Design, simulation, evaluation, andtechnological verification of arrayedwaveguide gratings

Dana SeyringerPatrick SchmidMichal BielikFrantisek UherekJozef ChovanAnton Kuzma

Design, simulation, evaluation, and technologicalverification of arrayed waveguide gratings

Dana Seyringer,a,* Patrick Schmid,a Michal Bielik,b Frantisek Uherek,c Jozef Chovan,c and Anton Kuzmac

aFachhochschule Vorarlberg GmbH, Research Centre for Microtechnology, Hochschulstrasse 1, Dornbirn, 6850 AustriabSlovak University of Technology, Institute of Electronics and Photonics, Ilkovicova 3, Bratislava, Slovakia 81219cInternational Laser Centre, Slovakia, Ilkovicova 3, Bratislava, Slovakia 84104

Abstract. We present the design, simulation, evaluation, and technological verification of various low-indexoptical demultiplexers based on arrayed waveguide gratings (AWGs). When designing such optical demulti-plexers, a set of input geometrical parameters must be first calculated. They are essential to create AWG layoutthat will be then simulated using commercial photonics tools. However, these tools do not support or support onlypartially such a fundamental calculation. Therefore, a new stand-alone tool called AWG-Parameters was devel-oped, which strongly reduces the time needed for the design. From the calculated geometrical parameters, theAWG layouts were created and simulated using three commercial photonic tools: Optiwave, (Ottawa, Ontario,Canada), Apollo Photonics, (Ancaster, Ontario, Canada), and R-Soft, (Pasadena, California). The designs werealso technologically verified. The simulated/measured transmission characteristics were evaluated by our newlydeveloped AWG-Analyzer tool. This tool provides calculations of AWG transmission parameters, which are alsomissing in commercial photonic tools. Additionally, the tool provides clear definitions of calculated transmissionparameters together with their textual and graphical representations. Finally, the transmission characteristicsand parameters achieved from different photonic tools were compared with each other and discussed in detail.The simulated results were also compared with the measurements. Very good agreement was achievedbetween theoretical (AWG-Parameters tool), simulated (commercial photonic tools), and fabricated AWG trans-mission parameters. © 2014 Society of Photo-Optical Instrumentation Engineers (SPIE) [DOI: 10.1117/1.OE.53.7.071803]

Keywords: optics; photonics; arrayed waveguide gratings; arrayed waveguide grating design; arrayed waveguide grating design tool;arrayed waveguide grating evaluation.

Paper 131620SSP received Oct. 24, 2013; revised manuscript received Nov. 25, 2013; accepted for publication Nov. 26, 2013; pub-lished online Jan. 21, 2014.

1 IntroductionWavelength division multiplexing (WDM) is the uncontestedcandidate for increasing capacity throughput of optical net-works. Arrayed waveguide gratings (AWGs) are the mostpromising devices for filters or multi/demultiplexers insuch WDM systems because of their low insertion loss, highstability, and low cost.1 There are various AWGs that can beused in optical networks, depending on their optical perfor-mance. They can have a different number of transmittingchannels, different channel spacings, and different opticalsignal shapes. It is also important to note that although thestructures of these optical demultiplexers are similar to eachother, each of them requires a different design approach. Forexample, the standard channel count (up to 40 channels) andstandard channel spacing (200 or 100 GHz) AWGs featurevery good transmission characteristics and are relatively easyto design.2 Nevertheless, the growing capacity demandsforce the designers to increase the AWG channel counts asfar as possible. To date, a number of large-scale AWGs withchannel counts of 400 or even more and narrow channelspacing (25 or 10 GHz or even less) have already been dem-onstrated.3,4 However, since the typical bending radius R ofsilica waveguides is around 2 to 25 mm (depending onrefractive-index difference), the size of these AWGs rapidlyincreases and this, in turn, leads to the deterioration in theiroptical performance caused by higher insertion loss and,in particular, higher channel crosstalk.2,5 Additionally, the

simulations of such large AWGs are accompanied by ahuge computational effort, and so, all in all, these AWGspresent for the designers the serious challenge. To avoid thisproblem, a new AWG generation is based mainly on high-contrast index planar waveguide technology, leading toa strong reduction in the AWG structure size.6,7

A special part of the AWG family creates so-called“cyclic” or “colorless” AWG with 100- or 50-GHz channelspacing and 8 or 16 output channels. Here, applying a specialdesign, such AWG will repeat its orders and can work in anypredefined channel band. Similarly, the coarse wavelengthdivision multiplexing (CWDM) optical demultiplexers, hav-ing flat-top shape of output transmitted signals and channelspacing of 20 nm (2500 GHz), require a special taper in thedesign to achieve a satisfying flat-top shape.

In this work, we will show that deep understanding ofthe AWG functionality, i.e., understanding of the strong rela-tionship between AWG geometrical parameters and theirtransmission parameters, together with the calculationsupport of our two software tools (AWG-Parameters andAWG-Analyzer) can ensure the designs of AWGs with verypromising performances.

