COMMISSION D: Electronics and Photonics(November 2013 - October 2016)
Edited by Katsutoshi Tsukamoto, Osaka Institute of Technology______________________________________________________________________________________
Electronics and Photonics cover huge scientific and technological fields. Among them, in this report, we haveselected several important topical areas, where in particular Japanese leading researchers have actively
contributed between 2013 and 2016. They overviewed recent remarkable advances in their specialized areas.
The following is a list of areas that the chapter editor has taken up.

D1. Quantum Cascade Lasers and Optical Antennas
Kenichi Kasahara
Ritsumeikan University
D2. Current-Injection Terahertz Lasing in a Distributed-Feedback Dual-Gate
Graphene-Channel Transistor
Taiichi Otsuji
Tohoku University
D3. Terahertz Photonic Crystals
Masayuki Fujita
Osaka University
D4. THz-Wave Emitters and Detectors Based on Diode Technology
Hiroshi Ito
Kitasato University
D.5 Millimeter-wave and submillimeter-wave antennas in standard CMOS technology
Eiichi Sano
Hokkaido University
D6. Space-Division-Multiplexed Optical Fiber Transmission
Hiroyuki Toda
Doshisha University
D7. Recent research trends in optical access network technologies
Jun-ichi Kani
NTT Corporation
D8. Optical Access Networks for broadband IoT services
Naoto Yoshimoto
Chitose Institute of Science and Technology
D9. Construction of resilient cyber physical system based on ICT
Katsumi Iwatsuki
Tohoku University1D1. Quantum Cascade Lasers and Optical Antennas
Kenichi Kasahara
Ritsumeikan University

Significant progress in mid-infrared distributed-feedback quantum cascade lasers (QCLs) capable ofcontinuous-wave operation above room temperature has allowed for the monitoring of trace gases relating toenvironmental and energy issues. To extend the application of QCLs, a modular waveguide mounting accessory
speciﬁcally designed for on-chip thin-ﬁlm GaAs/AlGaAs waveguides was reported [1]. GaAs slab waveguides
with dimensions of 5 × 10 mm of different thickness (6 and 10 μm) were combined with a tunable QCL providing
a wavelength range of 5−11 μm (1925−885 cm−1) within a single collimated beam serving as evanescent ﬁeld
sensing transducers in lieu of conventional attenuated total reﬂection crystals. The waveguide mount was designed
such that modular adaptation to any infrared optical assembly ensuring precise self-realignment was enabled. The
sensor response to 50 μL of ammonium perchlorate solutions with concentrations of 0.01, 0.05, 0.1, 0.5, and 1 M
using a 10 μm, and for comparison, a 6 μm thick GaAs thin-ﬁlm waveguide was compared to results recorded via
a Fourier transform infrared (FTIR) spectrometer. The QCL-based measurements revealed comparable results toconventional FTIR-based studies. The 6 μm waveguide provided a higher sensitivity in contrast to the 10 μm
structure, which was attributed to the enhanced intensity of the evanescent ﬁeld with increased mode conﬁnement.
However, it was found that the overall energy throughput was approximately ten times lower for 6 μm
waveguides compared to the 10 μm structures, because the coupling eﬃciency into the waveguide was reduced atsmaller waveguide dimensions.
On the other hand, the progress towards terahertz QCLs has been made since their birth in 2002 [2], andterahertz QCLs can now deliver milliwatts or more of continuous-wave coherent radiation throughout theterahertz range. However, it is still a challenge to increase their operating temperatures. One approach toovercome this problem is to use the Cherenkov difference-frequency phase-matching scheme. Intracavity
difference-frequency generation in mid-infrared QCLs with a dual-upper-state (DAU) active region was used to
get terahertz oscillation [3]. DAU active region design was theoretically expected to produce larger opticalnonlinearity for terahertz difference-frequency generation, compared to the active region designs of thebound-to-continuum type used previously. Fabricated buried heterostructure devices with a two-section buried
distributed feedback grating and the waveguide designed for Cherenkov difference-frequency phase-matching
scheme operated in two single-mode mid-infrared wavelengths at 10.7 µm and 9.7 µm and produced terahertz
output at 2.9 THz. The mid-infrared to terahertz conversion efficiency was 0.8 mW/W2 at room temperature.

To broaden the application of QCLs, photo-detectors exhibiting both high-sensitivity and high-speed areneeded in the mid-infrared range. Optical antennas generate optical hotspots, and enhance local field intensity
near the antenna, thereby allowing for efficient photon harvesting. Thus, sufficient absorption is obtainable evenfrom a thin absorption layer, and it is possible that a combination of optical antennas and a thin photo-absorption2

layer leads to the achievement of high-performance mid-infrared detectors with high-sensitivity and quick
response times. Field intensity becomes large near an antenna, and decreases rapidly with increasing distancefrom it. Accordingly, it is necessary to know the increased field distribution of the antenna in the vertical direction
for designing mid-infrared photo-detectors equipped with antennas. The finite-difference time-domain (FDTD)
method is a powerful tool for use in the design of optical devices, which can calculate electric fields in the depth
direction. However, it is still necessary to evaluate the appropriateness of the obtained results in a practicalmanner, because FDTD simulation necessitates the frequency dispersive nature of media in an analytical domain
and very fine discretization in space in order to correctly predict the electromagnetic distribution, particularly, inthe case of optical antennas including metals. The field enhancement of the antenna can be measured by observinga reflection or transmission spectra through a micro FTIR spectrometer. Local information on the fieldenhancement is also available if one uses a scattering near-field microscopy (s-SNOM). However, suchmeasurements alone are insufficient to ascertain the distribution of field enhancement normal to an antenna plane.
To this end, a thin Al2O3 layer was deposited on a Si substrate through atomic layer deposition (ALD) withthickness being controlled to an accuracy of ~1 nm, and dumbbell-shaped slot antennas (DSAs) were fabricatedon this [4]. By observing the spectral magnitude dependence of the SiO2 layer, which was naturally formed on the
Si layer, on the Al2O3 thickness, it was possible to actually gain an understanding of the distribution of enhanced
electric field parallel to the substrate surface in the depth direction. Fig. 1 shows normalized reflectivity vs.
wavenumber for antenna arrays on an Al2O3/Si substrate. The thicknesses of the ALD-made Al2O3 were 20, 40,
and 60 nm. The reflectivity-increase region of 1050~1250 cm-1 corresponding to the restrahlen band of native Si
oxide weakened as the Al2O3 was deposited and the thickness was increased. The band almost disappeared for a
60-nm Al2O3, meaning that the electric field in a natural SiO2 layer weakened with increases in the thickness of
Al2O3. Reflectivity increase around 750~1100 cm-1 was due to the restrahlen band of Al2O3 layer.

Fig. 1 Normalized reflectivity vs. wavenumber for antenna arrays on an Al2O3/Si substrate. The
lengths and gap width of a DNA were 1.5 µm and 0.1 µm, respectively. The thicknesses of the
ALD-made Al2O3 were 20, 40, and 60 nm [4].

3Experimental results were compared with FDTD calculation. Measured results were in reasonable quantitative
agreement with the FDTD calculation, but strictly speaking, it was found that there were some points which were
unaccountable for from the simulation, which wasn’t universal in predicting the electromagnetic distribution in
optical devices, particularly with antennas. Combing the FDTD method with the ALD technique allows for moreaccurate analysis of field enhancements in the perpendicular direction.
Next, this technique was applied to investigate the distribution of enhanced electric field normal to the substratesurface in the depth direction to develop high-performance mid-infrared detectors by combining the fieldenhancement effect of an optical antenna and intersubband transitions in quantum wells. Intersubband transitions
occur as electromagnetic waves with an electric field component perpendicular to the quantum well layers areirradiated. Circular slot antennas were formed on an ALD-made Al2O3/SiC substrate. Surface phonon polariton(SPhP) signals originating from the SiC substrate [5] were used to estimate the electric field perpendicular to thesubstrate by varying the Al2O3 thicknesses. SPhPs are bound to the interface between Al2O3 and SiC, and possess
s-polarization. Thus, it was possible to grasp the in-depth distribution of electric fields increased by opticalantennas by changing the thickness of the Al2O3 spacer layer (Fig. 2). The knowledge obtained in this study isavailable for designing high-performance mid-infrared detectors with high-sensitivity and quick response times.

Fig. 2 Normalized reflectivity of circular-slot antenna array on Al2O3/SiC substrate when the
thickness of Al2O3 was changed. The diameter of the antenna was 6 µm. When t = 0 nm, it was foundthat a decrease in reflectivity at 929 cm-1, caused by SPhP, appeared among the Restrahlen band of
SiC ranging from 800 to 970 nm. This dip almost disappeared when t = 100 nm, because the interfaceof Al2O3 and SiC was distant from the antenna, and field enhancement was weak [6].
References[1] M. Sieger, J. Haas, M. Jetter, P. Michler, M. Godejohann, and B. Mizaikoﬀ, “Mid-infrared spectroscopy
platform based on GaAs/AlGaAs thin film waveguides and quantum cascade lasers,” Anal. Chem., Vol. 88, pp.
2558−2562, 2016.
[2] R. Köhler, A. Tredicucci, F. Beltram, H. E. Beere, E. H. Linfield, A. G. Davies, D. A. Ritchie, R. C. Iotti, and
F. Rossi, “Terahertz semiconductor-heterostructure laser,” Nature, Vol. 417, pp. 156-159, 2002.

4

[3] K. Fujita, M. Hitaka, A. Ito, T. Edamura, M. Yamanishi, S. Jung, and M. A. Belkin, “Terahertz generation in
mid-infrared quantum cascade lasers with a dual-upper-state active region,” Appl. Phys. Lett., DOI:
10.1063/1.4923203, 2015.
[4] Y. Nishimura, T. Kawano, Y. Kunichika, K. Kasahara, T. Yaji, N. Ikeda, H. Oosato, H. Miyazaki, Y. Sugimoto,“Observation of the enhancement of electric fields normal to the surface using mid-infraed slot antennas andan atomic layer deposition technique,” Opt. Commun., Vol. 349, pp. 98-104, 2015.
[5] T. Wang, P. Li, B. Hauer, D. N. Chigrin, and T. Taubner, “Optical properties of single infrared resonant circular
microcavities for surface phonon polaritons,” Nano Lett., Vol. 13, pp. 5051-5055, 2013.
[6] J. Miyata, Y. Yamamoto, Y. Kunichika, T. Kawano, N. Umemori, K. Kasahara, N. Ozaki, N. Ikeda, H. Oosato,
and Y. Sugimoto, “Probing enhancement of an electric field perpendicular to an optical antenna surface using
SiC surface phonon polaritons,” Proceedings of 10th International Congress on Advanced Electromagnetic
Materials in Microwaves and Optics (Metamaterials 2016), Crete, Greece, Poster session II, No. 34, 17-22, Sep.
2016.

5

D2. Current-Injection Terahertz Lasing in a Distributed-Feedback Dual-Gate
Graphene-Channel Transistor
Taiichi Otsuji
Tohoku University

ABSTRACTThis paper reviews recent advancement on the research toward graphene-based terahertz (THz) lasers. Optical
and/or injection pumping of graphene can enable negative-dynamic conductivity in the THz spectral range, which
may lead to new types of THz lasers. A forward-biased graphene structure with a lateral p-i-n junction was
implemented in a distributed-feedback (DFB) dual-gate graphene-channel FET and observed a single modeemission at 5.2 THz at 100K. The observed spectral linewidth fairly agrees with the modal gain analysis based on
DFB-Fabry-Perrot hybrid-cavity-mode modeling. Although the results obtained are still preliminary level, theobserved emission could be interpreted as THz lasing in population-inverted graphene by carrier-injection.
Keywords: Graphene, terahertz, lasers, current injection, p-i-n junction, transistors

INTRODUCTION
Optical and/or injection pumping of graphene can enable negative-dynamic conductivity in the terahertz(THz) spectral range, which may lead to new types of THz lasers [1],[3]. In the graphene structures with p-i-njunctions, the injected electrons and holes have relatively low energies compared with those in optical pumping,so that the effect of carrier cooling can be rather pronounced, providing a significant advantage of the injection
pumping in realization of graphene THz lasers [4],[5]. We implement a forward-biased graphene structure with alateralp-i-njunctioninadistributed-feedback

dual-gate

graphene-channel

field-effecttransistor(DFB-DG-GFET) and experimentally observe a single mode emission at 5.2 THz at 100K [7]. The device exhibitsa nonlinear threshold-like behavior.

THEORY AND DESIGN
Current injection and population inversion in a dual-gate FET structure
When the intrinsic graphene is optically pumped with the photon energy Ω, interband transitions lead tothe generation of ‘hot’ photoelectron-photohole pairs at energy levels Ω / 2 above and below the Dirac point,
for electrons and holes, respectively [1]-[3]. At room temperature, these photo-excited ‘hot’ electrons and ‘hot’
holes are quasi-equilibrated with the low-energy conduction electrons and valence holes around the Dirac point in
the ultrafast time scale of tens of fs due to carrier-carrier scattering, resulting in the quasi-Fermi distribution asshown in Fig. 1 [3]. As a result, quasi-Fermi level of electrons and holes shift below and above the Dirac point,respectively. At the same time, the high-energy electrons and holes emit optical phonons, losing their energy.
Thus the quasi-Fermi distribution, which is originally spreading widely over the energy, shifts closer to the Diracpoint, resulting in rapid recovery of the quasi-Fermi level on a ps time scale as shown in Fig. 1 [3],[8]. After the6energy relaxation via optical phonon emissions, these non-equilibrium electrons and holes may lose their energies
by ‘interband’ optical phonon scattering, impurity-/disordered scattering, as well as acoustic phonon scattering, orbe recombined. Compared to the aforementioned ‘intraband’ optical phonon scattering, these scattering andrecombination processes have longer relaxation times. Therefore the nonequilibrium carriers are piled up aroundthe Dirac point if the pumping intensity is sufficiently high as shown in Fig. 1. This is a situation of carrier
population inversion in which the quasi-Fermi level for electrons and holes shifts above and below the Dirac point,respectively. It is noted that the Auger-type three-particle scattering processes are theoretically forbidden in theideal graphene [9], but may be dominant in the disordered low-quality graphene and/or under an intense pumping
leading to significant many-body effects [10]. The Auger-type scattering lowers the level of population inversion.

Fig. 1. Nonequilibrium carrier relaxation dynamics in optically pumped intrinsic graphene.

Optical pumping suffers from a significant heating of the electron-hole plasma, which suppresses the
population inversion [5],[8]. In the case of the optical pumping with a sufficiently low photon energy, however,
the electron-hole plasma can be cooled down [5]. Current injection pumping might allow us to substantiallyreduce the pumping threshold because the pumping energy could be of the order of “meV” for p-i-n junctions [4].
Figure 2 shows a dual gate p-i-n structure using a graphene-channel field effect transistor (DG-GFET). The gatebiasing (+Vg) controls the injection level, whereas the drain bias (Vds) controls the population inversion.

