WasedaUniversity-jobopening

【Faculty job openings】Department of Computer Science and Engineering

*English version follows Japanese

5/2に2022年度春学期分学費引落が成立しなかった場合は、7/1の第2回の学費引落にてお支払いください。

• 振替結果については、ご自身で通帳等にてご確認ください。
• 6月下旬に口座振替通知書（ハガキ）を発送の上、引落を行います。
• 休学の場合を除いて、請求額に変更はありません（追加料金等は発生しません）。
• 5/2の引落が成立しなかったことにより、直ちに退学になることはありません。
• 口座情報や学費負担者情報の変更を行う場合は、5月中をめどにご対応ください。
• 私費留学生の場合は、別途4/13に海外送金などに関する案内メールをお送りしていますので、ご確認ください。海外送金もしくはクレジットカード払いを希望の場合は、学費の支払い期日が通常とは異なります。詳細はメールに記載された通りです。

なお、2022年4月入学者は入学手続時に既に春学期分学費を納入済みのため、今回の引落の対象外となります（秋学期から引落が開始されます）。また、国の修学支援制度採用者や延長生の場合は、原則7/1が初回の請求日となります。
上記をはじめ、学費に関する詳細はこちらよりご確認ください。

In case the bank transfer for the tuition and fees for the AY2022 spring semester was not successful on May 2, please make your payment at the second bank transfer scheduled on July 1.

• Please confirm the result of the transfer in your bankbook, etc.
• The transfer will be made after sending a notification for bank transfer in the late June.
• There will be no change in the total amount (no additional fees, etc. will be charged), except in case of a student taking a leave of absence.
• You will not be immediately dismissed from the school due to the unsuccessful result of the transfer on May 2.
• If you wish to change your debit account or tuition payer information, please take necessary procedures by the end of May.
• If you are a privately financed international student, please check the separate information e-mail sent on April 13. If you wish a payment by international remittance or credit card, the due day differs from the originally planned. Please refer to the e-mail carefully.

Please note that students admitted in April 2022 are not eligible for this transfer since they have already paid tuition for the spring semester at the time of enrollment procedures (the transfer will begin in the fall semester). In addition, for students who are adopted by the New Aid System for Higher Education and Enchosei (extended students), the first bank transfer date is scheduled on July 1, in principle.
Please check here for more information on tuition and fees.

Center for English Language Education Faculty Recruitment Information

An international astronomer team has discovered the most distant galaxy candidate to date, named HD1, which is about 13.5 billion light-years away. This discovery implies that bright systems like HD1 existed as early as 300 million years after the Big Bang. This galaxy candidate is one of the targets of the James Webb Space Telescope launched late last year. If observations with the James Webb Space Telescope confirm its exact distance, HD1 will be the most distant galaxy ever recorded.

To understand how and when galaxies formed in the early Universe, astronomers look for distant galaxies. Because of the finite speed of light, it takes time for the light from distant objects to reach Earth. If an object is 1 billion light-years away, it means that the light left that object 1 billion years ago and had to travel for 1 billion years to reach us. Thus studying distant galaxies lets us look back in time.

The current record holder for the most distant galaxy is GN-z11, a galaxy 13.4 billion light-years away discovered by the Hubble Space Telescope. However, this distance is about the limit of Hubble’s detection capabilities.

HD1, a candidate object for the earliest/most-distant galaxy at 13.5 billion light-years away, was discovered from more than 1,200 hours of observation data taken by the Subaru Telescope, VISTA Telescope, UK Infrared Telescope, and Spitzer Space Telescope. “It was very hard work to find HD1 out of more than 700,000 objects,” says Yuichi Harikane, an assistant professor of ICRR, the University of Tokyo, who discovered HD1. “HD1’s red color matched the expected characteristics of a galaxy 13.5 billion light-years away surprisingly well, giving me a little bit of goosebumps when I found it.”

The team conducted follow-up observations using the Atacama Large Millimeter/submillimeter Array (ALMA) to confirm HD1’s distance. Akio Inoue, a professor at Waseda University, who led the ALMA observations, says, “We found a weak signal at the frequency where an oxygen emission line was expected. The significance of the signal is 99.99%. If this signal is real, this is evidence that HD1 exists 13.5 billion light-years away, but we cannot be sure without a significance of 99.9999% or more.”

