Language：English   Publishing type：Research paper (scientific journal)   Publisher：Optica Publishing Group  

We demonstrate unrepeated 200-km transmission of 40-Gbit/s 16-QAM signals using a digital coherent receiver, where the decision-directed carrier-phase estimation is employed. The phase fluctuation is effectively eliminated in the 16-QAM system with such a phase-estimation method, when the linewidth of semiconductor lasers for the transmitter and the local oscillator is 150 kHz. Finite-impulse-response (FIR) filters at the receiver compensate for 4,000-ps/nm group-velocity dispersion (GVD) of the 200-km-long single-mode fiber and a part of self-phase modulation (SPM) in the digital domain. In spite of the launched power limitation due to SPM, the acceptable bit-error rate performance is obtained owing to high sensitivity of the digital coherent receiver. (C) 2009 Optical Society of America

DOI： 10.1364/oe.17.001435

Publishing type：Research paper (international conference proceedings)   Publisher：SPIE  

DOI： 10.1117/12.3002195

Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

DOI： 10.1109/jlt.2023.3327267

Publishing type：Research paper (scientific journal)   Publisher：Optica Publishing Group  

Traversing each WSS in ultra-dense WDM networks narrows the signal spectra. Simulations and experiments demonstrate, for the first time to our knowledge, spectrum narrowing mitigation based on RNN. Numerical simulations show that the RNN-based demodulation with impairment-aware optical path control significantly enlarges the transmission distance. Transmission experiments in the extended C-band successfully confirm an extension of the transmissible distance of 16QAM signals by over 500 km.

DOI： 10.1364/ol.515414

Other Link： https://opg.optica.org/viewmedia.cfm?URI=ol-49-4-947&seq=0

Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

DOI： 10.1587/transcom.2023pnp0003

Publishing type：Research paper (international conference proceedings)   Publisher：Optica Publishing Group  

A novel cost-effective capacity enhancement method for resilient optical networks is proposed that introduces supplemental multi-band transmission and sharing of extended bands by backup paths. Numerical simulations confirm 17.7-35.3% enhancement on three real topologies.

DOI： 10.1364/ofc.2024.w1c.2

Publishing type：Research paper (international conference proceedings)   Publisher：Optica Publishing Group  

A novel and effective network capacity estimation method and an RSA algorithm suitable for elastic optical path networks are presented. The proposed algorithm successfully achieves the utilization penalty of just 5-16% from the bound.

DOI： 10.1364/ofc.2024.w3c.6

Publishing type：Research paper (international conference proceedings)   Publisher：Optica Publishing Group  

This paper proposes an optical circuit switch architecture using multi-band transmission. We experimentally confirmed the performance of 1,280×1,280 switch with 32-Gbaud dual-polarization QPSK signals aligned on 33-GHz grid in the C- and L- bands.

DOI： 10.1364/ofc.2024.m2g.3

Publishing type：Research paper (international conference proceedings)   Publisher：Institution of Engineering and Technology  

DOI： 10.1049/icp.2023.2536

Publishing type：Research paper (international conference proceedings)   Publisher：Institution of Engineering and Technology  

DOI： 10.1049/icp.2023.2583

Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.1109/psc57974.2023.10297168

Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.1109/psc57974.2023.10297165

Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.1109/psc57974.2023.10297285

Publishing type：Research paper (scientific journal)   Publisher：Optica Publishing Group  

A cost-effective and fast network capacity upgrade with what we believe to be novel bandwidth-variable virtual direct links (BV-VDLs) is proposed. BV-VDLs are designed to pass through the physical links expected to be heavily congested and realize very dense path accommodation. We adopt cut-set analysis to identify the links likely to be heavily congested; however, high computation loads will be incurred in finding all the cut-sets, and justifying these loads is essential. Thus a simple-path-search based graph splitting method is proposed that substantially shortens the calculation time. This strategy allows us to efficiently upgrade network capacity while keeping existing facilities unchanged and can be installed in networks in operation. The proposal is also valid for enhancing the capacity of new networks. The capacity enhancement can reach 13%–33% even in dynamic path operation scenarios on the real topologies of pan-European, North American, and Japanese networks.

DOI： 10.1364/jocn.485116

Publishing type：Research paper (scientific journal)   Publisher：Optica Publishing Group  

Fiber-granularity routing networks that adopt wavelength-granularity add/drop and a limited number of wavelength converters are proposed. The express parts of nodes can be optical switches, used as fiber cross-connects (FXCs), as they offer low-cost scaling. However, FXCs cannot route optical signals on a wavelength basis, unlike present wavelength cross-connects (WXCs). This routing restriction degrades the statistical multiplexing effect and decreases fiber utilization efficiency. The restriction is slightly relaxed by the use of wavelength-granularity add/drop operations; however, our numerical simulations show that the degradation can reach around 50% when the number of optical paths in a fiber is 25. This substantial degradation is cost-effectively alleviated by introducing wavelength/optical path grooming with a limited number of wavelength converters, which enhances routing flexibility. Numerical simulations show that our network architecture improves routing performance by 86% compared to fiber-granularity routing networks without wavelength converters, and the performance deterioration relative to present WXC-based networks is suppressed to ∼14%.

DOI： 10.1364/jocn.486314

Publishing type：Research paper (international conference proceedings)   Publisher：SPIE  

DOI： 10.1117/12.2649581

Publishing type：Research paper (international conference proceedings)   Publisher：Optica Publishing Group  

We propose a joint pre-/post-equalization scheme that uses Bayesian optimization and the LMS algorithm for mitigating the spectrum narrowing induced by repetitive node traversals. The transmissible distance and hop-count in ultra-dense WDM networks are extended.

DOI： 10.1364/networks.2023.netu3b.3

Publishing type：Research paper (international conference proceedings)   Publisher：Optica Publishing Group  

We investigate performance of symbol decision based on neural networks in the presence of severe quantization noise for the first time. Simulations show 2 dB improvement when 256-QAM signals are generated with 6-bit DACs.

DOI： 10.1364/iprsn.2023.jw2e.1

Publishing type：Research paper (international conference proceedings)   Publisher：Optica Publishing Group  

We propose a novel bundled-path-routing node architecture for multi-band optical networks and a network design algorithm based on graph degeneration. Feasibility is demonstrated through experiments on a prototype with 300.8 Tbps throughput.

DOI： 10.1364/ofc.2023.m4g.8

Publishing type：Research paper (international conference proceedings)   Publisher：Optica Publishing Group  

A cost-effective network expansion method through the use of virtual links in which multi-band transmission is introduced. Network capacity enhancement reaches over 10% just with the introduction of a few virtual links on each topology.

DOI： 10.1364/ofc.2023.m4g.4

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

A cost-effective network capacity expansion method with novel bandwidth-variable virtual direct links (BV-VDLs) is proposed. BV-VDLs are located to traverse physical links expected to be heavily congested and highly dense path accommodation to BV-VDLs is applied. This strategy allows us to efficiently upgrade network capacity while keeping existing facilities unchanged. The capacity enhancement in dynamic path operation scenarios on real topologies can reach 20%.

DOI： 10.1109/fnwf55208.2022.00061

Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.1109/rndm55901.2022.9927712

Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

DOI： 10.1109/lpt.2022.3196866

Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.23919/oecc/psc53152.2022.9850095

Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.23919/oecc/psc53152.2022.9849979

Publishing type：Research paper (international conference proceedings)   Publisher：SPIE  

DOI： 10.1117/12.2609376

Publishing type：Research paper (scientific journal)   Publisher：Optica Publishing Group  

Space-division multiplexing (SDM) is expected to increase the capacity of photonic networks. Reconfigurable optical add-drop multiplexers (ROADMs) for SDM-based networks must have high scalability in terms of port count. However, the ROADM architecture adopted in present networks cannot support large numbers of ports due to the limited port count of wavelength-selective switches. In this paper, we propose a ROADM architecture composed of space switches and wavelength-routing switches. Space switches have lower per-port cost than wavelength-routing switches. However, space switches cannot route optical paths on a wavelength basis, unlike wavelength-routing switches. By combining these two types of switches, the ROADM port count can be cost-effectively expanded virtually unlimitedly. Numerical simulations show the routing performance of our ROADM; they elucidate that the maximum fiber increment penalty is about 5% compared to the reference scheme. Experiments detail the transmission performance of the ROADM, where 32 Gbaud DP-4QAM/8QAM/16QAM signals are aligned on a 50 GHz grid, are examined.

