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Optics and Photonics: Essential Technologies for Our Nation (2013)

Chapter:3 Communications, Information Processing, and Data Storage

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Suggested Citation:"3 Communications, Information Processing, and Data Storage." National Research Council. 2013. Optics and Photonics: Essential Technologies for Our Nation. Washington, DC: The National Academies Press. doi: 10.17226/13491.
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3

Communications, Information Processing, and Data Storage

INTRODUCTION

Optics has become the way by which most information is sent over nearly all the distance that it travels. The remarkable growth of networks and the Internet over the past decade has been enabled by previous generations of optical technology. Optics is, furthermore, the only technology with the physical headroom to keep up with this exponentially growing demand for communicating information. The exceptionally high carrier frequency allows transmission of high bandwidths. Satisfying that future demand will, however, require the continued research and development (R&D) of new technology for optics and continued innovation in its interface with electronics. The use of optics will not be restricted to the traditional market of long-distance telecommunication. Increasingly optics will be used for ever-shorter distances, possibly providing few-millimeter or shorter links between the silicon chips themselves. One key driver for such shorter distances will be to attain sufficient density of communications. A second key driver will be to control the growing power dissipation that is due to switching/transmission elements and the environmental impact of information processing. This chapter summarizes the developments in optics over the last 15 years (i.e., since the publication of the National Research Council’s [NRC’s] Harnessing Light1) in information communications,

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1 National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, D.C.: National Academy Press.

Suggested Citation:"3 Communications, Information Processing, and Data Storage." National Research Council. 2013. Optics and Photonics: Essential Technologies for Our Nation. Washington, DC: The National Academies Press. doi: 10.17226/13491.
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processing, and storage and indicates key directions for the future for optics in these fields.

Communications

Optical communications networks provide the underlying high-capacity, ubiquitous connectivity that underpins the global Internet. Figure 3.1 characterizes the growth of communication and computing between 1986 and 2007, based on a broad collection of data.2 Approximately around the year 2000, Internet traffic took over from voice telephone as the single largest communication format for information. Now Internet traffic dominates completely. All of the long-distance communications on the Internet are over optical fiber.

Major advances in transmission techniques and technologies have allowed network providers to provide extremely cost-effective network upgrades that have kept pace with the extraordinary appetite for broadband Internet services. That growth, as exemplified in Figure 3.1, has driven network bandwidth demands by a factor of 100 over the last 10 years. That increase has been enabled by realizing the full potential of wavelength division multiplexing (WDM) that has resulted in fibers carrying as many as 100 separate wavelengths. In addition, the capacity per wavelength in commercially deployed terrestrial networks has increased from a maximum of 10 gigabits per second (Gb/s) per wavelength when the first edition of Harnessing Light was published in 1998, to 100 Gb/s today. As a result, per fiber transmission capacities in terrestrial systems today as high as 5-10 terabits per second (Tb/s) are possible.3 Transoceanic capacities have lagged somewhat behind terrestrial values because the long amplifier-only distances and the desire to extend the amplifier spacing have made upgrading to per wavelength capacities above 10 Gb/s problematic. Nevertheless, transoceanic per fiber capacities of approximately 1 Tb/s are typical. For the future there are expectations that this growth will continue as more video content calls for bandwidth and that there is a need for another factor-of-100 growth in the coming 10 years as well.

Major advances have also been achieved in both cost-effectively managing the large capacity in today’s WDM optical networks and in leveraging the value proposition of optical amplifiers to provide multi-wavelength amplification over network mesh and ring architectures. Reconfigurable, wavelength-routed networks—in which wavelength-defined units of capacity can be added, dropped, or switched

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2 Hilbert, M., and P. Lopez. 2011. The world’s technological capacity to store, communicate, and compute information. Science 332(6025):60-65.

3 Alferness, R.C. 2008. “Optical Communications—A View to the Future.” 24th European Conference on Optical Communications, Brussels, Belgium, September 22. Available at http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=4729111&tag=1. Accessed June 26, 2012.

Suggested Citation:"3 Communications, Information Processing, and Data Storage." National Research Council. 2013. Optics and Photonics: Essential Technologies for Our Nation. Washington, DC: The National Academies Press. doi: 10.17226/13491.
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image

FIGURE 3.1 Traffic In bytes per second (B/s) (1 byte = 8 bits) on the Internet, on voice telephone, and overall, 1986-2007. Also shown is the capacity to compute in general-purpose machines, expressed in millions of instructions per second (MIPS). SOURCE: MIPS graph based on data extracted from Hilbert, M., and P. Lopez. 2011. The world’s technological capacity to store, communicate, and compute information. Science 332(6025):60-65.

from one fiber route to another fiber route directly in the optical domain without the need for conversion to electronics—are now heavily deployed in long-haul terrestrial networks as well as metropolitan networks. Wavelength-routed networks provide cost-effective solutions because they allow data on wavelengths passing through a node at a multi-route network node to remain in the optical domain and benefit from the cost-effective multi-wavelength amplification enabled by optical amplifiers, rather than needing to be individually electronically regenerated. The large increase in capacity demand has ensured that a prerequisite for the economic viability of such networks—namely, that the capacity demand between any two node pairs on the network be at least as large as that which can be carried by a single wavelength—is met.

WDM optical networks require reconfigurable optical add/drop multiplexers (ROADMs) to, under network electrical control, drop or add wavelength channels at a node and to switch wavelength channels from one fiber route to another. ROADMs are key enablers that have evolved significantly in their functionality, providing increasing levels of flexibility, and in their capacity, or number of fiber ports and wavelengths per fiber, over the last decade. Further progress in these network elements and their enabling technologies will be essential to addressing the growing demand for capacity.

Suggested Citation:"3 Communications, Information Processing, and Data Storage." National Research Council. 2013. Optics and Photonics: Essential Technologies for Our Nation. Washington, DC: The National Academies Press. doi: 10.17226/13491.
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Ultimately, networks are no better than the access capacity that they provide to the end user, whether that customer is a business or a residence. Increasingly that access is through an optical link. The last decade has seen significant increase in the deployment of fiber in the access network, initially to the curb, but increasingly also directly to the business or home.

The United States is not a world leader in fiber to the home or business. It ranks roughly at number 11 among the countries in the world, with approximately 7 percent penetration. The penetration of fiber to the home in the United States is roughly 5 percent.4

Passive optical networks (PONs) are the primary broadband optical delivery architecture, providing the shared bandwidth of a fiber to multiple users (16 to 64 users). Initially, systems provided shared bandwidth of 2.5 Gb/s. New systems operating at a total bandwidth of 10 Gb/s are becoming available. Just as wavelength multiplexing has provided cost-effective bandwidth enhancement in long-haul and metropolitan networks, as capacity demand in the access network increases to enable new broadband services, it is expected that WDM will be employed for capacity expansion. A critical requirement will be robust, low-cost WDM optical components to operate in the outside plant. Research in this area at a somewhat modest level is ongoing; an example of U.S.-funded research in this area is the National Science Foundation (NSF) Engineering Research Center for Integrated Access Networks.5

In spite of the dramatic achievements that optics has brought to communication networks over the last decade (or, perhaps, because of them), the demand for higher bandwidth, both in the fixed and the mobile domains, continues to grow rapidly. Comparing projections from a number of sources, it seems conservative to suggest that network capacity demand will grow at the rate of at least a factor of 100 over the next 10 years, approximately following the recent historical trend shown in Figure 3.1.6 Ubiquitous video is the key driver. Increasingly that video is two-way as more end users upload video to sharing websites. Mobile video is also growing at an extremely rapid rate—92 percent compound annual growth rate according to a Cisco report7—that puts large bandwidth demand on the backhaul

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4 Montagne, R. 2010. “Understanding the Digital World.” Presented at the FTTH Council Europe Conference. Available at http://www.ftthcouncil.eu/documents/Reports/Market_Data_December_2010.pdf. Accessed June 26, 2012.

5 More information is available at the website of the Center for Integrated Access Networks at www.cian-erc.org. Accessed June 26, 2012.

6 Korotky, S.K., R.-J., Essiambre, and R.W. Tkach. 2010. Expectations of optical network traffic gain afforded by bit rate adaptive transmission. Bell Labs Technical Journal 1(4):285-296.

7 CISCO. 2012. “Cisco Visual Networking Index: Forecast and Methodology, 2010-2015.” White paper. Available at http://www.cisco.com/en/US/solutions/collateral/ns341/ns525/ns537/ns705/ns827/white_paper_c11-481360.pdf. Accessed June 26, 2012.

Suggested Citation:"3 Communications, Information Processing, and Data Storage." National Research Council. 2013. Optics and Photonics: Essential Technologies for Our Nation. Washington, DC: The National Academies Press. doi: 10.17226/13491.
×

network, which is increasingly requiring optical links. One can expect also that increasingly mobile backhaul will not be done by point-to-point links as is done today, but rather will be part of an extended-access network which serves end points that include wireless base stations, small enterprises, curb drops, and homes.

Broadband mobility will require ubiquitous, cost-effective backhaul, resulting in the convergence of wire-line and wireless networks that are currently often separate. The nature of traffic in an Internet- and video-driven network could suggest different overall network architecture, with merging between the metropolitan and access networks. Integration of data centers into the network is key today and will be even more so in the future, especially as cloud services become more pervasive. Already in 2003, the NRC’s Committee on Coping with Increasing Demands on Government Data Centers stated:

Recommendation: Data centers should aggressively adopt newer, more “bleeding edge” technical approaches where there might be significant return on investment. This should be done carefully to minimize the inevitable failures that will occur along the way. Even with the failures, the committee believes the cost savings and improvements for end users will be substantial when compared to the methods practiced today.8

Just as capacity demand has required the advantages of optics to be pushed further to the edges of the network, data centers will increasingly depend on optics for interconnection, and eventually for reconfigurability and switching. A future of “cloud services” in which most if not all digital services are performed by shared resources in the network is looking increasingly attractive. This new paradigm, which would depend on ubiquitous, instant, and highly reliable access to the network, could place demands on the network equivalent to those of the transition from voice to data in the late 1990s. The advent of WDM, made cost-effective with the optical amplifier, has enabled the cost-effective scaling of optical point-to-point transmission systems to meet the exponential demand growth. ROADMs and optical cross-connects have made it possible to leverage that advantage to multi-node ring and mesh networks. As a result, optical networks that underpin the Internet have been able to keep pace with exploding demand over the last 10 years. However, realistically, the several-orders-of-magnitude capacity increase resulting from many wavelengths will not be duplicated by simply adding even more wavelengths, because of limited optical amplifier bandwidth and fiber power limits in the fiber to mitigate transmission impairment.9

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8 National Research Council. 2003. Government Data Centers: Meeting Increasing Demands. Washington, D.C.: The National Academies Press.

9 Chraplyvy, A. 2009. “The Coming Capacity Crunch.” Plenary paper. 35th European Conference on Optical Communications, Vienna, Austria, September 21. Available at http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=5287305&tag=1. Accessed June 26, 2012.

Suggested Citation:"3 Communications, Information Processing, and Data Storage." National Research Council. 2013. Optics and Photonics: Essential Technologies for Our Nation. Washington, DC: The National Academies Press. doi: 10.17226/13491.
×

The optical communication industry is highly challenged to find the key new technologies, architectures, and techniques that will enable cost-effective growth in capacity to keep up with the capacity demand that video, including mobile video, will make on the network. Achieving that goal by simply increasing the number of wavelengths, or by increasing the bit rate per wavelength, appears unlikely with known or incrementally improved technology. Therefore, some fundamentally new optical solutions that build on top of today’s WDM optical networks will be urgently required in order to ensure that the global Internet is able to continue to serve as the engine for economic growth. It is estimated that without new technology developments, Internet growth will be constrained starting between 2013 and 2014.