Tominimize undesired tunneling current lowering the injection efficiency the distance between the dual gateelectrodes must be sufficiently long. The conductivity profiles calculated for typical dimensions and applied bias
conditions demonstrate the advantage of current injection for obtaining the THz gain compared to opticalpumping [5],[8].
According to Ref. [4], the optical conductivity of graphene channel under the gates and drain biases are
estimated as:

(a)

(b)

Fig. 2. Schematic cross-sectional view (a) and corresponding energy band diagram (b) of a lateral p-i-n junction in a
DG-GFET for current-injection lasing.
7=σ ω σ ωi ntra + σ ωinter ,(1)

σ ωintra ≈e 2ε Fτ⋅,
22π  1 + ω 2τ 2

(2)σ ωinter ≈ eV − 2e VF Vg  ω − eVds e2 ⋅ sinh ⋅ exp ,2
2k B T
2k B T 




(3)

where σ ωintra and σ ωinter are the intraband and interband component of the conductivity, respectively, e is theelementary charge, ε F is the Fermi energy reflecting the carrier doping level by applying Vg,  is the reduced
Planck constant, τ is the carrier momentum relaxation time, k B is the Boltzmann constant, and T is thetemperature.
Simulated σ ω for typical device geometrical parameters (the gate dielectric thickness of 50 nm , the gate
dielectric constant of 4.7, and the carrier momentum relaxation time τ of 2 ps) and various applied voltages Vg andVds are plotted in Fig. 3. As mentioned above, Vg determines the carrier injection level and corresponding ε F ,whereas Vds determines the level of population inversion (the amount of shifting the quasi-Fermi levels ofelectrons and holes). When the carrier injection level is relatively low at Vg ＝1.0 V, a weak Vds produces a net
gain (negative conductivity) in a wide THz frequency range. When the carrier injection level increases to Vg ＝10.0 V, a larger Vds is needed to produce a net gain. The lower cutoff frequency to get the net gain exhibits theblue-shifting For both cases when the Vds increases far above ε F , the potential slope along the channel becomesmanifested resulting in reduction of the carrier injection and corresponding gain (not shown in Fig. 3) [6].

Thus,the lower and upper threshold levels to get the net gain coexist.

Distributed feedback cavity structure
As seen in Fig. 3, the gain medium of graphene itself could not define the transition at single photon energy
specifically but exhibits a rather broadband gain spectrum due to the nature of continuum conduction/valenceband structures.

Thus to obtain a single-mode lasing one needs to install a high-Q laser cavity structure in the

dual-gate GFET. In this study, a pair of teeth-brash-shaped gate electrode was patterned to form a distributed
feed-back (DFB) cavity in which the active gain area and corresponding gain coefficient are spatially modulated[7],[12] (see Fig. 4(a)). Designed number of the grating periods N, the grating period Λ, the effective refractive
index neff , the Bragg wavelength λB , and the principal mode f P are 16, 12 μm, 2.52, 60.5 μm, and 4.96 THz,
respectively.

The modal gain of the in-plane waveguided modes in the DFB-DG-GFET was analyzed by taking

account of the DFB and the DG-stripline-induced in-plane Fabry-Perot hybridized cavity structures as shown in
Fig. 4(b) [7].

One can expect a single-mode lasing at the DFB fundamental mode frequency of 4.96 THz with aspectral Q factor of ~200.

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Fig. 3. Simulated optical conductivity for a DG-GFET structure at different Vg and Vds values. The gate dielectric
thickness and constant are assumed to be 50 nm and 4.7, respectively.

(a)

(b)

Fig. 4. (a) The DFB structure and its design parameters in a DFB-DG-GFET. (b) Numerically calculated Q factors andmodal gain for in-plane waveguided modes [7].

EXPERIMENTS
Device fabrication
Graphene was synthesized by the thermal decomposition of a C-face 4H-SiC substrate [13]. The Raman
spectrum confirmed a high-quality, a few layer non-Bernal stacked graphene (see Fig. 5(a)). The GFET was
fabricated using a standard photolithography and a gate stack with a SiN dielectric layer, providing an excellent
intrinsic field-effect mobility exceeding 100,000 cm2/Vs at 300K at the maximal transconductance [14]. A pair ofteeth-brash-shaped gate electrode was patterned to form a DFB cavity in which the active gain area andcorresponding gain coefficient are spatially modulated (see Figs. 5(b), (c)) [7].

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(a)

(b)

(c)

Fig. 5. (a) Raman spectrum of the epitaxial graphene on 4H-SiC. (b) Device cross-section. (c) Device SEM and photo
images [7].

The device exhibited ambipolar behaviors on its current-to-gate-voltages (Vg1, Vg2) characteristics as shown in
Fig. 6 [7]. With a complimentary biased Vg1 and Vg2 with a positive drain-source bias Vds the carrier population
can be inverted [4].

Fig. 6. Measured ambipolar current-voltage characteristics for a fabricated DFB-DG-GFET [7].

THz emission characteristics [7]THz emission from the sample was measured at 300 and 100K using a Fourier-transform spectrometer with a
4.2K-cooled Si bolometer. The background blackbody radiation was first observed under the zero-bias condition,
which was subtracted from the one observed under biased conditions. The observed spectra contain hugesystematic noise at around 3.6-4.2 THz due to misalignment of the Martin-Puplett interferometer so that it wasdisregarded (see Fig. 7(a)). No distinctive emission at 300 K, but rather sharp single mode emission at 5.2 THz
was observed at 100K (see Figs. 7(a), (b)) when Vds is positively applied to a certain level under symmetricelectron and hole injection conditions. The single mode emission exhibits a non-monotonic threshold-likebehavior as shown in Fig. 8 with the highest intensity ~10 μW. This peculiar phenomena might reflect the carrier
over-cooling effect under weak pumping [5]. Spectral narrowing with increasing the carrier injection around thethreshold was also observed. The emission spectra at Vds = 0.5 V can fit to the Lorentzian curve with a Q factor of
170 (a linewidth of 30.6 GHz), which fairly agrees with the simulated results shown in Fig. 4(b).

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Fig. 7. Observed emission spectra at 100K (a) in a broad range and (b) in a magnified range at around 5.2 THz [7].

Fig. 8. Nonlinear threshold-like behavior of the THz emission intensity vs. Vds [7].

Discussion
The conductivity analysis shown with Eqs. (1)-(3) suggests that much higher Q factors and/or higher qualityof graphene material having longer momentum relaxation times are/is needed to enable the single-mode intenselasing at room temperature. The Q factors of the DFB structure can be further increased by orders by increasing
the number of the DFB periods as well as by introducing a thick dielectric layer on top of the device to maximize
the spontaneously-emitted THz photon field at the graphene channel area.

These are future subjects.

CONCLUSION
A forward-biased graphene structure with lateral p-i-n junctions was implemented in a DFB-DG-GFET andexperimentally observed a single mode emission at 5.2 THz at 100K. The device exhibited a nonlinear
threshold-like behavior with respect to the current-injection level. The observed spectral linewidth fairly agrees
with the modal gain analysis based on DFB-Fabry-Perrot hybrid-mode modeling. Although the results obtained
are still preliminary level, the observed emission could be interpreted as THz lasing in population-inverted
graphene by carrier-injection.

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ACKNOWLEDGEMENTS
The author thanks V. Ya. Aleshkin, H. Wako, D. Yadav, H. Fukidome, M. Suemitsu, E. Sano, V. Mitin, and
M.S. Shur for their contributions. We also thank T. Palacios and Q. Hu for valuable discussions. This work was
financially supported in part by JSPS-KAKENHI (No. 23000008 and 16H06361), Japan. The work by A.D. was
supported by the Dynasty Foundation, Russia. The DFB-DG-GFET process was done at the Nano-Spin facility,
RIEC, Tohoku University, Japan.

References[1] Ryzhii, V., Ryzhii, M., Otsuji, T., “Negative dynamic conductivity in optically pumped graphene,” J. Appl.
Phys. 101, 083114-1-4, 2007.
[2] Otsuji, T., Boubanga-Tombet, S., Satou, A., Ryzhii, M. and Ryzhii, V., “Terahertz-wave generation usinggraphene-toward new types of terahertz lasers,” IEEE J. Selected Topics in Quantum Electron. 19,
8400209-1-9, 2013.
[3] Otsuji, T., Boubanga Tombet, S. A., Satou, A., Fukidome, H., Suemitsu, M., Sano, E., Popov, V., Ryzhii, M.,
and Ryzhii, V., “Graphene-based devices in terahertz science and technology,” J. Phys. D: Appl. Phys. 45,
303001-1-9, 2012.
[4] Ryzhii, M. and Ryzhii, V., “Injection and population inversion in electrically induced p-n junction ingraphene with split gates,” Jpn. J. Appl. Phys. (L) 46, 151-153, 2007.
[5] Ryzhii, V., Ryzhii, M., Mitin, V. and Otsuji, T., “Toward the creation of terahertz graphene injection laser,” J.
Appl. Phys. 110, 094503-1-9, 2011.
[6] Ryzhii, V., Semenikhin, I., Ryzhii, M., Svintsov, D., Vyurkov, V., Satou, A., and Otsuji, T., “Double
injection in graphene p-i-n structures,” J. Appl. Phys. 113, 244505-1-9, 2013.
[7] Tamamushi, G., Watanabe, T., Dubinov, A.A., Mitsushio, J., Wako, H., Satou, A., Suemitsu, T., Fukidome,
H., Suemitsu, M., Ryzhii, M., Ryzhii, V., and Otsuji, T., “Single-mode terahertz emission fromcurrent-injection graphene-channel transistor under population inversion,” the 74th Dev. Res. Conf. Dig., 1,
225-226, 2016.
[8] Satou, A., Ryzhii, V., Kurita, Y., and Otsuji, T., “Threshold of terahertz population inversion and negative
dynamic conductivity in graphene under pulse photoexcitation,” J. Appl. Phys. 113, 143108, 2013.
[9] Foster, M.S., and Aleiner, I.L., “Slow imbalance relaxation and thermoelectric transport in graphene,” Phys.
Rev. B 79, 085415-1-14, 2009.
[10] Winzer, T., and Malic, E., “Impact of Auger processes on carrier dynamics in graphene,” Phys. Rev. B 85,
241404(R)-1-5, 2012.
[11] Tani, S., Blanchard, F., and Tanaka, K., “Ultrafast carrier dynamics in graphene under a high electric field,”
Phys. Rev. Lett. 99, 166603-1-5, 2012.

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[12] Williams, W.S., Kumar, S., and Hu, Q., “Distributed-feedback terahertz quantum-cascade lasers with laterally
corrugated metal wavuguides,” Opt. Lett. 30, 2909-11, 2005.
[13] Fukidome, H., Kawai, Y., Fromm, F., Kotsugi, M., Handa, H., Ide, T., Ohkouchi, T., Miyashita, H., Enta, Y.,
Kinoshita, T., Seyller, T., and Suemitsu, M., “Precise control of epitaxy of graphene by microfabricating SiC
substrate,” Appl. Phys. Lett. 101, 041605, 2012.
[14] Satou, A., Tamamushi, G., Sugawara, K., Mitsushio, J., Ryzhii, V., and Otsuji, T., “A fitting model forasymmetric I-V characteristics of graphene field-effect transistors for extraction of intrinsic moblities,” IEEE
Trans. Electron Dev., 63, 3300-3306, 2016.

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D3. Terahertz Photonic Crystals
Masayuki Fujita
Osaka University
Terahertz (THz) electromagnetic waves, which are located in the frequency region between radio and infrared
light waves from 0.1 THz to 10 THz, have attracted considerable interest in the interdisciplinary fields of photonics
and electronics [1]. Currently, their unique potential applications such as in spectroscopic sensing, nondestructive
imaging, and broadband wireless communication, are being developing. However, most existing THz application
systems are composed of bulky and discrete components. Consequently, the exploration of techniques to control
THz waves in a thin, planar, and low-loss structure is indispensable toward the practical use of the THz technology
in various fields.
Photonic crystals, composed of a dielectric material with a periodic refractive-index distribution on a scalecomparable to the wavelengths of interest, are analogous to solid-state crystals with periodic potential distributions.
The photonic crystal can create an exotic dispersion relationship between the frequency and wave vectors to enable
the manipulation of photons or electromagnetic waves. Appropriate photonic-crystal designs show a photonic
bandgap (PBG) where no optical modes exist. Introducing an artificial defect into the photonic crystal by disturbing
the periodicity can enable the development of compact and low-loss photonic components in the PBG region. The
propagation of light can be changed according to the dispersion properties of a photonic crystal. At thephotonic-band edge, the group velocity approaches zero. The slow light or the standing-wave condition is thus
formed by the distributed-feedback effect to enhance the interaction between the light and matter, causing
resonance. Two-dimensional (2D) photonic-crystal slabs, which have a 2D periodic lattice of air holes on asemiconductor slab, are promising for practical applications owing to their simple planar structures.
Recently, the 2D photonic-crystal slab was studied as a platform for THz-wave manipulation with sufficientconfinement and efficient interaction [2–5]. Silicon photonic-crystal slabs are especially promising because they
can be easily fabricated with great accuracy using well-established semiconductor micromachining techniques, andthe absorption loss because of free carriers can be managed by varying the carrier density or resistivity.
A compact THz waveguide in the PBG region can be created by introducing a line defect in the photonic crystal
that disturbs the periodicity [6]. No radiation loss is theoretically predicted because the THz waves are completely
confined in both the vertical and in-plane directions by total internal reflection and the PBG effect, respectively. To
measure the propagation loss in the 0.3-THz band, photonic-crystal waveguides with a period of 240 µm andvarious propagation lengths were fabricated using a 200-µm-thick silicon substrate with a resistivity of ~20 kΩ⋅cm.
The transmission loss of the waveguides was measured using a THz spectroscopic system based oncontinuous-wave tunable electronic sources employing a WR-3 hollow waveguide. To efficiently couple the WR-3
and photonic-crystal waveguides, tapered structures were created at the end of the photonic-crystal waveguides.
When the tapered structure is directly plugged into the WR-3 waveguide, an extremely small insertion loss (lower
than 0.2 dB) is achieved, because the adiabatic change in the refractive index of the tapered structure reduces the14reflection. The measured propagation loss was less than 0.1 dB/cm in the frequency range of 0.326–0.331 THz, andthe minimum loss was 0.04 dB/cm at 0.330 THz. This value is two or three orders of magnitude smaller than thatreported for metallic lines in the 0.3-THz band. In addition, this value is the smallest ever reported for various
photonic-crystal waveguides, including those in the light-wave region. This indicates that the propagation loss inphotonic-crystal waveguides in the THz region is not determined by radiation loss arising from structural errors, asreported for the light-wave region, but by material absorption.
The ultralow-loss photonic-crystal waveguide technique demonstrated here is applicable to various compact andlow-loss THz components such as cavities, filters, diplexers, bends, branches, and couplers, may potentially lead tothe development of THz integrated circuits based on a photonic-crystal slab.
In the microcavity based on the photonic-crystal slab, Q factors over 9,000 in a cavity formed by a line defect of
three missing holes in a triangular-lattice pattern of air holes were reported for the 0.3-THz band [7]. Suchmicrocavities may be applicable to highly sensitive THz wave sensing systems for advanced spectroscopic
applications.

