5th Triangle Symposium on Advanced Information and Communication Technology (TriSAI 2010),
October 25-27, 2010, Beijing University of Posts and Telecommunications (BUPT), Beijing, P.R. China.

Crude future vision of 300-GHz optical micro-processor units
Yoshiyasu Ueno
Univ. of Electro-Communications (UEC)
Chofu city, Tokyo 182-8585, Japan
phone: +81-42-443-5207, e-mail: ueno@ee.uec.ac.jp

Abstract
Evolution in energy efficiency of many-core parallel
micro-processors through year 2025 looks unclear
nowadays, because the clock speed of new electroniccores has stopped to evolve. With reviewing on-going
applications-research activities in semiconductor optoelectronics, instead, the author estimates what
possibilities the potentially 100-times-faster serialprocessing micro-processors (e.g. 300-Gb/s) consisting of
all-optically-controlled gates and memories willcontribute, to the information communication technology(ICT).

1. Introduction: New semiconductor industries
The semiconductor industry has recently been
regarded as one of the most “competitive” industry,including cost competitions. It is mainly because of the
40-year-long maturity of crystalline-silicon nanotechnology, and also because such silicon integrated
circuits are utilized almost everywhere such as consumer-

level wireless terminals and industry-level giant data
centers. Now in this 21st century, in addition, it should be
noted that several other types of semiconductor-material
industries are newly attracting more attentions, too, as
follows.
1. Wireless terminals: electronic semicond. (Si)
2. Network servers: electronic semicond. (Si)
3. Data centers: electronic semicond. (Si)
4. Optical-network systems: optical semicond. (nonSi)
5. LED room lights: optical semicond. (non-Si)
6. Flat displays: electronic semicond. with LED lights
7. Solar batteries: optical semicond. (Si & non-Si)
8. Smart-grid networks: electronic semicond. (non-Si)
9. Super-express railway: electronic semicond. (nonSi)
10. Electric Vehicles: electronic semicond. (non-Si)
This list reminds us that these semiconductor
industries include many of those which are strongly

Zetta

Tr・Clk Product

1.E+21
1.E+201.E+19

Primary Energy Supply for generating
the annual total electricity supply in the country (EJ)

Exa

Moore’s magic?

1.E+181.E+171000.01.E+16Peta

1.E+15
1.E+14100.0

1.E+13

系列1

Japan
P.R. China

系列2

Japan

10.0

Tera

India

系列3

EU (15)
UK

系列5

USA

系列6

1.0

World

Pen 4
(2002)

1.E+11

系列4

UK

Core 2 Quad(2007)

1.E+12

1.E+10Giga

Pentium Pro(1996)

1.E+0980386(1985)

1.E+08

系列78086(1978)

1.E+07
1.E+06
1.E+05

0.1
1970

1980

1990

Year

2000

2010

Y. Ueno, February 2010
1.E+04

1960

1970

1980

1990

2000

Fig. 1. 40-year-long increases in international-level energy supplies (a)and indexes of micro-processor performance (b).

2010

2020

expected to save our world-level energy consumptions incoming decades. It reminds us, too, the fact that we are
already relying on several types of non-siliconsemiconductor industries, as well as silicon nanotechnology. For example, the future electronic
semiconductors in systems 8-10 will be non-silicon, thatis, wider-bandgap nitride-semiconductors, since thesehigh-power systems require robust semiconductor
circuits that are tolerant to ultra-large electric currents athigh temperatures. [Most of the active engineers,scientists, and students working in the above technicalareas 1-10 are, by the way, steadily interacting with eachother via international academic societies such as theJapan Society of Applied Physics (JSAP) and theInstitute of Electrical and Electronics Engineers (IEEE)
with “open” attitudes, since almost all of technical areas
1-10 have been covered by these broad-scope societies
since long.]
Optical semiconductors in systems 4-7 are consistingof crystalline arsenide, phosphate, and nitride materials.
After the 40-year-long history of research activities, optoelectronic properties and below-10-GHz responses ofthese optical semiconductors are already somewhat
mature and described well in standard textbooks [1, 2].
Opto-electronic R&D activities for their above-100-GHz
responses are, however, still very immature, and theirexperts exist in only limited number of research institutes
in the world, partially because of the instrumental
difficulties in such ultrahigh-speed characterizations.
In this talk, the author will review recent research
progresses in ultrafast all-optical semiconductor gatesand memories that have been studied and developed byseveral global research institutes, primarily for use in

