This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Lett. 97, 151101 (2010) and may be found at https://doi.org/10.1063/1.3499361.We present multioxide-aperture 980 nm-range vertical cavity surface emitting lasers (VCSELs) with highly temperature stable modulation characteristics operating error-free at 25 Gbit/s at 25 and 85°C. We perform small signal modulation experiments and extract the fundamental physical parameters including relaxation resonance frequency, damping factor, parasitic cut-off frequency, -factor, and -factor, leading to identification of thermal processes and damping as the main factors that presently limit high speed device operation. We obtain very temperature-insensitive bandwidths around 13–15 GHz. Presented results clearly demonstrate the suitability of our VCSELs for practical and reliable optical data transmission systems.EC/FP7/224211/EU/VISIT - Vertically Integrated Systems for Information Transfer/VISITDFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, Bauelement

Bimberg, Dieter

Blokhin, S. A.

Hofmann, W.

Lott, James A.

Moser, Philip

Mutig, A.

Nadtochiy, A. M.

Payusov, A.

Wolf, P.

Applied Physics Letters

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Appl. Phys. Lett. 97, 151101 (2010); https://doi.org/10.1063/1.3499361 97, 151101© 2010 American Institute of Physics.Highly temperature-stable modulationcharacteristics of multioxide-aperturehigh-speed 980 nm vertical cavity surfaceemitting lasersCite as: Appl. Phys. Lett. 97, 151101 (2010); https://doi.org/10.1063/1.3499361Submitted: 09 June 2010 • Accepted: 20 September 2010 • Published Online: 11 October 2010A. Mutig, J. A. Lott, S. A. Blokhin, et al.ARTICLES YOU MAY BE INTERESTED INThermal analysis of high-bandwidth and energy-efficient 980 nm VCSELs with optimizedquantum well gain peak-to-cavity resonance wavelength offsetApplied Physics Letters 111, 243508 (2017); https://doi.org/10.1063/1.500328885 °C error-free operation at 38 Gb/s of oxide-confined 980-nm vertical-cavity surface-emitting lasersApplied Physics Letters 100, 081103 (2012); https://doi.org/10.1063/1.368804081 fJ/bit energy-to-data ratio of 850 nm vertical-cavity surface-emitting lasers for opticalinterconnectsApplied Physics Letters 98, 231106 (2011); https://doi.org/10.1063/1.3597799Highly temperature-stable modulation characteristics of multioxide-aperture high-speed 980 nm vertical cavity surface emitting lasersA. Mutig,1,a J. A. Lott,2 S. A. Blokhin,1,b P. Wolf,1 P. Moser,1 W. Hofmann,1A. M. Nadtochiy,1,b A. Payusov,1,b and D. Bimberg11Institut für Festkörperphysik und Zentrum für Nanophotonik, Technische Universität Berlin,Hardenbergstrasse 36, 10623 Berlin, Germany2VI Systems GmbH, Hardenbergstrasse 7, 10623 Berlin, GermanyReceived 9 June 2010; accepted 20 September 2010; published online 11 October 2010We present multioxide-aperture 980 nm-range vertical cavity surface emitting lasers VCSELs withhighly temperature stable modulation characteristics operating error-free at 25 Gbit/s at 25 and85 °C. We perform small signal modulation experiments and extract the fundamental physicalparameters including relaxation resonance frequency, damping factor, parasitic cut-off frequency,D-factor, and K-factor, leading to identification of thermal processes and damping as the mainfactors that presently limit high speed device operation. We obtain very temperature-insensitivebandwidths around 13–15 GHz. Presented results clearly demonstrate the suitability of our VCSELsfor practical and reliable optical data transmission systems. © 2010 American Institute of Physics.doi:10.1063/1.3499361Human society’s constantly increasing demand for moreinformation, the on-going scaling of silicon-based integratedcircuits to ever smaller dimensions, and the intrinsic limita-tions of copper-based electrical data links have led to ourcurrent need for lower cost, power efficient, readily manu-facturable, reliable photonic devices. The unstoppable andprogressive penetration of optical communication links intotraditional copper interconnect markets greatly expands theapplications of vertical cavity surface emitting lasers VC-SELs for next-generations of board-to-board, module-to-module, and chip-to-chip interconnects.1,2 The continueddown-sizing of integrated photonics systems and quest forbroader bandwidths leads naturally to interest in very short-reach VSR data transmission systems residing, e.g., amongmicroprocessors, where