Optical Switching and Networking 5 (2008) 10–18www.elsevier.com/locate/osn

A comparison of optical buffering technologies

E.F. Burmeister∗, D.J. Blumenthal, J.E. Bowers

Electrical and Computer Engineering Department, University of California at Santa Barbara, CA 93106, United States

Received 15 April 2007; accepted 4 July 2007Available online 17 July 2007

Abstract

We describe the practical and fundamental limitations of the more prominent optical buffering approaches. The architecturalimplementation and needs of an optical packet switch are used as a foundation for the study. We also present initial results for abuffered, all-optical, 40 Gb/s packet switch.c© 2007 Elsevier B.V. All rights reserved.

Keywords: Optical packet switch; Buffers

1. Introduction

Optical circuit switches are a proven technologythat is expected to gain widespread use. However,optical packet switching technologies may soon becomeimportant with a role in optical routers as the scalingof electrical routers becomes limited. Packet switchingtechnologies must then look to meet and surpass theperformance of electrical switching. Although obvious,this benchmark is important for setting the foundationfor the requirements of many of the elements of opticalpacket switching. These requirements can be usedto determine the most promising technologies and toreveal the limitations. This is especially true for opticalbuffering, the main topic of this paper and one of themore controversial elements of optical switching.

Optical packet switching is a highly desirable ap-proach for future optical communication networkingdue to its efficient use of capacity and flexibility; how-ever it necessitates the implementation of contention

∗ Corresponding author. Tel.: +1 805 893 5828; fax: +1 805 8937990.

E-mail address: emily@ece.ucsb.edu (E.F. Burmeister).

1573-4277/$ - see front matter c© 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.osn.2007.07.001

and congestion resolution within core routers. Electri-cal routers use large banks of SRAM to resolve con-tention and hence there has been much research intoequivalent optical memory in the form of packet buffers.Alternatives to buffering such as deflection routingand wavelength conversion can be used to supplementoptical buffers, but severely limit network performanceif they are used as the sole solution. In this paper,the limitations of the most prominent optical bufferingapproaches are reviewed and initial results of a prac-tical, integrated, optical feedback buffer are presented.The paper begins by presenting an appropriate archi-tecture for optical packet switching to provide a back-ground for the buffers.

2. Optical packet switch architecture

Optical switching may become useful for datarouting in the core of the network, leaving electricalrouters for more intense processing at the edge of thenetwork. Label switching is used to allow the datapayload to remain in the optical domain while anelectrical data processor reads the address informationand makes the routing decisions. An example of a

E.F. Burmeister et al. / Optical Switching and Networking 5 (2008) 10–18 11

Fig. 1. Schematic of an optical data router comprised of synchronizers (Sync), buffers, packet forwarding chips (PFC), packet envelope detectors(PED), clock data recovery (CDR), deserializers (Deser), an arrayed waveguide grating router (AWGR), and control electronics.

basic optical data router for optical packet switching isshown in Fig. 1. This design is the basis of the LASORproject and features buffering for asynchronous, labeled40 Gb/s packets. The entire switch is described in detailin [1].

The system without synchronization or buffering wasdemonstrated using monolithic packet forwarding chips(PFCs) and evaluated using both Layer 1 and Layer2 performance metrics. The PFC performs 40 Gb/swavelength conversion of RZ payloads while rewriting10 Gb/s NRZ headers and wavelength switching on aper-packet basis. Layer 1 was demonstrated with error-free operation while Layer 2 showed header recoveryloss and packet switch throughput of over 90% [2].These results demonstrate the operation of the coreof the switch. However, without buffers placed beforethe PFCs, packets must be managed a priori to avoidcontention.

The remainder of this paper builds a comparison ofbuffering approaches and presents the approach chosenfor the LASOR project. Although synchronization is notexplicitly discussed, the synchronizer technology maybe built by using the same approach. Synchronizers ad-just the timing of packets within a time slot to align toa buffer clock if packet arrival is asynchronous. There-fore in general, synchronizers need smaller delays thanbuffers and the requirements are slightly less stringent.

3. Buffering requirements

Current routers use large capacities of electricalRAM to resolve contention. The state-of-the-art CiscoCRS-1 core router is based on linecards that operatewith 2 GB of memory. This capacity is currently notfeasible with any proposed optical buffering approach,but recent research has shown that much smallerbuffering capacities are adequate. Simulations show thatif access links are slower than the backbone network andthe traffic is smoothed, then only ten packet buffers peroutput port are needed for 80% throughput (Fig. 2) [3].

Fig. 2. Throughput as a function of buffer size for smoothed traffic isshown with the solid line [3].