HD1 is very bright, suggesting that bright objects already existed in the Universe only 300 million years after the Big Bang. HD1 is difficult to explain with current theoretical models of galaxy formation. Observational information on HD1 is limited and its physical properties remain a mystery. It is thought to be a very active star-forming galaxy, but it might be an active black hole. Either possibility makes it a very interesting object. In recognition of its astronomical importance, HD1 was selected as a target for the cycle 1 observations by the James Webb Space Telescope, launched last year. Yuichi Harikane, who is leading these observations, says, “If the spectroscopic observation confirms its exact distance, HD1 will be the most distant galaxy ever recorded, 100 million light-years further away than GN-z11. We are looking forward to seeing the Universe with the James Webb Space Telescope.”

This research will be published in the April 8, 2022 issue of The Astrophysical Journal as Yuichi Harikane, et al. “A Search for H-Dropout Lyman Break Galaxies at z~12-16”. This work was supported by the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT), the Japan Society for the Promotion of Science (17H06130, 19J01222, 20K22358, 21K13953), and the NAOJ ALMA Scientific Research Grant (2020-16B).

Journal: The Astrophysical Journal
Title: “A Search for H-Dropout Lyman Break Galaxies at z~12-16”
Authors: Yuichi Harikane, Akio K. Inoue, Ken Mawatari, Takuya Hashimoto, Satoshi, Yamanaka, Yoshinobu Fudamoto, Hiroshi Matsuo, Yoichi Tamura, Pratika Dayal, L. Y. Aaron Yung, Anne Hutter, Fabio Pacucci, Yuma Sugahara, and Anton M. Koekemoer
DOI：10.3847/1538-4357/ac53a9
URL：https://iopscience.iop.org/article/10.3847/1538-4357/ac53a9
https://ui.adsabs.harvard.edu/abs/2021arXiv211209141H/abstract

New Study Suggests an Alternative Technique for Determining the True Activity of Catalysts

Researchers from Japan perform reliable estimation of the activity of water-splitting catalysts with an unconventional technique

Electrolysis of water into hydrogen and oxygen is a potential source of clean hydrogen fuel. However, the process requires efficient electrocatalysts. Unfortunately, conventional techniques often overestimate their efficiency. Now, researchers from Japan demonstrate an alternative technique for gauging the electrocatalytic performance accurately, opening doors to a smooth transition from lab-scale studies to large-scale hydrogen fuel generation and commercialization of new catalysts with no activity loss issues from overestimation of activity with transient voltammetry techniques.

Electrolysis of water or “water electrosplitting” has received a great deal of attention recently owing to its potential as a clean source of hydrogen, the oft-touted fuel of the future. However, two issues have long stood in the way: the large amount of energy lost, and the cost of electrocatalysts (catalysts used for electrolysis). Fortunately, several new kinds of electrocatalysts have made their appearance, which could potentially solve these issues.

The screening of new electrocatalysts is conventionally performed with techniques such as “linear sweep voltammetry” (LSV) and “cyclic voltammetry” (CV), which involve applying a constantly changing voltage to an electrode and monitoring the resulting current. As this current depends on the rate of oxidation or reduction occurring at the electrode, the measured current readings can be used to determine the effect of an electrocatalyst on the speed of the electrolysis reaction.

However, an obvious drawback of these techniques is that they cannot accurately record the “steady-state” response of the electrocatalyst as it does not experience a particular applied voltage long enough to do so. As a result, substantially high current readings are often recorded, which do not reflect the true catalytic activity, hindering the development of efficient electrocatalysts and promotion of the same to large-scale processes.

In a new study published in the Journal of The Electrochemical Society, Assistant Professor Sengeni Anantharaj from Waseda University, Japan, along with his collaborators Dr. Subrata Kundu from CSIR-Central Electrochemical Research Institute, India, and Prof. Suguru Noda from Waseda University have now found a way around this problem, demonstrating an alternate technique called “sampled current voltammetry” (SCV) as a more reliable indicator of electrocatalytic performance at a constant steady-state applied voltage.

“Screening catalysts accurately is just as important as developing new catalysts for all energy conversion reactions,” says Anantharaj, speaking of his motivation. “Our work has highlighted a way to make accurate measurements of electrocatalytic activity previously not possible with conventional transient techniques.”