Information Processing

The main, and growing, use of optics in information processing is to connect information within and between information switching and processing machines. There are substantial physical reasons—specifically, reducing power dissipation and increasing the density of information communications—why optics is preferable and ultimately possibly essential for such connections.10 The idea of using optics for performing the logical switching in information processing—as in some kind of optical transistor—is one that has continuing research interest, although the criteria for success there are challenging,11 and no such use appears imminent. The last decade has seen sustained research interest in very advanced ideas such as quantum computing,12 although such ideas remain very much in fundamental research. Optics, however, is likely to have very important roles in any such quantum computing or quantum communications—for example, as the best means of communicating quantum states over any meaningful distance—and optics might also be important in future quantum gates. Quantum encryption, a means of sending key information with immunity to any eavesdropping, has seen first commercial optical systems. In the meantime, efforts are needed to create systems that utilize the best combination of optics and electronics to enable integrated systems for efficient information processing.

In the practical connection of information, optics has increasingly taken over the role of data communications within local networks for information processing systems such as data centers, supercomputers, and storage area networks, for

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10 Miller, D.A.B. 2009. Device requirements for optical interconnects to silicon chips. Proceedings of the IEEE 97:1166-1185.

11 Miller, D.A.B. 2010. Are optical transistors the next logical step? Nature Photonics 4:3-5.

12 Nielsen, M.A., and I.L. Chuang. 2000. Quantum Computation and Quantum Information. Cambridge, U.K.: Cambridge University Press.

Suggested Citation:"3 Communications, Information Processing, and Data Storage." National Research Council. 2013. Optics and Photonics: Essential Technologies for Our Nation. Washington, DC: The National Academies Press. doi: 10.17226/13491.
×

distances from approximately 100 meters to 10s of kilometers. Commercial approaches include optics in networking architectures such as Ethernet, Fiber Channel, and Infiniband, especially as data rates and data densities rise, and in active optical cables that are internally optical but externally have electrical inputs and outputs. Low cost is particularly critical for shorter distances. Consequently, such data communications have often used technologies different from the long-haul telecommunications approaches, such as vertical cavity surface-emitting lasers and laser arrays, and multi-mode (rather than single-mode) optical fibers for the shorter distances. The use of optics in such networks is likely to steadily increase because of continuing increases in network traffic and information processing.

As can be seen in Figure 3.1, the ability to compute information, as indicated by the growth of the computational power (expressed in millions of instructions per second [MIPS] that can be executed by the general-purpose silicon chips that are sold),13 has approximately the same growth rate as that of Internet traffic. The ability to connect information within information processing systems is, however, hitting increasingly severe limits. The ability of wires to interconnect does not scale well to higher densities, especially for information communications outside the silicon chips themselves (“off-chip” communications). This point is well understood by the semiconductor industry, as shown, for example, in its projections of an off-chip interconnect bottleneck.14 The interconnect capability of electrical wires between chips, or processor cores, is not going to scale to keep up with the ability to compute—sometimes called the bytes per flops (floating point operations per second) gap.15 Thus, although Internet bandwidth and raw computational power in MIPS or flops might continue to scale, the systems performance in between will not, unless a new interconnect technology is introduced that can scale in capacity.

High-performance silicon chips today, as characterized, for example, by the International Technology Roadmap for Semiconductors (ITRS) numbers,16 already have interconnect capabilities well into the range of multiple terabits per second per chip. To scale to keep up with processing power, future chips in the 2020 time frame would need 100s of terabits per second of interconnect capability, an amount of interconnect for one chip that is comparable to the entire Internet traffic today. It is worth noting that generally there is likely much more information sent inside information processing systems than is sent between them. One order-of-magnitude

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13 Hilbert M., and P. Lopez. 2011. The world’s technological capacity to store, communicate, and compute information. Science 332(6025):60-65.

14 Miller, D.A.B. 2009. Device requirements for optical interconnects to silicon chips.

15 Miller, D.A.B. 2009. Device requirements for optical interconnects to silicon chips.

16 More information is available through the International Technology Roadmap for Semiconductors at http://www.itrs.net/Links/2010ITRS/Home2010.htm. Accessed June 26, 2012.

Suggested Citation:"3 Communications, Information Processing, and Data Storage." National Research Council. 2013. Optics and Photonics: Essential Technologies for Our Nation. Washington, DC: The National Academies Press. doi: 10.17226/13491.
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estimate17 is that each byte traversing the Internet causes approximately 1 million bytes of data communications within data centers.

The large and increasing interconnected bandwidth inside information processing systems creates problems both for density of interconnects and for power sourcing and dissipation. Interconnecting one bit of information off a chip currently involves several to 10s of picojoules (pJ) of energy. A substantial portion (e.g., 50 to 80 percent) of all power dissipation on silicon chips is used to interconnect the information rather than to perform logic.18 Power dissipation in general is a severe limitation on information processing; for example, the clock rate on mainstream silicon chips has not changed substantially in recent years because higher clock rates would lead to too much power dissipation. A recent NRC study19 on supercomputing also emphasizes this problem with power dissipation and the need, in particular, to reduce interconnection power while maintaining or even increasing the byte-per-flops ratio.

Power dissipation in information processing is already environmentally significant. Electricity used in data centers in 2010 likely accounted for between 1.1 and 1.5 percent of total electricity use globally, and between 1.7 and 2.2 percent of electricity use in the United States.20 The power dissipation in interconnects in servers alone has been estimated to exceed solar power generation capacity.21 Of course, information processing and communication can have substantial environmental benefits in improving the efficiency of many activities and in reducing travel.22 But, if current growth trends in Internet traffic and in information processing continue, reducing the energy per bit processed and/or communicated will be crucial. Even though sending a bit over the Internet might take approximately 10 nanojoules (nJ)/bit23 compared to 10s of pJ/bit for off-chip interconnects, the vastly greater number of bits being sent inside the machine (approximately 106

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17 G. Astfalk. 2009. Why optical data communications and why now? Applied Physics A: Materials Science and Processing 95(4): 933-940.

18 Miller, D.A.B. 2009. Device requirements for optical interconnects to silicon chips.

19 National Research Council. 2010. The Future of Computing Performance: Game Over or Next Level? Washington, D.C.: The National Academies Press.

20 Koomey, Jonathan. 2011. Growth in Data Center Electricity Use 2005 to 2010. Oakland, Calif.: Analytics Press. Available at http://www.analyticspress.com/datacenters.xhtml. Accessed October 27, 2011.

21 Miller, D.A.B. 2009. Device requirements for optical interconnects to silicon chips.

22 The Climate Group. 2008. SMART 2020: Enabling the Low Carbon Economy in the Information Age. A report by the Climate Group on behalf of the Global eSustainability Initiative. Available at http://www.smart2020.org/_assets/files/02_Smart2020Report.pdf. Accessed June 26, 2012.

23 Tucker, R.S., R. Parthiban, J. Baliga, K. Hinton, R.W.A. Ayre, and W.V. Sorin. 2009. Evolution of WDM optical IP networks: A cost and energy perspective. Journal of Lightwave Technology 27:243-252.

Suggested Citation:"3 Communications, Information Processing, and Data Storage." National Research Council. 2013. Optics and Photonics: Essential Technologies for Our Nation. Washington, DC: The National Academies Press. doi: 10.17226/13491.
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times24) means that the power dissipation inside information processing machines may substantially dominate over telecommunications power. Hence reducing interconnect power inside machines is crucial for controlling power dissipation in information processing.

The research understanding of the reasons for, and the requirements on, optics and optoelectronics for such shorter connections is now relatively complete and consistent.25,26 Optics helps in two ways. First, optics can carry information at much higher density than is possible in electrical wires, as is essential for future scaling of interconnect capacity. Second, optics can fundamentally save energy in interconnects because it completely avoids the need for charging wires (the charging of electrical wires to the signal voltage is the dominant source of energy dissipation in electrical interconnects).

The key challenges in short-distance, high-density, low-energy optical interconnect are technological: optoelectronic devices must operate with very low energies—for example, 10 femtojoules (fJ)/bit—much lower than have been required for telecommunications and data communications until now. For cost and performance reasons, the optical and optoelectronic technologies have to be integrated with electronics in a mass-manufacturable process.

New technologies have been advancing in research and early product introductions over the last decade, especially in terms of devising ways of combining optics, optoelectronics, and electronics in silicon chips and platforms (silicon photonics).27,28 These technological opportunities are discussed below.

Data Storage

The past decade has seen the maturation of technologies such as the DVD (digital versatile disk) for the distribution of consumer video, and the emergence of third-generation disk technologies such as Blu-ray for higher density. Research continues on possible fourth-generation optical disks, including multi-layer and possible holographic recording. Optics may also have crucial roles to play in other potential emerging technologies such as HAMR (heat-assisted magnetic

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24 Astfalk, G. 2009. Why optical data communications and why now? Applied Physics A: Materials Science and Processing 95:933-940.

25 Miller, D.A.B. 2009. Device requirements for optical interconnects to silicon chips.

26 Astfalk, G. 2009. Why optical data communications and why now? Applied Physics A: Materials Science and Processing 95:933-940.

27 Lipson, M. 2005. Guiding, modulating, and emitting light on silicon—Challenges and opportunities. Journal of Lightwave Technolology 23(12):4222-4238.

28 Jalali, B., and S. Fathpour. 2006. Silicon photonics. Journal of Lightwave Technolology 24(12):4600-4615.

Suggested Citation:"3 Communications, Information Processing, and Data Storage." National Research Council. 2013. Optics and Photonics: Essential Technologies for Our Nation. Washington, DC: The National Academies Press. doi: 10.17226/13491.
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recording)29 for next-generation magnetic hard drives, for which near-field optics using nanophotonic technologies may provide the precise heating required.

The picture for the future of optical disks like DVDs is less clear because of the competition from other forms of storage, such as flash memory (memory sticks), and from networks. Whether optical disks will have the role in the distribution of data and entertainment, such as movies and games, which they have had in the past is debatable because of the growth of broadband networks as an alternative distribution channel—as, for example, in the growing use of video on demand over the Internet.

IMPACT EXAMPLE: THE INTERNET

As is now readily self-evident, the Internet has transformed the way in which society operates. Previous conceptualizations of distance and geography have disappeared in a maze of ubiquitous cellular telephones, e-mail, web browsing, and social networking. Indeed, the productivity of people coming of age after the mid-1990s is intertwined with the easily available communication and information found through the Internet.

Initially the Internet was not reliant on optical communications. However, data transfer rates during those formative times were painfully slow. Without optical technologies, conventional methods were utilized: users depended on slow, dial-up modems, e-mails occasionally took hours before arriving at their destination, and making intercontinental phone calls involved an annoying delay between speaking and listening.

Optics changed the Internet and data transfer by increasing the capacity of the system by nearly 10,000-fold over the past two decades. The nearly instantaneous nature of current optical-based communication allows for real-time video chats. Telepresence, telemedicine, and tele-education would not be possible without optics. In short, without optics, the Internet as we know it would not exist. As it was stated in 2006 by the NRC Committee on Telecommunications Research and Development: “Telecommunications has expanded greatly over the past few decades from primarily landline telephone service to the use of fiber optic, cable, and wireless connections offering a wide range of voice, image, video, and data services.”30

The same report continues:

Yet it is not a mature industry, and major innovation and change—driven by research—can be expected for many years to come.