DOI： 10.1364/jocn.450504

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Optica Publishing Group  

Efficient network design and control algorithms for fiber-granular routing networks are proposed. Routing performance of next-generation broad-bandwidth optical paths on fiber-granular routing networks with over 100×100 fiber-cross-connects is verified.

DOI： 10.1364/ofc.2022.w3f.6

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Optica Publishing Group  

We demonstrate a high-throughput optical circuit switch for intra-datacenter networks. DP-32QAM and Nyquist WDM are used to enhance the spectral efficiency of the switch. Experiments show the total switch throughput of 8.512 Pbps.

DOI： 10.1364/ofc.2022.th3a.2

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We propose a novel optical circuit switch architecture offering high reliability and high capacity. The proposed scheme substantially reduces the annual downtime of the switch with little additional hardware cost. Its transmission performance is experimentally confirmed by constructing part of a 1,536x1,536 optical switch. (C) 2022 The Author(s)

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We propose a reinforcement-learning-based network design and control algorithm that introduces reward variation dependent on maximum link utilization and link-adjacency embedding as input parameters. Up to 65%/20% capacity enhancement relative to first-fit and link-congestion-aware methods is verified. (C) 2022 The Author(s)

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

We propose an efficient network upgrade and expansion method that can make the most of the next generation channel resources to accommodate further increases in traffic. Semi-flexible grid configuration and two cost metrics are introduced to establish a regularity in frequency assignment and minimize disturbance in the upgrade process; both reduce the fragmentation in frequency assignment and the number of fibers necessary. Various investigations of different configurations elucidate that the number of fibers necessary is reduced about 10-15% for any combination of upgrade scenario, channel frequency bandwidth, and topology adopted.

DOI： 10.1587/transcom.2021ebp3027

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Optica Publishing Group  

The fast and widely tunable wavelength bank is a key enabler in creating wavelength-routing optical switches that do not use fast wavelength tunable lasers. A cost-effective design criterion needs to be developed before it can be applied to intra data center networks. In this paper, we develop a systematic method for designing a wavelength bank that yields high port-count and fast wavelength-routing optical switches for intra data center application. The wavelength bank is created with fixed-wavelength laser sources and wavelength-tunable filters with rapid wavelength selectivity. To optimize the optical switching system that uses the wavelength bank for supplying local oscillator (LO) lights for coherent detection, various parameters are analyzed, including effective bandwidth, laser output power, loss distribution, splitter port count, and optical amplifier gain. We carry out numerical simulations for optimizing the tradeoff between system performance and cost. To verify the designed wavelength bank, a silicon ring filter is newly fabricated with an average fiber-to-fiber insertion loss of 5.3 dB over a 22-nm bandwidth. Using 256-Gb/s DP-QPSK signals, experiments demonstrate a 1,024 1,024 optical switch that uses a fabricated silicon ring filter. The effectiveness of the scalable and fast-tunable LO bank is verified by achieving 262.1-Tb/s switch throughput with switching time under 18 µs.

DOI： 10.1364/oe.439469

Publishing type：Research paper (scientific journal)   Publisher：Optica Publishing Group  

We propose an ultradense wavelength-division-multiplexing (WDM) network architecture that dramatically enhances channel accommodation density to achieve extremely high spectral efficiency. The trade-offs between high spectral efficiency and severe signal impairment due to optical filtering are resolved by network optimization of the overlaid subnetworks based on fiber-granular routing and wavelength-granular add/drop operations. The spectrum narrowing induced by node operations is well controlled by the impairment-suppressing routing and wavelength assignment (RWA) method. Numerical simulations on several metro-scale network topologies verify that the proposal enhances the spectral efficiency by up to 31.0% compared to comparable dense WDM networks.

DOI： 10.1364/jocn.428571

Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.23919/ondm51796.2021.9492351

Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.1109/cleo/europe-eqec52157.2021.9542509

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

We investigate a large-scale and fast optical circuit switch system that uses digital coherent technologies. The achievable port count is expanded by introducing colorless detection which eliminates channel selecting filters at receivers. Wavelength selection is performed using a shared local oscillator (LO) bank, which is configured by combining a multi-wavelength optical source with Silicon-photonic tunable filters (TFs). Microsecond switching times are realized with the thermo-optic effect available with the Silicon-photonic TF. Design optimization of the proposed system handles complex level of freedom, i.e., available wavelength number, space switch port count, optical amplifier location and gain, and device loss. In addition, colorless coherent detection induces penalties, which further complicates switch scale assessment. In this paper, we develop a simulator to clarify how those parameters affect switch performance; the switch throughput is maximized with consideration of the degradation stemming from colorless detection. The design performance accurately matches results measured in various system scenarios. A dual-carrier 256-Gb/s DP-QPSK experiment successfully demonstrates a 1,856 x 1,856 optical switch system with switching time under 3.52 mu s. The resulting total throughput is 441 Tb/s assuming 7%-overhead HD-FEC. To the best of our knowledge, this is the first demonstration of 400-Tbps class large bandwidth optical switches that use TFs as wavelength selective devices for coherent detection.

DOI： 10.1109/jlt.2021.3050734

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

To improve the spectral efficiency and increase network capacity, wavelength signals must be multiplexed densely, i.e., ultra-dense wavelength-division-multiplexing (WDM) networks. However, the spectrum of each WDM signal is narrowed by the wavelength-selective switches placed in optical nodes, so the transmissible distance and node hop count of the signal are strictly limited. To counter this spectrum narrowing problem caused by optical node traversal in ultra-dense WDM networks, pre-filtering techniques have been proposed. This paper comprehensively investigates the performance of four pre-filtering techniques: conventional Nyquist filtering, pre-emphasis filtering, partial-response filtering, and the combination of pre-emphasis and partial-response filtering. We conduct extensive computer simulations and discuss the optimality of pre-filtering techniques. The simulation results show that the adaptive use of pre-filtering can substantially extend the maximum attainable transmission distance and hop counts of 4/16/64-QAM signals.

DOI： 10.1109/jphot.2021.3065139

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE  

Digital coherent reception is an attractive candidate for increasing the channel capacity without expanding the bandwidth needed. The receiver DSP circuit must be able to compensate for the IQ-phase/power mismatch with low computation power requirements. In this paper, we propose a novel DSP circuit suitable for short-reach transmission systems including intra-datacenter networks. The proposed DSP circuit offers polarization-mode demultiplexing, compensation for IQ-phase/power mismatch, and estimation of carrier phase and frequency offset. The proposed DSP circuit consists of three-stage one-tap FIR filters; therefore, complicated calculations are not required. Its good demodulation performance is confirmed by simulations.

DOI： 10.1117/12.2577542

Publishing type：Research paper (international conference proceedings)   Publisher：Optica Publishing Group  

We propose an RL-assisted RSA algorithm in elastic optical networks with multiple fibers per link. Numerical experiments show that the proposal can improve the performance of resource allocation in all topologies with different dimensions.

