590 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 30, NO. 4, FEBRUARY 15, 2012

Low Cost, Injection Molded 120 GbpsOptical Backplane

Paul Rosenberg, Member, IEEE, Sagi Mathai, Senior Member, IEEE, Wayne V. Sorin, Fellow, IEEE,Moray McLaren, Joseph Straznicky, Georgios Panotopoulos, David Warren, Terry Morris, and

Michael R. T. Tan, Member, IEEE

Abstract—A low cost, blind mate, injection molded optical back-plane is presented. The optical backplane consists of a 12 channeloptical broadcast bus operating at 10 Gbps/channel with six blindmate optical output ports spaced 1U apart.

Index Terms—Backplane, blind mate, broadcast, bus, network,optical interconnect, waveguide.

I. INTRODUCTION

E LECTRONIC systems with backplane architecture areapproaching operational rates of 25 Gbps. This frequency

regime presents technical challenges in the areas of signalintegrity, power management, and data rate scaling. Opticalinterconnects have the potential to improve performance ineach of these areas. However, electrical interconnects areunlikely to be replaced by optical interconnects until opticsdemonstrate a more compelling combination of performance,ease of use, reliability, and price. Direct replacement of copperinterconnects with optics requires a new generation of opticalsolutions along with optical interfaces that meet the cost chal-lenges of the “computercom” era. In this work, we describea low cost injection molded optical backplane comprised ofmultiple broadcast buses which allows all-to-all connectivitybetween connected modules. Each optical broadcast bus con-sists of 12 plastic hollow metal waveguides (HMWGs) [1], [2]with six blind mate drop ports. Commercially available opticalcomponents are used, including 12 channel transmitters (Tx)based on vertical cavity surface emitting laser (VCSEL) arrays,and receivers (Rx) based on PIN photodiode arrays.

II. BUS ARCHITECTURE

Electrical data buses were used widely in early computer ar-chitectures for connecting processors, memories, and IO. Asdata rates increased, signal integrity suffered due to electricalloading of bused transmission lines. Systems migrated awayfrom buses toward industry standard, point-to-point connectionssuch as PCIe and Infiniband. These links operate at high datarates, and proved to be easy to design-in and easy to upgrade.

Manuscript received July 19, 2011; revised September 26, 2011, October 25,2011; accepted November 17, 2011. Date of publication November 30, 2011;date of current version February 02, 2012.

The authors are with Hewlett-Packard Company, Palo Alto, CA 94304 USA(e-mail: paul.rosenberg@hp.com).

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

Digital Object Identifier 10.1109/JLT.2011.2177813

Point to point systems face limitations, however, including la-tency that can reach 200–300 ns, high power consumption, re-quirements for complex firmware/ucode, and issues with partreplication and data center sprawl.

Optical buses provide the benefits of electrical buses whileentirely eliminating the impact of electrical loading. When op-tical interconnects are confined inside a system chassis (therebyavoiding stringent eye safety power limits) significant excessoptical power is available. This is efficiently utilized by powersplitting, and routing to multiple drop points along an opticalbus. Transmitter sharing means that Tx cost is amortized acrossmultiple receivers. Reducing the number of transmitters alsocontributes to improved system reliability [3]. Optical transmis-sion offers reduced latency as propagation delay over typical in-side-of-rack link lengths is on the order of 1–2 ns.

The optical bus is an entirely passive device that, unlike aswitch, requires no core or IO power. No firmware program-ming is required in order to route data. A multi-bus architecturehas the potential to replace switch ICs, and the large, complex,and expensive backplanes that support them. Furthermore, theHMWG bus media can support bandwidth increases of one ortwo orders of magnitude as a result of increasing channel mod-ulation rates combined with wavelength division multiplexing(WDM). In contrast, the electrical infrastructure, consisting ofCu traces, electrical connectors, and one or more switch ICs re-quires costly redesign with every evolution in data rate. The fun-damental limitation in implementing the optical bus architectureis receiver proliferation. For a system comprised of N uniquenodes, receivers are required for non-blocking connectivity.

III. OPTICAL INTERCONNECTION FABRIC

The interconnection fabric of the optical backplane is shownin Fig. 1. Each module or linecard connects to every othermodule or linecard through an optical broadcast bus consistingof one or more HMWG assemblies. Each module can transmitto any module connected to its bus. Optoelectronic Tx are usedto convert the high speed electronic signals from a moduleinto digital light signals transferred to the optical bus fabric.Optoelectronic receivers on each module connect to the opticalbackplane through a blind mate optical connector, and convertreceived light signals into digital electronic data.

