Optical logic redux

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Optik

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Optik 117 (2006) 199–209 www.elsevier.de/ijleo

Optical logic redux H. John Caulfield, Chandra S. Vikram, Andrey Zavalin Conservative Optical Logic Devices, Fisk University, 1000 17th Ave., N., Nashville, TN 37208, USA Received 20 July 2005; accepted 5 November 2005

Abstract Twenty years ago IBM physicist Robert Keyes published a paper entitled ‘‘Optical Logic—in the light of computer technology.’’ It caused an instant furor in the fledgling optical logic community. Now, 20 years after that devastating critique, the field of optical logic has grown enormously. There are literally thousands of papers. Many of them are collected in a bibliography given here. Was Keyes’ critique wrong? Have opticists simply ignored what Keyes pointed out? Have new developments made some of his remarks not quite so relevant? We argue here that






Keyes was and still is mostly correct, but that may change in a few years Many researchers have indeed simply ignored what he said New developments in both optical logic and its applications open niches for optical logic that Keyes did not (and probably could not) anticipate
New and anticipated developments in electronics may increase the role for optics r 2006 Elsevier GmbH. All rights reserved. Keywords: Optical logic; Bibliography; All-optical; Conservative logic; SOA; Interferometric logic

1. Introduction In many ways, the mid-1980s were the heydays of optical computing, and optical logic was a just-emerging small part of that big field. Claims were made that were embarrassing to some of us even then about the future of ‘‘all optical computers.’’ At the heart of all computers is the logic, so optical logic was to be the way to keep Moore’s law forces going. Computer development has followed Moore’s law well over those two decades, but optics has had little to do with it. That was what Bob Keyes was trying to warn us about in his 1985 paper [1]. Corresponding author.

E-mail addresses: hjc@fisk.edu (H.J. Caulfield), cvikram@fisk.edu (C.S. Vikram), azavalin@fisk.edu (A. Zavalin). 0030-4026/$ - see front matter r 2006 Elsevier GmbH. All rights reserved. doi:10.1016/j.ijleo.2005.11.005

Yet optical logic did not die. There have been numerous papers (see the bibliography of this paper for what we believe to be a representative set of papers subsequent to his paper) and now some specialized applications. By revisiting the arguments from Keyes’ paper, we hope to provide the reader with some tools to help separate the relevant from the ‘‘merely academic’’ work in the field–the same goal as Keyes himself had. We will not provide even a partial list of irrelevant and ultimately unusable ideas that have been proposed. But, the reader should be able to make those judgments easily after reading this article. Bob and one of us (HJC) discussed his paper at length soon after he published it. He felt that the best thing he could do for the field of optical logic would be to offer a friendly but sound critique.

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There are areas where optical logic apparently does have a role, but that role still is not as components of an all optical computer. This update on the Keyes paper shows how a new nonlinear optical device (the SOA or Semiconductor Optical Amplifier) has made optical logic integrated optics practical and attractive. It also shows how interferometric logic combines profound new advantages with some equally dramatic disadvantages to become attractive in other applications. The result is that Keyes’ conclusion that optical logic for all optical computing is unattractive is still correct, but developments he could not have anticipated have opened up some attractive niche opportunities for optical logic. Other new developments point to a scenario wherein optical logic can become preferable for ultra high speed logic. We discuss those new developments below.

2. Overview of the Keyes paper The Keyes paper was narrowly focused on the use of nonlinear optical devices to replace transistors as the logic components of a general purpose computer. We know of no one now who seriously proposes such a thing, so obviously Keyes was right. So why revisit the paper now? Our reason is that most of his arguments apply to optical logic broadly not just for that narrow purpose. The arguments that seem peculiar to the general purpose computer (cascadability, low cost, reliability, small size, low power, uniform and controllable fanout, and so forth) still apply to the more limited applications for optical logic practiced and contemplated today. If that is so, then optical logic devices failing those tests dramatically have an extra burden of proof if their inventors seriously propose that their devices be used. Keyes remarked: ‘‘Attempts to introduce new technologies for logic gates often fail because a focus on devise speed diverts attention from other factors.’’ That is a cautionary warning readers (and perhaps even referees) in the field might attend. They have experiments involving thousands of dollar worth of equipment filling an optical table that achieve some logic operation at almost the speed of the transistors in my pc. Surely, the authors of such papers should offer us the readers some hope that these huge limitations might someday be overcome. Elsewhere he notes: ‘‘Circuits designed for widespread use and mass production must take into account the variability of device characteristics and operating conditions.’’ Just because a Ph.D. student can make one at the end of an arduous thesis effort does not mean that it can be produced.