DOI： 10.1364/psc.2021.w1b.3

Publishing type：Research paper (international conference proceedings)   Publisher：Optica Publishing Group  

We propose a novel joint modulation and demodulation optimization scheme without the differentiable channel model. Simulations show that our proposed scheme realizes higher phase noise tolerance than the conventional strategies.

DOI： 10.1364/psc.2021.tu5a.6

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Optica Publishing Group  

The introduction of next-generation paths and fiber-granular routing to dynamic optical networks is studied. Due to the severe deterioration of statistical multiplexing effect, the fiber utilization ratio decreases by 50+% from current levels.

DOI： 10.1364/oecc.2021.t2a.8

Publishing type：Research paper (international conference proceedings)   Publisher：Optica Publishing Group  

We experimentally evaluate the transmission performance of OXC structures based on flexible-waveband routing. 1600 km, 700 km, and 600 km transmission is demonstrated using 32 Gbaud DP-QPSK, DP-8QAM, and DP-16QAM on 37.5 GHz grid, respectively.

DOI： 10.1364/ofc.2021.th4e.3

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Optica Publishing Group  

We propose a novel method for measuring transmitter IQ skew. Numerical simulations show that its measurement error is less than 0.1% in the presence of the other IQ impairments. Proof-of-concept experiments confirm its effectiveness.

DOI： 10.1364/oecc.2021.t4b.7

Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.1109/ecoc48923.2020.9333047

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We demonstrate a 1,024x1,024 wavelength-routing optical switch with switching time under 18,us for 256-Gb/s DP-QPSK signals. Wavelength selection at the receivers is achieved by fast widely-tunable (22nm) local oscillators formed using a wavelength bank and newly fabricated silicon ring filters having 5.3-dB fibre-to-fibre insertion loss.

DOI： 10.1109/ecoc48923.2020.9333235

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We propose a novel pre-filtering scheme that combines pre-emphasis and partial-response coding to mitigate the spectrum narrowing caused by repeated optical-node traversal. Simulations show that our optimum pre-filtering scheme extends the maximum node-hop count and transmission distance by up to 13 and 1040 km, respectively.

DOI： 10.1109/ecoc48923.2020.9333222

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We propose a low-complexity DSP configuration for short-reach transmission systems. The proposed configuration uses three-stage one-tap filters to perform polarization-mode demultiplexing, phase/frequency estimation, and IQ-phase/power mismatch compensation. Its effectiveness is confirmed through extensive computer simulations.

DOI： 10.1109/oecc48412.2020.9273542

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We evaluate the feasibility of an optical circuit switch that suits SDM-based intra-datacenter networks. Simulations with 100-Gbps DP-QPSK signals and 200-Gbps DP-16QAM signals clarify the maximum switch throughput attainable with feasible hardware requirements.

DOI： 10.1109/oecc48412.2020.9273537

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

A novel routing, core and spectrum assignment (RCSA) algorithm based on reinforcement learning is proposed for space division multiplexing elastic optical networks (SDM-EONs). The algorithm introduces two metrics that measure the spare capacity of the network in terms of the inter-core crosstalk in multi-core fibers and the efficiency of spectrum resource utilization. Numerical simulations indicate that it can enhance the network capacity by up to 46.8% and 10.4%, compared with two conventional schemes.

DOI： 10.1109/oecc48412.2020.9273621

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

This paper presents a DSP configuration that partially merges the polarization demultiplexer, phase estimator, and frequency estimator to lower the computational complexity of digital coherent receivers for short-reach transmission such as intra-datacenter transmission. Its effectiveness is confirmed through intensive computer simulations.

DOI： 10.1109/ipc47351.2020.9252306

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

We analyze the cost of networks consisting of optical cross-connect nodes with different architectures for realizing the next generation large bandwidth networks. The node architectures include wavelength granular and fiber granular optical routing cross-connects. The network cost, capital expenditure (CapEx), involves link cost and node cost, both of which are evaluated for different scale networks under various traffic volumes. Numerical experiments demonstrate that the subsystem modular architecture with wavelength granular routing yields the highest cost effectiveness over a wide range of parameter values.

DOI： 10.1587/transcom.2019ebp3148

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

The spectral efficiency of photonic networks can be enhanced by the use of higher modulation orders and narrower channel bandwidth. Unfortunately, these solutions are precluded by the margins required to offset uncertainties in system performance. Furthermore, as recently highlighted, the disaggregation of optical transport systems increases the required margin. We propose here highly spectrally efficient networks, whose margins are minimized by transmission-quality-aware adaptive modulation-order/channel-bandwidth assignment enabled by optical performance monitoring (OPM). Their effectiveness is confirmed by experiments on 400-Gbps dual-polarization quadrature phase shift keying (DP-QPSK) and 16-ary quadrature amplitude modulation (DP-16QAM) signals with the application of recently developed Q-factor-based OPM. Four-subcarrier 32-Gbaud DP-QPSK signals within 150/162.5/175 GHz and two-subcarrier 32-Gbaud DP-16QAM signals within 75/87.5/100 GHz are experimentally analyzed. Numerical network simulations in conjunction with the experimental results demonstrate that the proposed scheme can drastically improve network spectral efficiency.

DOI： 10.1587/transcom.2019ebp3155

Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.23919/ondm48393.2020.9133005

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Optica Publishing Group  

We propose an extremely cost-effective optical-cross-connect (OXC) architecture that allows hitless port-count expansion with virtually no limit to meet incremental traffic growth at minimum cost. This is enabled by a new interconnection scheme of optical splitters and wavelength-selective switches (WSSs) in the express-switch part and that of transponder banks in the add/drop parts. The benefit is offset by slight constraint on routing flexibility, which is minimized with a newly developed interconnection algorithm. Extensive computer simulations are carried out to verify that the proposed OXC retains satisfactory routing capability without notable penalty even though smaller-degree WSSs ( ) are utilized: it is compared with present fixed-port-count OXCs using larger-degree WSSs ( ), which have no hitless extension capability. The proposed OXC with the proposed algorithm can drastically reduce OXC hardware cost. We analyze node-design parameters in terms of the routing performances of the OXC and search for the appropriate values. The effects of traffic churn ratio during network expansion and the geographic traffic distribution are investigated. The results verify the effectiveness and robustness of the OXC architecture and the expansion scheme; the number of WSSs needed and the necessary component WSS port count can be greatly reduced while the resultant link fiber increment is marginal (1%–5%), compared with the ideal OXC without any routing restriction.

DOI： 10.1364/jocn.384140

Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

DOI： 10.1587/transcom.2019ebp3026

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We experimentally demonstrate the transmission performance of a high-throughput OXC architecture that utilizes recently proposed spatially-jointed flexible waveband routing. Its feasibility is experimentally verified using 64-channel 400-Gbps dual-carrier DP-16QAM signals aligned with 75 GHz spacing in 4.8 THz of the full Cband. Our OXC design is shown to attain the net OXC throughput of 2.15 Pbps, OXC port count of 84, hop count of 7, and transmission distance of 700 km.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We propose a novel RSA algorithm for dynamically-changing flexible-grid networks. The proposed scheme can suppress spectral fragmentation and adapt to traffic-distribution change. Extensive simulations show that the fiber-utilization efficiency is increased by 1% to 57%.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

We propose a cost-effective metro network architecture with fiber-granular routing and path-granular add/drop operations together with its ILP-based design algorithm. The proposal alleviates the impact of filtering impairment while using already deployed OXC/ROADM nodes. Numerical simulations on several real-world metro topologies verify that it increases the spectral efficiency compared to the ideal method for DWDM networks.

