COMMISSION D: Electronics and Photonics (November 2007 October 2010)
Edited by Tadao Nagatsuma, Osaka University

Electronics and Photonics cover huge scientific and technological fields. Among them, in thisreport, we have selected several important topical areas, where in particular Japanese leading
researchers have actively contributed between 2007 and 2010. They overviewed recent
remarkable advances in their specialized areas.
The following is a list of areas that the chapter editor has taken up.
D1. Metamaterials
E. SanoHokkaido University
D2. Quantum Cascade Lasers
K. Kasahara
Ritsumeikan University
D3. Applications of THz Quantum Cascade Lasers
I. Hosako
National Institute of Information and Communications Technology
D4. Nonlinear Optical Waveguide for THz Tomography
K. Kawase
Nagaya University
D5. Photonic Millimeter-Wave Technology
H. ItoKitasato University
D6. Plasmonic Device Technology for Emission and Detection of THz Radiation
T. Otsuji
Tohoku University
D7. Integrated Josephson Voltage Standard Technology
S. Kohjiro
National Institute of Advanced Industrial Science and Technology
D8. High Capacity Optical Fiber Transmission Systems
H. TodaDoshisha University
D9. Radio-on-Fiber Technologies for Heterogeneous Wireless Services
K. Tsukamoto
Osaka University
D10. Broadband Access Networks with Wired and Wireless Integration/Convergence
K. Iwatsuki
NTT
D11. Body Area Network
Y. KadoKyoto Institute of Technology

D.1 Metamaterials
Eiichi Sano
Hokkaido University
Recently, there has been significant interest in researching and developing wirelesscommunication circuits for 3.1-10.6 GHz ultra-wideband, 24-GHz band, and 60-GHz band
applications. Modern CMOS technologies enable high-level integration of RF, physical layer(PHY), and medium access control (MAC) layer processors, thus reducing the size and cost of
wireless equipment. In recent years, several extensive attempts have been made to fabricate
antennas on Si substrates for inter-chip or intra-chip transmissions. However, a high-resistivity
float zone (FZ) and proton-bombarded Si substrate of more than 5 kWcm have been used tofabricate high-gain antennas. This is because the low-resistivity Si substrates commonly used in
CMOS processes degrade the antenna gain. When standard 10-Wcm substrates are used,transmission gains range from -50 to -60 dB for transmissions over a few centimeters. Thus, aningenious development is needed to increase the gains of antennas fabricated on low-resistivity
Si substrates.
An artificial dielectric layer (ADL) produces an extremely high permittivity. An ADL has
been developed to shield coplanar waveguides (CPWs) from Si substrate. Figure 1 shows aschematic of the ADL. Two identical metal patch arrays are formed on the two metal layers.
One patch array is shifted by one-half cell pitch to the other patch array. An insulating layer
with a dielectric constant ed and a thickness td is sandwiched between the two metal layers. The
dielectric constant along the surface of the ADL is given bye // = e d

A
tt d

(1)

where A is a quarter the area of a square metal patch and t is the total thickness of the ADL.
As found in Eq. 1, e// is increased by decreasing the layer thickness t and td and/or byincreasing the patch area. Fig.ure 2 shows potential distributions demonstrating the shielding
effect of ADL. The CPWs with and without ADLs were simulated with a finite-differencetime-domain (FDTD) electromagnetic simulator. The ADL was modeled as a uniform sheetwith e// of 104. The signal source was sine waves of 10 GHz. As shown in Fig. 2, the electricfield was terminated in the ADL. When an electromagnetic wave illuminates an ADL with highe// placed above a Si substrate, the electric field strength in the Si substrate reduces more thanwhen no ADL is on the Si substrate. Placing an ADL between the antenna and Si substrate is
expected to increases the antenna gain.
Three kinds of antennas were designed and fabricated: an antenna with ADL shielding, anantenna without ADL fabricated on a low-resistivity (~10 Wcm) Si substrate, and an antenna
without ADL fabricated on a high-resistivity (>1 kWcm) Si substrate. Figure 3 is a
microphotograph of fabricated antenna with the ADL. Figure 4 shows the measured andsimulated antenna gains. At 9 GHz, the ADL improved the antenna gain by 3-dB more than theantenna on the low-resistivity Si substrate. This is the higher gain ever archived for the antennas
fabricated on a low-resistivity Si substrate operating at around 9 GHz. Figure 5 compares theantenna gains achieved in this work with those reported in previous literature. Further
improvement will be possible by reducing the dielectric layer thickness to increase dielectric
constant and reducing the loss of poly-Si layer.

Fig.1. Schematic of artificial dielectric layer. (a) Top view. (b) Schematic eddy current flows forpatches with and without slots. (c) Cross-sectional view.

Electromagnetic field response for CPW (a) with ADL and (b) without ADL.

Fig. 3. Microphotograph of fabricated antenna with ADL.

0Gain (dB)

Fig. 2.

Symbol: measurementsLine:
simulation-5
High resistivity
ADL w/o loss
-10
ADL w loss
-15
-20
-25
-30

Low resistivity
7

8

9

10

11

Frequency (GHz)
Fig. 4. Comparison between measured and simulated antenna gains.

Fig. 5.. Comparison of fabricated antenna gains with those reported in the literatureliterature.

References
rtificial dielectric layer,”layer,[1] K. Takahagi and E. Sano, “High-gain silicon on-chip antenna with artificial
accepted for publication in IEEE Trans. Antenna & Propagation
Propagation.
[2] K. Takahagi and E. Sano, “High-gain silicon on-chip
chip antenna with artificial dielectric layer,”layer,
Proc. 2010 Asia-PacificPacific Radio Science Conf.
Conf.., paper DC1-5, 2010.
[3] M. Ikebe, D. Ueo, K. Takahagi, M. Ohuno, Y. Takada, and E. Sano, “AA 3.1
3.1-10.6 GHz RF
CMOS circuits monolithically integrated with dipole antenna,
antenna,” Proc. 2009 IEEE Int'l Conf.
Electronics Circuits and Systems , pp. 17-20, 2009.
[4] K. Takahagi, M. Ohno, M. Ikebe, and E. Sano, “Ultra-wideband
wideband silicon on-chiponantennaswith artificial dielectric layer
layer”, Proc. 2009 Int'l Sympo. Intelligent Signal Processing andCommunication Systems,, pp. 81
81-84, 2009.

D2. Quantum Cascade Lasers
K. Kasahara
Ritsumeikan University

Since they first appeared, the operation frequency of quantum cascade lasers (QCLs) has been

extended from the mid-infrared
infrared to the far
far-infrared
infrared range, and QCLs have become the mostdesirable tool for such applications as trace gas analysis owing to the continuous tunability of
their emission range. The progress of QCLs has been achieved by focusing on the active region
designs that make full use of wave function engineering. A continuous wave (cw) operation up
to above 60 °C has been reported for 7.9 mm InGaAs/InAlAs QCLs with single phonon
resonance-continuum (SPC) depopulation, exhibiting a threshold current density of 2.23 kA/cm2
and emitting a cw output power of 36 mW at 30 °C [1, 2]. In the SPC depopulation structure,the lower lasing state of the active region is designed to be separated by one phonon energy
from the top state of the miniband, enabling fast electron depopulation from the lower lasingstate and subsequent fast electron extraction. This structure can alleviate the problem caused byfluctuations of the energy separation between the lower states, which is usually observed with asingle phonon depopulation three quantum well-used design, leading to a successful commercial
mass production of QCLs.

Fig. 1. Schematic illustration of the proposed IDP scheme [3]

(a)

(b)

Fig. 2. (a) Schematic conduction band diagram and moduli squared of the relevant wave functions.
(b) Pulsed current-light output characteristics of the 8-mm-wide and 3-mm-long HR- coated buried
heterostructure QCL [5].

High stability for temperature changes has been demonstrated with 8 μm QCLs introducing
the indirect pump (IDP) [3]. Threshold current densities were 2.7 and 3.3 kA/cm2 for cavity
lengths of 4 and 1.5 mm, respectively. Respective characteristic temperatures T0 were as high as
243 and 303 K around room temperature. Maximum output powers were 362 and 50 mW.
T0-value of 303 K was the highest record ever reported with QCLs. In the IDP, electrons are
injected into the intermediate state, level 4 via electron tunneling from the ground state, level 1’
in the previous injector and then, are relaxed quickly down to the upper laser state, level 3 byLO-phonon emissions (Fig. 1). In this circumstance, the electron population n4 at level 4 is
expected to be low, and the backward tunneling current ( µ n4) is kept to be lower than theforward tunneling current ( µ n1) even when the electron population n3 at level 3 approaches ahigh value comparable to the injector doping density. High device performance of

InGaAs/InAlAs Fabry–Pérot QCLs based on the indirect pump scheme was also demonstrated
at 15 mm [4]. A low threshold-current-density of 3.5 kA/cm2, a high maximum output power of
216 mW, and a high slope efficiency of 346 mW/A, were obtained all at room temperature. The
observed extremely high T0 of 450 K over wide temperature range, 320–380 K was ascribed to astrong suppression of electron populations in injectors.
To generate new opportunities for broad gain band applications, homogeneous broad-gain
QCLs based on dual-upper-state design have been demonstrated at 8.4 mm [5]. The devices
exhibited a homogeneously wide electroluminescence spectrum of 330 cm−1 of which shapes
were insensitive to voltage changes as well as extremely high temperature insensitivity of thelaser. The strongly coupled two upper laser states and the lower laser state were associated withthe wave functions [Fig. 2 (a)]. The electron populations in the both upper laser states were kept
to be equal, because of the small energy gap between the two upper states of 20 meV. The
QCLs had a homogeneously broadened gain spectrum due not only to the appropriate energy
difference between the two states but also to equal oscillator strengths of the transitions from theboth upper laser states to the lower common laser state. A buried heterostructure was used inthem, and a high cw output power of 152 mW together with a high constant slope efficiency of
518 W/A at room temperature was obtained. In addition, the device performance was insensitive
to temperature change; T0-values of 306 K and constant slope efficiency over the wide
temperature range, 280–400 K [Fig. 2 (b)].

1

1

0 .5

b

a, b

00
-0 .5

-1

a

-1

-1 .5

-2

-2

-2 .5

400

500
Current (mA)

600

Fig. 3. a dependences on the current. The parameter, b = C/(1+a2)1/2, was also plotted in the figure.
b should not vary even if the current varies because it is determined only by such parameters as thereflectivities of QCL facet and the external mirror. b was ~0.35 throughout the measurements [6].

The linewidths of QCLs are broadened by the a-factor, as is the case of conventional
semiconductor laser diodes (LDs). Typical values for interband-using LDs lie in the range a =2-7. By contrast, it had been predicted that QCLs would have an a of zero as a result ofintersubband symmetric gains. However, deviation from zero is possible. The a-factor is closely
connected to the QCL gain profile and provides information on the shape of the gain.
Investigating the QCL’s a-factor, therefore, is of considerable importance, offering clues about
the physical parameters relevant to the gain, such as the electron distribution in the subbands.
Until now, however, only a limited number of experimental studies on the a-factor of the QCL

have been undertaken, particularly as to its current dependence. The a -factor of a commerciallyavailable 6.1 mm InGaAs/InAlAs QCLs with an SPC structure capable of cw operation with a
Peltier cooler, and its current dependence were investigated in detail [6]. The a was tended toincrease slowly with an increase in current, and was in the range of –1.8 to –1.7 (Fig. 3). The a,probably affected by detuning, was still smaller than that of the conventional 1.55 mm
semiconductor LDs.
Terahertz (THz) QCLs are expected to find applications in such areas as bio-sensing andsecurity in addition to environmental monitoring. To date, the highest recorded operating
temperatures for THz QCLs are 178 K in pulsed mode and 117 K in cw mode. To aim at theachievement of THz QCLs operating at higher temperatures, four-level system used QCLs has
been proposed [7]. It was found that both a lower threshold current and higher slope
efficiency can be realized in the four-level system compared with the conventional three-levelsystem. Furthermore, it was found that the four-level system has a higher operating
temperature tolerance than does the three level system. These results are expected to lead to thedevelopment of THz QCLs that can be operated at high temperatures.
References[1] K. Fujita,_S. Furuta, A. Sugiyama, T. Ochiai, T. Edamura, N. Akikusa, M. Yamanishi, and
H. Kan, “Room temperature, continuous-wave operation of quantum cascade lasers with single
phonon resonance-continuum depopulation structures grown by metal organic vapor-phase
epitaxy”, Appl. Phys. Lett., Vol. 91, 141121 (2007).
[2] K. Fujita, S. Furuta, A. Sugiyama, T. Ochiai, T. Edamura, N. Akikusa, M.Yamanishi, and H.
Kan, “High-Performance l~8.6 mm quantum cascade lasers with single phonon-continuum
depopulation structures”, IEEE J. Quantum Electron., Vol. 46, pp. 683-688 (2010).
[3] M. Yamanishi, K. Fujita, T. Edamura, and H. Kan, “Indirect pump scheme for quantum
cascade lasers: dynamics of electron-transport and very high T0-values”, Opt. Express, Vol. 16,
pp. 20748-20758 (2008).
[4] K. Fujita, M. Yamanishi, T. Edamura, A. Sugiyama, and S. Furuta, “Extremely highT0-values

(~450

K)

of

long-wavelength

(~15

mm),

low-threshold-current-density

quantum-cascade lasers based on the indirect pump scheme”, Appl. Phys. Lett., Vol. 97, 201109(2010).
[5] K. Fujita, T. Edamura, S. Furuta, and M. Yamanishi, “High-performance, homogeneous
broad-gain quantum cascade lasers based on dual-upper-state design”, Appl. Phys. Lett., Vol. 96,
241107 (2010).
[6] M. Ishihara, T. Morimoto, S. Furuta, K. Kasahara, N. Akikusa, K. Fujita and T. Edamura,“Linewidth Enhancement Factor of Quantum Cascade Lasers with a Single Phonon
Resonance-Continuum Depopulation Structure on a Peltier Cooler”, Electron. Lett., Vol. 45, pp.
1168-1169 (2009).
[7] N. Sekine and I. Hosako, “Design and analysis of high-performance terahertz quantum
cascade lasers”, Phys. Status Solidi C 6, pp. 1428–1431 (2009).

D3. Applications of THz Quantum Cascade Lasers
I. Hosako
National Institute of Information and Communications Technology
Since the terahertz quantum cascade laser (THz-QCL) has firstly demonstrated [1], many
people have been investigated and improved its performances. And some had demonstrated
imaging (THz-QCL as an illuminator) [2], local oscillator (for precision spectral sensing
purpose) [3] and even wireless communication [4] using the THz-QCL. In those demonstrations,almost THz-QCLs were operated in cryogenic temperature conditions below 77K (Liquid
nitrogen temperature) to achieve sufficient output power. For a cooling down to below 77K,
liquid helium or large mechanical coolers must be required. And those cooling equipments areusually bulky and expensive. The requirements of those cooling equipments limit a practicalityof the THz-QCL, even though lots of kinds of demonstration experiments have so far
conducted.
The recorded operating temperatures of the QCL (THz-QCLs and Mid IR-QCLs) aresummarized in Figure 1. In the THz-frequency range, GaAs/AlGaAs-material system is mainly
used for the QCL. On the other hand, InGaAs/InAlAs/InP-material system is mainly used for
the Mid-IR QCLs. The frequency gap between 5 THz and 10 THz is caused by strong phonon
absorption of GaAs/AlGaAs or InGaAs/InAlAs/InP-material systems. The highest operating
temperature of the THz-QCL was recorded as 186K at 3THz based on a resonant LO-phonon
type active structure and a double-metal waveguide structure [5]. The room temperature (evenin the thermoelectric cooling temperature) operation of the THz-QCLs has not yet achieved,
even though there is no theoretical limitations. At this moment, the most practical way to cool
down the THz-QCL is to use the liquid nitrogen for “real” applications.
3000 1500

3hn > kBT

hn < kBT
kB T

400

15 10 6hn =Operating Heat-Sink Temp. (K)500

Wavelength (mm)600 300 150 100 60 30

300
250 K; Peltier (DT=50K)

200
100

77 K; LN200.1

4 K; LHe

1
10Frequency (THz)100

Fig. 1. Operating temperatures of the QCLs are summarized.