A tempting error optical logic papers sometimes commit is not new. Keyes saw it even two decades ago and warned against it. Sadly, his warning is often ignored. He said: ‘‘One must also avoid the error of comparing laboratory experiments with contemporary commercial technology, rather than with the advanced technologies that will be available when the laboratory results have been reduced to practice.’’ Moore’s law shows no sign of failure. Let us move now to specific criticisms Keyes advanced against logic based on nonlinear optics. They concern size, reliability, and cascadability. On size, ‘‘Digital optical devices tend to be large, with dimensions in the ten to several hundred micrometer range.’’ That tendency has changed only slightly, and submicron optical devices seem problematical even in principle. The reliability requirement (in terms of fractions of operations of any logic element that are likely to fail) depends on the number of devices, how many operations per unit time are required of them, and mean time between failure for the system (as Keyes detailed in his paper). He concluded 20 years ago that a failure rate of 1010 per h or better was needed then. Twenty years of Moore’s inexorable law make the current needs much more severe. Cascadability is vital to digital computing. The problems with different input and output encryption and with unknown and variable fanout have already been noted. Keyes also pointed out that connection reliability was critical here. He also noted that discrete component connections system fare poorly as compared with integrated circuit connections. Level control (0 s being real 0 and 1 s being a predictable fixed value) also seemed easier in electronics than in optics.

3. Preconclusion It should be evident that Keyes’ concerns are still valid. Making a full all optical computer seems unlikely to happen. So the recent upsurge in publications in optical logic requires some explanation. Of course some of it is accidental or even deliberate ignoring of Keyes’ paper and arguments. But, we argue, there are understandable and sound reasons to look again at optical logic–hence the title of this paper.

4. Last two decades Thankfully, we note that no one now proposes to build all optical digital computers. But optical logic continues to flourish. Perhaps there are niches for optical logic other than the impossible one. And perhaps

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there are new kinds of optical logic that avoid the problems of the nonlinear optical methods pointed out by Keyes and reiterated here. And, of course, there are works that simply ignore the problems altogether. We want to look at the two legitimate reasons for continuing efforts in optical logic here: new niches and new implementation methods.

5. New niches Even 20 years ago, some optical logic proponents agreed with Keyes that the all optical digital computer was not a realistic goal. Their goals were more modest, namely specialized processors for some fixed purpose. The advantages sought are sometimes obtainable. They include the following.













Speed. This is what everyone pushes, but remember Keyes’ various cautions about it. Below, we will discuss systems of unlimited bandwidth (so far as the optics itself goes). Power Consumption. This can sometimes be an advantage. If there is considerable fanin and fanout to a gate, there can be a huge power consumption advantage to optics [2–7]. If a conservative optical logic gate [8–10] can be built, it is possible in principle to operate with zero energy consumption in the logic. For two logic gates (XOR and COINC) that is possible [11–13]. Immunity to Ultra Harsh Conditions. Some electronics must always be involved, but it can be concentrated in a small, well-shielded region, leaving the optical logic elements unshielded against ionizing radiation, Electro Magnetic Pulse (EMP) and so forth. Avoidance of Domain Switching (Optics-to-Electronics and Electronics-to-Optics). Some signals, e.g. those involved in optical communication, start out in the optical domain. For them, optical logic is quite attractive.

Niches for optical logic are likely to arise when one or more of those advantages are required. Consequently, it is not surprising that aspects mentioned above are under constant developments such as on low-power switching [14], hardware resources management [15], and highspeed low-energy processing [16].

6. New implementation methods: unanticipated developments in optical logic At least three advances in optical logic that Keyes could not have anticipated have changed the field dramatically since his paper.

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First, the Semiconductor Optical Amplifier or SOA has changed everything with regard to integrated optical logic. This fast and controllable nonlinear optical element is the basis for uncounted numbers of practical optical logic devices. A Google search for SOA in June 6, 2005 gave about 5,900,000 hits. Second, for two very useful gates (XOR and COINC); linear reversible, somewhat cascadable optical devices can be made. These are passive, so they consume no power and operate at the Input/Output (I/O) limit [11]. Third, the field of quantum computing seems to be progressing rapidly. There the hope is to use multiple entangled states for ‘‘quantum parallelism.’’ This would do some computations much faster than is ever possible conventionally. The reversible logic gates just discussed are quantum mechanical, but they operate only on single qubits. Other schemes suggest that linear optical devices can be used to make optical quantum computers [12]. A review of this paper in 20 years will surely be able to say whether or not that hope is realized. For now, it is simply an exciting hope.

7. How the physics of ultra high speed electronic computing may come to favor optical logic Since the beginning of electronic computing, it has been necessary to make both device size and inter-device spacing smaller to make the processing speed greater. Moore’s Law (not as Gordon Moore intended it but as it is most often used) incorporates this as it assumes speed goes up at the same geometric rate at which device density goes up. Nothing could be more logical, but perhaps that is about to change. As processing speed reaches the 10 s of GHz and beyond, the physics governing the electronics takes what was an unexpected turn. The electromagnet propagation becomes more and more like microwave propagation and less and less like wire-confined propagation. The basic phenomena are well known: 1. Accelerating charges produces electromagnetic radiation. 2. The amount of radiation is proportional to the acceleration. 3. The acceleration in a periodic charge motion is proportional to the frequency squared. 4. The current version of Moore’s Law says the clock speed (frequency) will double every 18 months. 5. That means the amount of power radiated will double every O(18) ¼ 4.24 months! 6. To keep that radiation from being picked up by other electronic components through evanescent wave means, components must be placed several wavelengths apart.