DOI： 10.1117/12.2545553

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

The introduction of quasi-Nyquist wavelength-division multiplexing (WDM) reduces unused frequency resources and hence attains high spectral efficiency. Applying quasi-Nyquist WDM to optical-path networks using wavelength-selective switches (WSSs) is hindered by two factors: the limited bandwidth resolution of WSS passbands and the significant spectrum narrowing occasioned by WSS traversal. In this paper, we propose a novel quasi-Nyquist WDM network architecture, where the bundle-based wavelength assignment and receiver-side partial-response spectrum shaping are simultaneously utilized so as to resolve the two problems. Extensive computer simulations show that the proposed network architecture increases the maximum attainable node-hop count compared to the conventional transmission systems.

DOI： 10.1117/12.2544195

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We propose a large-scale fast optical circuit switch created with Silicon-photonic tunable-filter-based LO bank and colorless coherent detection. Experiments verify 475.1-Tbps switch bandwidth (1,856 x 1,856 at 256 Gbps) and switching times under 3.52 mu s. (C) 2020 The Author(s)

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We propose a novel optical circuit switch architecture based on spatial super-channels We construct part of a 1,536X 1,536 optical switch and its performance is experimentally confirmed. The total throughput of the switch reaches 2.1 Pbps.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE Computer Society  

For efficient and dynamic path operations in transparent optical networks, routing and wavelength assignment (RWA) must be optimized in terms of not only link-resource utilization but also traffic distribution. In this paper, we propose a reinforcement-learning-based RWA algorithm that maximizes the number of paths to be accommodated to a network with pre-training using estimated traffic distributions. Numerical experiments elucidate that the number of paths accommodated increases by up to 9.1%.

DOI： 10.1109/ICTON.2019.8840405

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Optica Publishing Group  

Photonic networks based on wavelength-selective switches (WSSs) can transport wavelength-division-multiplexed (WDM) signals in a cost-effective manner. To accommodate the ever-increasing network traffic, the spectral efficiency should be maximized by minimizing the bandwidth of the guardbands inserted between WDM signals. Quasi-Nyquist WDM systems are seen as offering the highest spectral efficiency in a feasible way. However, highly dense WDM systems suffer from the signal-spectrum narrowing induced by the non-rectangular passbands of WSSs. Furthermore, widely deployed WSSs cannot process quasi-Nyquist WDM signals since the signal-alignment granularity does not match the passband resolution of the WSSs. This paper proposes a network architecture that enables quasi-Nyquist WDM networking with widely deployed WSSs. Through intensive network analyses based on computer simulations, we confirm that it has 30.8% higher spectral efficiency than conventional networks. Its feasibility is verified by transmission experiments on 72-channel 32-Gbaud/400-Gbps dual-carrier dual-polarization 16-ary quadrature-amplitude-modulation signals aligned with 66.6-GHz spacing in the full C-band. The net fiber capacity of 28.8 Tbps, the transmission distance of 900 km, and the hop count of 9 are attained by our proposed quasi-Nyquist WDM networking scheme. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

DOI： 10.1364/oe.27.018549

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

Link-level and node-level blocking in photonic networks has been intensively investigated for several decades and the C/D/C approach to OXCs/ROADMs is often emphasized. However, this understanding will have to change in the future large traffic environment. We herein elucidate that exploiting node-level blocking can yield cost-effective large-capacity wavelength routing networks in the near future. We analyze the impact of link-level and node-level blocking in terms of traffic demand and assess the fiber utilization and the amount of hardware needed to develop OXCs/ROADMs, where the necessary number of link fibers and that of WSSs are used as metrics. We clarify that the careful introduction of node-level blocking is the more effective direction in creating future cost effective networks; compared to C/D/C OXCs/ROADMs, it offers a more than 70% reduction in the number of WSSs while the fiber increment is less than similar to 2%.

DOI： 10.1587/transcom.2018ebp3140

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

We propose highly spectrally efficient grouped-routing networks with distance-adaptive modulation. The proposed scheme can realize highly dense wavelength-division-multiplexing networks under the constraint of the spectrum narrowing caused by optical-node traversals. Its effectiveness is confirmed by extensive computer simulations using 400-Gbps dual-polarization QPSK, 8QAM, and 16QAM signals. The fiber-utilization efficiency is improved by 18.0%.

DOI： 10.1109/lpt.2019.2892506

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

The use of the higher-order modulation format and narrower channel bandwidth can enhance the spectral efficiency of photonic networks. However, such spectrally efficient schemes cannot be utilized when the large margin is required to tolerate uncertainties in the performance of system components. Furthermore, the disaggregation of optical transmission system increases the required margin. In this paper, we propose highly spectrally efficient network architecture, where the required margin is minimized by transmission-quality-aware modulation-order/channel-bandwidth assignment based on optical performance monitoring. Up to 82% improvement in spectral efficiency for 400-Gbps DP-QPSK/DP-16QAM signals is confirmed through transmission experiments and network simulations.

DOI： 10.1587/comex.2018xbl0157

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE  

We propose novel network architecture that allows quasi-Nyquist WDM networks to be implemented with widely deployed WSSs. Network analyses show a 38% improvement in spectral efficiency. Transmission experiments using 69-channel 400-Gbps signals confirm its feasibility.

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE  

A spectral-resource-utilization-efficient and highly resilient coarse granular routing optical network architecture is proposed. The improvement in network resiliency is realized by a novel concept named loop inflation that aims to enhance the geographical diversity of a working path and its redundant path. The trade-off between the inflation and the growth in circumference length of loops is controlled by the Simulated Annealing technique. Coarse granular routing is combined with resilient path design to realize higher spectral resource utilization. The routing scheme defines virtual direct links (VDLs) bridging distant nodes to alleviate the spectrum narrowing effect at the nodes traversed, allowing optical channels to be more densely accommodated by the fibers installed. Numerical experiments elucidate that the proposed networks successfully achieve a 30+% route diversity improvement and a 12% fiber number reduction over conventional networks.

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

This paper evaluates performance of digital coherent receivers that executes partial-response spectrum shaping and MLSE to mitigate the inline-spectrum-narrowing impairment caused by traversing multiple optical nodes in photonic networks. Extensive computer simulations confirm that the maximum transmission distances and hop counts of DP 4-QAM and 16-QAM signals in highly dense WDM networks are substantially extended.

DOI： 10.1587/comex.2018xbl0162

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Institution of Engineering and Technology  

Intra- and inter-datacentre traffic is now exploding. To process both kinds of traffic cost-effectively, intra- and inter-datacentre converged network architectures that use cost-effective transponders are needed. In this paper, we propose a novel network architecture based on Kramers-Kronig (KK) receivers. KK-receiver-based systems can handle both intra- and inter-datacentre traffic more cost-effectively than coherent systems and attain higher transmission quality than legacy non-coherent systems. To confirm the feasibility of our proposed network architecture, we construct part of a 640×640 optical circuit switch and evaluate its performance when handling intra-datacentre traffic and inter-datacentre traffic
chromatic dispersion is managed in the receiver-side digital signal processing. 80 wavelengths in the C-band are fully evaluated through bit-error-ratio measurements.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We evaluate transmission characteristics of WSSs used in highly dense WDM networks. The passband measurement and transmission simulations show that spectrum narrowing enhanced by the deviation of the WSS passbands drastically shortens the reachable distance.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

This paper analyzes the performance of a commercially available wavelength-tunable laser for developing a large-scale (1,424x1,424) optical circuit switch intended for datacenter/HPC applications. The dynamic BER transitions of 100-Gbps DP-QPSK signals clarify the device feasibility.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

Optical-path networks based on wavelength-selective switches (WSSs) can cost-effectively process wavelength-division-multiplexed (WDM) signals. To deal with the continuously increasing network traffic, the spectral efficiency must be improved by minimizing guardband bandwidths. Quasi-Nyquist WDM systems are seen as offering the highest spectral efficiency. However, such highly dense WDM systems suffer from signal-spectrum narrowing induced by the non-rectangular passbands of WSSs. Furthermore, widely deployed WSSs cannot process quasi-Nyquist WDM signals since the signal-alignment granularity does not match the passband resolution of the WSSs. In this paper, we propose a network architecture that enables quasi-Nyquist WDM networking. First, multiple channels are bundled so that the total channel bandwidth matches the WSS-passband resolution. Second, the number of spectrum-narrowing events of each path is limited by our restriction-aware algorithm. These proposals allow a 100-GHz bandwidth to accommodate three 100-Gbps DP-QPSK signals aligned with 33.3-GHz spacing and a 200-GHz bandwidth to accommodate three 400-Gbps dual-carrier DP-16QAM signals aligned with 66.6-GHz spacing. Intensive network analyses confirm that the spectral efficiency is improved by up to 46.4%. Feasibility is verified by transmission experiments using 69-channel 400-Gbps dual-carrier DP-16QAM signals aligned with 66.6-GHz spacing in the extended C-band. The fiber capacity of 27.6 Tbps and the transmission distance of 800 km are attained by our proposed quasi-Nyquist WDM networking.

