We demonstrated 1-Tb/s-class transmissions of field-deployed large-core low-loss fiber links, which is compliant with ITU-T G.654.E, using our newly developed real-time transponder consisting of a state-of-the-art 16-nm complementary metal-oxide-semiconductor (CMOS) based digital signal processing application-specific integrated circuit (DSP-ASIC) and an indium phosphide (InP) based high-bandwidth coherent driver modulator (HB-CDM). In this field experiment, we have achieved record transmission distances of 1122km for net data-rate 1-Tb/s transmission with dual polarization-division multiplexed (PDM) 32 quadrature amplitude modulation (QAM) signals, and of 336.6 km for net data-rate 1.2-Tb/s transmission with dual PDM-64QAM signals. This is the first demonstration of applying hybrid erbium-doped fiber amplifier (EDFA) and backward-distributed Raman amplifier were applied to terrestrial G.654.E fiber links. We also confirmed the stability of signal performance over field fiber transmission in wavelength division multiplexed (WDM) condition. The Q-factor fluctuations respectively were only less than or equal to 0.052dB and 0.07dB for PDM-32QAM and PDM-64QAM signals within continuous measurements for 60 minutes.

This paper summarizes our recent studies on architecture, photonic integration, system validation and networking performance analysis of a flexible low-latency interconnect optical network switch (Flex-LIONS) for datacenter and high-performance computing (HPC) applications. Flex-LIONS leverages the all-to-all wavelength routing property in arrayed waveguide grating routers (AWGRs) combined with microring resonator (MRR)-based add/drop filtering and multi-wavelength spatial switching to enable topology and bandwidth reconfigurability to adapt the interconnection to different traffic profiles. By exploiting the multiple free spectral ranges of AWGRs, it is also possible to provide reconfiguration while maintaining minimum-diameter all-to-all interconnectivity. We report experimental results on the design, fabrication, and system testing of 8×8 silicon photonic (SiPh) Flex-LIONS chips demonstrating error-free all-to-all communication and reconfiguration exploiting different free spectral ranges (FSR₀ and FSR₁, respectively). After reconfiguration in FSR₁, the bandwidth between the selected pair of nodes is increased from 50Gb/s to 125Gb/s while an all interconnectivity at 25Gb/s is maintained using FSR₀. Finally, we investigate the use of Flex-LIONS in two different networking scenarios. First, networking simulations for a 256-node datacenter inter-rack communication scenario show the potential latency and energy benefits when using Flex-LIONS for optical reconfiguration based on different traffic profiles (a legacy fat-tree architecture is used for comparison). Second, we demonstrate the benefits of leveraging two FSRs in an 8-node 64-core computing system to provide reconfiguration for the hotspot nodes while maintaining minimum-diameter all-to-all interconnectivity.

We propose an ultra-low inter-core crosstalk (XT) multi-core fiber (MCF) with standard 125-μm cladding. We show the fiber design and fabrication results of an MCF housing four cores with W-shaped index profile; it offers XT of less than -67dB/km over the whole C+L band. This enables us to realize 10,000-km transmission with negligible XT penalty. We also observe a low-loss of 0.17dB/km (average) at a wavelength of 1.55μm and other optical properties compatible with ITU-T G.654.B fiber. We also elucidate its good micro-bend resistance in terms of both the loss and XT to confirm its applicability to high-density optical fiber cables. Finally, we show that the fabricated MCF is feasible along with long-distance transmission by confirming that the XT noise performance corresponds to transmission distances of 10,000km or more.

A high degree of freedom in spectral domain allows us to accommodate additional optical signal processing for wavelength division multiplexing in photonic analog-to-digital conversion. We experimentally verified a spectral compression to save a necessary bandwidth for soliton self-frequency shift based optical quantization through the cascade of the four-wave mixing based and the sum-frequency generation based spectral compression. This approach can realize 0.03 nm individual bandwidth correspond to save up to more than 85 percent of bandwidth for 7-bit optical quantization in C-band.