NEC Guidance and Electro-Optics Division and National Institute of Information andCommunications Technology (NICT) have developed a real-time terahertz imaging systemconsisting of the liquid nitrogen cooled THz-QCL illuminator and uncooled microbolometer
array detector[7]. As applications of the their real-time terahertz imaging system, a stand-off
imaging system that could be useful in a fire disaster relief and a label-free bio-materials
detection system have developed and demonstrated.

Fig. 2. Basic structure of micro-bolometer pixel.

Fig. 3. Schematic view of simulated fire experiment site. Burning test room was filled with flameand black smoke when the urethane blocks burned.

Liquid nitrogen cooled compact THz-QCL system has been developed for the external lightsource. By that system, the average output of a few micro watts can be obtained at 3.1 THz. The
emission power is large enough for the improved un-cooled micro-bolometer array (320 x 240
elements). The THz-QCL consists of the super-lattice structure of GaAs/AlGaAs, and thestructure of one active unit is almost same as the one written in the literature [6-8]. The active
layer consists of all 480 active units, and its thickness is about 25 μm. The resonator is
200-μm-wide single metal type waveguide, and length is 3 mm [7, 8]. A micro-bolometer arrayhas the structure shown in Figure 2. By performing various kinds of improvement shown below,
the improvement in sensitivity of about 2 orders magnitude was attained from the state before
improvement [9]. The improvements were as follows. (1) The index of absorption of terahertz
waves has been raised by tuning up the sheet resistance of the absorption film added by theaddition. (2) The arm that provides the thermal conduction of the bolometer has been extended
as an improvement of the thermal isolation structure. (3) The sensor package window material

was made from high resistance silicon with a small absorption of the terahertz waves, and theanti-reflection coating by parylene was given to both sides. By those improvements, the noise
equivalent power achieves about 8 times improvement from 250pW to 31pW in comparison
with before improvement [9]. As well as the improvement of the micro-bolometer array sensor,
optimizations of terahertz optics (a lens system and filters) are important for a real-time imaging
system. A Fresnel lens with anti-reflection coat using high resistance silicone was created, and a
metal mesh filter was used as a band path filter. The role of the filter is to prevent the emissionof the 10-μm-band to be emitted from the objects at room temperature. Although radiation of the
10-μm-band from the filter has always reached the sensor, since it hardly changes, the signal of
the portion can be excludable with processing in a sensor read-out circuit.
A stand-off (ca. 5 m in distance) terahertz imaging experiments were carried out undersimulated fire conditions (See Figure 3). Under the fire conditions which generated hot andblack smoke, terahertz images were obtained, while the smoke blocked visible light andsaturated signal in long wavelength infrared (LWIR) region. The experimental results show
advantage of terahertz over LWIR or visible. A video of the experiments will soon appear in
NICT channel in YouTube [10].

THz-imageSampleMB-Array(molar) 2x10 -3
4x10 -4
1x10 -2
DigoxinDigoxigenin

THz-QCL
THz-Camera system

Digotoxigenin
Detected area by using the THz-camera

Fig. 4. Real-time THz imaging system (left), THz-image of bio-material on a membrane (center),and a comparison between the conventional method and THz-image [12].

It is thought that biopolymers, such as protein and sugar protein, acted mutually considering
very weak interactions, such as a hydrogen bond and van der Waals forces, as mediation,changed the combined state by the thermal energy about room temperature, and have developed
its function. The energy of the terahertz wave is comparable as those weak couplings. Therefore,
using terahertz radiation, without the use of labeling, it is expected that the knowledge about adifference, folding, etc. of the structure of biopolymers are acquired. So, one of promising
application of the terahertz camera would be a label-free detection for a reaction of small
molecules with proteins. It is possible to detect a small change in the amount of bio-material byusing terahertz transmissive imaging, because very small amount of bio-material causes a large
absorption of the terahertz waves. The terahertz wave penetrated the porous PVDF sheets(membrane) and, using the living body materials such as an antigen / the antibody strongly
absorbing the terahertz wave, Ogawa et al. had developed a method to detect bio-materials

without using chemical labels [11]. By using the terahertz camera, a label-free detection
experiment of small molecule / protein reaction (Biotin [C10H16N2O3S, Molecular weight ca.
240] /Streptavidin [Molecular weight ca. 60,000]) were carried out only by placing themembrane to which bio-material adhered between the light source and sensor as shown in
Figure 4 in collaboration with Prof. Y. Ogawa of Kyoto University [12]. In the experiments,
detection sensitivity limit of the reaction was almost same as those of the conventional methods.
In the figure, comparison with the method using the conventional label is performed. The
terahertz images also have similar areas of the conventional label method. By the method
described here, the images can be retrieved in real time immediately after the antigen-antibody
reaction. It means that, since the work which attaches an indispensable label becomes
unnecessary in the conventional label method, the throughput of whole work can be improved.
Moreover, by that method, the development of the label set by each purpose itself becomes
unnecessary. Therefore, a result can be obtained, without waiting for development of a label.
Furthermore, this method is applicable also to the biopolymer of a small molecular weight.
Since it is difficult to develop a label to the biopolymer of a small molecular weight, thisterahertz-method is useful. Therefore, it can expect extensive application in fields, such as
biotechnology, medical treatment, and agriculture. The advantage of label-free detection methodby using the terahertz camera would be its throughput and low cost.
The real-time terahertz imaging system still has a large room to improve its performance.
The sensitivity of the un-cooled micro-bolometer array system could be improved by one orderof magnitude by optimizing the bolometer structure to the terahertz waves and improving
bolometer material. In light source, a milliwatt class output of the THz-QCL can be expected innear future. In total, a figure of merit of the real-time terahertz imaging system can be expected
to improve by 2 or 3 orders of magnitude.
References[1] R. Köhler, A. Tredicucci, F. Beltram, H. Beere, E. Linfield, A. Davies, D. Ritchie, R. Lotti,
and F. Rossi, “Terahertz semiconductor-heterostructure laser”, Nature, Vol. 417 (2002) 156.
[2] S. Kumar and A. Lee, ”Resonant-Phonon Terahertz Quantum-Cascade Lasers andVideo-Rate Terahertz Imaging,” IEEE Journal of Selected Topics in Quantum Electronics, Vol.
14, Issue 2 (2008) 333.
[3] D. Rabanus, U. U. Graf, M. Philipp, O. Ricken, J. Stutzki, B. Vowinkel, M. C.Wiedner, C.
Walther, M. Fischer, and J. Faist, “Phase locking of a 1.5 Terahertz quantum cascade laser anduse as a local oscillator in a heterodyne HEB receiver”, OPTICS EXPRESS Vol. 17, No. 3(2009) 1159.
[4] P. Grant, S. Laframboise, R. Dudek, M. Graf, A. Bezinger, H. Liu, “Terahertz free spacecommunications demonstration with quantum cascade laser and quantum well photodetector,”
ELECTRONICS LETTERS, Vol. 45, Issue 18 (2009) 952.
[5] S Kumar, Q. Hu, and J. Reno,” 186 K operation of terahertz quantum-cascade lasers basedon a diagonal design,” Appl. Phys. Lett., Vol. 94 (2009) 131105.
[6] B. S. Williams, S. Kumar, H. Callebaut, and Q. Hu, “Terahertz quantum-cascade laser
operating up to 137 K”, Appl. Phys. Lett., Vol. 83, No. 25, pp. 5142-5144 (2003).

[7] N. Sekine and I. Hosako, “Design and analysis of high-performance terahertz quantum
cascade lasers”, Physica Status Solidi (c), Vol. 6, pp. 1428-1431 (2009).
[8] I. Hosako, N. Sekine, M. Patrashin, S. Saito, K. Fukunaga, Y. Kasai, P. Baron, T. Seta, J.
Mendrok, S. Ochiai, and H. Yasuda, “At the Dawn of a New Era in Terahertz Technology,”
Proceedings of the IEEE, Vol. 95, No. 8, pp.1611-1623 (2007).
[9] N. Oda, H. Yoneyama, T. Sasaki, M. Sano, S. Kurashina, I. Hosako, N. Sekine, T. Sudou,
and T. Irie,“Detection of terahertz radiation from quantum cascade laser using vanadium oxide
microbolometer focal plane arrays”, Proc. SPIE, 6940, 69402Y-1-69402Y-12 (2008).
[10] http://www.youtube.com/user/nictchannel[11] Y. Ogawa, S. Hayashi, M. Oikawa, C. Otani and K. Kawase, “Interference terahertz
label-free imaging for protein detection on a membrane,” Optics Express, Vol. 16, No. 26,
pp.22083 - 22089 (2008).
[12] N. Oda, “Uncooled bolometer-type Terahertz focal plane array and camera for real-time
imaging”, Comptes Rendus Physique, Vol.11, pp.496-509 (2010).

D4. Nonlinear Optical Waveguide for THz Tomography
K. Kawase
Nagaya University
Abstract-Nonlinear optical terahertz wave generation is a promising method for realizing apractical source with wide frequency range and high peak power.

Unfortunately, many

nonlinear crystals have strong absorption in the terahertz frequency region.

This limits

efficient and widely tunable terahertz wave generation. The Cherenkov phase-matching method
is one of the most promising techniques for overcoming these problems. We propose aprism-coupled Cherenkov phase-matching method, in which a prism with a suitable refractive
index at terahertz frequencies is coupled to a nonlinear crystal. We demonstrate prism-coupled
Cherenkov phase-matching terahertz generation using the DAST and LiNbO3 crystals. WithDAST crystal, we obtain a spectral flat tunability up to 10 THz by difference frequency
generation. With LiNbO3 crystal, we observe a spectral flat broadband terahertz pulse
generation up to 5 THz pumped by a femto second fiber laser. The obtained temporal waveform
is an ideal half cycle pulse suitable for reflection terahertz tomography.
1. Cherenkov phase matching with prism coupler
Cherenkov THz radiation is generated inside a nonlinear crystal when the velocity of thepolarization excited by a DFG process is greater than the phase velocity of the radiated wave.
In general, many nonlinear crystals have strong dispersion between the optical and THz regions,and the refractive index at THz frequencies is larger than that at optical frequencies. This may
result in behavior that satisfies the Cherenkov-type phase matching condition, i.e.

lTHz
cos q crystal =

nTHz
=
l1l 2
2 Lc

lTHz

nTHz

( n1 l 2 - n 2 l1 )

»

nopt
nTHz
(1)

Where l1 and l2 are the wavelengths of the two signals contributing to the DFG process(w1 – w2 = wTHz), n1 and n2 (n1 = n2 @ nopt) are the refractive indices of the crystal at the pumpwavelengths, and Lc is the coherence length of the surface-emitted process (Lc = p/Dk, where
Dk = k1 – k2 and k is the wave vector). The Cherenkov angle, qcrystal, is determined by therefractive index at the pump frequencies and that at the THz frequency in the crystal, and so theangle is strongly dependent on the nonlinear material. The emitted THz radiation propagateswith the Cherenkov angle to the crystal–air interface, and if the angle is greater than a criticalangle, which is determined by the difference in the refractive index at the interface, the THz
wave undergoes total internal reflection at the interface. In order to prevent this, we use acladding material with a suitable refractive index formed into a prism. The angle of thecladding–nonlinear crystal interface, qclad, is important for practical applications and can be
determined using Snell’s law, as follows.

q clad =
=

æn
æn
ö p
p
p
æp
öö
- b = - arcsinçç THz sin (a )÷÷ = - arcsinçç THz sinç - q crystal ÷ ÷÷
2
2
è2
øø
è ncladè ncladø 2ænæp
æ n l - n l öööp- arcsinç THz sinçç - arccosçç 1 2 2 1 ÷÷ ÷÷ ÷ç2è nTHz (l2 - l1 ) ø ø ø÷è2
è ncladæ nl -n l ö= arccosçç 1 2 2 1 ÷÷
è nclad (l2 - l1 ) ø(2)where nclad is the refractive index of the cladding layer in the THz-frequency range. Thisequation is equivalent to a model in which the THz wave radiates directly into the cladding
layer.

It indicates that nclad should be larger than that of the nonlinear crystal at the pumpfrequency, and also that it is not necessary to take into account the refractive index of the crystal
at the THz frequency (as the equation does not depend on nTHz). We call the method as Prism
Coupled Cherenkov Phase Matching (PCC-PM). For the PCC-PM THz-wave generation, bycomparing the refractive index of the crystal at optical frequencies to that of Si (which isapproximately 3.4 across the whole of the THz-frequency region), we see that Si is suitable as a
Cherenkov radiation output coupler for many nonlinear crystals.
The organic nonlinear crystal DAST is attractive material for THz generation [1] because thecrystal has a very large nonlinear coefficient. DAST is also strongly absorbing in the THz
region, which has so far prevented efficient THz generation. The PCC-PC method, however,allows efficient THz generation with very high nonlinear efficiency. DAST has very similar
refractive indices in the near infrared and THz-frequency regions, and this means that collinear
phase matching can be achieved.

2. Widely tunable THz generation using DAST crystal
We performed an experiment to prove which is dominant for PCC-PM, Eq. 1 or 2. The
100-µm-thick DAST single crystal used was hexagonal so that we could achieve b-axis
propagation of two pump beams. A Si prism was coupled along the (001) plane of the DAST
crystal because as-grown DAST has a very flat surface in the plane. A dual-wavelength
potassium titanium oxide phosphate (KTP) optical parametric oscillator (OPO) with pulse
duration of 15 ns, pulse energy of 1.5 mJ, and 1300–1600-nm tunable range was used as a pump,as in our previous work. The polarizations of the two pumps were parallel to the a-axis of the
DAST crystal, which has the highest nonlinear coefficient component, and THz radiation was
generated parallel to the a-axis. The pump beam was focused to form a 46-mm diameter beamusing a cylindrical lens with a 50-mm focal length, resulting in a power density of about
50 MW/cm2.

The emitted THz radiation was detected using a liquid-helium-cooled Sibolometer with an electrical gain of 1000.

Si-bolometer output [V]101300 nm
1350 nm1400 nm
1450 nm

1
0.1
0.01
1E-3
1E-4

0

2

4

6

8

10

12

Frequency [THz]Fig.1. THz output spectra under pump wavelengths ranging from 1300 nm to 1450 nm.

Figure 1 shows the collected THz output spectra under different pump wavelengths. THz
radiation was generated at levels from less than 1 THz to more than 10 THz, with spectra thatwere almost independent of the pump wavelength proving Eq. 2 was right. When collinear
phase matching is used to generate THz radiation, a deep dip in the spectra is observed ataround 1.1 THz [1, 2]. This is suppressed here because the Cherenkov method results insurface emission, and we successfully obtained about 100 times higher signal compare to noiselevel of the detector.

Additionally, radiation was generated below 1 THz, which cannot be

generated using collinear phase matching with 1300–1450-nm pumping [3]. At frequencies
above ~10 THz, the detection efficiency of the bolometer falls off, and this explains the low
observed power at higher frequencies.

However, the collected power extends to higher

frequencies than were observed when using bulk lithium niobate in our previous works.

Although THz radiation generated far from the crystal–cladding interface interferes with thatgenerated near the interface, resulting in destructive interference, the radiation generated deep
within the crystal is attenuated due to the large absorption coefficient of the crystal. The
absorption coefficient of DAST crystal at around 2–10 THz is 100–200 cm-1. The beam
diameter was approximately 50 µm, and so the intensity of a THz-wave that propagates thisdistance will be attenuated by 30–60%.