DOI： 10.1117/12.2507449

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE  

The performance is analyzed of a highly cost-effective OXC that allows hitless port-count expansion to meet traffic-growth at minimum cost. Numerical experiments verify the OXC offers excellent routing capability even though it uses small-degree WSSs.

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

We present a large-scale optical circuit switch architecture using wavelength-tunable and bandwidth-variable silicon photonic filters. The attainable switch port count is quantitatively evaluated through extensive computer simulations. The requirements for constructing a 1,024 x 1,024 optical circuit switch are analjzed on 10-, 28-, and 56-Gb/s signals. The simulation results are verified through transmission experiments.

DOI： 10.1109/lpt.2018.2852328

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Optica Publishing Group  

DOI： 10.1364/jocn.10.000b82

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

We demonstrate a highly scalable subsystem-modular optical cross-connect (OXC) that uses sub-OXCs based on a newly developed single-module 6 × 6 wavelength-selective switch (WSS). Network simulations in dynamic-traffic scenarios assure the good routing performance of the architecture. Its technical feasibility is confirmed by transmission experiments using 96-wavelength 32-Gbaud dual-polarization quadrature phase-shift keying (DP-QPSK) signals. The developed 6 × 6 WSS allows each DP-QPSK signal to traverse 12 nodes when the internode distance is 100 km.

DOI： 10.1109/jlt.2018.2800082

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：OSA - The Optical Society  

We fabricate a fast wavelength-tunable and bandwidth-variable optical filter using silicon-photonics technologies. Part of a 1024x1024 switch is constructed and adaptive support of 10-Gbps, 28-Gbps and 56-Gbps signals is experimentally demonstrated.

DOI： 10.1364/NETWORKS.2018.NeTh1F.4

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE  

Extremely high-density WDM networks that bundle and route optical paths are proposed; each path comprises a Nyquist-shaped signal. Bundled-based routing realizes finer-granularity frequency assignment than the 1TU-T flexible grid and resolves spectrum narrowing at WSSs simultaneously. The number of fibers needed is reduced by 23% in a 16-node ring network.

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE  

This paper analyzes different optical cross-connect node architectures in terms of capital expenditure (CapEx) of networks. Numerical experiments demonstrate that the subsystem modular architecture yields the highest cost effectiveness over a wide range of traffic.

Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

The traffic volume processed within the datacenter increases exponentially. In a typical datacenter, top-of-rack (ToR) switches connected to servers are interconnected via multi-stage electrical switches. The electrical switches require power-consuming optical-to-electrical and electrical-to-optical conversion. To resolve the problem, a single optical switch needs to be introduced to offload large-capacity flows. The optical switch in the datacenter must have a large number of input/output ports to support many ToR switches. The combination of delivery-and-coupling (DC) switches and wavelength-routing (WR) switches comprised of 1xN non-cyclic arrayed-waveguide gratings (AWGs) can attain high-port-count switches. To further increase the port count, the system loss must be reduced or higher-power transmitters must be used. To overcome this difficulty, we propose novel optical-switch architecture in which nxN uniform-loss and cyclic-frequency (ULCF) AWGs are utilized for the WR-switch part, where the system loss can be reduced by the factor of n. To confirm the effectiveness of our proposal, 12x48 ULCF AWGs were newly fabricated with planar-lightwave-circuit (PLC) technology. Part of a 1,536x1,536 optical switch was constructed, and good transmission performance was experimentally confirmed by bit-error-ratio measurements in 96-wavelength 32-G-baud DP-QPSK signals in the full C-band. The throughput was 153.6 Tbps.

DOI： 10.1117/12.2288268

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE  

We propose an cost-efficient network framework, where datacenters are directly interconnected nation-wide with optical paths. In addition, grouped routing is applied to enhance the spectral efficiency. Its effectiveness is proven by sensitivity analyses of various parameter values. The numbers of fibers and transponders are reduced by 17% and 44%, respectively.

Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

We analyze the maximum transmission distance and hop count of M-QAM signals, where link and node transmission characteristics are jointly considered. With the modulation format optimally determined by the analyses, spectral efficiencies of ultra-dense wavelength-division-multiplexing (WDM) networks are maximized.

DOI： 10.1117/12.2288270

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE  

A spectral-utilization-efficient and multiple-failure-resilient coarse granular routing optical network architecture is proposed. This work is based on our previously proposal of shared path protected networks with dense channel accommodation to coarse granular (grouped routing) optical pipes. Its advances, which include enhanced path and pipe restoration, mean that the proposed networks not only resolve the optical filtering impairment issue at nodes but also achieve higher survivability. Numerical evaluations prove that the number of fibers needed is reduced by up to 20% relative to conventional networks with the same resiliency.

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE  

We demonstrate a spectrally efficient and highly resilient grouped- routing optical network, whose Q-factor margin is precisely controlled with optical performance monitoring. Its effectiveness is confirmed via network analysis and transmission experiments on 200-Gbps DP16QAM signals.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We demonstrate fast optical circuit switching from a monolithically integrated optical space switch and wavelength-tuneable filter on a silicon-photonic chip. Feasibility is confirmed by proof-of-concept experiments using 43-Gbps DQPSK signals. Switching time of 10 mu s is attained.

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE  

Efficient implementation strategy of new generation channel-speeds is presented where frequency fragmentation is minimized. Substantial improvement in frequency utilization or reduction in fiber number (similar to 15%) is verified for various network conditions and different channel-bandwidth allocations.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：OSA - The Optical Society  

This paper proposes a novel FM-noise spectrum measurement scheme for future ultra-dense digital coherent systems that is based on three-wave interference and DSP. The effectiveness of the proposed scheme is confirmed through simulations and experiments.

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE  

As one step in realizing next-generation flexible ROADMs, we demonstrate the effectiveness of a bandwidth-adaptable silicon-photonic filter, through transmission experiments on 10-Gbps OOK, 40-Gbps DQPSK, 100-Gbps DP-QPSK, 400-Gbps 2-subchannel DP-16QAM, and 1-Tbps 5-subchannel DP-16QAM signals.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：OSA - The Optical Society  

We propose a demodulation algorithm that uses duo-binary spectrum shaping and MLSE for mitigating the spectrum narrowing caused by traversing nodes. The maximum transmission distance and hop-count in ultra-dense WDM networks are drastically extended.

DOI： 10.1364/SPPCom.2018.SpW1G.5

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

This paper presents a novel demodulation method that performs quadrature duo-binary/quatemary/octemary spectrum shaping and MLSE for mitigating spectrum narrowing caused by traversing multiple optical nodes. The maximum hop count and transmission distance in ultra-dense WDM networks are substantially extended.