Fig. 1.

Prototype of THz transceiver using a photonic-crystal platform

A compact THz diplexer (frequency selector) was designed using a silicon photonic-crystal directional couplerof length comparable to the operation wavelength [8, 9]. The operation was demonstrated with the bandwidth of
2% and a 40-dB isolation between two input ports. Based on the photonic-crystal diplexer, a THz transceiver for
wireless communications employing frequency-division multiplexing was developed [10], as shown in Fig. 1. A
grating coupler was used for an input-output wireless interface for free-space communication. Resonant tunneling
diodes (RTDs) were used as THz receivers and transmitters, and were mounted on a photonic-crystal waveguide[11, 12]. Wireless transmission experiments of the transceiver were performed using an electric transmitter andreceiver based on a WR-3 horn antenna and hollow waveguide. The opened eye diagrams at 1.5 Gbit/s and an
oscillation spectrum were successfully observed at the RTD receiver (0.326 THz) and transmitter (0.354 THz),
respectively. However, the measured bit-error rate was limited to 5 × 10–4 due to low signal power. The mostimportant factor in improving the signal power is the enhancement of the coupling efficiency between the RTD andphotonic-crystal waveguide.
15

A photonic crystal exhibits not only the PBG effect but also the photonic band-edge effect due to the dispersion
relation. For the band-edge condition where the period matches the wavelength in the medium, the electromagnetic
waves propagating in various directions are combined by Bragg reflection to form a standing wave.
Simultaneously, the diffraction condition of the wave is satisfied in the direction normal to the photonic-crystal
plane, because the Bragg condition is also satisfied in that direction. This phenomenon can be utilized for thetrapping of a THz wave in a photonic-crystal slab [13], as shown in Fig. 2. An incident THz wave is trapped in thephotonic-crystal slab, and an in-plane resonant mode is excited. When the material of the slab shows no absorption,
the trapped wave is eventually released from the slab and observed from outside with a low damping decay. On theother hand, the introduction of appropriate free carriers in the slab by exciting or doping the material allows thetrapped THz wave to be efficiently captured within the slab by absorption, despite its structure being thinner than
the wavelength of the THz wave. Thus, the trapped THz wave can be controlled by changing the density of free
carriers through the change in pumping condition or the resistivity of the material.

Fig. 2.

Image of THz-wave trapping in a photonic-crystal slab

To investigate the dynamics of the THz-wave trapping and capture process for the 0.3-THz band, a silicon
photonic-crystal slab with a square lattice of air holes was employed. The temporal response of the transmission
wave was measured by using an optical-pump/THz-probe system [3]. The spectrogram for the sample without
pumping shows that the components trapped by the photonic crystal are slowly released from the slab at a resonant
frequency of ~0.3 THz. On the other hand, when the photonic crystal was pumped with a 775-nm laser lightsynchronized with the THz wave, the transmission signal was dramatically reduced owing to the effective
interaction between the trapped THz wave and the material absorption by excited free carriers.
To demonstrate the application of the THz-wave dynamic control in a photonic-crystal slab, on-off modulation
experiments of THz wireless transmission were performed [3]. A continuous THz wave was introduced into a
photonic-crystal slab and was modulated by the photoexcitation with a 775-nm laser modulated by a pulse-patterngenerator. The signal received through the photonic crystal was detected by a Schottky barrier diode module andthe received signal was analyzed using an oscilloscope. A signal was observed when the pumping laser was off,
while no signal was detected for the pump-on condition, owing to the capture of the THz wave in the photonic
crystal. This result successfully demonstrates the modulation of a THz wave in a photonic crystal. The modulation
speed was limited to below 1 MHz owing to the long carrier lifetime in silicon. The reduction in the carrier lifetime
16

would enable a faster modulation (beyond 10 GHz) by the introduction of ion implantation, a carrier removal
structure such as a PIN diode, or the use of other materials with faster carrier lifetimes, such as a
low-temperature-grown GaAs or graphene.
As an application of the capture of THz wave in a photonic-crystal slab, broadband absorbers, which are crucialcomponents for stabilizing THz systems by reducing multiple reflection and interferences, were developed [13, 14].
The THz-wave absorber was composed of a photonic-crystal slab and a reflection mirror. The broadband operation
was realized by using triple-band resonance involving three types of resonances—even and odd modes of thein-plane resonance, and Fabry-Pérot resonance between the photonic-crystal slab and the mirror. By superposing
the absorption peaks originating from these resonant modes, a flat-top broadband absorption spectrum can beachieved. To achieve high absorptivity, the resistivity of silicon was optimized to be ~6 Ω⋅cm to satisfy thematching condition. A measured peak absorptivity was over 99 % at 0.3 THz and the bandwidth (absorptivity ≥90%) was as broad as 17 % of the operation frequency. This performance was achieved despite the simplicity of thestructure, with an overall thickness of approximately half a wavelength. This absorber was applied to a THz
proximity wireless communication system to suppress the standing wave effect between the transmitter andreceiver. The photonic-crystal absorber successfully stabilized the communication system by reducing multiple
reflections.
In addition, preliminary results on THz sensing application using the THz-wave trapping of a photonic-crystal
slab were reported by the sensing of a single-layer graphene [4].
In conclusion, a photonic crystal is a potential candidate to form a core technology platform for various THz
devices and components. The results will prospectively lead to the realization of THz integrated circuits that reduce
the size of THz systems for use in advanced THz applications, including information and communication
technology, spectroscopic sensing, and nondestructive imaging. Based on the successful achievements, the CREST
research project entitled “Development of terahertz integrated technology platform through fusion of resonant
tunneling diodes and photonic crystals” funded by Japan Science and Technology Agency from 2015 was started inJapan.

References[1] T. Nagatsuma, S. Hisatake, M. Fujita, H. H. N. Pham, K. Tsuruda, S. Kuwano, and J. Terada,“Millimeter-wave and terahertz-wave applications enabled by photonics,” IEEE J. Quantum Electron., vol.
52, no. 1, pp. 0600912-1– 0600912-12, 2016.
[2] M. Fujita, K. Tsuruda, A. Suminokura, M. Yata and T. Nagatsuma, “Silicon photonic crystal as aterahertz-wave technology platform,” The 4th International Symposium on Photonics and Electronics
Convergence, no. E-3, 2014.
[3] M. Fujita, “Photonic-crystal slab for terahertz-wave technology platform,” Proc. SPIE, vol. 9757, pp.
97570W-1–97570W-8, 2016.

17

[4] M. Fujita, “Terahertz-wave manipulation using photonic-crystal slabs and its applications,” OYO BUTURI,
vol. 85, no.6, pp. 490–495, 2016.
[5] M. Fujita, “Terahertz photonic crystals and their applications,” The 12th International Symposium on
Photonic and Electromagnetic Crystal structures, 2016.
[6] K. Tsuruda, M. Fujita, and T. Nagatsuma, “Extremely low-loss terahertz waveguide based on silicon
photonic-crystal slab,” Opt. Express, vol. 23, no. 25, pp. 31977–31990, 2015.
[7] K. Tsuruda, K. Okamoto, S. Diebold, S. Hisatake, M. Fujita, and T. Nagatsuma, “Terahertz sensing based on
photonic crystal cavity and resonant tunneling diode,” Proc. PIERS, pp. 3922–3926, 2016.
[8] M. Yata, M. Fujita and T. Nagatsuma, “Diplexer for terahertz-wave integrated circuits using aphotonic-crystal slab,” International Topical Meeting on Microwave Photonics / The 9th Asia-PacificMicrowave Photonics Conference, no. Tu-C3, pp. 40–43, 2014.
[9] M. Yata, M. Fujita, and T. Nagatsuma, “Photonic-crystal diplexers for terahertz-wave applications,” Opt.
Express, vol. 24, no. 7, pp. 7835–7849, 2016.
[10] M. Fujita and T. Nagastuma “Photonic crystal technology for terahertz system integration,” Proc. SPIE, vol.
9856-21, pp. 98560O-1–98560O-10, 2016.
[11] A. Suminokura, K. Tsuruda, T. Mukai, M. Fujita and T. Nagatsuma, “Integration of resonant tunneling diodewith terahertz photonic-crystal waveguide and its application to gigabit terahertz-wave communications,”
International Topical Meeting on Microwave Photonics / The 9th Asia-Pacific Microwave Photonics
Conference, no. ThB-3, pp. 419–422, 2014.
[12] K. Tsuruda, M. Fujita, A. Suminokura, M. Yata and T. Nagatsuma, “Terahertz-wave integrated circuits
based on photonic crystals,” Proc. PIERS, pp. 2254–2259, 2015.
[13] R. Kakimi, M. Fujita, M. Nagai, M. Ashida and T. Nagatsuma, “A photonic-crystal terahertz-wave absorberand its application to terahertz communications,” The 4th International Symposium Terahertz NanoScience,
2014.
[14] R. Kakimi, M. Fujita, M. Nagai, M. Ashida and T. Nagatsuma, “Capture of a terahertz wave in aphotonic-crystal slab,” Nature Photon., vol. 8, no. 8, pp. 657–663, 2014.

18

D4. THz-Wave Emitters and Detectors Based on Diode Technology
Hiroshi Ito
Kitasato University

1. Introduction
Terahertz (THz) wave emitters and detectors are the key components for constructing various THz wave
systems. In particular, semiconductor-based diodes are important because they have good reliability andreasonable cost and because they are suitable for integration with other components. This report describes two
types of diode-based devices. One is a THz wave emitter (photomixer) consisting of a uni-traveling-carrier
photodiode (UTC-PD), and the other is a THz wave detector (rectifier) consisting of a Schottky barrier diode(SBD) or a Fermi-level managed barrier (EMB) diode.

2. THz-wave emitter
The photomixer is a promising device for generating continuous THz waves because it has several importantfeatures, such as wide frequency tunability, good frequency stability, narrow line width, and the capability forlong distance transmission of high frequency signals through low loss fibers. The basic requirements for thephotomixer are for them to have both high operation speed and large output current. In these regards, the UTC-PD
is one of the best solutions because it provides both a high 3-dB down bandwidth and a high saturation output
current. To transmit high frequency electromagnetic waves in a wide frequency range, the photomixer needs to be
integrated with a planar antenna, and there are two choices for this. One is a broadband antenna with constantcharacteristic impedance regardless of the operation frequency, and the other is a resonant antenna for increasing
the radiation efficiency at around the designated frequency while moderately reducing its operational bandwidth.

2.1 Broadband-antenna integrated UTC-PD
Self-complementary antennas, such as log-periodic antennas, log-spiral antennas, and 90º bowtie antennas, arefrequently used as broadband antennas. However, we have to be careful in using these antennas because thepolarization of the emitted signal from them sometimes strongly depends on the operation frequency. Figure 1
shows the dependence on frequency of the relative principal polarization axis angle (polarization angle) for theemitted signals from a log-periodic-antenna integrated UTC-PD and a 90º bowtie-antenna integrated UTC-PD [1].
The behaviors are quite different. The polarization angle of the log-periodic antenna device drastically changes
with frequency, while that of the bowtie antenna device remains almost the same. These differences come fromthe difference in the antenna electrode pattern, and the reflection symmetry in the antenna pattern is essential forstable and linear polarization characteristics [1, 2]. The precise control of the polarization characteristics is
especially important for applications that require polarization discrimination, such as radar, wirelesscommunication, ellipsometry [3, 4], imaging, and spectroscopy.

19

Polarization Angle (deg.)

150
120

log-periodic

90
60
30
0
bowtie
-30
100

1000
Frequency (GHz)

Fig. 1. Dependence on frequency of relative principal polarization axis angle for emitted signals from a
log-periodic-antenna integrated UTC-PD and a bowtie-antenna integrated UTC-PD.

Another important consideration for the UTC-PD to enable operations at very high-frequencies is the carriertransport in the device. We performed both analytical calculations and experimental characterization on the highfrequency characteristics of the UTC-PD photomixers, showing that locating its absorber on the anode side is
generally required to extend the 3-dB down bandwidth of a UTC-PD [5]. The optimized absorber design is
obtained using a certain partition of absorber thickness into p-type and depleted absorbers, depending on the total
absorber thickness. A comparison of the observed dependence on frequency of the output power in a UTC-PD
with calculated ones showed that the effective electron velocity can be as high as 6 × 107 cm/s in the depletion
layer [5]. This behavior results from the electron velocity overshoot effect that is sensitive to the field intensity.
Benefitting from such superior carrier transport, THz-wave emissions from a UTC-PD at frequencies of up to 3
THz were accomplished [6].

2.2 Resonant-antenna integrated UTC-PD
We investigated two types of antennas as resonant antennas, a slot antenna and a resonant bowtie antenna, forintegration with a UTC-PD. The former is effective for obtaining both an increased output power and a relatively
wide bandwidth, and the latter can be designed using a simple analytical calculation based on a discrete circuit
model including transmission lines.
Figure 2 shows the dependence on frequency of the output power generated from the fabricatedslot-antenna-integrated UTC-PD modules [6, 7]. The output power from the broadband bowtie-antenna-integrated
device is also plotted for comparison. The output power from the slot-antenna-integrated UTC-PDs was obviously
larger than that from the bowtie-antenna-integrated UTC-PD in a frequency range from about 900 GHz to 1.6
THz for the slot-A (narrow-slot) device and from 350 to 850 GHz for the slot-B (wide-slot) device. These
relatively wide bandwidths are due to the relatively weak resonance of the integrated slot antennas. The maximum
20

output power enhancement ratios were found to be about 2.3 at around 1.25 THz for the slot-A device and about
3.3 at around 650 GHz for the slot-B device.

102
bowtieslot-A
slot-B350(uW)
13050 (uW)

Output Power (µW)

700 (uW)

101

100

10-1
Ip = 6 mA
Vb = -0.4 V
10-2
300

1000

3000

Frequency (GHz)

Fig. 2. Dependences on frequency of output power generated from slot-antenna-integrated UTC-PD. The output
power from the bowtie-antenna-integrated UTC-PD is also plotted for comparison.

Another candidate is a resonant bowtie antenna with two connecting electrodes on both sides of the UTC-PD[8], which can be designed using an analytical calculation without using an electro-magnetic simulator. The
output power was enhanced at frequencies from about 400 to 700 GHz and the peak enhancement ratio was about
three at 550 GHz. These behaviors are fairly consistent with the analytical calculation, indicating that theresonant-bowtie antenna can be accurately designed without the electro-magnetic simulation.