future optical-network node systems. The author will,then, introduce their recent trial series of estimations, i.e.
what possibilities the potentially 100-times faster serialprocessing micro-processors (e.g. 300-Gb/s, consisting of
all-optical gates and memories) will contribute, to dataprocessing speeds in our information-communication
technology (ICT), with paying attention to their electricenergy consumption levels.

2.

History of electronic micro-processors

In the last 40 years, our world has changed
dramatically. Until 10 years ago, semiconductor devices,
systems, and their heat dissipations used to be notconsuming nor influencing any visible portion of the total
primary energy supply (TPES, from crude oil, coal,natural gas, nuclear powers, wind powers, and few
others) of ours, the mankind. Now in USA, data centers
are consuming 1.5% of its nation-wide electric-energy
supply [3], which corresponds to all outputs from 10
nuclear power reactors. In USA, the amount of primaryenergy supply (40 EJ/year= 40×1018 J/year) for
generating the nation-wide electric energy occupies
almost 40% of USA’s TPES (100 EJ/year) [4].
Figure 1(a) shows the increases over the last 40 years,
in primary energy supplies for generating electricity inselected countries including the above number (40EJ/year from USA), according to the statistics in ref. [4].
The primary energy supply for generating world-wide
electricity has reached 170 EJ/year. Silicon and othersemiconductor materials are playing or about to playroles in almost all electricity-consuming systems such as
computers, communication systems, back-lights ofdisplays, and now room lights. Many of the biggest
impacts to our world in the last 40
were, and those in coming 40 years
will be strongly influenced by
Optical data
semiconductor materials industry
Optical dataand particularly by the performanceof micro-processors.
Optical data
Instead of the number oftransistors which appears to follow
the Gordon Moore’s law [5], Fig.
1(b) in this work shows the productof clock frequency and transistor
number(Tr⋅Clk),
measuredfloating-point
operations
per
Drive energy (electric dc-bias)
second (FLOPS), and measured
Fig. 2. Fundamental elements of optical processors consist
instructions per second ((M)IPS) ofof optical logic gates and optical buffer memories [7-18].
micro-processors in the last 40

Distance in air, z (mm)Intensity (a. u.)

1.5

-10

0

+10

Gated waveform, 1540 nm
100100010111…

(a)

1.0

0.5

0

-75

-50

-25

0

+25

Wavelength (nm)

+20

+50

+75

Time (ps)15520Intensity (dBm/nm)

-20

1550

1548

1546

(b)

-20

550 GHz
-40
-60
193.0

193.5

194.0

Optical frequency (THz)

Fig. 3. 200-Gb/s data waveform generated by our III-V semiconductor optical gate (a)[15], andthe 550-GHz bandwidth of optical clock generated with a similar optical-gate scheme [12].
The ratio Δf/f0 of this bandwidth (550 GHz) to its center frequency (194 THz) is only 0.3%.

years since its birth in 1971. This figure suggests that the
big growth of the order of 1010 in FLOPS is proportional
not to transistor number Tr but to the product Tr⋅Clk. It
suggests next that the factor of 104 (among the factor
1010) came from growth in Clk, while the other, muchlarger factor of 106 came from the Moore’s law (growthin transistor number Tr).
The larger growth factor (106) from Tr than that (104)from Clk is quite symbolic; nearly 40 years we hadalready relied on, and will rely with coming many-coreprocessors on the even furthermore-folded parallelprocessing architectures which consist of command-level,
intra-core-level, intra-processorlevel, intra-sever level,
and intra-network level, rather than the transistor speed.
Many network applications will tolerate with morefolded parallel architectures, such as video streaming, inwhich time delays between packet arrivals and machineloads in packet-reconstruction procedures are acceptedsomehow. Other applications may not tolerate, however[6]. For such parallel-processing-intolerant applications,
it would be very difficult for skilled programmers to keep
the time-to-output efficiency in their tasks, and/or toenhance the system performance per unit energy in the
coming 40 years.
In the above majority R&D direction with developing
more and more cores, the time-domain speed oftransistors seem to completely stop to grow beyond 3
GHz. Instead of speed, the coming big challenges arelocated in the new few generations of ultra-finelithography, and newly in the increasing microwave
losses along the intra-processor microwave-guided
connections, rather than new challenges in transistors