The output voltage of the Si bolometer can beconverted to energy by considering that 1 V ≈ 20 pJ/pulse for low-repetition-rate detection andthat we have a gain of 1000 at the bolometer. The highest-energy THz pulse obtained wasabout 50 pJ (3.3 mW at peak). The obtained output energy was higher than that of obtained bycollinear phase matching condition using DAST crystal under almost same power density [1, 2].
3. Half cycle THz pulse generation using LiNbO3 crystal
As restricted by the temporal duration of the THz pulses used, the conventional axial
resolution of THz tomography remains a few tens of microns. The common ~10 mm thicknessof coatings in various industrial products and the stratum corneum of human skin require higher
depth resolution. The simple way to improve the axial resolution of tomographic imaging is
by generating broadband and short THz pulses using shorter optical pulses. We have developed
an all-fiber laser system that produces 17-fs optical pulses at a wavelength of 1.56 mm and havegenerated and detected broadband THz pulses ranging from 0.1 to 27 THz using DAST crystal.
The temporal waveform of the generated THz pulses has several multiple peaks [4].
Here, we demonstrated spectral flat broadband THz pulse generation using thin LiNbO3
crystal under PCC-PM condition pumped by a fiber laser. The obtained temporal THz
waveform was an ideal half cycle pulse which is suitable for reflection tomography. The
experimental setup is shown in Fig. 2.

The average power, pulse width, and repetition

frequency of the pump fs fiber laser were 200 mW, 17 fs, and 48 MHz, respectively. The opticalpulses were divided into two beams for THz wave generation and detection. The THz waves
were radiated from a LiNbO3 plate (10×5×1 mm). The pump beam was tightly focused as
close to the THz exit surface as possible using an asymmetrical lens with focal length of 11 mm.
The pump diameter before the asymmetrical lens was 1.2 mm (FWHM). A thin PET film
was sandwiched between Si prism and LiNbO3 crystal to avoid pump leakage into Si. The THz
pulses were detected by a 5-mm electrode gap LT-GaAs photoconductive antenna using
parabolic mirrors. The temporal waveform of the THz waves was traced by scanning a delay
stage whereby SHG pulses generated with a periodically- poled lithium niobate were used asprobe pulses. The signal obtained was then fed into a preamplifier and detected with a lock-in
amplifier. In this experiment, the THz wave path was not yet purged with dry nitrogen gas.

Fig. 2. Experimental setup for PCC-PM THz generation using LiNbO3 crystal pumped by a fs
fiber-laser.

Fig. 3. (left) The temporal waveform of the detected THz pulse. (right) The corresponding
spectrum obtained by Fourier transforming the temporal waveform. The spectrum is not smooth
due to water vapor absorption.

Figure 3 (left) shows the temporal waveform of the detected THz pulse. We successfully
obtained half cycle THz pulse with pulsewidth of 270 fs. Fig. 3 (right) shows the corresponding
spectrum obtained by Fourier transforming the temporal waveform. The spectrum extended up
to 5 THz with almost Gaussian profile which leads to an ideal half cycle pulse.
References[1] H. Ito, K. Suizu, T. Yamashita, A. Nawahara, and T. Sato, “Random frequency accessible
broad

tunableterahertz-wave

sourceusingphase-matched

4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) crystal,” Jpn. J. Appl. Phys. 46,
7321–7324 (2007).
[2] K. Suizu, K. Miyamoto, T. Yamashita, and H. Ito, “High-power terahertz-wave generation

using DAST crystal and detection using mid-infrared powermeter,” Opt. Lett. 32, 2885-2887(2007).
[3] T. Taniuchi, S. Ikeda, S. Okada, and H. Nakanishi, “Tunable Sub-Terahertz Wave
Generation from an Organic DAST Crystal,” Jpn. J. Appl. Phys. 44, L652-L654 (2005).
[4] J. Takayanagi, S. Kanamori, K. Suizu, M. Yamashita, T. Ouchi, S. Kasai, H. Ohtake, H.
Uchida, N. Nishizawa, and K. Kawase, “Generation and detection of broadband coherent
terahertz radiation using 17-fs ultrashort pulse fiber laser,” Opt. Express 16, 12859-12865(2008).

D5. Photonic Millimeter-Wave Technology
H. ItoKitasato University
Photonic generation of millimeter (mm)/sub-millimeter (sub-mm) waves or terahertz (THz)waves is a promising technology since it offers an extremely wide bandwidth at very highfrequencies, and is capable of low-loss transmission of these high-frequency signals throughoptical fibers. It is also advantageous for making the front-end component compact, light, andeasy-to-handle for practical use. There are two major approaches in this technology, i.e., apulsed operation of a device to simultaneously generate ultra-wideband frequency components,
and a continuous-wave operation to generate a signal having extremely high spectral (orfrequency) resolution with excellent stability.

The former is often used for time-domain

spectroscopy (TDS) employing low-temperature-grown GaAs photoconductors or nonlinear
optical crystals.

The latter is usually accomplished by photodiodes or nonlinear opticalcrystals, which offer relatively large average output power, less complex setup, and lesssensitivity against environmental fluctuation.
In reality, the use of long-wavelength (~1.55 mm) lights is important because we can use theabundant optical components developed for optical communications systems at a reasonablecost.

To achieve such scheme, an efficient O/E conversion device (photomixer) with

sufficiently high output power at very high frequencies is essential. The uni-traveling-carrier
photodiode (UTC-PD) [1-3], developed in NTT laboratories, Japan, is one of the best solutions,because it provides a high 3-dB bandwidth and a high saturation output power simultaneously.
To date, output powers of over 20 mW at 100 GHz, 0.5 mW at 350 GHz [4] and 10 mW at 1
THz have been achieved by the UTC-PD.
In some applications, such as reflection-geometry spectroscopy, reflection-geometry imaging,and bi-directional wireless communications [5], it is important to use a compact transceiverhead that has both transmitter and receiver functions. This makes the tight and therefore
difficult optical alignment of the components much easier.

For this purpose, a compact

transceiver module that integrates a UTC-PD, a Schottky barrier diode [6], and a planar
circulator circuit for operations at around 300 GHz [7] has been developed.

All thecomponents were assembled in a J-band (220-325 GHz) waveguide-output package.
Reflection-geometry THz imaging is suitable for security inspection and medical diagnosis
since biological substances, such as human body, has large absorption coefficient due tocontaining water. Thus, the imaging was performed using the sub-THz transceiver module [7]mentioned above, and it was confirmed that a practical contrast could be obtained for a
bandwidth of about 20 GHz. Based on these results, in-vivo imaging of a human finger was
successfully demonstrated at 270 GHz.
Another type of application is the TDS using pulsed mm-/sub-mm waves. For this purpose,a UTC-PD that is monolithically integrated with a coplanar waveguide (CPW) has beendeveloped [8]. A sub-THz (pico-second) electrical pulse was generated by a UTC-PD andpropagated through the CPW for the possible use in the proximity sensing of material properties
including liquid samples. It was found that the higher-frequency Fourier components in thepulse remain considerably even after 2 mm propagation through the CPW fabricated on an InP
substrate. These results demonstrated the possible use of the waveguide-integrated UTC-PD
as a proximity THz-TDS device for, at least, up to 400 GHz.
The photonic mm-wave ellipsometry system utilizing the UTC-PD has also been developed
for revealing complex relative dielectric constant of the biological substances. This techniqueenables us to obtain more qualitative information of the material compared with the imaging,and the complex relative dielectric constant of human skin was successfully measured in-vivo in
the F-band (90-140 GHz) [9]. Such information can only be obtained by the method other than
the imaging, so that the complementary use of the imaging and the ellipsometry should bepromising for analyzing the property of biological substances in the mm-/THz-wave range.
References[1] H. Ito and S. Kodama, “Uni-Traveling-Carrier Photodiode (UTC-PD) and PD-EAM Optical
Gate Integrating UTC-PD and Traveling Wave Electro-Absorption Modulator (Chapter 4),” in
Ultrafast All-Optical Signal Processing Device, edited by H. Ishikawa, Wiley (London, 2008),
pp. 89-153.

ISBN:978-0-470-51820-5[2] T. Nagatsuma, H. Ito, and T. Ishibashi, “High-Power RF Photodiodes and their Applications,”
Laser and Photonics Reviews, vol. 3, No. 1-2, pp. 123-137, 2009.
[3] H. Ito, “Photonic Generation of Millimeter-/Terahertz-Waves using Uni-Traveling-Carrier
Photodiodes,” Proc. Asia-Pacific Radio Science Conference (AP-RASC10), p. DC3-1, 2010.
[4] A. Wakatsuki, T. Furuta, Y. Muramoto, T. Yoshimatsu, and H. Ito, “High-Power andBroadband Sub-Terahertz Wave Generation Using a J-Band Photomixer Module with
Rectangular-Waveguide Output Port,” Proc. Int. Conf. on Infrared, Millimeter, and Terahertz
Waves, pp. 1999-1-2, 2008.
[5] T. Nagatsuma, “Towards 100-Gbit/s Wireless Using Terahertz Waves,” Extended Abst. Int.
Workshop on Terahertz Technol., pp. 121-126, 2009.
[6] H. Ito, F. Nakajima, T. Ohno, T. Furuta, T. Nagatsuma, and T. Ishibashi, “InP-Based
Planar-Antenna-Integrated Schottky-Barrier Diode for Millimeter- and Sub-Millimeter-Wave
Detection,” Jpn. J. Appl. Phys., vol. 47, No. 8, pp. 6256-6261, 2008.
[7] H. Ito, K. Yoshino, Y. Muramoto, H. Yamamoto, and T. Ishibashi, “Sub-Terahertz-WaveTransceiver Module Integrating Uni-Traveling-Carrier Photodiode, Schottky Barrier Diode, andPlanar Circulator Circuit,” IEEE J. Lightwave Tech., vol. 28, No. 24, pp. 3599-3605, 2010.
[8] H. Ito, H. Yamamoto, T. Furuta, and T. Ishibashi, “Generation and Propagation of
Sub-Terahertz Pulse Signal using Waveguide Integrated InP/InGaAs Uni-Traveling-Carrier
Photodiode,” Jpn. J. Appl. Phys., vol. 49, No. 3, pp. 032402-1 - 032402-5, 2010.
[9] H. Yamamoto, H. Ito, and M. Noshiro, “Millimeter-Wave Ellipsometry of Human Skin
Based on Photonics Technology,” IEICE Electron. Express 7, No. 13, pp. 964-968, 2010.

D6. Plasmonic Device Technology for Emission and Detection of THz Radiation
T. Otsuji
Tohoku University
1. Introduction“Terahertz” is an unexplored electromagnetic frequency band staying in between
microwaves and infrared lights in the sense that there is no commercially available
microelectronic device that can generate and detect electromagnetic waves over the entire
terahertz (THz) frequency band [1]. The terahertz frequencies fall in vibrations of most ofmolecules like water, protein, explosives, drugs, etc... Each substance has its own fingerprint in
the terahertz frequency range. Therefore terahertz electromagnetic waves are capable ofidentifying

thesubstance

by

using

spectroscopic

and/orimagingtechniques;transmission/absorption spectra is identical to the substance so that terahertz radiation is
powerful for those applications if a "compact," "room-temperature operating," "integrated
solid-state" terahertz light source is developed. Presently, terahertz inspection instruments arebeing equipped at the security gates in several international airports. However their physicalvolume of the hardware is quite large due to the lack of"compact," "room-temperature

operating," "integrated solid-state" terahertz light sources.

The technology could betransferred to billions of consumer markets like portable/handy terahertz camera/scanner.
The other aspect is the ultra-broadband/ultra-high-speed wireless communications [2].
Now mobile cellphones utilizes carrier frequencies around 3 GHz that can transfer data with
100-Mbit/s rate. It is insufficient to transfer HD-TV uncompressed video stream data. Soon orlater Ultra-High Definition TV system will become in service which requires 3 orders higher
data transmission rate. To accommodate such high capacity on the wireless comm. the carrier
frequency should be increased in the terahertz (1000 GHz) range.

Terahertz waves areseverely absorbed by water vapors but short-distant transmission over 100 m is possible. If a
compact, room-temperature operating solid-state terahertz light source is available, the terahertz
wireless communication becomes in our hand indeed [2].
In the last decade, development of compact, tunable and coherent sources operating at THz
frequencies has been one of the hottest issues of the modern THz electronics. Conventional
electron devices like field-effect transistors (FETs) increase their operating frequencies byscaling down the feature size, severely sacrificing the output power. Photonic devices likesemiconductor lasers, on the other hand, decrease their operating frequencies by scaling down
the transition energy from interband to inter-subband transition, severely suffering from thethermal noise. One can call such a difficult situation as “Terahertz Gap.” [1]The channel of a FET can act as a resonator for plasma waves (collective electronic
charge-density waves). When the plasma-wave frequencies are quantized into the resonances,
we call such a quantum as “plasmon.” The plasmon frequency depends on its dimension andeasily exceeds THz by setting the gate length in 100s nanometer scale. The study on
applications of plasmon FETs for THz imaging/spectroscopy started at the early ‘90s with thepioneering theoretical work of Dyakonov and Shur [3, 4]. One of the severe issues to cope with,however, is the quantum efficiency: how to efficiently couple the non-radiative nature ofplasmons to radiative electromagnetic waves.

Fig. 1. Cross sectional structure and SEM images of the plasmon-resonant DGG-HEMT emitter.
(after Ref. 6.)

2. Plasmon-Resonant Microchip Emitters2.1. Device structure and operation
To circumvent such a problem, we have introduced an interdigitated dual grating gate (DGG)
structure into a standard high-electron mobility transistor (HEMT) [5], shown in Fig. 1 [6].
The DGG can periodically localize the 2D plasmon in 100-nm regions with a micron interval.
When the DC drain-to-source bias is applied, 2D electrons are accelerated to produce a constantdrain-to-source current. Due to such a distributed plasmonic cavity systems in periodic 2Delectron-density modulation, the DC current flow may excite the plasma waves in eachplasmonic cavity, leading to self oscillation. The DGG acts also as a broadband THz antennathat converts non-radiative longitudinal plasmon modes to radiative transverse electromagnetic
modes.

Additionally, a vertical cavity structure is introduced in between the top DGG plane

and a THz mirror at the backside, working as a plasmon-resonant enhancer [5, 6].
2.2. THz emission performance and applications
We have designed and fabricated our original plasmon-resonant emitters (PREs) using
InGaP/InGaAs/GaAs material systems in a double-deck HEMT [6-8]. The intrinsic device area
has a geometry of 30 μm x75 μm, where the DGG pattern is replicated on the upper-deck
HEMT layer while the periodic plasmon cavities are configured on the lower-deck HEMT. The
DGG consists of 150-nm and 1850-nm lines aligned alternately with a spacing of 100 nm. The
number of DGG fingers is 37 (38) for the 1500-nm (1850-nm) grating. Experiments revealed
the performances of the PREs exhibiting 1-μW broadband emission ranging from 0.5 to 6.5 THz
at room temperature, shown in Fig. 2 [8].

Fig. 2. Measured emission spectra for fabricated PREs in comparison with a HP-Hg lamp at room
temperature. Double- and triple-chip PRE operation almost doubles and triples the emission power,
respectively. (after Ref. 8.)

Compared with a commercially available high-pressure mercury (HP-Hg) lamp, the PRE
exhibits a superior (two orders higher) energy conversion efficiency of 10-4 to 10-5 although theoutput power is by one order lower than the HP-Hg lamp5. We confirmed dual- and triple-chip
operation of PREs successfully doubles and triples the emission intensity as shown in Fig. 2.
The results encourage us to exceed the output power of the HP-Hg lamp by integrating thePREs in an array of 10 x 10 [8].
The device is introduced as a light source into a Fourier-transform THz spectrometer. The
water-vapor absorption lines are clearly observed with an excellent coincidence with the result
provided by NASA5 as shown in Fig. 3(a) [7]. Fingerprints of several sugar-group materials in
liquid are also successfully identified. As shown in Fig. 3(b), the poor S/N for a single-chip PRE
operation is greatly improved by triple-chip operation, almost approaching the result using
HP-Hg lamp [8].