Large-scale disasters can lead to a severe damage or destruction of optical transport networks including the data-plane (D-plane) and control and management-plane (C/M-plane). In addition to D-plane recovery, quick recovery of the C/M-plane network in modern software-defined networking (SDN)-based fiber optical networks is essential not only for emergency control of surviving optical network resources, but also for quick collection of information related to network damage/survivability to enable the optimal recovery plan to be decided as early as possible. With the advent of the Internet of Things (IoT) technologies, low energy consumption, and low-cost IoT devices have been more common. Corresponding long-distance networking technologies such as low-power wide-area (LPWA) and LPWA-based mesh (LPWA-mesh) networks promise wide coverage sensing and environment data collection capabilities. We are motivated to take an infrastructure-less IoT approach to provide long-distance, low-power and inexpensive wireless connectivity and create an emergency C/M-plane network for early disaster recovery. In this paper, we investigate the feasibility of fiber networks C/M-plane recovery using an IoT-based extremely narrow-band, and lossy links system (FRENLL). For the first time, we demonstrate a field-trial experiment of a long-latency/loss tolerable SDN C/M-plane that can take advantage of widely available IoT resources and easy-to-create wireless mesh networks to enable the timely recovery of the C/M-plane after disaster.

We developed a polarization-independent and reserved-band-less complementary spectral inverted optical phase conjugation (CSI-OPC) device using dual-band difference frequency generation based on highly efficient periodically poled LiNbO₃ waveguide technologies. To examine the nonlinearity mitigation in a long-haul transmission using a large number of OPCs, we installed a CSI-OPC device in the middle of a pure silica core fiber-based recirculating loop transmission line with a length of 320km. First, we examined the fiber-input power tolerance after 5,120-km and 6,400-km transmission using 22.5-Gbaud PDM-16QAM 10-channel DWDM signals and found a Q-factor improvement of over 1.3dB along with enhanced power tolerance thanks to mitigating the fiber nonlinearity. We then demonstrated transmission distance extension using the CSI-OPC device. The use of multiple CSI-OPCs enables an obvious performance improvements attained by extending the transmission distance from 6,400km to 8,960km, which corresponds to applying the CSI-OPC device 28 times. Moreover, there was no Q-factor degradation for the link in a linear regime after applying the CSI-OPC device more than 16 times. These results demonstrate that the CSI-OPC device can improve the nonlinear tolerance of PDM-16QAM signals without an excess penalty.

Mode-by-mode impulse responses, or spectral transfer matrix (STM) of birefringent fibers are measured by using linear optical sampling, with assist of polarization multiplexed probe pulse. By using the eigenvalue analysis of the STM, the differential mode delay and PMD vector of polarization-maintaining fiber are analyzed as a function of optical frequency over 1THz. We show that the amplitude averaging of the complex impulse responses is effective for enhancing the signal-to-noise ratio of the measurement, resulting in improving the accuracy and expanding the bandwidth of the measurement.

Communication networks for wide-scale distributed energy resources (DERs) including photovoltaics (PVs), wind, storage and battery systems and electric vehicles (EVs) will be indispensable in future power grids. In this paper, we compare optical fronthaul networks using existing optical ground wires (OPGWs) for centralized radio access network (C-RAN) architecture to realize cost effective wireless communication network expansion including low population area. We investigate the applicability of optical data transport technologies of physical layer split (PLS), analog radio-on-fiber (ARoF), and common public radio interface (CPRI). The deployment costs of them are comparatively analyzed. It was shown that physical layer split and analog radio-on-fiber with subcarrier multiplexing (SCM) result in lower cost than other technologies.