3. THz-wave detectors
A rectifier-based THz-wave detector has been widely used in numerous applications. A typical one is the SBD.
We investigated an InP-based SBD because it is suitable for zero-biased operation and integration with other
InP-based high-speed electron and photonic devices. Another topic is the development of a new type of THz-wave
detector named a Fermi-level managed barrier (FMB) diode.

3.1 Schottky barrier diode
Similar to the emitter, the polarization characteristics of the detector are also important for several applications,and they strongly depend on the antenna design. We fabricated an extended-bowtie-antenna-integrated InGaAsP
SBD [9] and accomplished a stable polarization angle within ±1.5° at frequencies from 80 to 600 GHz. The
degree of polarization was larger than 95% within this frequency range and more than 97.5% below 400 GHz.
The extinction ratio was less than 2.6% up to 600 GHz and less than 1.2% below 400 GHz. These results indicate
21that the fabricated SBD module has adequate linearly polarized detection characteristics in the sub-THz-wave
range. The polarization controlled UTC-PD [1] and the SBD [9] were successfully applied to the millimeter-wave
ellipsometry without utilizing an external polarizer [2].

3.2 Fermi-level managed barrier diode
Although the SBD is an excellent THz-wave detector, its characteristics may still need to be improved.
Because the Schottky barrier height is largely influenced by less-controllable surface states in semiconductors, its
characteristics are sometimes unstable and irreproducible. In addition, the rather large differential resistance of thesmall-sized SBD is also a concern in terms of the impedance matching among the input antenna, detector diode,and read-out circuit. The large resistance of the SBD also results in large thermal noise, which will cause
increased noise equivalent power (NEP) in THz-wave detection. To improve these points, we developed a novel
InP/InGaAs hetero-barrier rectifier named an FMB diode [10-14]. The primary feature of the FMB diode is that it
can easily achieve a very low barrier height, which is effective for decreasing diode resistance, increasing output
current, and attaining both low-noise detection and broadband operation. In addition, because it consists entirelyof semiconductor layers, its characteristics are stable, controllable, and reproducible.
Figure 3 shows a schematic band diagram of the FMB diode. The Fermi-level in heavily doped n-InGaAs islocated above the conduction band edge depending on the carrier density, which is called the “band-filling effect.”By using this phenomenon, we can control the barrier height at the InP/InGaAs hetero-interface from zero to avalue for the conduction-band discontinuity and can make the barrier height (differential resistance) of the diodevery small [10-14].

φBn
Efelectrode

P

InP

n-InP

n-InGaAs
Fig. 3. Schematic band diagram of Fermi-level managed barrier (FMB) diode.

An antenna-integrated InP/InGaAs FMB diode was fabricated and assembled in a quasi-optical package with a
pre-amplifier. Figure 4 shows the measured voltage sensitivity of the fabricated FMB diode module against
frequency [13, 14]. We confirmed signal detection at frequencies from 160 GHz to 1.4 THz with the fabricatedmodule. The peak voltage sensitivity obtained was as high as 3.2 MV/W at 300 GHz, and the sensitivity at 1 THz
was still considerably high at about 290 kV/W. We also evaluated the NEP of the FMB diode module and22obtained values as small as 3.0 pW/Hz0.5 at 300 GHz and 33 pW/Hz0.5 at 1 THz. These values are considerably
lower than those for commercially available zero-biased broadband SBD detectors, demonstrating that the FMB
diode is a promising solution for broadband and low noise THz-wave detection.

7

Voltage Sensitivity (V/W)

10

6

10

5

10

100

1000
Frequency (GHz)

Fig. 4. Dependence on frequency of voltage sensitivity for preamplifier-integrated FMB diode module.

4. Conclusion
Developments in diode-based THz wave emitters and detectors were described. A photomixer-based THz
wave emitter consisting of an antenna-integrated UTC-PD demonstrated a polarization-controlled output signal up
to 3 THz. A resonant antenna was also effective for increasing the output power from a UTC-PD while
maintaining a relatively wide bandwidth. A rectifier-based THz wave detector consisting of the SBD also
demonstrated polarization-controlled detection characteristics. A new type of THz-wave detector named an FMB
diode was also developed, and it demonstrated a broad operational bandwidth and a very low NEP of 3.0
pW/Hz0.5 at 300 GHz.

References[1]

H. Ito, T. Yoshimatsu, H. Yamamoto, and T. Ishibashi, “Photonic Terahertz-Wave Emitter Integrating
Uni-Traveling-Carrier Photodiode and Self-Complementary Planar Antenna,” Optical Engineering, Vol. 53,
No. 3, pp. 031209-1 - 031209-7, 2014.

[2]

H. Ito, T. Yoshimatsu, H. Yamamoto, and T. Ishibashi, “Widely Tunable THz-Wave Emitter with Linear
Polarization Characteristics Based on Antenna-Integrated UTC-PD,” Proc. SPIE, Vol. 9199, pp. 919908-1 919908-8, 2014.

[3]

H. Yamamoto and H. Ito, “Millimeter-Wave Ellipsometry Using Low-Coherence Light Source,” IEICE
Trans. Electron., Vol. E97-C, No. 5, pp. 460-462, 2014.
23

[4]

H. Yamamoto and H. Ito, “Millimeter-Wave Ellipsometry Using Interface-Planarization Prism,” IEICE
Trans. Electron., Vol. E98-C, No. 8, pp. 873-877, 2015.

[5]

T. Ishibashi, Y. Muramoto, T. Yoshimatsu, and H. Ito, “Uni-Traveling-Carrier Photodiodes for Terahertz
Applications,” IEEE J. Selected Topics in Quantum Electronics, Vol. 20, No. 6, pp. 3804210-1 3804210-10, 2014.

[6]

H. Ito, T. Yoshimatsu, H. Yamamoto, and T. Ishibashi, “Enhanced-Output-Power Broadband
Terahertz-Wave Emitter Based on Slot-Antenna-Integrated Uni-Travelling-Carrier Photodiode,” Electron.
Lett., Vol. 51, No. 21, pp. 1670-1671, 2015.

[7]

H. Ito, T. Yoshimatsu, H. Yamamoto, and T. Ishibashi, “Enhancement of THz-Wave Output Power Using
Slot-Antenna-Integrated Uni-Traveling-Carrier Photodiode,” Proc. SPIE, Vol. 9585, pp. 958506-1 958506-7, 2015.

[8]

T. Ishibashi, Y. Muramoto, T. Yoshimatsu, and H. Ito,” Generation and Detection of Continuous THz-wave
Radiation by Diode Technologies,” 2015 IEEE Photonics Conference (IPC) Proc., pp. 116-117, 2015.

[9] H. Ito, “Broadband Terahertz-Wave Detector Implementing Zero-Biased InGaAsP Schottky-Barrier Diode,”
Proc. SPIE, Vol. 9483, pp. 948307-1 - 948307-9, 2015.
[10] H. Ito and T. Ishibashi, “Fermi-Level Managed Barrier Diode for Broadband and Low-NoiseTerahertz-Wave Detection,” Electron. Lett., Vol. 51, No. 18, pp. 1440-1442, 2015.
[11] H. Ito and T. Ishibashi, Novel Fermi-Level Managed Barrier Diode for Broadband and Sensitive
Terahertz-Wave Detection,” Proc. International Conference on Infrared, Millimeter, and Terahertz Waves(IRMMW-THz 2015), pp. W1D3-1 - D1D3-2, 3133416-1 - 3133416-2, 2015.
[12] H. Ito and T. Ishibashi, “Broadband and High-Sensitivity Terahertz-Wave Detection Using Fermi-Level
Managed Barrier Diode,” Proc. SPIE, Vol. 9856, pp. 985624-1 - 985624-8, 2016.
[13] H. Ito and T. Ishibashi, “Low Noise Terahertz-Wave Detection by InP/InGaAs Fermi-Level Managed
Barrier Diode,” Appl. Phys. Express, Vol. 9, No. 9, pp. 092401-1 - 092401-3, 2016.
[14] H.

Ito

and

T.

Ishibashi,“Broadband

and

Low

NEP

Terahertz-Wave

Detection

Using

Preamplifier-Integrated Fermi-Level Managed Barrier Diode,” Proc. International Conference on Infrared,
Millimeter, and Terahertz Waves (IRMMW-THz 2016), pp. H4C.3-1-1 - H4C.3-2, 2016.

24

D.5 Millimeter-wave and submillimeter-wave antennas in standard CMOS technology
Eiichi Sano
Hokkaido University

The crowded bands of 2.4 and 5 GHz will push the operation bands of wireless communication systems to themillimeter (MM)-wave region (e.g., 60 GHz). Reduced sizes of antennas in the MM-wave region enable them to
be monolithically integrated with CMOS RF and baseband circuits. Furthermore, on-chip antennas in standard
CMOS processes can contribute to reducing costs. However, low-resistivity (~10 Ω-cm) Si substrates commonlyused in standard CMOS technology degrade the antenna gain. Figure 1 summarizes the antenna gain versus
frequency characteristics reported in the literature [1–19]. The antenna gains on low-resistivity substrates remain~10 dB lower than those on high-resistivity substrates. Thus, an ingenious development is needed to increase thegains of antennas fabricated on low-resistivity Si substrates. Since Pendry’s claim that materials with a negativerefractive index can act as perfect lenses [20], metamaterials have received an enormous amount of attention andinterest from both scientific and industrial communities. Artificial dielectric layers (ADL) [1, 2] and artificial
magnetic conductors (AMC) [10] are effective to shield the antennas from the lossy Si substrates. A metamaterial
antenna using a composite right/left-handed (CRLH) transmission line (TL) in standard CMOS technology was
proposed [11]. While the radiation frequency of a standard dipole antenna is determined by the antenna size, theradiation frequency of a CRLH antenna can be controlled by changing element values, which reduces the antenna
size [21, 22]. The reduced size can alleviate the influence of the Si substrate and increase the antenna gain.

10
High-resistivity substrate
Low-resistivity substrate

Antenna gain (dBi)

5

[7]

[6]

[15]

0

[19]
[18]
[17]
[16]

[13]
[14]
[10]

-5

[3]
[5]

-10
20 dB/dec

[4]

-15

[11]
[12]

[9]

[3]

-20

[2]

-25

[1]

-30

[8]

1

10

100

1000

Frequency (GHz)

Fig. 1. Antenna gain versus frequency characteristics. Solid symbols: measurements. Open symbols:
simulations.

The unit cell in a periodic CRLH TL is shown in Fig. 2. Shunt inductors LL and series capacitors CL as
left-handed elements are inserted in the right-handed transmission line. Applying the periodic boundary condition,
the dispersion characteristics for the CRLH TL are described by25

βp = cos −1 1 −



1
1  
1 

 ⋅  ωC R − ωLR −
 ,



2
ωC L  ωLL 


(1)

where β is the phase constant and p is the length of a unit cell [23]. A schematic dispersion diagram of the CRLH
TL is shown in Fig. 3.

Fig. 2. Unit cell of CRLH TL.

Fig. 3. Dispersion diagram of CRLH TL.

The structure of a metamaterial dipole antenna composed of 4 CRLH cells is shown in Figure 3. Since bothends of the lines are open, the amount of phase change between the two open ends must be π at the target
frequency. This means that βp must equal π/4 in the 4-cell configuration.

Fig. 4. Structure of metamaterial antenna.

A CRLH monopole antenna was implemented by using this concept in 180-nm CMOS with six metal layers.
Series capacitors CL were composed of metal-insulator-metal (MIM) capacitors between metal 5 and metal 6. The
parallel inductor LL was implemented only by a straight line of metal 6. The RH part of the CRLH monopole
antenna was formed of a parallel line without a ground plane under the line to prevent a possible reduction in theantenna gain caused by the eddy current flowing in the ground plane. The antenna was designed with afinite-difference time-domain (FDTD) electromagnetic simulator (Keysight Technologies, EMPro). Figure 5
shows a die photograph of the fabricated antenna. The CRLH unit cell was 300 × 420 µm.

26

Fig. 5. Die photograph of fabricated antenna.

The fabricated CRLH monopole antenna on a chip was measured using a vector network analyzer (VNA) anda RF probe. A standard 1-port calibration method was used for the return loss measurement. The antenna gain wasmeasured using the VNA and two types of horn antennas (20–40 GHz and 50–67 GHz ranges). Port 1 of the VNA
was connected to one of the fabricated antennas, and port 2 of the VNA was connected to the horn antenna. The
gain of the CRLH monopole antenna in each frequency range was evaluated by taking into account thetransmission S21 between two horn antennas and the gain of the horn antenna. Figure 6(a) shows the measured andsimulated return losses for the antenna. The designed impedance matching frequencies (|S11|) of the LH and RH
modes were about 28 GHz (–14.4 dB) and 62 GHz (–6.7 dB), respectively. The measured return loss was shifted
about 3 to 6 GHz to a higher frequency compared with the simulated one. Figure 6(b) shows the measured andsimulated antenna gains. The measured gains were –14.4 dBi at 39.3 GHz and –5.8 dBi at 66.7 GHz in LH modeand RH mode, respectively. Although the measured gain almost matched the designed gain, the measured gain
was ~7 dB smaller than the simulation in the 50–67 GHz range. This might be caused by the different radiation
pattern between the horn antenna and the fabricated antenna and/or the difference between fabricated silicon
substrate resistivity and the value (10 Ω-cm) assumed in the simulations. The measured gain is plotted in Fig. 1. It
is still lower than those of antennas on high-resistivity substrates.

27

(a)

(b)

Fig. 6. Measurement results of fabricated antenna.

The degradation in the gains of antennas on low-resistivity substrates can be alleviated in thesubmillimeter-wave (or terahertz-frequency) region. Figure 7 shows the calculated maximum efficiency and the
optimum radius of circular patch antennas composed of top metal (patch), bottom metal (ground), and 10-µm
thick SiO2. The Si substrate is shielded by the bottom metal. The maximum efficiency reaches 0.5 at 1 THz, and

1

10

0.1

1

0.01

0.1

Patch
10-µm thick SiO2

GroundOptimum radius (mm)Maximum efficiency

the radius is decreased to 50 µm.

Si substrate

0.001
10

0.01
1000

100

Frequency (GHz)

Fig. 7. Calculated maximum efficiency and optimum radius of circular patch antennas.

