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  • 1. University of Ljubljana Faculty for Mathematics and Physics Department of Physics Seminar Optical Computers Author: Žiga Lokar Mentor: prof. Igor Poberaj Ljubljana, 31.3.10Summary:This seminar will first explain benefits and problems with optical computers, then explain all vitalparts of a computer and how can they be replaced with optical equivalents. First, the seminar willdiscuss feasibility to completely replace copper wires with optical fibres in motherboards, thenexplain how is computation with photons possible. In the end, all optical storage and memory will bediscussed.
  • 2. 1. Contents2. Introduction ..................................................................................................................................... 3 SOA ...................................................................................................................................................... 33. Backplane (motherboard) ............................................................................................................... 44. Processing unit ................................................................................................................................ 65. Storage ............................................................................................................................................ 96. Memory ......................................................................................................................................... 117. Conclusion ..................................................................................................................................... 138. References ..................................................................................................................................... 14
  • 3. 2. IntroductionFor computing, light is currently used for signal transmission, mostly for long distancecommunication. It offers several benefits compared to electric signal transmission, most importantly,much higher bandwidth. Furthermore, energy requirements are lower and photons also interactweakly with electromagnetic field, meaning signal transmission is less prone to errors. The limitingfactor for long distance communication are currently electronic parts of the system. Signals need tobe cleaned of noise after some distance. This is usually realised with OEO (optic-electronic-optic)devices; signal is converted to electronic one, cleaned and retransmitted as an optical signal. Alloptical signal regeneration is also possible, experimentally demonstrated by various research groups[1, 2]. Another desire that would also greatly speed up long distance communication is fast all opticalsignal routing, also demonstrated [3].SOASemiconductor optical amplifiers are currently used mostly for signal amplification before detection,where only amplitude is increased. Using nonlinear effects exhibited by these devices, signal can bealso reshaped and retimed. This is called 3R regeneration.Image 1: Signal regeneration. 1R means amplification, 2R adds re-shaping while 3R adds re-timing as well. On x axis there is time, with amplitude on y axis.Such regenerators work at much higher bitrates than electronic signal regeneration and there is noneed to convert signals. But their problem is small amplification and regeneration for amount ofenergy needed. Only amplifiers are used in newer fibers, so there is no need to re-transmit the signalevery couple of kilometers, as it was needed in older fibers.All demonstrated technologies show promise for fast all optical systems for long distancecommunication, although there are still many serious problems how to scale the device and decreasepower consumption.For computers, on the other hand, small system size is also required. Seminar will explain currentstatus of all-optical computers.
  • 4. 3. Backplane (motherboard)Backplanes are vital parts of computers, however, they are not known under this name in personalcomputing. For personal computers, as well as quite many servers nowadays, term motherboard isused. These terms are not identical; motherboards offer some integrated processing, as well as asocket for the central processing unit, CPU. Backplanes are only used to route signals betweenvarious add-on cards, and a CPU is just one of the inserted cards if there is a central processing uniton the backplane at all. However, these differences are mostly semantic as both parts have identicalimportant role – interconnection between different components.I will use term backplane during this seminar, as work in this area is based on providing fastcommunication.Optical backplanes consist of similar parts than long distance communication. Signal source is aVCSEL, vertical-cavity surface-emitting laser. Benefit of these lasers is a low power requirement and avery high efficiency, but their maximum power is very low as well. Signal is modulated using Kerreffect, which will be discussed under processing unit. The signal propagates through optical fiber,similar to the one used in long distance communication. No regeneration is needed, as distance isshort. On the receiving point, a photodiode is used to convert optical signal back to electric one.Current demonstrations of optical backplanes do not plan to have optical signal routing, optics serveonly as a point-to-point link.Major benefit of the optical backplane is its very high transfer speed. Current demonstrations exceed10 Gbit/s/channel/fiber. Their major difficulty is its high cost of equipment, while there are someother, physical limitations.One of the problems is various sources of noise – noise on the laser, the detector and a fundamentalphysics limit, shot noise. This effect is known to anyone investing some time in photography. Whenthe number of photons collected is small, this causes a large variation of the signal. Underassumption there will be a Poisson distribution of photons and the detector has a constantpercentage to collect a photon, signal to noise ratio equals √ . This noise presents lowest possiblepower consumption, as system is characterized by minimum signal to noise ratio. Electric systemscurrently operate with less noise for short distance. Therefore, the advantage of decreasedinteraction with environment is reduced due to power requirement of the connection. This onlyholds true if the wire is very short, otherwise resistivity of wires greatly decreases performance –signal attenuation due to distance travelled is much higher for copper wires than it is for opticalfibers.