The broadcast bus consists of an array of 12 HMWGs with sixblind mate drop ports, or taps. Optical beam splitters positionedat each of the interconnection nodes are used to tap power fromall 12 bus waveguides. Each beam splitter has a unique value ofreflectivity, , and transmissivity, , chosen so as to distributethe transmitted optical power equally among the interconnected

0733-8724/$26.00 © 2011 IEEE

ROSENBERG et al.: LOW COST, INJECTION MOLDED 120 GBPS OPTICAL BACKPLANE 591

Fig. 1. Optical interconnection fabric for single module incorporating 12 waveguides and 6 taps.

Fig. 2. (a) Pellicle beam splitter fabricated on a silicon substrate. The siliconframe and pellicle window are 5.99 mm� 1.054 mm and 3.1 mm� 0.478 mm,respectively. Pellicle flatness is verified by surface profile measurements:(b) along its long and (c) short axes. The radius of curvature along both axes is� ��� mm.

modules. The maximum optical fan-out is determined by theTx power, the Rx sensitivity, and optical loss in the system. In-creasing the laser power or reducing the number of fan-outs al-lows for larger link margins and or longer bus lengths.

IV. KEY TECHNOLOGIES

A. Non-Polarizing Pellicle Beam Splitters

One key technology in the implementation of the opticalbackplane is the non-polarizing pellicle beam splitter shownin Fig. 2. Non-polarizing pellicle beam splitters have severalbenefits: 1) they enable the use of low cost, randomly polarizedlight sources such as multimode VCSELs, 2) compared to bulk

Fig. 3. Molded optical backplane with 1� 12 waveguide array and guide pins,1� 12 MMF fiber ferrule and mating connector, lens array, and beam splitteron 45 degree surface in the optical backplane. Arrows show the out coupling oflight from the backplane into the connector.

glass beam splitters, beam walk-off is negligible, and therefore,walk-off compensation is not required, 3) unwanted reflectionsand ghost images are eliminated, and 4) the uniform beamsplitter window mitigates the effects of mode partition noise ormode dependent loss.

The pellicle beam splitters are fabricated using conven-tional silicon microelectromechanical system (MEMS) andmature thin film dielectric optical coating technologies [4].As shown in Fig. 2, the semi-transparent pellicle window(3.14 mm 0.478 mm) is supported by a 250 m thick siliconframe (5.99 mm 1.054 mm), and is wide enough to spanall 12 HMWG bus channels. The multilayer thin film dielec-tric coatings are designed for polarization independence at850 nm wavelength with a wide optical bandwidth of 60 nm,and low film stress to prevent mechanical buckling. Pellicleflatness is verified by surface profile measurements of thesuspended membrane coated with the multilayer dielectric thinfilms. Along its short and long axes, the radius of curvature is

mm.

B. Plastic Injection Molded Optical Backplane

We utilize plastic injection molding, suitable for high volumeproduction, to fabricate the 30 cm long optical backplane. Thecore elements of the design are shown in Fig. 3. The metalizedplastic waveguide part incorporates 12 mirror-smooth, 3-sidedchannels with 150 m 150 m cross section, and features forthe placement and alignment of the beam splitters, mirrors, andoptical connector alignment pins. An uncoated section of theplastic part is shown in Fig. 4. Our approach allows for all ofthe necessary precision features to be formed on a single side ofthe mold tooling, thereby minimizing tolerances between crit-ical features. Long range precision alignment between taps is

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Fig. 4. Uncoated section of plastic optical backplane including: waveguidechannels, guide pin holes, and beam splitters reference surface.

Fig. 5. Isometric view of the 45 beam splitter reference surface on TiAucoated plastic optical backplane.

not required since tap-to-tap tolerance is taken up by mechan-ical ‘float’ built into the blind mate optical connectors. The openside of the optical channel is capped by a thin, flexible metal-ized plastic film with window openings for the light to exit ateach tap location. The pitch of the waveguide channels is set at250 m to match commercially available components, such asthe VCSEL and PIN arrays, microlens arrays, ribbon fibers, andoptical connectors.