2 AWG Functionality and FabricationThe standard AWG structure consists of input/output wave-guides, two couplers, and an array of waveguides (also calledphased array) with the constant path length difference dL,as shown in Fig. 1. One of the input waveguides carries

*Address all correspondence to: Dana Seyringer, E-mail: dana.seyringer@fhv.at 0091-3286/2014/$25.00 © 2014 SPIE

Optical Engineering 071803-1 July 2014 • Vol. 53(7)

Optical Engineering 53(7), 071803 (July 2014)

an optical signal consisting of multiple wavelengths λ1 − λn.The input coupler distributes the light among the array ofwaveguides. Then, the light propagates through the wave-guides to the output coupler. The length of arrayed wave-guides is chosen so that the optical path length differencebetween adjacent waveguides, dL, equals an integer multipleof AWG center wavelength λc of the demultiplexer. This lin-early increasing length of arrayed waveguides will causeinterference and diffraction when light mixes in the outputcoupler. As a result, each of the wavelengths λ1 − λn isfocused into only one of the N output waveguides.8

From a technological point of view, the low-index con-trast AWG is a planar waveguide structure usually obtainedon silicon substrate. It consists of SiO2 lower cladding oxidehaving refractive index ncl (see in Fig. 1—AWG fabrication).Chemical vapour deposition (CVD) process creates theactive layer (called “core”) with refractive index nc, wherenc > ncl. Optical lithography and dry etching then defineAWGwaveguide structure. The growth of the upper cladding(CVD process) having refractive index matching the lowercladding is the last technological step.2

3 8-100-GHz AWG DesignWe designed various low index AWGs with a typical refrac-tive index contrast of 0.75%, where ncl ¼ 1.445 and nc ¼1.456. The cross-section of waveguide structurewas 6 × 6 μm2

to ensure the single-mode propagation only (see Fig. 1). Theoptical demultiplexers were designed for the AWG centralwavelength λc ¼ 1550.12 nm.

The AWG designs start with the calculation of geometri-cal parameters that are essential to create the AWG layout.This layout can then be simulated by one of the commercialphotonic tools using BPM simulation. However, these toolsdo not support or support only partially such a fundamentalcalculation. Therefore, a new stand-alone tool called AWG-Parameters9 was developed, which strongly reduces the timeneeded for the design (Fig. 2 shows the user interface of thistool). This tool is based on the paper of Smit et al.8 thatexplains in detail the AWG functionality and the designrules and requirements.

As an example, we have chosen a standard 8-100-GHzAWG. The input for the AWG-Parameters tool isa set of input design parameters like

Technological parameters are taken to design AWGwaveguide structure (see “Material” window in Fig. 2,left):

1. Waveguide structure: waveguide width w ¼ 6 μm.2. Refractive indices: neffðeffective indexÞ ¼ 1.455003,

nout is the refractive index of the cladding (ncl ¼1.445).

Fig. 1 Arrayed waveguide grating (AWG) functionality and fabrication.

Optical Engineering 071803-2 July 2014 • Vol. 53(7)

Seyringer et al.: Design, simulation, evaluation, and technological verification of arrayed waveguide gratings

AWG-type parameters (see “Transmission Parameters←→AWG Parameters” window in Fig. 2, left):

1. Number of output waveguides (channels):Num ¼ 8.

2. AWG central wavelength (λc): LambdaðμmÞ ¼1.55012 μm.

3. Channel spacing: dfðGHzÞ ¼ 100 GHz.

Transmission parameters (see “Transmission Parameters←→ AWG Parameters” window in Fig. 2, left):

1. Adjacent channel crosstalk between output wave-guides (channels): CrðdBÞ ¼ −30 dB.

2. Adjacent channel crosstalk between arrayed wave-guides: CRaWðdBÞ ¼ −10 dB.

3. Uniformity over all the output channels (also callednonuniformity): LuðdBÞ ¼ 0.7 dB.

Pressing “Calculate,” the tool calculates all necessarygeometrical parameters in “Transmission Parameters←→ AWG Parameters” window (see Fig. 1 andFig. 2, left):

1. Number of arrayed waveguides: Na ¼ 100.951257.2. Minimum waveguide separation between input/

output waveguides: dxðμmÞ ¼ 19.328944 μm.3. Minimum waveguide separation between wave-

guides in phased array: dd ðμmÞ ¼ 8.97488 μm.4. Coupler length: LfðμmÞ ¼ 3007.77618 μm.5. Length increment: dLðμmÞ ¼ 111.621329 μm.

