ieee journal of quantum electronics, vol. 45, no. 4, …djm202/pdf/papers/... · bamiedakis et al.:...

IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 45, NO. 4, APRIL 2009 415

Cost-Effective Multimode Polymer Waveguides forHigh-Speed On-Board Optical Interconnects

Nikolaos Bamiedakis, Joseph Beals, IV, Richard V. Penty, Senior Member, IEEE, Ian H. White, Fellow, IEEE,Jon V. DeGroot, Jr., and Terry V. Clapp

Abstract—Cost-effective multimode polymer waveguides, suit-able for use in high-speed on-board optical interconnections, arepresented. The fundamental light transmission properties of thefabricated waveguides are studied under different launch condi-tions and in the presence of input misalignments. Low loss ( 0.04dB/cm at 850 nm) and low crosstalk � 30 dB� performance, re-laxed alignment tolerances � 20 m� and high-speed operation ata 10-Gb/s data rate are achieved. No degradation in the high-speedlink performance is observed when offset input launches are em-ployed. Moreover, a range of useful waveguide components thatadd functionality and enable complex on-board topologies are pre-sented. The optical transmission characteristics of the fabricatedcomponents are investigated and it is shown that excellent per-formance is achieved. Excess losses as low as 0.01 dB per wave-guide crossing, the lowest reported value for such components, andbending losses below 1 dB for 90-degree and S-shaped bends areobtained even with multimode fiber launches. Moreover, high-uni-formity power splitting and low-loss signal combining are achievedwith Y-shaped splitter/combiners while a variable splitting ratiobetween 30%–75% is demonstrated with the use of multimode cou-plers. Overall, the devices presented are attractive potential candi-dates for use in on-board optical links.

Index Terms—Multimode waveguides, optical interconnections,optical polymers, waveguide components.

I. INTRODUCTION

I N RECENT YEARS, there has been a continuing growth inthe demand for data communications link capacity. In turn,

this has driven the need for short-reach high-speed interconnectsthat are capable of operating at data rates greater than 10 Gb/s[1]–[3]. Existing interconnection technologies based on metalwiring technologies and using sophisticated electronic compen-sation techniques suffer from inherent disadvantages such aselectromagnetic interference, size-density, power and heat dis-sipation issues at high operating frequencies [4], [5]. Optical in-terconnects offer a promising solution to the performance bot-tleneck imposed by conventional electronic circuitry [6]–[8] andconstitute an intensive area of research both in industry and aca-demic institutions [9].

Manuscript received July 11, 2008; revised September 11, 2008. Current ver-sion published March 25, 2009. This work was supported by CAPE and DowCorning Corporation under the Project STOIC and by the EPSRC, U.K..

N. Bamiedakis, J. Beals, IV, R. V. Penty, and I. H. White are with the Elec-trical Engineering Division, Engineering Department, University of Cambridge,Cambridge CB3 0FA , U.K. (e-mail: [email protected]; [email protected];[email protected]; [email protected]).

J. V. DeGroot, Jr. and T. V. Clapp are with Dow Corning Corporation, Mid-land, MI 48686 USA.

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JQE.2009.2013111

The deployment of optical links in different levels of the in-terconnection hierarchy (e.g., card-to-card, module-to-module,chip-to-chip, or on-chip) is being thoroughly studied as differentrequirements and limitations exist at every level. The inevitablecoexistence of optics and electronics in future data systems how-ever imposes common key technological requirements that op-tics needs to satisfy at all interconnection levels, namely:

• cost effectiveness;• ability to be integrated into existing architectures;• compatibility with existing manufacturing processes of

conventional electronic circuitry.On-board optical interconnections, providing card-to-card

or module-to-module communications, constitute an area ofincreasing interest [10]–[13]. These applications require theuse of a low-cost material that can be integrated with standardprinted circuit boards (PCBs) while being able to withstand thehigh-temperature environments 250 C associated withthe lamination and soldering processes of PCB manufacturing.Polymer materials are potential candidates as they possessfavorable properties and can be sufficiently low-cost [14]–[16].Various types of polymer materials are being studied includingpolyimides [17], acrylates, such as SU-8 [18] and PMMA[19], and siloxanes [20] with particular attention being paidto whether they possess the required demanding thermal andmechanical properties and environmental stability. Moreover,multimode waveguides have received particular attention owingto their relaxed alignment tolerances that allow for reducedcosts for packaging and connectorization. Various fabricationtechniques are being employed to form waveguides and wave-guide devices, namely, photolithography [21], embossing [22],molding [23], direct writing [24], and laser ablation [25]. Fur-thermore, ways of efficiently and cost-effectively coupling lightin and out of the waveguides are being investigated. Severaldifferent approaches have been suggested and demonstrated:

• out-of-plane coupling employing 45 mirrors [26], [27],evanescent couplers [28] or grating couplers [13];

• butt-coupling using fiber ribbons [29], [30] or appropri-ately mounted VCSEL and photodiode arrays [31], [32].

The on-board integration of polymer waveguides and the ef-ficient implementation of specific on-board applications requirethe investigation of all relevant technological aspects and thesuccessful development of the corresponding key componentsand system architectures. As a result, this study presents a de-tailed study of a cost-effective technology based on multimodepolymer waveguides suitable for use in high-speed on-boardoptical interconnects. The polymer material employed exhibitsvery low intrinsic attenuation at the data communicationswavelength of 850 nm, allowing for the formation of cost-ef-

0018-9197/$25.00 © 2009 IEEE

Authorized licensed use limited to: CAMBRIDGE UNIV. Downloaded on July 7, 2009 at 10:53 from IEEE Xplore. Restrictions apply.

416 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 45, NO. 4, APRIL 2009

Fig. 1. (a) Schematic of cross section of waveguides and (b) photograph of a 1.4-m-long spiral waveguide illuminated by scattered red light.

fective waveguide devices on various substrates. The fabricatedwaveguides and waveguide components exhibit excellent trans-mission characteristics, indicating that they can be deployed asbuilding blocks of advanced system architectures.