A long-reach coexisting PON system (1G/10G-EPON, video, and TWDM-PON) that uses the Wavelength Selective-Asymmetrical optical SPlitter (WS-ASP) without any active devices like optical amplifiers is proposed. The proposal can take into account the subscriber distribution in an access network and provide specific services in specific areas by varying the splitting ratios and the branch structure in the optical splitter. Simulations confirm the key features of WS-ASP, its novel process for deriving the splitting-ratios and greater transmission distance than possible with symmetrical splitters. Experiments on a prototype system demonstrate how wavelengths can be assigned to specific areas and optical link budget enhancement. For 1G-EPON systems, the prototype system with splitting-ratio of 60% attains the optical link budget enhancement of 4.2dB compared with conventional symmetrical optical splitters. The same prototype offers the optical link budget enhancement of 4.0dB at the bit rate of 10G-EPON systems. The values measured in the experiment agree well with the simulation results with respect to the transmission distance.

Effective user accommodation will be more and more important in passive optical networks (PONs) in the next decade since the number of subscribers has been leveling off as well and it is becoming more difficult for network operators to keep sufficient numbers of maintenance workers. Drastically reducing the number of small-scale communication buildings while keeping the number of accommodated users is one of the most attractive solutions to meet this situation. To achieve this, we propose two types of long-reach repeater-free upstream transmission configurations for PON systems; (i) one utilizes a semiconductor optical amplifier (SOA) as a pre-amplifier and (ii) the other utilizes distributed Raman amplification (DRA) in addition to the SOA. Our simulations assuming 10G-EPON specifications and transmission experiments on a 10G-EPON prototype confirm that configuration (i) can add a 17km trunk fiber to a normal PON system with 10km access reach and 1 : 64 split (total 27km reach), while configuration (ii) can further expand the trunk fiber distance to 37km (total 47km reach). Network operators can select these configurations depending on their service areas.

We evaluated the noise properties of a periodically poled lithium niobite phase-sensitive amplifier (PSA) using a phase-locked local oscillator as a pump generated by an optical phase-locked loop (OPLL-LO). To examine whether or not the LO pump generated by an OPLL degrades the noise figure (NF) of the PSA, we compared the noise levels of a PSA using an OPLL-LO with that of one using a master local oscillator (M-LO) that utilizes the master light itself as a pump in the electrical domain. With the OPLL, the phase-locked local light had almost the same frequency noise components as the master light. We observed almost the same output noise spectra for the OPLL-LO PSA and M-LO PSA and confirmed the absence of excess noise components in the OPLL-LO PSA in the 0.1 to 20-GHz range. The OPLL-LO PSA exhibited low-noise amplification with an average NF of 1.7-dB at a 23.2-dB gain within an input power range of -31 to -21dBm, which is a feasible input power for repeater amplifiers used in the optical signal transmission systems. We also investigated the influence of the noisy master light, which emulates the accumulation of optical noise from the amplifiers in the transmission system. The OPLL-LO PSA was highly tolerant to the optical noise because the difference in the NF was negligibly small within a master light OSNR range of 5 to 55dB. These results indicate that the OPLL-LO PSA will be useful as a low-noise repeater amplifier for the spectrally efficient large-capacity photonic networks of the future.

In inter-data center networks where high transmission capacity and spectral efficiency are required, a 16QAM format is deployed. On the other hand, in intra-data center networks, a PAM4 format is deployed to meet the demand for a simple and low-cost transceiver configuration. For a seamless and effective connection of such heterogeneous networks without using optical-electrical-optical conversion, an all-optical modulation format conversion technique is required. In this paper, we propose an all-optical PAM4 to 16QAM modulation format conversion using nonlinear optical loop mirror. The successful conversion operation from 2 × 26.6-Gbaud PAM4 signals to a 100-Gbps class 16QAM signal is verified by numerical simulation. Compared with an ideal 16QAM signal, the power penalty of the converted 16QAM signal can be kept within 0.51dB.

