3rd International Symposium on Advanced Information andCommunication Technology (AICT 2008),
October 7-9, 2008,
Information and Communication Univ. (ICU), Daejeon, Korea.

Recent trends in the 200-Gb/s-class, 10-pJ/bit, compact,
all-optical semiconductor gate devices research
Yoshiyasu Ueno
University of Electro-Communications (UEC), Department of Electronic Engineering
1-5-1, Chofugaoka, Chofu city, Tokyo 182-8585, Japan
Tel: +81-42-443-5807, e-mail: ueno@ ee.uec.ac.jp, our laboratory’s website: http://www.ultrafast.ee.uec.ac.jp/Abstract
After outlining our research backgrounds such as therecent growths in the numbers of optical-fiber-LAN
subscribers in its leading countries Japan and Korea, wereview recent results about the speed and energy
consumption of ultrafast all-optical semiconductor
devices (and about a few optical buffer memories fromother institutes) under research, for use in ultrafast
large-capacity optical network-node systems in the nearfuture. One of the typical amount of dc-electric energy
consumption has turned out to be about 5 pJ/bit in the200-Gb/s all-optical gating operation, from oursystematically characterized results with using existingIII-V-semiconductor materials and devices from Japan,EU, and USA. Our standard experimental techniques forcharacterizing such semiconductor gates with precisely
handling 200-Gb/s optical data signals are quickly
introduced, too.

On the other hands in the world, the energy
consumption of the numbers of large-scale ultrafast
routers has started to appear as a bigger and bigger
bottle-neck. One of the symbolic examples is the Cisco’s
new high-end router (Fig. 2) released in 2006-2007,
which takes care of maximum 1,100 lines of optical 40Gb/s signals (i.e., nominal through-put of 92-Tb/s). Thissystem occupies about 70 of 2-meter-high heavy racks,
and then consumes 1-MW electricity (and maybe evenmore electricity for air-conditioning its machine room).4
The amount of electricity which Marconi was consumingfor his transmitting his Morse-code signals across theOcean in 1901 was, for comparison, supposed to be sometens of kW.

1. Introduction

Fig. 2: A part of the Cisco’s
92-Tb/s high-end router, CRS-1 (Ref. 4).
These two needs in front of us, that is, to enhance
the capacity of network systems and to drop their energy
consumptions will become stronger in the coming 10years. With taking into account this on-going
technological vision, several research institutes (most ofwhich are located in Japan and Europe) have started
intensively studying several groups of optical-signalprocessing semiconductor devices (i.e., ultra-short clockand-data generators, de-multiplexers, signal-wavelength
converters, XOR gates, 3R regenerators, buffer
memories, etc.) so that the extending research results willrealize (a) faster, (b) lower-energy-consumption, and also(c) highly integrated network systems.
The transparent-transmission bandwidth of therecent standard silica-fiber cables (“wires”) have already

1 Billion
109?

100 Million
108

Subscriber numbers

Subscriber numbers

It was December 1901 when Mr. Guglielmo Marconi
and his colleagues succeeded in their transmit-andreceiving the mankind-first, wireless, binary-digital
signals across the Atlantic Ocean.1 In the last 107 years
since that month, several big achievements from theelectric-and-electronic materials, devices, and systems
research have expanded our communication technology,
and they are supporting the latest world-wide infrastructures of ours, 24 hours a day.
Recently, the demands in network systems’ capacity
and speeds are globally rising up. For instance, thenumber of optical-fiber-link subscribers in Japan has
exceeded 10 millions in 2007 (Fig. 1).2 In Korea andJapan, the numbers of optical-fiber-link subscribers per
100 inhabitants have reached nearly ten.3 The opticalnetwork capacities between wireless base stations havecontinuously been strengthened by our industry, too.