  • 5. Image 2: Shot noise on a photograph.; 21.3.10Furthermore, frequency of light used in long distance communication fibers is 1550 nm. Thistherefore limits minimum diameter of the wire to about 750, as light cannot travel through awaveguide that is less than thick. Further consequence is the imposed limit on the laserdimensions, the very same 750 nm. This limit is much more important than the noise limit; itpresents a great difficulty to scale the system to on-chip communication, especially if routing isneeded.Despite all difficulties, end result is in favor of photonics with higher transfer speed and lower powerneed per GB/s, while cost problem remains. In HPC (high performance computing) connectionsbetween 2 backplanes have been optical for some time now, due to mentioned benefits. Migrationtowards optical backplanes has not started yet, although technology is ready and offers higher speedthan electronics. Shorter transfer length (even on chip optical communication) has beendemonstrated in laboratory, but practical use is not as close as it is for backplane opticalcommunication. [4, 5]
  • 6. Image 3: IBMs idea of merging photonics, memory and processing on the same chip in different layers. Estimated by IBM to appear around 2018. [4]For high performance computing, communication is therefore migrating towards photonics. Mainreasons are or need to become cost per bit for backplanes and communication between racks. Foron-chip communication, power per bit and maximum bandwidth is said to bring advantage tophotonic communication over copper wires.For personal use, optical communication is further away. The first announced optical connectionbetween devices is Intel’s Light Peak, optical connection between a PC and other devices, such asmonitors, TVs, hard disk drives and so on. All-optical motherboards or smaller connections are notexpected yet. 4. Processing unitOut of all components of an optical computer, processing unit is the most desired.As there are many nonlinear optical effects, many different possibilities have been tried in order tocreate an optical logic gate. They usually rely on Kerr effect. Kerr effect is a quadratic electro-opticaleffect, where material alters its refraction coefficient when electric field is applied. (1)Where λ is the wavelength of the light, K is is the Kerr constant and E is electric field.This principle is used to modulate signals for communication, as response of the material is veryswift. Electric field could also be a consequence of another beam, if the beam intensity is highenough, making the process a third order nonlinear effect. Third order optical nonlinearity meanspolarization depends on the cube of electric field, (2)
  • 7. Such transistors rely on cross phase modulation, where phase of one beam is influenced by another. { } (3) is the wavelength of light in free space, l length of the material where beams interacts, issecond field and is third order nonlinear tensor, where only part responsible for cross phasemodulation is taken.Response to the light is swift; therefore their possible operating frequency is high, much higher thanof current electronic ones. However, most materials have a very small nonlinear index. For areasonably small transistor (to compete with electronic ones in terms of frequency/size), a very highelectric field is needed, meaning powerful lasers are required. Optical transistor major problem isalso material inability to sustain laser power without damage. When frequency of pulses gets highenough to compete with silicon transistors, optical ones incinerate, limiting their maximum operatingfrequency to less than 1 Mhz. Either a material with very high nonlinearity needs to be found or amaterial needs to sustain very high power of lasers in order to increase this limit. First is preferred, asa very powerful laser would be also impractical for computing. Wall Street Journal (Jan. 30, 1990)labeled such a material "unobtainium,” as throughout all the years and funding, nothing was found.Situation has not improved much since then.Another material optical transistors could be based on are photonic crystals, or photonic bandgapcrystals they are also called. These crystals are made of a material with periodically varying refractionindex. While 1D photonic crystals have been known for over a century and frequently appear innature, first 2D crystal for optical wavelength has been fabricated in 1996. Their major problem is stillfabrication, as there is no known efficient method for creating such an array in 3D without defects.Currently, majority of research on this field is focused on producing the crystals, mainly through self-assembly. [7]For computation using photonic crystals, nonlinear defects in crystals are required. Even a simplescheme, where only 3 nonlinear rods are inserted in a 2D photonic crystal, exhibits bistabletransmission. This can be used to create a simple switching device, where one beam sets the path forthe second.