Researchers from Waseda University, Japan, suggest an alternate technique for measuring steady-state electrocatalytic activity more reliably over conventional transient techniques, opening up a potential route to efficient hydrogen generation from water splitting.
Photo courtesy: Sengeni Anantharaj from Waseda University

Before applying the SCV technique, the researchers analyzed the errors resulting from LSV. To show the deviation in current values, they used a steady-state technique called “chronoamperometry” (CA), which is the most accurate method of all yet time consuming to measure current at constant voltages and compared it to the values obtained from LSV.

To determine the activity of electrocatalysts used in electrolysis, they measured the current readings of both the oxygen-producing and hydrogen-producing half-cell reactions. Using a stainless-steel (SS) electrode, precipitated Co(OH)₂ (cobalt hydroxide), and platinum foil as catalysts in a KOH (potassium hydroxide) solution, the researchers found that the current density readings from LSV and CA differed significantly, with the difference growing wider at higher applied voltages.

Using the same setup, they then applied the SCV technique and recorded the current densities at various fixed voltages obtained from the steady-state CA responses. “To validate the suitability of SCV, we recorded the CA responses of the SS electrode at various regularly increasing voltages for 130 seconds, within which the SS interface was able to reach a steady state,” elaborates Anantharaj.

From the sampled current readings, the researchers found negligible difference compared to the steady-state CA technique, demonstrating the reliability of the SCV in correctly determining electrocatalyst’s behavior at different voltages. Additionally, while the SCV is particularly useful in the search for a suitable electrocatalyst for water electrosplitting, it can be used to screen electrocatalysts accurately for any electrochemical reaction.

“By addressing the long-standing problem of catalyst performance loss when promoted from the lab to the practical processes, our work could speed up the worldwide adoption of large-scale hydrogen generation from electrolysis,” comments Anantharaj.

It certainly appears we’re now one step closer to the wide adaptation of hydrogen-powered future!

Reference

Authors: Sengeni Anantharaj^1,2, Subrata Kundu³ and Suguru Noda^1,2
Title of original paper: Worrisome Exaggeration of Activity of Electrocatalysts Destined for Steady-State Water Electrolysis by Polarization Curves from Transient Techniques
Journal: Journal of The Electrochemical Society
DOI: 10.1149/1945-7111/ac47ec
Latest Article Publication Date: 5 January 2022
Affiliations:
¹Department of Applied Chemistry, School of Advanced Science and Engineering, Waseda University
²Waseda Research Institute for Science and Engineering, Waseda University
³Electrochemical Process Engineering (ECE) Division, CSIR-Central Electrochemical Research Institute (CECRI), India

早稲田大学_教員公募_211203_Eng

Unveiling Galaxies at Cosmic Dawn That Were Hiding Behind the Dust

Scientists serendipitously discover two heavily dust-enshrouded galaxies that formed when the Universe was only 5% of its present age

While investigating the data of young, distant galaxies observed with the Atacama Large Millimeter/submillimeter Array, Dr. Yoshinobu Fudamoto from Waseda University and the National Astronomical Observatory of Japan noticed unexpected emissions coming from seemingly empty regions in space that, a global research team confirmed, came actually from two hitherto undiscovered galaxies heavily obscured by cosmic dust. This discovery suggests that numerous such galaxies might still be hidden in the early Universe, many more than researchers were expecting.

A schematic of the results of this research. ALMA revealed a hitherto undiscovered galaxy as it is buried deep in dust (artist’s impression in upper right) in a region where the Hubble Space Telescope could not see anything (left). Researchers serendipitously discovered the new hidden galaxy while observing an already well-known typical young galaxy (artist’s impression in lower right)
Credit: ALMA (ESO/NAOJ/NRAO), NASA/ESA Hubble Space Telescope

When astronomers peer deep into the night sky, they observe what the Universe looked like a long time ago. Because the speed of light is finite, studying the most distant observable galaxies allows us to glimpse billions of years into the past when the Universe was very young and galaxies had just started to form stars. Studying this “early Universe” is one of the last frontiers in astronomy and is essential for constructing accurate and consistent astrophysics models. A key goal of scientists is to identify all the galaxies in the first billion years of cosmic history and to measure the rate at which galaxies were growing by forming new stars.