10 Million
107?

1 Million
106?
FTTH (100M)
DSL (10M)

100 thousand5

10Consumer marketin JapanCATV (8M)
MOBILE (200k)

10 thousand
104
2002/03

03/ 2002

03/ 2005

2008/03

03/ 2008

Month/Year

Fig. 1: Numbers of broadband subscribers in Japan.2

1characterize them in addition to limited numbers ofcommercially-available butterfly-packaged SOA’s.9, 10
Figure 5 shows a schematic view of the mostfundamental all-optical-gate structure which we havecurrently been focusing on. Even though its number of
all-optical functionalities in practical network-node
systems has been supposed to be relatively limited(wavelength conversion,5-8 optical 2R regeneration, etc.),
this fundamental gate scheme has been playing the mostpowerful role in our design-and-characterization studies
on these ultrafast semiconductor materials and devices.
We believe that this will be one of the best gate schemes
to further study fundamental principles of ultrafast
optical materials and devices.

reached a bandwidth wider than 100 THz as long astransmission distances of 10-20 km are concerned. These
super-high-quality optical cables, working passively as
ultrabroad-band communication waveguides, are alreadydeveloped very well, very thin, and their operations needrelatively little energy consumptions and fabrication
costs, as has been known well. Even in the latest network
systems, only 0.01-to-1 percentage (10-to-1,000 Gb/s) of
the full 100-THz bandwidth per one connected-cable isefficiently utilized, being currently limited by theexisting optical-and-electronic devices. What matters themost in the coming 10 years will be what kind of signal
processors we will develop and start replacing thepresent network systems with that innovative technology.

(b)

(a)
Holes
Optical pulses

Fig. 3: Transparent transmission bandwidth (100 THz)of standard silica-fiber cables.

Amplified pulsest(c)Electrons(d)Fig. 4: Research-level SOA chips which are placed
inside our all-optical gates and circuits. (a): microscope
photo, (b) and (c): lensed-fiber-connections to the chip,(d) ultrafast photon-electron interactions inside theSOA’s which drive our all-optical gates.

2. Speed and energy consumption of our
fundamental optical signal-processors
In this talk, I will outline our recent studies on thespeed and energy consumption of our all-opticalsemiconductor gates, for use as signal-processing units infuture network-node systems.
In our research group in the Univ. of ElectroCommunications, we are not crystal-growing nor
fabricating semiconductor-crystal chip structures. Everyyear, instead, we (the grad- and undergrad-students) arebuilding (a) proto-type all-optical semiconductor gatesand some “circuits,” (b) ultrahigh-frequency clock- anddata-signal sources, (c) ultrahigh-resolution optical
waveform- and spectrum-monitoring systems, (d) andother important experimental devices.
The semiconductor optical amplifier (SOA), in which
the ultrafast and extremely efficient photon-electroninteractions take place, is always the most importantmaterial in our building our all-optical gates.5-8 In 20042006, for example, we discussed with and received
several series of partially-customized research-level SOA
chips from three leading institutes in Japan, EU, andUSA, respectively, so that we can experimentally

SMZ-DISC方式

λ=λ1
λ=λ2

SOA

λ=λ2

local cw light

λ= λ2

MZI, ΔtFig. 5: Schematic view of the most fundamental alloptical-gate structure.5-82

mode-locked fiber laser.
Fig. 7 shows one of the experimental results that weachieved in May, 2008.10, 11 Fig. 7(a) shows a part of the200-Gb/s, 5,000-bit-long, binary-encoded data signal,generated in a manner similar to those in Fig. 6. We input
this data signal to one of our home-made all-opticalsemiconductor gate which was schematically shown inFig. 5. Figure 7(b) is the binary data signal measured at
the output of the semiconductor gate. The measured
waveform in Fig. 7(b) indicates that the speed of thesemiconductor gate we used in this experiment is as fast
as the speed of the data signal (200 Gb/s). From ourexperimental characterizations and analyses, we haverecently understood that the speed of the semiconductor
gate was strongly accelerated from 10-20 GHz up to 200
GHz up to now by three (i.e., one old and two new)significant physical factors.7, 10, 11 Due to ourunderstandings of these three physical factors, the qualityof the gated 200-Gb/s data-signal waveform was
dramatically improved from the previous poor one(which is not shown), to the significantly better one inFig. 7(b).

