DWDM Nanophotonic Interconnects:Toward Terabit/s Chip-Scale Serial Link

Tsung-Ching Huang∗, Cheng Li∗, Rui Wu†, Chin-Hui Chen∗, Marco Fiorentino∗,Kwang-Ting (Tim) Cheng†, Samuel Palermo‡, Raymond Beausoleil∗

∗Hewlett-Packard Laboratories, Palo Alto, California 94304-1123†University of California, Santa Barbara, CA‡Texas A&M University, College Station, TX

Abstract—Dense wavelength-division multiplexing (DWDM)is a promising technique to increase capacity of nanophotonicinterconnects for chip-scale serial links. This paper describesrecent progress in DWDM nanophotonic interconnects based onsilicon microring resonators, including a five-channel DWDMtransmitter using a quantum dot (QD) comb laser and carrier-injection modulators, as well as self-adaptive CMOS transmitterfor hybrid CMOS and photonic chips integration.

I. INTRODUCTION

High-performance computing requires high-bandwidth in-terconnects between microprocessor and memory, as well asamong microprocessor cores. Optical interconnects recentlyhave attracted much attention for high-speed chip-scale seriallinks because of its superior energy efficiency and low channelloss across a wide spectrum, which eliminates the need ofpower hungry channel equalizers that are usually requiredfor high-speed electrical transceivers. A nanophotonic inter-connect that is compatible with CMOS-VLSI infrastructureusing silicon optical components to modulate, transport, anddemodulate optical signals is expected to play a critical rolein the next-generation many-core processor architectures forbig-data and cloud computing [1], [2]. To further reducethe area overhead due to the introduction of on-chip opticalcomponents such as silicon waveguides, and also to increasethe channel capacity of the optical interconnects, DWDMnanophotonics is a promising candidate to realize the next-generation terabit/s serial links for chip-scale high-speed I/Owith minimum overhead [3].DWDM is widely used in fiber-optic telecommunication toincrease the system capacity without replacing existing back-bone optic network. With a multi-wavelength light sources, theoptical transmitter can multiplex N independent data streams,serialize them, and modulate them onto N different wave-lengths λ1...λN of light with 50GHz/100GHz channel spacing.These data streams can then be sent together and propagatethrough a optical fiber for a long distance (ex. ∼km). Onthe receiver side, optical filters tuned for N correspondingwavelengths, together with photodetectors, can serve as thede-multiplexer and de-serializer to convert multi-wavelengthserialized optical signals back to N independent electrical datastreams simultaneously.In order to realize chip-scale DWDM nanophotonic intercon-

nects for interchip and intrachip serial links as shown in Figure1, several key components are required: 1) a multi-wavelengthlight source, 2) an optical modulator, 3) an optical filter, and 4)a CMOS transceiver that can modulate, demodulate, and tune

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Fig. 1. Schematic of DWDM nanophotonic interconnects based on siliconmicroring modulators and drop filters.

optical modulators and filters to compensate for wavelengthdrifts and performance deviations due to process variations andenvironmental fluctuations. The multi-wavelength light sourcecan be implemented either by a distributed feedback (DFB)laser bank with an array of DFB laser diodes, or by InAs-GaAsbroad-spectrum QD laser diode. Implementing a DFB laserbank for DWDM nanophotonic interconnects is a significantchallenge due to area and power constraints, and this motivatesa single broad-spectrum comb laser such as [4], [5], [6].For chip-scale optical modulation, silicon microring resonatorsoffer several attractive advantages such as small footprint (ex.10-µm in diameter) and low power driving (ex. <1pJ/bit)compared with the Mach-Zehnder modulator. A microringresonator can be configured to be either an optical modulatorfor transmission or an optical drop filer for receiving. Themajor challenge with the silicon microring resonator, however,is its resonance wavelength λ dependence on process variationsand temperature fluctuations that requires in-situ tuning andclosed-loop compensations.In this paper, we first introduce silicon microring modulatorsfor DWDM transmission. The system demonstration of aDWDM transmitter with an off-chip comb laser and carrier-injection microring modulators [7], [8], as well as a designexample of a self-adaptive CMOS transmitter to stabilize thewavelength for microring modulators are also presented [9].