Various efforts have been made over the past decades to observe distant galaxies, which are characterized by electromagnetic emissions that become strongly redshifted (shifted towards longer wavelengths) before reaching the Earth. So far, our knowledge of early galaxies has mostly relied on observations with the Hubble Space Telescope (HST) and large ground-based telescopes, which probe their ultra-violet (UV) emission. However, recently, astronomers have started to use the unique capability of the Atacama Large Millimeter/submillimeter Array (ALMA) telescope to study distant galaxies at submillimeter wavelengths. This could be particularly useful for studying dusty galaxies missed in the HST surveys due to the dust absorbing UV emission. Since ALMA observes in submillimeter wavelengths, it can detect these galaxies by observing the dust emissions instead.

In an ongoing large program called REBELS (Reionization-Era Bright Emission Line Survey), astronomers are using ALMA to observe the emissions of 40 target galaxies at cosmic dawn. Using this dataset, they have recently discovered that the regions around some of these galaxies contain more than meets the eye.

While analyzing the observed data for two REBELS galaxies, Dr. Yoshinobu Fudamoto of the Research Institute for Science and Engineering at Waseda University, Japan, and the National Astronomical Observatory of Japan (NAOJ), noticed strong emission by dust and singly ionized carbon in positions substantially offset from the initial targets. To his surprise, even highly sensitive equipment like the HST couldn’t detect any UV emission from these locations. To understand these mysterious signals, Fudamoto and his colleagues investigated matters further.

In their latest paper published in Nature, they presented a thorough analysis, revealing that these unexpected emissions came from two previously unknown galaxies located near the two original REBELS targets. These galaxies are not visible in the UV or visible wavelengths as they are almost completely obscured by cosmic dust.  One of them represents the most distant dust-obscured galaxy discovered so far.

Distant galaxies imaged with ALMA, the Hubble Space Telescope, and the European Southern Observatory’s VISTA telescope. Green and orange colors represent radiations from ionized carbon atoms and dust particles, respectively, observed with ALMA, and blue represents near-infrared radiation observed with VISTA and Hubble Space Telescopes.
REBELS-12 and REBELS-29 detected both near-infrared radiation and radiation from ionized carbon atoms and dust. On the other hand, REBELS-12-2 and REBELS-29-2 have not been detected in the near-infrared, which suggests that these galaxies are deeply buried in dust.
Credit: ALMA (ESO/NAOJ/NRAO), NASA/ESA Hubble Space Telescope, ESO, Fudamoto et al.

What is most surprising about this serendipitous finding is that the newly discovered galaxies, which formed more than 13 billion years ago, are not strange at all when compared with typical galaxies at the same epoch. “These new galaxies were missed not because they are extremely rare, but only because they are completely dust-obscured,” explains Fudamoto. However, it is uncommon to find such “dusty” galaxies in the early period of the Universe (less than 1 billion years after the Big Bang), suggesting that the current census of early galaxy formation is most likely incomplete, and would call for deeper, blind surveys. “It is possible that we have been missing up to one out of every five galaxies in the early Universe so far,” Fudamoto adds.

The researchers expect that the unprecedented capability of the James Webb Space Telescope (JWST) and its strong synergy with ALMA would lead to significant advances in this field in the coming years. “Completing our census of early galaxies with the currently missing dust-obscured galaxies, like the ones we found this time, will be one of the main objectives of JWST and ALMA surveys in the near future,” states Pascal Oesch from University of Geneva.

Overall, this study constitutes an important step in uncovering when the very first galaxies started to form in the early Universe, which in turn shall help us understand where we are standing today.