The remainder of this paper is structured as follows. InSection II, the fundamental components of the technology(polymer multimode waveguides) and basic characteristics(e.g., loss, crosstalk performance, alignment tolerances, ro-bustness to temperature variation, and high speed operation)are presented. In Section III, the operation and performance ofkey functional blocks (waveguide components) under differentlaunch conditions are described. Finally, Section IV containsthe conclusions on the work presented.

II. POLYMER MULTIMODE WAVEGUIDES

A. Polymer Material

The siloxane materials OE-4140 (core) and OE-4141(cladding) developed by Dow Corning and employed in thiswork meet the key requirements of cost-effectiveness andcompatibility with existing manufacturing processes of con-ventional electronic circuitry. This siloxane polymer systempossesses excellent mechanical and thermal properties and isable to withstand the temperatures in excess of 250 C thatare required for solder reflow processes. Continuing migra-tion within the electronics industry to lead-free technologiesand high-melting-point solders such as AgSnCu render thisrequirement ever more acute [33]. The materials have beensubmitted to solder reflow and environmental stability testsand have successfully withstood temperatures up to 350 C.Additionally, no significant changes in intrinsic attenuation orrefractive index are observed after 1000 h at 85 C in an 85%relative humidity environment, nor after 500 thermal cyclesfrom 40 C up to 100 C [34].

Moreover, the polymer materials can be directly spun onto arange of substrates such as FR4, silicon, or glass and can be pat-terned by low-cost techniques such as photolithography and em-bossing. The refractive indexes of the bulk material are around1.5 at an 850-nm wavelength and can be tailored according to therequired application specific core-cladding index differences.

B. Multimode Waveguides

Core and cladding polymer materials, with bulk refractiveindexes of 1.52 and 1.5, respectively, are directly spun ontothe substrate while the multimode waveguides are patternedby conventional photolithographic techniques. The core layeris sandwiched between the bottom and top cladding layers

Fig. 2. Measured propagation loss as function of light wavelength for the fab-ricated waveguides.

[Fig. 1(a)]: the bottom cladding serves to mask any substrateroughness while the top cladding provides a protective layerover the waveguides and planarizes the sample surface. Allwaveguide samples are cut with a Disco 321 dicing saw. Thewaveguide cross section is chosen to be 50 50 m inkeeping with standard multimode fiber dimensions while theseparation between waveguides is 250 m, matching VCSELand photodiode array spacing and multimode fiber ribbons.Samples with varying separation distances have been realizedin addition in order to investigate the maximum achievablewaveguide density. Straight waveguides with lengths up to0.125 m and spiral waveguides with lengths of 1.4 m [Fig. 1(b)]have been fabricated and tested to derive their fundamentallight propagation properties.

A white light source and samples of different lengths areused to determine the propagation loss at different wavelengths(Fig. 2). Loss values are in the range 0.03–0.05 dB/cm at 850nm and 0.36–0.4 dB/cm at 1310 nm.

C. Transmission Characteristics

Crosstalk performance, input misalignment tolerances, andinsensitivity to temperature variation are examined. The indexdifference between core and cladding is 0.02 resulting in highlymultimoded guides. The light transmission characteristics areinvestigated in the case of SMF and MMF input launch condi-tions. In all cases, VCSEL sources operating at 850 nm are used.

Crosstalk performance is an important link parameter as itcan dictate the maximum achievable waveguide density. Two


BAMIEDAKIS et al.: COST-EFFECTIVE MULTIMODE POLYMER WAVEGUIDES FOR HIGH-SPEED ON-BOARD OPTICAL INTERCONNECTS 417

Fig. 3. Normalized received optical power at the output of a straight waveguideas a function of the position of the input fiber for varying waveguide lengths L.Input-output fiber: 50 �m MMF; separation � � 250 �m. Inset: schematic ofmeasurement setup. Gray features denote the relative position of the launch andadjacent waveguides.

Fig. 4. Normalized received optical power at the output of a straight wave-guide as a function of the position of the input fiber for varying waveguide sep-aration D. Input-output fiber: 50 �m MMF; sample length � � 10 mm. Inset:schematic of measurement setup.

types of crosstalk can be distinguished: intrinsic crosstalk thatis due to mode coupling between adjacent waveguides andextrinsic crosstalk that results from background light coupledinto the waveguide. Mode coupling between adjacent parallelwaveguides can be minimized by positioning the guides suf-ficiently far apart from each other. Extrinsic crosstalk can beattributed to the coupling of background scattered light due tosurface roughness and waveguide imperfections (waveguiderelated) and nonoptimized input coupling (coupling related).It is found that the level of induced crosstalk in adjacent par-allel waveguides increases with increasing propagation length(Fig. 3) and decreasing separation distance (Fig. 4). Longerruns of parallel waveguides and smaller separation distancesincrease both the strength of the mode coupling mechanism(intrinsic crosstalk) and the probability of any backgroundscattered light coupling into adjacent waveguides (extrinsiccrosstalk). However, in all cases, the recorded crosstalk levelsare sufficiently low for on-board interconnections. More specif-ically, for all separations studied (100 to 250 m), crosstalklevels are below 30 dB both for SMF and 50- m MMF inputs(Fig. 4) while for the longest straight waveguides that are mostsusceptible to crosstalk (lengths of 125 mm) measured valuesare approximately 50 dB for an SMF input and 35 dB for

Fig. 5. Normalized received optical power at the output of a straight wave-guide as a function of the position of the input fiber for the longest straightwaveguide sample and for both a SMF and a 50-�m MMF input. Sample length� � 125 mm, waveguide separation� � 250 �m, and output fiber type 50-�mMMF. Gray features denote the relative position of the launch and adjacentwaveguides.

Fig. 6. Normalized received power at the output of a straight waveguide as afunction of input fiber position and different operating temperatures.