  • 8. Image 4: Effects of nonlinear rods on transmission in a configuration shown in inset. Nonlinear rods are marked with black circles. [7]Another phenomenon that showed promise and is being researched for optical computers is asurface plasmon, a standing wave of free electrons on a metallic surface. When a photon hits metal,electrons ripple with a specific frequency. These standing waves propagate through a metal, butlosses are very high. If such waves are confined only to the metallic surface and most energy travelsthrough a dielectric layer that is in contact with metal, losses are dramatically reduced and the signalcan travel much further even through a very narrow waveguide, below conventional limit. [8]Although most research in this field is not useful for an all-optical computer at this stage, plasmonsare very promising for creating small lasers and waveguides. Both were demonstrated, but majoradvances are still needed in order to be of practical use.Optical logic gates also have the problem they need a lot of signal regeneration after gate. As opticalsignal regeneration is difficult, some predict that optical computing and possibly even routing isdead, or at least is not progressing the way scientists hoped for. [6]On the other hand, some are more optimistic: ““For the last five years or so it has been possible tobuild an optical computer chip, but with all-optical components it would have to measure somethinglike half a meter by half a meter and would consume enormous power. With plasmonics, we canmake the circuitry small enough to fit in a normal PC while maintaining optical speeds,” explainsAnatoly Zayats , a researcher at The Queens University of Belfast in the United Kingdom.” - Quotefrom [9].Current status: Such a computer is possible, but not feasible. Many significant improvements arerequired in order to compete with electronic processors for computers.
  • 9. 5. StorageWrite once, read many (WORM) optical storage has been around for quite a long time, since the firstCDs in 80s, which even enabled more storage density than magnetic disks at the time. CD wasfollowed by a DVD, this one by Blu-ray. After Blu-ray, holographic storage is predicted as the nextstop. First stop for holographic storage is to create a write once disk, same as with other types ofoptical storage. But, in order to make a proper computer storage using this technology, it is requiredthat a disk can also be rewritten multiple times. Rewritable holographic storage could be based onphotorefractive effect, an effect where a material alters its refractive index when exposed to light. Ifa material exhibits Pockels effect, change of refraction coefficient is (4)Where n is a refraction coefficient without applied electric field, r is an electro-optic coefficient and Eis the electric field.For the effect to be applied to holograms, process works in several steps. First, interference betweenreference and signal beams creates a pattern of light and dark fringes throughout the crystal. Inregions with bright fringes, electrons absorb photons and promote into conductive band. Electronsdiffuse around the crystal (or drift due to photovoltaic effect), while holes must remain stationary.Electrons may, with some probability, recombine with holes or fall back to the traps made byimpurities, where they cannot move. With more electrons in dark areas and holes in bright, there is anet electric field, called space charge field, which causes, via electro-optic effect, refractive index tochange, creating a grating. Grating reflects light, recreating the original signal beam pattern, whenreference beam strikes the grating at the same angle.The following setup does enable multiple writes, but every read would excite some of the trappedelectrons and decrease strength of the hologram. After several reads, image would not berecognizable anymore.The idea would need to be realized using two-photonic write in order to be practical. Two-photonicsetup uses 2 different dopants in order to create shallow and deep traps. When writing an image, apulse of short wavelength excites electrons to conduction band. They quickly fall back to a shallowtrap. From shallow traps, signal and reference beam can excite them back in conduction band, wherethey diffuse like before. Afterwards, electrons fall back, first to a shallow trap, then to a deep. Whenin deep traps, reference beam does not have enough energy to excite them back to conduction band,greatly increasing durability of the hologram.
