Yasuhiko Nakanishi

Bio: Yasuhiko Nakanishi is an academic researcher from Nippon Telegraph and Telephone. The author has contributed to research in topics: Optical amplifier & Avalanche photodiode. The author has an hindex of 9, co-authored 51 publications receiving 312 citations.

Transmission of 214-Gbit/s 4-PAM signal using an ultra-broadband lumped-electrode EADFB laser module

Abstract: Using a lumped-electrode EADFB laser with a modulation bandwidth of ∼59 GHz, we demonstrated single-wavelength single-polarization direct-detection 4-PAM transmission with the record net data rate of 200 Gbit/s.

214-Gb/s 4-PAM Operation of Flip-Chip Interconnection EADFB Laser Module

Abstract: We fabricated a Hi-FIT LE-EADFB laser module. Hi-FIT, which is a wire-free interconnection technique, provides a higher modulation bandwidth and a flatter frequency response than a conventional wire interconnection technique. The fabricated module has a 3-dB bandwidth of about 59 GHz and a sufficiently flat frequency response of less than 45 GHz. Using this module, we demonstrated a single-wavelength single-polarization direct-detection 4-PAM transmission with a record net data rate of 200 Gb/s. And with 4-PAM operation at 214 Gb/s, we obtained a BER of less than 3.8 × 10–3, which is an error-free condition using a 7%-OH HD-FEC code, even after a 10-km SMF transmission.

Silica-based, compact and variable-optical-attenuator integrated coherent receiver with stable optoelectronic coupling system.

Abstract: We demonstrate a compact and variable-optical-attenuator (VOA) integrated coherent receiver with a silica-based planar lightwave circuit (PLC). To realize the compact receiver, we integrate a VOA in a single PLC chip with polarization beam splitters and optical 90-degree hybrids, and employ a stable optoelectronic coupling system consisting of micro lens arrays and photodiode (PD) subcarriers with high-speed right-angled signal lines. We integrate a VOA and a coherent receiver in a 27x40x6 mm package, and successfully demodulate a 128-Gbit/s polarization division multiplexed (PDM) quadrature phase shift keying (QPSK) signal with a VOA-assisted wide dynamic range of more than 30 dB.

High Power and High Speed SOA Assisted Extended Reach EADFB Laser (AXEL) for 53-Gbaud PAM4 Fiber-Amplifier-Less 60-km Optical Link

Abstract: In our article to extend a 100 Gbit/s/λ fiber-amplifier-less optical link with a simple and cost-effective transmission system, we have fabricated an O-band wavelength range (1260–1360 nm) SOA assisted extended reach EADFB laser (AXEL) with a 300-μm-long DFB laser and 85-μm-long SOA. The length of the integrated electroabsorption modulator was set to 125 μm to obtain a section with low parasitic capacitance for high speed operation. The fabricated AXEL transmitter simultaneously achieved both the sufficient modulated light output of 8.9 dBm and an excellent quality eye diagram for a 53-Gbaud PAM4 signal with the TDECQ value of 1.52 dB. We investigated the effect of the integrated SOA on the AXEL's performance and found no significant degradation of the quality of the eye diagram or the bit error rate (BER) characteristics of the 53-Gbaud PAM4 back-to-back configuration, even when the AXEL achieved high modulated light output power. We were also able to extend the 53-Gbaud PAM4 fiber-amplifier-less optical link to 60 km by using the AXEL transmitter and APD receiver, and achieved error-free transmission assuming KP4-forward error collection (FEC). These results demonstrate that the enhanced modulated light output using the AXEL can successfully extend the 53-Gbaud PAM4 reach with a simple transmission setup.

High-speed III-V based avalanche photodiodes for optical communications—the forefront and expanding applications

Abstract: Avalanche photodiodes (APDs), i.e., semiconductor devices, which convert and amplify optical signals into electrical signals, are used for optical communications and for imaging and medical applications. The major requirements for APDs in optical communications are high-speed operation for high data rates and high-sensitivity operation for extending the transmission reach. This paper overviews the achievements of high-speed APDs for 100-Gbit/s optical communications, focusing on III-V material systems, which are advantageous in terms of band engineering. The outlook for APDs in future optical communications is also described.

