Engineering Materials For furthervolumes: http://www.springer.com/series/4288 Ali Rostami (cid:2) Hamed Baghban (cid:2) Reza Maram Nanostructure Semiconductor Optical Amplifiers Building Blocks for All-Optical Processing 123 Prof.AliRostami Reza Maram NanophotonicsSchool ofEngineering- Faculty ofElectrical and Emerging Technology Computer Engineering Universityof Tabriz Universityof Tabriz Bolv.29, Bahman Bolv.29, Bahman 51666Tabriz,Iran 51666Tabriz,Iran e-mail: [email protected] e-mail: [email protected] Hamed Baghban Faculty ofElectrical and Computer Engineering Universityof Tabriz Emam Khomeini Bolv.29,Bahman 51666Tabriz,Iran e-mail: [email protected] ISSN 1612-1317 e-ISSN1868-1212 ISBN 978-3-642-14924-5 e-ISBN 978-3-642-14925-2 DOI 10.1007/978-3-642-14925-2 SpringerHeidelbergDordrechtLondonNewYork LibraryofCongressControlNumber:2010937667 (cid:2)Springer-VerlagBerlinHeidelberg2011 Thisworkissubjecttocopyright.Allrightsarereserved,whetherthewholeorpartofthematerialis concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting,reproductiononmicrofilmorinanyotherway,andstorageindatabanks.Duplication ofthispublicationorpartsthereofispermittedonlyundertheprovisionsoftheGermanCopyrightLaw of September 9, 1965, in its current version, and permission for use must always be obtained from Springer.ViolationsareliabletoprosecutionundertheGermanCopyrightLaw. Theuseofgeneraldescriptivenames,registerednames,trademarks,etc.inthispublicationdoesnot imply, even in the absence of a specific statement, that such names are exempt from the relevant protectivelawsandregulationsandthereforefreeforgeneraluse. Coverdesign: deblik,Berlin Printedonacid-freepaper SpringerispartofSpringerScience+BusinessMedia(www.springer.com) Preface Theimportanceofsemiconductoropticalamplifiers(SOAs)askeycomponentsin optical communications and integrated optics, covering a wide range of applica- tions for the 1550- and 1300-nm optical windows, has grown in recent years. All-opticalsignalprocessing,includingwavelengthconversion,opticallogicgates andsignalregeneration,etc,isoneofthemostimportantenablingtechnologiesto realize optical switching, including optical circuit switching, optical burst switchingandopticalpacketswitchingandSOAsareverypromisinginall-optical signalprocessingsincetheyarecompact,easytomanufactureandpowerefficient. The need for all-optical elements for increasing the capacity of current and futurecommunicationnetworksandoptimizingtheoperationofopticalswitching networkshas been oneofthe main motivations for consideringSOAs asessential elements in all-optical switching scenarios in recent years. The present book tries tomarkasmallportionoftheSOAsandspeciallyquantum-dotSOAs(QD-SOAs) capabilities in the mentioned topics. In Chap. 1 of this book authors have tried to introduce different aspects of a SOAspeciallyaQD-SOAincludingstructural,opticalandelectricalspecifications of a QDSOA. Different definitions in the field of a SOA such gain-related mechanisms, SOA polarization characteristics, effect of impurity doping in the active region and fabrication requirements are presented in this chapter. Chapter2presentsageneraloverviewfordifferentsimulationmethodsofQD- SOAs. One of the most accurate ways of modeling a SOA is to solve the Semi- conductor Bloch Equation (SBE). However, this method is extremely time-con- suming. The computation time is not acceptable for the system applications of SOA-baseddevices,wheremanyopticalpulseshavetobetransmittedthroughthe SOAtoevaluatethesystemperformance.Simplifiedapproachesincludingcertain physicalprocessesphenomenologically,asitisdoneinrate-equationmodels,have much faster calculation speeds and are quite successful in explaining the experi- mental results.Althoughtheaccuracy forsub-picosecondpulsesis notasgoodas the SBE calculation. Numerical modeling is always necessary to understand the working principle of the devices and to optimize their performance. Physical modelingofcomplexdevicesincludingSOA,suchasall-activeMZIs,isnecessary v vi Preface inordertounderstandtheirpotentialandlimitations.Inthischapterthreedifferent methods for investigation of the QD-SOA performances based on rate-equation