II. MICRORING MODULATOR

A. Modulation Mechanism

The microring modulator is an example of a resonant filterconsisting of two bus waveguides and a ring waveguide. Ithas two output ports, i.e. the through port and the drop port.When an integer multiple of the incident light wavelengthmλI fits exactly around the ring waveguide perimeter p, i.e.mλI = p, a resonance condition is achieved. At resonance,

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Fig. 2. Modulation and tuning schemes of a microring modulator. FWHMstands for full width at half maximum and is typically around 0.05 to 0.15nm.

the optical power at the through port reaches its minimumvalue while the optical power at the drop port reaches itsmaximum value [2]. A microring modulator can use free-carrier plasma dispersion effect to change the silicon refractiveindex, which changes the phase of the light passing through it.By changing the free-carrier density, the resonance wavelengthof a microring modulator can be altered. This can be achievedby either injecting or removing carriers, which leads to twocommon types of microring modulators: the carrier-depletionmodulator based on reverse-biased PN-diode and the carrier-injection modulator based on forward-biased PIN-diode.Figure 2 shows the modulation and tuning schemes of amicroring modulator at the through port. Here T0, T1, andT2 represent the relationships between the optical power trans-mission versus resonance wavelength of a microring modulatorfor zero-bias, logic ”0” and logic ”1”, respectively. When thevoltage applied to the microring modulator changes, the res-onance wavelength of the modulator will change accordingly.Provided that the light source such as a continuous-wave (CW)laser provides a single fixed-wavelength of incident light, themodulator can therefore serve as an optical switch to turn onand off the light passing through it at the given wavelength.As shown in Figure 2, when the voltage applied to themodulator increases, the resonance wavelength decreases. Thedifference of the transmission optical power between logic ”1”and logic ”0” is the modulation depth that can significantlyimpact energy efficiency of a CMOS optical transceiver.

B. Carrier-Depletion Modulator

In a carrier-depletion modulator, the waveguide core isdoped and a depletion region is formed near the p-n junction.When the junction is reverse-biased, the depletion region willbe widened and the free carrier density in the core willdecrease. Therefore, the silicon refractive index will increaseand the resonance wavelength will redshift [10]. Compared toa carrier-injection modulator, the carrier-depletion modulatorcan achieve a faster modulation speed because of its electricfield induced majority carrier dynamics. However, due to lowerdoping concentration in the p-n junction, the change of therefractive index (∆n) is smaller than that of a carrier-injectiontype of modulators. The maximum achievable extinction ratio(ER) is therefore less than the carrier-injection modulator.An experimentally-validated analytical model of a carrier-depletion modulator is described in [11], [10].

Fig. 3. Microscope image of a carrier-injection microring modulator.

C. Carrier-Injection Modulator

In a carrier-injection p-i-n diode modulator as shown inFigure 3, free carriers such as electrons and holes are injectedinto the intrinsic region when the junction is forward-biased[7]. The carrier concentration within the waveguide corewill increase and the silicon core’s refractive index willdecrease. Therefore, the resonance wavelength will blueshift.Limited by the carrier lifetime between generation andrecombination, a carrier-injection modulator usually has alower modulation speed than a carrier-depletion modulator.On the other hand, the higher doping concentration of thecarrier-injection modulator enables a larger change of therefractive index (∆n) as well as a greater ER that can leadto a more energy efficient optical receiver design to maintainthe same bit-error-rate (BER). An experimentally-validatedanalytical model is described in [10].

III. DWDM MICRORING TRANSMITTER

We designed and fabricated a five-channel DWDM carrier-injection microring modulator-based transmitter on 130-nmsilicon-on-insulator (SOI) wafers [8] as shown in Figure 4.The microscope image of the DWDM transmitter with micror-ing modulators and fiber coupling is shown in the top figureand measured optical power transmission versus resonancewavelength is shown in the bottom figure. The quality factorQ is around 10,000 and the ER is greater than 15dB forthe carrier-injection modulators. Due to fabrication variations,black dotted curves in the bottom figure shows resonancewavelength drifts which make the microring modulator arrayunable to match the wavelengths of the comb laser. Thermaltuning using a heater is therefore required to tune each indi-vidual modulator for wavelength matching as shown as the redsolid curves. The channel spacing in the microring modulatorarray is set 160GHz in order to match 80GHz channel spacingof the comb laser, and also have sufficient guardband to avoidcrosstalk between channels.Figure 5 shows the spectrum of the comb laser before and afterSOA with 80GHz channel spacing [4], [5], [6]. Figure 6 showsoptical eye diagrams of channel #1 (top) and #2 (bottom) at10Gbps measured at 25 ◦C with an off-chip QD comb laserand on-chip carrier-injection microring modulators. The inputpattern is pre-emphasized PRBS31 and the external SOA is