As mentioned above, the injection molded part incorporatesfeatures for passive alignment of components that enable op-tical broadcast operation. To locate the pellicles along the bus,45 datum surfaces (Fig. 5) are formed orthogonal to the wave-guide axis at each drop port. Vacuum channels (Fig. 6), ex-tending between the back surface of the plastic part and the 45datum surface, are utilized to apply a suction force to the pel-licle frame, thereby drawing it against the 45 reference surface.In order to prevent damage to the pellicle during assembly, thevacuum channels are placed as far as practical from the pelliclewindow. Light cure adhesive is then dispensed into the reser-voirs (Fig. 4) adjacent to the vacuum channels, and cured topermanently secure the pellicle at the correct orientation. Thepellicle position along the waveguide length, and absolute anglewith respect to waveguide axis, are critical as they determine thespatial and angular characteristics of the reflected light signal

Fig. 6. Bottom view of the injection molded optical backplane showing thebeam splitter vacuum attach ports.

Fig. 7. Blind mate connector alignment pins assembled into a drop port in theoptical backplane.

as it intersects the blind mate optical connector. Measurementdata for the pellicle surfaces at all 6 tap locations shows angle

, and position relative to the datum guide pin holemm.

Precision holes, extending through the waveguide, are formedto hold metal guide pins that, in turn, capture and locate theblind mate optical connectors (Fig. 7). Measurement data forthese features at all 6 tap positions indicates mean hole diam-eter mm with range of mm. Two precisionslots (Fig. 8) are formed at one end of the backplane where op-tical signals enter the HMWG channels. These slots are centeredwith respect to the waveguide channel array and are sized andspaced to accept 700 um diameter guide pins used for MT fer-rule alignment. The slots allow passive alignment of an inputfiber ribbon connector based on MT ferrule geometry. All slotfeatures were measured to be within 3 m of their nominal de-sign value.

There were two main challenges in developing the injectionmolded part. First, in order to produce efficient optical cou-pling, the parts require a large number of mechanical tolerancesthat are extremely tight, especially over the 30 cm length ofthe part. For example, the spacing between optical channels isheld to m, and the diameter and true position tolerance ofthe holes that locate the optical connector guide pins is held to

m. Secondly, the extremely small waveguide channel fea-tures require surface roughness, , less than 40 nm. Note that

ROSENBERG et al.: LOW COST, INJECTION MOLDED 120 GBPS OPTICAL BACKPLANE 593

Fig. 8. MT guide pin alignment slots are incorporated at the input to the opticalbackplane for passive alignment of the input fiber MT ferrule.

the vertical wave guiding surfaces are internal channel walls,fabricated by mold tooling that cannot be ground or polishedby standard methods. The sections of the injection molding toolthat form the waveguide channels are made entirely by diamondturning, with no secondary surface polishing. of the top sur-face of the waveguide sidewalls is nm while the bottomsurface has nm. The top surface of the waveguide cor-responds to channels, formed by diamond turning in the mold,while the bottom surface of the waveguide corresponds to thepolished top surface of the mold.

Optical measurement of straight waveguides without gapsshow a propagation loss of less than 0.1 dB/cm. Combining thisdata with measurements from production parts incorporatingslots (needed for optical signals to exit the waveguide) we cal-culate that the excess loss from the 1 mm slots to be dB[4]. This result demonstrates that injection molding technologycan produce optically smooth surfaces over the entire length ofthe optical backplane [5]–[8].

Fig. 9 shows an output section of the waveguide assembly,along with the blind mate connector system. The optical con-nector is designed to provide a minimum float of mm in X,Y, and Z axes, as well as a minimum of of angular compli-ance in pitch, roll, and yaw. The blind mate connector also func-tions to absorb the impact of the mating action and provide suf-ficient spring force to ensure repeatable mating with the opticalbackplane. Guide pins and plates are fabricated from alloy 416stainless steel, hardened to Rockwell C 36-40. These parts werethen subjected to a dicronite surface treatment to reduce frictionand subsequent particle generation during optical mating anddemating [9]. Fig. 10 shows that the insertion loss repeatabilityof the blind mate connector is %.

The plastic parts are conformal coated with a reflective Ti/Aulayer before assembly into the backplane housing. The metalhousing provides means for supporting the plastic parts togetherwith the cover layer, guide pins and coarse alignment assembly.Small glass cover slips are used to seal the optical assembly fromdust. Fig. 11 show a photo of a completed optical bus assemblywhile Fig. 12 details the blind mate features on the outer housingof the optical bus assembly. In addition to providing environ-mental protection, the waveguide housing acts as a stiffener tomaintain straightness of the waveguide channels. Optical loss

Fig. 9. Rx blind mate connector assembly (right) and a backplane tap (left).Mating features for coarse and fine alignments are incorporated into each com-ponent, while float in X-Y-Z and angular compliance (pitch, yaw, and roll) aredesigned into the connector assembly.