Fig. 7. Normalized frequency response of a 1.4-m-long spiral waveguide foran SMF and a 50-�m MMF input.

a 50- m MMF input (Fig. 5). Moreover, the level of back-ground scattered light coupled into the waveguide remains thesame 32 dB when the light is launched directly into theregion between the waveguides (Fig. 5). The results are veryencouraging as they indicate that any input misalignment willnot severely impact the operation of adjacent waveguides.



Fig. 8. (a) Normalized frequency response of a 1.4-m-long spiral waveguide for different lateral offsets of an SMF input and received eye diagrams for (b) theback-to-back link and (c) the link with the spiral waveguide.

The relaxed alignment tolerances of multimode waveguidesare one of their key advantages as they allow for reduced con-nectorization and packaging costs. In low-cost systems, inputmisalignments are likely, and therefore an estimation of themaximum allowed values is required. The power received atthe waveguide output is recorded when the input fiber is offsetin both horizontal and vertical directions. The half power pointsare obtained at approximately 22.5 m for both directions foran SMF input (Fig. 6) and 17.5 m for a 50- m MMF input.

Finally, the stability of power transmission characteristics ofthe straight waveguide samples has been investigated for tem-peratures up to 90 C. Overall, a slight increase of the insertionloss (0.2 dB), a slight decrease in the half power misalignmenttolerance (5 m in total), and a slight increase in backgroundscattering noise (3 dB) are observed (Fig. 6). This slight perfor-mance degradation can be attributed to material stresses inducedby the increasing operating temperature. The results howeverclearly show that a large variation in the operating temperaturedoes not severely impact the waveguides’ transmission charac-teristics.

D. Data Transmission

The high-speed operation of a sample link comprising a mul-timode VCSEL source, a 1.4-m-long spiral waveguide, and aphotodiode is investigated using a network analyzer, a digitaloscilloscope, and a bit-error-rate tester (BERT). The VCSEL isdirectly modulated at a 10-Gb/s data rate by a short pseu-dorandom bit sequence (PRBS) to mimic the short codes (e.g.,8B10B, GbE) employed in datacommunication links. The fre-quency response of the link is obtained by recording the -pa-rameter with and without the waveguide. A flat frequency re-sponse is recorded up to at least the 7-GHz instrumentationlimit for both a SMF and a 50- m MMF input (Fig. 7). More-over, the response remains flat when the input is offset in boththe horizontal and vertical direction [Fig. 8(a)]. The receivedeye diagrams for the link with and without (back-to-back) thewaveguide are shown in Fig. 8(b) and (c). No significant ad-ditional noise or pulse dispersion is induced with the insertionof the waveguide. BER measurements confirm error-free oper-ation while the transmission power penalty for a BER isapproximately 0.5 dB.

The robustness of the link’s high-speed performance in thepresence of input spatial offsets is further examined. Offset

Fig. 9. FWHM of received electrical signal and normalized received opticalpower as a function of the input position for a restricted lens input and pulsetransmission via a 1.4-m-long spiral waveguide.

launch studies in other forms of multimode waveguides (suchas MMF) have shown that small variations in the position of theinput spot can lead to a significant degradation of the availablelink bandwidth [35]–[37]. A restricted launch scheme typicallygenerates a highly selective mode power distribution inside thewaveguide which can vary substantially between different inputpositions. The high-speed performance of the link employing a1.4-m-long spiral waveguide is thus studied when a free-spaceinput is used and the generated small input spot is offset in bothhorizontal and vertical directions. A 128-bit-long pattern at a10-Gb/s data rate with a unique “1” bit isemployed to imitate the transmission of a single pulse throughthe link. The optical signal is launched into the spiral waveguidevia a microscope objective (10 ), collected at the output endwith a 62.5- m MMF and delivered to the photodiode. Thereceived Gaussian-shaped electrical pulse is sampled with thedigital oscilloscope (every 0.1 ps) while its width is measuredfor the different input spot positions (Fig. 9).

No performance degradation is observed as the pulse’s full-width-at-half-maximum (FWHM) remains stable as the spot isoffset from the position of maximum power transmission. Therepeatability of the observations is confirmed by similar mea-surements carried out on different spiral waveguides. The re-sults indicate that, for the link parameters under consideration



Fig. 10. Photographs of fabricated components on silicon substrate: (a) waveguide crossings, (b) 90 bends, (c) S-bends, and (d) Y-shaped splitters/combiners.

Fig. 11. Insertion loss of waveguide crossings for SMF and 50-�m MMFinputs.

(waveguide lengths up to 1.4 m for these guides and data ratesup to 10 Gb/s), input offsets have no severe impact on the link’shigh-speed performance.

III. WAVEGUIDE COMPONENTS

Increased on-board functionality can be obtained and com-plex topologies enabled by the deployment of passive integratedwaveguide components in the optical layer. A series of usefulpolymer multimode components has hence been designed,fabricated and characterized: 90 and S-shaped bends, wave-guide crossings, Y-splitters/combiners, and directional couplers(Fig. 10). Due to the multimode nature of the devices, guidingbehavior strongly depends on the type of input excitation used.The fabricated components are thus characterized for both anSMF and an MMF launch.

A. Waveguide Crossings

Waveguide crossings constitute an important building blockfor any on-board architecture as their use can minimize the totallink lengths, maximize the available usable on-board area, in-crease the achieved interconnection density and eliminate theneed for additional optical layers. The induced loss per crossingdepends on the crossing angle and it has been shown that it de-creases for an increasing crossing angle [38]. Samples with avarying number of waveguide crossings (0 to 100) have beencharacterized with both SMF and 50- m MMF inputs. The lossvalues estimated from the slope of the insertion loss plots are0.006 and 0.01 dB/crossing, respectively (Fig. 11) and consti-tute the lowest reported values for polymer multimode wave-guide crossings. The devices benefit from the multimode nature

Fig. 12. Normalized received optical power at the output of a waveguide as afunction of the position of the input fiber and for a varying number of waveguidecrossings. Input: SMF—sample length: 20 mm. Gray features denote positionof waveguides.

