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. 106, 031114 (2015) and may be found at https://doi.org/10.1063/1.4906451.Passive mode-locking of two-section quantum-dot mode-locked lasers grown by metalorganic vapor phase epitaxy on InP is reported. 1250-μm long lasers exhibit a wide tuning range of 300 MHz around the fundamental mode-locking frequency of 33.48 GHz. The frequency tuning is achieved by varying the reverse bias of the saturable absorber from 0 to −2.2 V and the gain section current from 90 to 280 mA. 3 dB optical spectra width of 6–7 nm leads to ex-facet optical pulses with full-width half-maximum down to 3.7 ps. Single-section quantum-dot mode-locked lasers show 0.8 ps broad optical pulses after external fiber-based compression. Injection current tuning from 70 to 300 mA leads to 30 MHz frequency tuning.DFG, 43659573, SFB 787:  Halbleiter - Nanophotonik: Materialien, Modelle, BauelementeEC/FP7/EU/264687/Postgraduate Research on Photonics as an Enabling Technology/PROPHE

Arsenijević, Dejan

Bimberg, Dieter

Franke, D.

Kreissl, J.

Künzel, H.

Sadeev, Tagir

Applied Physics Letters

English

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Appl. Phys. Lett. 106, 031114 (2015); https://doi.org/10.1063/1.4906451 106, 031114© 2015 AIP Publishing LLC.1.55-μm mode-locked quantum-dot laserswith 300 MHz frequency tuning rangeCite as: Appl. Phys. Lett. 106, 031114 (2015); https://doi.org/10.1063/1.4906451Submitted: 06 November 2014 . Accepted: 12 January 2015 . Published Online: 22 January 2015T. Sadeev, D. Arsenijević, D. Franke, J. Kreissl, H. Künzel, and D. BimbergARTICLES YOU MAY BE INTERESTED INPassive mode-locking in 1.3 μm two-section InAs quantum dot lasersApplied Physics Letters 78, 2825 (2001); https://doi.org/10.1063/1.1371244High-power picosecond and femtosecond pulse generation from a two-section mode-lockedquantum-dot laserApplied Physics Letters 87, 081107 (2005); https://doi.org/10.1063/1.2032608Comparison of dynamic properties of InP/InAs quantum-dot and quantum-dash lasersApplied Physics Letters 109, 161104 (2016); https://doi.org/10.1063/1.49658461.55-lmmode-locked quantum-dot lasers with 300MHz frequencytuning rangeT. Sadeev,1,a) D. Arsenijevic´,1 D. Franke,2 J. Kreissl,2 H. K€unzel,2 and D. Bimberg1,b)1Institut f€ur Festk€orperphysik, Technische Universit€at Berlin, 10623 Berlin, Germany2Heinrich-Hertz-Institut, Einsteinufer 37, 10587 Berlin, Germany(Received 6 November 2014; accepted 12 January 2015; published online 22 January 2015)Passive mode-locking of two-section quantum-dot mode-locked lasers grown by metalorganicvapor phase epitaxy on InP is reported. 1250-lm long lasers exhibit a wide tuning range of300 MHz around the fundamental mode-locking frequency of 33.48 GHz. The frequency tuning isachieved by varying the reverse bias of the saturable absorber from 0 to 2.2 V and the gain sec-tion current from 90 to 280 mA. 3 dB optical spectra width of 6–7 nm leads to ex-facet opticalpulses with full-width half-maximum down to 3.7 ps. Single-section quantum-dot mode-lockedlasers show 0.8 ps broad optical pulses after external fiber-based compression. Injection currenttuning from 70 to 300 mA leads to 30 MHz frequency tuning.VC 2015 AIP Publishing LLC.