The terahertz-frequency region (100 GHz – 10 THz), namely, between millimeter-waves and far-infrared lightwaves, has attracted much attention owing to its wide range of applications (such as wireless communications andsensors). Terahertz waves, which pass through a wide variety of substances, such as plastics, fabrics, and paper,
can reveal hidden contents that cannot be seen by visible lights [24]-[26]. In addition, so-called fingerprint spectrain the terahertz frequencies can identify hazardous materials [24]. However, the lack of low-cost and small-size
microelectronics that generate sufficient power and detect faint signal (often called the “THz gap”) is one of the
major obstacles preventing terahertz applications from coming into wide use in our daily lives [27]. In the28microelectronics community, terahertz detection outperforms terahertz generation. Detection methods are
generally divided into two methods, namely, coherent (heterodyne) detection and incoherent (direct) detection.
Although heterodyne detection achieves higher sensitivity than direct detection, it requires a local oscillator andmixer that are difficult to construct with today’s technology, even if a sub-harmonic scheme is used. For thatreason, direct detection has been used. Terahertz direct detectors have primarily relied on specialized fabrication
technologies such as Schottky diodes [28], bolometers [29], and high-electron-mobility transistors [30]. Many ofthese technologies require additional process steps to make them compatible with CMOS technologies [31].
Silicon-CMOS process technologies are becoming a cost-efficient alternative. The main advantage of silicon
technologies is that they allow low-cost and large-scale integration of circuits with readout electronics and on-chipsignal processors [32]. Furthermore, the high-frequency capabilities of silicon technologies have steadily
improved through the guiding principle of scaling, which enables high integration level and low power
consumption at higher frequencies [33]. A CMOS cascode amplifier biased near the threshold voltage of a
MOSFET was proposed to increase the responsivity of terahertz direct detectors [19].
A block diagram of the terahertz imaging pixel –composed of a microstrip patch antenna, matching circuit,and detector– is shown in Fig. 8.
IDET

Vdd

Antenna
Matching circuit

Detector

S11
VOUT
f
IDET
VCM

Fig. 8. Block diagram of the terahertz imaging pixel.

For terahertz-wave detection, non-linearity of an nMOSFET was utilized. The nMOSFET was biased near thethreshold voltage. The circuit shown in Fig. 9 was used as an envelope detector. It consists of a cascode-amplifier
circuit and a subthreshold-biased operational amplifier (subVth-OP amp). The former consists of acommon-source nMOSFET M3 as the input stage driven by terahertz signal VIN with common-gate MOSFET M2.
And pMOSFET M1 is the load. The cascode-amplifier circuit operates as an envelope detector. The subVth-OP
amp operates as a feedback circuit, which determines the load resistance of M1 [34]. Output voltage of thedetector is fixed to common-mode voltage VCM at the DC level. Since the subVth-OP amp operates very slowly
with a large time constant, the feedback operation is established only at DC and very low frequencies. The
feedback circuit operates as a high-pass filter in the detector. Therefore, the envelope detector with the feedback
subVth-OP amp produces no DC offset voltage in its output.
29

Vdd
Cascode-amplifier

SubVth-OP amplifier
M4

M5

M1

VCM

M2

VIN

M3

VCM

M6

Vout

M8

M7

Fig. 9. Schematic of detector circuit.

Each pixel was equipped with a microstrip patch antenna formed by a metal-6 layer (top aluminum layer) anda ground plane (metal-1 layer) in the 180-nm CMOS technology. The dimensions of the antenna were determinedwith a FDTD electromagnetic simulator (Keysight Technologies, EMPro). The designed gain was 6.5 dBi at 0.85
THz. The antenna size was 130×148 µm. A microstrip line was used for the impedance-matching circuit because
it is difficult to use spiral inductors and metal-insulator-metal capacitors in the terahertz region. The width andlength of the microstrip line were respectively 5 and 41 µm. A die photo of the fabricated imaging array is shownin Fig. 10. It consists of a 4×4 pixel array and 2-bit row/column decoders. The total size of the circuit is 1035×745µm. A single pixel occupies 180×250 µm.

1 pixel

250 µm

745 µmRow/Column
Decoder

1035 µm

180 µmFig. 10. Die photo of fabricated imaging array.

An injection-seeded terahertz-wave parametric generator (is-TPG) was used as a frequency-tunableterahertz-wave source [35]. By changing the noncollinear phase-matching condition in MgO:LiNbO3 crystal, theterahertz-wave frequency was continuously tuned from 0.8 to 1.1 THz with a linewidth of approximately 4 GHz.
At the frequency of 0.905 THz, the terahertz-wave pulse energy was measured to be 132 nJ/pulse with a repetition
rate of 100 Hz, corresponding to the average power of 13.2 µW. According to the measured terahertz-wave beamprofile, the average power irradiated to the microstrip patch antenna was estimated to be 4.96 nW. Terahertz-wave
output was modulated at 31 Hz by an optical chopper, and lock-in detection with 10-100 Hz bandpass filter wasused. The measured responsivity of the fabricated imaging array is plotted in Fig. 11. The responsivity of the pixel
was calculated from detected output voltage divided by available power to the antenna. The peak responsivity at30

0.915 THz is 51.9 kV/W. And each pixel only draws 1 µA from a power supply (Vdd) of 1.5 V. Although thepeak frequency shifted from 850 to 0.915 THz, the measured responsivity was close to the designed responsivity.
Thus, the gain of the microstrip patch antenna was estimated to be close to the designed value of 6.5 dBi. The
double-peak property in the measured responsivity is under investigation.

Responsivity (kV/W)

60
50
40
30
20
10
0
0.8 0.85 0.9 0.95 1 1.05 1.1
Frequency (THz)

Fig. 11. Measured responsivity.

At modulation frequency of 31 Hz, output noise level was 18.6 µV/Hz1/2, and measured noise equivalent
power (NEP) was 358 pW/Hz1/2.

NEP can be decreased by increasing sampling rate. Since the measured

imaging array excluded an output buffer, the operation bandwidth of the detector with the extremely small current
was degraded by the parasitic capacitances in the measurement equipment. The operation bandwidth can beincreased by introducing an output buffer. By introducing an output buffer for each column (or row) andincreasing sampling rate, NEP of 42 pW/Hz1/2 is expected to be obtained at 10 kHz. These results suggest that
cost-efficient terahertz imaging is possible.

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33

D6. Space-Division-Multiplexed Optical Fiber Transmission
Hiroyuki Toda
Doshisha University
In this section, I summarize recent development of high capacity optical fiber transmission using
space-division-multiplexing (SDM) techniques and related technologies, reported during the period from
November 2013 to October 2016.

There has been significant interest in SDM techniques realized by MCF and

FMF, since it is expected to improve transmission capacity per fiber drastically.
In [1], 31-core MCF was developed for up to 4,000km transmission. In [2-5], MCFs with 125-µm cladding,which are compatible with conventional fibers, were developed.

In [6], transmission systems consist of

homogeneous single-mode MCF, where each core has the same parameter, were discussed. [7, 8] demonstrated
crosstalk reduction with bidirectional signal assignment in MCF, and showed possibility of gradual upgrade of
SMF-based line to MCF-based line.

Heterogeneous MCFs to reduce inter-core crosstalk with 30 cores [9] and32 cores [10] were demonstrated.
Coupled-core (CC) MCF described in [11-14] is another type of MCF, which permits strong mode coupling
between cores, thus allowing smaller core-to-core distance and higher spatial efficiency compared to uncoupled
MCFs.

CC-MCF also shows low DGD between spatial modes (SMD), which results in reducing MIMO-DSP

complexity, and therefore attractive for long-haul transmission. [15] demonstrated 4-core 125 µm cladding
CC-MCF with 0.158 dB/km propagation loss and 6.1 ps/km1/2 SMD.

[13] also demonstrated strongly-coupled

3-mode 7-core FM-MCF.
In order to further increase the transmission capacity, combining FMF and MCF technologies, referred as
FM-MCF is promising.

[16] and [17] review 3-mode 12 core FM-MCF.

FM-MCF with 306-µm diameter cladding was fabricated.

In [18] and [19], 3-mode 36-core

In [20], 6-mode 19-core FM-MCF with 318-µm

cladding, and in [21], 6-mode 19-core FM-MCF with 246 µm cladding were demonstrated.
that spatial channel numbers exceed 100 for those fibers.
fabricated for improving spatial efficiency [22].

It should be noted

6-mode 12-core FM-MCF with 227 µm cladding was[23] demonstrated moderately coupled 2-mode 6-core FM-MCF

with 125 µm cladding.
[24-27] review the current state of the MCFs and FM-MCFs.
In order to realize SDM transmission systems, development of SDM component is important. MC-EDFAs
were reported in [28, 29]. FM-EDFAs were reported in [30, 31].

Fiber connectors were demonstrated in [32, 33].

Fusion splicing of MCF was demonstrated in [34]. MCF cable was presented in [35].
mode MUX/DEMUX for FMF are necessary if conventional fiber devices are used.
MUX/DEMUX were proposed and demonstrated in [19, 36-42].
also important for realizing SDM transmission systems.

FI/FO for MCF and

Various types of FI/FO andParameter measurement of MCF and FMF is

Measurement techniques for distribution of mode field

diameter and refractive index, mode coupling at FMF splicing point, modal crosstalk of FMF, and mode coupling
distribution of FMF were proposed and demonstrated in [43], [44], [45] and [46], respectively.
34

In [47], fiberfuse propagation, that limits maximum launched power to the fiber, in FMF was observed and examined.
Many transmission experiment using MCF or FM-MCF were reported.

Using 7-core EDFA and 7-core MCF,

120.7 Tbit/s (7-SDM 180-WDM 95.8 Gb/s) 204 km transmission [48], and 51.1 Tbit/s 2,520 km transmission [49,50] were demonstrated.

2.15 Pbit/s 31 km transmission was demonstrated using 22-core MCF in [51].

In [52],

20-WDM PDM-16QAM 96 Gbit/s signals were transmitted over 1,600 km using low-crosstalk heterogeneous
32-core MCF.

The aggregate spectral efficiency was 201.46 bit/s/Hz.

In [53], 1,200 km transmission

experiment was made using 19-core EDFA and 19-core MCF.
Transmission capacity can be further improved by using FM-MCF.

In [54], 20-WDM PDM-32QAM 105

Gbit/s signals were transmitted over 40 km using 3-mode 12-core FM-MCF.
was 247.9 bit/s/Hz.

The aggregate spectral efficiency

In [55, 56], 20-WDM PDM-QPSK 40 Gbit/s signals were transmitted over 527 km with

3-mode 12-core FM-MCF. In [18] and [19], 403.7 Tbit/s 5.5 km transmission was demonstrated using 3-mode
36-core FM-MCF.

Using 9.8-km 6-mode 19-core MCF, 140.7 Tbit/s 7,300 km transmission were demonstrated

by using super Nyquist WDM [57].

Spectral efficiency of 345 bit/s/Hz was obtained in [58].

transmission was demonstrated in [59, 60].

2.05 Pbit/s

The spectral efficiency was improved by 947 bit/s/Hz by using

DP-64QAM modulation format [61].
SDM transmission technologies were reviewed in [62-69].
List of Abbreviations
DGD

Differential group delay

DP

Dual polarization

DSP

Digital signal processing

EDFA

Erbium-doped fiber amplifier

FI/FO

Fan-in/fan-out

FMF

Few-mode fiber

FM-MCF

Few-mode multicore fiber

MCF

Multicore fiber

MIMO

Multiple-input multiple-output

MUX/DEMUX

Multiplexer/demultiplexer

PDM

Polarization-division multiplexing

QAM

Quadrature amplitude modulation

QPSK

Quadrature phase-shift keying

SMF

Single-mode fiber

WDM

Wavelength-division multiplexing

References[1] Tetsuya

Nakanishi,

Tetsuya

Hayashi,

Osamu
35

Shimakawa,andTakashi

Sasaki,“Spatial­Spectral­Efficiency­enhanced Multi­Core Fiber,” Tech. Digest of OFC 2015, Th3C.3, Los Angeles,
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[11] R. Ryf, N. K. Fontaine, M. Montoliu, S. Randel, S. H. Chang, H. Chen, S. Chandrasekhar, A. H. Gnauck, R.
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36Fibers with Low Differential Modal Properties,” Tech Digest of ECOC 2015, We.1.4.1, Valencia, Sept.
27-Oct. 1, 2015.
[13] T. Sakamoto, T. Mori, M. Wada, T. Yamamoto, F. Yamamoto, and K. Nakajima, “Coupled Few-mode
Multi-core Fibre for Ultra-high Spatial Density Space Division Multiplexing,” Tech Digest of ECOC 2016,
W.2.B.3, Düsseldorf, Sept. 18-22, 2016.
[14] Tetsuya Hayashi, Haoshuo Chen, Nicolas K. Fontaine, Takuji Nagashima, Roland Ryf, René-Jean Essiambre,and Toshiki Taru, “Effects of Core Count/Layout and Twisting Condition on Spatial Mode Dispersion in
Coupled Multi-Core Fibers,” Tech Digest of ECOC 2016, W.2.B.5, Düsseldorf, Sept. 18-22, 2016.
[15] Tetsuya Hayashi, Yoshiaki Tamura, Takemi Hasegawa, and Toshiki Taru, “125-µm-cladding Coupled
Multi-core Fiber with Ultra-low Loss of 0.158 dB/km and Record-low Spatial Mode Dispersion of 6.1
ps/km1/2,” Tech. Digest of OFC 2016, Th5A.1, Anaheim, Mar. 20-24, 2016.
[16] Yusuke Sasaki, Yoshimichi Amma, Katsuhiro Takenaga, Shoichiro Matsuo, Kunimasa Saitoh and Masanori
Koshiba, “Few-mode Multicore Fibre with 36 Spatial Modes (Three modes (LP01, LP11a, LP11b) × 12 cores),”
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modes (Three modes (LP01, LP11a, LP11b) × 12 cores),” Journal of Lightwave Technology, vol. 33, no. 5, pp.
964-970, 2015.
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Yoshinari Awaji, Naoya Wada, Tetsuya Hayashi, Tetsuya Nakanishi, Tatsuhiko Watanabe, Yasuo Kokubun,
Taketoshi Takahata, and Tetsuya Kobayashi, “Realizing a 36-core, 3-mode Fiber with 108 Spatial Channels,”
Tech. Digest of OFC 2015, Th5C.2, Los Angeles, Mar. 22-26, 2015.
[19] Jun Sakaguchi, Werner Klaus, José Manuel Delgado Mendinueta, Benjamin James Puttnam, Ruben Soares
Luís, Yoshinari Awaji, Naoya Wada, Tetsuya Hayashi, Tetsuya Nakanishi, Tatsuhiko Watanabe, Yasuo
Kokubun, Taketoshi Takahata, and Tetsuya Kobayashi, “Large Spatial Channel (36-Core × 3 mode)
Heterogeneous Few-Mode Multicore Fiber,” Journal of Lightwave Technology, vol. 34, no. 1, pp. 93-103,
2016.
[20] Tetsuya Hayashi, Takuji Nagashima, Kazuhiro Yonezawa, Yuta Wakayama, Daiki Soma, Koji Igarashi,
Takehiro Tsuritani, and Takashi Sasaki, “6-mode 19-core Fiber for Weakly-coupled Mode-multiplexed
Transmission over Uncoupled Cores,” Tech. Digest of OFC 2016, W1F.4, Anaheim, Mar. 20-24, 2016.
[21] T. Sakamoto, T. Matsui, K. Saitoh, S. Saitoh, K. Takenaga, T. Mizuno, Y. Abe, K. Shibahara, Y. Tobita, S.
Matsuo, K. Aikawa, S. Aozasa, K. Nakajima, Y. Miyamoto, “Low-loss and Low-DMD Few-mode
Multi-core Fiber with Highest Core Multiplicity Factor,” Tech. Digest of OFC 2016, Th5A.2, Anaheim, Mar.
20-24, 2016.
[22] Taiji Sakamoto, Takashi Matsui, Kunimasa Saitoh, Shota Saitoh, Katsuhiro Takenaga, Shoichiro Matsuo,
Yuki Tobita, Nobutomo Hanzawa, Kazuhide Nakajima, and Fumihiko Yamamoto, “Few-mode Multi-coreFibre with Highest Core Multiplicity Factor,” Tech Digest of ECOC 2015, We.1.4.3, Valencia, Sept. 27-Oct.
37