Fig. 13. Measured bending loss for 90 bends for SMF and 50-�m MMF in-puts and theoretically computed values using a ray tracing model and a uniform(overfilled) and a Gaussian (underfilled, � � � ) ray power distribution.

of the guides as the light is primarily guided in the center re-gion of the guide. The use, however, of MMF launches resultsin higher loss values as a MMF launch typically couples a largeramount of to higher order modes that are more susceptible tocrossing losses.

Moreover, the induced crosstalk in the intersectingwaveguides is below the sensitivity of the power meter,thus indicating crosstalk levels better than 60 dB. However,crosstalk in adjacent parallel waveguides increases with anincreasing number of crossings (Fig. 12). The worst valuesobtained from the waveguides with the largest number ofcrossings (100) are of the order of 30 dB for an SMF and

25 dB for a 50- m MMF input which corresponds to a 15-dB



Fig. 14. (a) Normalized received power at next adjacent bent waveguide for different radii of curvature and input types and (b) schematic of the measurementsetup (� � 250 �m, output fiber: 50-�m MMF).

Fig. 15. Schematic of S-shaped bends generated by (a) two circular arcs of constant radius R and (b) a raised cosine function. (c) Measured bending loss for raisedcosine S-bends with a lateral offset D of 10 mm as a function of the minimum radius of curvature � for an SMF and a 50-�m MMF input.

performance degradation. The recorded values are howeversufficiently low for on-board interconnections.

B. Waveguide Bends

Waveguide bends enable the use of curved optical paths andnon co-linear topologies. 90 bends and raised cosine S-bendscan provide interconnection between points with a perpendic-ular orientation and a lateral displacement respectively. Further-more, their deployment eliminates the need for in-plane mirrorswhich require additional fabrication steps and are very suscep-tible to surface roughness and sidewall verticality issues. Theloss induced by the curved optical path depends on the wave-guide index step, waveguide dimensions and mode power distri-bution within the guide. High-order modes are more susceptibleto radiation loss at waveguide bends and thus MMF launchestypically result in higher bending loss values. Fig. 13 shows thebending loss for the fabricated 90 bends as a function of thebend radius for both a SMF and a 50- m MMF input. Theo-retical loss values computed for the fabricated waveguide pa-rameters using a simple ray tracing model are also shown. Themodel assumes a uniform and a Gaussian ray power distribu-tion inside the guide to approximate an overfilled and an under-filled launch, respectively, and calculates the bending loss in-duced by the curved path [39]. The experimental data and thetheoretical values agree well for large radii of curvature. Forsmall radii ( 5 mm), however, a difference occurs mainly dueto the transition losses that become significant and that are nottaken into account by the ray tracing approach. Overall, bending

losses below 1 dB are achieved for radii of curvature larger than4.5 mm for an SMF input and larger than 8 mm for a 50- mMMF input. Depending on the application requirements, avail-able board area, launch conditions, and system power budget,an appropriate bend radius can be chosen.

The increased radiation loss generated by the curved op-tical path raises concerns regarding crosstalk in neighboringwaveguides. Therefore, samples comprised of nested wave-guide bends are employed to estimate the induced crosstalklevels in adjacent guides. The input fiber is aligned with eachsample waveguide in order to achieve maximum power trans-mission while the output fiber is employed to measure thepower received at the output of the next adjacent waveguidemost susceptible to radiation [Fig. 14(b)]. The measurement isrepeated for both SMF and 50- m MMF inputs. In both cases,excellent crosstalk behaviour is achieved as the recorded levelsare below 40 dB [Fig. 14(a)]. These results demonstrate theweakness of the coupling of radiated light into neighboringwaveguides while being consistent with the excellent crosstalkperformance obtained from straight and crossed waveguides.

A lateral offset in the optical path can be achieved with theuse of S-shaped waveguide bends. To minimize the inducedbending losses various shape profiles have been studied in-cluding simple curves made by two circular arcs of constantradius [Fig. 15(a)] and smooth curves generated by sine orcosine functions [40]–[42]. Smooth curves such as raisedcosine S-bends [Fig. 15(b)] exhibit a gradual variation in theradius of the curvature along their length thereby minimizing



Fig. 16. Near-field images at the output of (a) a 65-mm-long straight wave-guide, (b) a 30-mm-long straight waveguide with 90 crossings and (c) a 90bent waveguide �� mm� with an SMF input.

Fig. 17. Schematic of Y-shaped components: (a) 1 � 2 and (b) 1 � 8 splitter.

the transition losses associated with the mode mismatches atthe interface of the straight and curved sections and at theinflection point of S-bends with arcs of fixed radius. Dependingon the required lateral displacement, the minimum radius ofcurvature of the raised cosine bend can be chosen appropriatelyso that the total bend length and the induced bending loss areminimized. In Fig. 15(c), the excess loss of the raised cosinebends for the different input types for samples with a lateraldisplacement of 10 mm is shown. The use of S-bends with aminimum radius of curvature larger than 2.5 mm and 5 mm foran SMF and a 50- m MMF input, respectively, ensures excesslosses below 1 dB.

Mode mixing is an important issue in such short-lengthwaveguides as the assumptions typically made in optical fibersdo not necessarily hold [43]. It should be noted that the useof passive waveguide components can increase the amount ofinduced mode mixing in the guides. Near-field images takenat the output of straight waveguide samples and the aforemen-tioned waveguide components in the case of a restricted launchinput (SMF) suggest that, although the effect is not significantfor straight waveguides, the more complex structures sufferfrom increased mode mixing (Fig. 16).