The tool also offers function “Calculate” (see “Calculate”window in Fig. 2). This is a very important function allowingus to calculate the geometrical parameters from the

transmission parameters (“Properties → dimension”) andvice versa (“Dimension → properties”). This function isused to easily adjust calculated geometrical parameters tothe technological requirements. For example, the minimumwaveguide separation in the phased array, ddðμmÞ ¼8.974880 μm (see Fig. 2, left), is very difficult to reach tech-nologically in comparison to ddðμmÞ ¼ 9 μm, achieved usingthe function “Dimension → properties” (see Fig. 2, right).Since all parameters depend on each other, usually a few iter-ations are needed to achieve both the satisfying AWG inputdesign and also the geometrical parameters, as shown inFig. 2, right. In spite of this fact, this calculation is veryfast and strongly reduces the time needed for theAWGdesign.This is because the calculations of the parameters are numeri-cal (for more information on these calculations, see Ref. 8)and by simply pressing “Calculate,” one can immediatelyget the calculated parameter values. Another advantage ofthe tool is that by changing one parameter one can get promptresponse of all other parameters. This is very important whenoptimizing the existing design.The geometrical parameters,calculated as AWG-Parameters tool, were used in the com-mercial photonic tools to create AWG layouts and to dothe BPM simulations (Fig. 3 shows the AWG layout createdby Apollo Photonics tool). The output of the simulations is anAWG spectral response, so called transmission characteris-tics. They are the basis for the calculation of AWG transmis-sion parameters (used also as an input in theAWG-Parameterstool). These parameters define the performance of AWG andalso determine its suitability for a particular application. Thecalculations of the transmission parameters were performedby AWG-Analyzer tool (see Sec. 4.1).

4 8-Channel 100-GHz AWG Simulation andEvaluation

As a first step, we created the identical waveguide structure(shown in Fig. 1) using all three commercial photonic tools:

Fig. 2 Calculated geometrical parameters of eight-channel 100-GHz AWG (left) and after a few iterations(right) using AWG-Parameters tool.

Optical Engineering 071803-3 July 2014 • Vol. 53(7)

Seyringer et al.: Design, simulation, evaluation, and technological verification of arrayed waveguide gratings

Apollo Photonics, Optiwave, and R-Soft. Then, the AWGgeometrical parameters (output from AWG-Parameterstool, see Fig. 2, right) were used to create the identicalAWG layout by each photonic tool. For all simulations,we used the same calculation conditions. Finally, the AWGdesign was fabricated and measured on the chip by an exter-nal foundry. The simulated/measured transmission character-istics are shown in Fig. 4. From these characteristics, it is

evident that the simulation performed by Optiwave tool isvery similar to the measurement. The simulated characteris-tics from Apollo and R-Soft tools are similar to each other,but they differ from the measured transmission characteris-tics in much better background isolation (BX).

All transmission characteristics were evaluated usingAWG-Analyzer tool. The output of this evaluation is a setof transmission parameters describing the optical properties

Fig. 3 Software tools used to design, simulate, and evaluate AWGs.

Fig. 4 Measured and simulated AWG transmission characteristics of 8-channel 100-GHz AWG.

Optical Engineering 071803-4 July 2014 • Vol. 53(7)

Seyringer et al.: Design, simulation, evaluation, and technological verification of arrayed waveguide gratings

of the designed AWG. Before we discuss all these parame-ters, let us first describe the functionality of AWG-Analyzertool.10 To this purpose, we picked out the transmission char-acteristics achieved by Optiwave tool.

4.1 AWG-Analyzer Tool

When defining the performance of AWG, a set of transmis-sion parameters must be considered. These parameters areextracted by analyzing AWG transmission characteristics:simulated/measured AWG spectral response for both thetransverse electric and the transverse magnetic polarizationstates, as shown in Fig. 5 (“Diagram” window). While themeasurement method adhered by most AWG vendors isthe deployment of Mueller matrix method,11,12 the waythat vendors specify the performance of a device from themeasured curves differs widely. Additionally to this, it isalso important to note that the output from all commerciallyavailable AWG design tools is the simulated transmissioncharacteristics only, and none of them supports (or supportsonly partially) the software-aided calculation of transmission

parameters. Therefore, the only possibility is to manuallydetermine these parameters directly out of the transmissioncharacteristics graph. The major problems of the manualevaluation are that it is very unhandy, time intensive, andcan be imprecise due to reading errors especially for high-channel number, narrow channel spacing, or colorlessAWGs. As a consequence of the manual evaluation, thedesign, simulation, and fabrication of AWGs can becomea complicated matter, because the modifications of layoutor fabricated device always require a detailed evaluationof the current simulation or measured data. Besides the men-tioned disadvantages of the manual evaluation, a furtherproblem is that the evaluation of AWGs is not underlyinga uniform standard. This means that the definitions ofAWG transmission parameters, by now, are not standardized.This ends in the fact that without enclosed definitions no onecan exactly determine the performance of a present AWG. Tosolve these problems, a new software tool was developed.Figure 5 shows its user interface. It is divided into threewindows

Fig. 5 User interface of AWG-Analyzer software tool showing simulated (Optiwave tool) AWG transmis-sion characteristics of 8-100-GHz AWG.