[http://dx.doi.org/10.1063/1.4906451]Passively mode-locked semiconductor lasers (MLLs)are able to emit as low as fs-short optical pulses at frequen-cies up to tens of GHz without any external frequencysource.1 MLLs are of largest importance, e.g., for ultra-highdata rate optical transmission, signal processing, clock sour-ces, and biomedicine.1 Significant improvements of the per-formance of MLL as well as continuous-wave (CW) laserparameters have been achieved in last decades, thanks toimplementing zero-dimensional (quantum dot) structuresinto the active layer.2,3 Their inherent properties allow toproduce lasers with increased temperature stability,4 lowthreshold current,5 fast carrier dynamics,6 inhomogeneouslybroadened spectrum, and low amplified spontaneous emis-sion (ASE).7 Most advantages of QD MLL have been dem-onstrated for GaAs-based ones, operating at 1310 nmwavelength, targeting short-range telecom applications,where optical fiber exhibits zero dispersion.8 Long-haul opti-cal transmission systems operate in the 1550 nm range,where fiber losses are minimal. At this wavelength matureand cost effective InP based growth technology is common,but the brake-through results of QD MLLs are much scarceras compared to their 1310 nm counterparts. Presentlyreported MOVPE (metal organic vapour phase epitaxy)grown monolithic two-section QD MLLs operate at 10.5 GHzand emit strongly chirped pulses several tens of ps broad.9Quantum dashes (QDashes), elongated quantum dots, arebelieved to have properties close to QDs and have beeninvestigated intensively in several groups. Reports on mode-locking observed in QDash single-section lasers show bothex facet sub picosecond10 and ps range pulse emission aftercompression with low timing jitter.11,12 In Ref. 13 chemicalbeam epitaxy grown single-section QD laser demonstratesex facet 295 fs pulses at 50 GHz. The mechanism of pulsegeneration in such single-section lasers is not well under-stood. Thus, proper device engineering as a solution toobtain direct ex-facet (i.e., without compression techniques)fs-pulse emission is not yet available, constraining furthersystem implementation of such lasers. QDash two-sectionlasers, however, demonstrate ex facet ps-pulse emission andrepetition frequency up to 20 GHz (Ref. 14) with better con-trol of output pulse train characteristics. The present resultsobtained for InP based QDash MLLs as well as for GaAsbased 1310 nm QD MLLs confirm the advantages of the 3-Dcarrier confinement approach, but also reveal that QD basedfull advantages have not been yet realized at 1550 nm.In this paper, we demonstrate ex-facet ps-range opticalpulses at ultra-high repetition rate of InP/InAs based two-section QD MLL. Furthermore, we report on single-sectionQD MLLs (or SSMLL) made of the same wafer material andlocated on the same laser bar as two-section QD MLLs. Wecompare their mode-locking properties and also discuss themechanism of pulse propagation.The laser structures are grown by metalorganic vaporphase epitaxy (MOVPE) on n-type (001) InP substrate. Theactive region consists of 7 stacked QD layers in anIn0.78Ga0.22As0.47P0.53 matrix. This structure is enclosed byan In0.82Ga0.18As0.40P0.60 waveguide. Finally, single-modelaser buried heterostructures are formed by deep etchingthrough the active region and regrowth of p/n-blocking andcontact layers. Details of material growth, processing, andresults of material characterisation can be found in Ref. 15.Single- and two-section devices are processed from thiswafer material. Their length is 1.25 mm and the ridge widthis 1 lm in both cases, no coating is applied to the facets. Forthe two-section devices, the ratio between gain section (GS)and absorber section (AS) is 5.6:1 (15% for the absorber).For electrical isolation of the GS and AS, sections are sepa-rated by a 20 lm gap without impacting the optical wave-guide properties. Single-section lasers are driven only byelectrical pumping of the GS. In the two-section lasers, thesections are operated independently: reverse bias is appliedto AS and forward bias to the GS.Light-current curves measured at room temperature areshown in Fig. 1(a) both for single- and two-section lasers.The threshold current density is 1.57 kA/cm2, the differentiala)Electronic mail: tagir@mailbox.tu-berlin.deb)Also at King Abdulaziz University, 22254 Jeddah, Saudi Arabia.0003-6951/2015/106(3)/031114/4/$30.00 VC 2015 AIP Publishing LLC106, 031114-1APPLIED PHYSICS LETTERS 106, 031114 (2015)quantum efficiency is 0.32, maximum output power is34 mW, the series resistance is 3.7X, and turn-on voltage isequal to 0.78 V for single-section devices. For two-sectionlasers, an increased threshold current is observed for ASopen-circuit and increasing reverse bias due to increasingabsorption. An optical spectrum of a single-section devicemeasured at 270 mA (13Ithr) shown in Fig. 1(b), exhibits apeak wavelength at 1533 nm, and a 3 dB 6–7 nm spectralwidth. The spectrum is broad and uniform due to the inho-mogeneous broadening of the QDs.Passive mode-locking in two-section lasers is first ana-lysed by recording radiofrequency (RF) spectra for eachpair of Uabs/Iinj from 2.2 V to 0 V and from 90 mA to280 mA, resulting in a RF vs. Uabs/Iinj map (shown inFig. 2(a)), considering only pure mode-locking operation,i.e., RF signal-to-noise ratio (SNR) larger than 30 dB in thewhole range. The region of mode-locking (ML) is limited by(a) Q-switching (shaded area below left), characterized bythe presence of low-frequency oscillations (Fig. 2(a), inset)and their beating products; and (b) CW-lasing, as a resultof weak absorption at low Uabs (0 V to 0.2 V) and high cur-rent (Iinj> 190 mA). 300 MHz is the tuning range of repeti-tion frequency under ML conditions. The repetitionfrequency decreases with increasing current Iinj at fixedabsorber voltage Uabs. The thermal expansion of materialand reduced refractive index nrefr due to carrier escape(caused by temperature induced carrier heating and band gapshrinkage) are the reasons. Second, an increase of reverseUabs at fixed Iinj leads to an increase of repetition frequency.We interpret this increase by enhanced absorption leading toa reduced number of photons in the cavity. This in-turnreduces the number of stimulated transitions, meanwhile theexciton population increases due to the pump, thus nrefrdecreases.16 The reduction of nrefr leads to the increase ofrepetition frequency.Single-section lasers also exhibit mode-locking. Theirrepetition frequency depends on current. The frequency gen-erally increases with current (Fig. 2(b)), corresponding to areduced nrefr due to material dispersion17 of the waveguide,induced by a red shift of optical spectra by 10 nm inthe 100–300 mA range. The waveguide thermal expansion(cavity length increases) is the reason for RF decrease at cur-rents beyond 250 mA. The flat region in the middle and theoscillations observed are presently under more detailedinvestigation. We point out that the repetition frequency ofSSMLL is tunable by 30 MHz, which is ten times lowerthan that of the two-section laser mentioned above.Optical pulse emission of both two- and single-sectionMLLs is now examined using second-harmonic generationbased autocorrelation techniques. For two-section MLLautocorrelation (AC) traces are recorded at operating param-eters with stable mode-locking (RF SNR> 30 dB), and with-out pulse compression (i.e., ex-facet measurement). PulseFWHM map after deconvolution assuming Gaussian shapepulses is shown in Fig. 3(a). An AC-trace for one particularpair of operating parameters Iinj¼ 200 mA, Uabs¼1.0 V isshown in the inset of Fig. 3(a). We point to the minimummeasured pulse FWHM of 3.7 ps. The FWHM increaseswith increased current at fixed Uabs, because at higher cur-rent the absorber bleaches faster as pulses carry more power,also gain is enhanced leading to the broadening of the netgain window.18 On the other hand, at higher Uabs and con-stant Iinj, pulses get shorter due to reduced absorber recoverytime as carriers experience a faster