C. Y-Splitters/Combiners

Y-splitters/combiners are useful components for applicationsthat require signal multicasting (e.g., for clock distribution) orcombining multiple signals. The device design consists of aninput arm, an up-tapering region, and two S-bends that providethe required separation between the output arms. The length ofthe S-bend sections is appropriately chosen to minimize the in-duced bending loss. Multiple stage Y-splitters are created bycascading a series of Y-junctions (Fig. 17).

Multimode polymer 1 2, 1 4 and 1 8 Y-shaped deviceshave been fabricated on various substrates and characterized inthe case of an SMF and an MMF input when used as splittersand combiners.

When used as splitters, MMF launches are preferred, as thosetypically result in coupling power to a large number of modesinside the input arm, and therefore a relatively uniform and ro-bust power splitting can be achieved even in the presence ofinput misalignments. The maximum power imbalance between

Fig. 18. Maximum imbalance between output arms of the 1� 8 Y-splitter for50- and 62.5-�m MMF input (waveguide core within �25 �m).

the output arms is lower than 1 and 1.2 dB for the 1 8 de-vices for a 50- and 62.5- m MMF launch, respectively, and forall positions of the input fiber (Fig. 18). Excess losses due to thecurved sections and the acute edge at the diverging point of theoutput arms are found to be as low as 0.8 dB.

However, the devices exhibit a differing behavior when usedas combiners as the multimode nature of the guides allows signalcombining without the 3-dB penalty/join, which is normally ob-served in equivalent single-mode devices [44]. This advantagestems from the fact that, in multimode combiners, the modalvolume is typically underfilled at the input of the combining re-gion and results in the excitation of local supermodes that canefficiently couple to guided modes in the output arm. The lossinduced in the combining region depends on the efficiency of thecoupling of the excited supermodes to the guided modes of theoutput arm as they propagate through the down-tapered region.As higher order modes exhibit stronger coupling to radiationmodes, MMF launches lead to significantly higher loss valuesfor the multimode combiners when compared to the values ob-tained with a SMF input. The lower order modes that carry themajority of power in that case can propagate through the struc-ture without significant coupling to radiation modes at the com-bining regions and without excessive bending loss at the curvedsections. For example, for an 8 1 combiner, the use of a SMFinput results in an insertion loss for all arms of approximately4 dB (Fig. 19). This clear power advantage (a 5-dB improvementover similar single mode device) and asymmetry in the deviceoperation (a 6-dB difference from insertion loss values whenused as a 1 8 splitter) can be exploited in optical systems thatrequire enhanced performance for the combining function.

Table I summarizes the recorded total insertion loss values(including coupling, propagation and splitting losses) for thetwo configurations (splitter/combiner) and for the different inputtypes. The results demonstrate the differing behavior of the de-vices when used as splitters and combiners.

D. Directional Couplers

Cost-effective multimode directional couplers suitable forhigh-speed on-board interconnects have been designed, fabri-cated and characterized. Typical coupler devices employed inintegrated systems are star couplers and evanescent couplers.Star couplers on the one hand offer a uniform splitting ratio as aresult of the mode mixing and power redistribution between the



Fig. 19. Insertion loss of an 8-way device used as a splitter and a combiner fora SMF input.

TABLE ITOTAL INSERTION LOSS OF FABRICATED DEVICES WITH DIFFERENT INPUT

TYPES WHEN USED AS A SPLITTER (SPLIT) AND A COMBINER (COMB)

Fig. 20. Illustration of the cross section of fabricated parallel waveguides inproximity for a (a) large �D/H � 0.5� and (b) small (D/H � 0.5) separationaspect ratio. (c) Schematic of a fabricated polymer coupler.

large number of modes within the coupler branch and usuallyrequire long device lengths [45], [46]. On the other hand,the design of evanescent couplers necessitates positioning theguides in proximity to allow for evanescent field coupling [47].In the case of polymer multimode waveguides, however, thelarge index steps employed and waveguide dimensions lead tothe requirement for a narrow gap on the order of a few microns.In such cases, where the separation aspect ratio is smaller thanthe resolution of the process will allow, sidewall deformationoccurs and as a result high losses and crosstalk levels are ob-served [Fig. 20(a) and (b)]. Additional fabrication steps have tobe undertaken (etching/post-processing) increasing the overallfabrication costs and reducing the total yield.

The fabrication limitation is overcome by merging the cou-pler branches in a single wide main branch. The device designconsists of input/output S-shaped branches and the main cou-pler branch [Fig. 20(c)]. Some waveguide deformation still oc-curs at the junction points of the input-output arms [points A-Bin Fig. 20(c)] but only for a short waveguide length.

Fig. 21. Top view of (a) a simple model of a multimode coupler, (b) exampleof light propagation inside main coupler branch, and (c) corresponding intensityplots at cross sections A, B, and C.

The operation of the device resembles a multimode inter-ference (MMI) device with multimoded input and output arms[48], [49]. The power coupled to each one of the output armsdepends on the mode interference of the guided modes at theend of the coupler branch. Therefore, the length of the cou-pler section and the input mode distribution both strongly af-fect the power splitting behaviour. Close to the input arm-cou-pler branch interface, the light spatial intensity oscillates dueto strong mode interference caused by the small mode phasedifferences (Fig. 21). The magnitude of oscillation graduallydecreases with increasing branch lengths as the relative modephase differences become more uniformly distributed and themode power distribution becomes smoother due to the inducedmode mixing within the guide. Utilising short branch lengths (ofthe order of a few millimetres) in contrast to the long lengths ofstar couplers, high splitting-ratios between the output couplerarms can be achieved.

A simple model based on the geometry shown in Fig. 21(a)is utilized to explore the power splitting behavior of the device.The guided modes in the arms and the main branch of the cou-pler are computed with the help of the effective index methodwhile the power coupled into the output ports is found by calcu-lating the overlap integrals of the modes at the input and outputinterfaces. The phase shift of each guided mode in the main cou-pler branch is taken into account. Even in the case of a uniforminput mode power excitation with random phases, it is foundthat power oscillations between the output arms exist up to atleast 4 mm of length for the main coupler branch.