the high temperature stability of thelasers is indispensable. In the recent years, great progresswas achieved in increasing both the data rate and the tem-perature stability of VCSELs. At room temperature bit ratesof 32 Ref. 3 to 38 Gbit/s Refs. 4 and 5 850 nm, 35Gbit/s 980 nm,6 and 40 Gbit/s 1100 nm7 have been dem-onstrated. While 850 nm is the current standard wavelengthfor local and storage area networks and other short-reachoptical link systems, potential competitive standards at 980and 1100 nm have many critical advantages for VSR systemsincluding: smaller operational voltages, deeper potentialwells, and transparency of the GaAs substrate, which is im-portant for bottom-emitting VCSELs. Hereby the wavelengthof 980 nm is preferable for Si-based photodetectors, sincethe absorption coefficient of Si is much larger at 980 nm ascompared to longer wavelengths.Because the free carrier absorption coefficient increaseswith the wavelength, tolerances for adjustment of dopinglevels in the epitaxial structure for 980 nm are more strictlyas compared to 850 nm. Additionally the application ofhighly strained materials for the active region and thus theproper strain handling creates new challenges, which are nor-mally absent in 850 nm devices. While at elevated tempera-tures of 85 °C or even above bit rates of 25 Gbit/s have beendemonstrated for VCSELs operating at 850 Ref. 3 and1100 nm,7 however, at 980 nm the highest bit rate at 85 °Cwas still limited to 20 Gbit/s,8 where VCSELs with InGaAslayers grown in the submonolayer growth mode by molecu-lar beam epitaxy were applied. Here we indeed present 980nm-range oxide-confined VCSELs operating error-free atdata bit rates of up to 25 Gbit/s at temperatures as high as85 °C, which were grown by metal-organic chemical-vapordeposition MOCVD with conventional multiple quantumwells QWs, thus applying the mature and well approvedlarge-scale mass production techniques. To facilitate ouranalysis of the limiting physical processes inside the VC-SELs we perform small and large signal modulation experi-ments at various temperatures enabling us to determine thedominant factors that control the VCSEL’s speed, notably atelevated temperatures.The VCSEL structure is grown on an 001-oriented 2°-off n-doped GaAs-substrate using MOCVD. DopedAl0.12Ga0.88As /Al.90Ga0.10As distributed Bragg reflectorsDBRs with 20 nm thick linear graded interfaces with 24/37periods for the top/bottom mirror are employed. The activeregion contains five 4.2 nm thick compressively strainedIn0.21Ga0.79As QWs with 6 nm thick GaAs0.88P0.12 tensilestrained barrier layers. The 4.7% smaller lattice constant ofGaAs0.88P0.12 as compared to GaAs partially compensates thelarger lattice constant of In0.21Ga0.79As and the correspond-ing lattice constant mismatch of 1.55%. Carefully designingof the layer thicknesses leaves both the local and the globalstrain well below values critical for the high quality growth.To improve the temperature stability of the gain and differ-ential gain optimized detuning of 15 nm between the QWphotoluminescence peak and the cavity resonance was ap-plied, helping to compensate the gain decrease at elevatedtemperatures and leading to the very temperature insensitiveaElectronic mail: mutig@sol.physik.tu-berlin.de.bOn leave from the St. Petersburg Physical Technological Centre for Re-search and Education RAS, Khlopina 8/3 and Ioffe Physical-TechnicalInstitute RAS, Polytekhnicheskaya 26, Saint-Petersburg, 194021 RussianFederation.APPLIED PHYSICS LETTERS 97, 151101 20100003-6951/2010/9715/151101/3/$30.00 © 2010 American Institute of Physics97, 151101-1threshold current behavior. To facilitate electrical and opticalconfinement and to reduce electrical parasitics two 30 nmthick Al0.98Ga0.02As oxide aperture layers3 with tapered fin-gers are formed by selective wet oxidation. The VCSELs areplanarized with 10 m thick bisbenzocyclobutene to re-duce the parasitic capacitances. Ground-source-ground metalcontact pads are evaporated for ease of on-wafer high-frequency probe characterization.We show in Fig. 1 the measured output power-current-voltage characteristics for a VCSEL with a 10 m diameteroxide aperture and the corresponding values for differentialefficiency and threshold current. The maximum output poweris 4.3 mW at 20 °C and decreases little to 2.6 mW at 85 °C.The maximum differential efficiency decreases by only 18%from 0.34 W /A at 20 °C to 0.28 W /A at 