  • 10. Image 5: Two-photonic write, described above. [10]Additionally, storage could be written on in the Fourier space, meaning a single point of data doesnot map to a single part of crystal, but rather to the whole, thus decreasing defects due toimperfections.Using all these techniques, holographic rewritable storage could at least compete with magneticdisks with respect of reliability. Polymeric holographic storage, on the other hand, is among the mostdurable WORM storages and could compete even with magnetic tape in terms of reliability.There is also a possibility to create several holograms on the same space using different angle ofreference beam, or some other way of multiplexing signal (discussed in [10]). This increases storagedensity, but decreases speed as reflection is weaker. Practical upper limit is around one hundredholograms in a material 1 mm thick.As the capacity of magnetic disks increase, holographic storage seems unable to bring any significantadvantages. First announced holographic disk would offer only 300 GB of capacity with up to 1.6 TBto follow. Magnetic disks offer more capacity now than proposed storage could in the future. Butthere is one major possible benefit of holographic storage – speed. Holographic drive enables readingand writing whole set of data in parallel, enabling very high read and write speed. On the other hand,all current data is written sequentially.Due to lack of any moving parts, access time would be very low as well compared to magnetic drives,where needle needs to move. Announced products, on the other hand, would be first severelylimited by write speed of only 20MB/s, which was said to increase later on, to be comparable withcurrent disks. [11]
  • 11. The other possible benefit compared to current drives is longevity and reliability, both mostly valuedfor permanent storage and not as useful for home computers as speed and capacity are. Magnetictape is currently dominant in this space with no real alternative.Major difficulty of holographic storage and the reason it has not appeared yet is the material. Thematerial needs to have traps of correct energy, enable high write density and speed, would bewithout defects that decrease performance and cheap enough to make, in order to be in commercialproducts. Most researched material was Lithium Niobate, while many others have been tried inlaboratories, including proteins. Polymers are also currently used, but for permanent holograms.One of the most noticeable companies, working on holographic storage, InPhase technologies, spunoff Bell Labs in 2000. The company made several claims when will they release their storage, first as300GB disks with up to 1.6TB later, but nothing was released to the market. Recently, the companyhad their assets seized, as they were unable to pay taxes. Similarly, Optware made several claimsabout their upcoming products, while none actually appeared on the market in the end, and thecompany does not exist anymore either.General Electric and IBM continue research on holographic storage, but neither announcedupcoming products yet. 6. MemoryMemory for systems seeks to have as high bandwidth as possible with lowest latency possible, whilehaving the largest size possible. These requests are mutually excluding, so compromises are made,usually in favor of speed and latency. Optical competition to traditional memory seem very promisingas replacement, theoretically enabling high bandwidth and low latency, while being able to scale sizeas well. Such optical system could be based on several effects. First possibility was discussed withhard disks and is holographic data storage. Further option is a temporary storage of light pulses.This storage relies on light to excite electrons in conduction band, generating pairs of electron-hole.Electric field is applied to modulate potential and trap electrons and holes in spatially separatepotential minimums, limiting recombination. This energy can be later re-emitted in a short flash oflight, when potential is released. Due to the mechanism, only energy can be stored, coherence is notpreserved. The idea was shown to work with temperatures around 100K, light was stored for several , while energy was released on demand. Further improvements could enable such storage for alonger period of time and at a room temperature. As coherence cannot be preserved, this is not asinteresting storage possibility as the holographic one. [12]In addition to these possibilities, light can be also slowed down or trapped in a resonator. Resonatorbased storage would require mirrors with total reflectivity as there would be 300 reflections ofmirrors if we wanted to store light in 1 m long resonator for only 1 . Similarly, fiber based delayrequires low signal attenuation and ability to release stored pulses on demand at once. Bothsolutions are possible, but more difficult to realize, compared to holographic storage. Anotherpossibility is to stop light and release it on demand.Light stopping is a process in which light is slowed from vacuum speed at least two decades down.However, for a practical memory cell, speed needs to be reduced to 0 on demand, otherwise