Fig. 3. (a) Measured absorption spectrum of atmospheric water vapor in comparison with the data
provided by NASA [5] (after Ref. 7.), (b) transmission spectra for maple syrup liquid measured byusing single- and triple-chip PRE(s),RE(s), and by using a highhigh-pressurepressure mercury lump (after Ref. 8.).

Resonant THz Detectors
3. Plasmon-Resonant
3.1. Device structure and operation
The plasmon resonant emitter device also works for a broadband highhigh-sensitive
sensitive THz detector[9]. Resent works done with collaboration with CNRS, Montpellier demonstrated excellent
terahertz imaging hidden materials inside an envelope [9]. Our device structure is a
InGaP/InGaAs/GaAs single-deck
deck DGG
DGG-HEMT [5-8, 10] (see Fig. 4). The DGG is formed with
65-nm-thick Ti/Au/Ti by a standard lift
lift-off
off process. Four different structures with a DGG were
fabricated. The metal fingers of one grating gate G1 are of the same length LG1 = 100 nm in all
structures, while grating gate G2
2 was designed to have fingers of different lengths LG2 = 300,
800, 1300, and 1800 nm for different structures. The spacing between the grating
grating-gate fingers is
100 nm for all four structures. The whole length of the channel between the source and drain
contacts covered by the DGG is about 80 µm with the channel width of 30 µm in each structure.

(b)SourceLG1 LG2

Gates

Drain
d

2D Channel
65 nm Ti/Au/Ti
40 nm ud In0.48 Ga0.52 P
17 nm Si-d In0.48 Ga0.52P
3 nm ud In0.48Ga0.52P
15 nm ud In0.2Ga0.8As
200 nm ud GaAs
SI GaAs

Fig. 4. (a) Top view and (b) schematic side view of the DGG-HEMT with indicated heterostructurematerial systems. The external THz radiation is incident normally from the top. (after Ref. 10.)

It is known that hydrodynamic nonlinearities in 2D electron channels can be used for
detection of THz radiation [3]. Resonant THz detection is related to the excitation of highquality plasma waves (plasmons) in 2D electron channel, whereas a broadband non-resonant
THz detection is related to overdamped plasmons in depleted regions of the channel. In the bothresonant and non-resonant cases, THz photoresponse originates from the nonlinear dynamics of
2D electron fluid described by the hydrodynamic equations [3]¶V(x,t)¶V(x,t) V(x,t) e+ V(x,t)++E(x,t) = 0,
¶t
¶x
τ m*
e

¶
¶
N ( x, t ) j ( x, t ) = 0,
¶t
¶x

(1)

(2)

where E(x,t) is the in-plane electric field depending on the time t and coordinate x in 2D electron
system, τ is the electron momentum relaxation time, j (x,t) = -e N(x,t)V(x,t) is the density of
induced electric current, N(x,t) and V(x,t) are hydrodynamic electron density and velocity in 2D
electron channel, and e and m* are the electron charge and effective mass, respectively. There
are two different nonlinear terms in the system Eqs. (1) and (2): the second term in the Euler
equation Eq. (1) describing the nonlinear electron convection in 2D electron fluid and the
product N(x,t)V(x,t) defining the current density in the continuity equation Eq. (2). Time averageof the nonlinear current yields the detection signal.
3.2. THz detection performance and applications
The transfer characteristics of the structure with LG2 = 300 nm are shown in the inset of
Fig. 5 for drain-to-source voltage 0.5 V [10]. Each DGG structure was irradiated at normal
incidence by THz beam at frequency 0.24 THz. We measured a photovoltaic DC response
between the source and drain contacts (the source contact was grounded and no drain bias DC
voltage was applied) at room temperature. Since the quality factor is estimated to be smaller
than unity, ωτ = 0.25, for the incident radiation frequency ω/2π = 0.24 THz, the non-resonant
detection should be expected in our experiments at room temperature. Terahertz radiation was
generated by a Backward Wave Oscillator (BWO) with maximum output power of 5 mW. The

radiation was linearly polarized and chopped at 170 Hz. The radiation-induced photovoltage ΔU
was measured as a function of the gate voltage UG1 or/and UG2 as 170 Hz ac voltage componentof the source-to-drain voltage using the standard lock-in technique. Figure 5 demonstrates thephotoresponse measured as a function of the gate voltage UG1 (for UG2 = 0) and that as afunction of the gate voltage UG2 (for UG1 = 0) for the structure with LG2 = 300 nm when the THz
electric field was directed across the grating-gate fingers (i.e., in the source-to-drain direction).

Drain Current (A)500µPhotoresponse (V)

250µ
200µ

UG1=0

150µ

400µ UG2=0
300µ
200µ

UG1=0

100µ
-3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0

Gate Voltage U G (V)

100µ
50µ

UG2=0
0
-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

Gate Voltage UG (V)Fig. 5. The photoresponse at 0.24 THz in the structure with LG2= 300 nm as a function of the gate
voltage UG1 (for UG2 = 0) and as a function of the gate voltage UG2 (for UG1 = 0). The inset shows
the transfer characteristics of the DGG-HEMT. (after Ref. 10.)Figure 6 shows the dependence of the photoresponse on the azimuthal angle j between theTHz electric field and the source-to-drain direction for the structure with LG2 = 1300 nm [10].
The strongest photoresponse in all structures was observed when the electric-field vector of theincident THz wave was directed across the grating-gate fingers (i.e., in the source-to-drain
direction). This demonstrates that the coupling between THz radiation and 2D electron channel
is ensured by the DGG structure.
90
120

60
30

150

0

180

330

210
300

240
270

Fig. 6. Photoresponse at 0.24 THz as a function of the azimuthal angle j between the electricfield vector of the incident THz wave and the source-to-drain direction (solid curve). It follows thecos2(j) dependence (dashed curve) with the strongest photoresponse occurred for j = 0. (after Ref.
10.)

We performed raster-scan imaging in transmission mode with the measurement setup as
shown in Fig. 7. The Gaussian-shaped focal spot has a radius of 710 μm. The power of thetransmitted radiation is detected by our InP-HEMT with an active plasmonic gate length of 100
nm and a cutoff frequency of 268 GHz.

Fig. 7.

THz raster-scan imaging setup. A InP-based HEMT works for plasmonic THz detection.

(a)

(b)
Fig. 8. THz raster scan imaging for (a) Acer Monspessulanum and Ginkgo Biloba (left: photo
image, right: THz image), and (b) a tea bag (left: photo image, right: THz image).

Figure 8 shows typical results for the samples (a) leaves which are called Acer
Monspessulanum (Acer from Montpellier) and Ginkgo Biloba and (b) a tea bag with the
package. The image of the leaves was successfully obtained with good contrast. From the image(b) and (c), the THz radiation transmitted through the package so that the tea leaves could be
identified. The more water contains the more the THz radiation is absorbed, resulting in darker
color. The amount of water contained in the tea bags appears in the images (b) and (c) as its
darkness.
4. Conclusion
In conclusion the DGG-HEMT-based plasmon-resonant microchip emitters/detectors havebeen developed and revolutionized. Further improvements on its emission power and detection
sensitivity by arrayed integration and/or novel breakthroughs like metamaterial antennae enable
the device to be of a promising candidate of potential products of billions of consumer markets
like portable/handy terahertz cameras/scanners as well as transmitters/receiver s for THz
wireless communication systems.
References[1] M. Tonouchi, "Cutting-edge terahertz technology," Nature Photon. 1, 97-105 (2007).
[2] M. Dyakonov and M. Shur, Shallow water analogy for a ballistic field effect transistor: newmechanism of plasma wave generation by dc current, Phys. Rev. Lett. 71, 2465-2468 (1993).
doi:10.1103/PhysRevLett.71.2465[3] T. Nagatsuma, Challenges for ultrahigh-speed wireless communications using terahertz
waves, International J. Terahertz Science and Technology, 3, 55-65 (2010).
[4] M. Dyakonov, and M. Shur, Detection, mixing, and frequency multiplication of terahertz
radiation by two dimensional electronic fluid, IEEE Trans. Electron Devices 43, 380-387 (1996).
doi:10.1109/16.485650[5] T. Otsuji, M. Hanabe, T. Nishimura and E. Sano, A grating-bicoupled plasma-wave
photomixer with resonant-cavity enhanced structure, Opt. Express 14, 4815-4825 (2006).
doi:10.1364/OE.14.004815[6] T. Otsuji, Y. M. Meziani, T. Nishimura, T. Suemitsu, W. Knap, E. Sano, T. Asano, V.V. Popov,
Emission of terahertz radiation from dual-grating-gates plasmon-resonant emitters fabricatedwith InGaP/InGaAs/GaAs material systems, J. Phys.: Condens. Matter 20, 384206 (2008).
doi:10.1088/0953-8984/20/38/384206[7] Y. Tsuda, T. Komori, A. El Fatimy, T. Suemitsu, and T. Otsuji, Application of plasmonic
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A52-A57 (2009). doi:10.1364/JOSAB.26.000A52[8] T. Otsuji, "Plasmon-resonant microchip emitters and detectors for terahertz sensing andspectroscopic applications," Proc. SPIE. 7671, 767102 (2010).
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D7. Integrated Josephson Voltage Standard Technology
S. Kohjiro
National Institute of Advanced Industrial Science and Technology
Abstract-A compact Programmable Josephson Voltage Standard (PJVS) system based on acryocooler is under development at the National Institute of Advanced Industrial Science andTechnology (AIST). Its key technology is a superconducting integrated circuit which generates
the accurate voltage of 2 V based on the RF Josephson effect. The circuit chip consists of 65536
niobium-nitride-based Josephson junctions operating at 12 K, i.e., 3 times as large as the typicaloperating temperature of conventional niobium-based Josephson voltage standards. Since the
PJVS chip requires cooling power of less than 100 mW, it is possible to use a compact-size
cryocooler, which can be mounted inside a standard 19-inch rack-mount chassis. By combining
the 2 V PJVS chip with a high-precision semiconductor voltage multiplier circuit, the accurate
and arbitrary voltages up to 20 V are expected to be available not only to metrology institutes
but also to manufacturing industries.
1. Introduction
Josephson junctions (JJs), junction sandwiches of superconductor/insulator (or normal
conductor)/superconductor,

exhibitequally spaced

constant

voltagesteps

on theirvoltage-current (V –I) curves under the irradiation of microwave of frequency f. Step voltages
are given by

V = mn

h
f
2e ,

(1)

where m is the number of junctions in series, n the harmonics number, h Planck’s constant, ande the unit charge. This means the voltage accuracy of JJs is in principle equal to the frequency
accuracy of the microwave source and independent of temperature, humidity, materials of JJs,
etc. From this reason, JJs are used for the primary DC voltage standard at national metrology
institutes around the world. Because a JJ can produce only very small voltages of 10-200 mV,
the array of 104-106 JJs should be developed to increase the output voltage up to ~10 V required
as a calibration source for digital multimeters. Technologies of superconducting integrated
circuits are necessary for JJ arrays with enough numbers of junctions and operating
successfully.
The Programmable Josephson Voltage Standard (PSVJ) systems [1-3] have the inherent
advantages over the conventional Josephson voltage standard, i.e., shorter settling time (<1 μs)
and larger voltage step width (>1 mA). Though the PJVS systems are now widely used as theprimary voltage standards at national metrology institutes, semi-conductor (Zener) voltage
standards are still the only choice for the secondary voltage standards. At the National Instituteof Advanced Industrial Science and Technology (AIST), cryocooler-based PJVS systems have
been

under

the

development,utilizingtheniobium-nitride-based(NbN/TiN/NbN)

Josephson-junction arrays operating at 10 K [4,5]. This temperature is significantly higher than
4 K, at which conventional niobium (Nb)-based Josephson voltage standards operate.
Recently, the stable operation of the PJVS chip at 12 K has been demonstrated. The increase of
2 K in the operating temperature has enabled us the use of the smallest two-stage
Gifford-McMahon (GM) cryocooler available on the market.
This paper reports a low cost, compact PJVS system under development at the AIST, aiming
to be a replacement for the conventional Zener secondary voltage standards.
2. System Description
The new PJVS system, designed as a secondary standard, requires periodical calibration by
the primary standards, and need to be sufficiently compact for the transportation to thecalibration institutes. To adopt a compact cryocooler which dominates both volume and power
consumption of voltage standard system, two-stage voltage-output circuits are employed. The
first-stage is a JJ array which produces accurate voltages of 0-2 V based on Eq. (1). The
second-stage is a precision semiconductor voltage multiplier with a factor of 10. While theintrinsic accuracy of the PJVS will be degraded by the use of semiconductor circuits, it is still
accurate enough for the secondary voltage standard.

(a)(b)
Fig.1. Block diagram (a) and layout (b) of a compact PJVS system integrated into a 19-inchrack-mountable case. A superconducting integrated circuit consisting of 65536 Josephson junctions
in chip area of 14.7 mm×4.7 mm is installed in a cryostat and operates at 12 K.

The dimensions of the smallest commercially available cryocooler (CTI model22) is 290 mm
×240 mm×160 mm, which can be mounted into a 19-inch rack-mount chassis except for the
helium(He)-gas compressor. Other system components to be assembled into the 19-inchrack-mount chassis are (1) a panel-mount PC as the system controller, (2) temperature controller
to stabilize the operating temperature of the PJVS chip at 12K, (3) 24-channel bias current

source, (4) 16GHz, 500mW microwave source, and (5) the ×10 voltage multiplier circuit. The
block diagram and layout of the compact PJVS system are shown in Fig. 1.

Fig.2. Equivalent circuit of dual-channel 1-V PJVS circuits.

Fig. 2 shows an equivalent circuit of the dual-output 1-V PJVS circuit, where the total output
voltage of the chip is 2 V. The microwave signal with f = 16 GHz is split into 16 arrays
including 4096 NbN/TiN/NbN Josephson junctions each, and two of these have binary
sub-arrays those work as digital-to-analog converters (DACs) with resolution of 10 bits. The
polycrystalline NbN films for the electrode were prepared by an rf-magnetron sputter at ambient
temperature on a 3-inch-diameter Si wafer, and the critical temperature of the 200-nm-thick
NbN film was 15.9 K. The fabrication process was as same as that of a 10-V PJVS circuit
except for the number of barriers per junction stack [6].
3. Experimental Results
Fig. 3(a) shows a V–I characteristics of the dual-output PJVS circuit including 65536
NbN/TiN/NbN JJs with microwave irradiation of 16 GHz measured at 12.0 K. The constantvoltage at 2.168 V is observed in the current width, i.e., step height, of 0.7 mA at 12.0 K, whichis larger than the typical fluctuation of bias current due to external noise. Fig. 3(b) shows theoperating area for the temperature and microwave current, where the current widths of bothconstant voltage steps of n=0 and n=1 are larger than 1 mA, i.e., the bias point is enough stableon the 1st step with typical external noise. The temperature range was about 1 K (12.0 ± 0.5 K)
and the microwave power range was about 3 dB (24.5 ± 1.5 dBm).