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29 Kryder, M.H., E.C. Gage, T.W. McDaniel, W.A. Challener, R.E. Rottmayer, G. Ju, Y.-T. Hsia, and M.R. Erden. 2008. Heat assisted magnetic recording. Proceedings of the IEEE 96:1810-1835.

30 National Research Council. 2006. Renewing U.S. Telecommunications Research. Washington, D.C.: The National Academies Press, pp. 1-20.

Suggested Citation:"3 Communications, Information Processing, and Data Storage." National Research Council. 2013. Optics and Photonics: Essential Technologies for Our Nation. Washington, DC: The National Academies Press. doi: 10.17226/13491.
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    Without an expanded investment in research, however, the nation’s position as a leader is at risk. Strong competition is emerging from Asian and European countries that are making substantial investments in telecommunications R&D.

    For many telecommunications products and services that are now commodities, the United States is at a competitive disadvantage compared with countries where the cost of doing business is lower. Continued U.S. strength in telecommunications, therefore, will require a focus on high-value innovation that is made possible only by a greater emphasis on research. Expansion of telecommunications research is also necessary to attract, train, and retain research talent.

    Telecommunications research has yielded major benefits such as the Internet, radio frequency wireless communications, optical networks, and voice over Internet Protocol. Promising opportunities for future research include enhanced Internet architectures, more trustworthy networks, and adaptive and cognitive wireless networks.31

The 2002 NRC study Atoms, Molecules, and Light: AMO Science Enabling the Future stated:

Internet optical backbone link capacity increased a hundredfold between 1995 and 1998, to 20,000 trillion bits per second. While the communications industry has recently suffered a downturn, the potential for increasing demand for capacity in the 21st century remains, with features and services such as HDTV, broadband communications, and advanced home security systems becoming more available through the use of optical fiber, wireless, satellite, and cable connections.32

The use of optics, however, is not always obvious to the casual observer. Often someone can be seen using a cell phone to perform an Internet search. Where is the optics? The cell phone uses a wireless radio connection to a local cell tower, but that radio signal is converted to an optical data stream and sent along the fiber optic network across the planet. The data search itself, such as through Google, relies on data centers in which clusters of co-located computers talk to each other through high-capacity optical cables. In fact, there can be as many as a million lasers in a given data center.

During the introduction to the 2009 Nobel Prize Lecture by Dr. Charles Kao on the innovation of optical fiber communications, the Physics Committee Chair said: “The work has fundamentally transformed the way we live our daily lives.”33

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31 National Research Council. 2006. Renewing U.S. Telecommunications Research. pp. 1-20.

32 National Research Council. 2002. Atoms, Molecules, and Light: AMO Science Enabling the Future. Washington, D.C.: The National Academies Press, pp. 7-8.

33 More information on the Nobel Prize is available at http://www.nobelprize.org/nobel_prizes/physics/laureates/2009/. Accessed June 26, 2012.

Suggested Citation:"3 Communications, Information Processing, and Data Storage." National Research Council. 2013. Optics and Photonics: Essential Technologies for Our Nation. Washington, DC: The National Academies Press. doi: 10.17226/13491.
×

TECHNICAL ADVANCES

Communications

The past 20 years have seen an explosion in technical achievements in the field of optical communications. Whereas an undersea optical cable was deployed in 1988 and could accommodate 280 Mb/s,34 there exist today commercial systems that can carry 5 Tb/s per fiber strand. This represents an astonishing increase of four orders of magnitude in performance in commercial systems, and R&D laboratories can transmit more than an order of magnitude higher than that.35 This technological improvement was driven primarily by the exponential growth in capacity demand, primarily due to Internet usage. Looking to the future, it is expected, as stated above, that the network traffic demand will grow by a factor of 100 over the next decade,36 necessitating technology advances that can keep pace. However, the research community is struggling to envision technical paths for achieving this vision.

In the mid-1990s, leading laboratories achieved the transmission of 1 Tb/s and 0.1 Tb/s respectively over hundreds and thousands of kilometers of single-mode optical fiber.37 The key advances that enabled these results and that were captured by the NRC’s 1998 Harnessing Light38 study include those described below.

Wavelength Division Multiplexing (WDM)

By simultaneously transmitting different channels of data on different wavelengths by means of WDM, the system capacity can be dramatically increased. Moreover, wavelength-selective components enable highly efficient network routing, such that specific wavelength channels can be added or dropped at intermediate nodes without disrupting the other optical channels.39

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34 Runge, P.K. 1990. Undersea lightwave systems. Optics and Photonics News 1(11):9-12.

35 Zhu, B., T.F. Taunay, M. Fishteyn, X. Liu, S. Chandrasekhar, M.F. Yan, J.M. Fini, E.M. Monberg, and F.V. Dimarcello. 2011. 112-Tb/s space-division multiplexed DWDM transmission with 14-b/s/hz aggregate spectral efficiency over a 76.8-km seven-core fiber. Optics Express 19(17):16665-16671.

36 Winzer, P.J. 2010. Beyond 100G Ethernet. IEEE Communications Magazine 48(7):26-29.

37 Kerfoot, F.W., and W.C. Marra. 1998. Undersea fiber optic networks: Past, present, and future. Journal on Selected Areas in Communications 16:1220-1225.

38 National Research Council. 1998. Harnessing Light.

39 Kaminow, I.P., C.R. Doerr, C. Dragone, T. Koch, U. Koren, A.A.M. Saleh, A.J. Kirby, C.M. Ozveren, B. Schofield, R.E. Thomas, R.A. Barry, D.M. Castagnozzi, V.W.S. Chan, B.R. Hemenway, D. Marquis, S.A. Parikh, M.L. Stevens, E.A. Swanson, S.G. Finn, and R.G. Gallager. 1996. A wideband all-optical WDM network. Journal on Selected Areas in Communications 14:780-799.

Suggested Citation:"3 Communications, Information Processing, and Data Storage." National Research Council. 2013. Optics and Photonics: Essential Technologies for Our Nation. Washington, DC: The National Academies Press. doi: 10.17226/13491.
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Erbium-Doped Fiber Amplifiers

Broadband (>3 THz), low-noise optical amplifiers—erbium-doped fiber amplifiers (EDFAs)—enabled the amplifying of many channels simultaneously and the cascading of numerous stages, thereby making long-distance WDM transmission practical.40

Dispersion Management

Chromatic dispersion and nonlinearities are fiber-based effects that significantly degrade transmitted data. However, by judiciously alternating between positive and negative dispersion element, the accumulated dispersion can be compensated, and the phase-matching needed for generating inter-channel nonlinear effects can be minimized.41

New Engineering Approaches Needed

Similar to the experience in other areas of technology, new engineering approaches are required in order to achieve new eras of technical advancement in data transmission. Some of the key directions over the past decade42 that have enabled the continued achievement include approaches borrowed from the radio community, although these approaches were not possible until recent device performance increases. They are described below.

Higher-Order Modulation Formats. Until recently, deployed systems employed simple, amplitude on/off-keyed digital data encoding. However, phase-shift keying (PSK) of the optical carrier wave produces a better receiver sensitivity and is relatively tolerant to degrading effects such as nonlinearities.43 The fiber bandwidth is precious because the available spectrum is rapidly filling up, and so spectral efficiency, measured in bits per second per hertz, becomes more critical; as another benefit, spectrally narrow channels are more robust to dispersion effects. A typical example is quadrature-phase-shift-keying (QPSK), which encodes two bits of information on each symbol, thereby doubling the spectral efficiency. This can be

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40 Li, T.Y. 1993. The impact of optical amplifiers on long-distance lightwave telecommunications. Proceedings of the IEEE 81:1568-1579.

41 Kurtzke, C. 1993. Suppression of fiber nonlinearities by appropriate dispersion management. IEEE Photonics Technology Letters 5:1250-1253.

42 Kaminow, I.P., Tingye Li, and A.E. Willner. 2008. Optical Fiber Telecommunications V B: Systems and Networks. New York, N.Y.: Academic Press.

43 Winzer, P. J., and R.-J. Essiambre. 2007. Advanced modulation formats for high-capacity optical transport networks. Journal of Lightwave Technology 24(12):4711-4720.

Suggested Citation:"3 Communications, Information Processing, and Data Storage." National Research Council. 2013. Optics and Photonics: Essential Technologies for Our Nation. Washington, DC: The National Academies Press. doi: 10.17226/13491.
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further extended to even higher I/Q (in-phase/quadrature) data constellations—for example, quadrature-amplitude modulation (QAM) of 16 to 512,44,45,46 as seen in Figure 3.2, in which data are encoded on both phase and amplitude of the optical carrier. Additionally, independent data can be transmitted along the two orthogonal polarization axes of the optical fiber, that is, polarization-division-multiplexing, doubling again the capacity and spectral efficiency.47

Coherent Systems Utilizing Digital Signal Processing. In heterodyne receivers, ubiquitous in the radio world, a weak data signal is mixed with a local oscillator that has much higher power. Following the same approach, an optical coherent system utilizes a narrow line-width laser as the local oscillator to mix with a weak incoming optical data signal. The balanced detectors can be used to recover both amplitude and the phase information, that is, the time history of the data channel’s wave. Coherent systems (1) exhibit better receiver sensitivity than direct detection and (2) can utilize sophisticated electronic digital-signal-processing (DSP) techniques to equalize and compensate for many impairments, such as chromatic- and polarization-based degradations.48,49,50

Concepts That Might Improve Capacity

Recently, due to technology advances, two other concepts have emerged that might further improve capacity for the next several years, as discussed below.

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44 Zhou, X., and J. Yu. 2009. Multi-level, multi-dimensional coding for high-speed and high-spectral-efficiency optical transmission. Journal of Lightwave Technology 27(16):3641-3653.

45 Schmogrow, R., D. Hillerkuss, S. Wolf, B. Bäuerle, M. Winter, P. Kleinow, B. Nebendahl, T. Dippon, P.C. Schindler, C. Koos, W. Freude, and J. Leuthold. 2012. 512QAM Nyquist sinc-pulse transmission at 54 Gbit/s in an optical bandwidth of 3 GHz. Optics Express 20:6439-6447.

46 Okamoto, S., K. Toyoda, T. Omiya, K. Kasai, M. Yoshida, and M. Nakazawa. 2010. “512 QAM (54 Gbit/s) Coherent Optical Transmission over 150 km with an Optical Bandwidth of 4.1 GHz.” Conference paper. 36th European Conference and Exhibition on Optical Communication, Turin, Italy.

47 Gnauck, A.H., P.J. Winzer, A. Konczykowska, F. Jorge, J.Y. Dupuy, M. Riet, G. Charlet, B. Zhu, and D.W. Peckham. 2012. Generation and transmission of 21.4-Gbaud PDM 64-QAM using a novel high-power DAC driving a single I/Q modulator. Journal of Lightwave Technology 30:532-536.

48 Taylor, M.G. 2004. Coherent detection method using DSP for demodulation of signal and subsequent equalization of propagation impairments. IEEE Photonics Technology Letters 16(2):674-676.

49 Savory, S.J. 2010. Digital coherent optical receiver: Algorithms and subsystems. Journal of Selected Topics in Quantum Electronics 16(5):1164-1179.

50 Salsi, M., O. Bertran Pardo, J. Renaudier, W. Idler, H. Mardoyan, P. Tran, G. Charlet, and S. Bigo. 2011. “WDM 200 Gb/s Single-Carrier PDM-QPSK Transmission Over 12,000 km.” Conference paper. 37th European Conference and Exhibition on Optical Communication, Geneva, Switzerland. Available at http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=6065939. Accessed June 26, 2012.