Optical Engineering 071803-5 July 2014 • Vol. 53(7)

Seyringer et al.: Design, simulation, evaluation, and technological verification of arrayed waveguide gratings

1. Textual representation of raw data: the content of theoriginal input file (consisting of simulated or measureddata) is displayed in “Raw Data” window (Fig. 5,bottom left).

2. Graphical representation of raw data: is displayed in“Diagram” window (Fig. 5, top left).

3. AWG transmission parameter view: this view shows atable containing all evaluated transmission parameters,their units, and their associated channels (Fig. 5, right).They are listed and explained in Table 1. The evalu-ation of the transmission characteristics is performedby calculating each transmission parameter for eachchannel separately. The worst parameter value over allthe output channels, considering both polarizationstates, is then presented in “AWG TransmissionParameters” window.

Besides the evaluation of transmission characteristics, thetool offers some other supporting functions

1. User-defined passband value: the parameter “pass-band” is the range of wavelengths surrounding thecenter wavelength for any given channel. Most ofthe AWG transmission parameters (listed below thisparameter in the table in Fig. 5, right) are calculatedwithin this passband. It usually equates to 25% ofthe respective channel spacing. In the AWG-Analyzertool, this parameter is implemented as a variable inputquantity (see Fig. 5).

2. Help for each calculated AWG transmission param-eter: by showing its textual and graphical definitions(see Fig. 6). All transmission parameter calculationsimplemented in the evaluation algorithm of the AWG-Analyzer software tool are performed according tothese definitions. Moreover, these textual and graphi-cal definitions are implemented in the software’s helpsystem and, therefore, available every time the userneeds specific information about how the AWG trans-mission parameters are calculated.

3. Recognition of various spectrums: the tool recognizesif the evaluating file contains the data of single spec-trum transmission characteristics (Fig. 6), multiplespectrum transmission characteristics [consistingmultiple orders of the phased array response witha free spectral range (FSR), as presented in Fig. 7,or so called colorless spectrum transmission character-istics, etc. (see Fig. 8).

4. AWG tutorial: for the teaching purposes, a tutorialabout AWGs (starting from its functionality goingthrough the AWG types, design, fabrication, testing,packaging, etc.) is also implemented in the tool(Fig. 8).

4.2 8-100-GHz AWG Evaluation

The simulated and measured transmission characteristics arethe basis for the calculations of transmission parameters.

Table 1 Transmission parameters calculated from the simulated/measured transmission characteristics using arrayed waveguide grating (AWG)-Analyzer tool.

Nr. Channels Number of output channels

LambdaCh Channel center wavelength

LambdaITU The nearest ITU wavelength to channel center wavelength

dLambdaITU Channel center wavelength shift to the nearest ITU wavelength

Ch. Spacing Wavelength/frequency spacing between two neighbor channels

FSR Free spectral range

B@0.5dB, B@1dB,B@3dB, B@20dB

Bandwidth of the transmitted optical signal at −0.5, −1, −3, and −20 dB drop from transmission peak

Passband Passband is defined as a symmetrical wavelength range around each channel center wavelength usuallydefined as 25% of the channel spacing (most of the transmission parameters are calculated within this passband)

PBu Passband uniformity

pIL Peak insertion loss: the loss in power measured from channel transmission peak to the 0-dB reference line

IL Insertion loss of a channel measured within the passband to the 0-dB reference line

pILU Peak insertion loss uniformity: the difference between maximum and minimum peak insertion losses overall the output channels

ILU Insertion loss uniformity calculated within the passband of each channel over all the output channels

BX Background crosstalk

AX Adjacent channel crosstalk (also called the channel isolation)

nAX Nonadjacent channel crosstalk

Optical Engineering 071803-6 July 2014 • Vol. 53(7)

Seyringer et al.: Design, simulation, evaluation, and technological verification of arrayed waveguide gratings

These parameters should correlate with the theoretical trans-mission parameters used in the AWG design. The theoreticaltransmission parameters are the output of the AWG-Parameters calculation (see Fig. 2, right, parameter valuesLambda, Cr, Lu, and df) and are presented in the columnTheoretical parameters in Table 2. As can be seen, thereis neither insertion loss parameter (IL) nor nonadjacent chan-nel crosstalk parameter (nAX) nor background crosstalkparameter (BX) in this calculation, since these parametersdepend mainly on the fabrication process. Transmissionparameters calculated from the simulated transmission char-acteristics are listed in the columns OPTIWAVE, APOLLO,and R-SOFT, and the transmission parameters calculatedfrom the measured characteristics (obtained from the techno-logical foundry) are presented in MEASUREMENT column.