Fig. 10. Insertion loss repeatability measurement of the blind mate connector.

is inversely proportional to the radius of curvature of the wave-guides as shown in Fig. 13. Optical loss of Er polarized light (Efield parallel to the plane of curvature) is given by the formula[10]

dB (1)

Losses due to bending are negligible due to the ability of theassembly to maintain curvature of less than 1 degree of arc.

C. Competing Optical Backplane Technologies

Today’s leading optical backplane architecture is representedby products such as Molex Flexplane™, which consists of apoint to point arrangement of individual fibers and fiber ribbons,captured between two sheets of polyimide, and terminated withMT optical ferrules, see Fig. 14. In fabricating these systems,some automation is used to route the optical fibers. But the con-nectors must be assembled manually, and the systems tend to becustom in nature with almost no design modularity. By contrast,the hollow metal waveguide backplane is designed with a stan-dard configuration for bladed system architectures. Rx outputsignals are located with ‘1U’ (1.75 ) spacing along the lengthof the waveguide part. All Tx signals are routed to the top of thewaveguide. This construction allows blades to be entirely inter-changeable since optical signals are transmitted and received atthe same location on every blade.

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Fig. 11. Complete 30 cm long optical backplane assembly with and input fiber pigtail and 6 taps.

Fig. 12. Detail of a blind mate connection point on the optical backplane as-sembly with coarse and fine alignment features.

Fig. 13. Optical loss in bent hollow metal waveguide as function of bend radius.

Another technology under development for optical back-planes involves lithographic fabrication of waveguides in apolymer layer buried in the interior of the backplane circuitboard [11]. The backplane will also likely incorporate copperlayers for traditional transmission of some data and power.This approach transfers the optical signals to the interior ofthe PCB, thereby minimizing the clutter of optical fibers andreducing impediments to the flow of cooling air. Since polymerwaveguide cores can be spaced more closely than the standard250 um pitch of MTP/MPO multi-fiber connectors, this tech-nology has the potential to offer increased signaling density,see Fig. 15.

However, before this technology is broadly commercializedit will be necessary to develop and qualify a great deal of

Fig. 14. Optical backplane implemented in Molex Flexplane™ technology.

Fig. 15. Concept for optical backplane based on waveguides buried in printedcircuit board structure.

optical infrastructure including; i) polymer waveguide integra-tion with standard PCB fabrication processes; ii) integrationand alignment of precision optical connectors with embeddedwaveguides; iii) solder reflow and flux cleaning processescompatible with integrated optics; iv) robust processes forinspecting and cleaning embedded optical interfaces.

V. OPTICAL SYSTEM

The key optical system design requirement is the delivery ofsufficient optical power at each receiver for error free operation.The total transmission loss along the optical broadcast bus is ex-pected to be approximately 14 dB from the output of a VCSELarray to the input at the photodiode array. The inherent loss dueto the 1 6 fan-out is approximately 8 dB, resulting in an av-erage excess loss of 6 dB due to lens coupling, optical connectorloss, propagation loss (0.1 dB/cm) along the HMWG, and gap

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Fig. 16. Tap loss distribution along the optical backplane normalized to its input optical power.

Fig. 17. Eye diagrams at10 Gb/s from all 6 taps, all 12 channels active. Average extinction ratio � ��� dB, average received optical power 150 �W.

loss ( dB/gap) due to diffraction. Since the path length toeach tap is different, the individual excess losses increase fromabout 4.2 dB at the tap nearest to the input to about 8.2 dB at themost distant tap. If losses due to the waveguide, and other ex-cess losses, were zero, the tap losses would all be identical, cor-responding to increasing reflectivity values of 1/6, 1/5, 1/4, 1/3,1/2 and 1. With an expected VCSEL average modulated poweroutput of dBm; the received power at each photodetector isapproximately dBm. As receiver sensitivity is dBm,a link margin of dB is available for a 10 Gbps link. It isimportant to determine the HMWG propagation loss and gaplosses accurately, as they determine the unique tap specifica-tions (beamsplitter ) which yield the most uniform powerdistribution [2]. A large link margin is desirable in order to ac-commodate these variations.