  • 12. maximum storage time would be limited by memory length. Using a material with very highrefraction coefficient would also result in very high losses; therefore this is not a practical solution.Possibility is to use Rabi oscillations, a quantum phenomenon where two states are coupled throughelectric field.Rabi oscillationsSuppose we have a system with two eigenstate energies, ⟩ , ⟩ , and we shine alight with frequency of We start perturbatively, where perturbative term ofhamiltonian is [ ⟩⟨ ⟩⟨ ] (5)New wave function is a linear superposition of our ground states. | ⟩ ⟩ ⟩ (6)Inserting wave function in Schrödinger equation, we get relations between coefficients A and B: ̈ ̈ ( ) (7)Probability of excited state therefore varies with a typical frequency, called Rabi frequency. (8)Frequency can also be written in generalized form, when field frequency is not equal to energydifference, √ | | (9)Intuitively, light behaves as if the matter was periodically absorbing and then re-emitting photonsdue to stimulated emission. Each such cycle is called a Rabi cycle. [14]If we calculate eigenstates for the new Hamiltonian, we see that states split due to polarization,energy difference between new states equals . These new states are called dressed states.Therefore, matter is more transparent to the light it would be without state splitting.We can use the effect with 3 states in configuration, where 1 wave, called coupling, couples states2 and 3, while second (called probe) couples 1 and 3. As states 2 and 3 require much higher energy,they are not occupied at start. First, coupling wave is activated, but as states 2 and 3 are empty,there is no oscillation. Probe wave is then activated adiabatically. Due to probe wave, state 3 splits.Oscillations interfere, resulting in impossibility to have any particles in state 3. Therefore, absorptionis not only a superposition of 2 absorption lines, but its imaginary part does fall to 0 in the middle,while real part of the refraction index has a very large derivative.
  • 13. Image 6: Real and imaginary part of refraction index without coupling field (dashed line) and with coupling field (solid line) [13]If we deactivate coupling wave with light in material, energy is stored in state 2, meaningpropagation of light stops, till coupling wave is reactivated, when the light resumes propagation. Asthere is spontaneous state transition through emission, strength of the signal decays exponentially.Currently, lowest speed in a viable material was 80 , achieved in ruby vapour at 360K. Lower speedsand even total stop were achieved, but vapour needed to be cooled to several hundred nK, waybelow practical usage for storage. Light was also stopped only for some . Using current memoryproduction techniques and materials, light was stopped to 1/100 c. Therefore, it is possible to slow oreven stop light using this process, but it is not currently useful for memory.Out of all mentioned technologies, only holographic seems achievable in not too distant future, whilenone are actually close to commercial production. 7. ConclusionOptical computing is, at the moment, possible, but not practical. For servers, process of migrationtowards optical connections even on a smaller scale has already started, while other systemcomponents are currently not being focused on. For personal computers, situation is even lesspromising, as technology is driven by its cost, not absolute performance for any price. Estimationswhen all-optical computers could appear are currently only guesses, as technology is not ready evenfor prototype computers, let alone massive commercial production.
  • 14. 8. References[1]:; 21.3.2010[2]:; 21.3.2010[3]: M. Takenaka, K. Takeda, Y. Kanema, M. Raburn, T. Miyahara, H. Uetsuka, Y. Nakano, 320Gb/soptical packet switching using all-optical signal processing by an MMI-BLD optical flip-flop, Proc. 32ndEur. Conf. Opt. Commun. (ECOC 2006), TH4.5.2, Cannes, France, 2006.[4]: 21.3.2010[5]: Board-to-Board Optical Interconnection System Using Optical Slots; In-Kui Cho et al., IEEEPhotonics Technology Letters, Vol. 16, no. 7, July 2004[6]: All-Optical Computing and All-Optical Networks are Dead; Charles Beeler, El Dorado Ventures andCraig Partridge, BBN[7]:, 21.3.2010[8]: Lasers go nano, Francisco J. Garcia-Vidal and Esteban Moreno, NATURE|Vol 461|1 October 2009[9]: 21.3.2010[10]: Holografsko shranjevanje podatkov, Blaz Kavcic, Ljubljana, November 2007[11]:, 21.3.2010[12]: Semiconductor based photonic memory cell, S. Zimmermann, et al., Science 283, 1292 (1999)[13]: Pocasna svetloba, Martin Strojnik, May 2008[14]: “Laser Electronics”, J. Verdeyen, 3rd ed., Chapt. 14.