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7. In silicon, 10 GHz produces a wavelength of about 1 cm. The good news is that 100 GHz (roughly the year 2020 according to Moore’s Law) goes down to 1 mm. 8. That is still huge compared to an effective wavelength of about 0.5 microns in silicon. At that point, the design considerations reverse. Higher speed requires greater spacings, not smaller! Thus the profound advantage electronic logic enjoys in density over its optical counterparts will decrease steadily as the processing speed of the logic increases. Of course Keyes did not anticipate such a development, and we may soon begin to see the slow emergence of optics for very fast processing.

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Fig. 1. Year wise number of publications in optical logic (limitations described in the text) with titles starting with words ‘‘All-optical.’’

8. Selected bibliography We have assembled a bibliography of almost 1000 technical papers in optical logic since the Keyes paper. The fact that so much has been done and is still being done, testifies to the health of the field. Indeed, the publication rates seem to have increased significantly in the resent years. We attach here (Appendix A) a selection (2000 onwards) from those papers and intends to put the whole bibliography online as an aid to research. However, we are including only journal publication with rare exceptions of a few Ph.D. thesis and special compressive reports. Also excluded are conference publications at the moment. We are not listing on quantum, fuzzy, as well as molecular logic gates. No pretense is made that this bibliography is complete or even that it includes all of the truly important papers. What we do believe, however, is that it gives a fair representation of what has been done in optical logic over the last two decades. They are grouped very roughly. An interesting observation is titles with starting with the words: ‘‘All-optical’’. All-optical logic is what is needed to avoid electronic-to-optical and optical-toelectronic conversions, so it is matched to situations such as optical communication where the signal is already optical. So, ‘‘all optical’’ may be taken as a clue that the authors are aiming at such a practical application. Papers containing ‘‘all optical’’ later in the title were not included in the data we analyzed and now present in the following figures and table. Fig. 1 shows progression in two decades. Although we have 11 such articles to-date in 2005 we anticipate a conservative another 5 in the remaining part. Fig. 2 is a graphical representation of the data presented in Fig. 1. Raw as well as smooth plots show tremendous increase in the numbers in the recent years. The smooth plot on semilogarithmic scale show almost linear response since 1997. Thus, the number of publications per year (N)

against the year (Y) since 1997 can be fitted into: N ¼ 0:9428 þ 4:5634 exp½0:1627ðY  1997Þ.

(1)

It is interesting to see that since 1997, the publication rate (N) has been doubling every 3.75 years on the average! It has been fairly steady in the 3.50–4.0 range. Table 1 shows publication source based data of such articles. Although total number of such publications is roughly equally divided among physics and electrical engineering journals, the two electrical engineering journals very remain popular for articles starting with ‘‘All-optical’’.

9. Conclusions Keyes’ main point was that a general purpose all optical computer was not going to be practical. So far as we know, that is now universally accepted. Our suggestion here is that many of the observations Keyes made are valid outside the domain of all optical digital computers. They apply to optical logic broadly. Nevertheless there appear to be niches where optical logic can be helpful. In addition, the technology has changed a lot in 20 years. A wonderful way to do optical nonlinear operations—the SOA—is dramatically changing the field. New less-than-universal reversible optical logic gates are now being developed that literally cannot be beat for speed (I/O limited bandwidth) and power consumption (I/O power consumption is all there is). Finally, early work in optical quantum computing holds immense promise, if it can be brought to successful implementation. Those remarks apply to computers as they have been since before Keyes wrote his paper and still apply today. But the path toward a reversal of fortune for optical

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Publication titles

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Electronics Letters IEEE Photonics Technology Letters Optics Communications Optics Express Applied Optics, Applied Physics Letters, JOSA B Optics Letters Jpn. J. Applied Physics, J. Lightwave Technology Chinese Physics, IEEE J. Quantum Electronics, IEEE J. Selected Topics in Quantum Electronics, Optical Engineering Caltech Ph.D. thesis, Chemical Physics, Current Applied Physics, Electronics and Communications in Japan, Fiber and Integrated Optics, IEEE Optical Communication, IEEE Transactions on Nanoscience, Microwave and Optical Technology Letters, J. Applied Physics, Optical and Quantum Electronics, Optik

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logic has become clear and will become clearer and clearer. In a decade, we may very well find that optical logic is the key to ultra high speed computation.

Acknowledgment Work done under Contract No. HQ00604C0010 for the United States Missile Defense Agency.

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