Wavelength division multiplexing (WDM) scheme is used widely in photonic metro-core networks. In a WDM network, wavelength continuity constraint is employed to simply construct relay nodes. This constraint reduces the wavelength usage efficiency of each link. To improve the same, an all-optical wavelength converter (AO-WC) has been attracting attention in recent years. In particular, an AO-WC is a key device because it enables simultaneous conversion of multiple wavelengths of signal lights to other wavelengths, independent of the modulation format. However, each AO-WC requires installation of multiple laser sources with narrow bandwidth because the lights emitted by the laser sources are used as pump lights when the wavelengths of the signal lights are converted by the four-wave mixing (FWM) process. To reduce the number of laser sources, we propose a remote pumped AO-WC, in which the laser sources of the pump lights are aggregated into several relay nodes. When the request for the wavelength conversion from the relay node without the laser source is conveyed, the relay node with the laser source transmits the pump light through the optical link. The proposed scheme enables reduction in the number of laser sources of the pump lights. Herein we analyze the distortion of the pump light by propagating it through the optical link We also evaluate the effect of the noise in optical amplifiers and nonlinearities in optical fibers using numerical simulations employing the representative parameters for a practical WDM network.

Impact of sampling frequency and the number of quantization bit of analog-to-digital conversion (ADC) in a direct detection lightwave system using Kramers-Kronig (KK) relation, which has been attracting attention in recent years, are numerically investigated. We studied the effect of spectral broadening caused by nonlinear operations (logarithm, square root) of the KK algorithm when the frequency gap (shift frequency) between the modulated signal and the optical tone is varied. We found that reception performances depend on both the ADC bandwidth and the relative positions of the optical tone and the spectrum. Spectral broadening caused by the logarithm operation of the KK algorithm is found to be the dominant factor of signal distortion in an ADC bandwidth limited system. We studied the effect of the number of quantization bit on the error vector magnitude (EVM) of KK relation based reception in a carrier-to-signal power ratio (CSPR) adjustable transmission system. We found that performances of KK relation based receiver can be improved by increasing the number of quantization bits. For minimum-phase-condition satisfied KK receiver, the required number of quantization bit was found to be 5 bits or more for detection of QPSK, 16-QAM and 64-QAM-modulated signal after 20-km transmission.

We show that imperfection in an IQ-modulator degrades the accuracy of the auto bias control (ABC) circuit connected to the modulator's complementary port. Theoretical analyses show that the IQ-modulator constructed by a nested Mach-Zehnder modulator with a low extinction ratio can distort a constellation of modulated light observed at the complementary port. We propose an auto calibration technique for the ABC circuit that can effectively suppress this degradation. Experimental results using 32-Gbaud, 16-QAM signals showed the measured Q-factor improved by 0.5dB with our proposed technique.

Through novel rotation writing method of Bragg grating in multicore fiber, its strain vector characteristics are analyzed. The relation between the rotation angle and the strain curvature sensitivity is obtained. Reconstruction of strain vector is verified.

Massive multiple-input multiple-output (MIMO) is an enabling technology for next-generation wireless systems because it provides significant improvements in data rates compared to existing small-scale MIMO systems. However, the increased number of antennas results in high computational complexity for data detection, and requires more efficient detection algorithms. In this paper, we propose a new data detector based on a box-constrained complex-valued dichotomous coordinate descent (BCC-DCD) algorithm for large-scale MIMO systems. The proposed detector involves two steps. First, a transmitted data vector is detected using the BCC-DCD algorithm with a large number of iterations and high solution precision. Second, an improved soft output is generated by reapplying the BCC-DCD algorithm, but with a considerably smaller number of iterations and 1-bit solution precision. Numerical results demonstrate that the proposed method outperforms existing advanced detectors while possessing lower complexity. Specifically, the proposed method provides significantly better detection performance than a BCC-DCD algorithm with similar complexity. The performance advantage increases as the signal-to-noise ratio and the system size increase.

For insertion and deletion channels, there are many coding schemes based on low-density parity-check (LDPC) codes, such as spatially coupled (SC) LDPC codes and concatenated codes of irregular LDPC codes and markers. However, most of the previous works have problems, such as poor finite-length performance and unrealistic settings for codeword lengths and decoding iterations. Moreover, when using markers, the decoder receives log-likelihood (LLR) messages with different statistics depending on code bit position. In this paper, we propose a novel concatenation scheme using protograph-based LDPC code and markers that offers excellent asymptotic/finite-length performance and a structure that controls the irregularity of LLR messages. We also present a density evolution analysis and a simple optimization procedure for the proposed concatenated coding scheme. For two decoding scenarios involving decoding complexity, both asymptotic decoding thresholds and finite-length performance demonstrate that the newly designed concatenated coding scheme outperforms the existing counterparts: the irregular LDPC code with markers, the SC-LDPC code, and the protograph LDPC code, which is optimized for an additive white Gaussian noise channel, with markers.