(a)

(b)
Fig. 3. (a) V–I characteristics of the dual-output PJVS circuit including 65536 NbN/TiN/NbN
Josephson junctions with microwave irradiation of 16 GHz measured at 12.0 K. The step height, i.e.,current width which exhibits constant voltage, is larger than 0.5 mA. (b) The n = 1 step height asfunctions of temperature and microwave current. The operating area was obtained at 12.0 ± 0.5 K
with the n = 1 step height of about 1 mA.
4. Conclusion
A compact programmable Josephson voltage standard (PJVS) system using a 2 V PJVS chip isunder development for the purpose of the new-generation secondary voltage standard withhigher accuracy and less calibration frequency than present semiconductor (Zener) standards. It
is demonstrated that the superconducting chip operates at 12 K with enough margins for theexternal noise. The chip requires moderate cooling power of <100 mW, enabling the use ofsmall-size cryocooler. All the main components of the PJVS systems, excluding the He-gas
compressor, have been assembled inside a standard 19-inch rack-mountable case. By the use of

a precision semiconductor voltage-multiplier circuit, the PJVS system can generate arbitrary
voltages up to 20 V.
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D8. High Capacity Optical Fiber Transmission Systems
H. TodaDoshisha University
In this section, I summarize recent development of high capacity optical fiber transmission
systems and related technologies.
Digital coherent technique is now indispensable for high capacity transmission systems. All of
the experiments described here without notice are related to digital coherent. One of the recent
trends is real-time coherent optical receiver and real-time digital signal processing [1, 2].
Compensation of impairments due to optical nonlinearity of transmission fiber is investigated [3, 4].
Error correction is important technique to greatly improve the performance of optical transmission
systems [5]. Real-time FPGA prototype [6, 7] was demonstrated with soft decision LSI [8].
For ultra-high capacity systems, 69.1 Tbit/s (432 WDM x 171-Gbit/s) transmission over 240 km
was demonstrated using PDM-16-QAM modulation [9]. High optical spectral efficiency can beachieved with multi-level modulation format. 6.4 bit/s/Hz with PDM 16-QAM [10], 9.0 bit/s/Hz
with PDM 64-QAM [11], 11.9 bit/s/Hz with PDM 256-QAM [12], and 13.2 bit/s/Hz with PDM
512-QAM [13] were obtained.

On the contrary, a review of direct-detection (incoherent)

multi-level modulation scheme as a simple alternative to the coherent schemes was made in [14].
Optical QAM signal can be obtained with an optical IQ modulator, which is used for optical QPSK
modulation, driven by multi-level electrical signals. Efforts have been made to generate optical

QAM signal with a single dual-drive Mach Zehnder modulator [15]. Other efforts are being madeto realize optical QAM modulator for high baud-rate QAM modulation. A hybrid configuration ofsilica planar lightwave circuit and LiNbO3 phase modulators for 64 QAM modulation [16], andmonolithically integrated quad Mach-Zehnder IQ modulator for 16 QAM modulation [17] aredemonstrated.
Multi-core fiber [18, 19] is also one of the key trends for increasing transmission capacity.
Efforts are being made for reducing propagation loss and inter-core crosstalk. Very recently, record
transmission capacity of 109 Tbit/s was achieved by using multi-core fiber. Modulation format
used in that experiment was QPSK. Therefore, increase of the transmission capacity can be easily
expected if more advanced modulation format is used.
CO (coherent optical) -OFDM has been studied for optical fiber transmission systems because of
its high dispersion tolerance and high spectrum efficiency [20, 21]. 10 WDM x 121.9-Gbit/s
transmission is demonstrated over 1,000-km standard single mode fiber without any inline
dispersion compensation [22].

7.0-bit/s/Hz spectral efficiency was obtained with 32-QAM

subcarrier modulation [23]. No guard-interval CO-OFDM scheme was demonstrated for ultimate
spectral efficiency [24, 25]. 13.5 Tbit/s (135 WDM x 111-Gbit/s) transmission over 6,248 km wasdemonstrated by the use of distributed Raman amplification [26].

Bit-rate-flexible all-optical

OFDM transceiver [27] and integrated-optic OFDM demultiplexer [28] were demonstrated.
For ultra-high speed transmission, OTDM can be used in conjunction with the digital coherent
technique [29]. 640 Gbit/s dual-polarization QPSK transmission [30, 31] was demonstrated. For
incoherent system, 640 Gbit/s single-polarization DPSK transmission with time-domain optical
Fourier transformation was demonstrated [32].
New wavelength of 1μm region for optical fiber communication is exploited with photonic
crystal fiber [33, 34], while applicability to conventional telecommunication band is investigated [35,36]. Efforts are continuously being made to obtain low propagation loss and enlarged effective
area [37].

List of Abbreviations
DPSK

Differential Phase-Shift Keying

FPGA

Field-Programmable Gate Array

OFDM Orthogonal Frequency Division Multiplexing
OTDM Optical Time Division Multiplexing
PDM

Polarization Division Multiplexing

QAM

Quadrature Amplitude Modulation

QPSK

Quadrature Phase-Shift Keying

WDM

Wavelength Division Multiplexing

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Tech. Digest of ECOC 2010, We.6.A.3, Torino, Sept. 19-23, 2010.
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D9. Radio-on-Fiber Technologies for Heterogeneous Wireless Services
K. Tsukamoto
Osaka University
1. Introduction
Ubiquitous network should offer users the environment to access any communication service

at any time, any place, and any situations. The demand of users for various multimedia services
will be increasing more and more; therefore full IP connectivity will be required toaccommodate the variety of contents. However, such diversification appears not only inapplication services but also in air interfaces of wireless access methods. The heterogeneous
wireless networks also strongly depend on users’ different demands for applications, quality,
latency, and moreover users’ situations such as indoor, outdoor, and fast/slow mobility.
Therefore, the platform for heterogeneous wireless network will become a key issue to realize
ubiquitous networks.
In current wireless networks, various operators independently overlaid their own radio basestations and networks. This leads redundant equipments and investments on infrastructures, andprevents the quick start of a new wireless service. From the viewpoint of the improvement in theradio frequency utilization, microcellular or picocellular architecture is much effective, however,
the implementation of different types of base stations and networks provided by different
operators prevent them from adopting microcellular due to its large investment. Therefore, thefuture ubiquitous wireless networks will need a cross frequency platform, which can becommonly used by various types of wireless services under the multi-air interferences
coexistence. In addition, various kinds of high-throughput mobile internet services should be
provided at any time and any place. It will be requested that mobile throughput should beequivalent to the optical fiber internet access service up to Gigabit per second [1-4]. The radio
regulation (RR) strictly also limits the transmitting power and bandwidth of RF signal. The
higher radio frequencies will be assigned to new wireless services because of the lack of
bandwidth at present. The new higher radio frequency bands such as microwave, millimeter
wave, and terahertz-wave are promising as resources for higher-throughput wireless
local/personal area networks. To accommodate a large number of access points, and also
increasing mobile traffic as increased opportunities accessing to mobile video streaming, HDV
download, wireless cloud services, or so on, advanced FMC (fixed and mobile convergence)
between broadband optical access and mobile access, will become more and more important [5].
RoF (Radio on Fiber) technologies are available to realize a universal platform for transparent
forwarding various types of wireless services under the multi-air-interferences coexistence inwireless cloud service environments. To realize future user centric wireless cloud service
networks, software definable radio network (SDRN) has been proposed [6], that can be
configured with the combination of RoF entrance networks, software definable radio gateway,
wireless service over IP, and radio agents.
2. Radio on Fiber Technologies
Radio on Fiber (RoF) link shown in Fig. 1 has a function of transferring radio signals into
remote stations with keeping their radio formats. Consequently, RoF link becomes a hopeful
candidate of a common platform for various wireless access networks. For example, mobile
carriers have widely installed RoF feeder systems in underground, in-building, and tunnels tosolve the dead zone problems in these areas [7-8] as shown in Fig. 2. Especially, in public
spaces such as underground stations and shopping center, three carriers commonly use a RoF
feeder to provide 2G (800MHz, 1.5GHz) and 3G (2GHz) services together in Japan. For

broadcasting applications, some feasibility experiments have been executed for TTL SFN relay
link [9], power-supply less RoF repeater for terrestrial SFN [10] and gap filler systems in rural
areas for terrestrial digital broadcasting. Recently, gap filler systems for mobile reception of
one-segment digital TV have been applied in underground [11]. Reference [12] summarizes
various applications of RoF technologies in mobile communications and broadcasting systems.
Recently, RoF applications has been developed to solve digital divide problem. Reference [13]reported the convergence project of RoF and CATV which was experimentally demonstrated
the simultaneous transmission of ISDB-T, WiMAX, and WiFi signals into rural areas where
broadband fiber-infrastructures have not yet been constructed due to their high cost and a low
population. A similar convergence of RoF and CATV, which is called as RFoG (Radio
Frequency over Glass) has been approved as a standard of the Society of Cable andTelecommunications Engineers (SCTE), SCTE 174 2010. RFoG has started to be demonstrated
in urban optical CATV systems. In order to improve the transmission quality of RF signals
against nonlinearities or reflection noises in RoF link, recently, digital RoF systems have beendeveloped [14].

Fig. 1. Radio on Fiber (RoF) links.

Fig. 2. RoF applications for mobile communications.

When RoF equips photonic routing functions, any radio signal can be forwarded to its
destination control station [6]. Such RoF networks have been called as “Virtual Radio Free
Space Network (VRFSN)”. By the use of RoF, the radio access architecture can easily employ
microcell, picocell or femtocell systems. Then, the RBS in each radio zone equips only theconverter between radio and optical signals. The RBS requires neither the modulation functions
nor demodulation functions of radio signals. The radio signals converted into optical signals aretransmitted via a RoF link with the benefit of its low transmission loss and broadband.
Therefore, RoF links can be independent of the radio signals format and can provide many
universal radio access methods. This means that VRFSNs are very flexible to the modificationof radio signal formats, the opening of new radio services, or the accommodation of somedifferent types of radio signal formats. A remote radio control station (RCS) executes functionsof modulation and demodulation of radio and other controls such as channel allocations, handover processing and so on. Such concentrated execution of their complicated function makes
radio access networks more simplified and cost effective, and promises easy realization ofrecent advanced radio techniques, such as hand over control, or interference cancellations.
Universal RBSs and entrance links mean a realization of layer 1 routing shown in Fig. 3, that is
especially effective in private areas at in-house, in-building, and underground at urban areas,
and rural areas with no broadband fiber-infrastructures and a low population. The layer 1
routing can be also realized RoFSO (Radio on Free Space Optics) [15, 16] or RoR (Radio on
Radio)[6] networks, which can provide a free space for heterogeneous wireless services in Free
Space Optics or millimeter wave radio. In each network, radio signals are converted into opticalfree-space optic signals or MMW signals with wideband frequency conversion.

Fig. 3. Layer 1 Routing in Radio on Free Space Network.

3. Software Definable Radio Networks
Figure 4 illustrates the concept of extended VRFSN, called Software Definable Radio Network(SDRN) and its configuration [6]. The SDRN is composed with Radio on Free Space (RoFSx)
networks, universal RBS and software definable control server (SDRCS) in IP network forvarious types of wireless services, and SDRGW (Software Definable Radio Gateway), which
provides a seamless connectivity between local RoF networks and IP network. Followings arefeatures:1) Virtual Radio Free Space Transporting Networking: RoFSx (Fiber, Optics, Radio, and LCXetc) networks can transparently connect multi-dimensional radio spaces with photonic intensity(1 dimension).
・ Universality for multi carrier operation: Various types of radio terminals can access a RBS,
and their RF signals are transparently transported in a RoFSx network, which provides atransparent entrance network between RBS and IP network, implements virtual free spaces forany radio signals in fiber, FSO, and MMW by equipping routing function to transfer the radio
signal to its desirable SDRGW.
・ SDRGW and WoIP: SDRGW has functions of air-interface conversion and IP packeterizationof wireless data and control signals. The later function is that the datagram in any radio signal
are converted to IP packet, which are transported to its center station altogether with the control
channel signal. This can realize a global cross layer platform on the IP network forheterogeneous wireless services networks, called “Wireless Service over IP (WoIP)”[6].

DRoF techniques is effective to realize RF signals over IP networks, where various types ofwireless services can be transparently supported by using IP routing abilities[17]2) Heterogeneous Radio Smart Space Construction: RoFSx networks can easily provide
microcellular architecture, and have an ability of delivering appropriate radio frequency
resources to users with the minimum power at any place and any time. With several distributed
universal BS, macro diversity reception, interference canceling, or SDMA can be easily realized.
Therefore, more improvement in the frequency efficiency can be expected.
3) User Centric Radio Space Control Agent: The SDRN can realize an easy spectrum delivery
to users, but its operation should be performed not only to enhance frequency efficiency but also
to satisfy users’ and operators’ demands. Even if accessing the same wireless services, user
utilities depend on their mobility and situations. In order to realize such seamless session
handover or service handover in heterogeneous wireless, we need the Radio Agent on Layer 7,
which controls layer 1-3 according to users’ and operators’ demands, and radio regulations.

Fig. 4. Concepts of Software Definable Radio Networks.

4. Ubiquitous antenna architecture distributed by RoF
As for the recent wireless radio technology
technology, multi-antennaantenna technologies are used in almost all
new wireless radio access such as MIMO (multiple(multiple-input multiple-output)
output) towards highthroughput and high reliable communication [18-20].
]. The heterogeneous wireless access iseffective to achieve higher dadatata rate and wider area coverage. RoF is effective to construct
heterogeneous networks to provide wireless cloud services with low cost. Figure 5 illustrates
MIMO system with ubiquitous antenna distributed by RoF. The ubiquitous antenna system is akind of distributed antenna system (DAS) using RoF network [21-28].
]. This network is
constructed with a centralized control station (CCS) and plural remote antenna elements
connected with CCS via RoF link. A remote antenna element haa only a function as a media
converter that provides radio--to-optical (R/O) and optical-to-radio
radio (O/R) conversion. The otherfunctions of radio modulation, demodulation, diversity, and handover are processed in the CCS.
Especially in MIMO, multiplexing and decomposition process of plural spatial streams is
processed at the CCS. At the remote antenna, heterogeneous RF services are transmitted forheterogeneous wireless services. In this configuration, the separation among multiple antenna
elements can be easily expanded so that MIMO spatia
spatiall filtering process overcome correlated
fading channel [18, 26].