Suggested Citation:"3 Communications, Information Processing, and Data Storage." National Research Council. 2013. Optics and Photonics: Essential Technologies for Our Nation. Washington, DC: The National Academies Press. doi: 10.17226/13491.
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image

FIGURE 3.2 Recorded experimental data constellation of optical 1024-quadrature-amplitude-modulation (QAM) signal (60 Gb/s) using coherent optical transmission over 150 km. The two axes are for in-phase and quadrature directions, with the total constellation representing data points encoded in amplitude and phase without (a) and with (b) digital nonlinear compensation. SOURCE: Reprinted, with permission, from Koizumi Y., K. Toyoda, M. Yoshida, and M. Nakazawa. 2012. 1024 QAM (60 Gb/s) single-carrier coherent optical transmission over 150 km. Optics Express 20(11):12508-12514.

Space Division Multiplexing (SDM). After time, wavelength, and polarization multiplexing, space multiplexing is now being investigated to further increase the transmission capacity. In a space-division-multiplexing system, each independent data channel is carried by an orthogonal spatial dimension. Two emerging approaches include using the following: (1) a special multi-core fiber, each individual core of which transmits an independent data stream;51 and (2) a few-mode fiber, with each independent data channel on one of the orthogonal spatial modes (see Figure 3.3). In both approaches, the key challenge is the crosstalk. Unique challenges for multi-core fiber systems include the following: (1) further increasing the number of cores, (2) decreasing the inter-core nonlinear effects, and (3) developing multi-mode/multi-core network elements, such as an erbium-doped fiber that has matched multi-cores, so that all cores can be amplified simultaneously in a single fiber element. For multi-mode systems, unwanted mode conversions among different spatial modes are a natural occurrence which leads to several-channel crosstalk.

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51 The Climate Group. 2008. SMART 2020: Enabling the Low Carbon Economy in the Information Age. A report by the Climate Group on behalf of the Global eSustainability Initiative. Available at http://www.smart2020.org/_assets/files/02_Smart2020Report.pdf. Accessed June 26, 2012.

Suggested Citation:"3 Communications, Information Processing, and Data Storage." National Research Council. 2013. Optics and Photonics: Essential Technologies for Our Nation. Washington, DC: The National Academies Press. doi: 10.17226/13491.
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image

image

FIGURE 3.3 (a) Space division multiplexing (SDM) using multi-core fiber. SOURCE: Reprinted, with permission, from Matsuo, S., K. Takenaga, Y. Arakawa, Y. Sasaki, S. Tanigawa, K. Saitoh, and M. Koshiba. 2011. Crosstalk behavior of cores in multi-core fiber under bent condition. IEICE Electronics Express 8(6):385-390. (b) Multi-mode fiber. SOURCE: Reprinted, with permission, from Ryf, R., S. Randel, A. Gnauck, C. Bolle, A. Sierra, S. Mumtaz, M. Esmaeelpour, E. Burrows, R. Essiambre, P. Winzer, D. Peckham, A. McCurdy, and R. Lingle. 2012. Mode-division multiplexing over 96 km of few-mode fiber using coherent 6×6 MIMO processing. IEEE Journal of Lightweight Technology 30:521-531. (c) Bit error rate performance and recovered data constellations of the quadrature-phase-shift-keying (QPSK) signal tributaries after SDM transmission using four spatial modes in a single few-mode fiber. SOURCE: Reprinted, with permission, from Al Amin, Abdullah, An Li, Simin Chen, Xi Chen, Guanjun Gao, and William Shieh. 2011. Dual-LP11 mode 4×4 MIMO-OFDM transmission over a two-mode fiber. Optics Express 19:16672-16679.

Suggested Citation:"3 Communications, Information Processing, and Data Storage." National Research Council. 2013. Optics and Photonics: Essential Technologies for Our Nation. Washington, DC: The National Academies Press. doi: 10.17226/13491.
×

This problem can be partially solved by utilizing a multiple-input, multiple-output (MIMO) approach. MIMO is a popular technique in radio-frequency (RF) systems and can untangle some of the crosstalk between modes using digital signal processing.52,53 However, it is unclear as to the upper limit of performance and number of modes that can be supported.

Orthogonal Frequency Division Multiplexing (OFDM). OFDM uses a single carrier wave and multiple orthogonal subcarrier waves; it has produced some of the most exciting results in the RF world. Each subcarrier that carries independent data can be densely packed in the spectrum, since orthogonality is maintained by advanced electronic processing in the transmitter/receiver.54 OFDM is spectrally efficient and is tolerant to many fiber-based impairments, but it does require a fair amount of high-speed electronic data signal processing, such as fast Fourier transforms and analog-digital converters.55

Device and Subsystem Achievements

These recent systems-level advances have been enabled by device and subsystem achievements, which include those described below.

High-Speed Local and Access Area Networks. The advances of local area networks (LANs) made it possible to deliver the high bandwidth from long-haul optical networks to desktops. Started with light-emitting diodes (LEDs) and Fabry-Perot lasers as transmitters, multi-mode optical fibers and fiber ribbon cables became major LAN standards in the mid-1990s. In the late 1990s, direct modulated gigabits

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52 Kaminow et al. 1996. A wideband all-optical WDM network.

53 Ryf, R., A. Sierra, R. Essiambre, A. Gnauck, S. Randel, M. Esmaeelpour, S. Mumtaz, P.J. Winzer, R. Delbue, P. Pupalaikis, A. Sureka, T. Hayashi, T. Taru, and T. Sasaki. 2011. “Coherent 1200-km 6 × 6 MIMO Mode-Multiplexed Transmission over 3-CoreMicrostructured Fiber.” Conference paper. European Conference and Exposition on Optical Communications. Available at http://www.opticsinfobase.org/abstract.cfm?URI=ECOC-2011-Th.13.C.1. Accessed June 26, 2012.

54 Shieh, W., and I. Djordjevic. 2009. Orthogonal Frequency Division Multiplexing for Optical Communications. New York, N.Y.: Elsevier/Academic Press.

55 Liu, X., S. Chandrasekhar, B. Zhu, P.J. Winzer, A.H. Gnauck, and D.W. Peckham. 2011. 448-Gb/s reduced-guard-interval CO-OFDM transmission over 2000 km of ultra-large-area fiber and five 80-GHz-Grid ROADMs. IEEE Journal of Lightwave Technology 29:483-490.

Suggested Citation:"3 Communications, Information Processing, and Data Storage." National Research Council. 2013. Optics and Photonics: Essential Technologies for Our Nation. Washington, DC: The National Academies Press. doi: 10.17226/13491.
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per second vertical cavity surface emitting laser (VCSEL)56,57,58,59,60 transmitters became the dominant transmitters due to the low-drive voltage and low manufacturing cost. At present, 10 Gb/s VCSELs are being deployed and 40 Gb/s have been demonstrated in laboratories.61,62 Research efforts toward wavelength-tunable and 100 Gb/s direct modulated lasers will be important for next-generation broadband communications.63,64

High-Speed Electronic Circuits. Electronic circuits are an “optical system’s best friend.” High-speed, high-output-power and linear electronics are clearly needed to drive the optical modulators, and high-speed logic circuits are crucial to achieving the signal processing in coherent, MIMO, and OFDM systems.65,66 Good electronics can help mitigate many of the problems produced in the optical domain, thereby enabling better system performance.

Photonic Integration. In general, a system with better performance requires more components, which might make the system even more complex. For example, the transceiver in a system using higher-order modulation formats is much more complicated

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56 Soda, H., K. Iga, C. Kitahara, and Y. Suematsu. 1979. GaInAsP/InP surface emitting injection lasers. Japanese Journal of Applied Physics 18:2329-2330.

57 Watanabe, I., F. Koyama, and K. Iga. 1986. Low temperature CW operation of GaInAsP/InP surface emitting laser with circular buried heterostructure. Electronics Letters 22:1325-1327.

58 Jewell, J.L., S.L. McCall, Y.H. Lee, A. Scherer, A.C. Gossard, and J.H. English. 1989. Lasing characteristics of GaAs microresonators. Applied Physics Letters 54:1400-1402.

59 Chang-Hasnain, C.J., J.P. Harbison, G. Hasnain, A. Von Lehmen, L.T. Florez, and N.G. Stoffel. 1991. Dynamic, polarization, and transverse mode characteristics of vertical cavity surface emitting lasers. IEEE Journal of Quantum Electronics 27:1402-1409.

60 Maeda, M.W., C.J. Chang-Hasnain, J.S. Patel, C. Lin, H.A. Johnson, and J.A. Walker. 1991. Use of a multi-wavelength surface-emitting laser array in a 4-channel wavelength-division-multiplexed system experiment. IEEE Photonics Technology Letters 3:268-269.

61 Müller, M., W. Hofmann, A. Nadtochiy, A. Mutig, G. Böhm, M. Ortsiefer, D. Bimberg, and M.-C. Amann. 2010. B1.55 m high-speed VCSELs enabling error-free fiber-transmission up to 25 Gbit/s. Proceedings of International Semiconductor Laser Conference (ISLC), September 26-30, Kyoto, Japan, pp. 156-157.

62 Westbergh, P., J.S. Gustavsson, B. Kögel, A. Haglund, and A. Larsson. 2011. Impact of photon lifetime on high-speed VCSEL performance. IEEE Journal of Selected Topics in Quantum Electronics 17(6):1603-1613.

63 Chang-Hasnain, C.J. 2000. Tunable VCSEL. IEEE Journal of Selected Topics in Quantum Electronics 6(6):978-987.

64 Taubenblatt, M.A. 2012. Optical interconnects for high-performance computing. Journal of Lightwave Technology 30(4):448-457.

65 Li, T.Y. 1993. The impact of optical amplifiers on long-distance lightwave telecommunications. Proceedings of the IEEE 81:1568-1579.

66 Kurtzke, C. 1993. Suppression of fiber nonlinearities by appropriate dispersion management. IEEE Photonics Technology Letters 5:1250-1253.

Suggested Citation:"3 Communications, Information Processing, and Data Storage." National Research Council. 2013. Optics and Photonics: Essential Technologies for Our Nation. Washington, DC: The National Academies Press. doi: 10.17226/13491.
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than an on-off keying (OOK) system,67 because it requires multiple lasers, modulators, couplers, and balanced detectors, as seen in Figure 3.4. This scenario has benefited greatly from advances in photonic integrated circuits on both III-V68 and silicon materials.69

Optical Fiber Modifications. Although the attenuation of optical fiber reached its near-optimum point of 0.2 decibels per kilometer (dB/km) decades ago, modifications to the fiber itself have produced critical changes to the optical wave propagation properties that enabled systems advances. By varying the core and cladding size and composition, key changes in the amount and spectral slope of chromatic dispersion have managed the accumulation of dispersion and nonlinear effects.70 Moreover, large-effective-area fiber has reduced the power density, thereby reducing the build-up of nonlinear effects.71 Furthermore, creating nanostructures in the cladding of the fiber has enabled the fiber to be very tightly wound without incurring bending losses, thus enabling ubiquitous deployment of fiber without the need for highly skilled technicians.

Potential Innovation. Device and subsystem innovations that herald the next wave of communication systems could include those described below.

Silicon photonics. At the time of the publication of Harnessing Light in 1998,72 the community did not pay much attention to silicon as a photonics material. In terms of performance and the ability to interact with light, III-V materials were far superior in performance. However, significant advances in the past few years in creating silicon-based photonic elements have excited the industry. Modulators, waveguides, couplers, detectors, multiplexers and filters have all been demonstrated.73 (See Figure 3.5.)

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67 OOK is the simplest form of amplitude-shift keying (ASK) modulation, in which digital data are represented by the presence or absence of a carrier wave.