As can be seen, the AWG central wavelength λc (parameterLambda) agrees to the theoretical value λc ¼ 1550.12 nmonly in the Apollo simulation. This deviation is a result

of the effective refractive index used in the waveguide struc-ture simulation, in which the calculated value is slightly dif-ferent in each photonic tool. Further, insertion loss (IL)calculated within the passband (¼25% of channel spacing)is similar for all simulations and lies between 2.624 dB(Optiwave) and 2.000 dB (R-Soft). The small deviationsresult from the slightly different optical signal shapesonly. Measured insertion loss reached 6.438 dB. This param-eter is always higher than its simulated value since it alsoincludes the coupling losses between the fibers and wave-guides and the propagation losses in the AWG structure(these losses are usually not included in the simulation).Insertion loss uniformity (ILu) is similar in all simulations(ILu ¼ 0.52, 0.76, and 0.529 dB) in the measurement(ILu ¼ 0.694 dB), and it correlates well with its theoreticalvalue (Lu ¼ 0.712248 dB). This is because this parameterdepends mainly on the AWG structure itself. Here again,the small deviations result from the slightly different optical

Fig. 7 Multiple spectrum transmission characteristics [consisting multiple orders of the phased arrayresponse with a free spectral range (FSR)].

Fig. 6 Textual and graphical definitions are implemented in the software’s help system and available anytime.

Optical Engineering 071803-7 July 2014 • Vol. 53(7)

Seyringer et al.: Design, simulation, evaluation, and technological verification of arrayed waveguide gratings

signal shapes. Adjacent channel crosstalk (AX, calculatedwithin the passband) is very similar for Apollo and R-softcharacteristics (about −50 dB), since these are nearly iden-tical. Adjacent channel crosstalk from Optiwave simulationequals −42.024 dB, and it is about 10 dB better than thechannel crosstalk calculated from the measured characteris-tics (−32.476 dB). This difference is a result of the side lobespresent in the measured characteristics, originating fromthe fabrication imperfections (see Fig. 4).

5 Application of AWG-Parameters Tool in Design of8-Channel 100-GHz Colorless AWG

The colorless or cyclic AWG uses its periodical response torepeat its orders, and this way it can cover any predefinedchannel band (see Fig. 8). In other words, the same AWGcan work on channels 1 to 8 or 9 to 16, etc. In the frequencydomain, this period is called FSR (see Fig. 7). The conditionfor cyclic AWG is that the FSR should equal N times thechannel spacing (df), where N is the number of output chan-nels, i.e., FSR ¼ 8 × 100 GHz ¼ 800 GHz.

5.1 8-Channel 100-GHz Colorless AWG Design

To design the colorless AWG, we again used AWG-Parameters tool. This tool offers one more important func-tion: it calculates all additive parameters needed for thecalculation (see in Fig. 9, the “Computed data” window).One of these parameters is dfsr parameter carrying the infor-mation about FSR. As can be seen, dfsr ¼ 800 GHz, a nec-essary condition for 8-channel 100-GHz colorless AWG.

5.2 8-Channel 100-GHz Colorless AWG Simulation

The geometrical parameters, calculated using AWG-Parameters tool (Fig. 9), create the input for the AWG layoutof 8-channel 100-GHz colorless AWG. This layout was cre-ated and simulated using Optiwave and Apollo photonic tools.The simulated transmission characteristics (Optiwave tool) arepresented in Fig. 10(a). We would like to point out that thesimulated characteristics from Apollo tool were nearly iden-tical with the Optiwave simulated results. The only differencewas again in the background crosstalk (BX) that was about

Table 2 Transmission parameters calculated from the simulated/measured transmission characteristics of eight-channel 100-GHz AWG.

Transmission parameters Theoretical parameters MEASUREMENT OPTIWAVE APOLLO R-SOFT

Number of output channels (Num) 8 8 8 8 8

AWG central wavelength (Lambda) (nm) 1.55012 1547.50 1549.00 1550.12 1550.70

Channel spacing (df) (GHz) 100 100 100 100 100

Insertion loss (IL) (dB) — 6.438 2.624 2.306 2.000

Insertion loss uniformity (Lu, ILu) (dB) 0.712248 0.694 0.520 0.760 0.529

Adjacent channel crosstalk (Cr, AX) (dB) −29.33894 −32.476 −42.024 −50.426 −50.264

Nonadjacent channel crosstalk (nAX) (dB) — −43.28 −46.413 −51.050 −50.309

Background crosstalk (BX) (dB) — −54.793 −54.571 −71.050 −73.309

Fig. 8 AWG tutorial is implemented for the teaching purposes starting from its functionality going throughthe AWG types, design, fabrication, testing, packaging, etc.

Optical Engineering 071803-8 July 2014 • Vol. 53(7)

Seyringer et al.: Design, simulation, evaluation, and technological verification of arrayed waveguide gratings

5 dB better for Apollo characteristics (−59.7895 dB)compared with Optiwave transmission characteristics(−54.76 dB).

The AWG design was also technologically verified (usingthe external foundry), and measured transmission character-istics are shown in Fig. 10(b). As can be seen, very goodagreement was achieved between simulated and measured

transmission characteristics. Only the background crosstalk(BX) is about 10 dB higher in the measured transmissioncharacteristics.

