AbstractSilicon photonics has evolved in the last years into a vi-brant ecosystem of foundries and design houses. This hasenabled a broader set of commercial applications besidesthe traditional telecom fields, like optical chip intercon-nects, sensor interrogation or microwave filters andbeamformers. Basic devices like Arrayed Waveguide Grat-ings (AWGs), Mach-Zehnder Interferometers (MZI), Mul-timode Interference Couplers (MMIs), or Ring Resonators(RRs) have already become standard building blocks, pro-viding a reliable and customizable basis for more complexintegrated optical systems.

Keywords: Silicon photonics, photonic integration, Ar-rayed Waveguide Grating, AWG, Mach-Zehnder Interfer-ometer, MZI, Multimode Interference Coupler, MMI, RingResonator.

1. Introduction

Photonic integration allows for the miniaturization of op-tical components and systems into a single chip, with thecorresponding gains in size and volume, robustnessagainst mechanical or thermal stress, or the issues of mis-alignments and optical cleanliness commonly found infree space bulk or micro optics. Multiple material tech-nologies have been used along the years to achieve suchmonolithic integration, each of them more suited for acertain type of devices: lithium niobate for modulators,silica (also known as planar lightwave circuit, PLC) for pas-sive devices like couplers, splitters and gratings, or indiumphosphide for active components like lasers or amplifiers.

While all these technologies have been researched andalready taken intro production since aproximately 10

years ago, it is silicon photonics that has gained a lot oftraction now, due to the opportunity of implementing itinto semi-standard complementary metal oxide semicon-ductor (CMOS) processes, with the large cost reductionsthis implies, and the corresponding increase in produc-tion volumes, system scalability and complexity. More-over, this also allows for an easier integration of siliconphotonic components along with electronic control func-tions [1].

There are currently several international silicon pho-tonic foundries offering their services through stand-alone, dedicated wafer runs, or as part of what isknown as multi-project wafer runs, where wafer spaceis shared among multiple users to decrease cost, at theexpense of a tight fab schedule (see Fig. 1). Processstandardization has allowed for a business ecosystemthat is quickly evolving to mimic that of the electronicsworld, where end users and independent designhouses alike submit their chip designs to genericfoundries. Photonic design kits (PDK) are usually of-fered by each foundry, implementing the most com-mon optical components as basic building blocks thathave been thoroughly tested and are ready to be di-rectly plugged into more complex designs. Additionally,design houses develop their own advanced BuildingBlocks (BBs) on top of these PDKs, offering extendedfunctionalities and savings in cost and developmenttime for their customers, the end users.

Regarding end user applications, as silicon is transparentin the infrared above 1.1 um, it is a good material tech-nology for optical communications in the O to U wave-length bands (1270-1675 nm), from long-range telecomto shorter datacom links. Moreover, due to the CMOS

5Waves - 2013 - year 5/ISSN 1889-8297

Basic silicon photonic building blocks forcommercial applicationsI. Artundo1, P. Muñoz1,2, D. Domenech1,2, J.H. Den besten1, J. Capmany1,2

Correspondence author: <inigo.artundo@vlcphotonics.com>

1 VLC Photonics S.L.Ed. 9B, Desp. 05 – UPV, Camino de vera s/n 46022 Valencia

2 iTEAM (GCOC)Ed. 8G, Acc. D, 4P – UPV, Camino de vera s/n 46022 Valencia

compatibility, low power consumption and extreme com-pactness, silicon photonic links and devices are very ap-propriate for chip-to-chip and on-chip opticalinterconnects. However, silicon photonics is not restrictedonly to the communications field, as there are alreadycommercial examples of optical chips for e.g. fiber sensorinterrogation, microwave signal generation and process-ing, or biophotonic diagnostic chips.

Independently of the application, there are several pho-tonic components that are usually found in optical sys-tems due to the basic functionality they perform. Thesebasic building blocks (BBs) are already mature in mostplatforms, and can be found in most PDKs ready to beplugged into more complex layouts. We will now de-scribe some of them, and introduce some end applica-tions that include these BBs as an example of maturedevices that are ready to go into production.