The Tx optoelectronic engines located on each linecard in-corporate off-the-shelf 1 12 VCSEL arrays and microlenses,and are pigtailed to 1 12 multimode fiber ribbons. The op-tics are carefully designed to preserve the étendue of the lasersources through the optical system. Since the low loss modes ofthe HMWG propagate at grazing angles, a low numerical aper-ture (NA) input beam is required to excite them. An off-the-shelfmicrolens array is used at the HMWG entrance to efficiently ex-cite low loss modes. Precise angular alignment of the low NA

input beams to the optical axis of the HMWG is essential forlow propagation loss. The optical connector alignment slots in-corporated at the input to the backplane (see Fig. 8) ensure pre-cise angular and spatial alignment of the input beams.

Eye safety is a concern for optical interconnects in datacen-ters. For example, active optical cables for rack to rack inter-connects must adhere to eye safety standards due to potentialfiber breakage which leads to radiation exposure. Since the laserbeams leaving the Tx are enclosed by the rack and transportedwithin the optical backplane encased inside the rack, the Txlaser power can be higher than eye safety limits for devices andoptical media operating outside an enclosure [12]. Therefore,the excess electrical energy required to drive lasers to higherbit rates can be efficiently utilized to increase the fan-out, linkmargin, and alignment tolerances. The laser power tapped outof the optical backplane must adhere to eye safety limits. Thesebeams are directed away from the backplane and have the po-tential to escape the rack at unpopulated slots. Since the tappedpower is kept well below the eye safety limit, unpopulated slotsdo not present eye safety concerns.

VI. EXPERIMENTAL RESULTS

Fig. 16 shows the measured optical loss from the input portof the optical bus to each tap output for all 12 waveguides. A

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single VCSEL was used to excite each waveguide and the outputfrom each tap was measured using a large area detector. The av-erage channel loss is between to dB. This measure-ment combines all sources of optical loss including couplingloss to the HMWG, tap loss, propagation loss and gap loss. Itdoes not include the coupling loss from the optical bus backthrough the blind mate connector and the short fiber ribbon stubto the photodiode receiver array. We estimate this loss to be be-tween 1.5 to 2 dB, based on measurements on the blind mateconnector. These measurements verify our link budget assump-tions of the total channel loss to be around 14 dB. With dBmof VCSEL output power, we anticipate at least dBm ofpower at the photodetector. With a receiver sensitivity of around

dBm, we have sufficient link margin for the reliable oper-ation of the bus. This number is also corroborated by the mea-sured tap powers in excess of dBm for a dBm Tx op-tical power.

High speed characterization of the optical bus was performedas follows. A commercially available Snap12 optical engine,with a 1 12 10 Gbps VCSEL array and laser driver IC, wasmodified to allow placement of a custom microlens array andfiber ferule pigtailed to a 50 m GIF multimode fiber ribbon.The pigtailed output was directly attached to the input end ofthe optical bus using a standard MT ferrule and an off-the-shelfoptic.

Each VCSEL in the array was driven with an output opticalpower in excess of 3 mW. It is interesting to note that in orderfor the VCSELs to operate at 10 Gbps, they must be biasedat a current density which can easily provide over 3 mW ofoutput power even at 75 C. The Snap12 transmitter was directlymodulated from a 12 channel Agilent ParBert. A custom blindmate fiber ribbon connector, based on a single 1 12 MT fer-rule, was used to couple the 12 output beams from the opticalbus into a Snap12 receiver module. The 12 optical signals hadan average extinction ratio of around 5.2 dB and an averagepower of around 150 W as measured from an optical fiberribbon breakout cable. The 10 Gbps eye diagrams from all 12waveguide outputs at each tap are shown in Fig. 17. A BER of

was measured on the first 4 channels in tap 1 using aPRBS sequence. A BER of was measured on the

same 4 channels at tap 6. The measurements shown in Fig. 16were obtained by moving the blind mate optical connector toeach of the tap outputs on the optical bus with all 12 channelsactive.

VII. CONCLUSION

In summary, we have demonstrated a low cost, 12 channel,6 tap optical backplane operating at a data rate of 10 Gbps perchannel. The heart of the system consists of injection moldedhollow metal waveguides intersected by non-polarizing pelliclebeam splitters that broadcast optical data through blind mate op-tical connectors, utilizing transmitters and receivers fabricatedfrom off-the-shelf parts. To the best of our knowledge, this isthe first demonstration of a parallel, high speed, optical back-plane operating at 120 Gbps manufactured by low cost injec-tion molding. The production cost for the entire optical back-plane assembly (waveguides, beamsplitters, and supporting me-chanical elements) is estimated at less than $10. This approachhas the potential to significantly reduce the cost difference be-tween the physical media associated with electrical and opticalbackplanes.

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