68 Welch, D.F., F.A. Kish, R. Nagarajan, C.H. Joyner, R.P. Schneider, V.G. Dominic, M.L. Mitchell, S.G. Grubb, T.-K. Chiang, D. Perkins, and A.C. Nilsson. 2006. The realization of large-scale photonic integrated circuits and the associated impact on fiber-optic communication systems. Journal of Lightwave Technology 24:4674-4683.

69 Asghari, M., and A.V. Krishnamoorthy. 2011. Silicon photonics: Energy-efficient communication. Nature Photonics 5:268-270.

70 Miller, D.A.B. 2009. Device requirements for optical interconnects to silicon chips. Proceedings of the IEEE 97:1166-1185.

71 Lewotsky K. 1996. Large-effective-area fiber minimizes nonlinearities. Laser Focus World 32:16.

72 National Research Council. 1998. Harnessing Light.

73 Xu, Q., B. Schmidt, S. Pradhan, and M. Lipson. 2005. Micrometre-scale silicon electro-optic modulator. Nature 435:325-327.

Suggested Citation:"3 Communications, Information Processing, and Data Storage." National Research Council. 2013. Optics and Photonics: Essential Technologies for Our Nation. Washington, DC: The National Academies Press. doi: 10.17226/13491.
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image

FIGURE 3.4 (a) 28-Gbaud InP square or hexagonal 16-QAM modulator. SOURCE: Reprinted, with permission, from Doerr, C.R., L. Zhang, P. Winzer, and A.H. Gnauck. 2011. “28-Gbaud InP Square or Hexagonal 16-QAM Modulator.” Conference paper. Optical Fiber Communication Conference (OFC), Los Angeles, Calif., March 6, 2011. Available at http://www.opticsinfobase.org/abstract.cfm?URI=OFC-2011-OMU2. (b) Monolithically integrated InP dual-port coherent receiver for 100-Gb/s PDM-QPSK signal. SOURCE: Reprinted, with permission, from Houtsma, V.E., N. Weimann, T. Hu, R. Kopf, A. Tate, J. Frackoviak, R. Reyes, Y. Chen, C.R. Doerr, L. Zhang, and D. Neilson. 2011. “Manufacturable Monolithically Integrated InP Dual-Port Coherent Receiver for 100G PDM-QPSK Applications.” Conference paper. Optical Fiber Communication Conference (OFC), Los Angeles, Calif., March 6, 2011. Available at http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=5875296.

   Even active elements such as lasers have been reported by wafer bonding74 or direct growth.75,76 This makes it possible to mass-produce cost-effective, chip-scale integrated photonic circuits by using the massive silicon manufacturing infrastructure. An important application is in the mass production of active cables, in which silicon photonics and fiber take the place of coaxial cable to provide tens of gigahertz of bandwidth with extremely low loss over 10s of meters.

Low power consumption. As data communications increase at an exponential rate, the power consumption by the communication infrastructure is growing rapidly. Moreover, it has been well known that although electronic power consumption scales with increased data rate, the power consumption of photonics does not. In view of both of these facts, the optical communications community is taking very seriously the mantle of using optics in novel ways to dramatically reduce the power consumption. This will require new integrated device technologies, such as photonic-crystal

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74 Liang, D., and J.E. Bowers. 2010. Recent progress in lasers on silicon. Nature Photonics 4:511-517.

75 Kunert, B., S. Zinnkann, K. Volz, and W. Stolz. 2008. Monolithic integration of Ga(NAsP)/(BGa) P multi-quantum well structures on (0 0 1) silicon substrate by MOVPE. Journal of Crystal Growth 310(23):4776-4779.

76 Chen et al. 2011. Nanolasers grown on silicon.

Suggested Citation:"3 Communications, Information Processing, and Data Storage." National Research Council. 2013. Optics and Photonics: Essential Technologies for Our Nation. Washington, DC: The National Academies Press. doi: 10.17226/13491.
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image

image

FIGURE 3.5 (a) Intel creates the world’s first end-to-end silicon photonics connection with integrated lasers. SOURCE: Image available at http://www.demonstech.com/2010/07/intel-creates-worlds-first-end-to-end.xhtml. (b) Kotura’s chip-scale 10×10 Gb/s integrated silicon transmitter. SOURCE: Image available at http://www.gazettabyte.com/home/2009/10/26/photonic-integration-bent-on-disruption.xhtml. Reprinted with permission.

Suggested Citation:"3 Communications, Information Processing, and Data Storage." National Research Council. 2013. Optics and Photonics: Essential Technologies for Our Nation. Washington, DC: The National Academies Press. doi: 10.17226/13491.
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   structures, as well as novel system and switching architectures to create a “green” network.77,78

Reconfigurable network elements. Electronic systems are not static, and yet optical systems tend to be deployed in a fairly static fashion. Tunable and reconfigurable network elements will provide needed flexibility to optical systems, such that the network can optimize its bandwidth allocation and quality of service in a robust fashion.79,80 Filters, (de)multiplexers, routers, switches, dispersion compensators, and lasers all fall into this category.81

Networking

The topics discussed above deal primarily with the core issues of increasing the capacity and distance performance of optical communications systems—the solutions to this need has fueled the dramatic momentum of worldwide communications. However, the power of optics has also been unleashed to enable dramatic advances in multi-user networking. Not only do parallel wavelengths readily provide capacity enhancement, they also enable wavelength-dependent routing.82 In a WDM network, the wavelength can define the network routing such that a specific wavelength channel can appear at its destination by traversing wavelength-dependent network elements. A network node can detect only the specific wavelength and traffic meant for that node and pass the other wavelengths not meant for that destination without disturbance. Key benefits include these: (1) lower network delay and latency for non-dropped traffic, and (2) lower “cost” to detect only the dropped traffic rather than all the traffic entering the node.

A key element in this network revolution is the wavelength-selective optical add/drop multiplexer (OADM). Moreover, the OADM has evolved to become dynamic,

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77 Nishimura, S., K. Shinoda, Y. Lee. G. Ono, K. Fukuda, F. Yuki, T. Takemoto, H. Toyoda, M. Yamada, S. Tsuji, and N. Ikeda. 2011. Components and interconnection technologies for photonic-assisted routers toward green networks. IEEE Journal of Lightwave Technology 17:347-356.

78 Kilper, D. 2011. “Tutorial: Energy Efficient Networks.” Conference paper. Optical Fiber Communication Conference (OFC), Los Angeles, Calif., March 6. Available at http://www.opticsinfobase.org/abstract.cfm?uri=OFC-2011-OWI5. Accessed June 26, 2012.

79 Gringeri, S., B. Basch, V. Shukla, R. Egorov, and T.J. Xia. 2010. Flexible architectures for optical transport nodes and networks. IEEE Communications Magazine 48(7):40-50.

80 Saleh, A.A.M., and J.M. Simmons. 2011. Technology and architecture to enable the explosive growth of the internet. IEEE Communications Magazine 49(1):126-132.

81 Poole, S., S. Frisken, M. Roelens, and C. Cameron. 2011. “Bandwidth-Flexible ROADMs as Network Elements.” Conference paper. Optical Fiber Communication Conference (OFC), Los Angeles, Calif., March 6, 2011. Available at http://www.opticsinfobase.org/abstract.cfm?URI=OFC-2011-OTuE1. Accessed June 26, 2012.

82 Feuer, M.D., S.L. Woodward, P. Palacharla, X. Wang, I. Kim, and D. Bihon. 2011. Intro-node contention in dynamic photonic networks. Journal of Lightwave Technology 29(4):529-535.

Suggested Citation:"3 Communications, Information Processing, and Data Storage." National Research Council. 2013. Optics and Photonics: Essential Technologies for Our Nation. Washington, DC: The National Academies Press. doi: 10.17226/13491.
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such that a reconfigurable OADM (ROADM) can accommodate changing traffic patterns.83 A myriad of wavelength-selective and tunable components have enabled such advances.84,85 The speed of reconfiguration might be as long as once a day or as short as nanoseconds, depending on the application.

These applications include circuit and packet switching. Circuit switching has limited speed requirements and is considered fairly easy to implement but is somewhat inefficient in terms of capacity utilization. As bandwidth becomes more precious in the network, packet switching, which is common in today’s Internet, has fast switching requirements and is much harder to implement optically but is quite capacity-efficient. In the 5- to 10-year horizon, it is likely to become clearer as to the specific technologies that might be used for optical packet switching as well as the perceived benefits of implementing it in the optical domain.

As time has passed, optical fiber has moved ever closer to the end user. Even 30 years ago, it was clear that optical fiber would dominate long-distance communications. At present, optical fiber is being deployed around the world to enable individual users to access the broadband Internet. Fiber to the x, in which x can represent many things, including curb, home, or office, will only accelerate as costs decrease.86 In fact, access networks are quite sensitive to cost, given that only a few users will share the cost of the optical components, which are generally assumed to be more expensive at present than electronic ones. An exciting area of R&D is in passive optical networks, such that gigabits per second data rates can be deployed at low cost to many simultaneous users in a neighborhood.87 Advances in low-cost integrated optical components and efficient optical access architectures will be crucial to achieving this vision; note that RF-over-fiber, in which a radio-frequency subcarrier is encoded and is transmitted on an optical carrier wave, represents an interesting and potentially exciting approach to bridge the gap between wireless signals and WDM networks.88 It should be emphasized that the exponential growth in wireless traffic only accelerates the need for optics in access networks, since wireless hubs will typically connect in the ground to the larger network through optical fiber. Clearly, the optics and photonics community needs

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83 Lewotsky, K. 1996. Large-effective-area fiber minimizes nonlinearities. Laser Focus World 32:16.

84 Asghari, M., and A.V. Krishnamoorthy. 2011. Silicon photonics: Energy-efficient communication. Nature Photonics 5:268-270.

85 Lewotsky, K. 1996. Large-effective-area fiber minimizes nonlinearities. Laser Focus World 32:16.

86 Lee, C., W.V. Sorin, and B. Kim. 2006. Fiber to the home using a PON infrastructure. IEEE Journal of Lightwave Technology 24:4568-4583.

87 Skubic, A., J. Chen, J. Ahmed, L. Wosinska, and B. Mukherjee. 2009. A comparison of dynamic bandwidth allocation for EPON, GPON, and next-generation TDM PON. IEEE Communications Magazine 47(3):S40-S48.

88 Chen, L., K. Preston, S. Manipatruni, and M. Lipson. 2009. Integrated GHz silicon photonic interconnect with micrometer-scale modulators and detectors. Optics Express 17(17):15248-15256.

Suggested Citation:"3 Communications, Information Processing, and Data Storage." National Research Council. 2013. Optics and Photonics: Essential Technologies for Our Nation. Washington, DC: The National Academies Press. doi: 10.17226/13491.
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to bring forward technologies for the next-order-of-magnitude capacity increases in optical networks very soon.