A 50- and 62.5- m MMF are used to launch light in each oneof the input arms of multimode couplers with varying branchlengths (0 to 4.5 mm). For both input types, power splitting ra-tios between 30% and 75% are obtained while symmetric be-havior is observed for both input arms (Fig. 22). The varia-tion of the splitting ratio with changing input position is alsoinvestigated by recording the minimum and maximum spittingratios for each device as the launch is shifted in both directions(nonfilled points in Fig. 22). For both 50- and 62.5- m MMF in-puts, a small variation is observed ( 2.5% in average), demon-strating the insensitivity of the power splitting to input misalign-ments (which are likely in low-cost integrated optical systems).



Fig. 22. Fraction of output power received at the antisymmetric port as a func-tion of the coupler branch length for a 50-�m MMF input. Nonfilled points rep-resent the minimum and maximum recorded values when changing the positionof the input fiber while filled points correspond to the average recorded values.

The devices’ excess loss is 0.8 dB which may be mainly at-tributed to the bending losses and the waveguide deformationat the acute points A and B [Fig. 20(c)].

IV. CONCLUSION

Siloxane polymer waveguides are suitable candidates foruse in on-board optical interconnects as they offer a cost-ef-fective high-performance technology that can be integratedwith conventional PCBs. A detailed study of the transmissioncharacteristics of fabricated polymer multimode waveguidesunder different input configurations and of a series of usefulwaveguide components is presented. The waveguides exhibitlow loss of approximately 0.04 dB/cm at 850 nm and an excel-lent crosstalk performance of below 30 dB, even for adjacentwaveguides, and for all input configurations. Relaxed alignmenttolerances of approximately 20 m, insensitivity to operatingtemperature variations, and high-quality data transmission at10 Gb/s via 1.4-m-long spiral waveguides are demonstrated.The robustness of the high-speed operation is confirmed evenin the presence of input spatial offsets. Moreover, low-loss andlow-crosstalk waveguide crossings and bends, high-uniformityY-splitters/combiners with differing behaviors between thetwo configurations, as well as multimode couplers achievinghigh-power splitting ratios are presented. Overall, the wave-guide devices exhibit a very encouraging performance andconstitute a valuable component basis. Their deployment canprovide improved performance in various system applicationsand enable novel on-board interconnection architectures.

REFERENCES

[1] A. F. J. Levi, “Optical interconnects in systems,” Proc. IEEE, vol. 88,no. 6, pp. 750–757, Jun. 2000.

[2] A. F. Benner et al., “Exploitation of optical interconnects in futureserver architectures,” IBM J. Res. Develop., vol. 49, no. 5, pp. 759–782,2005.

[3] A. F. Benner, P. K. Pepeljugoski, and R. J. Recio, “A roadmap to 100G Ethernet at the enterprise data center,” IEEE Trans. Commun. Mag.,vol. 45, no. 11, pp. 10–17, Nov. 2007.

[4] M. Horowitz, C.-K. K. Yang, and S. Sidiropoulos, “High-speed elec-trical signaling: Overview and limitations,” IEEE Micro, vol. 18, no. 1,pp. 12–24, Jan.–Feb. 1998.

[5] D. A. B. Miller, “Physical reasons for optical interconnection,” Int. J.Optoelectron., vol. 11, pp. 155–168, 1997.

[6] D. A. B. Miller, “Rationale and challenges for optical interconnects toelectronic chips,” Proc. IEEE, vol. 88, no. 5, pp. 728–749, May 2000.

[7] D. Huang, T. Sze, A. Landin, and R. LytelH. L. Davidson, “Opticalinterconnects: Out of the box forever?,” IEEE J. Sel. Topics QuantumElectron., vol. 9, no. 2, pp. 614–623, Mar./Apr. 2003.

[8] E. D. Kyriakis-Bitzaros, N. Haralabidis, M. Lagadas, A. Georgakilas,Y. Moisiadis, and G. Halkias, “Realistic end-to-end simulation of theoptoelectronic links and comparison with the electrical interconnec-tions for system-on-chip applications,” J. Lightw. Technol., vol. 19, no.10, pp. 1532–1542, Oct. 2001.

[9] S. Uhlig and M. Robertsson, “Limitations to and solutions for opticalloss in optical backplanes,” J. Lightw. Technol., vol. 24, no. 4, pp.1710–1724, Apr. 2006.

[10] L. Schares et al., “Terabus: Terabit/second-class card-level optical in-terconnect technologies,” IEEE J. Sel. Topics Quantum Electron., vol.12, no. 5, pp. 1032–1044, Sep.–Oct. 2006.

[11] A. Glebov et al., “Integration technologies for pluggable backplane op-tical interconnect systems,” Opt. Eng., vol. 46, p. 015403, 2007.

[12] C. Berger et al., “Optical links for printed circuit boards,” in Proc. IEEELEOS, 2003, vol. 1, pp. 61–62.

[13] R. T. Chen, L. Lin, C. Choi, Y. J. Liu, B. Bihari, L. Wu, S. Tang, R.Wickman, B. Picor, M. K. Hibb-Brenner, J. Bristow, and Y. S. Liu,“Fully embedded board-level guided-wave optoelectronic intercon-nects,” Proc. IEEE, vol. 88, no. 6, pp. 780–793, Jun. 2000.

[14] H. Ma et al., “Polymer-based optical waveguides: Materials, pro-cessing, and devices,” Adv. Mater., vol. 14, pp. 1339–1364, 2002.

[15] L. Eldada and S. W. Shacklette, “Advances in polymer integrated op-tics,” IEEE J. Sel. Topics Quantum Electron., vol. 6, no. 1, pp. 54–68,Jan.–Feb. 2000.

[16] W. H. Wong et al., “Polymer devices for photonic applications,” J.Cryst. Growth, vol. 288, pp. 100–104, 2205.