Thus, optical buffers may present a realistic solution.The remainder of this paper describes the needs thatthe architecture places on a buffer and reviews possibleapproaches.

Optical buffers must meet certain requirements toachieve acceptance. It is first necessary that a bufferingapproach can impart a delay of at least the lengthof the packet payload in order to provide contentionresolution. It is important for the buffering deviceto be bit rate scalable to 10 Gb/s or greater tooffer an advantage over electrical domain counterparts.For acceptable network loads, they should have thecapability to store packets of no less than 40 byteswith guard bands no more than several nanosecondslong. Packet payload length is one of the more difficultchallenges for many buffering approaches, but unlessthe ratio of the payload length to the overhead of theheader and guardbands is reasonable, optical buffersand packet switches will not afford an advantage. Inaddition, it is desirable to require less header processingfor a given amount of payload. In order to accommodate

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short guard bands, buffers must be able to switch orreset in less than several nanoseconds.

Along with the above requirements, there are alsoother considerations that increase the probability ofsuccess of a given optical buffer approach. As usual,low cost is a main issue. Optical packet switches mustlower the cost per bit for data transmission to make themadvantageous. It is also necessary that optical buffershave low power consumption, low heat production,and a small footprint—the main challenges facing thescalability of electrical packet switches. For both costand footprint, it is obviously important that the numberand complexity of components included in a givenbuffer architecture must be kept to a minimum. Inorder to make the optical buffer more flexible, it isdesirable for the buffer to be transparent to packetlength and to provide dynamically variable storage time.Apart from these architectural considerations, it is alsodesirable that the buffer device impart the least possibledispersion and optical power penalty.

4. Buffering approaches

In this paper, we review the advantages andlimitations of two types of transparent optical bufferingapproaches and one approach based on the electricalconversion of packets. Transparent optical bufferingapproaches rely on delaying packets by increasingtotal transmission time, either by decreasing the groupvelocity (slow light buffers), or increasing the physicallength (delay line buffers). Slow light buffers can bedivided into two types; devices using material-basedresonances and those using coupled resonant structures(CRS). Delay line buffers are also categorized into twosubsets; feedforward and feedback.

Slowing mechanisms proposed for optical buffersuse strong resonances between electromagnetic waves(CRS) or between an electromagnetic field and apolarizable medium (material-based). Examples of thetwo types are shown in Fig. 3. In CRS such as gratingsand photonic crystal defects, the group velocity isreduced by lengthening the light path through repeatedreflections; the group velocity decreases drastically inthe vicinity of the photonic bandgap. The band structurefor polarizable media buffers looks very similar tothat of the appropriate CRS dispersion curve withbandgaps created from strong material resonances. Forthe application of optical buffering, electromagneticallyinduced transparency (EIT) devices using resonanceswith exciton excitations in semiconductors are studiedfor their compactness. Population oscillation is analternative material-based slow light technology similar

Fig. 3. (a) A photonic crystal waveguide—an example of a coupledresonator structure that can be used for slow light. (b) Energy levelsproviding resonances for electromagnetically induced transparency.

Fig. 4. Schematics of (a) feedforward and (b) feedback buffers.

to EIT. It utilizes a carrier population grating to createthe material-based resonance. A more detailed overviewof slow light buffer mechanisms can be found in [4,5].

Delay line buffers provide a practical solution andhave demonstrated the best results. The two types areshown in Fig. 4. Data in a feedforward buffer is sentthrough a given delay line only once. The buffers musttherefore use the same number of delay lines as thedesired variation in delay times. Feedback buffers areoperated to use the same delay line repeatedly. In afeedback buffer the length of the delay line determinesthe resolution of possible delays and in general shouldbe made the length of the packet payload. The numberof delay variations and maximum storage time aredetermined by the maximum number of recirculations.

5. Buffering comparisons

The three types of buffering approaches are nowstudied to find the practical and theoretical limitations.This information is then used to contrast the advantages.

E.F. Burmeister et al. / Optical Switching and Networking 5 (2008) 10–18 13

Fig. 5. Photonic RAM schematic [6].

Before comparing transparent optical buffers, it isnecessary to consider the challenges in the electricaldomain. Although speed appears to limit the scalabilityof electrical RAM, recent research shows that silicon-based CMOS RAM can be used as a storage mediumfor optical packets at data rates up to 40 Gb/s by usinga combination of optical and electrical components [6].Because bits are stored electronically, such a designoffers very long storage times, large capacity, andrandom access at arbitrary times. However, loss andcomponent complexity have limited the design topackets of less than 10 bytes [6]. The buffer uses serialto parallel conversion of packets, and therefore eachpacket must be split into the same number of streamsas bits in the packet. The maximum amount of splitting,and therefore maximum number of bits is determined bythe amount of power needed to accurately read the bits.The restriction on maximum packet size greatly limitsthe maximum load of the network.