R&D Example Areas

In general, the field of optical communications is quite rich in terms of innovations and applications. Brief descriptions of example areas of R&D that have the potential for large impact include the following:

Quantum communications. Quantum optical communications typically use the quantum state of a single photon or a pair of entangled photons—which can be widely separated while maintaining their inherent quantum connection—in order to achieve ultimately secure and highly efficient communications.89 For example, if information is encoded on the photon’s state of polarization (SOP), then any eavesdropper would necessarily disturb the photon’s SOP and be detected. Quantum key distribution (QKD) might be extremely important for links requiring ultimate security. Moreover, for systems in which single photons are carrying information, the bits per photon can be quite efficient. Practical single-photon transmitters and detectors are being actively pursued, and the quantum amplifier/repeater remains an elusive research goal.90,91

Free-space communications. Free-space communications have existed for decades as radio. However, there has been the desire, also for decades, to make use of optics in free-space communications due to its much higher frequency range and data transmission capacity.92 Lasers are also highly directional, such that they are power-efficient in terms of divergence and are relatively secure since they are difficult to intercept. Especially for satellite communications at higher bandwidths, there is the potential for dramatic decreases in the crucial parameters of size, weight, and power (SWaP). Satellite or high-altitude free-space laser communications are attractive because of the lack of weather attenuation issues, even at optical wavelengths. The Department of Defense is especially interested in free-space laser communications,

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89 Treiber, A., A. Poppe, M. Hentschel, D. Ferrini, T. Lorünser, E. Querasser, T. Matyus, H. Hübel, and A. Zeilinger. 2009. A fully automated entanglement-based quantum cryptography system for telecom fiber networks. New Journal of Physics 11:045013. Available at http://iopscience.iop.org/1367-2630/11/4/045013. Accessed June 26, 2012.

90 Kimble, H.J. 2008. The quantum Internet. Nature 453:1023-1030.

91 Gisin, N., and R.T. Thew. 2010. Quantum communication technology. Electronics Letters 14:965-967.

92 Killinger, D. 2002. Free space optics for laser communication through the air. Optics and Photonics News 13(10):36-42.

Suggested Citation:"3 Communications, Information Processing, and Data Storage." National Research Council. 2013. Optics and Photonics: Essential Technologies for Our Nation. Washington, DC: The National Academies Press. doi: 10.17226/13491.
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   as discussed in Chapter 4 of this report, because of the need for high-bandwidth communication with moving platforms.

Information Processing

In the technology for optics for interconnection, the past decade has seen significant progress in three main areas:

• Evolutionary advances in optoelectronics made from III-V semiconductors,

• The emergence of silicon photonics as a potential new technology base, and

• Nanophotonics for very-high-performance optics and optoelectronics, some with quite novel capabilities and opportunities.

Evolutionary Progress in III-V Device Technology

Technology for optical data links continued to evolve in the last decade, with continued improvements in speed and reduced power dissipation for device technologies such as VCSELs. Integration of devices in larger functional units has continued in III-V technology.93 Such predominantly III-V approaches can integrate multiple active optoelectronic devices, such as lasers, optical amplifiers, modulators, switches, and detectors for highly functional units for optical networking.

Silicon Photonics

The potential for the use of silicon-based optical technologies (so-called silicon photonics, as mentioned above) has advanced substantially.94 Such silicon photonics can be manufactured in silicon complementary metal oxide semiconductor (CMOS) fabrication facilities, thereby potentially reducing manufacturing costs and allowing direct integration with low-cost silicon electronics. Such an approach offers serious possibilities for an integrated electronics/optoelectronics/optics platform that could address problems of manufacturability and cost for future high-volume applications such as local optical networks or interconnects.

Silicon itself and/or related CMOS materials such as silicon nitride or silicon dioxide can all be used as very effective optical waveguides on the surface of a

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93 Blumenthal, D.J., J. Barton, N. Beheshti, J.E. Bowers, E. Burmeister, L.A. Coldren, M. Dummer, G. Epps, A. Fang, Y. Ganjali, J. Garcia, B. Koch, V. Lal, E. Lively, J. Mack, M. Masanovic, N. McKeown, K. Nguyen, S.C. Nicholes, H. Park, B. Stamenic, A. Tauke-Pedretti, H. Poulsen, and M. Sysak. 2011. Integrated photonics for low-power packet networking. IEEE Journal of Selected Topics in Quantum Electronics 17:458-471.

94 Soref, R. 2006. The past, present, and future of silicon photonics. IEEE Journal of Selected Topics in Quantum Electronics 12:1678-1687.

Suggested Citation:"3 Communications, Information Processing, and Data Storage." National Research Council. 2013. Optics and Photonics: Essential Technologies for Our Nation. Washington, DC: The National Academies Press. doi: 10.17226/13491.
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silicon chip. Additionally, very close integration of devices such as photodetectors and output devices has the potential to improve system performance, especially when power dissipation is particularly important, as it is in optical interconnect applications.

Optical modulators using silicon as the active material can be made in linear (e.g., Mach-Zehnder) forms that have been integrated into products or in more aggressive high-quality-factor ring-resonator forms that also can allow the selection and switching of different wavelength channels. Ring resonators are still largely in research, in part because of the requirements for precise tuning and thermal stabilization. Most silicon photonics work is targeted at the telecommunications wavelengths (e.g., in the so-called C band near 1550 nanometers [nm]) where silicon itself is transparent. Incorporating photodetectors requires the addition of germanium, which absorbs light efficiently in some of the C band. Germanium is already known to be compatible with silicon CMOS technology, as it is already in use for other purposes in modern CMOS systems.

Other silicon-compatible modulator concepts based on optical absorption changes in germanium structures are starting to emerge from research. These devices might offer very low operating energies without the tuning of resonators. Approaches include Franz-Keldysh bulk germanium modulators95 and, more aggressively, so-called Quantum-Confined Stark Effect (QCSE) modulators96 based on the use of thin germanium “quantum well” layers.

At the time of this writing, there is still no room-temperature-operated electrically-driven laser that can be grown directly on silicon, although research is continuing on various approaches, including attempts to make germanium or related materials (such as germanium-tin alloys) into the so-called direct bandgap materials that are generally required for efficient lasers. Lasers have been successfully demonstrated based on the hybridization of III-V materials onto silicon after growth.97 Viable III-V light sources on silicon by direct growth onto the

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95 Liu, J., M. Beals, A. Pomerene, S. Bernardis, R. Sun, J. Cheng, L.C. Kimerling, and J. Michel. 2008. Waveguide-integrated, ultralow-energy GeSi electro-absorption modulators. Nature Photonics 2:433-437.

96 Roth, J.E., O. Fidaner, E.H. Edwards, R.K. Schaevitz, Y.-H. Kuo, N.C. Helman, T.I. Kamins, J.S. Harris, and D.A.B. Miller. 2008. C-band side-entry Ge quantum-well electroabsorption modulator on SOI operating at 1 V swing. Electronics Letters 44:49-50.

97 Fang, A.W., H. Park, O. Cohen, R. Jones, M.J. Paniccia, and J.E. Bowers. 2006. Electrically pumped hybrid AlGaInAs-silicon evanescent laser. Optical Express 14:9203-9210.

Suggested Citation:"3 Communications, Information Processing, and Data Storage." National Research Council. 2013. Optics and Photonics: Essential Technologies for Our Nation. Washington, DC: The National Academies Press. doi: 10.17226/13491.
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silicon are still a subject of research, although some nanophotonic structures offer promise.98,99

It is an open question whether the laser light sources need to be integrated on the silicon. Use of off-chip lasers removes some power dissipation from the chip and allows centralized stabilization of wavelengths and/or pulse timing, at the expense of having to align the laser light to the chip with some additional optics. Approaches exist for hybridizing III-V materials onto silicon substrates and/or optical structures, and such approaches offer one intermediate approach for integrating light sources on the chip. However, amplifiers on the chip may be very desirable in order to increase functionalities, given that the Si-based optical components are still fairly lossy, which limits the number of components that can be integrated. This argues for monolithic integration of active components with optical gain.

Although the various output device options (modulators and lasers) for direct modulation are still emerging from research, they do offer serious possibilities for devices that could meet the energy per bit requirements100 to allow optics to replace wires even at relatively short distances such as chip-to-chip interconnects. It is conceivable that one technology platform could emerge here that would allow the manufacture of systems for applications ranging from chip-to-chip all the way to long-distance optical networks, but much research and technology development remain to be done.

Nanophotonics

The ability to controllably fabricate structures on a deeply sub-wavelength scale—for example, using the same lithographic techniques employed for silicon CMOS electronics—has opened up a broad range of possibilities in optics and optoelectronics. Areas include photonic crystal structures, metamaterials, nanometallics/plasmonics, nanoresonators, nanolasers, and new classes of optical components based on sub-wavelength nonperiodic design.

The basic physics of various novel nanophotonic structures has advanced substantially. Dielectric nanoresonators of various kinds achieved very high quality factors. Nanometallic structures have allowed the concentration of light to very small, sub-wavelength volumes and the guiding of light in sub-wavelength circuits.101 Metamaterials, based on sub-wavelength patterning, allow the engineering of effective materials with optical properties unlike those of conventional

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98 Kunert, B., S. Zinnkann, K. Volz, and W. Stolz. 2008. Monolithic integration of Ga(NAsP)/(BGa) P multi-quantum well structures on (0 0 1) silicon substrate by MOVPE. Journal of Crystal Growth 310(23):4776-4779.

99 Chen et al. 2011. Nanolasers grown on silicon.

100 Miller, D.A.B. 2009. Device requirements for optical interconnects to silicon chips.

101 Brongersma, M.L., and V.M. Shalaev. 2010. The case for plasmonics. Science 328:440-441.

Suggested Citation:"3 Communications, Information Processing, and Data Storage." National Research Council. 2013. Optics and Photonics: Essential Technologies for Our Nation. Washington, DC: The National Academies Press. doi: 10.17226/13491.
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materials.102 Novel nanophotonics approaches to optical components such as very compact wavelength splitters have emerged, including the concept of nonperiodic design for function.103 Extremely sensitive optical and optoelectronic device structures have been demonstrated, including lasers, photodetectors, modulators, and switches, with some down to single-photon sensitivity.104 With such nanophotonics approaches, there is demonstrated potential for very low threshold lasers,105 very low energy modulators, and very low capacitance photodetectors. Novel approaches to optoelectronic devices exploiting nanoscale growth techniques may circumvent some of the difficulties (such as crystal lattice constant matching) that often limit more conventional approaches to device fabrication.106 As the size of many of these new optics and photonics elements shrinks, it certainly opens up for the possibility to produce systems that utilize the best of optics and electronics to enable integrated systems to seamlessly provide solutions in many of today’s fields.

Data Storage

The advent of nanometallic structures to concentrate light to deeply sub-wavelength volumes has opened up new opportunities for optics in data storage, both for sub-wavelength optical reading and writing and for the use of optics to concentrate light for other storage approaches, as in heat-assisted magnetic recording, whereby the light provides the very localized heating above the Curie temperature to enable correspondingly localized changes in magnetic state. This emerging technology is apparently a serious contender for future mainstream magnetic hard-drive technologies.107

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102 Chen, H., C.T. Chan, and P. Sheng. 2010. Transformation optics and metamaterials. Nature Materials 9:387-396.

103 Liu, V., Y. Jiao, D.A.B. Miller, and S. Fan. 2011. Design methodology for compact photonic-crystal-based wavelength division multiplexers. Optics Letters 36:591-593.

104 Fushman, I., D. Englund, A. Faraon, N. Stolz, P. Petroff, and J. Vuckovic. 2008. Controlled phase shifts with a single quantum dot. Science 320(5877):769-772.

105 Ellis, B., M.A. Mayer, G. Shambat, T. Sarmiento, J. Harris, E.E. Haller, and J. Vuèkoviæ. 2011. Ultralow-threshold electrically pumped quantum-dot photonic-crystal nanocavity laser. Nature Photonics 5:297-300.

106 Chen et al. 2011. Nanolasers grown on silicon.

107 Kryder, M.H., E.C. Gage, T.W. McDaniel, W.A. Challener, R.E. Rottmayer, G. Ju, Y.-T. Hsia, and M.R. Erden. 2008. Heat assisted magnetic recording. Proceedings of the IEEE 96:1810-1835.