Reference

Authors: Y. Fudamoto^1,2,3, P. A. Oesch^1,4, S. Schouws⁵, M. Stefanon⁵, R. Smit⁶, R. J. Bouwens⁵, R. A. A. Bowler⁷, R. Endsley⁸, V. Gonzalez^9,10, H. Inami¹¹, I. Labbe¹², D. Stark⁸, M. Aravena¹³, L. Barrufet¹, E. da Cunha^14,15, P. Dayal¹⁶, A. Ferrara¹⁷, L. Graziani^{18,20, 27}, J. Hodge⁵, A. Hutter¹⁶, Y. Li^21,22, I. De Looze^23,24, T. Nanayakkara¹², A. Pallottini¹⁷, D. Riechers²⁵, R. Schneider^18,19,26,27, G. Ucci1⁶, P. van der Werf⁵, C. White⁸
Title of original paper: Normal, Dust-Obscured Galaxies in the Epoch of Reionization
Journal: Nature
DOI: 10.1038/s41586-021-03846-z
Affiliations:
¹Department of Astronomy, University of Geneva
²Research Institute for Science and Engineering, Waseda University; ³National Astronomical Observatory of Japan
⁴Cosmic Dawn Center (DAWN), Niels Bohr Institute, University of Copenhagen
⁵Leiden Observatory, Leiden University
⁶Astrophysics Research Institute, Liverpool John Moores University
⁷Sub-department of Astrophysics, The Denys Wilkinson Building, University of Oxford
⁸Steward Observatory, University of Arizona
⁹Departmento de Astronomia, Universidad de Chile
¹⁰Centro de Astrofisica y Tecnologias Afines (CATA)
¹¹Hiroshima Astrophysical Science Center, Hiroshima University
¹²Centre for Astrophysics & Supercomputing, Swinburne University of Technology
¹³Nucleo de Astronomia, Facultad de Ingenieria y Ciencias, Universidad Diego Portales
¹⁴International Centre for Radio Astronomy Research, University of Western Australia
¹⁵ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D)
¹⁶Kapteyn Astronomical Institute, University of Groningen
¹⁷Scuola Normale Superiore
¹⁸Dipartimento di Fisica, Sapienza, Universita di Roma
¹⁹INAF/Osservatorio Astronomico di Roma
²⁰INAF/Osservatorio Astrofisico di Arcetri
²¹Department of Astronomy & Astrophysics, The Pennsylvania State University
²²Institute for Gravitation and the Cosmos, The Pennsylvania State University
²³Sterrenkundig Observatorium, Ghent University
²⁴Dept. of Physics & Astronomy, University College London
²⁵Cornell University
²⁶Sapienza School for Advanced Studies
²⁷INFN, Roma, Italy

Nissan and Waseda University in Japan testing jointly developed recycling process for electrified vehicle motors

New process efficiently recovers high-purity rare-earth compounds from motor magnets, practical application targeted for mid-2020s toward carbon neutral goal

YOKOHAMA, Japan　–　Nissan Motor Co., Ltd. and Waseda University today announced the start of testing in Japan of a jointly developed recycling process that efficiently recovers high-purity rare-earth compounds from electrified vehicle motor magnets. The testing is aimed at enabling practical application of the new process by the mid-2020s.

The automotive industry is promoting vehicle electrification to tackle climate change and to realize a carbon-neutral society. Most motors in electrified vehicles use neodymium magnets, which contain scarce rare-earth metals such as neodymium and dysprosium. Reducing the use of scarce rare earths is important not only because of the environmental impact of mining and refining, but also because the shifting balance of supply and demand leads to price fluctuations for both manufacturers and consumers.

=>Press Release

To use limited and valuable resources more effectively, since 2010 Nissan has been working from the design stage to reduce the amount¹ of heavy rare-earth elements (REEs) in motor magnets. In addition, Nissan is recycling REEs by removing magnets from motors that do not meet production standards and returning them to suppliers. Currently, multiple steps are involved, including manual disassembly and removal. Therefore, developing a simpler and more economical process is important to achieve increased recycling in the future.

Since 2017, Nissan has been collaborating with Waseda University, which has a strong track record of researching non-ferrous metal recycling and smelting. In March 2020 the collaboration successfully developed a pyrometallurgy process that does not require motor disassembly.

Process overview:

1. A carburizing material and pig iron are added to the motor, which is then heated to at least 1,400 C and begins to melt.
2. Iron oxide is added to oxidize the REEs in the molten mixture.
3. A small amount of borate-based flux, which is capable of dissolving rare-earth oxides even at low temperatures and highly efficiently recovering REEs, is added to the molten mixture.
4. The molten mixture separates into two liquid layers, with the molten oxide layer (slag) that contains the REEs floating to the top, and the higher density iron-carbon (Fe-C) alloy layer sinking to the bottom.
5. The REEs are then recovered from the slag.