Fig. 4. (Top) microscope image of a five-channel DWDM microringmodulator-based transmitter; (bottom) measured optical power transmissionversus resonance wavelength at the through port. Black dotted curves showthe original resonance wavelength as fabricated and red solid curves show theresonance wavelength after thermal tuning. The channel spacing is set to be160GHz.

Fig. 5. Measured spectrum of the QD comb laser output before and aftersemiconductor optical amplifier (SOA) with 80GHz channel spacing. ASErepresents the amplified spontaneous emission of the SOA.

included to compensate for 10dB insertion loss due to theoptical filter. The optical filter is required in order to measurethe individual channel without on-chip DWDM drop filters.No major difference is observed among DWDM channels. Thewall-plug efficiency of the comb laser is around 10% and theenergy efficiency is 3 pJ/bit at 25 ◦C.

IV. HYBRID INTEGRATED DWDM TRANSMITTER

Figure 7 illustrates a hybrid integrated platform of DWDMnanophotonic interconnects. CMOS and photonic chips areflip-chip bonded with micro-bumps to reduce the parasiticsassociated with wire-bonding for high-speed signaling. Aphoto of the current version of the hybrid integrated DWDMtransmitter is shown in Figure 8 where a CMOS chip in 65-nm bulk process is wire-bonded to a photonic chip in 130-

Fig. 6. Measured optical eye diagrams of DWDM transmitter channel #1(top) and channel #2 (bottom) at 10Gbps and 25 ◦C. Input pattern is PRBS31.

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Fig. 7. Schematic of a hybrid integrated nanophotonic interconnects.

nm SOI process. The CMOS chip can provide nonlinear pre-emphasis driving signals for carrier-injection modulators, aswell as generate bias voltages to modulators for wavelengthstabilization and matching to the CW laser wavelength. Con-ventional thermal tuning with the resistive heater can onlygenerate redshift, i.e. increasing the resonance wavelength.The tuning speed is usually slow (∼ms) and the tuning poweris high (> 40µW/GHz in wavelength shifts). On the otherhand, by increasing the bias-voltage applied to the anode

Fig. 8. Photo of the hybrid integrated transmitter with wire bonding [9].

Fig. 9. Measured optical eye diagram for wire-bonded hybrid integratedDWDM transmitter at 9Gbps and 25 ◦C.

terminal of the p-i-n diode in the carrier-injection modulator,the resonance wavelength can be blueshifted to a shorterwavelength due to accumulation of free carriers. By monitoringthe optical power transmission at the drop port of the microringmodulator with an off-chip photodetector (PD), a closed-loopwavelength stabilization can be formed to first search forresonance wavelength and then lock the bias-voltage of themodulator for optimized ER. Measured optical eye diagramof the wire-bonded hybrid integrated DWDM transmitter isshown in Figure 9. The energy efficiency of the hybridintegrated transmitter is 0.48 pJ/bit and the ER is 9.2dB. MoreCMOS design details can be found in [3], [9].

V. CONCLUSION

In this paper, we briefly introduce silicon microring mod-ulators including carrier-depletion and carrier-injecting mod-ulators. We also show the DWDM transmitter with QD-based comb laser and carrier-injection modulators. The 10Gbpsoptical eye diagrams at different DWDM channels reveal nomajor difference. The wire-bonded hybrid integrated DWDMtransmitter including a CMOS chip to drive microring mod-ulators and provide bias voltages for resonance wavelengthstabilization is also presented with highest data rates up to9Gbps. We believe that DWDM nanophotonic interconnectscan pave the way to realize terabit/s interchip and intrachiphigh-speed serial links for high-performance computing.

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