(a)(b)(c)
Fig. 6: Ultrafast 200-Gb/s optical-data signal,
Experimentally generated in our lab.
(a) Measured binary-encoded waveforms, (b) block
diagram of our home-made data-signal source andall-optical waveform-monitoring system, (c) part ofour experimental setup.10, 11
Figure 6(a) shows one of the typical binary-encoded
1.55-μm-band, 200-Gb/s optical data signal’s waveforms,
experimentally generated in our laboratory.10, 11 The
distance between the adjacent two bits is adjusted to 5 ps,
the width of each one-bit pulse is approximately 1 ps,
and the length of encoded data pattern is 5,000 bits. The
1/16 seed pulses of this ultrahigh-frequency data signal isgenerated by a commercial, research-level mode-locked
fiber laser and an adiabatic-soliton compressor, operating
at the base frequency of 12.5 GHz in our laboratory. We
are building and improving our ultrafast waveformmonitoring system. The high-precision, high-resolution
all-optical waveform-monitoring system (Figs. 6(b) and6(c)) was developed in our laboratory, too, and used inmeasuring the 200-Gb/s data signals such as those in Fig.
6(a).10, 11 This monitoring system is sub-harmonicallyrunning at the 1:5,000 frequency (40 MHz) with respect
to the data frequency (200 GHz). In the original schemeof ours in Fig. 6(c), the optical reference pulses requiredin the scheme are synchronously triggered by a train ofshort pulses from a commercial, research-level 40-MHz

Fig. 7: 200-Gb/s optical-data signals.
(a) Data signal (λ= 1560 nm), measured at the input ofan all-optical gate, and, (b) wavelength-converted
data signal (λ= 1540 nm), measured at the output of
the all-optical gate.10, 11
The other important subject, the energy consumptionof the all-optical semiconductor gate, has been studied asa function of the all-optical gating speed, along with our3

104

measuredB#4 1100 μm
B#3 500 μm

103(a)

102
10

100

1000

Bandwidth, B (GHz)
nonlinear semiconductors
nonlinear silica fibers>100 pJ/bit

1 nanono-e-o conversion

coo- nve
e- no c tio
on nave lrsioElectric-energy consumption,EB (Joule/bit)

Electric dc power consumption, POP(mW, @ΔΦNL=0.3 π)above-mentioned 200-Gb/s gating experiments. The dcelectric energy consumption, which is originated fromthe dc-electric-bias-current injection into the SOA chip,
has turned out to increase linearly with the opticallyaccelerated gating speed (Fig. 8(a)).9, 11 The typicalnumbers we obtained with the latest SOA-chip design
were; 20-pJ/bit consumption in 200-Gb/s gating and 10pJ/bit consumption in 100-Gb/s gating. When weadopted one of the two new acceleration factors to theall-optical gate, the energy consumption dropped from
20-pJ/bit to below 5-pJ/bit in the above-mentioned latest
200-Gb/s gating experiment in Fig. 7(b). This reduced
level of dc-electric energy consumption approximately
matches to that in the world-record-fast 320-Gb/s
wavelength conversion experiment reported by Y. Liu, etal., from the Netherlands in Ref. 8. (Approximately
2×107 quantum electron-hole pairs are consumed per onebit.)
These levels of energy consumption that we haverecently induced from our experimental research resultswith existing SOA chips seem to us to be not bad, as we
had been expecting before them. First, the electricenergy-consumption levels of optical buffer memories,
under separately on-going global research, are close to
the above levels. The energy consumption level of a
micro-ring memory reported by M.T. Hill, et al. in 2004,
was 1 pJ/bit in its 25-Gb/s operation.12 That of bistable
VCSEL memory, reported by H. Kawaguchi, et al. in2006, was 3 pJ/bit in its 10-Gb/s operation.13 That ofbistable PC/QD-SMZ-switch memory is expected to beless than 1 pJ/bit in its 10-to-40 Gb/s operation.14 Second,the energy consumption of Cisco’s CRS-1 router is 30 nJ
per one-bit routing. That of the latest computer-CPU
processors from Intel and AMD is 1 nJ per one
instruction.15 Beyond the existing practical applications
in large-scale ultrafast network-node systems, depending
upon the on-going research progresses in terms of theenergy consumption of all-optical gates and buffer
memories, researchers will finally be able in the near
future to fabricate and demonstrate the mankind-first
integrated optical-signal-processor chip in a manner
similar to the historical Intel 4004 chip (Fig. 9) but, withperformance 100 times faster than the latest Intel chip
and without increasing its energy consumption.

