he chief goal of any designproject is to produce anoptimized design that hasthe highest possible per-formance given any required

design constraints, whether physicalor financial. RSoft Design Grouptools have always supported optimiza-tion through automated parameterscanning and by user-created pro-grams using the scripting interface.However, with the increasing impor-tance of optimization across thedevice and system levels, RSoft isnow releasing a Multi-variableOptimization and Scanning Tool,(MOST), solely dedicated to theautomation and analysis of parameter

scanning and optimization. This toolis designed to work with RSoft’s exist-ing line of design and simulation soft-ware packages including BeamPROP,F u l l W A V E ,B a n d S O LV E ,G r a t i n g M O D ,D i f f r ac tMOD,LaserMOD, andOptSIM.

While a proto-type can usuallybe improved bya trial and errorsearch for betterdesigns, themost efficient

and effective means to approachdesign evolution is to utilize optimiza-tion algorithms. Optimization algo-rithms enable engineers to quickly

find design solu-tions that satisfyspecific require-ments. For exam-ple, the cross-sec-tion of a micro-structured fibercan be designedsuch that the fiberhas specific de-sired dispersioncharacteristics ora taper-based modeconverter can be

E C O C 2 0 0 4 S t o c k h o l m , S w e d e n B o o t h 2 1 8 S e p t e m b e r 2 0 0 4

Volume 3 Number 2

RSoft Introduces Advanced Optimizer andComplete VCSEL Design Solution

T

continued on page ii

n the current design para-digm, VCSEL designs andthe optical systems that usethem are simulated sepa-rately. However, it is

important for cost and time-to-marketreduction, as well as performanceoptimization, that VCSEL designersare able to simulate their new deviceswithin the context of a system, andthat system designers have thoroughand accurate VCSEL models. It is alsoimportant that laser driver circuitdesigners have accurate VCSEL mod-els due to the tight nonlinear couplingbetween the VCSEL and the laser driv-er circuit.

While thorough and accurate VCSELmodels do exist for system simula-tions, these models have many param-eters, making them difficult to cali-brate for real devices and to includeeffects such as jitter, self-heating, andspatial hole burning. To tackle thisproblem, RSoft introduces a newhybrid simulation methodology thatnot only allows the extraction of realis-tic system-level VCSEL models fromboth measured and simulated devicedata, but can also provide the equiva-lent circuit-level models for use withSPICE and other EDA tools.

The hybrid simulation procedure

begins either with RSoft’s activedevice tool, LaserMOD™, which simu-lates the VCSEL at the microscopiclevel or measured data. Next, theOptSim™ Best-Fit Laser Toolkit™directly extracts the system modelparameters from this data. RSoft’ssystem tool, OptSim, is then used tosimulate the system performance withthe extracted VCSEL model. Once thedesired performance criteria areachieved, a circuit model for theVCSEL can be exported for use inelectronic circuit simulations withtools such as SPICE. This is extreme-ly valuable for accurate laser driver cir-cuit simulations and design optimization.

A Complete Design Strategy: Using Comprehensive Mixed-level Designand Simulation for VCSEL-based Optical Transmission Systems

I

continued on page ii

The MOST TM Advanced Optimizer for Photonic Design

Evolution of a 40 variable system using a Genetic Algorithm

created which has the lowest couplingloss within a maximum device length.

Optimization algorithms also allowengineers to explore systems which arehighly complicated and have either alocal or a broad parameter space.Local optimization routines allow theuser to quickly find an optimal designgiven a few initial guesses at parame-ter values. Global optimization rou-tines, though computationally moredemanding than local algorithms,allow the identification of global maxi-ma/minima in a broad parameter spaceand allow the user to create non-obvi-ous solutions to difficult problems.

MOST includes a number of excitingfeatures: It is fully integrated withexisting RSoft products and providesdialogs for specifying optimizationsand scans over an arbitrary number ofvariables. The package includes alarge number of single and multi-

dimensional optimization algorithmsincluding root-finders, minimizers,simplex search routines, genetic algo-rithms, and other stochastic routines.Moreover, a simple but flexible APIallows users to add custom algorithmsthrough their own C++ DLL’s or load-able Python scripts. It is fully integrat-ed with existing RSoft software andprovides dialogs for specifying opti-mizations and scans over an arbitrarynumber of variables. A graphical dis-play indicates the overall convergenceof the algorithm and current values ofthe parameters.

