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Author

Takuro Fujii

Other affiliations: Keio University
Bio: Takuro Fujii is an academic researcher from Nippon Telegraph and Telephone. The author has contributed to research in topics: Laser & Photonic crystal. The author has an hindex of 15, co-authored 141 publications receiving 1134 citations. Previous affiliations of Takuro Fujii include Keio University.


Papers
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Journal ArticleDOI
TL;DR: In this paper, an embedded active region structure in which the wavelength-scale active region is buried with an InP PhC slab was proposed to improve the thermal resistance of the device.
Abstract: Lasers with ultra-low operating energy are desired for use in chip-to-chip and on-chip optical interconnects. If we are to reduce the operating energy, we must reduce the active volume. Therefore, a photonic crystal (PhC) laser with a wavelength-scale cavity has attracted a lot of attention because a PhC provides a large Q-factor with a small volume. To improve this device's performance, we employ an embedded active region structure in which the wavelength-scale active region is buried with an InP PhC slab. This structure enables us to achieve effective confinement of both carriers and photons, and to improve the thermal resistance of the device. Thus, we have obtained a large external differential quantum efficiency of 55% and an output power of ?10?dBm by optical pumping. For electrical pumping, we use a lateral p?i?n structure that employs Zn diffusion and Si ion implantation for p-type and n-type doping, respectively. We have achieved room-temperature continuous-wave operation with a threshold current of 7.8??A and a maximum 3?dB bandwidth of 16.2?GHz. The results of an experimental bit error rate measurement with a 10?Gbit?s?1 NRZ signal reveal the minimum operating energy for transferring a single bit of 5.5?fJ. These results show the potential of this laser to be used for very short reach interconnects. We also describe the optimal design of cavity quality (Q) factor in terms of achieving a large output power with a low operating energy using a calculation based on rate equations. When we assume an internal absorption loss of 20?cm?1, the optimized coupling Q-factor is 2000.

157 citations

Journal ArticleDOI
TL;DR: In this paper, the authors demonstrate an MZ modulator with a 250µm-long InGaAsP/Si metal-oxide-semiconductor (MOS) capacitor phase-shifter and obtain a VπL of 0.09
Abstract: Hybrid silicon optical modulator brings efficiency benefits. Demand for more transmission capacity in data centres is increasing due to the continuous growth of Internet traffic. The introduction of external modulators into datacom networks is essential with advanced modulation formats. However, the large footprint of silicon photonics Mach–Zehnder (MZ) modulators will limit further increases in transmission capacity1,2,3,4. To overcome this, we introduce III–V compound semiconductors because the large electron-induced refractive-index change, high electron mobility and low carrier-plasma absorption are beneficial for overcoming the trade-offs among the voltage–length product (VπL), operation speed and insertion loss of Si MZ modulators. Here, we demonstrate an MZ modulator with a 250-µm-long InGaAsP/Si metal-oxide–semiconductor (MOS) capacitor phase-shifter and obtain a VπL of 0.09 Vcm in accumulation mode, an insertion loss of ∼1.0 dB, a cutoff frequency of ∼2.2 GHz in depletion mode and a 32-Gbit s–1 modulation with signal pre-emphasis. These results are promising for fabricating high-capacity large-scale photonic integrated circuits with low power consumption.

135 citations

Journal ArticleDOI
TL;DR: The fabrication method is a promising way to fabricate high-efficiency lasers at a low cost and the use of a lateral current injection structure is important for forming a p-i-n junction using bonded thin film.
Abstract: We describe the growth of InP layer using an ultrathin III-V active layer that is directly bonded to SiO₂/Si substrate to fabricate a buried heterostructure (BH) laser. Using a 250-nm-thick bonded active layer, we succeeded in fabricating a BH distributed feedback (DFB) laser on SiO₂/Si substrate. The use of a lateral current injection structure is important for forming a p-i-n junction using bonded thin film. The fabricated DFB laser is directly modulated by a 25.8-Gbit/s NRZ signal at 50°C. These results indicate that our fabrication method is a promising way to fabricate high-efficiency lasers at a low cost.

108 citations

Journal ArticleDOI
TL;DR: In this paper, a high-bandwidth InP-based Mach-Zehnder modulator and in-phase/quadrature (IQ) modulators were proposed by combining an n-i-p-n heterostructure and a capacitively loaded traveling wave electrode.
Abstract: We report novel high-bandwidth InP-based Mach–Zehnder modulator and in-phase/quadrature (IQ) modulators that we realized by combining an n-i-p-n heterostructure and a capacitively loaded traveling wave electrode. The extremely low electrical and optical loss structure enhances the 3-dB electro-optic bandwidth of over 67 GHz without degrading other properties such as driving voltage and optical loss. The modulator also exhibits a static extinction ratio of over 24 dB with a V π of less than 1.5 V for the entire C-band. Furthermore, we demonstrate the first 120-Gbaud rate IQ modulation without optical pre equalization, and 100-Gb/s non-return-to-zero on-off keying modulation with a dynamic extinction ratio of over 10 dB.