85 °C. Theapplied redshifted detuning of nominally 15 nm between theQW photoluminescence peak wavelength 965 nm andthe cavity resonance wavelength 980 nm resulted in aminimum threshold current appearing at a temperaturearound 50 °C as seen in Fig. 1b. The threshold current of584 A at 20 °C, decreasing to 504 A at 50 °C, andagain increasing to 579 A at 85 °C. The variation in thethreshold current across the whole range from 20 to 85 °C isless than 16% of the minimum value. The detuning effectdecisively contributes to the improved temperature stabilityof the device by keeping the threshold current density to aminimum. The small threshold current density helps to main-tain a large differential gain9 and thus high speed VCSELoperation. The differential resistance of the VCSEL is45  at 12 mA, practically perfectly matched to the stan-dard impedance of 50 . The thermal resistance is2.2 K /mW, which can be further improved in the futureby using, e.g., binary alloys in the bottom DBR.To determine the factors that control high speed opera-tion, we measure the small signal modulation response S21and input impedance S11 using a network analyzer and acalibrated photodetector at three temperatures of 20, 50, and85 °C. These data are used to extract the relaxation reso-nance frequency fr, damping factor , D-factor, K-factor,parasitic cut-off frequency fp, and –3 dB frequency f−3 dB.These key physical parameters are commonly used to char-acterize the high speed properties of VCSELs and are basedon the standard laser diode rate equation formalism.9Small signal modulation response for different currentsat 20 and 85 °C with corresponding curve fits are shown inFig. 2. All fits show excellent agreement with the measureddata. The maximum parasitic cut-off frequency extractedfrom the measured S11 data is at all temperatures larger than22 GHz. In Fig. 3 we present the extracted values of therelaxation resonance frequency fr and the –3 dB-frequencyf−3 dB as a function of the forward bias current for 20, 50,and 85 °C. The maximum bandwidth is 15.3 GHz at 20 °Cand decrease to 14.5 GHz at 50 °C. At 85 °C the maximumbandwidth is still 13.2 GHz, which is only by 2 GHzlower than the value at 20 °C. The overall change in themaximum bandwidth within the investigated temperaturerange is only 2 GHz, which is a result of the gain-cavitydetuning and the highly temperature stable multi-QW activeregion. The maximum relaxation resonance frequency is12.9 GHz at 20 °C and decreases to 9.8 GHz at 85 °C.This decrease corresponds to higher temperatures inside theVCSEL at higher ambient temperatures internal temperatureat 12 mA at 85 °C is 140 °C, leading to saturation of therelaxation resonance frequency at lower currents.The K-factor that characterizes the damping limitationcan be extracted by fitting a linear function to a plot of thedamping factor as a function of the squared relaxation reso-nance frequency,9 as shown in Fig. 3b. The K-factor is inthe range of 0.4 ns between 20 and 85 °C and is temperatureinsensitive. One of the main reasons for the relatively largeK-factor is a relatively high mirror reflectivity, since theK-factor increases with the photon life time.9 On the otherhand, the high mirror reflectivity reduces the mirror lossesand thus the threshold carrier density, which increases thedifferential gain and thus the relaxation resonance frequency.Since the relaxation resonance frequency is lower than thebandwidth Fig. 3a, especially at 85 °C, a proper compro-mise between the acceptable high damping and the reason-FIG. 1. Color online Output power-current-voltage L-I-V characteristicsof a 980 nm double oxide-aperture VCSEL at temperatures from 20 to85 °C a, and the corresponding extracted values for the maximum differ-ential efficiency and threshold current b.FIG. 2. Color online Magnitude of the small signal modulation response2-port scattering parameter S21 for different applied forward bias currentsand the corresponding curve-fits at 20 °C a, and at 85 °C b.151101-2 Mutig et al. Appl. Phys. Lett. 97, 151101 2010able high relaxation resonance frequency has been found.The extracted values of the K-factor correspond to a maxi-mum damping limited bandwidth of 21–23 GHz, clearlydemonstrating its noticeable limiting effect.The important D-factor can be obtained by fitting a lin-ear function to the extracted squared relaxation resonancefrequencies as a function of the current above threshold.9 TheD-factor is practically constant over the whole temperaturerange, and is 3.5 GHz /sqrtmA. The high temperaturestability of the D-factor is caused by the gain—cavity detun-ing, which prevents the decrease in the differential gain athigher temperatures.Next, we performed data transmission experiments usinga nonreturn to zero NRZ data format with a 27−1 pseu-dorandom binary sequence PRBS in a standard back-to-back measurement configuration BTB 3 m fiber. InFig. 