MOST is highly flexible not only in themanner of optimization but also in thetarget to be optimized. While it is easyto optimize simple scalar quantitiessuch as coupled power, the tool can beconfigured to optimize any vector ormatrix-like quantity in terms of a “met-ric” or figure-of-merit. For instance,the integrated difference between a

target and a simulated transmissionspectrum can be minimized.

Perhaps most significantly, MOST isfully ready for clustered computation.Global optimization problems, in par-ticular, can be hugely demanding ofcomputational resources. Users withmultiple copies of RSoft tools can useMOST to automatically distribute largescans and certain optimizations acrossa network. This effectively extendscluster capabilities beyond FullWAVEto the entire RSoft device suite on bothWindows and UNIX platforms.Through load-balancing, it is not evennecessary for the various machines tohave the same performance.

While nothing can replace the knowl-edge and experience of a good opticalengineer, optimization routines allowexperienced engineers to explore newand untried solutions to their problems.

Introducing MOST continued from page i

The LaserMOD simulation engine pro-vides a self-consistent solution of theelectro-thermal transport and opticalfield propagation in semiconductorlasers such as VCSELs. The devicestructure and material parame-ters are digitized on a non-uni-form mesh. Many importanteffects such as self-heating,gain guiding and saturation,multi-mode competition, andpackaging parasitics are allaccounted for in such ascheme. Simulation resultsinclude steady-state solutionsfor analyzing CW performance,and transient solutions for ana-lyzing the modulation responseof the device.

The OptSim Best-Fit Laser Toolkitextracts a corresponding system-levellaser model from LaserMOD simulationresults, such as the material gain ver-sus temperature and carrier density,via a proprietary parameter optimiza-

tion procedure. Measured results ormanufacturer data sheets can also beused to generate the system-levelmodel. At this point, a comprehensivenonlinear electrical circuit model can

be exported to many popular EDA toolson the market such as SPICE3,PSpice, ADS, Spectre, and Hspice.

After the calibrated parameters are inplace, the OptSim VCSEL model, themost advanced model of its kind, isavailable for VCSEL-based optical sys-

tem simulation. This model is basedon multimode rate equations whichconsider the thermal dependence andspatial distribution of carrier density.Various performance analyses such as

eye diagrams, Q and BER esti-mates, and spatial couplingeffects are provided when usedin conjunction with ModeSYS, amultimode simulation tool.

This compressive mixed-levelmethodology allows, for the firsttime in a commercial tool suite,the accurate and consistentdescription of a VCSEL at bothdevice-and system-levels andprovides an interface to elec-

tronic circuit design. It enables previ-ously unavailable design paths, andthus make the overall design and opti-mization cycle of VCSEL-based opticaltransmission systems much more effi-cient.

A Complete Design Strategy continued from page i

–

–

ii

RSoft Design Group Interview with Dr.Kurt Hingerl, University of Linz

Professor Kurt Hingerl is a researcher at the University of Linz in Linz,Austria. After earning his PhD at theUniversity of Linz, Prof. Hingerl did hispost-doctoral research at the University ofIllinois and Bellcore. After this, his careerpath has taken him from environmentalengineering to surface physics and now tophotonics.

RSoft: Professor Hingerl, please tell us a little bit about your position andthe research you do at the University of Linz.

My group is working on basic problems related to a new kind of mate-rial, photonic crystals, also called PhCs (or photonic band-gap materi-als). PhCs are periodic dielectric (or sometimes metallic) structuresthat have a photonic band gap (PBG) for photons, i.e. that light propa-gation is forbidden at certain frequencies. By scaling the size of theperiodicity, the PBG can be tuned from the microwave to the UV region.This property enables one to control (guide and split) light in a way thatis almost impossible with conventional optics or with conventional inte-grated optics.

We are active in theoretical and numerical techniques of light propaga-tion, but also performing experimental work as one of the major usersof the clean room at the University of Linz, especially electron beamlithography and structural processing.

One of our recent achievements is the “invention” of curvilinear pho-tonic crystals, as recently published in Appl. Phys. Lett, issue 7th ofJune 2004. Other works in progress are concerned with add- dropMultiplexers with PhCs, the fabrication of 3D PhCs, the use of nanoim-print-lithography, etc.