107 citations

Journal ArticleDOI
TL;DR: In this paper, a membrane distributed reflector laser on a low-refractive-index and high-thermal-conductivity silicon carbide substrate was proposed to achieve a 42 GHz relaxation oscillation frequency.
Abstract: Increasing the modulation speed of semiconductor lasers has attracted much attention from the viewpoint of both physics and the applications of lasers. Here we propose a membrane distributed reflector laser on a low-refractive-index and high-thermal-conductivity silicon carbide substrate that overcomes the modulation bandwidth limit. The laser features a high modulation efficiency because of its large optical confinement in the active region and small differential gain reduction at a high injection current density. We achieve a 42 GHz relaxation oscillation frequency by using a laser with a 50-μm-long active region. The cavity, designed to have a short photon lifetime, suppresses the damping effect while keeping the threshold carrier density low, resulting in a 60 GHz intrinsic 3 dB bandwidth (f3dB). By employing the photon–photon resonance at 95 GHz due to optical feedback from an integrated output waveguide, we achieve an f3dB of 108 GHz and demonstrate 256 Gbit s−1 four-level pulse-amplitude modulations with a 475 fJ bit−1 energy cost of the direct-current electrical input. Directly modulated membrane distributed reflector lasers are fabricated on a silicon carbide platform. The 3 dB bandwidth, four-level pulse-amplitude modulation speed and operating energy for transmitting one bit are 108 GHz, 256 Gbit s−1 and 475 fJ, respectively.

99 citations


Cited by
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Journal ArticleDOI
24 Sep 2018-Nature
TL;DR: Monolithically integrated lithium niobate electro-optic modulators that feature a CMOS-compatible drive voltage, support data rates up to 210 gigabits per second and show an on-chip optical loss of less than 0.5 decibels are demonstrated.
Abstract: Electro-optic modulators translate high-speed electronic signals into the optical domain and are critical components in modern telecommunication networks1,2 and microwave-photonic systems3,4. They are also expected to be building blocks for emerging applications such as quantum photonics5,6 and non-reciprocal optics7,8. All of these applications require chip-scale electro-optic modulators that operate at voltages compatible with complementary metal–oxide–semiconductor (CMOS) technology, have ultra-high electro-optic bandwidths and feature very low optical losses. Integrated modulator platforms based on materials such as silicon, indium phosphide or polymers have not yet been able to meet these requirements simultaneously because of the intrinsic limitations of the materials used. On the other hand, lithium niobate electro-optic modulators, the workhorse of the optoelectronic industry for decades9, have been challenging to integrate on-chip because of difficulties in microstructuring lithium niobate. The current generation of lithium niobate modulators are bulky, expensive, limited in bandwidth and require high drive voltages, and thus are unable to reach the full potential of the material. Here we overcome these limitations and demonstrate monolithically integrated lithium niobate electro-optic modulators that feature a CMOS-compatible drive voltage, support data rates up to 210 gigabits per second and show an on-chip optical loss of less than 0.5 decibels. We achieve this by engineering the microwave and photonic circuits to achieve high electro-optical efficiencies, ultra-low optical losses and group-velocity matching simultaneously. Our scalable modulator devices could provide cost-effective, low-power and ultra-high-speed solutions for next-generation optical communication networks and microwave photonic systems. Furthermore, our approach could lead to large-scale ultra-low-loss photonic circuits that are reconfigurable on a picosecond timescale, enabling a wide range of quantum and classical applications5,10,11 including feed-forward photonic quantum computation. Chip-scale lithium niobate electro-optic modulators that rapidly convert electrical to optical signals and use CMOS-compatible voltages could prove useful in optical communication networks, microwave photonic systems and photonic computation.