themselves. The microwave-connection-loss issue isinherent and will be enhanced more by the still
expanding parallel-processing architectures. [The modern
MPU’s internal-connection-loss issue is inherently large,
because the bodies of all “electronic data signals” inside
high-speed MPU’s are not at all electron currents but
electro-magnetic waves which are inherently absorbed bymetallic wires themselves and also easily dissipated(radiated)tooutsidestrongly-bending
metal/dielectric/silicon waveguides.]An ultimately alternative direction has been widely
recognized so forth, to be the extremely-quantum signalprocessing system (called as quantum computer) sincenearly 20 years ago. It has been believed, however, totake care of a very limited range of data-processing types
only, such as prime factorizations for breaking RSAencrypted security data in extremely shorter times.

3. 300-GHz optical gates and memories for
future network systems
It seems safer for us in coming 40 years to have an
option other than the above two R&D directions (that is,furthermore-parallel-processing architectures with fixed
speed or new quantum computer). We do have one option,in fact; Near the middle point of the above twoalternative directions, a group of international research
institutes have been studying in the third direction, that is,
optical signal-processing devices and materials [7-18].
In Fig. 2, the elementary functions that this group of
institutes has always been keeping in their minds are
symbolically drawn. More specifically, this group of
institutes has been trying to develop optical de-Optical pulses

Amplified pulsest(a)Electrons(b)

Optical intensity

Holes

(c)

Propagation distance inside SOA,z (μm)

Fig. 4. Top view (a) and physics (b) of semiconductor optical waveguide-amp material, being
coupled with input and output optical fibers. The typical width and depth of amp-waveguidestructures are only 1 × 3 μm 2, while the length of this amp was 1,100 μm.
One of the most interesting subjects is to improve the energy efficiency in the processes of
optically accelerating the gates [20], taking into account the undesirable amp-gain competition
between the two separate-wavelength input components, which had been decreasing theenergy efficiency in conventional schemes (c).

multiplexers, optical wavelength converters, optical 3Rrepeaters, synchronized optical clock sources, opticalbuffer memories, etc. so that these devices and subsystems work in the near-future optical communication
systems. Some of recent research results from ourinstitute (UEC, Tokyo) are shown in Fig. 3. The typical
optical data speed has reached nearly 200-to-300 Gb/s
[11-13, 15]. The typical electric-dc-bias energy
consumption has been improved dramatically to less than
10 pJ/bit [14], from much larger energies 10-to-20 years
ago. The sizes of elementary gates and buffers are, incontrast, apparently much behind (larger than) modern
electronic processors; The typical sizes are presently ofthe order of 500×500 μm2 (before incorporating several
types of nano-photonic wave-guiding techniques inwhich
Si-compatible
electron-beam-lithography
techniques are applied [19]).

4. 300-GHz optical gates and memories for
optical micro-processors in future
Almost all of previous research goals had been
limited to those within optical-communication-node
systems, and therefore the goals had not yet been
expanded to an optical-processor vision, mostly becauseof their spatial sizes and partially because of theirelectric-bias-energy levels which might still look behind
electronic processors. Nontheless, being triggered by theabove-mentioned status of electronic micro-processors,
we have started estimating the potential performance of
optical processors, with taking into account potential

progresses in their energy consumption and size, frommaterial physics viewpoints (i.e., semi-classicallyquantum opto-electronics with III-V semiconductormaterials). Thus estimated potential performance isconsisting of general pros and cons of optical processors.
Depending upon these pros and cons, the group ofparallel-intolerant signal-processing applications will be
specified better, hopefully in collaboration with computer
specialists. This type of activities will support the longterm R&D of all-optical logic-gate devices for use inultrafast network-node systems, as well.