Suggested Citation:"3 Communications, Information Processing, and Data Storage." National Research Council. 2013. Optics and Photonics: Essential Technologies for Our Nation. Washington, DC: The National Academies Press. doi: 10.17226/13491.
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MANUFACTURING

Communications

Optical communications network equipment includes functional elements such as transmissions systems; optical networking elements, including add/drop multiplexers; crossconnects; and network-management software systems. Substantial numbers of circuit boards for electrical multiplexing, electrical cross-connects, and control functions are also required. These optical systems and network elements are assembled from optical modules (for example, optical transmitters and receivers) that in turn are built from optical components, including lasers, optical modulators, photodetectors, optical fiber amplifiers, and others.

At the systems’ vendor level, which is a roughly $15 billion annual business, there have been significant changes in the industry since Harnessing Light108 was written. Nortel, a major Canadian vendor, has gone out of business. Lucent Technologies has been acquired by Alcatel to form Alcatel-Lucent, with headquarters in Paris, France. Of the top 10 optical network systems vendors, two—Cisco and Tellabs—are headquartered in the United States. Alcatel-Lucent, with significant R&D operations in the United States, and Huawei, headquartered in Shensen, China, have vied for the market-leader position over the last several years. ZTE, also from China, is also in the top 5.109 In the optical module and component business, nearly half of the market share is owned by U.S.-based companies: Finisar and JDSU are the market number one and number two leaders, respectively.110 The growth and fabrication of the III-V semiconductor chips to build the basic laser and detector discrete devices are done in the United States, Japan, and Taiwan and are increasingly in areas like Singapore and Southeast Asia. There is little evidence that companies in China have mastered this highly specialized technology at this time, but work in the research laboratory is ongoing there.

At the circuit board and optical module level, manufacturing assembly is increasingly being done in Asia. However, some U.S. companies continue to do assembly of leading-edge boards—such as 100 Gb/s transmitters and receivers—in the United States. Infinera, a metropolitan and long-haul optical networking company based in the United States and a leading proponent of photonic integrated circuits (PICs) to provide transmitter and receiver arrays for WDM transmission

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108 National Research Council. 1998. Harnessing Light.

109 Ovum. 2011. “Ovum: ZTE Ranked Third in Growing Global Optical Networking Market.” Ovum. Available at http://www.tele.net.in/news-releases/item/7010-ovum-zte-ranked-third-ingrowing-global-optical-networking-market. Accessed July 26, 2012.

110 Ovum. 2012. “Oclaro Combines with Opnext to Challenge Finisar for No. 1.” Available at http://ovum.com/2012/03/27/oclaro-combines-with-opnext-to-challenge-finisar-for-no-1/. Accessed July 26, 2012.

Suggested Citation:"3 Communications, Information Processing, and Data Storage." National Research Council. 2013. Optics and Photonics: Essential Technologies for Our Nation. Washington, DC: The National Academies Press. doi: 10.17226/13491.
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systems, produces the PIC chip in the United States. Because PICs provide on-chip integration of basic components that otherwise would be done by hand, this technology eliminates much of the manually intensive activity of assembling and connecting individual components that is currently done more cost-effectively in low-labor-cost regions. More highly integrated optical modules will be needed for next-generation systems from a functional point of view. By being the technology leader in this area, the United States could potentially increase the value added in the United States that, with today’s modules made up of discrete devices, is moving offshore.

Optical components for communications networks still involve several technologies—indium phosphide (InP), CMOS, lithium niobate, and silica. At the same time, volumes addressed by each technology are rather limited. The company and/or nation that finds a consolidation technology (if possible) could have substantial advantage. Some might suggest that silicon photonics has the potential to displace other passive technologies as well as non-lasing electro-optic technologies such as lithium niobate.

Also worth noting is anecdotal evidence which suggests that as the manufacturing of optical systems equipment or optical modules moves to lower-cost manufacturing regions, the development function tends eventually to move to that region as well.

Information Processing

Current optoelectronic systems are dominated by III-V semiconductors in systems with generally small integration levels. To make the transition to a new technology such as silicon-based photonics with high integration levels is difficult, and it appears to need high volumes to justify it.111 There is, therefore, a “chicken and egg” issue with this major potential technology shift. There is substantial medium-to-long-term potential for a very high volume market for optical interconnections, one that could help retain a U.S. technology and market leadership in information processing equipment, but it may need a shift to a low-cost, possibly silicon-based technology to satisfy that market. The same silicon photonics platform, if successfully adopted, also has the capability to create a broad range of nanophotonic structures.

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111 Fuchs, E., R. Kirchain, and S. Liu. 2011. The future of silicon photonics—Not so fast?: The case of 100G Ethernet LAN transceivers. Journal of Lightwave Technology 29(15):2319-2326.

Suggested Citation:"3 Communications, Information Processing, and Data Storage." National Research Council. 2013. Optics and Photonics: Essential Technologies for Our Nation. Washington, DC: The National Academies Press. doi: 10.17226/13491.
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Data Storage

At present, the substantial majority of manufacturing for optical data storage products, such as DVDs, is likely in Asia.112 Given that the impact of a next generation of optical disk storage faces strong competition from networks as a distribution medium for video, it is not clear that a major push to manufacture such a technology in the United States is a strong choice. There is significant research in the United States in future areas such as the HAMR optically assisted hard-drive technology, which may influence some future manufacture of such magnetic hard drives in the United States.

At this time, beyond the strong possibilities in HAMR and some continuing use of optical disks such as DVDs, the path for other optical data storage approaches is less clear, although there is continuing development of holographic storage for commercial archiving, with possible capacities in the 500 Gb to 1 Tb range in a disk of a size comparable to that of current DVDs.113 Other approaches remain in research at the present.

ECONOMIC IMPACT

Communications networks have taken on a role well beyond people-to-people voice communications; they provide the information and trade routes of the new global digital economy. They will also likely provide the sensor integration and distributed control for the power grid, transportation, and freight networks and smart enterprises and cities in the future. Optical interconnects are very likely to have a key role in future information processing technology and systems, allowing performance to continue to scale. Leadership in communications networks and interconnects, including the underpinning optical networks, both in the R&D and in commercialization, deployment, and effective use, will be absolutely essential to maintaining and enhancing economic growth and improving the quality of people’s lives in the 21st century. These optical technologies will be increasingly indispensable for the technology of the information age.

This critical importance of communications networks to the future has driven action in countries around the world. Several countries, including Australia for example, have mandated broadband fiber to the home. Europe continues to provide

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112 Esener S.C., M.H. Kryder, W.D. Doyle, M. Keshner, M. Mansuripur, and D.A. Thompson. 2012. International Technology Research Institute, World Technology (WTEC) Division. 1999. WTEC Panel Report on the Future of Data Storage Technologies. Available at http://www.wtec.org/pdf/hdmem.pdf. Accessed August 1, 2012.

113 More information is available at General Electric’s Global Research website at http://ge.geglobalresearch.com/blog/breakthrough-in-micro-holographic-data-storage/. Accessed June 26, 2012.

Suggested Citation:"3 Communications, Information Processing, and Data Storage." National Research Council. 2013. Optics and Photonics: Essential Technologies for Our Nation. Washington, DC: The National Academies Press. doi: 10.17226/13491.
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substantial research funding, as do Korea and Japan. However, the single most important threat to U.S. leadership, especially with respect to network equipment development and manufacture, is likely to come from China, where the government-sponsored Huawei ranks as number 1 or number 2 in global market share,114 followed closely by ZTE, another systems vendor from China. Already extremely innovative in its products, China is now focused on research and innovation. Its paper submissions to international optics journals and to the premier global optical communications conferences have increased substantially in the last 5 years.115 There is particularly strong research in Europe and recently also in Japan in the emerging silicon photonics platforms for networks and interconnects.

Given that labor-intensive industries will continue to migrate their manufacturing to low-labor-cost regions, it is imperative that the United States stay at the leading edge of optical technology at the component, platform, and system levels. This approach appears to have served the U.S. electronics industry well. To keep a substantial portion of the value chain in the United States, it will be important for U.S. enterprises to have the critically enabling intellectual property that provides a barrier of entry without financial compensation. To have that intellectual property, it is essential to be at the leading edge of enabling the fundamental and applied research. Suggested areas of focus on the component side include very high speed electronics and optical components, including modulators and detectors at operating rates of 400 Gb/s and 1 Tb/s; advanced signal processing to overcome transmission impairments for coherent systems; and on-chip integration that provides increased functionality while reducing size, power consumption, and cost. Such integrated chips also do not require the substantial manual assembly that is now being performed in low-labor-cost regions. In the systems and networks area, finding a new approach to cost-effectively achieve several-orders-of-magnitude long-haul and metropolitan distance transmission capacity and to rapidly get that technology to market will be critical in order for the United States to maintain a strong global leadership position. Such technology evolution will also underpin the increasing move to optics inside information processing that will be essential for the continued scaling of an information-driven economy. The successful development of an integrated platform technology, such as some version of silicon photonics that can service a broad range of applications and integration with electronics, could be a major enabler for U.S. economic impact.

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114 Huawei. 2010. Milestones. Available at http://www.huawei.com/en/about-huawei/corporate-info/milestone/index.htm. Accessed July 26, 2012.

115 Cao, J. 2012. A new journal in optics and photonics—Light: Science and Applications. Editorial. Light: Science and Applications 1:Online. Available at http://65.199.186.23/lsa/journal/v1/n3/full/lsa20123a.xhtml. Accessed July 26, 2012.

Suggested Citation:"3 Communications, Information Processing, and Data Storage." National Research Council. 2013. Optics and Photonics: Essential Technologies for Our Nation. Washington, DC: The National Academies Press. doi: 10.17226/13491.
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Comparison Between the United States and the Rest of the World

For decades the United States has been the envy of the entire world in terms of R&D in high-tech fields, including optics and photonics for communications. For decades the most prestigious international journals, conferences, and professional societies have been based in the United States. Many of the most impactful R&D advances came from the U.S. industrial laboratories (e.g., AT&T Bell Laboratories, Corning, RCA Sarnoff Labs, IBM), with several Nobel Prizes as well. The U.S. university system was unrivaled, with student researchers flocking to the United States from countries around the world. The combined R&D in optics from corporate laboratories and federally funded university labs set the bar high in terms of quality and quantity. Critical advances were made in the United States, including low-loss fibers, semiconductor lasers, optical amplifiers, and information theory.

The scenario has changed somewhat in the past 20 years. In general, corporate laboratories no longer enjoy steady, long-term funding, and the main sources of university research funding have not grown at a pace consistent with that in the rest of the world. The main sources of university funding in communications and information processing, such as the NSF and the Department of Defense (DOD), tend to fund a relatively small percentage of new proposals, particularly with a 10-year focus.

In the area of optical research for information processing, U.S. efforts are generally comparable in size to efforts in Europe and Japan in the emerging technologies for optics in information processing, but the United States has no overall lead against these research competitors. The United States lacks the larger European framework projects that help tie together a broad range of players from academia to industry. Both Europe and Japan are making substantial investments in silicon photonics technology research now.

Trends in terms of the United States in relation to the rest of the world include the following:

Research funding. Nations all over the world view the United States as a benchmark in terms of research funding. In order for countries to compete, many nations focus on specific, strategic areas in which to invest in long-term photonics research. The United States tends not to make such strategic, long-term “bets.” The effect is that in any given area, the United States will share leadership with, and perhaps be surpassed by, research in specific countries. High-speed communications in Germany, integrated photonics in Japan, and access technologies in China are all examples of thrusts centered in various countries.