？

1 pico100M

1G10GOp
ticalw(le ith n signg on na
th= lin l-p
1 m ear roc
5～30pJ/bit, H sili ess
NL ca- ing
Optical signal-processing
F fibeγ = rswith nonlinear semiconductors
30W -1(length << 1 mm)
⋅k m

100G

1T

Bit-rate, BR (b/s)

10T(b)-1)

Fig. 8: Energy consumption of the all-opticalsemiconductor gate as a function of
the optically-accelerated gating speed.
(a) Measured dc-electric power consumptions,(b) comparisons with the alternative ultrafast devices,that is, the research-level ultrafast o-e-o devices
and the research-level highly-nonlinear-fiber(HNLF) devices

Fig. 9: Mankind-first integrated signal-processor chip,Intel 4004, released in 1971.
Clock frequency was 500 kHz. The energy consumption
is unfortunately unclear. On the 3×4 mm2 surface of asilicon chip, 2,300 transistor gates was integrated. 16 of
4-bit-long resister memories were included.

3. Other on-going topics
Within the range of our laboratory’s on-going
research activities, we have been exploring one of thepotential applications of our all-optical gates, that is,experimental studies of ultrafast optical clock-pulse
sources with using the all-optical gates.16-18 Figure 10
shows one of the most recently achieved results.18 In thisseries of experiments, a stable train of 10-GHz-repetition,2-ps-long, single-longitudinal-mode, nearly-Fouriertransform-limited optical-clock-pulses was generated

4

from our original ring-laser-cavity scheme, whichcontains the all-optical semiconductor gate structure in amanner very similar to that in Fig. 5.
Separately from this talk, two other talks aboutclosely related on-going research results will be given by
their respective graduate students, in this year’s
symposium on Advanced Information andCommunication Technology (AICT).19, 20

10.

11.

12.

13.

Fig. 10:
10-GHz, 2-ps, nearly-Fourier-transformlimited mode-locked pulses generated by our originalring-laser-cavity scheme that contains the all-opticalsemiconductor gate.16-18

14.

References
1.
2.

3.
4.
5.

6.

7.

8.

9.

Biography and Nobel lecture text of Guglielmo Marconi,
Nobel Foundation , http://nobelprize.org/
Statistical data released by the Telecommunications
Bureau (http://www.tele.soumu.go.jp/e/), the Ministry ofInternal Affairs and Communications, Japan, on March 18,2008.
Statistical data from the Organization for Economic Cooperation
andDevelopment(OECD),
http://www.oecd.org/sti/ict/broadband/
Product brochure of the carrier routing system (CRS-1),
October 2006, Cisco Systems, Inc., San Jose, CA, USA.
Yoshiyasu Ueno, Shigeru Nakamura, and Kazuhito Tajima,"Record low-power all-optical semiconductor switch
operation at ultrafast repetition rates above the carrier
cutoff frequency," Opt. Lett. vol. 23, no. 23, pp. 18461848, Dec. 1998.
Y. Ueno, S. Nakamura, and K. Tajima, 'Nonlinear phaseshifts induced by semiconductor optical amplifiers withcontrol pulses at repetition frequencies in the 40-160 GHz
range for use in ultrahigh-speed all-optical signalprocessing,' J. Opt. Soc. Am. vol. B19, no. 11, pp. 25732589, Nov. 2002.
M.L. Nielsen, J. Mørk, R. Suzuki, J. Sakaguchi, and Y.
Ueno, 'Experimental and theoretical investigation of theimpact of ultra-fast carrier dynamics on high-speed SOAbased all-optical switches,' Optics Express, vol. 14, no. 1,pp. 331-347, Jan. 2006.
Y. Liu, E. Tangdiongga, Z. Li, Huug de Waardt, A. M. J.
Koonen, G. D. Khoe, Xuewen Shu, Ian Bennion, and H. J.
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Jun Sakaguchi, Ferran Salleras, Kohsuke Nishimura, andYoshiyasu Ueno, "Frequency-dependent electric dc powerconsumption model including quantum-conversion

15.
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18.