Slow light devices offer many desirable higher-leveladvantages, but the fundamental limitations that areoutlined here (Fig. 5) will inhibit their use as buffers.Slow light buffers are heavily researched becausethey potentially offer a compact solution that hascontinuously variable storage times and can handleasynchronous packets of varying lengths. However,dispersion, bandwidth, and loss are fundamental issuesthat will limit the use of slow light devices as buffers.

Slow light shows promising results if data rate andpacket length are ignored. Fig. 6 shows the relative com-pactness and loss tradeoffs for variations on slow lightcompared to integrated feedback buffers. Electromag-netically induced transparency under extreme cooling(7 K) and ideal coupled resonators show the best resultsin this figure because of the large degree of slowing theyimpart. However, the three regions show a lot of over-lap in terms of the degree of delay per loss, despite thevery large effective group index of the slow light meth-ods. This is due to very high losses which will end up

Fig. 6. Size and loss tradeoffs for slow light buffers compared tofeedback buffers.

being one of the limiting factors. Fig. 6 uses recent re-sults in the field for the lowest propagation losses andthe highest indices of refraction. The best slowing re-sults were commonly obtained using optimized pulsesin short devices. The data used for the figure is presentedin Table 1.

In order to quantify the limitations for slow lightbuffers with given data rates and packet lengths,equations are presented relating the index, or degree ofslowing, with these parameters. The same relationshipswere also developed for basic recirculating buffersfor comparison. The recirculating buffer simply hasa constant index across the bit rate range. Therefore,dispersion places only a limit on the delay in numberof bits. However, the maximum and minimum bit ratescan be defined by the dispersion limit and practical sizelimit with several assumptions. The simple equation,

BL |D| 1λ ≤ 1, (1)

is used to find the approximate dispersion limit bychoosing a conservative length of 10 m for thetotal maximum buffer delay. The value used for the

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Table 1Comparison of propagation loss and index of refraction

Structure Material system Propagation loss (dB/cm) Max index of refraction Reference

Slow light - Material EIT-QD (7 K) 107 5 × 107 [7]Slow light - Material EIT-QD (300 K) 105 200 [7]Slow light - Material PO-QW 105 105 [5,8]Slow light - Structural Ideal 0.1 103 [4]Slow light - Structural Si 3.5 100 [9]Recirculating SiO2 0.02–0.04 1.3–1.6 MeasuredRecirculating Si 0.13–0.35 3.0–3.5 [10,11]

Table 2Comparison of operable bit rates

Structure Material system Packet length = 10 bits Packet length = 200 bits IndexMin. bit rate (Bps) Max. bit rate (Bps) Min. bit rate (Bps) Min. bit rate (Bps)

Slow light - Material EIT-QD 25 G 110 G – – Eq. (5)Slow light - Material PO-QW 20 G 1 T 400 G – Eq. (8)Slow light -Structural

Ideal 3 G 18 T 75 G 1 T Eq. (9)

Slow light -Structural

Si 30 G 18 T 700 G 1 T Eq. (9)

Recirculating SiO2 1 G 1 T 20 G 1 T ng ≈ 1.5Recirculating Si 500 M 50 G 10 G 50 G ng ≈ 3

second-order dispersion, D, is 900 ps/nm km forsilicon waveguides [12] and 50 ps/nm km for silica(measured). Along with a spectral source width of 2 nm,this yields a limit of up to 50 Gb/s for silicon and1 Tb/s for silica waveguide delays. There also exists aminimum bit rate for a practical, integrated recirculatingbuffer. The length of the delay line must be longer thanthe size of the packet, but also must be short enoughsuch that the lumped loss is not too large. Therefore,for a packet length of 10 bits and a maximum singlecirculation length of 2 m, the bit rate must not beless than 500 Mb/s for silicon and 1 Gb/s for silica.For a packet length of 200 bits, the minimum bit rateincreases to 10 Gb/s and 20 Gb/s for silicon andsilica respectively. These results are later summarizedin Table 2.