modeli.e.numericalmethods,equivalentcircuit-modelingmethodsandanalytical methods is briefly described. Chapter 3 of the book covers different techniques toward utilizing the high- speed operation capabilities of SOAs for high-bit-rate signal processing. This chapter reviewsthe mostrecent techniques inthe field ofbulk, quantumwell and alsoquantumdot-basedSOAsandgivesaninsightforpossiblefutureoptimization methods for increasing the response of SOA-based devices for high-speed operations. Chapter 4 covers the applications of SOAs in all-optical logic gates and subsystems which seem to be essential elements in all-optical signal processing scenarios. In this chapter it has been tried to introduce different techniques for realization of SOA-based optical units and hence, it hasn’t suffice to QD-SOA based structures. This diversity of introduced structures may provide inspiration for novel ideas in eager readers. Authors believe that investigation of QD-SOA specifications in the beginning chapters and presentation of practical methods based on different types of SOAs provides this ability for researchers who are interested in the topics. Finally, in Chap. 5 of the present book, recent progresses in all-optical signal processing and switching with considering SOAs as one of the main elements in the proposed structures are presented. Although these applications are not the wholeSOAbasedarchitecturesforopticalswitchingandsignalprocessing,asmall part of recent development in this field is has been reviewed in this chapter. Iran, October 2010 Ali Rostami Contents 1 Quantum-Dot Semiconductor Optical Amplifiers, Basic Principles, Design Methods, and Optical Characterizations . . . . . . 1 1.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.2 Operation Principles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1.3 SOA Gain. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.3.1 Gain Saturation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.3.2 Confinement Factor . . . . . . . . . . . . . . . . . . . . . . . . . . 8 1.4 Refractive Index. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 1.5 Linewidth Enhancement Factor . . . . . . . . . . . . . . . . . . . . . . . 10 1.6 Comparison of Operating Characteristics. . . . . . . . . . . . . . . . . 12 1.6.1 Amplified Spontaneous Emission. . . . . . . . . . . . . . . . . 16 1.6.2 Noise Figure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 1.7 Polarization Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 1.8 Doped QD-SOAs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 1.8.1 p-doped QD-SOAs. . . . . . . . . . . . . . . . . . . . . . . . . . . 28 1.8.2 n-doped QD-SOAs. . . . . . . . . . . . . . . . . . . . . . . . . . . 37 1.9 Fabrication Process. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 1.9.1 Quantum-Dot Growth. . . . . . . . . . . . . . . . . . . . . . . . . 38 1.9.2 Epitaxial Structure of QD-SOA. . . . . . . . . . . . . . . . . . 40 1.9.3 Waveguide Requirements of QD-SOA . . . . . . . . . . . . . 43 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 2 Simulation Methods of QD-SOAs . . . . . . . . . . . . . . . . . . . . . . . . . 53 2.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 2.2 Numerical Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 2.3 Equivalent Circuit Methods. . . . . . . . . . . . . . . . . . . . . . . . . . 58 2.4 Analytical Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 vii viii Contents 3 Techniques Toward High Speed Operation of SOAs . . . . . . . . . . . 71 3.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 3.2 Gain Recovery Improvement Techniques in Bulk and QW-SOAs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 3.2.1 Carrier Reservoir. . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 3.2.2 Optical Pulse Injection and Holding Beam . . . . . . . . . . 75 3.2.3 Optical Filtering . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 3.2.4 Active Region Modification . . . . . . . . . . . . . . . . . . . . 87 3.3 Gain Recovery Improvement Techniques in QD-SOAs. . . . . . . 90 3.3.1 Two-Photon Absorption-assisted Recovery . . . . . . . . . . 