Testing has shown that this process can recover 98% of the motors’ REEs. This method also reduces the recovery process and work time by approximately 50% compared to the current method because there is no need to demagnetize the magnets, nor remove and disassemble them.

Going forward, Waseda and Nissan will continue their large-scale facility testing with the aim of developing practical application, and Nissan will collect motors from electrified vehicles that are being recycled and continue to develop its recycling system.

Nissan will continue to contribute to the building of a cleaner, safer and more inclusive society as part of its efforts to develop a sustainable society. Through its Nissan Green Program 2022, Nissan is addressing four priority issues: climate change, resource dependency, air quality and water scarcity. Nissan will continue to aim for carbon neutrality and zero new material resource use, and will simultaneously promote the use of electrified vehicles and the recycling and reduced use of REEs.

¹ The Nissan Note e-POWER produced in FY2020 uses magnets with 85% fewer heavy REEs than the Nissan LEAF produced in FY2010.

World’s First Transparent Fiber–Millimeter-wave–Fiber System in 100-GHz Band

Using Low-Loss Optical Modulator and Direct Photonic Down-Conversion

【Highlights】

• A 100-GHz band fiber–millimeter-wave–fiber transparent system was constructed based on direct millimeter-wave-to-optical conversion using a low-loss optical modulator with direct photonic down-conversion.
• 70-Gbit/s high-capacity transmission over the transparent fiber–millimeter-wave–fiber system at 101 GHz was demonstrated using 64-QAM OFDM.
• This demonstration opens the door for transparent fiber–millimeter-wave systems in the field of high-capacity, low-latency, and low-power consumption communications in the 5G and beyond era.

【Abstract】

The National Institute of Information and Communications Technology (NICT, President: TOKUDA Hideyuki, Ph.D.), Sumitomo Osaka Cement Co., Ltd. (President: MOROHASHI Hirotsune), and Waseda University (President: TANAKA Aiji) jointly developed the first transparent fiber–millimeter-wave–fiber system in the 100-GHz band using a low-loss broadband optical modulator with direct photonic down-conversion. The developed broadband modulator and photonic down-conversion technology were utilized to successfully demonstrate a high-speed transmission of more than 70 Gbit/s over a wired and wireless converged system consisting of two optical fiber links and a 20 m radio link at 101 GHz.

The utilization of a low-loss broadband optical modulator for the direct conversion of a millimeter-wave signal to an optical signal^*1 significantly simplified the millimeter-wave radio receiver because it included only a radio front end and an optical modulator. In addition, by adopting direct photonic down-conversion technology*² for simultaneous detection and down-conversion of the signal to the microwave band, the fiber-radio receiver and the subsequent digital signal processing could be considerably simplified, thus rendering the proposed system a promising solution for high-capacity, low-latency, and low-power consumption fiber–wireless transmission in 5G and beyond networks.

The results of this demonstration were published as a post-deadline paper presentation at the 2021 International Conference on Optical Fiber Communications (OFC 2021).

【Background】

Fiber–wireless systems in high-frequency bands are a promising technology for inter-building connections, disaster recovery, and mobile transport networks, especially in 5G and beyond era. To date, most systems rely on the use of electronics-based receivers for radio-to-optical conversion, which generally feature less bandwidth and complicated antenna sites. Achieving fully transparent radio–optical conversion using photonic solutions is promising for increasing the transmission capacity and simplifying the antenna sites. However, the frequency of radio links in the previous systems that utilized the photonic conversion method was limited to below 90 GHz owing to the limited bandwidth of optical modulators. Recently, a plasmonic modulator was employed to realize a transparent bridge system in high-frequency bands. Generally, plasmonic modulators exhibit high insertion loss, which requires the use of optical amplifiers. However, this increases the optical noise, system cost, and the antenna site complexity.

On the other hand, most of the previous systems utilized coherent detection by using free-running lasers for signal detection at the fiber-radio receiver, which significantly increased the system complexity, frequency offset, and phase noise of the detected signal, thus requiring complicated digital signal processing algorithms for signal recovery. Therefore, employing a direct photonic down-conversion technology to simultaneously detect and down-convert the signal to the microwave band using a coherent two-tone optical signal generation*³ is promising for simplifying the system and reducing the cost and power consumption.