5.3 8-Channel 100-GHz Colorless AWG Evaluation

The calculated transmission parameters confirm this agree-ment (see Table 3). Insertion loss parameter (IL), calculated

Fig. 10 Simulated (a) and measured transmission characteristics (b) of 8-channel 100-GHz colorlessAWG.

Fig. 9 Calculated geometrical parameters of eight-channel 100-GHz colorless AWG. The dfsr (GHz)parameter shows 800-GHz value that equals 8 × 100 GHz; a necessary condition for cyclic 8-channel100-GHz AWG.

Optical Engineering 071803-9 July 2014 • Vol. 53(7)

Seyringer et al.: Design, simulation, evaluation, and technological verification of arrayed waveguide gratings

from the measured transmission characteristics, is slightlyhigher and it is caused mainly by the coupling losses inthe phased array and between the fibers and input/outputwaveguides. The simulated and measured insertion lossuniformity parameters (ILu) are similar to each other (sim-ulation: ILu ¼ 2.38 dB and measurement: ILu ¼ 2.4 dB);however, they feature a strong deviation from their theoreti-cal value (Lu ¼ 0.927 dB, see Fig. 9). This is because thetapers were used in the phased array to collect the lightand were used this way to keep the losses as small as pos-sible. On the other hand, these tapers alter the far-fielddistribution in the output coupler, and this in turn determinesthe uniformity value (for more information, see Refs. 2 and8). Unfortunately, the presence of the tapers in the phasedarray cannot be considered in the AWG-Parameters tool.Regarding the adjacent channel isolation (AX), there areonly small deviations (simulation: AX ¼ −33.054 dB andmeasurement: AX ¼ −32.4 dB) from the theoretical value(Cr ¼ −31.395 dB, see Fig. 9) caused by the slightly differ-ent shapes of the optical signals in the simulated and mea-sured transmission characteristics.

6 Design, Simulation, and Evaluation of128-Channel 10-GHz AWG

As mentioned at the beginning, the AWGs can vary in num-ber of transmitting channels (4, 8, 16, 32, 40, 64, 128, etc.),in channel spacings (100 GHz, 50 GHz, 25 GHz, etc.), or canhave different optical signal shapes (Gaussian, flat-top, andsemi-flat-top). However, the structures of these opticaldemultiplexers are also similar to each other; each of themrequires a different design approach. To show this difference,we designed narrow channel spacing 128-channel 10-GHzAWG.

6.1 128-Channel 10-GHz AWG Design

The 128-channel 10-GHz AWG was also designed by apply-ing AWG-Parameters tool. This tool offers the possibility tochange any parameter, as explained in Sec. 3 and shown inFig. 2, with immediate response of all other parameters. Thisway, it is much easier to find satisfying transmitting param-eters with technologically appropriate geometrical parame-ters of the AWG structure. In the AWG-Parameters tool,insertion loss uniformity was set to Lu ¼ 0.938 dB and

adjacent channel crosstalk was Cr ¼ −29.329 dB.Figure 11(a) shows the layout of this narrow channel spacingAWG (size cca. 10 × 7 cm2) together with the standard8-channel 100-GHz AWG (size cca 1.8 × 1.5 cm2). Ascan be seen, the structure of 10-GHz AWG got muchlarger; however, it still fits onto a 6 in. wafer [shown inFig. 11(b)].

We would like to point out that the most critical part ofany AWG structure is the phased array consisting of a par-ticular number of arrayed waveguides. While in the 8-chan-nel AWG structure there are 96 relatively short-arrayedwaveguides placed far from each other, in the 128-channelAWG, there are 823 long-arrayed waveguides placed veryclosely to each other. This becomes a problem becausethe technological imperfections of these waveguides inducethe phase errors in the propagating light, causing the deterio-ration of the transmission characteristics.13 Therefore, tokeep these errors as small as possible, the phased arrayshould be designed very small. However, the numbers, thelengths, and the positions of the arrayed waveguides cannotbe set freely because of the dl parameter, i.e., the lengthdifference between any two neighbor arrayed waveguides(see Sec. 2 and Fig. 1). This parameter is one of the most

Table 3 Transmission parameters, calculated from theoretical (AWG-Parameters tool), simulated, and measured transmission characteristics ofeight-channel 0-GHz colorless AWG.

Colorless AWG: Transmission parameters Theoretical parameters Simulated parameters Measured parameters

Lambda (λc ) ITU-grid ITU-grid ITU-grid

Channel spacing (df) (GHz) 100 100 100

Insertion loss (IL) (dB) — 4.76 6.12

Insertion loss uniformity (Lu, ILu) (dB) 0.927 2.38 2.4

Adjacent channel crosstalk (Cr, AX) (dB) −31.395 −33.054 −32.4

Nonadjacent channel crosstalk (nAX) (dB) — −52.038 −40.94

Background crosstalk (BX) (dB) — −54.76 −43.97

Fig. 11 Layout of the 128-channal 10-GHz AWG together with8-channel 100-GHz AWG (a) and wafer layout of two 128-channel10-GHz AWGs placed on the 6 in. wafer (b).