escape from the QDs tothe barrier.19,20AC measurements are also carried out for SSMLL.Fig. 3(b) shows the measured pulse width after 150 m ofstandard single-mode fiber used for compression.21 Weobserve a Lorentzian shape (pulses in Fig. 3(b), inset) withthe lowest pulse FWHM  0.8 ps (Fig. 3(b)). At low current,FIG. 1. Light-current characteristics ofsingle-section and two-section devices(a); (b) optical spectrum of a single-section laser at 270 mA.FIG. 2. (a) Repetition frequency colourmap of two-section MLL, inset—Q-switched mode-locking regime atIinj¼ 220 mA, Uabs¼1.9 V. (b)Single-section MLL repetition fre-quency as a function of injectioncurrent.031114-2 Sadeev et al. Appl. Phys. Lett. 106, 031114 (2015)the pulses are broad, because along with lower opticalFWHM, the efficiency of v(3) is not enough to set up phasecorrelation between optical modes.22 With increased current(80–280 mA) more optical modes are excited and intra-cavity nonlinearities are strong enough to lock their phases,leading to short pulse formation, thus a flat region of thecharacteristic in Fig. 3(b) is built-up. At currents larger than280 mA pulse FWHM increases again, which is probablydue to the increased number of excited modes inducingunsynchronized phase interference, but, in particular, a largera-factor and hence increased chirp.23,24We propose the following simplified model for pulsepropagation dynamics in SSMLL: during propagation alongthe cavity two-photon absorption (TPA) creates non-uniformbut periodic exciton distributions in the cavity. Since highphoton densities within the pulse are reduced via TPA, thephoton density is increased by stimulated emission fromexcitons which are electrically pumped and whose distribu-tion is uniform.25 Non-uniform distribution of carrier densityand third-order nonlinearities are responsible for a complexdependence of refractive index on current, resulting in pulsephase modulation and large chirp which are actuallyobserved in SSMLL.21Timing jitter is characterized through the RF-linewidth26and along with high repetition rate, low timing jitter of thepulse train is important for any application. A huge differ-ence between RF-linewidths in two- and single-sectionMLLs has been observed. For two-section lasers, it is in theorder of 2–3 MHz, whereas for single-section MLLs it doesnot exceed 300 kHz. The reason for the one order of magni-tude difference is that the absorber section induces additionallosses, thus reducing the ratio between stimulated and spon-taneous emission, resulting in an increase of optical phasenoise. This in turn broadens the RF-line and increases thetiming jitter.21,26To conclude, we demonstrate monolithic two-sectiontrue QD MLLs at 1.55 lm with better performance than yetreported. The lasers exhibit a wide range of operating param-eters with stable mode locking. 300 MHz of the repetitionfrequency tuning for RF SNR> 30 dB is observed.Autocorrelation shows ex-facet Gaussian shape optical pulseemission. Lowest optical pulse FWHM is 3.7 ps without anycompression at 33.5 GHz repetition rate. We also investi-gated the ML properties of single-section lasers made of thesame wafer material. The repetition frequency of single-section mode-locked laser is found to be less tuneable,30 MHz around 33.68 GHz. The FWHM measured afterfibre compression demonstrates two local minima at130–150 mA and 260–270 mA of approximately 0.8 ps. Wediscussed the variation of the pulse FWHM and repetitionfrequency under varying operating parameters in both typesof devices. The control of pulse width and repetition fre-quency is more efficient in two-section lasers.This work has been supported by CollaborativeResearch Centre 787 (SFB 787) of German ResearchFoundation (Deutsche Forschungsgemeinschaft-DFG) andSeventh