This paper proposes a noise reduction technology for a specific frequency band that uses the pulse coding controlled method to automatically set the notch frequency in DC-DC switching converters of communication equipment. For reducing the power levels at the frequency and its harmonics in the switching converter, we often use a frequency-modulated clock. This paper investigates a technology that prevents modulated clock frequency noise from spreading into protected frequency bands; this proposed noise reduction technology does not distribute the switching noise into some specified frequency bands. The notch in the spectrum of the switching pulses is created by the Pulse Width Coding (PWC) method. In communication devices, the noise in the receiving signal band must be as small as possible. The notch frequency is automatically set to the frequency of the received signal by adjusting the clock frequency using the equation F_n = (P+0.5)F_ck. Here F_n is the notch frequency, F_ck is the clock frequency, and P is a positive integer that determines the noise spectrum location. Therefore, simply be setting the notch frequency to the received signal frequency can suppress the noise present. We confirm with simulations that the proposed technique is effective for noise reduction and notch generation. Also we implement a method of automatic switching between two receiving channels. The conversion voltage ratio in the pulse width coding method switching converter is analyzed and full automatic notch frequency generation is realized. Experiments on a prototype circuit confirm notch frequency generation.

This paper proposes two algorithms, namely Server-User Matching (SUM) algorithm and Extended Server-User Matching (ESUM) algorithm, for the distributed server allocation problem. The server allocation problem is to determine the matching between servers and users to minimize the maximum delay, which is the maximum time to complete user synchronization. We analyze the computational time complexity. We prove that the SUM algorithm obtains the optimal solutions in polynomial time for the special case that all server-server delay values are the same and constant. We provide the upper and lower bounds when the SUM algorithm is applied to the general server allocation problem. We show that the ESUM algorithm is a fixed-parameter tractable algorithm that can attain the optimal solution for the server allocation problem parameterized by the number of servers. Numerical results show that the computation time of ESUM follows the analyzed complexity while the ESUM algorithm outperforms the approach of integer linear programming solved by our examined solver.

The machine-to-machine (M2M) service network platform that accommodates and controls various types of Internet of Things devices has been presented. This paper investigates program file placement strategies for the M2M service network platform that achieve low blocking ratios of new task requests and accommodate as many tasks as possible in the dynamic scenario. We present four strategies for determining program file placement, which differ in the computation method and whether the relocation of program files being used by existing tasks is allowed or not. Simulation results show that a strategy based on solving a mixed-integer linear programming model achieves the lowest blocking ratio, but a heuristic algorithm-based strategy can be an attractive option by allowing recomputation of the placement when the placement cannot be obtained at the timing of new task request arrival.

We propose a novel non-orthogonal multiple access (NOMA)-based optimal multiplexing method for multiple downlink service channels to maximize the integrated system throughput. In the fifth generation (5G) mobile communication system, the support of various wireless communication services such as massive machine-type communications (mMTC), ultra-reliable low latency communications (URLLC), and enhanced mobile broadband (eMBB) is expected. These services will serve different numbers of terminals and have different requirements regarding the spectrum efficiency and fairness among terminals. Furthermore, different operators may have different policies regarding the overall spectrum efficiency and fairness among services. Therefore, efficient radio resource allocation is essential during the multiplexing of multiple downlink service channels considering these requirements. The proposed method achieves better system performance than the conventional orthogonal multiple access (OMA)-based multiplexing method thanks to the wider transmission bandwidth per terminal and inter-terminal interference cancellation using a successive interference canceller (SIC). Computer simulation results reveal that the effectiveness of the proposed method is especially significant when the system prioritizes the fairness among terminals (including fairness among services).