90 3.3.2 Control Pulse-assisted Recovery . . . . . . . . . . . . . . . . . 94 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 4 Applications and Functionalities. . . . . . . . . . . . . . . . . . . . . . . . . . 109 4.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 4.2 SOA-MZI Gate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 4.3 SOA-MZI Transfer Function . . . . . . . . . . . . . . . . . . . . . . . . . 112 4.4 Michelson Interferometer . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 4.5 Wavelength Conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 4.5.1 XGM-Based Wavelength Conversion. . . . . . . . . . . . . . 114 4.5.2 XPM-Based Wavelength Conversion . . . . . . . . . . . . . . 116 4.5.3 FWM-Based Wavelength Conversion. . . . . . . . . . . . . . 117 4.5.4 Wavelength Conversion in SOA-BPF Configuration. . . . 118 4.6 All-Optical Regeneration. . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 4.7 Logic Gates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 4.7.1 XOR Gate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 4.7.2 AND Gate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128 4.7.3 OR Gate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 4.7.4 NOR Gate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130 4.7.5 XNOR Gate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134 4.7.6 NAND Gate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136 4.7.7 NOT Gate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138 4.8 All-Optical Multiplexing and Demultiplexing . . . . . . . . . . . . . 138 4.8.1 SOA-MZI-Based Multiplexing. . . . . . . . . . . . . . . . . . . 139 4.8.2 SOA-MZI-Based Demultiplexing. . . . . . . . . . . . . . . . . 141 4.9 Data Format Conversion. . . . . . . . . . . . . . . . . . . . . . . . . . . . 144 4.9.1 NRZ-to-RZ Data Format Conversion . . . . . . . . . . . . . . 144 4.9.2 NRZ-to-PRZ Data Format Conversion . . . . . . . . . . . . . 146 4.9.3 RZ-to-NRZ Data Format Conversion . . . . . . . . . . . . . . 148 4.10 All-Optical Flip-Flop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149 4.11 All-Optical PRBS Generation . . . . . . . . . . . . . . . . . . . . . . . . 153 4.12 All-Optical Clock Recovery. . . . . . . . . . . . . . . . . . . . . . . . . . 155 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 Contents ix 5 Applications of SOA-Based Circuits in All-Optical Signal Processing and Switching. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163 5.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163 5.2 All-Optical Header/Payload Separation. . . . . . . . . . . . . . . . . . 163 5.3 All-Optical Correlator. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165 5.4 All-Optical Packet Routing . . . . . . . . . . . . . . . . . . . . . . . . . . 170 5.5 All-Optical Header Processing. . . . . . . . . . . . . . . . . . . . . . . . 172 5.6 All-Optical Packet Switching Based on In-Band Filtering. . . . . 175 5.7 All-Optical Self-Routing Node and Network Architecture. . . . . 177 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182 Chapter 1 Quantum-Dot Semiconductor Optical Amplifiers, Basic Principles, Design Methods, and Optical Characterizations 1.1 Introduction Thedevelopmentofsemiconductoropticalamplifiers(SOAs)happenedsoonafter the invention of the semiconductor laser. A SOA is very similar to a semicon- ductor laser without (or with negligible) optical facet feedback. An incoming signal injected into the SOA propagates along its optical waveguide and is amplified by stimulated emission. The optical gain is achieved by inverting the carrier population in the active region via electrical pumping. During the 1990s due to the development of the erbium doped optical amplifier (EDFA) the popu- larityoftheSOAasalinearamplifierdeclinedastheEDFAsprovidedhighergain without the detrimental nonlinearities associated with an SOA. During the development of SOAs, there were three main challenges related to SOAsperformancemakingthemacceptableforpracticalapplications:polarisation sensitivityreduction,opticalfeedbackreduction,anddecreasingthenoiselevelof SOAs. However, attentions turned to SOAs in the late 1990s as SOA design techniques