The largest limiting factor for electromagneticallyinduced transparency is the transparency bandwidth,however we demonstrate here that even if this issueis resolved, dispersion will still greatly limit thedevice performance. Although second-order dispersionis eliminated through the use of twin resonances inEIT, third-order dispersion must still be considered.As bit rate or length increases, less dispersion isallowable and therefore not as much slowing is possible.These parameters determine the allowable dispersionand therefore the maximum group index. The equationsfor maximum propagation in the presence of third-order

dispersion, calculating third-order dispersion, and EITgroup index are taken from [4] and shown below.

B [β3Lmax]1/3= .324 (2)

β3 =6Ωpn

cΩ4 (3)

ng = n + nΩ2

p

Ω2 . (4)

The three equations are combined to yield therelationship between EIT group index and bit rate.

ng = n

(1 +

Ωp

9.4 × B√

N

). (5)

Ωp is the plasma frequency and is estimated to beapproximately 2 × 1013 Hz using a value of 21 A forµ21/e from [6]. The minimum bit rate is determinedfrom the maximum time delay due to the coherence timeand the packet length [4].

Population oscillation is studied as an alternativeto electromagnetically induced transparency due to itslarger linewidth, however it is demonstrated that it isstill too narrow to be used in a practical buffer. Theequation for the slow-down factor and optical transferfunction are taken from [5].

S(ω0) =n2

n1+

c1α(ω0)

n1ω1/2(6)

E.F. Burmeister et al. / Optical Switching and Networking 5 (2008) 10–18 15

Fig. 7. Maximum index as a function of bit rate for (a) 10 bit packets,(b) 200 bit packets.

ω−3d B

ω1/2=

√ln(2)

L1α(ω0). (7)

Together they yield the equation for the maximum groupindex as a function of bit rate and packet length.

ng =1

16π2

c1α(ω0) ln(2)

N × B. (8)

An approximate value of 106 m−1 is taken from [8] forthe absorption dip depth.

Third-order dispersion is also responsible forlimiting the maximum operable bit rate in slow lightresonator structures. Using the maximum bit ratesfrom [4], the relationship between index and speed isknown.

ng = nBmax

B

(1 −

n2

n2g

)−1/2

Fig. 8. Index as a function of packet length at the minimum operablebit rate.

×1

ln(

nHnL

)+ ln

[ngn

(1 +

√1 −

n2

n2g

)] . (9)

The minimum bit rate for the resonators is determinedby the maximum time delay and the packet length. Themaximum time delay is determined from the waveguideloss at the point where half of the optical power isdissipated [4].

Fig. 7(a) plots the previous equations to demonstratethat bit rate greatly restricts both subsets of slowlight approaches. Material-based resonator devices andcoupled resonator structures are limited by linewidthand third-order dispersion as bit rate increases andby dephasing (loss) for low bit rates. It can alsobe seen that as the bit rate increases, the index ofrefraction decreases. The maximum bit rate is the pointat which it is no longer possible to subject the data tothe dispersive slowing phenomena without damagingthe data. Integrated recirculating buffers implementingeither silicon or silica delay lines are limited at lowbit rates due to practical size considerations and arelimited in length by second-order dispersion. Fig. 7(b)shows the effect of requiring operability for morereasonable packet lengths of 200 bits (25 bytes).Although this packet length is still not long enoughfor reasonable throughput, it is close to the maximumlength that can be used with any slow light approach—only an ideal coupled resonator structure would beable to operate in this regime. The packet lengthdefines the minimum required delay time to performthe buffering needed to resolve contention and thereforethe minimum length of the device. As the devicebecomes longer, less dispersion is acceptable. Fig. 8shows the maximum slowing achievable with increasing

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Fig. 9. Schematic of feedback configuration allowing speed-up andsimultaneous read/write.

packet length; demonstrating the strong decrease inperformance as delay time increases. However, it isclear from Fig. 7 that integrated recirculating buffers area suitable approach, capable of operating at the requiredbit rates and packet lengths.

Over the last decade, there have been several bufferarchitectures that have achieved good results, but whosesuccess will be limited by either large componentcounts or component complexity. Optical storage of atleast 0.1 ms has been demonstrated using feedforwardarchitectures such as a folded-path buffer [13] and fiberBragg gratings [14], as well as feedback architecturesemploying parametric nonlinearity in fiber [15] andfiber nonlinear loop mirrors [16]. High data rate resultswere achieved as early as 1998 using a compensatingfiber loop buffer to achieve 20 µs of storage at40 Gb/s [17]. However, scalability, footprint, and powerconsumption are large issues for future core routersrequiring at least 5 buffers for each of more than 16ports.