2. Silicon waveguides and couplers

Silicon photonics is a general term that mostly refers tothe lithographic process performed over Silicon-on-In-sulator (SOI) wafers, using Silica (SiO2) as the insulationlayer between a thin device layer and the silicon sub-strate layer. The high contrast index of silicon versusthe silica layer allows for the fabrication of optical

waveguides with different geometries in scales fromtens of nm to hundreds of �m (see Fig. 2). Dependingon these geometries, propagation losses and bendingradii can vary, but for standard strip waveguides (alsoknown as silicon nanowires) of 220 nm high and 450nm wide, these are usually around 2.5 dB/cm and 5�m, and lower values can be achieved through carefuldesign. mm2

Simple waveguides allow for more advanced structureslike splitters (also referred as Y-junctions), parallel cou-plers, delay lines or spiral light sinks. An example of atelecom device designed with basic waveguide BBs is theoptical code division multiple access (OCDMA) demulti-plexer shown in Fig. 3, where 450 nm wide buried wave-guides, separated 200 nm apart, where used as parallelwaveguide couplers along with spiral delay lines of 10ps. The coupling ratio was controlled here by the straightcoupling section length and the longitudinal offset. Ad-ditionally, phase shifters were implemented by usingthermo optic heaters, resulting in a total chip size of 8×9mm2 [2].

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Figure 1. Photonic integration ecosystem, from application to chip design and fabrication.

Figure 2. Basic silicon waveguide BBs with differentgeometries.

Figure 3. OCDMA decoder chip including delay lines andwaveguide couplers, along with thermo-optic heaters tointroduce phase shifts, designed by VLC Photonics.

3. Arrayed Waveguide Gratings

The arrayed waveguide grating (AWG) is a very versatileBB, and depending on its configuration, it can be usefulfor multiplexing and demultiplexing different wave-lengths, switching or N×N routing, and as part of anadd–drop filter.

The AWG is based on three regions, the first and last actas passive star coupler slabs, and the middle one is anarray of rib waveguides of progressively increasinglength, to allow multiple beam interference at the outputby introducing incremental phase shifts to the raysemerging from the waveguides. Silicon AWGs benefitfrom the very small bending radii allowed by SOI, yieldingmuch more compact devices than the common PLCAWGs previously used in telecom. Currently reportedAWGs in SOI reach channel counts as high as 50 wave-lengths, with channel spacings ranging from 0.2 to 3.6nm (25 to 400 GHz at 1550 nm).

There are multiple design variations depending on the re-quirements of the end device, but the most critical designfeatures of an AWG BB will be the minimum bending ra-dius of the rib waveguides, the straight lengths (largeenough to taper the waveguides into the slab in a losslessmanner), and the minimum waveguide separation at thecentre (as to avoid coupling interaction). The length ofthe waveguide array should always be kept to a mini-mum as to minimise phase errors. Fig. 4 shows multipleAWG designs at different foundries.

As an example of an application for these AWG BBs, Fig.5 shows a microwave photonic beamformer chip, wheresingle side-band signals coming out of a Mach-Zehndermodulator are demultiplexed by an AWG and phase-shifted by means of tunable all-pass ring resonators. TheAWG was designed with 4 input and 4 output channels,and the additional channels at the input side were usedfor testing purposes. The AWG channel spacing was setto 3.2 nm with a Gaussian response and a FSR of 22.4

nm, enough to locate several resonances of the ring res-onators into the passband. The cross section of the wave-guides was 220×450 nm width, and for all thewaveguide apertures to the slab region of the star cou-plers in the AWG, an extra shallow etch process wasbeen employed to reduce the insertion losses [3].© 2013WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

4. Mach-Zehnder Interferometers

Interferometers are the core building block to many op-tical systems, and one of the most frequently used con-figurations is the Mach–Zehnder interferometer (MZI). Asany interferometer, it is used to measure phase shifts be-tween two arms through power interference at the com-bined output. Depending on the length difference ofeach arm, MZIs can be symmetric or asymmetric (see Fig.6). By dynamically varying the relative phase of the twoarms of the interferometer (e.g. by using heaters or byinserting an optical phase modulator into one of the in-terferometer arms), the intensity at the output of the MZIcan be modulated.

A MZI can serve as the basic BB of several devices suchas thermo-optic modulators or filters. Microring-en-hanced MZI structures are used as advanced BBs for sens-ing and switching applications, using ring resonatorscoupled to one arm of the MZI to give a resonant en-hancement of the phase shift in that arm.

5. Multimode Interference Couplers

A Multimode interference (MMI) coupler is based on theself-imaging property of a multimode waveguide, wherean input field profile is reproduced in single or multiple im-ages at periodic intervals along the propagation direction

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Silicon allows for small footprint and CMOS compatiblephotonic integrated circuits.

Figure 4. Different AWG BBs designed at multiplefoundries: LETI (up left), IME (up right), and IMEC (down).