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Optica Publishing Group  

Reliable coarse granular routing optical network architectures that maximize fiber frequency utilization efficiency are presented. The architectures combine coarse granular routing and fine granular add/drop operations, where optical paths are carried by virtual optical pipes. Higher efficiency than conventional fine granular routing networks is achieved by dense path packing in the frequency domain. We propose two architectures that apply shared protection with different levels of granularity and develop static network design algorithms for the two different shared protection schemes. The design algorithms neutralize the impairment caused by adjacent path dropping in the frequency domain. Numerical experiments verify that the necessary number of fibers in a network can be reduced by up to 20% due to the improved spectral utilization efficiency.

DOI： 10.1364/jocn.9.000864

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

We propose a novel optical node architecture that fully utilizes M x M wavelength-selective switches (WSSs), where the subsystem modular architecture is adopted and the M x M WSS ports are optimally allocated to inter-node fibers and add/drop fibers in accordance with the traffic distribution. The proposed architecture can accommodate the maximum incoming (outgoing) fiber number of 60 in USNET and 50 in the pan-European network where fiber-utilization efficiency offset is less than 3% compared with the completely unrestricted node. The number of necessary M x M WSSs is reduced by up to 26% compared with the previously proposed scheme.

DOI： 10.1109/lpt.2017.2707474

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Optica Publishing Group  

To create reliable high-capacity photonic networks, we propose a novel optical cross-connect (OXC) architecture that offers failure resiliency and port scalability, simultaneously. The proposed OXC employs the subsystem-modular structure to attain the port scalability, where the use of 1xM wavelength selective switches (WSSs) or MxM WSSs is considered. Furthermore, by introducing an intra-node protection mechanism suited to each OXC architecture, our proposed scheme offers high reliability while retaining the port scalability. Through computer simulations, we evaluate the total number of WSSs needed in a network and the annual path downtime due to WSS failures. The proposed OXC architecture can drastically decrease the annual path downtime with just a small number of WSSs. (C) 2017 Optical Society of America

DOI： 10.1364/oe.25.017982

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Optica Publishing Group  

A novel compact OXC node architecture that combines WSSs and arrays of small scale optical delivery-coupling type switches ("DCSWs") is proposed. Unlike conventional OXC nodes, the WSSs are only responsible for dynamic path bundling ("flexible waveband") while the small scale optical switches route bundled path groups. A network design algorithm that is aware of the routing scheme is also proposed, and numerical experiments elucidate that the necessary number of WSSs and amplifiers can be significantly reduced. A prototype of the proposed OXC is also developed using monolithic arrayed DCSWs. Transmission experiments on the prototype verify the proposal's technical feasibility. (C) 2017 Optical Society of America

DOI： 10.1364/oe.25.015838

Other Link： https://www.osapublishing.org/viewmedia.cfm?URI=oe-25-14-15838&seq=0

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Optica Publishing Group  

A new optical cross-connect (OXC) configuration that yields hitless port-count expansion to meet traffic growth at minimum cost is presented. We introduce a control algorithm that uses a simple criterion for scaling up OXC port count. The scheme exploits the difference between node degree and fiber degree. Numerical experiments verify that our node configuration offers almost complete routing capability even though it uses substantially fewer wavelength-selective switches of small degree.

DOI： 10.1364/jocn.9.00a263

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

We develop a design method for creating a large-scale optical switch consisting of two sub-switch parts, i.e., delivery-and-coupling switches and wavelength-routing switches based on cyclic arrayed-waveguide gratings, where the available optical-power budget is optimally allocated to the two sub-switches to maximize the entire switch scale. The power budget necessary for each sub-switch is quantitatively evaluated via extensive computer simulations. The simulation results show that a 1000 x 1000 switch scale can be realized when the available optical-power budget is 23 dB. The simulation results are verified via proof-of-concept experiments.

DOI： 10.1109/jphot.2016.2635384

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Dynamic control of coarse/fine granular routing optical networks is proposed. The routing scheme exploits virtual direct links, which enhances fiber frequency utilization and eliminates the need to control intermediate nodes. its effectiveness is numerically verified.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We propose a novel scheme to compensate the spectrum narrowing caused by optical nodes. Simulations show that our combination of jointly optimized pre- and post-compensation filters and inline power compensation can drastically enlarge the maximum node-hop count and transmission distance.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We present a wavelength-tunable band-pass filter composed of multi-stage cascaded asymmetric Mach-Zehnder interferometers. The fabricated seven and eight-stage filters exhibit full tunability in the C-band, and 3 dB on-chip loss, and 0.27 nm (33 GHz), 0.14 nm (18 GHz) bandwidth respectively.

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

This paper presents a fast and large-scale optical circuit- switch architecture for intra-datacenter applications that uses a combination of space switches and wavelength-routing switches are utilized. A 1,440 × 1,440 optical switch is designed with a fast-Tunable laser, 8 × 8 delivery-And-coupling switch, and a 180 × 180 wavelength-routing switch. We test the bit-error-ratio characteristics of all ports of the wavelength-routing switch using 180-wavelength 10-Gbps signals in the full C-band. The worst switching time, 498 microseconds, is confirmed and all bit-error ratios are acceptable.

DOI： 10.1587/transcom.2017obi0002

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We demonstrate a highly scalable subsystem-modular OXC, where a newly developed single-module 6x6 WSS is utilized as a sub-OXC. Network simulations in dynamic-traffic scenarios assure its good applicability. Its technical feasibility is confirmed by transmission experiments using 32-Gbaud DP-QPSK signals.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We propose a highly reliable and large-scale OXC architecture that consists of MxM WSSs. The proposed scheme can drastically reduce the annual downtime of optical paths stemming from WSS failures while retaining excellent cost-effectiveness.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

Ever-increasing intra-datacenter traffic will spur the introduction of high-baud rates and high- order modulation formats. Increasing symbol rates and modulation levels decreases tolerance against transmission impairment that includes chromatic dispersion. Transmission distance in warehouse-scale datacenterscan be several kilometers, and then management of chromatic dispersion is necessary. Dispersion-compensating fibers are widely deployed in backbone networks, however, applying them in datacenters is not cost-effective since wavelength channels are coarsely multiplexed. In digital coherent systems, signal distortion due to chromatic dispersion can be resolved in digital domain; however, it will take long time before coherent systems can be introduced in datacenter networks because of their high cost. In this paper, we propose a novel impairment mitigation method employing machine learning. The proposed method is effective even after non-coherent detection and hence it can be applied to cost-sensitive intra-datacenter networks. The machine learns optimum symbol-decision criteria from a sequence of dispersed training signals, and it discriminates payload signals in accordance with the established decision criteria. With the scheme, the received signals can be demodulated in the presence of large chromatic dispersion. The transmission distance thus can be extended without relying on costly optical dispersion compensation. Since information of transmission links is not a priori required, the proposed scheme can easily be applied to any datacenter network. We conduct transmission experiments using 400-Gbps channels each of which comprises 8-subcarrier 28-Gbaud 4-ary pulse-amplitude-modulation (PAM-4) signals, and confirm the effectiveness of the proposed scheme.

DOI： 10.1117/12.2253456

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We fabricate an optical tunable filter for gridless signal drop in ROADMs. Its effectiveness is confirmed by experiments on 10-Gbaud intensity-modulated signals with 50-GHz spacing and 32-Gbaud dual-polarization QPSK and 16QAM signals with 33.3-GHz spacing.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

We propose introducing aligned frequency assignment to each bandwidth channel, i.e., the semi-flexible grid, with the goal of minimizing frequency slot fragmentation under dynamic flexible-grid network expansion considering the expected need for channel capacity upgrades. In semi-flexible grid networks, for each set of channels having the same frequency bandwidth, a regular grid is defined where its spacing is the same as the required bandwidth and channels are aligned to their corresponding grids. A network expansion algorithm is developed that maximizes the efficiency of semi-flexible grid assignment to achieve efficient channel bandwidth upgrading. Numerical experiments prove that the number of fibers necessary and the degree of fragmentation in the frequency domain are reduced by 15% and 80%, respectively, compared to conventional flexible grid networks accommodating the same traffic.