1, 2015.
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Multi-Core

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FutureDense

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38

Naoto Itou, Ken Osato and Shoichiro Matsuo, “Multicore-fiber Cable with Core Density of 6 cores/mm2,”
Tech. Digest of OFC 2014, W4D.3, San Francisco, Mar. 9-14, 2014.
[36] Yusaku Tottori, Hiroshi Tsuboya and Tetsuya Kobayashi, “Multi Functionality Demonstration for Multi
Core Fiber Fan-in/Fan-out Devices using Free Space Optics,” Tech. Digest of OFC 2014, Th2A.44, San
Francisco, Mar. 9-14, 2014.
[37] T. Watanabe and Y. Kokubun, “Fan-in/Fan-out Three-dimensional Polymer Waveguide for Uncoupled
Multi-core Fibers,” Tech Digest of ECIO 2014, Tu3bI2, Nice, June 24-27, 2014.
[38] Masato Yoshida, Toshihiko Hirooka, and Masataka Nakazawa, “Fused Type Fan-out Device for Multi-core
Fiber Based on Bundled Structure,” Tech. Digest of OFC 2016, Tu3I.2, Anaheim, Mar. 20-24, 2016.
[39] H. Uemura, Y. Sasaki, S. Nishimoto, T. Uematsu, K. Takenaga, K. Omichi, R. Goto, S. Matsuo and K.
Saitoh, “Mode Multiplexer/Demultiplexer Based on a Partially Elongated Multi-Core Fiber,” Tech. Digest of
OFC 2014, Tu3D.3, San Francisco, Mar. 9-14, 2014.
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Saitoh, “Multicore fibre-based Mode Multiplexer/Demultiplexer for Three-Mode Operation of LP01, LP11a,and LP11b,” Tech Digest of ECOC 2014, Tu.4.1.4, Cannes, Sept. 21-25, 2014.
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Soma,

Koki

Takeshima,

Koji

IgarashiandTakehiro

Tsuritani,“Degenerate

Modes

Multiplexer/Demultiplexer using Mach- Zehnder Interferometer with Image Inversion,” Tech Digest of
ECOC 2014, We.1.1.6, Cannes, Sept. 21-25, 2014.
[42] Nobutomo Hanzawa, Kunimasa Saitoh, Taiji Sakamoto, Takashi Matsui, Kyozo Tsujikawa, Takui Uematsu,
and Fumihiko Yamamoto, “Four-mode PLC-based mode multi/demultiplexer with LP11 mode rotator on one
chip for MDM transmission,” Tech Digest of ECOC 2014, We.1.1.1, Cannes, Sept. 21-25, 2014.
[43] M. Ohashi, H. Kubota, Y. Miyoshi, R. Maruyama, and N. Kuwaki, “Longitudinal Fiber Parameter
Measurements of Two-Mode Fiber Links by using OTDR,” Tech Digest of ECOC 2014, Th.1.4.5, Cannes,
Sept. 21-25, 2014.
[44] Masato Yoshida, Toshihiko Hirooka, and Masataka Nakazawa, “Mode Coupling Measurement at a Splice
Point between Few- Mode Fibers Using a Synchronous Multi-Channel OTDR,” Tech. Digest of OFC 2016,
Th1J.4, Anaheim, Mar. 20-24, 2016.
[45] Takayuki Mizuno, Hidehiko Takara, Manabu Oguma, Takayuki Kobayashi, and Yutaka Miyamoto, "Modal
Crosstalk Measurement Based on Intensity Tone for Few-Mode Fiber Transmission Systems,” Tech. Digestof OFC 2014, W3D.5, San Francisco, Mar. 9-14, 2014.
[46] Masataka Nakazawa, Masato Yoshida, and Toshihiko Hirooka, “Measurement of Mode Coupling
Distribution Along a Few- Mode Fiber Using a Synchronous Multi-Channel OTDR,” Tech. Digest of OFC
2014, W3D.7, San Francisco, Mar. 9-14, 2014.
[47] Nobutomo Hanzawa, Kenji Kurokawa, Kyozo Tsujikawa, Takayoshi Mori, Masaki Wada, and Fumihiko
Yamamoto, “Fiber fuse propagation in LP11 mode in few­mode fiber,” Tech. Digest of OFC 2015, W4I.5,
Los Angeles, Mar. 22-26, 2015.
39

[48] H. Takara, T. Mizuno, H. Kawakami, Y. Miyamoto, H. Masuda, K. Kitamura, H. Ono, S. Asakawa, Y.
Amma, K. Hirakawa, S. Matsuo, K. Tsujikawa, and M. Yamada, “120.7-Tb/s (7 SDM/180 WDM/95.8 Gb/s)
MCF-ROPA Unrepeatered Transmission of PDM-32QAM Channels over 204 km,” Tech Digest of ECOC
2014, PD3.1, Cannes, Sept. 21-25, 2014.
[49] Koki Takeshima, Takehiro Tsuritani, Yukihiro Tsuchida, Koichi Maeda, Tsunetoshi Saito, Kengo Watanabe,
Toru Sasa, Katsunori Imamura, Ryuichi Sugizaki, Koji Igarashi, Itsuro Morita, and Masatoshi Suzuki,“51.1-Tbit/s MCF Transmission Over 2,520 km Using Cladding Pumped 7-Core EDFAs,” Tech. Digest of
OFC 2015, W3G.1, Los Angeles, Mar. 22-26, 2015.
[50] Koki Takeshima, Takehiro Tsuritani, Yukihiro Tsuchida, Koichi Maeda, Tsunetoshi Saito, Kengo Watanabe,
Toru Sasa, Katsunori Imamura, Ryuichi Sugizaki, Koji Igarashi, Itsuro Morita, and Masatoshi Suzuki,“51.1-Tbit/s MCF Transmission Over 2,520 km Using Cladding-Pumped Seven-Core EDFAs,” Journal ofLightwave Technology, vol. 34, no. 2, pp. 761-767, 2016.
[51] B. J. Puttnam, R. S. Luis, W. Klaus, J. Sakaguchi, J.-M. Delgado Mendinueta, Y. Awaji, N. Wada, Yoshiaki
Tamura, Tetsuya Hayashi, Masaaki Hirano and J. Marciante, “2.15 Pb/s Transmission Using a 22 Core
Homogeneous Single- Mode Multi-Core Fiber and Wideband Optical Comb,” Tech Digest of ECOC 2015,
PDP.3.1, Valencia, Sept. 27-Oct. 1, 2015.
[52] T. Mizuno, K. Shibahara, H. Ono, Y.Abe, Y. Miyamoto, F. Ye, T. Morioka, Y. Sasaki, Y. Amma, K.
Takenaga, S. Matsuo, K. Aikawa, K. Saitoh, Y. Jung, D. J. Richardson, K. Pulverer, M. Bohn, and M.
Yamada, “32-core Dense SDM Unidirectional Transmission of PDM- 16QAM Signals Over 1600 km Using
Crosstalk-managed Single-mode Heterogeneous Multicore Transmission Line,” Tech. Digest of OFC 2016,
Th5C.3, Anaheim, Mar. 20-24, 2016.
[53] Jun Sakaguchi, Werner Klaus, Benjamin J. Puttnam, José Manuel Delgado Mendinueta, Yoshinari Awaji,
Naoya Wada, Yukihiro Tsuchida, Koichi Maeda, Masateru Tadakuma, Katsunori Imamura, Ryuichi Sugizaki,
Tetsuya Kobayashi, Yusaku Tottori, Masayuki Watanabe, and R. V. Jensen, “19-core MCF transmission
system using EDFA with shared core pumping coupled via free-space optics,” Optics Express, vol. 22, no. 1,
pp. 90-95, 2014.
[54] T. Mizuno, T. Kobayashi, H. Takara, A. Sano, H. Kawakami, T. Nakagawa, Y. Miyamoto, Y. Abe, T. Goh,
M. Oguma, T. Sakamoto, Y. Sasaki, I. Ishida, K. Takenaga, S. Matsuo, K. Saitoh, and T. Morioka, “12-core× 3-mode Dense Space Division Multiplexed Transmission over 40 km Employing Multi-carrier Signals
with Parallel MIMO Equalization,” Tech. Digest of OFC 2014, Th5B.2, San Francisco, Mar. 9-14, 2014.
[55] K. Shibahara, T. Mizuno, H. Takara, A. Sano, H. Kawakami, D. Lee, Y. Miyamoto, H. Ono, M. Oguma, Y.
Abe, T. Kobayashi, T. Matsui , R. Fukumoto, Y. Amma, T. Hosokawa, S. Matsuo, K. Saito, H. Nasu, and T.
Morioka, “Dense SDM (12­core 3­mode) Transmission over 527 km with 33.2­ns Mode­Dispersion
Employing Low­Complexity Parallel MIMO Frequency­Domain Equalization,” Tech. Digest of OFC 2015,
Th5C.3, Los Angeles, Mar. 22-26, 2015.
[56] Kohki Shibahara, Takayuki Mizuno, Hidehiko Takara, Akihide Sano, Hiroto Kawakami, Doohwan Lee,40

Yutaka Miyamoto, Hirotaka Ono, Manabu Oguma, Yoshiteru Abe, Takayuki Kobayashi, Takashi Matsui,
Ryohei Fukumoto, Yoshimichi Amma, Tsukasa Hosokawa, Shoichiro Matsuo, Kunimasa Saitoh, Hitoshi
Nasu, and Toshio Morioka, “Dense SDM (12-Core × 3-Mode) Transmission Over 527 km With 33.2-ns
Mode-Dispersion Employing Low-Complexity Parallel MIMO Frequency-Domain Equalization,” Journal ofLightwave Technology, vol. 34, no. 1, pp. 196-204, 2016.
[57] Koji Igarashi, Takehiro Tsuritani, Itsuro Morita, Yukihiro Tsuchida, Koichi Maeda, Masateru Tadakuma,
Tsunetoshi Saito, Kengo Watanabe, Katsunori Imamura, Ryuichi Sugizaki, and Masatoshi Suzuki,“Super-Nyquist-WDM transmission over 7,326-km seven-core fiber with capacity-distance product of 1.03
Exabit/s·km,” Optics Express, vol. 22, no. 2, pp. 1220-1228, 2014.
[58] Koji Igarashi, Daiki Souma, Yuta Wakayama, Koki Takeshima, Yu Kawaguchi, Takehiro Tsuritani, Itsuro
Morita,and

Masatoshi

Suzuki,“114

Space­Division­Multiplexed

Transmission

over

9.8­kmWeakly­Coupled­6­Mode Uncoupled­19­Core Fibers,” Tech. Digest of OFC 2015, Th5C.4, Los Angeles,
Mar. 22-26, 2015.
[59] D. Soma, K. Igarashi, Y. Wakayama, K. Takeshima, Y. Kawaguchi, N. Yoshikane, T. Tsuritani, I. Morita,
and M. Suzuki, “2.05 Peta-bit/s super-nyquist-WDM SDM transmission using 9.8-km 6-mode 19-core fiberin full C band,” Tech Digest of ECOC 2015, PDP.3.2, Valencia, Sept. 27-Oct. 1, 2015.
[60] Koji Igarashi, Daiki Soma, Yuta Wakayama, Koki Takeshima, Yu Kawaguchi, Noboru Yoshikane, Takehiro
Tsuritani, Itsuro Morita, and Masatoshi Suzuki, “Ultra-dense spatial-division-multiplexed optical fibertransmission over 6-mode 19-core fibers,” Optics Express, vol. 24, no. 10, pp. 10213-10231, 2016.
[61] Daiki Soma, Yuta Wakayama, Shohei Beppu, Koji Igarashi, Takehiro Tsuritani, Hidenori Taga, Itsuro
Morita, and Masatoshi Suzuki, “665 and 947 b/s/Hz Ultra-highly Aggregate-Spectral-Efficient SDM/WDM
Transmission over 6-Mode 19-Core Fibre Using DP-16QAM/64QAM Signals,” Tech Digest of ECOC 2016,
Th.3.C.2, Düsseldorf, Sept. 18-22, 2016.
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Multicore Fibers,” Tech. Digest of OFC 2014, Tu2J.1, San Francisco, Mar. 9-14, 2014.
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Fibers,” Tech. Digest of OFC 2014, Tu2J.2, San Francisco, Mar. 9-14, 2014.
[64] Akihide Sano, Hidehiko Takara, Takayuki Kobayashi, and Yutaka Miyamoto, “Crosstalk-Managed HighCapacity Long Haul Multicore Fiber Transmission With Propagation-Direction Interleaving,” Journal ofLightwave Technology, vol. 32, no. 16, pp. 2771-2779, 2014.
[65] Koji Igarashi, Takehiro Tsuritani, Itsuro Morita, and Masatoshi Suzuki, “Ultra-Long-Haul High-Capacity
Super-Nyquist-WDM Transmission Experiment Using Multi-Core Fibers,” Journal of Lightwave
Technology, vol. 33, no. 5, pp. 1027-1036, 2015.
[66] Takayuki Mizuno, Hidehiko Takara, Akihide Sano, and Yutaka Miyamoto, “Dense Space Division
Multiplexed Transmission over Multi-core and Multi-mode Fiber,” Tech. Digest of OFC 2015, Th1D.2, Los
Angeles, Mar. 22-26, 2015.
41

[67] Takayuki Mizuno, Hidehiko Takara, Akihide Sano, and Yutaka Miyamoto, “Dense Space Division
Multiplexing Long Haul Transport System Using Multi-core / Multi-mode Fibre,” Tech Digest of ECOC
2015, Th.1.2.2, Valencia, Sept. 27-Oct. 1, 2015.
[68] Takayuki Mizuno, Hidehiko Takara, Akihide Sano, and Yutaka Miyamoto, “Dense Space-Division
Multiplexed Transmission Systems Using Multi-Core and Multi-Mode Fiber,” Journal of Lightwave
Technology, vol. 34, no. 2, pp. 582-592, 2016.
[69] Takayuki Mizuno, Hidehiko Takara, Kohki Shibahara, Akihide Sano, and Yutaka Miyamoto, “Dense Space
Division Multiplexed Transmission Over Multicore and Multimode Fiber for Long-haul Transport Systems,”
Journal of Lightwave Technology, vol. 34, no. 6, pp. 1484-1493, 2016.