Figure 5. Optical beamformer chip in SOI, including anAWG and multiple ring resonators, courtesy of Universi-tat Politècnica de València designers.

Figure 6. Different Mach-Zehnder interferometer BBsdesigned by VLC Photonics.

of guide [4]. Thus, MMIs can provide signal routing andcoupling over wide optical bandwidths, while being largelypolarization insensitive. Additionally, they usually showgood excess losses and crosstalk, and its phase relationscan be utilized to obtain phase dependent switching. Whilethey have relatively low port counts, silicon MMI couplersprove to be the most robust splitting devices when com-pared to directional couplers, Y-branches or star couplers.

In a MMI coupler BB, light is launched from a number ofN input waveguides into a central section, which is awaveguide designed to support a sufficiently high num-ber of modes. After the multimode interference, light iscaptured back into a number M of ouput waveguides,providing effectively the N×M coupling functionality.Common configurations for MMI BBs are e.g. 1×2, 1×4,2×2 or 4×4, and this will determine the device's shapeand length, from a few tens of �m to a few mm, as de-picted in Fig. 7. Coarse multiplexing functionality can alsobe achieved, with usual wavelength channel spacingsranging from 3.2 up to 34 nm.

Silicon MMI BBs have mainly commercial application asbroadband couplers with arbitrary power ratios and in-creasing port counts. MMIs are also a fundamental BB tobuild Mach-Zehnder delay interferometers, used in dif-ferential phase-shift keying (DPSK) demodulation and all-optical wavelength conversion, and on thermo-opticaland electro-optical switches.

6. Ring Resonators

The strong light confinement achieved in SOI waveguidesallows for very small losses at sharp bends, with radii ofonly a few micrometres. Thus, very compact rings res-onators can be made and used in multiple configurations(e.g. coupled resonator, all-pass filter, cascaded).

The design trade-offs for a ring resonator BB are betweenresonantly enhanced group delay, device size, insertionloss and operational bandwidth.

Most of the developments towards commercial applica-tion of ring resonator BBs are in the direction of opticalswitching matrix, optical filters (as shown in Fig. 8, [5])or thermally reconfigurable multiplexing devices, andthere are currently efforts towards their use for opticalbuffering applications [6] and label-free biosensing.

7. Conclusions

Silicon photonics is an optical integration platform with vastopportunities ahead, due to its potential CMOS integrationto reduce cost and power consumption, miniaturize func-tionalities, and scale up production. Multiple basic buildingblocks like the ones described here are already available atmost foundries and included in their corresponding pho-tonic design kits. More advanced funcionalities are beingcustomized by emerging design houses at the request ofend users, extending the number of optical applicationsthat can benefit from their integration in silicon. Whilethere are certainly challenges ahead, the flourishing siliconphotonics ecosystem is progressing and maturing rapidly,opening its doors to new fields and markets.

Acknowledgments

The authors would like to thank the Optical Communi-cations Group of the iTEAM Institute from the UniversitatPolitecnica de Valencia, Spain, for some of the chip ex-amples illustrated in this article.

References

[1] Graham T. Reed and Andrew P. Knights, "Silicon Pho-tonics: an introduction," Wiley, ISBN 0-470-87034-6,2004.

[2] Rocío Baños, Daniel Pastor, David Domenech, "Code-Tunable Direct Sequence Coherent OCDMA devicebased on Silicon On Insulator," Proc. of 15th ICTONConference, June 2013.

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VLC Photonics building block library has been tested inall the current the Silicon fabs.

Figure 7. Multiple MMI coupler BBs, fabricated at dif-ferent foundries: IMEC (up left), IME (up right), and LETI(down).

Figure 8. CROW filters composed of parallel and apodi -zed racetrack shaped ring resonators, courtesy of Univer-sitat Politècnica de València designers.

[3] J.D. Domenech, I. Artundo, P. Muñoz, J. Capmany, P.Dumon and W. Bogaerts, "Design, fabrication andcharacterization of a narrow band microwave pho-tonics beamformer in Silicon-On-Insulator," Proc. ofECIO, Sitges (Spain), 2012.

[4] Lucas B. Soldano and Erik C. M. Pennings, "OpticalMulti-Mode Interference Devices Based on Self-Imag-ing: Principles and Applications," Journal of Light-wave technology, Vol. 13(4), pp. 615, Apr 1995.

[5] J. Capmany, P. Muñoz, J. Domenech, and M. Muriel,"Apodized coupled resonator waveguides," Opt. Ex-press, Vol. 15, pp. 10196-10206, 2007.