Optical Engineering 071803-10 July 2014 • Vol. 53(7)

Seyringer et al.: Design, simulation, evaluation, and technological verification of arrayed waveguide gratings

important parameters driving demultiplexing properties ofAWG. Therefore, any change in the dl parameter causesthe changes in the input design parameters, particularly inCr, Lu, df, and Lambda (explained in Sec. 3 and Fig. 2),and those in turn drive the values of geometrical parametersagain. Simply spoken, compared with the standard AWGs,there is much less freedom in calculating appropriate geo-metrical parameters that can ensure a set of satisfying trans-mission parameters. Therefore, the changes in the dlparameter have to be done very carefully.

6.2 128-Channel 10-GHz AWG Simulation andEvaluation

The designed AWG structure was simulated using anOptiwave tool and evaluated using an AWG-Analyzertool. The simulation of the AWG structure took about 2months (to compare, the simulation of 8-channel 100 GHzAWG took 1 to 2 days using Optiwave tool and a fewhours using Apollo Photonics tool). The simulated transmis-sion characteristics of this AWG can be seen in Fig. 12.Figure 12(a) shows the transmission characteristics of all

128 output channels. Figure 12(b) shows the transmissioncharacteristics only of one output channel.

The simulated transmission characteristics were thenevaluated using an AWG-Analyzer and are shown in Fig. 13.The most important parameters are

1. Adjacent channel crosstalk between output wave-guides, AX ¼ −28.692 dB, that features very goodagreement with the theoretical value, Cr¼−29.329 dB.

2. Insertion loss parameter calculated within passbandIL ¼ −2.271 dB. Its value is always higher thanthe peak insertion loss (pIL ¼ −1.339 dB) (forparameter explanations see Sec. 4.1).

3. Difference between the maximum and minimum inser-tion losses over all output channels is a peak insertionloss uniformity, pILu ¼ 0.969 dB. Its theoreticalvalue in AWG-Parameters tool: Lu ¼ 0.938 dB, thatis again a very good agreement.

4. Parameter ILu equals 0.968 dB, and it is the sameparameter as pILu but insertion loss is taken overthe 25% passband.

Fig. 12 Simulated transmission characteristics of the 128-channel 10-GHz AWG obtained fromOptiwave photonics tool (a) and simulated transmission characteristics of only one channel (b).

Optical Engineering 071803-11 July 2014 • Vol. 53(7)

Seyringer et al.: Design, simulation, evaluation, and technological verification of arrayed waveguide gratings

5. Nonadjacent channel crosstalk nAX ¼ −32.947 dBand background crosstalk BX ¼ −65.701 dB [seeFig. 12(b)] ensure a good channel isolation.

7 ConclusionIn this article, we presented the design, simulation, evalu-ation, and technological verification of different low indexcontrast AWGs like the standard 8-channel 100-GHz AWG,narrow channel spacing 128-channel 10-GHz AWG, and8-channel 100-GHz colorless AWG. To design AWG struc-tures (i.e., to calculate the geometrical parameters based onthe technological and transmission input parameters), weused our stand-alone AWG-Parameters tool. This tool canbe used to design any type of AWG. It saves strongly thetime needed for the calculations and is also very useful forthe AWG design optimization.

For the AWG layout simulations, we used three differentcommercial photonic tools: Optiwave, Apolo Photonics, andR-Soft. The designs were also technologically verified. Thesimulated transmission characteristics of 8-channel 100-GHzAWG were compared with each other and also with the mea-sured characteristics. We showed that all three tools providevery similar simulation results; however; the best agreementwas achieved between Optiwave simulations and the mea-surements. The transmission parameters calculated fromthe transmission characteristics confirm this agreement.On the other hand, the Apollo and R-Soft photonic toolsare much faster than the Optiwave tool and therefore, the

simulation results can be obtained in a significantly shortertime.

To evaluate the simulated/measured transmission charac-teristics, the AWG-Analyzer tool was used. As presented,this tool is very useful, particularly when evaluatingsimulated/measured transmission characteristics of high-channel count AWGs. It saves time and guarantees exactcalculation of all transmission parameters. Finally, we wouldlike to conclude that very good agreement was achievedbetween theoretical (AWG-parameters tool), simulated(commercial photonic tools), and fabricated AWG transmis-sion parameters.

AcknowledgmentsThis work was supported by project No. SK-AT-0011-12from Slovak Research and Development Agency ofMinistry of Education, Science, Research and Sport of theSlovak Republic and SK 14/2013 from Austrian Agencyfor International Cooperation in Education and Research(OeAD-GmbH).

References

1. A. Kaneko et al., “Design and applications of silica-based planar light-wave circuits,” J. Sel. Topic Quant. Electron. 5(5), 1227–1236 (2002).

2. K. Okamoto, “Planar lightwave circuits,” in Fundamentals of OpticalWaveguides, P. L. Kelley, I. P. Kaminow, and G. P. Agrawal, Eds.,Academic Press (2000).