42

D7. Recent research trends in optical access network technologies
Jun-ichi Kani
NTT Corporation

1. Background
One of the backgrounds to trigger recent researches in optical access is the rapid progress of the mobile
wireless access services.

To fundamentally increase the bandwidth in the mobile access, dense deployment ofsmall sells is an essential approach [1].

Then, optical access networks to connect many antennas for the small

cells to a central node become important [2].

Another background is that expansion of broadband applications

and diversification of user devices are bringing various requirements to the network.

To accommodate suchvarious requirements on demand, it is needed make it easier to add and replace functions in access network
equipment [3].

In addition to address these two issues, it is important to think about how to further increase thebasic performance of optical access networks to prepare to any future upgrades. The following sections review
recent researches from these three aspects.

2. Optical access for the mobile fronthaul (MFH)
Centralized Radio Access Network (C-RAN) architecture is considered as a valid solution to support asustainable growth of the mobile access in the 5th generation mobile communications [4, 5].

In the C-RAN

architecture, An Remote Radio Head (RRH) is located at each remote antenna site while baseband processing of
the wireless transmission is centralized into Base Band Units located at a central node.

The section between the

RRHs and the BBUs is called Mobile Front-Haul (MFH).
Optical access is the primal solution to realize the MFH in terms of providing high and stable performance inthe 5G RAN, in which several Gbit/s to 10 Gbit/s speed is assumed in the wireless communication [1].

Passive

Optical Network (PON) systems provide point-to-multipoint (PtMP) fiber connections in a cost effective manner,so they can effectively support small cells especially when they are densely deployed. Thus, various studies
have been reported on how to apply PON technologies to MFH [6-9].
One of the focuses among such studies is cooperation between optical systems and mobile wireless systems tomaximize the performance in total.
[6].

Figures 1 illustrates such cooperation in the dynamic bandwidth allocation

As shown in the figure, the Optical Line Terminal (OLT) at the center node in PON dynamically allocatesbandwidth to each Optical Network Unit (ONU) at the antenna side while the BBU dynamically allocatesbandwidth to each User Equipment (UE).

By sharing the allocation information from the BBU to the OLT inreal time, the OLT can allocate bandwidth to each ONU in just proportion in advance to the arrival of thecorresponding upstream signals.
network.

This method can minimize the latency in the upstream transmission in the

Another example of such cooperation is that in the power-saving control; a method has been reported

to minimize the latency by coordinating the ONU-sleep control in PON with the discontinuous reception (DRX)
to save power in UE in the mobile system [7].
43Bandwidth allocation
RRH

ONU

Bandwidth allocation
PON
OLT

RRH

BBU

Core
network

ONU

Sharing the allocation
information
Figure 1 – Cooperation between optical and mobile wireless systems; (a) cooperative bandwidth allocation and (b)cooperative power-saving control

3. Modularization and virtualization
Expansion of broadband applications and diversification of user devices are bringing various requirements tothe network.

However, in the current systems, it is not easy to add or replace functions to accommodate newservice requirements on demand.
issue.

Access network modularization and virtualization are the keys to address thisFigures 2(a) and 2(b) show the first step and the second step of the evolutions in a view [3].

In the firststep, access network functions are implemented as software modules with common interfaces so that eachfunction can be added or replaced depending on requirements.
network.

This approach can be applied in the current

In the second step, some of the functions are “virtualized,” i.e. moved to the cloud.

in the era of Software Defined Network (SDN) in future.

This can happen

In addition, deeper softwarization will further increase

the flexibility and potentially realize “fully programmable” optical access.
Related works include Central Office Re-architected as Datacenter (CORD) promoted by ON.Lab and several
industry players including AT&T [10], Flexible Access Network Architecture (FASA) announced by NTT [11, 12],and Future-X Network announced by Nokia Bell Labs [13].

Within the above-mentioned purpose, FASA

relatively focuses on the modularization while CORD relatively focuses on the virtualization.
Challenges for the deeper softwarization include real-time implementation of 10-Gbit/s-class Forward Error
Correction and Encryption [14] as well as that of the Multi-Point MAC Control (MPMC) function in an Ethernet
PON system [15].

44… Software module
Access network
equipment

Access network equipment
Bandwidth
controlPowersavingOAM

OAMBandwidth
control…

Virtualization

…

Power
savingInterface

Interface

PON OLT
HardwareServerDeep
softwarization

Access control
Encryption
PHY processing
OpticsHardwareAccess control
Encryption
PHY processing
Optics(b)(a)

Figure 2 – Access network modularization and virtualization; (a) step 1 and (b) step 2

4. Basic performance increase
Researches to increase the basic performance, e.g. capacity and reach, in optical access are continuously active
as well.

Such researches will guarantee any upgrades of optical access systems including the new directions

mentioned above.

Enhancing the peak rate (per wavelength channel) to more than 10 Gbit/s is one of thenear-term targets [16].
Among various technical items, how we can use the digital-coherent transmission and reception technologies
in optical access is one of the active study topics [17-22].

It has been reported that we can apply thedigital-coherent transmission and reception in the burst-mode upstream transmission in PON throughimplementing a burst-mode optical pre-amplifier and some appropriate digital signal processing functions [22].

References[1] T. Asai, “5G Radio Access Network and its Requirements on Mobile Optical Network,” proceedings of 2015
International Conference on Optical Network Design and Modeling (ONDM), pp. 7-11, 2015.
[2] J. Kani, S. Kuwano, J. Terada, “Options for future mobile backhaul and fronthaul,” Optical Fiber
Technology, Vol. 26, pp. 42-29, 2015.
[3] J. Kani, “Solutions for future mobile fronthaul and access-network convergence,” OFC2016, paper W1H.1,
Mar. 2016..
[4] K. Kiyoshima et al, “Commercial Development of LTE-Advanced Applying Advanced C-RAN Architecture– Expanded Capacity by Add-on Cells and Stable Communications by Advanced Inter-cell Coordination,”
NTT DOCOMO Technical Journal, Vol. 17, No. 2, pp. 10-18, 2015.
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Cloudification,” IEEE Access, Vol. 2, pp. 1030-1039, 2014.
45

[6] T. Tashiro, S. Kuwano, J. Terada, T. Kawamura, N. Tanaka, S. Shigematsu, and N. Yoshimoto, “A Novel
DBA Scheme for TDM-PON based Mobile Fronthaul,” OFC2014, paper Tu3F.3, Mar. 2014.
[7] N. Iiyama, J. Kani, J. Terada, N. Yoshimoto, “A study on cooperation of sleep control and DRX control in
mobile fronthaul based on TDM-PON,” IEICE General Conference, B-8-55, Niigata, March, 2014 (inJapanese).
[8] A Pizzinat, P. Chanclou, F. Saliou, T. Diallo, “Things you should know about fronthaul,” IEEE J. Lightwave
Technol. Vol. 33, pp. 1077-1083, 2015.
[9] N. Shibata, T. Tashiro, S. Kuwano, N. Yuki, J. Terada and A. Otaka, “Mobile front-haul employing
Ethernet-based TDM-PON system for small cells,” OFC2015, paper M2J.1, Mar. 2015.
[10] Larry Peterson, IEEE Software Defined Networks - Newsletter, “CORD: central Office Re-architected as a
Datacenter (CORD),” November 2015.
[11] NTT press release, “Introducing the New FASA Concept for Future Access Systems – With accessequipment modularization, NTT enables service providers to begin service immediately –,” Feb 2016:
http://www.ntt.co.jp/news2016/1602e/160208a.html[12]NTT Access Network Service Systems Laboratories, “Flexible Access Network Architecture – White Paper,”May 2016: http://www.ansl.ntt.co.jp/e/global/FASA/index.html[13] “The Future X Network – A Bell Labs Perspective,” CRC Press, March 2016.
[14] T. Suzuki, S.-Y. Kim, J. Kani, K.-I. Suzuki, A. Otaka, T. Hanawa, “Parallelization of Cipher Algorithm on
CPU/GPU for Real-time Software-Defined Access Network,” Proceedings of APSIPA Annual Summit andConference 2015, pp. 484-487, Dec 2015.
[15] K. Nishimoto, M. Tadokoro, T. Mochida, A. Takeda, T. Tanaka, and T. Inoue, “Virtualization of EPON OLT
functions and collision suppression techniques for Multi-Point MAC Control,” Optical Fiber Communication
Conference, OSA Technical Digest (Optical Society of America, 2016), paper W3F.1, 2016.
[16] P. P. Iannone, A. H. Gnauck, D. T. van Veen, and V. E. Houtsma, “Increasing TDM Rates for Access
Systems Beyond NG-PON2,” J. Lightwave Technol. 34, 1545-1550, 2016.
[17] N. Yoshimoto, J. Kani, S. -Y. Kim, N. Iiyama, J. Terada, “DSP-based optical access approaches forenhancing NG–PON2 systems,” Commun. Mag., Vol. 51, No. 3, pp. 58–64, 2013.
[18] H. Rohde, E. Gottwald, S. Rosner, E. Weis, P. Wagner, Y. Babenko, D. Fritzsche, H. Chaouch, “Trials of a
Coherent UDWDM PON Over Field-Deployed Fiber: Real-Time LTE Backhauling, Legacy and 100G
Coexistence,” J. Lightwave Technol. Vol. 33, pp. 1644-1649, 2015.
[19] J. Prat, I. Cano, M. Presi, I. Tomkos, D. Klonidis, G. Vall-llosera, R. Brenot, R. Pous, G. Papastergiou, A.
Rafel, E. Ciaramella, “Technologies for cost effective udWDM-PONs,” IEEE J. Lightwave Technol., vol. 34,
no. 2, pp. 783-791, 2016.
[20] S. -Y. Kim, J. Kani, K. -I. Suzuki, A. Otaka, “OLT Receiver for Power Normalization of Burst OFDM
Signals Enabling OFDM/TDMA-PON,” Photon. Technol. Lett., Vol. 26, No. 24, pp. 2469-2472, 2014.
[21] S.-Y Kim, N. Iiyama, J. Kani, R. Koma, T. Suzuki, K.-I. Suzuki, A. Otaka, “Real-time demonstration of 30
46

Gbit/s hierarchical star 8-QAM passive optical network employing fractionally-spaced signed-error radiusdirected equalization,” IET Electron. Lett., Vol. 52, No. 7, pp. 544-546, 2016.
[22] R. Koma, M. Fujiwara, J. Kani, S. -Y. Kim, T. Suzuki, K. -I. Suzuki, A. Otaka, H. Mori, T. Wada, “22-dB
Dynamic Range, Real-Time Burst-Mode Reception of Digital Coherent 20-Gb/s QPSK PON Upstream
Signals,” ECOC2016, Presentation M.1.E.4, Sept 2016.

47

D8. Optical Access Networks for broadband IoT services
Naoto Yoshimoto
Chitose Institute of Science and Technology

INTRODUCTION
Recently, wired-wireless converged networks with sensors have been eagerly investigated to provide various
cloud services using big data handling techniques [1] [2]. Video monitoring is a kind of attractive wireless sensors.
High definition video technology such as 4K has been drastically developed and various 4K cameras are widely
diffused.

Since such high quality video cameras can be monitored human activities and traffic condition clearly,

it will be expected to construct secure and safety community toward future IoT era. Although high definition
video cameras constantly output large size of data (giga-bit class), quite efficient data compressed technique forhigh definition video data such as H.265/HEVC has been progressed [3].
video camera compressed to moderate capacity (mega-bit class).

Therefore, we can transmit data of 4K

In this report, Distributed 4K-Video camera

monitoring on already deployed Ethernet PON (Passive Optical Network) system is proposed.
advantage. The first is cost-effectiveness.

This is someSince 1G- Ethernet PON (1G-EPON) has massively been installed,

it is easily and quickly utilized for new broadband services.

The second is upgradability to 8K anduncompressed signal compared with wireless access such as Long Term Evolution (LTE), because PON system
has authorized upgrade scenario toward 10G-Ethernet PON (10G-EPON) and Next-generation PON-2(NG-PON2).

The third is capability of service co-existence with current provided broadband services such as

WiFi spot service.

In broadband service, downstream traffic is generally large than upstream one.

side, in 4K-video monitoring service, upstream traffic is large than downstream one.
be efficiently used at not only downstream and but also upstream.

On the other

Therefore, bandwidth canFigure 1 shows an image of secure andubiquitous community based on optical access network using widely distributed 4K video cameras.

4K video

cameras are located in the office area and residential area as well as WiFi base stations (for outside Internet
service), and are connected with optical fiber cable deployed for FTTH (Fiber to the Home).

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NETWORK CONFIGURATION
Figure 2 shows proposed 4K video monitoring and data transmission system based on 1G-EPON.

Servicearea is up to 10 km as typical size of community and number of connected 4K video monitors is up to 32 as equal
to typical splitting ratio of 1G-EPON systems.

At the 4K-transmitter, compressed 4K video data is packetizedand transmitted through gigabit Ethernet interface with variable frame size in accordance with traffic condition
and buffering capacity.

Since the data direction is mainly upstream, at the Optical Network unit (ONU),

packetized 4K video data should be burst mode transmission on the Time-Division Multiplexing Access (TDMA).
The burst cycle is set to 10 msec and the data load is controlled by changing the burst size flexibly as shown in theinserted right side block in Fig.2.

In PON span, the upstream packets are controlled by the both fixed anddynamic bandwidth allocation technique.

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SIMULATION
In order to confirm the feasibility of the proposed network configuration for 4K-video monitoring service, we
evaluated the performance of 1G-EPON systems by using PON traffic simulation “SimOliver.”The simulation parameters of 4K video monitoring service and Internet service are listed in Table 1.

Thequeue set had two priorities; the data traffic of 4K-video monitoring service is assigned to “high priority” and thatof Internet service is assigned “low priority.”

The bandwidth for high priority was reserved to 15 Mbps.

The

upstream data rate of 4K-video is set to 30 Mbps according to the typical transmission rate of compressed 4K
video data by H.265.

The flame size of the upstream data was fixed to 1518 Byte to easily control bandwidth

allocation at the PON system for efficient transmit thorough the PON span.

On the other side, the upstream anddownstream data rate of Internet service was set to 1.5 Mbps and 30 Mbps, respectively, which values isaccording to the typical transmission rate of Internet service.

The flame sizes of the both directions were varied

from 64 to1518 Byte randomly.
Firstly, a case of dedicated 4K-video service (all 32 ONUs are connected to 4K-video) was simulated.
Figure 3 shows the simulated result of mean packet delay as a function of network load.

