84784701 Silicon Photonics

8/12/2019 84784701 Silicon Photonics

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Silicon photonics can be defined asthe utilization of silicon-based

materials for the generation(electrical-to-optical conversion),guidance, control,and detection (optical-to-electricalconversion) of light to communicateinformation over distance. The mostadvanced extension of this conceptis to have a comprehensive set ofoptical and electronic functionsavailable to the designer asmonolithically integrated buildingblocks upon a single siliconsubstrate.

Within the range of fibreoptic telecommunication wavelength(1.3 ?m to 1.6 ?m), silicon isnearly transparent and generallydoes not interact with the light,making it an exceptional medium for

guiding optical data streamsbetween active components. But nopractical modification to silicon hasyet been conceived which gives


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efficient generation of light. Thus itrequired the light source as anexternal component which was a

drawback.

There are two parallel approachesbeing pursued for achieving opto-electronic integration in silicon. Thefirst is to look for specific caseswhere close integration of an opticalcomponent and an electronic circuitcan improve overall systemperformance. One such case wouldbe to integrate a SiGephotodetector with aComplementary Metal-Oxide-

Semiconductor (CMOS)transimpedance amplifier. Thesecond is to achieve a high level ofphotonic integration with the goal ofmaximizing the level of opticalfunctionality and opticalperformance. This is possible by

increasing light emitting efficiency ifsilicon. The paper basically dealswith this aspect.


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AbstractIn its everlasting quest to deliver more data fasterand on smaller components, the silicon industry is

moving fullsteam ahead towards its final frontiers of size,device integration and complexity. As the physicallimitations ofmetallic interconnects begin to threaten thesemiconductor industry's future, researches areconcentrated heavily onadvances in photonics that will lead to combiningexisting silicon infrastructure with opticalcommunicationstechnology, and a merger of electronics andphotonics into one integrated dual functionaldevice. Optical

technology has always suffered from its reputationfor being an expensive solution. This promptedresearch intousing more common materials, such as silicon, forthe fabrication of photonic components, hence thename siliconphotonic.

IntroductionDuring the past few years, researchers at Intelhave been


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actively exploring the use of silicon as the primarybasisof photonic components. This research has

establishedIntels reputation in a specialized field calledsiliconphotonics, which appears poised to providesolutionsthat break through longstanding limitations ofsilicon asa material for fiber optics.

In a major advancement, Intel researchers havedeveloped a silicon-based optical modulatoroperating at50 GHz - an increase of over 50 times theprevious

research record of about 1GHz (initially 20MHz).Thisis a significant step towards building opticaldevices thatmove data around inside a computer at the speedoflight. It is the kind of breakthrough that ripples

across anindustry over time, enabling other new devicesandapplications. It could help make the Internet run


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faster,build much faster high-performance computersand

enable high-bandwidth applications like ultra-high-

definition displays or vision recognition systems. Intels research into silicon photonics is an end-to-endprogram to extend Moores Law into new areas. In addition to this research, Intels expertise infabricatingprocessors from silicon could enable it to createinexpensive, high performance photonic devicesthatcomprise numerous components integrated ononesilicon die. Siliconizing photonics to develop and

build optical devices in silicon has the potential tobringPC economics to high-bandwidth opticalcommunications. Another advancement in siliconphotonics is the demonstration of the firstcontinuoussilicon laser based on the Raman Effect. This

researchbreakthrough paves the way for making opticalamplifiers, lasers and wavelength converters toswitch a


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signals color in low-cost silicon.Fiber optic communication is well established todaydue

to the great capacity and reliability it provides.However, the technology has suffered from areputationas an expensive solution. This view is based inlargepart on the high cost of the hardware components.Thesecomponents are typically fabricated using exoticmaterials that are expensive to manufacture. Inaddition,these components tend to be specialized andrequirecomplex steps to assemble and package. These

limitations prompted Intel to research theconstructionof fiber-optic components from other materials,such assilicon. The vision of silicon photonics arose fromtheresearch performed in this area. Its overarching

goal isto develop high-volume, low-cost opticalcomponentsusing standard CMOS processingthe same


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manufacturing process used for microprocessorsandsemiconductor devices

What Is Silicon Photonics?Photonics is the field of study that deals with light,especially the development of components foropticalcommunications. It is the hardware aspect of fiberoptics, and due to commercial demand forbandwidth, ithas enjoyed considerable expansion anddevelopmentduring the past decade. Fiber-optic communication,asmost people know, is the process of transporting

data athigh speeds using light, which travels to itsdestinationon a glass fiber. Fiber optics is well establishedtodaydue to the great capacity and reliability it provides.However, fiber optics has suffered from its

reputation asan expensive solution. This view is based in largeparton the high price of the hardware components.