【Achievements】

In this work, we demonstrated the first transparent fiber–millimeter-wave–fiber system in the 100-GHz band (see Fig. 1) using two key element technologies: (i) a low-loss broadband optical modulator, and (ii) direct photonic down-conversion. For direct conversion of a millimeter-wave signal to an optical signal, we fabricated and employed a broadband modulator*⁴ for operation up to 110 GHz. This was achieved by performing Ti diffusion on the x-cut thin-film lithium niobate in the low dielectric constant layer. In addition, we employed a photonic down-conversion method based on a coherent two-tone optical signal generation technology to simultaneously detect and down-convert the signal to the microwave band. This significantly simplified the system and reduced the frequency offset and phase noise, as compared to systems utilizing coherent detection. Using the technologies developed in this study, we successfully transmitted 64-quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) signal*⁵ with a line rate of 71.4 Gbit/s over a system consisting of two fiber links and a 20 m radio link at 101 GHz.

The system consists of the following key element technologies:

• A broadband optical modulator with a low half-wave voltage and low loss in the high-frequency band for direct conversion of millimeter-wave signals to optical signals.
• Direct photonic detection and down-conversion of signals to the microwave band utilizing coherent two-tone optical signal generation based on optical modulation technology.
• High-spectral efficiency 64-QAM OFDM signal transmission.

The optical carrier for data modulation at the antenna site was remotely generated and distributed from the fiber-radio receiver, which significantly simplified the antenna site and eased its operation and management. In addition, owing to the use of direct photonic down-conversion and detection technology at the fiber-radio receiver, the frequency offset and phase noise of the detected signal could be largely suppressed. This considerably reduced the receiver complexity and the subsequent digital signal processing. The proposed system is promising for high-speed, low-latency, and low-power-consumption communication links in 5G and beyond networks.

【Future Prospects】

In the future, we will further study the millimeter-wave-to-optical conversion device and fiber wireless technology that were developed in this study to further increase the radio frequency and transmission capacity. In addition, we will promote international standardization activities and social implementation activities related to fiber wireless communication systems.

The paper containing the results of this demonstration was published at the 2021 International Conference on Optical Fiber Communication (OFC 2021, June 6 (Sun.) to June 11 (Fri.)), one of the largest international conferences in the field of optical fiber communications. It was highly evaluated and was presented in the Post Deadline session, which is known to release the latest important research achievements, on June 11 (Fri) 2021 local time.

【References】

International Conference: Optical Fiber Communications (OFC 2021) June 2021,
paper F3C.4 (Post Deadline Paper)

Title: Transparent Fiber–Radio–Fiber Bridge at 101 GHz using Optical Modulator and Direct Photonic Down-Conversion

Authors: Pham Tien Dat, Yuya Yamaguchi, Keizo Inagaki, Masayuki Motoya, Satoshi Oikawa, Junichiro Ichikawa, Atsushi Kanno, Naokatsu Yamamoto, Tetsuya Kawanishi

【Glossary】

*1 Direct millimeter-wave to optical conversion

It is a technology that converts a wireless signal in the millimeter-wave band to an optical signal without down-conversion of frequency. On the contrary, in the electronics-based conversion method, the millimeter-wave signal needs to be down-converted to a lower frequency signal in the microwave band before its conversion into an optical signal. The direct conversion of a millimeter-wave signal to an optical signal can be realized using a broadband optical modulator or plasmonic modulator. This significantly simplifies the antenna site.

*2 Direct photonic down-conversion

It is a technology used for detecting and down-converting a millimeter-wave signal to a microwave band signal using optical signals from the same light source. In this technology, a two-tone optical signal consisting of the two optical sidebands with a frequency separation that is approximately equal to the frequency of the millimeter-wave signal is generated from a single light source. One of the sidebands is modulated by the millimeter-wave signal, and an optical double-sideband carrier-suppressed signal is generated. One of the modulated sidebands is selected using optical filtering. Finally, the modulated and unmodulated sidebands are combined and input to a low-speed photodetector to be converted to an electrical signal in the microwave band.

*3 Coherent two-tone optical signal generation

This technology generates two coherent optical signals from the same light source using optical modulation technology. In particular, an optical signal consisting of odd or even order harmonic sidebands is generated by applying a clock signal to an optical modulator and controlling the bias voltage.