5. More-recent research activities of opticalgates
The amount of dc-electric power consumption Pgate
of a typical, 200-Gb/s-level optical gate is of the order ofPgate = 300 mA × 2.0 V = 600 mW, 70% of which is
occupied by the Joule heat energy discipation from its
internal Ohmic resistor components (which are always
located outside the optically active semiconductor regionof the gate). The dc power consumption for generating
input optical signals will not dominate in the total
consumption. As a consequence, the energy consumption
per one gate unit per one data-bit is derived to be 3 pJ/bit.
The number of continuously injected quantum electrons
is calculated from the amount of dc current to be of theorder of 1 × 107. The amount of electron consumption
and consequently the energy consumption are generally
influenced first by the gate scheme and second by thescheme of optical acceleration [8, 11, 14]. Previous

futureHigher-Hetero(proposal)

presently

0.87 μm
pump laser

presently

Direct-Mod(DR laser)

Bandgap Energy, Eg (eV)

2.5

1.5
1.0

(920 meV)

InGaAs/AlGaAs

all-optical’s

2.0

0.98 μm

GaAs/AlGaInP

1.31 μm1.55 μm

1.31 μm

InGaAs/InP

InGaAsP/InP(550 meV)(400 meV)(760 meV)

AlGaInAs/InP

Larger-density
electrons

920 meV(750 meV)(= kBT× 35)

550 meV*) 伝導帯の状態密度もやや増加する。

0.5
0.0

Fig. 5. Long-term proposal of a new lattice-matched hetero-structure system GaAs/AlGaInP
that will activate exponentially-larger-density excited electrons in all-optical gates (with0.87-μm optical data), and consequently improve both energy efficiency and spatial size of
optical gate units, since this system expands the hetero-barrier energy (ΔEg= ΔEc+ΔEv)
from conventionally 550 meV up to 920 meV (=kB T×35).

research activities from this viewpoint had, however,been relatively poor in related research institutes in theworld. One of the purposes of our recent research [14, 16,
20, 21] is to make the above-mentioned electric-energyconsumption levels clearer and then more efficient, stepby step.
The new original viewpoints, to our knowledge, fromwhich we are or we will be experimentally studying are,for example:
1. Linear-optical-spectrum-synthesis gate scheme
which removes inherent output waveform
distortions more effectively [16].
2. Degenerate-wavelength, polarization-discriminating
scheme which will accelerate the gate more
effectively [20], than conventional two-wavelength
input schemes [11-13, 15] (Fig. 4).
3. To move from conventional active-layer structures(bulk and MQW’s) of semiconductor optical amps,
to one or two new groups of photonic structures fordesigning more energy-saving refractive-index
modulations.
4. To characterize the dependences of the amounts ofboth refractive-index- and gain-modulations on the
optical-data’s wavelength (i.e., optical frequency)

5.

[21], when those optical data are input to a group of
semiconductor structures at applicable ranges ofelectron-excitation levels, particularly forsupporting the above viewpoints 2 and 3.
To move from the conventional InGaAs-core,
InGaAsP-clad system (with 1.55-μm-wavelength
optical data), to the new GaAs-core, AlGaInP-cladstill-lattice-matched system (with photonic-interconnection-compatible 0.87-μm-wavelength data),
for improving both energy efficiency and gate size,
furthermore (Fig. 5).

In this talk, with introducing the [speed, energy, andsize] dimensions of all-optical gates and memories underresearch, we will present an outline of the abovementioned computational performance estimations
towards mankind-first “300-GHz optical micro-processor
units.” Any sudden ideas, weird ideas, or fundamental
questions are always welcome during and after the talk.

Acknowledgments
We thank Dr. Naoya Wada, Dr. Satoshi Shinada, andtheir co-workers in the National Institutes of Information
and Communications Technology (NICT), Koganei city,

Tokyo, Japan, for their long good collaboration with us in
UEC since early 2008.

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