RoF.
Fig. 5. MIMO system with ubiquitous antenna distributed by RoF

Therefore, the distributed MIMO antenna can increase the throughput in cell because itemphasize rich scattering environ
environment
ment and reduce the correlation among four signals overwireless multipath channels [1818, 26].
Recently, reference proposed a new configuration for RoF
RoF-MIMOMIMO DAS over WDM-PON,
WDM
and reported some results of basic experimental examination. Fully utilization of optical
wavelength resources is effective to accommodate a lot of remote station that are required forRoF-MIMO DAS with higher wireless capacity. Since the capacity of a wavelength channel islarge enough for heterogeneous wireless signals, such as various types of wireless services,
different radio frequency, some RF signals with different destinations. Then, multiplexing
method is required for not only remote antennas, but also RF services. References proposes a
new architecture to multiplexing method on RoF link based on the combination of optical time
division multiplexing (OTDM) and wavelength division multiplexing (WDM).
References[1] K. Prince, et al., “Converged Wireline and Wireless Access Over a 78-km Deployed Fiber
Long-Reach WDM PON”, IEEE PTL, 21, no.17, pp. 1274-1276 (2009).
[2] W-T. Shaw, et al., “An Ultra-Scalable Broadband Architecture for Municipal Hybrid
Wireless Access Using Optical Grid Network”, OFC/NFOEC 2009, OThP2 (March, 2009).
[3] K. Iwatsuki, et al., “Access and Metro Networks based on WDM Technologies,” IEEE JLT,
22, no.11, pp.2623-2630 (2004).
[4] K. Iwatsuki, “Application and Technical Issues of WDM-PON,” SPIE, OptEast, 7620-11,(2010).
[5] Junichi Kani, “Trends in Next Generation Optical Access Networks and a Proposed Hybrid
Optical/wireless Wide-area Access Network,” PIERS Online, Vol. 4, No. 1, 101-105, (2008).
[6] S. Komaki, ''Microwave Technologies for Software Radio Networks'', Proceedings of
APMC2003, Vol.3, No.FC7-1, pp.1780-1785 (November 2003).
[7] Y. Ito and Y. Ebine: “Radio on Fiber System for Triple Band Transmission in Cellular
Mobile Communication”, Tech. Digest of Int. Topical Meeting on Microwave Photonics, MWP
2000, TU1.2 Oxford, pp.35-38 (2000).
[8] Y. Horiuchi, “ROF Application to 3G Mobile Systems in Offices and Outdoors,” MWP
2005, p.3, Oct. 2005.
[9] M. Maeda, NHK STRL R&D No.90, pp.28-39 (March 2005).
[10] Y. Toba, et. al., "Development of a wide dynamic ranged radio wave receiving systems
with an optical modulators and DFB laser", IEICE Trans. on Elec., Vol.J88-C, No.2, pp.99-106(2005).
[11] http://www.nhkitec.co.jp/001topics/2005/002/index[12] T. Kuri , Y. Horiuchi , T. Nakatogawa , and K. Tsukamoto: ''Communication andBroadcasting Systems with Microwave and Millimeter-Wave Photonics Technologies'', IEICE
Transactions on Electronics, Vol.J91-C, No.1, pp.11-27, (January 2008) in Japanese.
[13] K. Kumamoto, K. Yasukawa1, K. Inagaki, T. Higashino, K. Tsukamoto, and S.
Komaki :''CONVERGECE OF BROADCASTING AND COMMUNICATION UTLIZINGCATV NETWORK'', Proc. of URSI AP-RASC2010, Toyama, Vol.1, No.DC2-3, p.1(September 2010).
[14] A. Nirmalathas, P. A. Gamage, C. Lim, D. Novak, R. Waterhouse, and Y. Yang, “Digitized
RF transmission over Fibers”, IEEE Microwave Magazine, June, pp.75-81 (2009)[15] Kamugisha Kazaura, Kazuhiko Wakamori, Mitsuji Matsumoto, Takeshi Higashino,
Katsutoshi Tsukamoto, and Shozo Komaki :''RoFSO : A Universal Platform for Convergence ofFiber and Free-Space Optical Communication Networks'', IEEE Communications Magazine,Vol.48, No.2, pp.130-137 (February 2010).
[16] Katsutoshi Tsukamoto, Akira Hashimoto, Yuji Aburakawa, and Mitsuji Matsumoto :''The
case for free space - Exploring the prospect of free-space optical links for backhaul
applications-'', IEEE Microwave Magazine, Vol.10, No.5, pp. 84-92 (August 2009).
[17] Bo Hu, Takeshi Higashino, Katsutoshi Tsukamoto, and Shozo Komaki :''Proposal of an RF
Transmission over the IP Based Network and its SNR Performance Evaluation'', Proc. of
Asia-Pacific Microwave Photonic Conference (APMP)2009, Vol.1, No.63, pp.1-4, (April 2009).
[18] A. Van Zelst and J. Hammerschmidt, “A Single Coefficient Spatial Correlation Model for
Multiple-input Multiple-output MIMO Radio Channel,” Proc. of URSI-GA, vol.1 (August,2002).
[19] E. Biglieri, R. Calderbank, A. Constantinides, A. Goldsmith, A. Paulraj, H.V. Poor, MIMO
Wireless Communications, Cambridge University Press, New York (2007).
[20] S. Karachontzitis, and D. Toumpakaris, “Efficient and Low-cost Complexity User
Selection for the Multiuser MISO Downlink, ” Proc. of PIMRC2009, vol.1, no.1, pp.1-5 (Sep.
2009).
[21] A. Kobyakov, D. Thelen, A. Chamarti, M. Sauer, and J. Winters, “MIMO Radio Signals
over Fiber in Picocells for Increased WLAN Coverage,” Proc. of OFC/NFOEC 2008.
[22] S.P. Monteiro, N.J. Gomes, A. Gameiro, and T. Kawanishi, “Next-generation Distributedand Heterogeneous Architectures: The FUTON Project,” Proc. of APMP2009, vol.1, no.1,
pp.1-4 (April 2009).
[23] C. Liu, and A. Seeds, “Transmission of MIMO Radio Signals over Fibre Using a Novel
Phase Quadrature Double Sideband Frequency Translation Technique,” Technical digest ofinternational Topical Meeting on MWP2008/APMP2008, vol.1, No.1, pp.23-27 (Oct. 2008).
[24] K. Yonezawa, S. Okamura, K. Tsukamoto, and S. Komaki, “Pre-weighting Space Division
Mutiplexing Reception Based on RoF Ubiquitous Antenna System,” Technical digest ofinternational topical meeting on MWP 2002, P3-10, pp.249-252 (Nov. 2002).
[25] L. Hong Hai, et. al.: ''Performance Improvement of Radio-on-Fiber Ubiquitous Antenna
System Using Sub-carrier Resource Management'', Proc. of SPIE -Broadband Access Common.
Technol., Vol.6390, ppB1-B11(Oct. 2006).
[26] T. Yamakami, T. Higashino, K. Tsukamoto, S. Komaki, “An Experimental Investigation of
Applying MIMO to RoF Ubiquitous Antenna System,” Technical digest of international Topical
Meeting on MWP2008/APMP2008, vol.1, No.1, pp.201-204 (Oct. 2008).
[27] S. Okamura, M. Okada, K. Tsukamoto, and S. Komaki, “Investigation of RoF link noise
influence in Ubiquitous Antenna System,” IEICE Trans. Electron., Vol. E86-C, no.8,
pp.1527-1535, (Aug. 2003).
[28] K. Tsukamoto, et al., "Convergence of WDM Access and Ubiquitous Antenna Architecturefor Broadband Wireless Services," PIERS Online, 6, no. 4, pp. 385-389 (April 2010).

D10. Broadband Access Networks with Wired and Wireless Integration/Convergence
K. Iwatsuki
NTT
Abstract-This report reviews recent research activities on the applications and technical issuesof WDM-PON that is the attractive approach for next generation access network considering thewired and wireless integration/convergence. First, we describe two candidate applications of
WDM-PON: one is a long-reach WDM-PON based on wavelength routing for metro/access andwired/wireless integration, and the other is a short-reach WDM-PON for co-existence withcurrent PON systems and broadband mobile backhaul networks.

We identify that therealization of colorless optical network units (ONUs) of WDM-PON is an important technicalissue for both applications, and an effective protection function is another important issue,
especially for the long-reach WDM-PON. We introduce several colorless ONU approaches aswell as introduce our recent research results to resolve the issues raised by some colorless ONU
approaches. In the second half of this report, we describe two application: one is the wide-area
wired/wireless integrated access network based on long-reach WDM-PON with loop-back type
colorless ONUs, and the other is the wired/wireless integrated access based on short-reach
WDM-PON coexisting with a current PON infrastructure and broadband mobile backhaul withtunable-type colorless ONUs.
1. Introduction
The recent explosive growth of the Internet has triggered the introduction of a broadband
access network based on fiber-to-the-office (FTTO) and fiber-to-the-home (FTTH). This trend
will dramatically accelerate from now due to further progress in and adoption of contents
delivery, TV telephony, and IP-TV services. To deal with the various demands, access andmetro networks must be scalable in terms of capacity and accommodation, and flexible withregard to physical topology [1].
Figure 1 shows the transmission speeds of core and access networks over time.

The

transmission speed of optical access has increased by around 100 times in the last decade. In
order to accommodate the huge traffic from the broadband access networks, the transmission
capacities of core networks are being dramatically accelerated with the use of time-divisionmultiplexing (TDM) and wavelength-division multiplexing (WDM) technologies; so that the
total transmission capacity of WDM can reach up to 3 Tbps (40 Gbps x 80 wavelengths). As
for the wireless access, the transmission speeds have also increased more than 100Mbps withwireless LAN (IEEE 802.11n), and accelerated up to 100Mbps mobile access with the Long
Term Evolution (LTE).

We have been challenging the broadband mobile access beyond

100Mbps with the 4G mobile standardization.
The most traditional optical multiple-access technology is time-division multiplexed access(TDMA) and TDM-based optical access networks have been deployed widely.

In current

TDMA optical networks, total bandwidth of around one gigabit is shared by several tens ofusers via an optical power splitter [2].

In Japan, the full-scale introduction of GE-PON

systems has been started, and the number of FTTH subscribers exceeded 15 million in 2009.
As the latest trend in standardization activities on the next generation optical access, IEEE
started developing the standard of 10 Gb/s Ethernet Passive Optical Network (10G-EPON) in
P802.3av Task Force in 2006; completion of its work is expected by 2009 [3]. The FSAN (fullservice access network) forum, which submitted B-PON and G-PON proposals to ITU-T, isstudying various technologies and architecture options for next generation optical accesssystems (e.g. WDM, 10 Gbit/s and increased reach/split) and how an operator would evolvefrom a deployed ITU-T G-PON (or IEEE GE-PON) to a next generation access system. [4].

10T

Bit rate (bps)

1T

40Gx8010Gx8010Gx48100G

WDM
100GbE

10G

10G

2.4G

1G

WDMWDM- PON ？
GbE

1.6G
100BASE-T

400M

100M
10M

10GbE
TDM

10BASE-T

1M
1980

Ethernet

1985

1990

10G-EPON

G-EPON
B-PON

Optical 802.11a
Access

1995

LTE
802.11n802.11g

802.11b
STM-PON
Wireless
LAN

2000

4G

WiMAX

Mobile3G(HSDPA)

2005

2010

Year
2015

Fig. 1. Transmission speeds of core and access networks.

The further enhancement of TDM access technologies beyond ten gigabits is facing various
barriers to cost-effective realization: the history of the core network has proven this.

In

addition, a higher-speed TDMA requires higher-rate burst-mode circuits. Given this, one of
the most attractive candidates for future optical access next to 10G-EPON and 10G-PON is
WDM-PON, in which each signal from/to each user occupies a different wavelength.
WDM-PON is a physically shared system, but a logically unshared system. This provides
certain advantages as follows.
1. The bandwidth for each user can be easily upgraded.
2. Various services can be provided per wavelength.
From the viewpoint of advantage 2, integration/convergence with wired and wireless accesswill flexibly provide broadband services in next generation access networks.

The recent

progress of cloud computing needs a broadband wireless access easily connecting to highdefinition videos and rich Web applications with smart phones and tablet computers anywhere,
at any time as shown in Fig. 2, so that one of the most important targets for next generation

wireless access is to realize a higher throughput equivalent to current optical access. Recent
proposals have aimed to provide current wireless and/or wired services over WDM-PON[5]-[8].
Application
servers

Smart home

Data storage Video delivery
servers
servers

Cloud computing

High speed
wireless
ONU

FTTH

Broadband networks

Net TV
ONU: Optical Network UnitHigh speed
wireless

Net book/Smart book

High speed
wireless

High speed
wireless

Smart phone
TabletFig. 2. Broadband networks.

This report reviews the technical issues of WDM-PON, some solutions, and theirapplications to simultaneously achieve wired and wireless access on the same platform.
Section 2 describes our focus on two candidate applications of WDM-PON: one is WDM-PON
based on wavelength router for metro/access integration and green field, and the other is a
short-reach WDM-PON co-existing with current PON systems.

We identify that therealization of colorless optical network units (ONUs) is an important technical issue for both
applications, and an effective protection function is another important issue, especially for the
long-reach WDM-PON.

Next, we review several ‘colorless-ONU’ approaches as well as

address several issues raised by some colorless ONU approaches.

We also describe along-reach WDM-PON architecture with a wavelength shifted protection scheme as well as loop
back type colorless ONUs. In Section 3, we show a wide-area wired/wireless integrated accessnetwork based on long-reach WDM-PON architecture.

Section 4 describes the otherarchitecture to provide wired and wireless services based on a short-reach WDM-PON thatcoexists with a current PON system and broadband mobile backhaul.
2. Technical issues of WDM-PON
Figure 3 summarizes the application and technical issues for two options to implement
WDM-PON: one is to use wavelength routers and the other is to use power splitters in the
optical distribution networks (ODNs).

The wavelength-router-based WDM-PON, where

wavelength allocation is basically static, is applicable to short-reach access for green field (e.g.
broadband mobile backhaul) and long-reach access due to the low loss-budgets of the
wavelength routers. The power-splitter-based WDM-PON is applicable to migration scenarios
from legacy PONs, since they use the same ODNs as the existing infrastructure.

Here,dynamic wavelength allocation is possible [9], because the power splitters are transparent towavelength. The researches of WDM-PON are mainly focused on two applications: one is along-reach WDM-PON based on wavelength routing for metro/access integration, and the otheris a short-reach WDM-PON for co-existence with current PON systems.
A pp lic at io n
W a ve le n gt h-ro u te rb a se d W D M -P O N
ON U
OLT

ON U

Is su e• Lo n g re ach(m e tro /ac ce ss• C o lo rle ss O N U
inte gra tio n)• Prote ction fo r lo ng• Sho r t r ea ch (last o nerea chm ile ) fo r gree n fie ld

N ote• W avelengtha llo ca tion isba sica lly sta tic .

ON U
P o w er-sp litt e r-b a se d
W DM -P O N
ON U
OLT

• Sho r t r ea ch (last o nem ile ) fo r m igra tio n• C o lo rle ss O N U
fro m le gac y PO N s

ON U• Dyna m ic
w ave lengtha llo ca tion ispo ssible [9 ].

ON U

Fig. 3. Applications and technical issues for two types of WDM-PON.

The common technical issue in both types of WDM-PON is the colorless ONU; the ONUs
should be colorless (in other words, no ONU is wavelength specific) to reduce the costs ofoperation, administration, and maintenance (OA&M) functions, as well as the production cost,
since mass production becomes possible with just one specification. Another issue, especiallyfor the long-reach WDM-PON, is a protection function that can counter feeder fiber breaks.
The rest of this section describes the various colorless-ONU techniques and the protection
function for the long-reach WDM-PON.
Local em ission
W ave le ngth
tu ning

W avelength supply

Spectru m slicing

Inje ction lo cking

1 fiber
O NU
configura tio n

N um be r o f fibe rs
C oh erency of
upstre am -signal
light

Rx

W
D
M

TLD

1
Coherent

Typical transm ission~1~10
0 Gbps o r ove r
ra teTechn ical iss ues

Wa ve leng th control
at O NU

Low-cost
imple m en ta tion

Rx

W
D
M

B LS

2 fiber

W
D
M
W
D
M

Rx

Loop ba ck

1 fiber

FP-LD
Rx
FP-LD

2 fiberRx

W
DM

M od +Am p

W
D
M

Rx
M od+Amp

1

1 or 2

In cohe re nt

Depe nds on the lockin gcondition a nd th e type of see dligh tDe pends on the type o f see dligh t< 1 Gbps (ASE -se eded/1-fibe r)~1 G bps(A SE-s ee ded/1-fibe r/direct-m od)~10 Gbps(LD -see ded/2-fibe r/ext-m od)•Back reflection•SNR of see d ligh t•In sufficien t lo cking [14],[15]•Back refle ction•SN R of se ed light~ 2.5 G bps(~10 Gbps
with FE C [12])Bea t noise of BLS

1 or 2In th e case of pow e r-splitte r-ba se d W DM-PO N, the w avele ngth con trol of TLD a nd w avele ngth sele ctors
a re ne ede d. In the ca se of w avele ngth-rou te r-based W DM -PO N, no w avelen gth sele ctors a re ne e de d,a nd th us only the wa velen gth control of TLD is n eede d.

TLD : Tunable laser diode, B LS: Broadband light source, Rx: ReceiverMMod:
od: M odulator, Am p: Optical amplifier:Wavelen gth selector (W S)

Fig. 4. Approaches to realize colorless ONUs. FEC; forward error correction; FP-LD; Fabry–Perotlaser diode; ASE, amplified spontaneous emission; SNR; signal-to-noise ratio.