4 we show the measured bit error rates BER at the databit rate of 25 Gbit/s and temperatures of 25 and 85 °C. Thedrive current and the peak-to-peak modulation voltage Vp-pare held constant in both data transmission experiments at 12mA and 0.8 V, respectively. This makes the expensive driverelectronics redundant, reducing the application costs. At bothtemperatures error-free data transmission with BERs smallerthan 10−12 is demonstrated. Received optical power for thelowest BER is at both temperatures smaller than –1 dBm,well below 1 mW. The power penalty for the lowest BER forthe higher temperature of 85 °C in respect to 25 °C issmaller than 0.8 dBm and is caused by the reduction of theoutput power at higher temperatures Fig. 1a. These resultsclearly demonstrate the maturity of the presented devices forfuture high speed VSR interconnects, where high tempera-ture stability is indispensable.To summarize and conclude, 980 nm VCSELs withhighly temperature stable modulation characteristics are pre-sented. We demonstrate maximum bandwidths around 13–15GHz that are very temperature insensitive. By using a stan-dard small signal analysis, we identify a saturation of therelaxation resonance frequency, due to the limits of heat dis-sipation in our current VCSELs, and damping as the stron-gest factors that limit high speed operation. We measureerror-free data transmission at data bit rates of up to 25Gbit/s at temperatures of up to 85 °C, and thus demonstratethe suitability of our VCSELs for practical high speed andhigh temperature stable short-reach optical link applications.We acknowledge funding by the EU FP7 program underAgreement No. 224211, the Deutsche Forschungsgemein-schaft Sfb 787, the State of Berlin 100100 Optics andgrants of St. Petersburg Government and Ministry of Educa-tion and Science of Russia.1D. Collins, N. Li, D. Kuchta, F. Doany, C. Schow, C. Helms, and L. Yang,Proc. SPIE 6908–09 2008.2F. E. Doany, L. Schares, C. L. Schow, C. Schuster, D. M. Kuchta, and P.K. Pepeljugoski, Proceedings of the IEEE Optical Fiber CommunicationConference and Exposition, Anaheim, CA, USA, 2006.3P. Westbergh, J. S. Gustavsson, Å. Haglund, A. Larsson, F. Hopfer, D.Bimberg, and A. Joel, Proceedings of CLEO/IQEC, IEEE Standards Of-fice, 2009.4S. A. Blokhin, J. A. Lott, A. Mutig, G. Fiol, N. N. Ledentsov, M. V.Maximiv, A. M. Nadtochiy, V. A. Shchukin, and D. Bimberg, Electron.Lett. 45, 501 2009.5A. Mutig, S. A. Blokhin, A. M. Nadtochiy, G. Fiol, J. A. Lott, V. A.Shchukin, N. N. Ledentsov, and D. Bimberg, Appl. Phys. Lett. 95, 1311012009.6Y.-C. Chang and L. A. Coldren, IEEE J. Sel. Top. Quantum Electron. 15,704 2009.7N. Suzuki, T. Anan, H. Hatakeyama, K. Fukatsu, K. Tokutome, T. Aka-gawa, and M. Tsuji, IEICE Transactions on Electronics E92-C, 942,2009.8A. Mutig, G. Fiol, K. Pötschke, P. Moser, D. Arsenijevic, V. A. Shchukin,N. N. Ledentsov, S. S. Mikhrin, I. L. Krestnikov, D. A. Livshits, A. R.Kovsh, F. Hopfer, and D. Bimberg, IEEE J. Sel. Top. Quantum Electron.15, 679 2009.9L. A. Coldren and S. W. Corzine, Diode Lasers and Photonic IntegratedCircuits Wiley, New York, 1995.FIG. 3. Color online Relaxation resonance frequency fr and –3 dB fre-quency f−3 dB vs VCSEL forward bias current a, and damping factor  vsfr-squared with corresponding linear fits b, both at temperatures of 20, 50,and 85 °C.FIG. 4. Color online Bit error rates at 25 Gbit/s at 25 and 85 °C at thecorresponding VCSEL drive current of 12 mA and VP-P of 0.8 V for a BTBmeasurement configuration with a 27−1 PRBS in the conventional NRZexcitation scheme.151101-3 Mutig et al. Appl. Phys. Lett. 97, 151101 2010

Highly temperature-stable modulation characteristics of multioxide-aperture high-speed 980 nm vertical cavity surface emitting lasers

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Highly temperature-stable modulation characteristics of multioxide-aperture high-speed 980 nm vertical cavity surface emitting lasers

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