1,358 citations

Journal ArticleDOI
TL;DR: The maturity of high-volume semiconductor processing has finally enabled the complete integration of light sources, modulators and detectors in a single microwave photonic processor chip and has ushered the creation of a complex signal processor with multifunctionality and reconfiguration similar to electronic devices.
Abstract: Recent advances in photonic integration have propelled microwave photonic technologies to new heights. The ability to interface hybrid material platforms to enhance light–matter interactions has led to the development of ultra-small and high-bandwidth electro-optic modulators, low-noise frequency synthesizers and chip signal processors with orders-of-magnitude enhanced spectral resolution. On the other hand, the maturity of high-volume semiconductor processing has finally enabled the complete integration of light sources, modulators and detectors in a single microwave photonic processor chip and has ushered the creation of a complex signal processor with multifunctionality and reconfigurability similar to electronic devices. Here, we review these recent advances and discuss the impact of these new frontiers for short- and long-term applications in communications and information processing. We also take a look at the future perspectives at the intersection of integrated microwave photonics and other fields including quantum and neuromorphic photonics. This Review discusses recent advances of microwave photonic technologies and their applications in communications and information processing, as well as their potential implementations in quantum and neuromorphic photonics.

532 citations

Journal ArticleDOI
TL;DR: This work shows that the next major interconnect dissipations are in the electronic circuits for receiver amplifiers, timing recovery, and multiplexing, and it can address these through the integration of photodetectors to reduce or eliminate receiver circuit energies, free-space optics to eliminate the need for timing andmultiplexing circuits, and using optics generally to save power by running large synchronous systems.
Abstract: Optics offers unique opportunities for reducing energy in information processing and communications while simultaneously resolving the problem of interconnect bandwidth density inside machines. Such energy dissipation overall is now at environmentally significant levels; the source of that dissipation is progressively shifting from logic operations to interconnect energies. Without the prospect of substantial reduction in energy per bit communicated, we cannot continue the exponential growth of our use of information. The physics of optics and optoelectronics fundamentally addresses both interconnect energy and bandwidth density, and optics may be the only scalable solution to such problems. Here we summarize the corresponding background, status, opportunities, and research directions for optoelectronic technology and novel optics, including subfemtojoule devices in waveguide and novel two-dimensional (2-D) array optical systems. We compare different approaches to low-energy optoelectronic output devices and their scaling, including lasers, modulators and LEDs, optical confinement approaches (such as resonators) to enhance effects, and the benefits of different material choices, including 2-D materials and other quantum-confined structures. With such optoelectronic energy reductions, and the elimination of line charging dissipation by the use optical connections, the next major interconnect dissipations are in the electronic circuits for receiver amplifiers, timing recovery, and multiplexing. We show we can address these through the integration of photodetectors to reduce or eliminate receiver circuit energies, free-space optics to eliminate the need for timing and multiplexing circuits (while also solving bandwidth density problems), and using optics generally to save power by running large synchronous systems. One target concept is interconnects from ∼1 cm to ∼10 m that have the same energy (∼10 fJ/bit) and simplicity as local electrical wires on chip.

485 citations

Journal ArticleDOI
TL;DR: In this paper, the authors review recent advances in integrated photonic neuromorphic systems, discuss current and future challenges, and outline the advances in science and technology needed to meet those challenges.
Abstract: Research in photonic computing has flourished due to the proliferation of optoelectronic components on photonic integration platforms. Photonic integrated circuits have enabled ultrafast artificial neural networks, providing a framework for a new class of information processing machines. Algorithms running on such hardware have the potential to address the growing demand for machine learning and artificial intelligence in areas such as medical diagnosis, telecommunications, and high-performance and scientific computing. In parallel, the development of neuromorphic electronics has highlighted challenges in that domain, particularly related to processor latency. Neuromorphic photonics offers sub-nanosecond latencies, providing a complementary opportunity to extend the domain of artificial intelligence. Here, we review recent advances in integrated photonic neuromorphic systems, discuss current and future challenges, and outline the advances in science and technology needed to meet those challenges. Photonics offers an attractive platform for implementing neuromorphic computing due to its low latency, multiplexing capabilities and integrated on-chip technology.

480 citations

Journal ArticleDOI
TL;DR: Recent advances in integrated photonic neuromorphic neuromorphic systems are reviewed, current and future challenges are discussed, and the advances in science and technology needed to meet those challenges are outlined.
Abstract: Research in photonic computing has flourished due to the proliferation of optoelectronic components on photonic integration platforms. Photonic integrated circuits have enabled ultrafast artificial neural networks, providing a framework for a new class of information processing machines. Algorithms running on such hardware have the potential to address the growing demand for machine learning and artificial intelligence, in areas such as medical diagnosis, telecommunications, and high-performance and scientific computing. In parallel, the development of neuromorphic electronics has highlighted challenges in that domain, in particular, related to processor latency. Neuromorphic photonics offers sub-nanosecond latencies, providing a complementary opportunity to extend the domain of artificial intelligence. Here, we review recent advances in integrated photonic neuromorphic systems, discuss current and future challenges, and outline the advances in science and technology needed to meet those challenges.

454 citations