Feedback buffers are beneficial for their lowcomponent count and small footprint. Thus far,integrated switches as developed by our group [18]and Chi et al. [19] show promise of offeringa practical solution. Recirculating buffers meet allnecessary requirements. Limitations resulting fromnoise accumulation can be reduced with optical filteringas performed in optical transmission systems. Althoughfeedback buffers are optimized for a set packet length,solutions are being examined to manage this issueexternally. Most importantly, the devices described hereoffer a solution that combines a compact footprintwhile allowing practical throughput. In addition, thefeedback buffer is the only approach which can be easilyconfigured to offer simultaneous reading and writing aswell as speed-up (Fig. 9).

Our group investigated several designs to implementa recirculating optical packet buffer. The chief concernwas a switch that would meet all of the bufferrequirements. Several Mach-Zehnder Interferometric

Fig. 10. SEM and schematic of a fabricated SOA gate matrix switchwirebonded to an aluminum nitride submount.

Fig. 11. Switching rise times as a function of the optical input powerto the device and the injected current.

(MZI) switches and several Semiconductor OpticalAmplifier (SOA) gate matrix switch designs weretested. The switches were tested for extinction,crosstalk, 40 Gb/s power penalty, switching speed, andinsertion loss. As expected, the MZI switches couldnot meet the stringent extinction and crosstalk metricsrequired for use in a cascaded element.

The semiconductor optical amplifier (SOA) gatematrix switch developed and tested by our groupis shown in Fig. 10. It features high extinction(>40 dB), low crosstalk (< −40 dB) and fast switchingtimes as shown in Fig. 11; therefore meeting therequirements needed for optical buffer performance.As previously mentioned, the ability to switch withinseveral nanoseconds allows for increased throughput.Further, the switch demonstrated error-free operationwith power penalties of less than 1 dB for 40 Gb/s datafor all four port configurations, as shown in Fig. 12.

6. Comparison

The technologies for optical buffering that weredescribed in the last section and are amenable tointegration are compared in Table 3. This tabledemonstrates the difficulty in finding a successfulbuffering approach. Slow light buffers would degradenetwork performance by limiting packet length andtherefore network load. The technologies suffer fromlosses and dispersion which yield a low bandwidth-delay product. This results in impractical bit rates

E.F. Burmeister et al. / Optical Switching and Networking 5 (2008) 10–18 17

Table 3Overall comparison of buffer approaches

Metrics Slow light Delay lineEIT-QD CRS-Si CRS-ideal Silicon Silica

Maximum delay 28 bits 160 bits 700 bits 7000 bits 50,000 bits

Packet length = 10 bits

Max storage density 18 bits/cm 95 bits/cm 95 bits/cm 60 bits/cma 60 bits/cma

Max delay density 1 ns/cm 3 ns/cm 30 ns/cm 0.1 ns/cm 0.05 ns/cmLoss/Delay 105 dB/ns 1 dB/ns 0.004 dB/ns 1.3 dB/ns 0.4 dB/ns

Packet length = 200 bits

Max storage density – 60 bits/cma 60 bits/cma 60 bits/cma 60 bits/cma

Max delay density – 0.08 ns/cm 0.4 ns/cm 0.1 ns/cm 0.05 ns/cmLoss/Delay – 4 dB/ns 0.2 dB/ns 1.3 dB/ns 0.4 dB/ns

a Density taken at 700 Gb/s, the largest bit rate used for comparable slow light density calculations.

Fig. 12. BER as a function of optical power at 40 Gb/s RZ 27-1. AnSHF 50-Gb/s BERT was used for all measurements.

and capacities. Feedforward buffers have shown goodresults and do not place any limit on packet lengths,but may be impractical for implementation dueto high component counts. Combination electrical-optical buffers using CMOS photonic RAM are alsoimpractical due to component count and complexity.Lastly, feedback buffers provide a compact solution forintegration, but place some constraints on packet length.However, as an overall solution, integrated recirculatingbuffers offer the most practical compromise and will beused in the LASOR optical switch.

7. Summary

This paper covers the challenge of optical bufferingbeginning with a foundation in the architecture and

progress of a 40 Gb/s optical packet switch. Threegeneral types of optical buffering were compared forpractical implementation in an optical packet switchedrouter. The limitations of several buffering approacheswere studied and found to be primarily due tocomponent complexity and scalability or, in the caseof slow light devices, the inability to reach the delaytime and data rates required of a practical optical buffer.Recirculating buffers using SOA gate matrix 2 × 2switches meet all requirements and offer a compactsolution with several architectural advantages.

Acknowledgments

This work is supported by LASOR award #W911NF-04-9-0001 under the DARPA/MTO DoD-N program.The authors would like to thank Garry Epps from Ciscofor the helpful discussions.

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