[17] N. Kawakami, J. Kobayashi, M. Hikita, A. Kudo, F. Yamamoto, andS. Imamura, “Filter-embedded four-channel WDM module fabricatedfrom fluorinated polyimide,” J. Lightw. Technol., vol. 24, no. 6, pp.2388–2393, Jun. 2006.

[18] M. P. Immonen et al., “Investigation of environmental reliability ofoptical polymer waveguides embedded on printed circuit boards,” Mi-croelectron. Rel., vol. 47, pp. 363–371, 2007.

[19] D. Bosc et al., “Strengthened poly(methacrylate) materials for op-tical waveguides and integrated functions,” Opt. Mater., vol. 30, pp.1514–1530, 2008.

[20] S. Kopetz et al., “Polysiloxane optical waveguide layer integrated inprinted circuit board,” Electron. Lett., vol. 40, pp. 668–669, 2004.

[21] H. Schröder et al., “Waveguide and packaging technology for opticalbackplanes and hybrid electrical-optical circuit boards,” presented atthe Proc. of SPIE, 2006, paper 612407, 6124.

[22] H. Mizuno et al., “Low-loss polymeric optical waveguides with largecores fabricated by hot embossing,” Opt. Lett., vol. 28, pp. 2378–2380,2003.

[23] D. K. Cai et al., “Optical absorption in transparent PDMS materialsapplied for multimode waveguide fabrication,” Opt. Mater., vol. 30,pp. 1157–1161, 2008.

[24] H. Suyal et al., “Direct laser written polymer structures and passivedevices for photonics applications,” in Proc. CLEO Europe, 2005, p.505.

[25] G. Van Steenberge, N. Hendrick, E. Bosman, J. Van Erps, H.Thienpont, and P. Van Deele, “Laser ablation of parallel opticalinterconnect waveguides,” IEEE Photon Technol. Lett., vol. 18, no. 9,pp. 1106–1108, May 2006.

[26] M. P. Immonen, M. Karppinen, and J. K. Kivilahti, “Fabricationand characterization of polymer optical waveguides with integratedmicromirrors for three-dimensional board-level optical interconnects,”IEEE Trans. Electron. Pkg. Manuf., vol. 28, no. 4, pp. 304–311, Oct.2005.

[27] G. Van SteenbergeP. GeerinckS. Van PutJ. Van KoetsemH. OttevaereD.MorlionH. TheinpontP. Van Deele, “MT-compatible laser-ablated in-terconnections for optical printed circuit boards,” J. Lightw. Technol.,vol. 22, no. 9, pp. 2083–2090, Sep.–Oct. 2004.



[28] J. J. Yang et al., “Array waveguide evanescent ribbon coupler forcard-to-backplane optical interconnects,” Opt. Lett., vol. 32, 2007.

[29] H.-D. Bauer et al., “Polymer waveguide devices with passive pig-tailing: An application of LIGA technology,” Synthetic Metals, vol.115, pp. 13–20, 2000.

[30] S. Park, J.-M. Lee, and S. C. Koo, “Fabrication method for passivealignment in polymer PLCs with U-grooves,” IEEE Photon. Technol.Lett., vol. 17, no. 7, pp. 1444–1446, Jul. 2005.

[31] T. Lamprecht et al., “Passive alignment of optical elements in a printedcircuit board,” in Proc. ECTC, 2006, pp. 761–767.

[32] Y.-J. Chang, D. Guidotti, L. Wan, T. K. Gaylord, and G.-K. Chang,“Board-level optical-to-electrical signal distribution at 10 Gb/s,” IEEEPhoton. Technol. Lett., vol. 18, no. 17, pp. 1828–1830, Sep. 2006.

[33] J. H. Lau and K. Liu, “Global trends in lead-free soldering,” Adv. Pack-aging, vol. 13, no. 1, 2004.

[34] J. V. DeGroot, Jr., “Cost effective optical waveguide components forprinted circuit applications,” Proc. SPIE, vol. 6781, Passive Compo-nents and Fiber-Based Devices IV, 2007.

[35] L. Raddatz, I. H. White, D. G. Cunningham, and M. C. Nowell, “Anexperimental and theoretical study of the offset launch technique forthe enhancement of the bandwidth of multimode fiber links,” J. Lightw.Technol., vol. 16, no. 3, pp. 324–331, Mar. 1998.

[36] L. Raddatz, I. H. White, D. G. Cunningham, and C. Nowell, “Influenceof restricted mode excitation on bandwidth of multimode fiber links,”IEEE Photon. Technol. Lett., vol. 10, no. 4, pp. 534–536, Apr. 1998.

[37] P. Pepeljugoski, M. J. Hackert, J. S. Abbott, S. E. Swanson, S. E.Golovich, A. J. Ritger, P. Kolesar, Y. C. Chen, and P. Pleunis, “Devel-opment of system specification for laser-optimized 50 �m multimodefiber for multigigabit short-wavelength LANs,” J. Lightw. Technol.,vol. 21, no. 5, pp. 1256–1274, May 2003.

[38] D. A. Zauner et al., “High-density multimode integrated polymer op-tics,” J. Opt. A, Pure Appl. Opt., vol. 7, pp. 445–450, 2005.

[39] C. Winkler et al., “Loss calculations in bent multimode optical waveg-uides,” Opt. Quantum Electron., vol. 11, pp. 173–183, 1979.

[40] L. D. Hutcheson et al., “Comparison of bending losses in integratedoptical circuits,” Opt. Lett., vol. 5, pp. 276–278, 1980.

[41] R. Baets and P. E. Lagasse, “Loss calculation and design of arbitrarilycurved integrated-optic waveguides,” J. Opt. Soc. Amer., vol. 73, pp.177–182, 1983.

[42] K. T. Koai and P.-L. Liu, “Modeling of Ti:LiNbO waveguide devices.Part II. S-shaped channel waveguide bends,” J. Lightw. Technol., vol.7, no. 7, pp. 1016–1022, Jul. 1989.