DOI： 10.1117/12.2250489

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We propose a novel network architecture that enables intra- and inter-datacenter converged flow management. By adopting our proposed single large-scale optical circuit switch, intra- and inter-datacenter traffic can be transported without any blocking.

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE  

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

In this paper, we propose a machine-learning-based novel dynamic performance control method for routers that supports several performance levels. The method utilizes support vector machine (SVM) to determine performance level change points. We utilize a traffic normalization technique with a corresponding performance threshold that allows us to apply the same SVM to different traffic volumes. This technique shortens the learning process for control. Several experiments on real Internet traffic data sequences prove that the method yields higher energy efficiency than conventional methods, that is, our previously proposed frequency decomposition-based method and a conventional traffic prediction method based on auto-regressive moving average model with optimal parameter values given by Akaike's information criterion. We also evaluate the impact of several key parameters such as traffic measurement interval and the number of packet processing engines. The results clarify the proper ranges of parameter values that attain significant power reductions while keeping packet loss to acceptable levels.

DOI： 10.1109/tnsm.2016.2605640

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Optica Publishing Group  

We propose a highly scalable and compact optical-node architecture that combines the subsystem-modular optical cross-connect (OXC) and transponder-bank add/drop, where multiple M x M wavelength-selective switches are effectively introduced. Numerical experiments verify that the proposed architecture offers large-scale nodes with substantially relaxed hardware requirements while holding the fiber-utilization offset to a minimal level.

DOI： 10.1364/jocn.9.000a18

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

We propose a novel optical switch architecture for intra-datacenter interconnection. The proposed switch consists of asymmetric-port-count delivery-and-coupling (DC) switches and wavelength-routing (WR) switches based on arrayed-waveguide gratings (AWGs). To compensate for the insertion loss of the DC switch part and to suppress the passband-frequency deviation inherent in multiple-input-port AWGs, we employ a combination of an aggregation coupler, an erbium-doped fiber amplifier, and a single-input-port AWG for the WR-switch part. Moreover, we can reduce complicated fiber connections between the DC switches and the WR switches by making their port counts asymmetric. To confirm feasibility of the proposed switch architecture, we fabricate a 24 x 4 DC switch and a 16 x 8 DC switch with planar-lightwave-circuit technologies and evaluate the overall switching system. We construct part of 768 x 768 optical switch and part of 1536 x 1536 optical switch with the fabricated DC switches. Their excellent transmission characteristics are experimentally confirmed.

DOI： 10.1109/jphot.2016.2529842

Other Link： http://xplorestaging.ieee.org/ielx7/4563994/7416022/7406687.pdf?arnumber=7406687

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

We propose a novel and highly scalable optical-node architecture utilizing MxN asymmetric-multiport wavelength-selective switches (WSSs), within which we implement subsystem-modular optical cross-connects (OXCs) and a transponder-bank-based add/ drop part. Although the proposed add/drop part has a bank-based structure, it can realize flexible routing through the routing capability of the MxN WSSs. Numerical experiments verify that the proposed node architecture offers almost the same routing performance as the equivalent fully flexible node. Moreover, the proposed configuration attains substantial hardware reductions in terms of the number of WSS components in the OXC part, as well as that of optical amplifiers in the add/drop parts.

DOI： 10.1109/jphot.2016.2547838

Other Link： http://xplorestaging.ieee.org/ielx7/4563994/7416022/7442532.pdf?arnumber=7442532

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Optica Publishing Group  

In order to mitigate the signal spectrum narrowing caused by optical filtering at nodes, an adequate guard band is needed between optical channels, which degrades the frequency utilization of optical fibers. In this study, we propose a grouped routing based network architecture that minimizes spectrum narrowing while greatly improving spectral efficiency. Coarse granular routing at GRE (grouped routing entity) level is employed at each ROADM node, but fine granular add/drop is adopted to retain high frequency utilization. Optical channels are packed densely in each GRE, and sufficient guard bands are inserted between GREs. As a result, signal spectrum narrowing is minimized and efficient spectrum utilization is achieved. Network design/control algorithms that support both static and dynamic traffic growth are developed. Extensive simulations demonstrate the effectiveness of the proposed architecture. To implement the scheme, current LCOS-based ROADMs are applied without any hardware changes; only the control schema are modified. (C) 2016 Optical Society of America

DOI： 10.1364/oe.24.006213

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

We propose novel large-scale optical-switch architectures that utilize delivery and coupling (DC) switches and wavelength routing (WR) switches based on the cyclic arrayed-waveguide gratings (AWGs). To suppress the passband-frequency deviation inherent in cyclic AWGs, we propose two-stage WR structures in which multiple small-scale cyclic AWGs are cascaded to create a large-scale WR switch. We introduce two configurations for constructing a WR switch; the degrees of the first-and second stage cyclic AWGs are coprime or not. We fabricate both types of optical switches with planar lightwave circuit technologies. A 270 x 270 switch comprised of coprime cyclic AWGs and a 180 x 180 switch comprised of cyclic AWGs whose degrees have a common divisor are compactly fabricated. Transmission experiments confirm their excellent performance.

DOI： 10.1109/jlt.2015.2489221

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

\A new OXC configuration that yields hitless port-count expansion to meet traffic growth at minimum cost is presented. Numerical experiments verify the node configuration offers almost complete routing capability even though it uses small-degree WSSs.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We propose a novel optical switch configuration that utilizes erbium-doped fiber amplifiers (EDFAs) and asymmetric-port-count cyclic arrayed-waveguide gratings (AWGs). By optimizing the number of utilized input ports of the cyclic AWGs, our proposed scheme can reduce total switch loss and hence relax requirements for EDFAs in terms of gain and maximum output power. To verify effectiveness of the proposed scheme, we fabricate a 90x90 cyclic AWG using planar-lightwave-circuit technology and construct part of a 720x720 optical switch. Good transmission characteristics are confirmed via bit-error-ratio measurements.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

We propose a novel optical network architecture that uses waveband virtual links, each of which can carry several optical paths, to directly bridge distant node pairs. Future photonic networks should not only transparently cover extended areas but also expand fiber capacity. However, the traversal of many ROADM nodes impairs the optical signal due to spectrum narrowing. To suppress the degradation, the bandwidth of guard bands needs to be increased, which degrades fiber frequency utilization. Waveband granular switching allows us to apply broader pass-band filtering at ROADMs and to insert sufficient guard bands between wavebands with minimum frequency utilization offset. The scheme resolves the severe spectrum narrowing effect. Moreover, the guard band between optical channels in a waveband can be minimized, which increases the number of paths that can be accommodated per fiber. In the network, wavelength path granular routing is done without utilizing waveband virtual links, and it still suffers from spectrum narrowing. A novel network design algorithm that can bound the spectrum narrowing effect by limiting the number of hops (traversed nodes that need wavelength path level routing) is proposed in this paper. This algorithm dynamically changes the waveband virtual link configuration according to the traffic distribution variation, where optical paths that need many node hops are effectively carried by virtual links. Numerical experiments demonstrate that the number of necessary fibers is reduced by 23% compared with conventional optical path networks.