Framework Programme through PROPHET InitialTraining Network.1D. Bimberg, J. Phys. D: Appl. Phys. 38, 2055 (2005).2D. Bimberg, Electron. Lett. 44, 168 (2008).3C. Gilfert, V. Ivanov, N. Oehl, M. Yacob, and J. P. Reithmaier, Appl.Phys. Lett. 98, 201102 (2011).4O. B. Shchekin and D. G. Deppe, Appl. Phys. Lett. 80, 3277 (2002).5R. L. Sellin, C. Ribbat, M. Grundmann, N. N. Ledentsov, and D. Bimberg,Appl. Phys. Lett. 78, 1207 (2001).6O. Karni, K. J. Kuchar, A. Capua, V. Mikhelashvili, G. SeRk, J. Misiewicz,V. Ivanov, J. P. Reithmaier, and G. Eisenstein, Appl. Phys. Lett. 104,121104 (2014).7J. Gomis-Bresco, S. Dommers-V€olkel, O. Sch€ops, Y. Kaptan, O.Dyatlova, D. Bimberg, and U. Woggon, Appl. Phys. Lett. 97, 251106(2010).8M. Kuntz, G. Fiol, M. L€ammlin, D. Bimberg, M. G. Thompson, K. T. Tan,C. Marinelli, R. V. Penty, I. H. White, V. M. Ustinov, A. E. Zhukov, Y.M. Shernyakov, and A. R. Kovsh, Appl. Phys. Lett. 85, 843 (2004).9M. J. R. Heck, A. Renault, E. A. J. M. Bente, Y.-S. Oei, M. K. Smit, K. S.E. Eikema, W. Ubachs, S. Anantathanasarn, and R. Notzel, IEEE J. Sel.Top. Quantum Electron. 15, 634 (2009).10G.-H. Duan, A. Shen, A. Akrout, F. Van Dijk, F. Lelarge, F. Pommereau,O. LeGouezigou, J.-G. Provost, H. Gariah, F. Blache, F. Mallecot, K.Merghem, A. Martinez, and A. Ramdane, Bell Labs Tech. J. 14, 63(2009).11K. Klaime, C. Calo, R. Piron, C. Paranthoen, D. Thiam, T. Batte, O.Dehaese, J. Le Pouliquen, S. Loualiche, A. Le Corre, K. Merghem, A.Martinez, and A. Ramdane, Opt. Express 21, 29000 (2013).12K. Merghem, C. Calo, R. Rosales, X. Lafosse, G. Aubin, A. Martinez, F.Lelarge, and A. Ramdane, IEEE J. Quantum Electron. 50, 275 (2014).13Z. G. Lu, J. R. Liu, P. J. Poole, Z. J. Jiao, P. J. Barrios, D. Poitras, J.Caballero, and X. P. Zhang, Opt. Commun. 284, 2323 (2011).14E. Sooudi, G. Huyet, J. G. McInerney, F. Lelarge, K. Merghem, R.Rosales, A. Martinez, A. Ramdane, and S. P. Hegarty, IEEE PhotonicsTechnol. Lett. 23, 1544 (2011).15D. Franke, J. Kreissl, W. Rehbein, F. Wenning, H. Kuenzel, U. W. Pohl,and D. Bimberg, Appl. Phys. Express 4, 014101 (2011).16B. R. Bennett, R. A. Soref, and J. del Alamo, IEEE J. Quantum Electron.26, 113 (1990).17P. Runge, R. Elschner, and K. Petermann, IEEE J. Quantum Electron. 46,644 (2010).18H. Haus, IEEE J. Sel. Top. Quantum Electron. 6, 1173 (2000).19E. A. Viktorov, T. Erneux, P. Mandel, T. Piwonski, G. Madden, J. Pulka,G. Huyet, and J. Houlihan, Appl. Phys. Lett. 94, 263502 (2009).FIG. 3. (a) Pulse width colour map oftwo-section laser (a), in the inset AC-trace at Iinj¼ 200 mA, Uabs¼1.0 V.(b) Pulse width with AC-trace (inset)after 150 m of single-mode fiber as thefunctions of current measured forsingle-section device.031114-3 Sadeev et al. Appl. Phys. Lett. 106, 031114 (2015)20D. B. Malins, A. Gomez-Iglesias, S. J. White, W. Sibbett, A. Miller, andE. U. Rafailov, Appl. Phys. Lett. 89, 171111 (2006).21R. Rosales, K. Merghem, A. Martinez, A. Akrout, J.-P. Tourrenc, A.Accard, F. Lelarge, and A. Ramdane, IEEE J. Sel. Top. Quantum Electron.17, 1292 (2011).22R. Rosales, S. G. Murdoch, R. T. Watts, K. Merghem, A. Martinez, F.Lelarge, A. Accard, L. P. Barry, and A. Ramdane, Opt. Express 20, 8649(2012).23M. Osinski and J. Buus, IEEE J. Quantum Electron. 23, 9 (1987).24S. Schneider, P. Borri, W. Langbein, U. Woggon, R. L. Sellin, D.Ouyang, and D. Bimberg, IEEE J. Quantum Electron. 40, 1423(2004).25H. Ju, A. V. Uskov, R. N€otzel, Z. Li, J. M. Vazquez, D. Lenstra, G. 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1.55-μm mode-locked quantum-dot lasers with 300 MHz frequency tuning range

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1.55-μm mode-locked quantum-dot lasers with 300 MHz frequency tuning range

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