developed, and thus its possibilities for integration and cost effec- tiveness led the SOA to become a competitive component in comparison to the EDFA.ThedesignofSOAsdevelopedintwodirections:asalinearamplifier,itis needed to reduce optical nonlinearities of SOA and as a nonlinear medium; the nonlineareffectsshouldbeexploitedforuseinvarietyofapplicationssuchasall- optical signal processing. The advantages of SOAs are their versatility and pos- sibility of monolithic integration with other optical components like passive waveguides and couplers to perform more complex functions. They are compact, electrically pumped and have a large optical bandwidth. Moreover, they allow a wideflexibilityinthechoiceofthegainpeakwavelength.Inlinearoperationsuch as a power booster, as an inline amplifier and as a preamplifier EDFAs are the dominantamplifiersspeciallyinlong-haulsystemsastheyhavelowernoiselevels andmuchbettercrosstalkpropertiesformulti-channelamplificationincomparison to SOAs. However, the SOA offers a cost competitive alternative to the EDFA A.Rostamietal.,NanostructureSemiconductorOpticalAmplifiers, 1 EngineeringMaterials,DOI:10.1007/978-3-642-14925-2_1, (cid:2)Springer-VerlagBerlinHeidelberg2011 2 1 Quantum-DotSemiconductorOpticalAmplifiers when used as an inline amplifier in metro networks, as a power booster and as a preamplifier. Also, in nonlinear operation they can perform all-optical signal processing due to their strong nonlinearities and their fast dynamics. AdventofnewSOAgenerationinthelastdecade(i.e.quantumwell-SOAsand quantum dot-SOAs) has promised enormous improvements over traditional bulk- SOAs. SOAs with quantum wells or dots in their active region have presented higher output power, lower threshold current, good temperature stability, lower noise characteristics and interesting nonlinear properties compared with bulk SOAs. Quantum-dot SOAs (QD-SOAs) specially have attracted great interest recently due to interesting specifications of quantum dots and have developed alongwithquantumdotlasersinrecentyears. Low-thresholdcurrent,highoutput saturation power, fast gain dynamics and low noise level of QD-SOAs have been provedandithasemphasizedthattheseelementscanbeutilizedasbuildingblocks of all-optical systems. Multi-channel operation capability of QD-SOAs such as multi-channel amplification and wavelength conversion provides a great chance for development of WDM network as well as demonstration of all-optical networks. In this chapter, a brief overview of the operation principles of SOAs will be given and to prevent presentation of redundant subjects in the field of SOAs that have previously discussed in other valuable and outstanding books, specifications of QD-SOAs will be introduced as the main roadmap of this chapter. Optical properties of QD-SOAs, fabrication methods, polarisation-sensitivity, and prop- erties of doped QD-SOAs are the main subjects that will be covered during the next pages. 1.2 Operation Principles The operation principle of the SOA lies in the creation of an inversion in the carrierpopulationusedtoamplifytheinputopticalsignalviastimulatedemission. The population inversion is achieved by electric current injection into the SOA. Figure 1.1showsthesimplifiedbandstructureofadirect-gapsemiconductor.The conduction band and the valence band are separated by the band-gap energy E . g Thecurrentinjectionleadstofreeelectron–holepairgenerationintheconduction bandandvalenceband,respectively.Inquasi-equilibriumtherelaxationtimesfor transitions within either of the bands are much shorter than the relaxation time between the two bands. So, the carrier distribution within each band can be described by two quasi-Fermi levels denoted by E and E . The position of the fc fv quasi-Fermilevelsisdeterminedbythecurrentinjection.Ifthecurrentinjectionis sufficientlylargetheseparationbetweenthequasi-Fermilevelsexceedstheband- gapenergy(E - E [E )andthesemiconductoractsasanamplifierforoptical fc fv g frequencies(m)withE \hm\E - E .Theabsorptionprocessdominatesover g fc fv stimulated emission for photon energies larger than DE = E - E (hm[DE) f fc fv f and the material acts as an attenuator [1].