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20.

5

efficiencies in ultrafast all-optical semiconductor gates,"
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Jun Sakaguchi, Takehiro Nishida, and Yoshiyasu Ueno,"200-Gb/s wavelength conversion using a delayedinterference all-optical semiconductor gate assisted by
nonlinear polarization rotation," submitted to Opt. Comm.,
Elsevier, the Netherlands, August 2008.
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all-optical semiconductor gates towards low-powerconsumption operation,” PhD thesis (in English), Univ.
Electro-Communications, June 2008.
Martin T. Hill, Harmen J.S. Dorren, Tjibbe de Vries,Xaveer J.M. Leijtens, Jan H. den Besten, Barry
Smalbrugge, Yok-Siang. Oei, Hans Binsma, Giok-Djan
Khoe, and Meint Smit, “A fast low-power optical memory
based on coupled micro-ring lasers,” Nature vol. 432, pp.
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polarization bistable VCSEL,” CLEO/QELS 2006, May21-26, 2006, Long beach, USA, paper no. CWG7.
Kiyoshi Asakawa, Yoshimasa Sugimoto, Yoshinori
Watanabe, Nobuhiko Ozaki, Akio Mizutani, Yoshiaki
Takata, Yoshinori Kitagawa, Hitoshi Ishikawa, Naoki
Ikeda, Koichi Awazu, Xiaomin Wang, Akira Watanabe,
Shigeru Nakamura, Shunsuke Ohkouchi, Kuon Inoue,
Martin Kristensen, Ole Sigmund, Peter I. Borel, and Roel
Baets, “Photonic crystal and quantum dot technologies for
all-optical switch and logic devices,” New J. Physics, vol.
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Philip E. Ross, “Why CPU frequency stalled,” the data,IEEE Spectrum, April 2008 issue, p. 52.
R. Suzuki, T. Ohira, J. Sakaguchi, and Y. Ueno, '40-GHz
mode-locked pulse generation with a new scheme of SOAbased pulse generation,' CLEO/QELS 2006, May 21-26,
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R. Nakamoto, R. Suzuki, T. Ohira, J. Sakaguchi, and Y.
Ueno, "160-GHz-class optical-clock circuit withsemiconductor optical amplifiers,' AICT symposium,September 17-19, 2007, BUPT, Beijing, China, student
session.
Ryoichi Nakamoto, Hiroyuki Takeuchi, Jun Sakaguchi,and Yoshiyasu Ueno, "1.55-um, mode-locked, singlelongitudinal-mode, 10-GHz, 2-ps, ultra-short optical pulse
train from our original semiconductor-based pulse-source
scheme," Topical Conference on Nanophotonics (NANO),
Optical Society of America, May 26-29, 2008, Southeast
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Hiroyuki Takeuchi, Ryoichi Nakamoto, Jun Sakaguchi,
Takashi Ohira, and Yoshiyasu Ueno, "Physical factors thatdefine the preciseness required in our original,
semiconductor-laser cavity scheme for generating 5picosecond, 40-GHz, optical clock pulses," AICTsymposium, October 7-9, 2008, ICU, Daejeon, Korea,student session.
Takehiro Nishida, Ryoichi Nakamoto, Jun Sakaguchi,
Satoshi Shinada, Naoya Wada, and Yoshiyasu Ueno,"Experimental
researchofa300-Gb/s-class,semiconductor-based all-optical gate which contains an
optical-spectrum’s amplitude-and-phase synthesizer,"
AICT symposium, October 7-9, 2008, ICU, Daejeon,Korea, student session.