Berg, Tommy Winther

Mørk, Jesper

Semiconductor quantum dot amplifiers are predicted to exhibit superior characteristics such as high gain, and output power and low noise. The analysis provides criteria and design guidelines for the realization of high quality amplifiers

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Downloaded from orbit.dtu.dk on: Dec 17, 2017Theoretical analysis of quantum dot amplifiers with high saturation power and lownoise figureBerg, Tommy Winther; Mørk, JesperPublished in:ECOC 2002 ProceedingsLink to article, DOI:10.1109/ECOC.2002.204457Publication date:2002Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Berg, T. W., & Mørk, J. (2002). Theoretical analysis of quantum dot amplifiers with high saturation power andlow noise figure. In ECOC 2002 Proceedings (Vol. 2, pp. 1-2). IEEE. DOI: 10.1109/ECOC.2002.204457All-optical Technology 4.3.6Theoretical analysis of quantum dot amplifiers with high saturation powerand low noise figureTommy W. Berg and Jesper MorkCOM, Technical University of Denmark, Build. 345V, DK-2800 Kgs. Lyngby, DenmarkPhone: +45 4525 6379, Fax: +45 4593 6581, E-mail: twb com.dtu.dkAbstract Semiconductor quantum dot amplifiers are predicted to exhibit superior characteristics such as highgain, and output power and low noise. The analysis provides criteria and design guidelines for the realization ofhigh quality amplifiers.IntroductionThe dominance of the Erbium Doped Fibre Amplifier(EDFA) in optical communication systems, owes totheir higher output power and lower noise figure (NF)compared to semiconductor amplifiers. Quantum Dot(QD) lasers have improved immensely the past fewyears and now exhibit properties like low thresholdcurrent density and high output power [1]. However,there has been only little work on QD amplifiers,perhaps owing to the small modal gain, requiring longwaveguides, which is traditionally known to lead tostrong Amplified Spontaneous Emission (ASE)saturation. We show here, that QD amplifiers mayoperate in an EDFA-like regime, with improvedcharacteristics compared to bulk and Quantum Well(QW) amplifiers, despite the longer waveguides. Dueto differences in fabrication method, materialcomposition etc., the QD parameters may varysignificantly, and we, therefore, perform an extensiveanalysis of key parameters, extending earlier work [2].The modelThe analysis is based on a detailed rate equationmodel, which previously has been shown to agreewell with measured gain dynamics [3]. Full account ofpropagation effects is taken, including ASE. Carriersare assumed to be injected directly into the WeftingLayer (WL) and then relax through the Excited State(ES) of the dots into the Ground State (GS) through acombination of phonon and Auger processes.Thermal carrier excitation and escape from the dotsensure that the system will reach the correct thermalequilibrium given sufficient time. Effects like spectralhole burning and Fermi blocking are inherent parts ofthe model.Basic device propertiesThe fundamental properties of QD amplifiers are firstillustrated by presenting results for a referencedevice, which will serve as the starting point for thefollowing analysis of the importance of different keyparameters. For this purpose, we have chosen anInAs/GaAs device, with device parameters found inthe literature. The gain spectrum under full inversion(proportional to the effective density of states of thedevice) is illustrated in Fig. 1(b), together with thegain spectrum for the current density of 1.75kA/cm2used for all results presented here. The structure isseen to exhibit an inhomogeneously broadened GStransition, an ES transition, and a WL in the shape ofa QW. Key parameters include: maximum modal gain22.5cmn' (GS), 36.5cmr1 (ES), and 117cm' (WL), lowwaveguide loss of 2cm' [1], phonon dominatedcarrier capture time of 1 ps, and a maximum Augerdominated intra dot relaxation time of 1 50fs [3]Im 50-.G 40 -coV- 30.M20-CZ 10.Ecn o10 -m0 8-0 6m 4a2)z 0 -I II II I II II w,--o 1 2 3 4 5 6 7 8Amplifier length [mm]( L-6mm/ 1f,- S' _ ES\,o' -~unsat302010 Co010-20a-3012010080 'E2060 ,40 CD20O1.1 Pht1.2 e ei311 Photorn energy [eViFIg.1: (a) Small signal gain and input and outputpower required to reduce the gain by 3dB. (b) Noisefigure for different device lengths and gain spectrumunder full inversion (dashed line) and for a currentdensity of 1. 75kA/cr2 (dash-dotted line).Fig. 1(a) illustrates the length dependence of thesmall signal gain, and saturated (3dB) output power,P3dB, of the reference device. As in bulk and QWdevices, the gain saturates due to ASE, but in thiscase at a much slower rate due to the small modalgain, which leads to a high maximum small signalgain of 47.5dB. The saturation length is on the orderof 6mm, which is much longer than for a bulk/QWdevice. Also the saturated output power is seen to bevery high, in the range of 20-25dBm for all lengths.The high P3dB is a result of the combination of lowmodal gain, resulting in a low stimulatedrecombination rate locally, and the comparatively fastcarrier capture and relaxation, resupplying the activeAuthorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on March 17,2010 at 09:47:16 EDT from IEEE Xplore.  