RSoft: Now, we understand that you and the university are part of a largecollaboration among researchers throughout Europe. Please tell us moreabout this.

We have been very active in measuring (with ellipsometric methods)and calculating (with FDTD, plane wave expansion, etc.) the polariza-tion optical response of nano-structured surfaces and all this is doneeither in international projects. So we are currently involved in twoEuropean Community projects (5th and 6th Framework program) andindependently also pursue co-operations with groups at CREOL,Florida, Univ. Brno, Czech republic, Univ. Rome III, Italy, and Univ.Bath, UK.

RSoft: What made you chose RSoft Design Group as the vendor of choice fordesign and simulation software?

Basically three reasons were the important ones for us:

RSoft FDTD code runs on 64 bit supercomputers, in our case Altix andSGI Origin machines, and especially for 3D computations the 2GB limitof Pentium based systems can be overcome easily. The fully integratedCAD system allows one to switch to BandSOLVE, DiffractMOD,BeamPROP, FullWAVE, GratingMOD, all within the same structure. The really good support and continued development!

RSoft: In what way has RSoft’s line of design and simulation software aidedin these collaborative efforts? Given this research, what ways hasBeamPROP and FullWAVE been utilized for specific designs of photonicdevices?

One example is e.g. – for curvilinear PhCs- the integrated user interfaceand the scanning property: after obtaining the band gap maps for thecurvilinear PhCs it was a simple change to obtain the FDTD simulationresults.

BeamPROP is currently used for student education. Novel photonicdevices are difficult to be understood and optimized by analytical tech-niques alone. Although it is of utmost importance for students tounderstand the basic equations, they only get a feeling for photonics ifthey are able to work “hands on” with simulation software to see, howtheir designs really perform.

RSoft: What types of research have you been pursuing with LaserMOD?

We plan to enter into VCSELs out of IV-VI compounds, (PbTe and relat-ed materials), where the thermal management is of high importance.

RSoft: Your research is targeted toward photonic bandgap structures anddevices. How has RSoft’s products helped you pursue this research?

I can only highlight it again: the integration of FullWAVE withBandSOLVE within one CAD layout is really helpful!

It is not our vision to develop software, but to apply this softwaretowards innovative designs and RSoft turned out to be the ideal part-ner for these goals.

RSoft: How do you see the need and demand for photonic software in futureapplications?

Especially if one works with high index materials and low index mate-rials with high index contrasts, like the Si/SiO2, but also with theAlGaAs/Al2O3 oxide system, fast BPM techniques which are able tohandle this high index contrast will become important for industrialpurposes. Approaches beyond effective index approximations shouldbe developed by close collaboration between mathematicians andusers.

At the same time optimization tools integrated into the existing soft-ware will become important in industrial environments to automatedesign projects.

Nevertheless, creative new approaches, where the computer code isthen used to verify and respectively falsify, these models will alwaysdemand new extensions of the available software tools.

RSoft: Yes, in fact at ECOC 2004 we plan to present a new integrated opti-mizer and new capabilities to handle high index contrast structures. Anyfinal thoughts regarding RSoft’s products and their integration with yourresearch?

Just continue with your kind of customer support and development tocontinue to aid the industry!

RSoft: Thank you for your time and we wish you good luck in your research!

iii

–

2 0 0 E x e c u t i v e B o u l e v a r dO s s i n i n g , N Y 1 0 5 6 2 U S A

P h o n e : 9 1 4 . 9 2 3 . 2 1 6 4E m a i l : i n f o @ r s o f t d e s i g n . c o m We b : w w w . r s o f t d e s i g n . c o m

U K O f f i c e :R S o f t D e s i g n U K , L t d .4 4 1 7 8 7 4 7 7 9 7 8i n f o @ r s o f t d e s i g n . c o . u k

I N B R I E F

Application papers on our website

RSoft Design Group has developed a collection oftechnical papers authored by both our technicalstaff and our customers which describe many areasof design applications using our software. Please goto our website www.rsoftdesign.com to access thesereferences. If you would like to submit a paper to belisted on our site, please emailinfo@rsoftdesign.com.

