nanotechnology in 2007 from the Universitat Wurzburg, Wurzburg,where he is currently working toward the Ph.D. degree at the Technische Physik.

During his studies, he went for one year to Vancouver as an exchangestudent at the University of British Columbia, Vancouver, BC, Canada. Hewas with Technologico de Monterrey for three months and was involved inthe field of quantum dot morphology research. His current research interestsinclude the growth and characterization of semiconductor quantum dots andtheir application in single-quantum-dot-based devices.

Mr. Schneider is a member of the Deutsche Physikalische Gesellschaft(DPG).

Sven Hofling (M’04) was born in 1976. He received the diploma in appliedphysics from the University of Applied Science, Coburg, Germany, in 2002. Heis currently working toward the Ph.D. degree at the Technische Physik, Univer-sitat Wurzburg, Wurzburg, Germany.

For one year, he was a student at the Fraunhofer Institute of Applied Solid-State Physics, Freiburg, Germany. For six months, he was at LG Laser Tech-nologies GmbH, Kleinostheim, Germany, where he was mainly involved in thecharacterization of InGaN-based light emitting devices. His current research in-terests include the design, fabrication, and characterization of low-dimensionalelectronic and photonic nanostructures, including quantum wells and quantumdots, high-electron-mobility structures, high-quality-factor photonic cavities,and semiconductor lasers.

Mr. Hofling is a member of the IEEE Laser and Electro-Optics Society, theGerman Physical Society (DPG), and the European Physical Society (EPS).

Alfred Forchel (M’04) was born in Stuttgart, Germany, in 1952. He receivedthe Diploma and the Ph.D. degree in physics from Stuttgart University in 1978and 1982, respectively, and the Dr. Habil. degree in 1988.

From 1983 to 1989, he was In-Charge of the scientific and technical plan-ning of the Microstructure Laboratory, Stuttgart University. In 1990, he becamea Full Professor of physics at the Universitat Wurzburg, Wurzburg, Germany,where he is the Chair of the Technische Physik, and is also with the Microstruc-ture Laboratory. He has authored or coauthored more than 800 papers mainlyconcerning the optical and electronic properties of compound semiconductornanostructures. His current research interests include the development of novelIII–V semiconductor structures for optoelectronic devices and the investigationof low-dimensional photonic and electronic structures.

Martin Kamp was born in Lippstadt, Germany, in 1971. He received the M.A.degree from Stony Brook University, Stony Brook, NY, in 1995, and the Ph.D.degree in semiconductor lasers with lateral feedback structures in 2003.

He studied physics at the Universitat Wurzburg, Germany. He is currentlyincharge of the Nanophotonics Group of the Technische Physik in Wurzburg,Germany. He has authored or coauthored over 100 papers relating to semicon-ductor nanostructures and optoelectronic devices. His current research interestsinclude the development of high-brightness single-mode lasers, high speed di-rectly modulated lasers and photonic crystal structures for basic studies, andapplications in optoelectronic devices.

Mr. Kamp is a member of the German Physical Society (DPG).

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Mode-Controlled Tapered Lasers Based on Quantum Dots