At this case, Fixed

Bandwidth Allocation (FBA) mode is more suitable, because only 4K-video traffic continuously transmitted andthe frame size was set to fixed value at the 4K-transmitter as mentioned above.

As the network load increased,

the mean packet delay gradually increased, and the network load exceeded to 0.8, the delay rapidly increases.
As a result, the burst packet interval should be controlled as equal to the network load of less than 0.8.

Secondly,a case of shared 4K-video service (32 ONUs are connected to 4K-video and the additional ONUs are connected toInternet service) was simulated for multiple service coexistence.
Figure 4 shows the simulated result of mean packet delay as a function of connected Internet service users on
the same PON branch.

In this simulation, there are four types of bandwidth allocation scheme as listed in Fig. 4.

The Priority Dynamic Bandwidth Allocation (P-DBA) means that 4K-video service is set priority and the total
bandwidth is assigned by DBA mode. On the other hand, the Non Priority DBA (Non P-DBA) means that both
services are fairly treated and the total bandwidth is assigned by DBA mode.
the total bandwidth is assigned by FBA mode.

P-FBA and Non P-FBA shows thatAs shown in Fig. 4, even priority based FBA, the value of the50

mean packet delay was high, because the traffic pattern of Internet service is assumed to be random access, not tobe fixed rate access.
still low.

By using DBA mode, as the number of Internet user increased, the mean packet delay was

Therefore, in case of both 4K-video and Internet service provisioning, DBA mode is more suitable forhighly efficient traffic management.

In such way, hybrid bandwidth allocation scheme by using both FBA andDBA according to various service provisioning and traffic condition is valuable for multiple service accessnetworks in near future.

References[1] N. Yoshimoto, “Operator Perspective on Next-Generation Optical Access for High-Speed Mobile Backhaul,”
OFC2013, OTu3E.1, March 2013.
[2] K. Iwatsuki and K. Tsukamoto, “Next-generation resilient access networks,” Proc. SPIE VOl. 9387, pp.
9387-2-1-8, San Francisco, CA, February 2015.
[3] M. Ikeda, “H.265/HEVC encoder for UHDTV,” 20th Asia and South Pacific Design Automation Conference(ASP-DAC2015), pp. 686-688, Chiba, Japan, January 2015.

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D9. Construction of resilient cyber physical system based on ICT
Katsumi Iwatsuki
Tohoku University

ABSTRACT
While development of science and technology has built up the sophisticated civilized society, it has also
resulted in quite a few disadvantages in global environment and human society.

The common recognition has

been increasingly shared worldwide on sustainable development society attaching greater importance to thesymbiotic relationship with nature and social ethics. After the East Japan Great Earthquake, it is indispensable
for sustainable social development to enhance capacity of resistance and restoration of society against natural
disaster, so called “resilient society.”
Our society consists of various Cyber Physical Systems (CPSs) that make up the physical systems by fusingwith an Information Communication Technology (ICT).
realize resilient society.

We describe the proposed structure of CPS in order to

The configuration of resilient CPS consisting of ICT and physical system is discussed to

introduce “autonomous, distributed, and cooperative” structure, where subsystems of ICT and physical system are
simultaneously coordinated and cooperated with Business Continuity Planning (BCP) engine, respectively. We
show the disaster response information system and energy network as examples of BCP engine and resilient CPS,
respectively. We also propose the structure and key technology of resilient ICT.
Keywords: Cyber physical system (CPS), Business Continuity Planning (BCP), Resilience, Network Function
Virtualization (NFV), Software Defined Network (SDN), Full coherent transmission technology, Wavelength
division multiplexing Passive optical network (WDM-PON)

INTRODUCTION
Though natural disasters as yet unpreventable, the key issue in the conventional disaster management study
has been improving disaster resistance.

Disaster prevention technology of course has its own limitations.

Recently, attention has been increasingly paid to disaster mitigation technology based on the idea that upon theoccurrence of a disaster, restoration and reconstruction should be conducted swiftly and effectively to minimize
adverse impact due to disasters.

To realize sustainable social development, it is indispensable to enhancecapacity of resistance and restoration of society against disaster by combing the disaster prevention technology
and disaster mitigation technology.

We name the society resilient to disaster “Resilient Society” [1].

By fusing with the Information Communication Technology (ICT), a physical system becomes smarter as a
Cyber Physical System (CPS).

Our society consists of various CPSs based on the physical systems Xi, such astransportation, robot, energy network, city, and so on, as shown in Fig.1.

To realize resilient society, we need to

improve resilience of CPS itself with introducing the autonomous, distributed, and cooperative operation, andsimultaneously to coordinate and cooperate each CPS with a business continuity planning (BCP) engine, as shownin Fig.2.
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Figure 1. Social system consisting of various cyber physical systems (ICT x Xi).

Figure 2. Resilient society realized with resilient CPS.

When the disaster occurs, we lose and recover the function and performance of system as shown in the redline of Fig. 3.

The function and performance changes over the time as shown in blue line of Fig. 3, withimproving resilience of system.

The framework of resilience in Ref. [2] shows that four fundamental properties,“Robustness”, “Redundancy”, “Resourcefulness”, and “Rapidity” in Fig. 3 are required to achieve the resilience.
The approach to four fundamental properties is as follows:•

Approach of Robustness; we make elements and/or subsystems composed of system stronger against adisaster. The autonomous system can achieve Robustness.

•

Approach of Redundancy; we don’t aggregate functionality of system in one place.
system can achieve Redundancy.

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The distributed•Approach of Resourcefulness and Rapidity; we configure the system again with resources that are inserted
from the outside.

The cooperative operation of system easily to insert the resources can achieve

Resourcefulness and Rapidity.

Figure 3. Four fundamental properties to achieve resilience.

REALIZATION OF RESILIENT CYBER PHYSICAL SYSTEMThe CPS consists of ICT and physical system Xi.

The ICT and Xi are composed of subsystem of ICTn andXin (n = I, j, k, m), respectively, as shown in Fig. 4.

Among ICTn and Xi, the information and physical quantityof χ , for example current in power grid, is distributed, respectively.

To realize the resilient CPS, the four

fundamental properties are achieved with introducing autonomous, distributed, and cooperative operation to both
ICTn and physical system Xin.

Subsystems of ICTn and Xin simultaneously are coordinated and cooperated witha Business Continuity Planning (BCP) engine, respectively, as shown in Fig.4.

The BCP engine collects andanalyzes environmental data, so as to grasp the disaster situations and operating condition of ICTn and Xin.

Figure 4. Configuration of resilient CPS.
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Figure 5 shows the conceptual diagram of BCP engine to support the resilient society.
broadcasting, social media, Xi with sensors are gathered through network as big data.

Various data from

The big data are analyzed

with information technologies, such as deep learning, natural language processing, speech recognition, and so on,
and then we can understand operating conditions of CPSs with the visualized information from BCP engine bysmart devices.

Figure 5. Conceptual diagram of BCP engine to support the resilient society.

We introduce the disaster response information system in Fig.6, as an example of BCP engine.

As for thedisaster response against the East Japan Great Earthquake, inefficient disaster-information gathering and sharing
based on papers is a critical issue.

To prepare the Tokyo Metropolitan Earthquake and Tokai-Tonankai-Nankai

Earthquake, predicting the occurrence in near future, the development of information systems to support disaster
response and recovery will be an urgent issue.

To make efficiently a database of disaster information andresponse record, and to utilize effectively the information and accumulate lessons for future by analyzing the huge
database, we apply the natural language processing and speech recognition technologies, so that computers assist
to structure various kinds of disaster information and automatically make the database, as shown in Fig. 6.

Then,

we solve the bottleneck of disaster-information input into the database with keyboard in actual work, so as toshare the accurate and quick recognition of present situation in the case of disaster [3].

Figure 6. Disaster response information system.
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We also introduce the resilient energy network in Fig.7 [4], as an example of resilient CPS.

The fundamentalof power grid is multi-port power routers, which joint solar panels, batteries, and households with having small
solar panels and batteries, as a ring topology.
a core unit and AC/DC converter units.

The router is composed of three-way isolated DC/DC converter as

The router can distribute power in three directions and perform thepower interchanging function regardless of DC and AC, with the use of resilient ICT.
surplus electricity in the power grid is lent to power-shortage households.

In the usual time, theIn the time of disaster, each household

is operating with self-sufficient power supply or power supply from surviving power grid.

Figure 7. Resilient energy network.

REALIZATION OF RESILIENT ICT
Figure 8 shows whole picture of resilient ICT.

Environmental data from sensor networks are analyzed withthe distributed cloud computing which operates as the BCP engine, so as to grasp the information of disastersituations and operating conditions of networks, such as wireless network, Wi-Fi network, ITS network, Ad-hoc
network, and so on.

Using the information from the distributed cloud computing, we can autonomously andcooperatively operate various networks, even in the time of huge disaster, and then we can enhance resilience of
networks.

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Figure 8. Whole picture of resilient ICT.

Since existing wired and wireless access networks have been constructed independently, we cannot
cooperatively operate wired and wireless access networks, and then the resilience is insufficient.

The keytechnology of cooperative operation of networks is to virtualize into the physical layer with “full coherent
transmission” together with the Network Function Virtualization (NFV) and/or Software Defined Network (SDN)of access networks, as shown in Fig. 9 [5].

Figure 9. Resilient network with technical convergence of wired and wireless.

Optical transmission technologies
Figure 10 shows the configuration of NG-PON2.

NG-PON2 is constructed based on WDM-PON, which is aphysically shared system, but a logically unshared system.
TDM-PON.

Several wavelengths of WDM-PON are adapted for

The other wavelengths of WDM-PON are used as point-to-point WDM.

The wavelength

allocation of WDM is going to be standardized in Full Service Access Network (FSAN).

The point-to-point

WDM can be applied to accommodate broadband mobile access networks.

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Figure 10. Configuration of NG-PON2.

Figure 11 shows the fundamental configuration of digital coherent optical transmission.
emitted from coherent light source is modulated by QAM signals with an IQ modulator.

The optical wave

The modulated opticalwave is transmitted through an optical fiber, and then homodyne detection is achieved with a coherent receiverand local oscillator.

The transmitted signal is demodulated with an analog digital converter (A/D) and digital

signal processor.

Figure 11. Fundamental configuration of digital coherent optical transmission.

Wireless transmission technologies
Figure 12 shows the rapid increase of mobile traffic over the past time and also the prediction in near future.
The mobile traffic of 5G commercially starting at 2020 is predicted more than1000 times as compared with that of
3G.

The main traffic after LTE system is data and video downloaded from the cloud through the internet.

we will develop technologies for 5G mobile networks to accommodate a huge traffic.

So,

One of the mostpromising approaches is to achieve a small cell with distributed antenna network (DAN), as shown in Fig. 13[6]-[9].

A large number of antennas are distributed and covered in the existing a macro cell, and many antennas

are accommodated in several signal processing centers (SPCs) with optical links. Each SPC is connected with
optical links as well.

The issue of DAN is how to cost-effectively and flexibly achieve a lot of antennas andoptical links connecting between each antenna and SPC.

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Figure 12. Rapid increase of mobile traffic.

Figure 13. Realization of small cell with distributed antenna network.

Convergence of wired and wireless with coherent technology
We can adopt same modulation format as wireless transmissions for optical fiber transmissions with
developing the digital coherent technology.

When we apply frequency convertor from optical to wireless carrier

with preserving the carrier coherence, optical signals transmitting through optical fibers can be radiated from anantenna as wireless signals, as shown in Fig. 14. We call this technology “full coherent transmission.” The media
independent transmission can be achieved with the full coherent transmission, so that we will construct cost-effective
and flexible access networks together with the SDN and/or NFV.

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Figure 14. Full coherent technology with convergence of wired and wireless.

SUMMARY
We described the proposed structure of resilient CPS consisting of ICT and physical system in order to realize
resilient society.

The configuration of resilient CPS was discussed to introduce “autonomous, distributed, andcooperative” structure into both ICT and physical system, where subsystems of ICT and physical system are
simultaneously coordinated and cooperated with BCP engine, respectively [10].

We showed the disaster

response information system and resilient energy network as examples of BCP engine and resilient CPS,
respectively.

With the use of the information analyzing data from sensor, broadcasting and social media, we canautonomously and cooperatively operate various networks, so as to realize the resilient ICT. We also described
the key technology of resilient ICT to virtualize into the physical layer with full coherent transmission withconvergence of wired and wireless.

ACKNOWLEDGEMENT
The author is grateful thanks to Profs. F. Adachi, M. Nakazawa, N. Kato, and Associate Prof. H. Nishiyama of
Tohoku University, and Prof. Y. Kado of Kyoto Institute of Technology for their fruitful discussions.

REFERENCES AND LINKS[1] K. Iwatsuki, and H. Hayashi, “The Realization of Resilient Society with Information Technology Revolution,”

J. Disaster Research, Vol.5 No.6, pp.622-626, 2010.
[2] https://mceer.buffalo.edu/meetings/AEI/presentations/01Bruneau-ppt.pdf[3] K. Iwatsuki, “The projects of disaster-resistant information communication network at the Research

Organization of Electrical Communication, Tohoku University,” Proc. of SPIE, Vol.8646, No.86460B,
invited paper, SPIE Photonics West 2013, 8646-9, 2013.

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[4] Y. Kado, K. Iwatsuki, and K. Wada, “Multi-port power router and its impact on resilient power grid systems,”

Proc. of SPIE, Vol.9772, No. 9772-18, invited paper, SPIE Photonics West 2016, 2016.
[5] K. Iwatsuki and K. Tsukamoto, “Next Generation Resilient Access Networks,” invited paper, SPIE

Photonics West 2015, San Francisco, vol.9387, p.93870J-1, 2015.
[6] F. Adachi, “Wireless Optical Convergence Enables Spectrum-Energy Efficient Wireless Networks,” Proc.

2014 International Topical Meeting on Microwave Photonics/the 9th Asia Pacific Microwave Photonics(MWP/APMP 2014), pp.51-56, Sapporo, Japan, 20-23 Oct. 2014.
[7] F. Adachi, et al, “Distributed Antenna Network for Gigabit Wireless Access,” International Journal of

Electronics and Communications (AEUE), Elsevier, Vol. 66, Issue 6, pp.605-612, 2012.
[8] F. Adachi, et al, “Recent Advances in Single-carrier Distributed Antenna Network,” WirelessCommunications and Mobile Computing, Vol.11, Issue 12, pp.1551–1563, Dec. 2011.
[9] F. Adachi, et al, “Recent Advances in Single-carrier Frequency-domain Equalization and Distributed

Antenna Network,” IEICE Trans. Fundamentals, Vol.E93-A, No.11, pp.2201-2211, Nov. 2010.
[10] K. Iwatsuki, “Cyber physical system based on resilient ICT,” invited paper, SPIE Photonics West 2016, San

Francisco, vol.9772, p.9772-17, 2016.

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