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Opticaldevices typically have been made from exoticmaterials

such as gallium arsenide, lithium niobate, andindiumphosphide that are complicated to process. Inaddition,many photonic devices today are hand assembledandoften require active or manual alignmentto connect thecomponents and fibers onto the devices. This non-automated process tends to contribute significantlytothe cost of these optical devices.

Silicon photonics research at Intel hopes toestablish thatmanufacturing processes using silicon canovercomesome of these limitations. Intels goal is tomanufactureand sell optical devices that are made out of easy-

to-

manufacture silicon. Silicon has numerous qualitiesthatmake it a desirable material for constructing small,


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low-cost optical components: it is a relativelyinexpensive,

plentiful, and well understood material forproducingelectronic devices. In addition, due to thelongstandinguse of silicon in the semiconductor industry, thefabrication tools by which it can be processed intosmallcomponents are commonly available today.BecauseIntel has more than 35 years of experience insilicon anddevice fabrication, it finds a natural fit in exploringthe

design and development of silicon photonics.

Silicon photonics is the study and applicationof photonic systems which use silicon as anopticalmedium. It can be simply defined as the photonictechnology based on silicon chips. Siliconphotonics can

be defined as the utilization of silicon-basedmaterialsfor the generation (electrical-to-opticalconversion),


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guidance, control, and detection (optical-to-electricalconversion) of light to communicate information

overdistance. The most advanced extension of thisconcept isto have a comprehensive set of optical andelectronicfunctions available to the designer asmonolithicallyintegrated building blocks upon a single siliconsubstrate.The goal is to siliconize photonics-specifically tobuildin silicon all the functions necessary for opticaltransmission and reception of data. The goal is

then tointegrate the resulting devices onto a single chip.Ananalogy can be made that such optical chips holdthesame relationship to the individual components asintegrated circuits do to the transistors that

constitutethem: they provide a complete unit that can bemanufactured easily and inexpensively usingstandard


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silicon fabrication techniques. Intel has recentlybeenable to demonstrate basic feasibility to siliconize

manyof the components needed for opticalcommunication.The most recent advance involves encoding high-speeddata on an optical beam.There are two parallel approaches being pursuedforachieving optoelectronic integration in silicon. Thefirstis to look for specific cases where close integrationof anoptical component and an electronic circuit can

improveoverall system performance. One such case wouldbeto integrate a Si-Ge photo-detector with aComplementary Metal-Oxide-Semiconductor(CMOS) trans-impedance amplifier. The second isto

achieve a high level of photonic integration withthegoal of maximizing the level of optical functionalityand


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optical performance. This is possible by increasinglightemitting efficiency if silicon.

Why Silicon Photonics?Fiber-optic communication is the process oftransportingdata at high speeds on a glass fiber using light.Fiberoptic communication is well established today dueto thegreat capacity and reliability it provides. However,thetechnology has suffered from a reputation as anexpensive solution. This view is based in large part

onthe high cost of the hardware components. Thesecomponents are typically fabricated using exoticmaterials that are expensive to manufacture. Inaddition,these components tend to be specialized andrequire

complex steps to assemble and package.These limitations prompted Intel to research theconstruction of fiber-optic components from othermaterials, such as silicon. The vision of silicon


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photonics arose from the research performed inthisarea. Its overarching goal is to develop high-

volume,low-cost optical components using standardCMOSprocessingthe same manufacturing processused formicroprocessors and semiconductor devices.Silicon presents a unique material for this researchbecause the techniques for processing it are wellunderstood and it demonstrates certain desirablebehaviors. For example, while silicon is opaque inthevisible spectrum, it is transparent at the Infra-redwavelengths used in optical transmission, hence it

canguide light. Moreover, manufacturing siliconcomponents in high volume to the specificationsneededby optical communication is comparativelyinexpensive.Silicons key drawback is that it cannot emit laser

light,and so the lasers that drive optical communicationshavebeen made of more exotic materials such as


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indiumphosphide and gallium arsenide. However, siliconcan

be used to manipulate the light emitted byinexpensivelasers so as to provide light that hascharacteristicssimilar to more-expensive devices. This is just onewayin which silicon can lower the cost of photonics.Silicon photonic devices can be made usingexisting semiconductor fabrication techniques, andbecause silicon is already used as the substrateformost integrated circuits, it is possible to createhybrid

devices in which the optical and electroniccomponents are integrated onto a singlemicrochip.The propagation of light through silicon devices isgoverned by a range of nonlinear opticalphenomenaincluding the Kerr effect, the Raman effect, Two

PhotonAbsorption and interactions between photons andfreecharge carriers. The presence of nonlinearity is of


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fundamental importance, as it enables light tointeractwith light, thus permitting applications such as

wavelength conversion and all-optical signalrouting, inaddition to the passive transmission of light.Within the range of fiber optic telecommunicationwavelength (1.3 m to 1.6 m), silicon is nearlytransparent and generally does not interact withthelight, making it an exceptional medium for guidingoptical data streams between active components.