[6] Fengnian Xia Lidija Sekaric and Yurii Vlasov, "Ultra-compact optical buffers on a silicon chip," NaturePhotonics, Vol. 1, pp. 65-71, Jan 2007.

Biographies

Iñigo Artundo obtained the M.Sc.in Telecom Engineering at the Uni-versidad Publica de Navarra (Pam-plona, Spain) in 2005, and receivedhis Ph.D. in Applied Physics andPhotonics at the Vrije UniversiteitBrussel (Brussels, Belgium) in 2009.He has been involved in several na-tional and European research proj-ects and networks of excellence

focused on reconfigurable optical interconnects, the de-sign, fabrication and characterization of micro-optic de-vices, and on flexible access and in-building fiber networkarchitectures. He has worked as a reviewer for several sci-entific journals and national funding agencies. He holdsspecializations in Business Financing, Commercial Man-agement and Research, and Strategic Marketing. He is amember of IEEE, SPIE and COIT.

Pascual Muñoz obtained theM.Sc. degree in TelecommunicationEngineering from Universitat Poli -téc nica de Valencia (UPVLC) in1998. After one year in the industryas IT Engineer, he returned toUPVLC to obtain his Ph.D degree in2003, where he developed refer-ence models for the design andmodeling of integrated optics Ar-

rayed Waveguide Grating (AWG) devices. The modelswere validated in collaboration with the company AlcatelOptronics (Livingston, UK). After his Ph.D. he moved toTechnische Universiteit Eindhoven (TUe), where he wasengaged on the design of AWG-based ASPICs for ultra-short pulses processing. Back into UPVLC, Pascual wasappointed as coordinator for the "Joint Research Activityon InP AWG based devices" (2007-2009) in the ePIXnetnetwork of excellence, where several research institutesand companies collaborated to develop multi-wave-length lasers for optical burst/packet switched networks.From 2008, Pascual was an Associate Professor at UPVLC.

José David Domenech receivedthe B.Sc. degree in Tselecommuni-cations and the M.Sc. degree inTechnologies, Systems and Net-works of Communication from theUniversidad Politecnica de Valencia(UPVLC) in 2006 and 2008 respec-tively. He is currently working to-wards the Ph.D. degree in optics at

the Telecommunications and Multimedia Applications In-stitute (iTEAM) from the Universidad Politécnica de Valen-cia, inside the Optical and Quantum CommunicationsGroup, and his research has been focused in the use of in-tegrated ring resonators for microwave photonics applica-tions. Since 2006, he has been working on the design ofintegrated optic circuits in Indium-Phosphide/Silicon Ni-tride/SOI technologies within several European and na-tional research projects.

Jan Hendrik den Besten graduatedin Physics and Dutch Literature at theUniversity of Groningen in 1999 andreceived his Ph.D. degree from theEindhoven University of Technologyin 2004 for his work on the model-ling and design, the technology andcharacterization of RF-modulatorsand multi-wavelength lasers in InP.

From 2004-2006 he coordinated a "facility access activity"in the European "Network of Excellence" ePIXnet. In prac-tice, this meant the coordination of researchers throughoutEurope cooperating with researchers in Eindhoven in thedevelopment of various ASPICs. He helped shaping the"Joint European Platform for InP-based Photonic IntegratedComponents and Circuits" (JePPIX), about which he wrotean MBA-thesis (TiasNimbas Business School, 2006). From2006-2011, he worked as development analyst and seniordesigner at ASML.

José Capmany obtained the M.Sc.degrees in Telecommunications En-gineering and Physics from Universi-dad Politecnica de Madrid (UPM)and UNED respectively. He holds aPh.D. in Telecommunications Engi-neering from UPM and a Ph.D. inPhysics from the Universidad deVigo. In 1991 he moved to the Uni-

versidad Politecnica de Valencia (UPVLC) where he hasbeen a full Professor in Photonics and Optical Communi-cations since 1996. In 2002 he was appointed Director ofthe Institute Of Telecommunications and Multimedia(iTEAM) at UPVLC. He has been engaged in many researchareas within the field of photonics during the last 25 yearsincluding Microwave Photonics, Integrated Optics, FiberBragg Grating design and fabrication, Optical Networksand, most recently Quantum Information systems. He haspublished over 375 papers in scientific journals and con-fererences. He is a Fellow of the Institute of Electrical andElectronic Engineers (IEEE) and the Optical Society ofAmerica (OSA). In 2012, he was awarded with the ReyJaime I on New Technologies.

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