[43] F. E. Doany et al., “Measurement of optical dispersion in multimodepolymer waveguides,” in Dig. LEOS Summer Top. Meet., 2004, pp.31–32.

[44] R. Baets and P. E. Lagasse, “Calculation of radiation loss in integrated-optic tapers and Y-junctions,” Appl. Opt., vol. 21, pp. 1972–1978, 1982.

[45] R. Griffin, J. D. Love, P. R. A. Lyons, D. A. Throncraft, and S. C.Rashleigh, “Asymmetric multimode couplers,” J. Lightw. Technol., vol.9, no. 11, pp. 1058–1517, Nov. 1991.

[46] S. Musa et al., “Low-cost multimode waveguide couplers for multi-mode fibre-based local area networks,” in Proc. IEEE LEOS AnnualMeeting, 2000, vol. 2, pp. 468–469.

[47] J. Kobayashi, T. Matsura, S. Sasaki, and T. Maruno, “Directional cou-plers using fluorinated polyimide waveguides,” J. Lightw. Technol., vol.16, no. 4, pp. 610–614, Apr. 1998.

[48] L. B. Soldano and E. C. M. Pennings, “Optical multi-mode interferencedevices based on self-imaging: Principles and applications,” J. Lightw.Technol., vol. 13, no. 4, pp. 615–627, Apr. 1995.

[49] I. Ilic, R. Scarmozzino, R. M. Osgood, Jr., J. T. Yardley, K. W. Beeson,and M. J. McFarland, “Modeling multimode-input star couplers inpolymers,” J. Lightw. Technol., vol. 12, no. 6, pp. 996–1003, Jun.1994.

Nikos Bamiedakis received the Diploma degree in electrical and computerengineering from National Technical University of Athens (NTUA), Athens,Greece in 2003. He is currently working toward the Ph.D. degree at the Centrefor Photonic Systems, University of Cambridge, Cambridge, U.K.

His research focuses on the design of single-mode and multimode waveguidesand waveguide components for high-speed optical interconnects.

Joseph Beals IV received the B.S. degree in electrical engineering from BrownUniversity, Providence, RI, in 2004. He is currently working toward the Ph.D.degree at the Centre for Photonic Systems, University of Cambridge, Cam-bridge, U.K.

Following his undergraduate work he was a Senior Research Assistant withBrown University, conducting research into optoelectronic components for animplantable neural-recording device. His research focuses on polymer opticalwaveguides for high-bandwidth interconnect systems.

Richard V. Penty (SM’08) received the Ph.D. degree from the University ofCambridge, Cambridge, U.K., in 1989. His doctoral work focused on opticalfiber devices for signal processing applications.

He returned to the University of Cambridge in 2001, having been a Pro-fessor with the University of Bristol. His research interests include high-speedoptical communications systems, WDM switched networks, optical amplifiers,high-speed and high-power semiconductor lasers, and radio-over-fiber and LANsystems. He is an author or coauthor of over 500 refereed journal and confer-ence papers and is Editor-in-Chief of IET Optoelectronics

Ian H. White (S’82–M’83–SM’00–F’05) received the B.A. and Ph.D. degreesin engineering from the University of Cambridge, Cambridge, U.K., in 1980and 1984, respectively.

He was appointed as a Research Fellow and Assistant Lecturer with the Uni-versity of Cambridge before he became a Professor of physics with the Univer-sity of Bath, Bath, U.K., in 1990. In 1996, he joined the University of Bristol,Bristol, U.K., where he was a Professor of optical communications, the Headof the Department of Electrical and Electronic Engineering, and the Deputy Di-rector with the Centre for Communications Research. He returned to the Uni-versity of Cambridge in October 2001 as van Eck Professor of Engineering andHead of the School of Technology. He is the Head of Photonics Research withthe Electrical Engineering Division, University of Cambridge. He is the authoror coauthor of 550 publications and holds 28 patents. He is currently an Hon-orary Editor of Electronics Letters.

Jon V. DeGroot, Jr., received the B.S. degree in chemical engineering fromIowa State University, Ames, in 1989 and the Ph.D. degree in chemical engi-neering and materials science from the University of Minnesota, Minneapolisin 1994.

After completing his undergraduate studies, he worked for a short period oftime with 3M Corporation, St. Paul, MN. Since 1994 he has been with DowCorning Corporation, Midland, MI, working in the areas of elastomers and re-inforcement in rubber and sealants focused on structure/property relationships,materials characterization and rheology. Since 2001, he has been focused ondevelopment of materials for waveguides and other photonic applications and isthe technical leader for Dow Corning’s waveguide development efforts.

Terry V. Clapp received the B.S.Hons (1st) degree in applied sciences fromBrighton Polytechnic (now the University of Brighton), Brighton, U.K., in 1980and the M.S. degree in chemistry and solid-state physics and the Ph.D. degree inalumino silicate catalysts from the University College of Wales, Aberystwyth,in 1981 and 1985, respectively, and the B.A. degree in mathematics from theOpen University, U.K., in 1987.

Between 1994 and 2001, he worked with a number of organizations, includingNortel and Nortel Networks, to develop fiber-optic technology, planar waveg-uides, and advanced photonics components and technology. He was also a Se-nior Technical Consultant within Nortel High Performance Optical ComponentsSolutions. In the fall of 2001, he left Nortel to join the University of Cambridge,Cambridge, U.K., as one of the principals of what became the Centre for Ad-vanced Photonics and Electronics (CAPE). In 2003, he joined Dow CorningCorporation, Midland, MI, and, while remaining an embedded researcher inCAPE, participated in the development of Dow Corning’s Lightwave Manage-ment actions. He is currently the Director of the University of Cambridge’s In-tegrated Knowledge Centre.


ieee journal of quantum electronics, vol. 45, no. 4, …djm202/pdf/papers/... · bamiedakis et al.:...

Documents