Alsooptical data transmission allows for much higherdatarates and would at the same time eliminate

problemsresulting from electromagnetic interference. Thetechnology may also be useful for other areas ofopticalcommunications, such as fiber to the home.

Physical Properties

A. Optical Guiding and Dispersion Tailoring

Silicon is transparent to infrared light withwavelengthsabove about 1.1 microns. Silicon also has a very


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high refractive index, of about 3.5. The tight opticalconfinement provided by this high index allows formicroscopic optical waveguides, which may have

cross-

sectional dimensions of only a few hundrednanometers.This is substantially less than the wavelength of thelightitself, and is analogous to a sub wavelength-diameteroptical fiber. Single mode propagation can beachieved, thus (like single-mode optical fiber)eliminating the problem of modal dispersion. Thestrong dielectric boundary effects that result fromthistight confinement substantially alter the optical

dispersion relation. By selecting the waveguidegeometry, it is possible to tailor the dispersion tohavedesired properties, which is of crucial importancetoapplications requiring ultra-short pulses. Inparticular,

the group velocity dispersion (that is, the extent towhich group velocity varies with wavelength) canbeclosely controlled. In bulk silicon at 1.55 microns,


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isknown as silicon on insulator. It is named after thetechnology of silicon on insulator in electronics,

whereby components are built upon a layerof insulator in order to reduce parasitic capacitanceandso improve performance.

B. Kerr NonlinearitySilicon has a focusing Kerr nonlinearity, in thatthe refractive index increases with optical intensity.This effect is not especially strong in bulk silicon,but itcan be greatly enhanced by using a siliconwaveguide toconcentrate light into a very small cross-sectional

area.This allows nonlinear optical effects to be seen atlowpowers. The nonlinearity can be enhanced furtherbyusing a slot waveguide, in which the highrefractive

index of the silicon is used to confine light into acentralregion filled with a strongly nonlinear polymer. Kerrnonlinearity underlies a wide variety of optical


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phenomena. One example is four-wave mixing,whichhas been applied in silicon to realize both optical

parametric amplification and parametricwavelengthconversion. Kerr nonlinearity can also causemodulationinstability, in which it reinforces deviations from anoptical waveform, leading to the generation ofspectral-sidebands and the eventual breakup of thewaveforminto a train of pulses.

C. Two-Photon AbsorptionSilicon exhibits Two Photon Absorption (TPA), in

which a pair of photons can act to excite anelectron-hole pair. This process is related to the Kerr effect,andby analogy with complex refractive index, can bethought of as the imaginary-part of a complex Kerrnonlinearity. At the 1.55 micron telecommunication

wavelength, this imaginary part is approximately10%of the real part.The influence of TPA is highly disruptive, as it both


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wastes light, and generates unwanted heat. It canbemitigated, however, either by switching to longer

wavelengths (at which the TPA to Kerr ratio drops),orby using slot waveguides (in which the internalnonlinear material has a lower TPA to Kerrratio). Alternatively, the energy lost through TPAcan bepartially recovered by extracting it from thegeneratedcharge carriers.

D. Free Charge Carrier InteractionsThe free charge carriers within silicon can bothabsorb

photons and change its refractive index. This isparticularly significant at high intensities and forlongdurations, due to the carrier concentration beingbuilt upby TPA. The influence of free charge carriers isoften

(but not always) unwanted, and various meanshavebeen proposed to remove them. One such schemeis


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to implant the silicon with helium in order to

Conclusion

It is clear that an enormous amount of work, corresponding to huge capital investments, is still required before silicon photonics can beestablished as akey technology. However, the potential meritsmotivatebig players such as Intel to pursue thisdevelopmentseriously. If it is successful, it can lead to a very powerful technology with huge benefits forphotonicsand microelectronics and their applications.

Although research in the area of planar optics insiliconhas been underway for several decades, recentefforts atIntel Corporation have provided betterunderstanding ofthe capabilities of such devices as silicon

modulators,

ECLs and SiGe detectors. Silicon modulatorsoperatingat 50 GHz have demonstrated several orders of


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magnitude improvement over other known Si-basedmodulators, with theoretical modeling indicating

performance capabilities beyond 1 THz. Through

further research and demonstration of novel siliconphotonics devices, integrated silicon photonics hasaviable future in commercial optoelectronics.

84784701 Silicon Photonics

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