Professors. Several nations have invested in hiring distinguished researchers to prominent, well-paid professorial positions. Oftentimes, these researchers

Suggested Citation:"3 Communications, Information Processing, and Data Storage." National Research Council. 2013. Optics and Photonics: Essential Technologies for Our Nation. Washington, DC: The National Academies Press. doi: 10.17226/13491.
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   have made their reputations in the United States, only to be drawn away in their prime. Countries including Australia, Canada, China, and Germany have hired excellent researchers in communications with generous research funding. This trend, if it accelerates, could have a profound impact.

China. When observing statistics of prestigious optical communications journals, it is undeniable that the quantity of research from China is growing rapidly. Looking at “optical communication” in the Scopus database, there are 15,003 journal publications from 1969 to 2012 from 21 common optical communications journals;116 in total, the United States represents 3,909 of those, and China, 1,303. However, since 2000 the U.S. scientists have published 42 percent of their work, or 1,642 publications, whereas Chinese scientists have published 85 percent of their work, or 1,108 publications. Most of the Chinese publications have appeared since 2000. Moreover, the quality gap, according to rejection ratio statistics, is narrowing. Using the number of citations that a paper receives as a metric of quality also points to China’s gaining on the United States. A comparison between the two countries with respect to publications related to the three keywords (Optics, Photonics, and Communication) in the Scopus database, using the highest-cited papers, gives the following results: currently there are 2.3 citations of a U.S. paper for each citation that a Chinese paper receives; however, 5 years ago this number was 2.8, or 16 percent higher.117 Furthermore, China is aggressively funding research in optical communications and has a foothold as home to some of the world’s largest communications companies. China is poised to make great strides in the coming decade.

FINDINGS AND CONCLUSIONS

In spite of great progress in optical communications over the last three decades, the optics and photonics community faces a great challenge if optical communications networks are to continue to satisfy the insatiable global demand for information

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116 The selected journals were these: Optics Communications, IEEE Photonics Technology Letters, Optics Express, Journal of Lightwave Technology, Optical Engineering, Microwave and Optical Technology Letters, Journal of Optical Communications, Physical Review A Atomic Molecular and Optical Physics, Optics Letters, Photonic Network Communications, Applied Optics, Applied Physics Letters, Journal of the Optical Society of America B Optical Physics, Fiber and Integrated Optics, Optical and Quantum Electronics, Proceedings of SPIE the International Society for Optical Engineering, IEEE Transactions on Communications, IEEE Transactions on Microwave Theory and Techniques, IEEE Proceedings Optoelectronics, Journal of Optical Networking, and Photonics Spectra.

117 Data from SciVerse Scopus, www.scopus.com. Data derived using the top 25 papers and averaging over 5-year periods. Accessed March 21, 2012.

Suggested Citation:"3 Communications, Information Processing, and Data Storage." National Research Council. 2013. Optics and Photonics: Essential Technologies for Our Nation. Washington, DC: The National Academies Press. doi: 10.17226/13491.
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processing, data storage, bandwidth, and ubiquitous connectivity that, in turn, drive global economic growth.

Key Finding: The growth in bandwidth demand over the next 10 years is expected to be at least another 100-fold, possibly much more. It is important to note that the previous 100-fold gain in capacity that came very naturally with wavelength division multiplexing has been used up; growth by means of higher bit rates per wavelength comes more slowly; hence without a new breakthrough, increases in data transmission capacity will stall.

Key Finding: Cloud services not only drive capacity demand, but also make critical the role of large data centers. This will usher in a new and important era for short-distance optical links in massive numbers to provide cost- and energy-efficient high-density interconnects inside data centers and in information processing systems generally.

Key Finding: Silicon-based photonic integration technologies offer great potential for short-distance applications and could have great payoff in terms of enabling continued growth in the function and capacity of silicon chips if optics for interconnection could be seamlessly included in the silicon CMOS platform. It is also highly likely that integrated optoelectronics is a critical development area with significant growth potential for continuing the advance of defense systems.

Finding: Nanophotonic technologies promise very compact and high-performance optics and optoelectronics that could allow such platforms to continue to scale to higher density and performance. Information processing in general and data centers will also require massive and exponentially growing data storage.

Finding: Magnetic storage will continue as the primary data storage technology, and optics-assisted magnetic techniques may play an important future role.

Finding: Communications networks and information processing consume a relatively small fraction of the global energy budget (approximately 2 percent). However, with the growing network demands, this percentage will likely grow significantly unless successful action is taken to reduce systems’ energy requirements.

Finding: Leveraging networks with applications like telepresence to reduce energy-hungry activity such as travel would reduce total energy consumption.

Finding: Optics has the potential to increase the energy efficiency in networks by, for example, replacing high-energy-loss electrical conductors in data centers and wireless backhaul or, perhaps, even on silicon chips.

Suggested Citation:"3 Communications, Information Processing, and Data Storage." National Research Council. 2013. Optics and Photonics: Essential Technologies for Our Nation. Washington, DC: The National Academies Press. doi: 10.17226/13491.
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Conclusion: The committee believes that a strong partnership among industry, universities, and government agencies will be crucial to overcoming technical challenges and to ensuring that the United States leverages that knowledge to gain market leadership.

Finding: Many of the optical communications successes over the last 10 years are built on earlier research that came from research laboratories of vertically integrated companies, as well as strongly supporting government agencies. The industry is no longer integrated, but instead is segmented by material fabrication, components, modules, systems, network providers, and content and service providers. In this fragmented environment there has been a reduction in industrial research laboratories, because the reduced scale makes it difficult for companies to capture the value that is prudently needed to justify investing in research.

Finding: Today’s broadband access by individual users in the United States is neither high-capacity nor available at reasonable cost to a large fraction of the population. Bandwidths of 1 Gb/s represent the current state of the art in broadband access to the home in leading installations today.

RECOMMENDATIONS AND GRAND CHALLENGE QUESTIONS

Key Recommendation: The U.S. government and private industry, in combination with academia, need to invent technologies for the next factor-of-100 cost-effective capacity increase in long-haul, metropolitan, and local-area optical networks.

The optics and photonics community needs to inform funding agencies, and information and entertainment providers, about the looming roadblock that will interfere with meeting the growing needs for network capacity and flexibility. There is a need to champion collaborative efforts, including consortia of companies, to find new technology—transmission, amplification, and switching—to carry and route at least another factor-of-100 capacity in information over the next 10 years.

This key recommendation leads directly to the first grand challenge question:

1. How can the U.S. optics and photonics community invent technologies for the next factor-of-100 cost-effective capacity increases in optical networks?

The first recommendation of the chapter states a goal for increased capacity; the next recommendation offers a path to help achieve that goal, especially with respect to very short range communication, such as that required inside a data center.

Key Recommendation: The U.S. government, and specifically the Department of Defense, should strive toward harmonizing optics with silicon-based electronics

Suggested Citation:"3 Communications, Information Processing, and Data Storage." National Research Council. 2013. Optics and Photonics: Essential Technologies for Our Nation. Washington, DC: The National Academies Press. doi: 10.17226/13491.
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to provide a new, readily accessible and usable, integrated electronics and optics platform.

They should also support and sustain U.S. technology transition toward low-cost, high-volume circuits and systems that utilize the best of optics and electronics in order to enable integrated systems to seamlessly provide solutions in communications, information processing, biomedical, sensing, defense, and security applications. Government funding agencies, the DOD, and possibly a consortium of companies requiring these technologies should work together to implement this recommendation. This technology is one approach to assist in accomplishing the first key recommendation of this chapter concerning the factor-of-100 increase in Internet capability.

The second key recommendation in this chapter leads to the second grand challenge question:

2. How can the U.S. optics and photonics community develop a seamless integration of photonics and electronics components as a mainstream platform for low-cost fabrication and packaging of systems on a chip for communications, sensing, medical, energy, and defense applications?

In concert with meeting the first grand challenge, achieving the second grand challenge would make it possible to stay on a Moore’s law-like path of exponential performance growth. The seamless integration of optics and photonics at the chip level has the potential to significantly increase speed and capacity for many applications that currently use only electronics, or that integrate electronics and photonics at a larger component level. Chip-level integration will reduce weight and increase speed while reducing cost, thus opening up a large set of future possibilities as devices become further miniaturized.

The size and number of data centers in the United States and globally is expected to grow dramatically over the next decade to address the needs of a global digital society, especially if cloud services become more pervasive. It is clear that these data centers will be the focal point for the development and deployment of new optics and photonics communications technologies, and as such will be very important for the economy.

Key Recommendation: The U.S. government and private industry should position the United States as a leader in the optical technology for the global data center business.

Suggested Citation:"3 Communications, Information Processing, and Data Storage." National Research Council. 2013. Optics and Photonics: Essential Technologies for Our Nation. Washington, DC: The National Academies Press. doi: 10.17226/13491.
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Optical connections within and between data centers will be increasingly important in allowing data centers to scale in capacity. The committee believes that strong partnering between users, content providers, and network providers, as well as between businesses, government, and university researchers, is needed for ensuring that the necessary optical technology is generated, which will support continued U.S. leadership in the data center business.

Recommendation: The U.S. government and private industry, in conjunction with academia, should strive to develop technology to have optics take over the role of communicating and interconnecting information, not just at long distances, but at shorter distances as well, such as inside information processing systems, even to the silicon chip itself, thereby allowing substantial reductions in energy consumption in information processing and allowing the performance of information processing machines and systems to continue to scale to keep up with the exploding growth of the use of information in society.

Recommendation: The U.S. government and private industry, in conjunction with academia, need to encourage the exploitation of emerging nanotechnology for the next generation of optics and optoelectronics for the dramatic enhancement of performance (size, energy consumption, speed, integration with electronics) in information communications, storage, and processing.

Recommendation: The optics and photonics community needs to position the United States in broadband to the home and business space. The U.S. government should pursue policies that will enable at least gigabits per second broadband access to the substantial majority of society at a reasonable cost by 2020.

Recommendation: A multi-agency and cross-discipline effort is recommended to identify the opportunities and optical technologies to significantly increase the energy efficiency in communications networks, information processing, and storage. In addition, new ideas for the use of energy-efficient optical approaches to displace current energy-hungry practices—for example, travel—should be identified and supported. Greater focus and support in this area, especially at the fundamental level where companies are less likely to invest and where payoff could be huge, will be important.

Suggested Citation:"3 Communications, Information Processing, and Data Storage." National Research Council. 2013. Optics and Photonics: Essential Technologies for Our Nation. Washington, DC: The National Academies Press. doi: 10.17226/13491.
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Optics and photonics technologies are ubiquitous: they are responsible for the displays on smart phones and computing devices, optical fiber that carries the information in the internet, advanced precision manufacturing, enhanced defense capabilities, and a plethora of medical diagnostics tools. The opportunities arising from optics and photonics offer the potential for even greater societal impact in the next few decades, including solar power generation and new efficient lighting that could transform the nation's energy landscape and new optical capabilities that will be essential to support the continued exponential growth of the Internet.

As described in the National Research Council report Optics and Photonics: Essential Technologies for our Nation, it is critical for the United States to take advantage of these emerging optical technologies for creating new industries and generating job growth. The report assesses the current state of optical science and engineering in the United States and abroad--including market trends, workforce needs, and the impact of photonics on the national economy. It identifies the technological opportunities that have arisen from recent advances in, and applications of, optical science and engineering. The report also calls for improved management of U.S. public and private research and development resources, emphasizing the need for public policy that encourages adoption of a portfolio approach to investing in the wide and diverse opportunities now available within photonics.

Optics and Photonics: Essential Technologies for our Nation is a useful overview not only for policymakers, such as decision-makers at relevant Federal agencies on the current state of optics and photonics research and applications but also for individuals seeking a broad understanding of the fields of optics and photonics in many arenas.

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