*4 Broadband optical modulator using thin-film lithium niobate

A broadband Mach–Zehnder modulator (MZM) can be fabricated using a thin substrate. In this work, we fabricated a broadband MZM, in which Mach–Zehnder interferometer waveguides were fabricated by Ti diffusion on the x-cut thin-film lithium niobate in the low dielectric constant layer. This was done to achieve ripple-free operation and maximized electro-optic responsivity up to 110 GHz. By thinning the substrate, as shown in Figs. 3(a) and (b), the frequency ripple due to mode coupling between the coplanar guided mode and substrate mode can be suppressed. The electrodes were also optimized to reduce electrical propagation loss to attain high sensitivity. The optical insertion loss, including fiber pigtails, is approximately 4.6 dB at 1550 nm. The half-wave voltage at 100 GHz is approximately 6.7 V, demonstrating a sufficiently low value for high-sensitivity conversion of a millimeter-wave signal to an optical signal at the antenna site.

*5 OFDM 64-QAM signal

OFDM is a digital multi-carrier modulation scheme that uses multiple subcarriers within the same single channel. Instead of transmitting a high-rate data stream using a single subcarrier, OFDM uses a large number of closely spaced orthogonal subcarriers that are transmitted in parallel. In this work, subcarriers are modulated with 64 QAM symbols, each of which consists of six input data bits.

Appendix

1. Configuration of the proposed system

Fig. 4 shows a schematic diagram of the proposed system, which includes six main parts: a fiber-radio transmitter, a millimeter-wave radio transmitter, a millimeter-wave radio receiver, a fiber-radio receiver, millimeter-wave-to-optical conversion, and signal down-conversion and detection.

(1) Fiber-radio transmitter

This block generates and modulates signals. A two-tone optical signal with a frequency separation of 91 GHz was generated using optical modulation technology. The two optical sidebands were separated, and one of them was modulated by a 10 GHz radio signal. The bias voltage to the modulator was controlled to generate only the upper modulation sideband. The modulated and unmodulated sidebands were combined to form a 101-GHz radio-over-fiber (RoF) signal.

(2) Millimeter-wave radio transmitter

After transmitting over a 20-km single-mode fiber, the RoF signal was up-converted to a 101-GHz millimeter-wave radio signal using a high-speed photodetector. The generated radio signal was emitted into free space using a millimeter-wave antenna.

(3) Millimeter-wave radio receiver

The millimeter-wave signal was received by another millimeter-wave antenna, amplified, and converted to an optical signal using the developed high-speed optical modulator.

(4) Fiber-radio receiver

Another two-tone optical signal with a frequency separation of 84 GHz between the two sidebands was generated. One of the sidebands was transmitted to a millimeter-wave radio receiver for data modulation.

(5) Millimeter-wave-to-optical conversion

The optical carrier signal generated at (4) was modulated by the 101 GHz millimeter-wave signal obtained from (3), and the bias voltage to the modulator was controlled to generate a double-sideband suppressed carrier signal. The modulated signal was transmitted to the fiber-radio receiver using a 10-km single-mode fiber link.

(6) Signal down-conversion and detection

One of the modulated sidebands from (5) was selected using optical filtering and combined with the unmodulated sideband of the generated two-tone optical signal from (4) to form an RoF signal with a center frequency of 17 GHz (= 101–84GHz). The signal was converted to a microwave band signal using a low-speed photodetector.

2. Experimental results

In the demonstration, an OFDM signal at 10 GHz was generated and transmitted over the system. The performance measured in terms of the error vector magnitude (EVM) for the 64-QAM OFDM signal is plotted in Fig. 5(a) for different signal bandwidths. Considering a forward error correction overhead of 20 %, which requires an EVM value of 11.2 %, a satisfactory transmission performance was experimentally confirmed for the OFDM signal with a bandwidth of 14 GHz or smaller. This confirmed that a line rate of 71.4 Gbit/s could be attained when transmitting a 14-GHz bandwidth signal that consisted of 4096 subcarriers, of which, 15 % were inactive at the band edges. The superior performance of the system using a 20 m radio link could be attributed to the better power adjustment of the fiber-radio transmitter. An example of the received signal constellation is shown in Fig. 5 (b).

Waseda-MMech2021-TenureTrackEN

Waseda-MMech2021-TenureEN