Many colorless proposals have been reported [10]–[18]. The proposals can be summarizedinto two main schemes as shown in Fig. 4: local emission and wavelength supply. In the caseof the power-splitter-based WDM-PON, wavelength selectors (WSs) are needed. Note that noWSs are needed in the case of the wavelength-router-based WDM-PON, because thewavelength from/to each ONU is selected by the wavelength-router. We select the colorless
ONU scheme suitable for the application shown in Fig.3. Enabling technologies for the two
schemes are described in this section.
2.1. Local emission scheme
There are two local emission approaches: a wavelength tuning [10] and spectrum slicing [11],[12]. The ONU of the wavelength tuning approach consists of a tunable laser diode (TLD) asa transmitter (Tx), an optical receiver (Rx) with WS, and a WDM coupler that divides/combines
the upstream and downstream signals. The wavelength-tunable scheme is expected to achievea large loss budget and high speed operation at 10 Gbps, because a coherent light is used. To
achieve Tx and Rx tunability with ease for WDM-PONs based on the power-splitter, a tunablefiber ring laser is another option, where the wavelength selector might be simultaneously used
for selecting the downstream wavelength. We have reported the feasibility of the fiber ringlaser using a semiconductor optical amplifier (SOA) and an optical tunable filter (OTF), which
operated over a 20-nm wavelength range with accuracy of +/- 1.75 GHz [19].
The configuration of the ONU in the spectrum slicing approach is similar to that in the
wavelength tuning approach except that a broadband light source (BLS) with WS is usedinstead

of

the

TLD.

The

advantageofthespectrumslicing

approachinwavelength-router-based WDM-PONs is its light sources, e.g. superluminescent diodes (SLDs)
under quasi-athermal operation without precise wavelength control can be used. A particular
noise factor in the spectrum slicing approach is the signal-signal beat noise.

Thesignal-to-noise ratio (SNR) of the signal-signal beat noise can be expressed as being
proportional to the ratio of the data rate to sliced bandwidth [12]. To realize high speed
operation, broadening the sliced bandwidth is inevitable; it follows that the number of available
channels decreases assuming the light source has a fixed bandwidth. Broadening the sliced
bandwidth also increases the fiber dispersion penalty. The use of forward error correction(FEC) can effectively overcome this drawback. The 10-Gb/s, eight channel spectrum-sliced
DWDM transmission with the channel spacing of 200 GHz was confirmed through the use of
FEC [12].
2.2. Wavelength supply scheme
There are two approaches to wavelength supply scheme: injection locking [13]-[15] and loop
back approach [16]-[18]. In the injection locking approach, the ONU consists of a Fabry-Perotlaser diode (FP-LD) as the Tx, an Rx with WS, and a WDM coupler that divides/combines the
upstream and downstream signals and/or the injection light. The spectrum sliced light or laser
light is injected into the FP-LD, so as to lock the wavelength of the upstream signal. The
relative intensity noise (RIN) of the upstream signal (injection locked light) depends on theinjection locking condition [14], [15].
In the loop back approach, the ONU consists of an optical modulator (Mod) and opticalamplifier (Amp) as the Tx, an Rx with WS, and a WDM coupler that divides/combines the
upstream and downstream signals and/or continuous wave (CW) light. The CW light supplied
from the OLT is modulated and amplified by the Mod and Amp, respectively, to generate theupstream signal.

Reflection-II
modulated signal

Reflection-I

Wavelength
Supply module

CW

Tx
Rx

X

Rx

Reflection point

WDM-OSU……

Mod+AmpW
D
M

Wavelength-routeror power-splitter
ONU with loop back approach

WDM filter

OLT: Optical line terminal, OSU: optical subscriber unit, ONU: Optical network unit

Relative received power

OLT
CW
Modulated signal

SNR is maximized
SignalRe f-Iion
l ec tRef

I

le c tioRn -I +

efle

Optimum gain

c tio

n II

Reflection-I

ONU gain(b)(a)
Fig. 5. Impact of back reflection in loop back approach with one fiber configuration. (a) Path of two
back reflection light, (b) Increase in the impact of Reflection-II with ONU gain.

To avoid the degradation due to back reflection of the upstream signal, one option is to basewavelength supply on the two-fiber configuration; the long-reach WDM-PON described in
Section 3 assumes this configuration.

In the one-fiber configuration, two types of backreflection, the back reflection of continuous wave (CW) light from the OLT (reflection I) andthat of the modulated signal from the ONU (reflection II), as shown in Fig. 5 (a), must be
considered, because the interference between the signal and back reflection degrades thereceived SNR due to intensity noise [20]. The impact of reflection-II increases strongly with
ONU gain, so that the received SNR falls as ONU gain is increased, as shown in Fig. 5 (b) [20].
To reduce the impact of reflection-II, we employ a gain-saturated SOA in the ONU [21].
2.3. Protection function
In the long-reach WDM-PON, the failure of the feeder fiber carrying the WDM signals may
cause the loss of data for all users. Thus, a protection function for the feeder fiber is required
to realize the reliable WDM-PON.

Several approaches have been proposed to achieve theprotection needed [22]–[24]; they use 3-dB optical couplers to split (or combine) the path of
WDM signals to/from both the working and protection fibers in the OLT or in the wavelengthrouter. Note that the OLT and the wavelength router are typically located in the central office(CO) and in the access node (AN), respectively. This protection scheme, unfortunately, has a
low loss budget, because of the use of the 3-dB optical couplers [22]-[24]. To address thisissue, we have proposed and demonstrated a wavelength-shifted protection scheme utilizing thecyclic property of the 2×N athermal arrayed-waveguide grating (AWG) and two wavelength
allocations, one each for working and protection, which does not use 3-dB optical couplers forprotection [25]. While this protection scheme can be applied to WDM-PONs with various
types of colorless ONU, the loop-back approach is the most suitable choice because we do notneed any protection function in the ONUs.

3. Long-reach WDM-PON
3.1 Wide-area wired/wireless integrated access network
The growth of e-businesses is accelerating strong demands for broadband services over gigabit,
for business users. Such business users need wide-area broadband access networks that offer
high-speed connection services between several local area networks [16]. The long-reach
WDM-PON promises a wide-area access network that realizes consolidated operation to reduce
OA&M cost, because it can offer from gigabit to ten gigabit-class guaranteed-bandwidth
services according to user demand.
Figure 6 illustrates the wide-area wired/wireless integrated access network based on long-reach
WDM-PON [26]. OLTs and an optical carrier supply module (OCSM) are located in thecenter node; the OCSM is an optical frequency comb generator that supplies multi-wavelength
carriers to several OLTs. The OLT has interfaces for radio-on-fiber access (RoF IFs) as wellas those for high speed optical access such as 10 Gigabit Ethernet (10GE IFs). The RoF
approach simplifies the system architecture by directly delivering broadband signals of radio
frequency through optical fibers.

Center nodeServers

RF term

…

L2SW

…

OFCG

OLT: Optical line terminal
ONU: Optical network unit
G-MUX: Wavelength-Group multi/demultiplexer
lMUX: Wavelength multi/demultiplexer
OCSM: Optical carrier supply module

RoF IFs 10GE IFs

OLTBuilding/
Apartment
Remotenode

Remotenode

Metro area
e.g. ~40 km

G-MUX

10GE
l
M
U
X

ONU

HUB

…

G-MUX

…

Building/
Apartment…

Building/
Apartment

Access area
e.g. 10 km

BS

Building/
Apartment

Fig. 6. Wired/wireless hybrid wide-area access network.

One of the most attractive RoF approaches over 100Mbps with the millimeter band is toheterodyne two optical frequencies whose difference corresponds to the radio frequency (RF),e.g. 60 GHz [27]. By modulating one of two optical frequencies before fiber transmission, theRF signal can be extracted by heterodyne detection after fiber transmission. This approach caneliminate not only the local RF generator in each base station (BS), but also achieve better
fiber-transmission performance than sub-carrier modulation (SCM) approach due to the fiber
dispersion. However, the two optical frequencies must be precisely controlled to generate astable RF signal after detection.

The OCSM that can inherently provide such precise

frequency control between any two wavelengths are an attractive candidate for this purpose aswell as for multiplexing many different RF signals.
This network uses DWDM wavelength channels (e.g. 64 channels) and the group
multi/demultiplexer (G-MUX) in each remote node divides/combines the DWDM channels
into/from several groups of wavelengths (e.g. 8 wavelengths x 8 groups). Each wavelength
group is dedicated to a building/apartment, where a wavelength pair can provide either opticalaccess via an ONU or RoF access with a BS (e.g. 1 x 10GE-ONU and 3 x RoF BSs in eachbuilding/apartment).
3.2. System Configuration of wide-area wired/wireless integrated access network
Figures 7 (a) and (b) show the configuration and optical spectrum of the OCSM that wereported previously [28]. By modulating the CW lights of seed laser diodes (LDs l1 to ln in
the figure) with a phase modulator (PM) and an intensity modulator (IM) using 25-GHz radio
frequencies with appropriate amplitudes, we can obtain multi-wavelength optical carriers as themodulation sideband as shown in Fig. 7 (b); the spectrum consists of four wavelength groups
where each group consists of eight wavelengths with 25-GHz spacing. Typically, low-cost
optical filters for group multi/demultiplexing require an adequate guard-band between
neighboring groups. As shown in the spectrum, this OCSM is very suitable for such a group
multi/demultiplexing because one can easily adjust the spacing between neighboring

Oscillator
LD l1
LD l2

…

M
U
X

PM

IM

LD ln

Relative optical power (dB)wavelength groups by adjusting the seed wavelengths.
-20
-30
-40
-50
-60
-70
-80
1575

LD: Laser Diode, MUX: Wavelength Multiplexer
PM: Phase Modulator, IM: Intensity Modulator

1580

1585

1590

Wavelength (nm)

(b)

(a)

Fig. 7. (a) Configuration and (b) optical spectrum of optical carrier supply module (OCSM).

Figure 8 shows a detailed configuration of the proposed network system. Wavelength pairs to
be used for an ONU or a BS are lk1/lk3, lk2/lk4, lk5/lk7, lk6/lk8 where k is the group number (k =1 ~ n).

In the case of the OCSM shown in Fig. 7 (b), n is 4; we can increase n by adding seedwavelengths to OCSM. The center node consists of n sets of modulation arrays (Mod array), n
sets of receiver arrays (Rec array) and group multi/demultiplexers (G-MUX/G-DMX). Each
Mod

array

multi/demultiplexes

the

eight

wavelengths

in

each

group

by

using

arrayed-waveguide gratings (AWGs), and lk1, lk2, lk5, lk6 are modulated by 10Gbit/s binary
NRZ data (for optical access) or 10GHz-band IF data (for RoF access). In the remote node,
the G-MUX/DMX divides/combines signals by wavelength groups. In the building, AWGs
multi/demultiplex the eight wavelengths. The BS receives the RoF signal by heterodyning the
wavelength pair (e.g. lk1/lk3) to generate a wireless signal with 60-GHz RF as well astransmitting the RoF signal by remotely modulating and transmitting one of the wavelengthpairs (e.g. lk3) upstream [29]. The ONU simply receives one modulated wavelength pair (e.g.
lk2) as well as modulating/demodulating another wavelength pair (e.g. lk4).

Figure 9summaries the signal spectra of OLT, ONU, and OLT. By using this system configuration, we
can realize a wide-area access network that provides both high-speed optical access andfiber-wireless access flexibly depending on need.

BS

mod

mod

l11
l12
l13
l14
l15
mod
l16
l17
l18

OLT

mod

G-DMX

AWG

G-MUX

G-DMX

mod

AWG

OFCG

Mod array 2
…

l-MUX

l11
l12
l13
l14
l15
l16
l17
l18

Mod array 1

10GHz-band IF data or
10Gbps binary data

AWG

PD
mod

Center node
Remote
node

AWG

ONU
UNI

Building
l11
l12
l13
l14
l15
l16
l17
l18

PD

G-MUX

Mod array n
Rec array 1

G-MUX

AWG

G-DMX

10GHz-band IF data or
10Gbps binary NRZ
l11 data
l12
l13
PD
l14
PD
l15
l16
l17
PD
l18

PD

Rec array 2…

Rec array n

Fig. 8. System configuration of optical/wireless hybrid wide-area access network.

OLT output

l1 1
9.6 GHz25 GHz

l 12

l 13

l 14

10G NRZ downstream
l 12

l 14

W avelength

ONU input
W avelength
10G NRZ upstream

ONU output
l 11

l1 3

BS input

59.6 GH z

BS output

60.0 GHz

10.0 GHz

10GH z IF signal in OLT

W avelength

Wavelength

W avelength

Fig. 9. Signal spectra of OLT, ONU and BS.

4. Short-reach WDM-PON
4.1. Co-existence with current PON system
Figure 10 illustrates the migration to WDM-PON, that accommodates wired and wirelessservices, from the current PON system via the common power-splitter-based ODN.

The

WDM-OLT and the WDM-ONU in Fig. 10 for WDM-PON can be simply added withoutchanging the ODN: the WDM-OLT comprises multiple optical subscriber units (OSU), each ofwhich communicates with a WDM-ONU. The detailed wavelength allocation is also shown in
Fig. 10.

According to the standardization bodies, the upstream and downstream wavelengthsof the current PON systems range from 1260 to 1360 nm and from 1480 to 1500 nm,
respectively. The wavelengths for video use lie in the range of 1550 to 1560 nm. The
simplest approach to wavelength allocation for WDM-PON is to use the L band, which islocated in the wavelength region that is longer than that reserved for video use. On theassumption that many wavelengths will be needed to accommodate a large number of users,
DWDM technologies will play an important role in efficiently utilizing the limited wavelength
region available.
WDM-PON

Existing PON

13
60

V-ONU

12
60

ONU

UpstreamVideo use (1550-1560nm)
Downstream

14
8
15 0
00

Home
Existing PON

nm

Central office
Existing PON

OLT
V-OLT

WDM-ONU
WDM-OSU(1Gbps binary data)

Power splitterBS

WDM-OSU(Microwave RF signal)

WDM-OLT
WDM-ONU

Fig. 10. Migration to WDM-PON from current PON system over power-splitter-based optical
distribution network (ODN).

To construct an easy to use power-splitter-based WDM-PON for mass users, plug-and-play
functionality is essential [10]. Plug-and-play means automatic wavelength control of the ONU,
which is as critical as the precise time control needed in the current PON based on TDMA.
We demonstrated wavelength-tuning colorless ONUs with the plug-and play function [10], fromthe viewpoint of realizing the transmitter without WS. Wavelength control can be achieved
with initial wavelength setting and wavelength stabilization.

The former remotely andautomatically establishes the initial wavelengths between each ONU and the OLT. After the
initial wavelength is set, wavelength stabilization is carried out. This is especially critical if
the channel spacing is narrow, and thus the multi-wavelength monitoring technique [30] isrequired.

Fig. 11. Performance of the colorless O
ONU with tunable DWDM-SFP
SFP for the WDM-PON
WDM
co-existing
existing with a legacy PON, (a) optical spectra, (b) loss budget
budget.

Figs. 11 (a) and (b) show the optical spectra and loss budget of the colorless ONU based on a
SFP (Small Form-Factor
Factor Pluggable) for the WDM
WDM-PON that can co-exist
exist with a legacy PON[31]. The DFB-LDs
LDs in SFPs are directly modulated with 1.25Gbps binary data. The 3-nm
tunable range, which provides 8 channels with 50 GHz spacing, was verified in [[29]:wavelength tuning was realized via temperature cocontrol
ntrol of the laser in the SFP. The loss
budget between the WDM-OSU
OSU and colorless ONU (WDM
(WDM-ONU in Fig. 1010) was reported in[30] as over 36 dB, which is sufficient to support the adoption of legacy ODNs. To provide
wireless services with microwave band sim
simultaneously over the WDM-PON,
PON, digitized RoF
techniquess [32] are available to directly modulate the DFB
DFB-LDLD in SFP with digitized RF signals.
4.2. Broadband mobile backhaul
Because of the strict limitation imposed by the radio frequency spectrum,, the increase
increa of thewireless access throughput needs to the reduction of cell size, and thus wired access networks
accommodate a huge number of base stations (BSs)) to cover a wide area. The femto-cellarchitecture provided by broadband optical access network is a promisingromising approach to realize
the next generation broadband wireless access
access. We have proposed the radio over fiber (RoF)
distributed antenna system (DAS)
DAS) over WDM-PON architecture [33] to combine WDM access[1] and a DAS with multiple input multiple output (MIMO) technologies [34]], as shown in Fig.
12.