Restrictions apply. All-optical Technology 4.3.6states. It is the combination of low modal gain andhigh P3dB, which causes the slow saturation from ASEand hence allows the large small signal gain.The NF of the device as function of photonenergy is shown in Fig. 1(b). At the centre of the GStransition (where the optical signal is injected), the NFis seen to increase from below 3dB (L=1 mm) to 4.4dB(L=6mm). The fact that the noise figure is below theordinary lower limit of 3dB simply reflects the smalltotal gain of the 1mm device [4]. The remarkably goodnoise properties result from the high GS inversion,even at low WL densities. As the ASE starts tosaturate the amplifier, the GS inversion decreases,resulting in a rapid increase of the noise figurebeyond the saturation length.Parameter variationLowering of the maximum GS modal gain to 14cmr(solid circles in Fig. 2) implies a slower increase of thesmall signal gain with length, and hence a longersaturation length. Secondly the device exhibits ahigher P3dB due to the smaller stimulatedrecombination rate locally. The higher P3dB in turnleads to higher tolerance towards ASE and hence ahigher maximum obtainable gain, on the order of50dB. Finally the smaller modal gain leads to a slightincrease of the NF compared with the referencedevice for lengths between 2 and 5mm, due to theincreased relative importance of the waveguidelosses. For devices longer than 5mm, the NF of thereference device increases rapidly due to ASEsaturation, which decreases the inversion, whereasthe device with small modal gain is far from ASEsaturation and thus exhibits a better NF in this region.The improved NF is, however, at the cost of a muchlower total gain and when compared at the samegain, the device with the highest modal gain exhibits asmaller NF in all cases.By increasing the waveguide losses to 10cm1(triangles) the P3dB is lowered by a few dBmcompared to the reference device, which leads to asmaller maximum gain of the device. However, theNF increases strongly because the modal gain is nowonly twice the waveguide losses. It is thus essential toreduce the waveguide losses in QD amplifiers withsmall modal gain in order to avoid an unacceptablyhigh NF.Realization of fast carrier capture and relaxationis also essential, as evidenced by the results foreither slow capture (open squares) or slow intra dotrelaxation (open circles) of 20ps. In both cases theeffect is seen to be a significant reduction of the P3dB,most severely for the latter of the two. Again the lowerP3dB leads to a shorter saturation length and hence alower maximum gain. It is thus important that QDinjection dynamics are fast enough to sustain the rateof stimulated emission from the dots, leading to a highP3dB-1 2 3 4 5 6 7 8Amplifier length [mm]Fig. 2: Small signal gain (a), saturated outputpower, P3dB, (b), and noise figure at the centre of theGS transition (C) versus amplifier length for 6 differentdevices.The energy separation between GS, ES, and WLgoverns the extent of QD inversion relative to the WLoccupation. The effect of reducing the levelseparation (marked with stars) is seen to be a sub-linear increase of the gain with length, because theGS population is strongly influenced by the rapid ASEsaturation of the upper levels exhibiting higher modalgain. This also leads to a smaller P3dB. In general, thelevel splitting of the dots should be as large aspossible and certainly larger than the thermal energyin order to avoid the limitation arising from the ASEsaturation of the upper levels.ConclusionWe have shown that QD amplifiers can be expectedto exhibit superior performance regarding small signalgain, saturated output power and noise figure.Furthermore, we have provided criteria and designguidelines for the realization of such devices.References1 F. Klopf et al., IEEE Photon. Technol. Lett., 13(2001), p. 7642 T.W. Berg, J. Mork, Optical Amplifiers and theirApplications, Vancouver (2002), accepted3 T.W. Berg, et al., IEEE Photon. Technol. Lett., 13(2001), p. 5414 E. Desurvire, Optical Fiber Technol., 5 (1999), p. 40Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on March 17,2010 at 09:47:16 EDT from IEEE Xplore.  Restrictions apply. 

Theoretical analysis of quantum dot amplifiers with high saturation power and low noise figure

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Theoretical analysis of quantum dot amplifiers with high saturation power and low noise figure

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