Digital Signal Processing for Short-Reach Optical Communications: A Review of Current Technologies and Future Trends

Abstract: Driven primarily by cloud service and data-center applications, short-reach optical communication has become a key market segment and growing research area in recent years. Short-reach systems are characterized by direct detection-based receiver configurations and other low-cost and small form factor components that induce transmission impairments unforeseen in their coherent counterparts. Innovative signaling and digital signal processing (DSP) play a pivotal role in enabling these components to realize their ultimate potentials and meet data rate requirements in cost-effective manners. This paper presents an overview of recent DSP developments for short-reach communications systems and discusses future trends.

Monolithic Silicon Photonic Integrated Circuits for Compact 100 $^{+}$ Gb/s Coherent Optical Receivers and Transmitters

Abstract: We present silicon photonic integrated circuits (PICs) based coherent optical transmitters and receivers for high-speed long-distance fiber optical transmission. High-degree photonic integration is achieved by monolithically integrating silicon electro-optic modulators, germanium photo detectors, silicon nitride-assisted on-chip polarization rotators, thermal phase shifters, and various passive silicon optical devices on a single wafer platform. We demonstrate the use of these PICs for modulating and detecting 112-Gb/s polarization-division-multiplexed quadrature phase-shift keying (PDM-QPSK) and 224-Gb/s PDM 16-ary quadrature amplitude modulation (PDM-16-QAM) signals. Transmission and coherent detection of a 112-Gb/s PDM-QPSK signal over 2560-km standard single-mode fiber is also demonstrated. The high-degree photonic integration for silicon PICs promises small-form-factor and low-power-consumption transceivers for future coherent systems that demand high cost efficiency and energy efficiency.

Volterra and Wiener Equalizers for Short-Reach 100G PAM-4 Applications

Abstract: Unlike ultralong coherent optical systems that seriously suffer from fiber nonlinearities, short-reach noncoherent systems such as data center interconnections, which utilize small, cheap, and low-bandwidth components, are sensitive to nonlinearities that are mainly produced by devices responsible for electrical signal amplification, modulation, and demodulation. One of the most promising schemes for these applications is the four-level pulse amplitude modulation format combined with intensity modulation and direct detection; however, it can be significantly degraded by linear and nonlinear intersymbol interference. Linear and nonlinear signal degradation can efficiently be handled by different types of equalizers. In many cases, the straightforward linear equalizer cannot lower the error rate at the acceptable level. Therefore, much stronger equalizers based on nonlinear models such as the Volterra series are proposed. Volterra filter that can also be orthogonalized by the Wiener model is well described in the existing literature, and, in this paper, we investigate the most critical points related to high-speed Volterra filter design and implementation. Several experiments are carried out in order to indicate filter requirements/complexity, acquisition, and stability. We also provide a simple guidance for filter complexity reduction and useful hints for channel acquisition.

200 Gbps/Lane IM/DD Technologies for Short Reach Optical Interconnects

Abstract: Client-side optics are facing an ever-increasing upgrading pace, driven by upcoming 5G related services and datacenter applications. The demand for a single lane data rate is soon approaching 200 Gbps. To meet such high-speed requirement, all segments of traditional intensity modulation direct detection (IM/DD) technologies are being challenged. The characteristics of electrical and optoelectronic components and the performance of modulation, coding, and digital signal processing (DSP) techniques are being stretched to their limits. In this context, we witnessed technological breakthroughs in several aspects, including development of broadband devices, novel modulation formats and coding, and high-performance DSP algorithms for the past few years. A great momentum has been accumulated to overcome the aforementioned challenges. In this article, we focus on IM/DD transmissions, and provide an overview of recent research and development efforts on key enabling technologies for 200 Gbps per lane and beyond. Our recent demonstrations of 200 Gbps short-reach transmissions with 4-level pulse amplitude modulation (PAM) and discrete multitone signals are also presented as examples to show the system requirements in terms of device characteristics and DSP performance. Apart from digital coherent technologies and advanced direct detection systems, such as Stokes–vector and Kramers–Kronig schemes, we expect high-speed IM/DD systems will remain advantageous in terms of system cost, power consumption, and footprint for short reach applications in the short- to mid- term perspective.

Directly modulated membrane lasers with 108 GHz bandwidth on a high-thermal-conductivity silicon carbide substrate

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.