The provision of various wireless services per wavelength in WDM-PON
WDM
can beaccommodated with RoF-DASDAS technologies
technologies..

This technical convergence will achieve theuniversality for various types of broadband air-interfaces
interfaces expected to the Gbps class throughput
throughput,
the flexibility for non-uniform
uniform traffic distribution
distribution, and the user-mobility
mobility in wireless service area.
area

WDM-PON
technologies

Distributed antenna system
BS (Base Station)

BS

BS

BS

BS

BS

BS

BS

BS

Optical Fiber

Internet

CS(Center Station)

Optical Fiber

Optical Fiber
MIMO
technologies

RoF
technologies

Fig. 12. Broadband mobile backhaul based on WDM-PON.

Figure 13 illustrates a configuration of WDM-PON, where two wavelengths are assigned toeach BS as an up- and down-stream. We employ a 2 x N arrayed waveguide gratings (AWGs)
as a wavelength multi/demultiplexer, to achieve a low dissipation for accommodating a large
number of BSs in single feeder fibers. At the CS, four MIMO RF signals for different fourcells are multiplexed in the MIMO signal processing in Fig. 12. Each BS consists of an opticalcirculator without any wavelength selectivity to make it easy to add upstream signals and drop
downstream signals. Optical filters, such as fiber gratings, at the BS are used to select awavelength assigned to the BS. This configuration achieves the transparency of transmission
lines through its lack of optical filters, and thus has scalability of upgrading wavelength
channels in the future. To decrease the costs of operation, administration, and maintenancefunctions, as well as the production cost, we can introduce wavelength tunability in LDs andfiber gratings of BSs; that is colorless BS.

2xN AWG
One
wavelength
shift

BS

BS

BS

BS

BS

BS

BS

Cell

Downstream

CS

BS

MIMO

BS

BS

BS

BS

BS

BS

BS

BS

BS

BS

BS

BS

Upstream
Optical branch

Tｘ

CS
RF
RF
RF

Tx
Tx

Fiber
grating

・・・

・・・

MIMO
signalprocessing

BS

Tx

Circulator
RF
RF

Rｘ

・・・

・・・

MIMO
Rx
Rx
signalprocessing
RF
Rx

Fig. 13. Configuration of WDM-PON.

At the CS and BSs, four MIMO RF signals can be multi/demultiplexed over a wavelength,
and transmitted by using subcarrier-multiplexing (SCM) technique. As for the multiplexing
techniques of four MIMO RF signals, we can employ frequency division multiplexing (FDM),

time division multiplexing (TDM), or code division multiplexing (CDM). Since the FDM
needs the frequency conversions of MIMO RF signals at CS and BS, the transparence of radio
air-interfaces is diminished. Thus, the TDM or CDM is suitable to multiplex the MIMO RF
signals. As compared with the TDM, CDM is complicated, and the sampling rate is higher
than that of TDM. By using TDM, each MIMO RF signal is simply bandpass-sampled [35]and multiplexed with time domain, so as to achieve the transparency for different types ofbroadband radio air-interfaces.

In addition, the TDM can easily extend to digitized RoF

approach with the use of analog/digital and digital/analog converter, which mitigates therequirement for linear device characteristics of optical components [32].
Figure 14 shows our experimental setup to transmit downstream signals of RoF-DAS over
WDM-PON with optical TDM and the signal transition diagram at each component. We
assumed that 2 x 2 MIMO signal transmission in a cell. Two wavelengths were used to realize
2 x 2 MIMO signal transmission; two 802.11n RF signals in the 2.4 and 5 GHz bands were
multiplexed in each wavelength with optical TDM and transmitted to the BS.
The two continuous lightwaves emitted from DFB-LDs were wavelength-multiplexed in Fig.
13 (a), and modulated with a lithium niobate Mach-Zehnder modulator (LN-MZM) driven witha return to zero (RZ) pulse of 100 ps pulse width at a 1 GHz repetition rate from a pulse patterngenerator (PPG) in Fig. 13 (b). Two wavelength signals were amplified with an erbium doped
fiber amplifier (EDFA) and demultiplexed with an AWG of 25 GHz wavelength spacing.
Each wavelength was divided with a 3 dB coupler and modulated with each LN-MZM driven
by 802.11n RF signals with 40 MHz bandwidth whose center frequencies were 2.422 GHz (ch3) and 5.230 GHz (ch 46) from the vector signal generator (VSG) in Fig. 13 (c). At the output
of each LN-MZM, we obtained the bandpass-sampled RF signals of the 2.4 and 5 GHz bands.
The modulation and coding scheme (MCS) index of 802.11n RF signals was 15 [36] i.e.
64-quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing(OFDM) signals. We employed optical delay lines and 3 dB couplers to optically multiplex
the bandpass-sampled RF signals and dummy RF signals of an adjacent cell, as shown in Fig.
13 (d).

The optically multiplexed RF signals were wavelength-multiplexed again andtransmitted through the optical feeder fiber as a downstream signal.
The transmitted signals from the CS were distributed to each BS with the AWG, and thewavelength assigned at the BS was dropped by the 3-port circulator. The downstream signal
dropped at the BS was converted by the photo diode transimpedance amplifier (PD-TIA), anddemultiplexed with a high-speed electrical switch (SW) in Fig. 13 (f). The SW was driven bya control signal with a 300 ps pulse width at 1 GHz repetition rate from the PPG. The
demultiplexed RF signals were converted to original RF signals with the band pass filter (BPF),
which were transmitted as wireless signals from antennas at BS in Fig. 13 (g).

Center Station (Tx)
Synchronized with BS

1552.524 nm(193.1 THz)
DFB-LD

VSG

ANT1

VSG

ANT2

(2.4/5GHz)

FadingSimulator(2.4/5GHz)lD1

10G PPG

Delay

PC

LN-MZMPC

LN-MZM

1GHz

Delay

100ps width(a) RZ
pulse

lD1

DFB-LD

(d)

(c)

PC

(b)

lD2

Delay
EDFA

1553.329 nm(193 THz)

AWG

(a)
lD1
lD2
CW
lightwave

100ps (b)tPC

LN-MZMPC

LN-MZM(d) Dummy signals

RZ pulse
modulatedtlD1

Synchronizedwith CSt

5 GHz ANT2Superimposed
RF signals(Bandpass-sampling)tt

2.4 GHz ANT2t

Upstream port(i)

PD-TIA(O/E)

From feeder fiber
10G PPG

t

t

t

(f)
SW

Cont.

2.4 GHz

(ii)

(iii)

BPF
BPF
5 GHz

t

1000psObjective cell

Adjacent cellt

Dummy signal
generation and OTDMtt
1000psObjective cell

Adjacent cell

Transmitted optical
WDM/OTDM signals

(g)
ANT1
ANT1

(iv)
(v)
VSA
(2.4GHz/5GHz)

Base Station 2 (Rx)
lD2

t

t

t

(e)

for adjacent cell

Base Station 1 (Rx)VOA

To feeder fiber

Delay

(c)

5 GHz ANT1

t

(e)
EDFA

Delay

lD2

2.4 GHz ANT1

1000pst

Delay

LN-MZM

Upstream port

VOA

2.4 GHzPD-TIA(O/E)
10G PPG

SW

BPF
BPF

Cont.

5 GHz

ANT2

(g)

(f)
Time division
demultiplexing

ANT2

2.4 GHz ANT1tt

5 GHz ANT1tt

RF signal regenerationand transmission over air

2.4 GHz ANT2tt

5 GHz ANT2ttFig. 14. Experimental setup and signal transition diagram.

Figures 15 (a) and (b) show the analyzed RF signals of lD1 and lD2, respectively. The
EVM values of -22.7 and -21.9 dB were obtained, respectively. The optical powers at the
PD-TIA input and the electrical powers at the VSG output were the same values without themultipath fading. Figure 14 (c) and (d) depict the the RF spectrum of of lD1 and lD2. The
experimental results confirm the basic feasibility of optical TDM and bandpass-sampled MIMO
RF signal transmission in the proposed RoF-DAS over WDM-PON architecture.
(a)(b)
2.4 GHz bandλD1 constellation

2.4GHz bandλD2 constellation

EVM : -25.6 dB(c)

EVM : -24.1 dB(d)

2.4 GHz bandλD1 spectrum

2.4 GHz bandλD2 spectrum

Fig. 15. Expermental results of MIMO RF signal transmission without multipath fading. (a) and(b) constellation of 2.4 GHz band transmitted RF signal in lD1 and lD2, respectively. (c) and (d)2.4 GHz band transmitted RF spectrum in lD1 and lD2, respectively.

5. Summary
We have described the technical issues, solutions, and applications of WDM-PON. The
WDM-PON can construct scalable and flexible networks to provide wired and/or wirelessservices, so it is the leading candidate for the next generation optical access systems beyond 10
Gbps TDMA systems.

The main technical issue of WDM-PON was how to realize thecolorless ONU. Two candidates, local emission and wavelength supply, were summarized andtheir features were explained in detail. The requirements and technical issues related to theprotection function for the feeder fibers of WDM-PON were elucidated. We described theapplications of long- and short-reach WDM-PON. The former offers wide-area wired/wireless
integrated access networks that can accommodate the large number of business users expected.
The latter enables migration from the current PON systems by utilizing the current
infrastructure.

The requirements and technical issues associated with the plug-and-play

function, which remotely and automatically establishes the initial wavelengths of each ONU,
were discussed.

The wireless services can be also accommodated with the short-reach

WDM-PON utilizing the microwave band RoF techniques. We also mentioned broadband
mobile backhaul utilizing short-reach WDM-PON based on wavelength router. The radio overfiber (RoF) distributed antenna system (DAS) over WDM-PON architecture was proposed to
combine WDM access and a DAS with multiple input multiple output (MIMO) technologies.
The preliminary experiments were demonstrated to confirm the feasibility of the proposed
network architecture.
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D11. Body Area Network
Y. KadoKyoto Institute of Technology
The advancement in sensing, embedded computing, and wireless communication has
accelerated the progress of adopting sensor networks to form Wireless Body Area Networks(BAN or WBAN). BAN integrates wireless sensors, RFID tags, and other pervasive devices
within and around human bodies for constructing diverse and practical systems for human
computer interaction, physiological monitoring, and provision of entertainment. In the past few
years, much of the research in the area of BAN has focused on challenges related to channel
model, reliability, flexible network architectures, wearable transceiver designs, lightweight
security solutions, and lifestyle applications.
An important step in the development of BAN is the characterization of the electromagnetic
wave propagation from devices that are close to or inside the human body. As the antennas for
BAN applications are placed on or inside the body, the BAN channel model needs to take into
account the influence of the body on the radio propagation. The IEEE 802.15.6 Task Group isdeveloping a communication standard optimized for low power devices and operation on, in or
around the human body to serve a variety of applications [1]. To develop more detailed models,
the static path loss and impulse response models for BAN were also reported [2].
The human body is not an ideal medium for radio frequency wave transmission. It ispartially conductive and consists of materials of different dielectric constants, thickness, andcharacteristic impedance. Therefore depending on the frequency of operation, the human bodycan lead to high losses caused by power absorption, central frequency shift, and radiation
pattern destruction [1]. The absorption effects vary in magnitude with both frequency of appliedfield and the characteristics of the tissue. This makes it difficult to drive a simple path loss
model for BAN and design antennas of wearable transceivers.
One of the most promising approach to avoid this problem is a near-body quasi-static
electric-field communication system that uses the surface of the human body as a datatransmission path [3], [4], [5], [6]. A key aspect of the technology is the behavior of analternating current (AC) electric field, which can be summarized in terms of three components
produced around the body when an AC signal is applied to a parallel-plate electrode attached tothe body. These components, which are all related to distance r from the body, are: thequasi-static electric field (proportional to 1/r3), the induced electric field (proportional to 1/r2),
and the emitted electric field (proportional to 1/r). The near-body electric-field communications
technology uses the quasi-static electric field, which attenuates steeply with r [6].
As shown in Figures 1 and 2, the system consists of a transmitter that emits an ACelectric-field signal modulated by input data from an electrode and a receiver that uses anotherelectrode to read the weak AC electric field on the body induced by the signal and demodulates
it to recover the data from it. The electronic devices in contact or in close proximity to thehuman body can use its conductive properties to establish body coupled communication
between each other. This human centric communication paradigm can be used for BAN toreduce the impact of interference on/from RF systems, to avoid the fading effect that the bodyhas on radio systems and to enable power efficient wireless links.
To suppress electromagnetic radiation, NTT adopted a baseband transmission system with a
newly developed electrooptic (EO) sensor in which the quasi-static electric field is directly
modulated by 10 BASE-T data signal. They succeeded in 10-Mb/s data transmission throughthe human body and discussed the advantages of using the EO sensor for intrabodycommunication [7], [8], [9]. For this technology to come into wide personal use, the transceivermodule must be compact and inexpensive, so the electronic components should be fabricated as
LSI chips. NTT has developed a small and low cost multi-chip module for miniaturization ofcommunication device [10], [11]. The communication quality of the prototype terminals
implementing them was evaluated [12], [13].
The body coupled communication implementations using a wideband correlation-basedtransceiver [14], an inductive coupling transceiver [15], and a scalable double-FSK modulationtransceiver have been successively reported. The main applied field of these transceivers is
wearable body sensor network.

References[1] K. Y. Yazdandoost and K. Sayrafian-Pour, “Channel Model for Body Area Network (BAN),”
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IEICE Transactions on Electronics, Vol. E93-C, No. 3, pp. 234-243, 2010.

[7] A. Sasaki, A. Furuya, and M. Shinagawa, “Study of semiconductor electro- optic modulators

for sensing extremely-low-frequency electrical signals”, Sensors and Actuators A, vol. 151, no.
1, pp. 1-8, 2009.
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[10] Y. Kado and M. Shinagawa, “RedTacton Near-body Electric-field Communications
Technology and Its Applications”, NTT Technical Review, Vol. 8, No. 3, Mar. 2010.
[11] R. Kawano, F. Morisawa, M. Shinagawa, and Y. Kado, “LSIs for an Advanced Compact
Electric-field Communication Module”, NTT Technical Review, Vol. 8, No. 3, Mar. 2010.
[12] A. Sasaki, R. Kawano, T. Ishihara, and M. Shinagawa, “Modeling of Human-bodyNear-field Communication and Evaluation of Communication Quality”, NTT Technical Review,Vol. 8, No. 3, Mar. 2010.
[13] T. Minotani and M. Shinagawa, 2010, “Highly Efficient Signal-Inducing Transceiver and
Evaluation over Human Body”, AP-RASC'10, Toyama, BCK-1,2010.
[14] A. Fazzi, S. Ouzounov, and J. Homberg,” A 2.75mW Wideband Correlation-BasedTransceiver for Body-Coupled Communication,” Dig. Tech. Papers of 2009 IEEE International
Solid-State Circuits Conference, pp. 204-205, Feb. 2009.
[15] S. Lee, J. Yoo, H. Kim, and H.-J. Yoo, “A Dynamic Real-time Capacitor CompensatedInductive Coupling Transceiver for Wearable Body Sensor Network,” 2009 Symposium on
VLSI Circuits Digest of Technical Papers, pp. 42-43, 2009.