3. K. Takada et al., “12.5 GHz spaced 1.28 Tb∕s (512-channel×2.5 Gb∕s) super-dense WDM transmission over 320 km SMF usingmultiwavelength generation technique,” in Proc. ECOC 2000, PD3.8(2000).

Fig. 13 AWG-Analyzer tool presenting calculated transmission parameters of 128-channel 10-GHzAWG.

Optical Engineering 071803-12 July 2014 • Vol. 53(7)

Seyringer et al.: Design, simulation, evaluation, and technological verification of arrayed waveguide gratings

4. K. Takada et al., “1-GHz-spaced 16-channel arrayed-waveguide gra-ting for a wavelength reference standard in DWDM network systems,”J. Lightwave Technol. 20(5), 850–853 (2002).

5. K. Okamoto, “Functional PLC devices for optical-layer signal process-ing,” in Proc. ECOC 2002, Copenhagen, Denmark (2002).

6. Y. Hibino, “Recent advances in high-density and large-scale AWGmulti/demultiplexers with higher index-contrast silica-based PLCs,”J. Sel. Top. Quant. Electron. 8, 1090–1101 (2002).

7. H. Li et al., “Design of 1 × 8 silicon nanowire arrayed waveguide gra-ting for on-chip arrayed waveguide grating demodulation integrationmicrosystem,” Opt. Eng. 51(12), 123001 (2012).

8. M. K. Smit et al., “PHASAR-based WDM-devices: principles,design and applications,” J. Sel. Top. Quant. Electron. 2, 236–250(1996).

9. D. Seyringer and M. Bielik, “AWG-Parameters: new software toolto design arrayed waveguide gratings,” Proc. SPIE 8627, 862716(2013).

10. D. Seyringer and P. Schmid, “A new software tool is developedto evaluate the measured/simulated transmission characteristicsof optical multilexers/demultiplexers,” Proc. SPIE 8167, 81671D(2011).

11. D. Derickson, Fiber Optic Test and Measurement, D. Derickson, Ed.,Prentice Hall, Upper Saddle River, New Jersey (1998).

12. W. A. Shurcliff, Polarized Light: Production and Use, HarvardUniversity Press, Cambridge, Massachusetts (1966).

13. P. Munoz et al., “Analytical and numerical analysis of phase andamplitude errors in the performance of arrayed waveguide gratings,”J. Sel. Top. Quant. Electron. 8(6), 1130–1141 (2002).

Dana Seyringer is a project manager at the Research Centre forMicrotechnology at Vorarlberg University of Applied Sciences. Shereceived her first PhD in microelectronics at Slovak University ofTechnology in 1996, and her second PhD at the JK University LinzAustria, in 1998. She is internationally known for her work onsimulation methods and is the author of numerous papers in thefield of crystal growth and passive optical components fortelecommunications.

Patrick Schmid received the master’s degree in mechatronics fromVorarlberg University of Applied Sciences in 2011. For the next twoyears he extended his scientific work as a scientific assistant in theResearch Centre for Mechatronics (Vorarlberg University) working onpower converter control for wind power plants. Currently he works as

a project manager at Obrist Powertrain Company in Vorarlberg. He isauthor and co-author of various papers in the field of AWG and FPGAsystem design.

Michal Bielik received his master’s degree in electronics from theInstitute of Electronics and Photonics (Faculty of ElectricalEngineering and Information Technology, Slovak University ofTechnology) in Bratislava in 2007. His thesis was related to the devel-opment and testing of the AWG-Parameters software tool. He isauthor and co-author of various papers in the field of AWG designand simulation.

Frantisek Uherek is a professor at the Slovak University ofTechnology in Bratislava. He is an expert in the field of photonics,lasers, optical communication, and sensor systems. He publishedmore than 150 scientific papers and is (was) the coordinator ofmore than 30 scientific and application-oriented projects. He was per-sonally involved in the establishment of the International Laser Centrein Bratislava, now acting as a director of the centre.

Jozef Chovan received the MSc and PhD degree in electrical engi-neering from Slovak University of Technology in Bratislava, in 1999and 2003, respectively. In 2003, he joined International Laser Centre,where he worked as a researcher. In 2004, he was promoted to headof the information technology laboratory, where he had responsibilityfor the research in optical communication systems, all-optical signalprocessing, and optical sensing and research cooperation with Slovakphotonics industrial partners.

Anton Kuzma received his master’s degree in electronics from theInstitute of Electronics and Photonics (Faculty of ElectricalEngineering and Information Technology, Slovak University ofTechnology) in Bratislava in 2011. He is a PhD student at theInstitute of Electronics and Photonics. His thesis research is relatedto the design of progressive photonic structures for photonic sensors,photonic devices for optical communication systems, and photonicintegrated circuits.

Optical Engineering 071803-13 July 2014 • Vol. 53(7)

Seyringer et al.: Design, simulation, evaluation, and technological verification of arrayed waveguide gratings