ATS in Turino Italy has referenced its use of OptSimin a talk at ECOC 2004, entitled “Impact ofElectronic Equalization on Advanced ModulationFormats in Dispersion-Limited Systems” by V. Curri,R. Gaudino and A. Napoli.

ecently this year RSoftDesign Group announcedthe release of its newestcomponent level package,DiffractMODTM, a powerful,

easy to use tool for the design andanalysis of periodic and diffractivestructures. The software is based onthe well-known Rigorous CoupledWave Analysis (RCWA) algorithm.While RCWA is currently an industrystandard, DiffractMOD has been pow-erfully enhanced with the latestbreakthroughs and state of the arttechniques, including ModalTransmission Line (MTL) and FastFourier Factorization (FFF). Theseunique new features allow the soft-ware to converge quickly and providefor the greatest stability. An exten-sive range of materials and devicescan be designed using this advancedsimulation technology.

One important application area thatwill benefit fromDiffractMOD is opticalmetrology where theprofile of a periodicstructure can becharacterized by usingoptical scatterometryand spectrometry.The RCWA algorithmprovides the neces-sary accuracy andefficiency for suchoptical metrology toolsespecially as the Critical Dimension(CD) decreases for semiconductormanufacturing. Additionally more andmore manufacturers are now makingstandard advanced process controls(APCs) based on real time opticalmetrology where software holds animportant role. DiffractMOD can beutilized for both the design andimplementation of metrology systemsby utilizing the RCWA algorithm as anefficient way of addressing theseneeds.

DiffractMOD is integrated directlyinto the RSoft CAD in order to simu-late these structures types in astraightforward and easy manner.

Once the structure is created in theCAD tool, the powerful RCWA tool willoutput all of the spectral informationat any order, and can even output thefields at various positions throughoutthe device. This type of analysisallows for the “fine-tuning” of metrol-ogy devices where the line-widths andpositions of spectral response peaksare critical.

Another important use of DiffractMODis to simulate photonic bandgap(PBG) applications. PBG devices arean exciting area of research wherescientists and engineers are lookingto create structures which benefitfrom their low loss and dispersioncapabilities. DiffractMOD can ana-lyze an incoming beam on a diffrac-tive structure and quickly give theresponse of the device. This is veryuseful for a fast and accurate analysisof the propagation of a field through aPBG device acting either as a diffrac-

tive structure or as afilter. To furtherextend this analysis,DiffractMOD can beused with other RSoftsimulation tools. Resultsfrom DiffractMODcan be used in conjunc-tion with FullWAVE’sability to producetime dependent resultsof the field andBandSOLVE’s simu-

lations of the complete bandgap ofthe device. The results of these threetools, can be combined to produce acomplete solution for PBG baseddevices.

In addition to metrology and PBGapplications, DiffractMOD can beused for many other periodic and dif-fractive structures. The software isapplicable to optical signal process-ing, sub-wavelength gratings, anti-reflection structures, filters, diffrac-tive optical elements (DOEs), andbeam splitting, and other such opti-cal devices. For further informationon these or the previous applications,please contact RSoft.

R

–

RSoft Design Group Includes Software in New Bookby Govind P. Agrawal

Furthering its strong relationships with the aca-demic community, RSoft Design Group is includinga demonstration version of its award-winning soft-ware, the RSoft Photonics CAD Suite™, includingsystem design, in a new book by fiber-optics lumi-nary Dr. Govind P. Agrawal. The first book in a two-volume engineering resource, LightwaveTechnology: Components and Devices, publishedby John Wiley and Sons, describes a multitude oftoday’s silica- and semiconductor-based opticaldevices. Conceived and written by the foremostexpert and bestselling author in the fiber opticfield, the text provides detailed, in-depth coverageof both theoretical and practical aspects of the sci-ence, including fiber optics, passive and activefiber components, planar waveguides, semiconduc-tor lasers and amplifiers, optical modulators, pho-todetectors, WDM components, and space- andtime-domain switching. Included with this book isa CD-ROM containing simulation examples basedon RSoft’s software that accompany and illustratethe concepts discussed in each of the book’s chap-ters. The wide range of examples make use ofBeamPROP™, FullWAVE™, GratingMOD™,BandSOLVE™, LaserMOD™, and OptSim™products.

iv

DiffractMOD: The Tool of Choice for Optical Metrology and a Complementary Solution for

Photonic Crystal Design