DOI： 10.1117/12.2209466

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Optimum symbol-decision boundaries created using machine learning approaches such as support-vector machine (SVM) can enhance system performance in the presence of nonlinear signal distortion. However, they fail if the received training signals for boundary creation include cycle slips induced by laser phase noise. In this paper, we propose a novel decision-boundary generation algorithm that is tolerant to cycle slips. Our proposed scheme groups rotationally symmetric constellation symbols and detects cycle slips by monitoring the phase differences between symbols in the same group. Through numerical analysis, we confirm that our proposed decision-boundary creation technique is immune to cycle slips.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We propose a fast and large-scale optical circuit-switch architecture for intra-datacenter applications. A 1,440x1, 440 optical switch is designed and tested with 180-wavelength signals in the full C band; the worst switching time is 498 microseconds.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

Intra-datacenter traffic is growing more than 20% a year. In typical datacenters, many racks/pods including servers are interconnected via multi-tier electrical switches. The electrical switches necessitate power-consuming optical-toelectrical (OE) and electrical-to-optical (EO) conversion, the power consumption of which increases with traffic. To overcome this problem, optical switches that eliminate costly OE and EO conversion and enable low power consumption switching are being investigated. There are two major requirements for the optical switch. First, it must have a high port count to construct reduced tier intra-datacenter networks. Second, switching speed must be short enough that most of the traffic load can be offloaded from electrical switches. Among various optical switches, we focus on those based on arrayed-waveguide gratings (AWGs), since the AWG is a passive device with minimal power consumption. We previously proposed a high-port-count optical switch architecture that utilizes tunable lasers, route-and-combine switches, and wavelength-routing switches comprised of couplers, erbium-doped fiber amplifiers (EDFAs), and AWGs. We employed conventional external cavity lasers whose wavelength-tuning speed was slower than 100 ms. In this paper, we demonstrate a large-scale optical switch that offers fast wavelength routing. We construct a 720x720 optical switch using recently developed lasers whose wavelength-tuning period is below 460 mu s. We evaluate the switching time via bit-error-ratio measurements and achieve 470-mu s switching time (includes 10-mu s guard time to handle EDFA surge). To the best of our knowledge, this is the first demonstration of such a large-scale optical switch with practical switching time.

DOI： 10.1117/12.2211998

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We investigate the effectiveness of coarse and fine granular hybrid optical-path routing networks implemented with virtual direct links. Numerical and experimental evaluations show that our network control scheme can substantially improve the product of spectral efficiency and hop count compared to present single-granular networks.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We proposed a novel elastic optical network architecture employing Grouped Routing that enhances frequency utilization. With a developed network design algorithm, necessary fiber number was further reduced +20% compared with conventional elastic optical path network.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We fabricated and characterized a polarization-independent tunable band-pass filter, composed of a four-stage Si-wire asymmetric Mach-Zehnder interferometer. The filter exhibited full tunability in the C-band, 2.2 nm bandwidth, and 2 dB on-chip loss.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

An effective solution to the continuous Internet traffic expansion is to offload traffic to lower layers such as the L2 or L1 optical layers. One possible approach is to introduce dynamic optical path operations such as adaptive establishment/tear down according to traffic variation. Path operations cannot be done instantaneously; hence, traffic prediction is essential. Conventional prediction techniques need optimal parameter values to be determined in advance by averaging long-term variations from the past. However, this does not allow adaptation to the ever-changing short-term variations expected to be common in future networks. In this paper, we propose a real-time optical path control method based on a machine-learning technique involving support vector machines (SVMs). A SVM learns the most recent traffic characteristics, and so enables better adaptation to temporal traffic variations than conventional techniques. The difficulty lies in determining how to minimize the time gap between optical path operation and buffer management at the originating points of those paths. The gap makes the required learning data set enormous and the learning process costly. To resolve the problem, we propose the adoption of multiple SVMs running in parallel, trained with non-overlapping subsets of the original data set. The maximum value of the outputs of these SVMs will be the estimated number of necessary paths. Numerical experiments prove that our proposed method outperforms a conventional prediction method, the autoregressive moving average method with optimal parameter values determined by Akaike's information criterion, and reduces the packet-loss ratio by up to 98%.

DOI： 10.1117/12.2209463

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Intra-datacenter traffic is growing more than 20% a year. In typical datacenters, a large number of top-of-rack switches are interconnected via multi-tier electrical switches. The electrical switches necessitate power-consuming optical-to-electrical (OE) and electrical-to-optical (EO) conversion. In addition, the power consumption increases with the growing traffic. To overcome this problem, optical switches that eliminate costly OE and EO conversion and enable low-power-consumption switching are being investigated. The most important requirement for the optical switch is scalability. It must have a high port count cost-effectively so as to construct reduced tierintra-datacenter networks since flatter network simplifies control complexity and reduces latency. Among various optical switches, we focus on a combination of delivery-and-coupling switches and wavelength-routing (WR) switches since the total switch scale can be given by a product of both switch scales. We have been working on WR switches based on tunable lasers and arrayed- waveguide gratings (AWGs). The AWG-based wavelength routing can offer low- power switching since AWG is a passive device. The AWG-based switch can be classified into two types: Either cyclic AWG or non-cyclic AWG is utilized. The former scheme utilizes multiple small-scale cyclic AWGs to construct a large-scale WR switch. Although passband-frequency deviation of cyclic AWGs is unavoidable, we can effectively suppress it thanks to the use of small-scale cyclic AWGs. The latter one exploits non-cyclic and single-input-port AWG to perfectly avoid frequency deviation. To compensate the shortage of the number of the input ports, a coupler for wavelength-signal aggregation and an optical amplifier for coupler-loss compensation are introduced. Thanks to the introduction of optical amplifier, we can also compensate the loss of the coupler in the DC-switch part, which enables us to enlarge the DC-switch scale. In this paper, we report optical-switch configurations which are recently developed and implemented by our group and discuss their features in terms of scalability.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We present a novel dynamic optical path control algorithm for Grouped Routing that can greatly enhance fiber frequency utilization. Careful grooming is shown to effectively mitigate the routing penalty even with dynamic optical path control.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

To offset the impairment caused by imperfect optical filtering at ROADMs/OXCs, guard bands must be inserted between optical channels. The resulting degradation in frequency utilization detracts from the benefit of next-generation elastic optical path networks. To overcome this problem and achieve an ICT infrastructure with the required level of resiliency, we add path-granularity-level shared protection to our coarse granular optical routing networks with fine granular add/drop. Optical paths are densely packed in coarse granular optical pipes, but the filtering impairment generated by the routing process is greatly mitigated, which results in enhanced fiber frequency utilization. The impairment caused by path dropping is carefully minimized for all possible backup paths, which is made possible by our newly developed network design algorithm. Numerical experiments show that the number of fibers in a network is substantially reduced over conventional shared protected networks.

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

A novel resilient coarse granularity optical routing network architecture that adopts finely granular protection and finely granular add/drop is presented. The routing scheme defines optical pipes such that multiple optical paths can be carried by each pipe and can be dropped or added at any node on the route of a pipe. The routing scheme also makes it possible to enhance frequency utilization within pipes, by denser path packing in the frequency domain, as we recently verified. We develop a static network design algorithm that simultaneously realizes the independence of working and backup paths and pipe location optimization to efficiently carry these paths. The design algorithm first sequentially accommodates optical paths into the network, then tries to eliminate sparsely utilized fibers and iteratively optimizes frequency slot/wavelength assignment in each coarse granular pipe so as to limit the impairment caused by dropping the optical paths adjacent in the frequency domain. Numerical experiments elucidate that the number of fibers in a network can be reduced by up to 20070 for 400Gbps channels without any modification in hardware.

DOI： 10.1587/transcom.2015ebp3482

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We provide a guideline on the size of the MxM WSS needed to create future scalable OXCs. Analyses show that 10x10 WSSs can create an OXC that is scalable to 40x40 with marginal routing-performance offset.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We experimentally demonstrate a 2,160x2,160 optical circuit switch with a switching time of less than 500 mu s, aiming at fast and non-hierarchical intra-datacenter interconnection. The switch comprises delivery-and-coupling switches and wavelength-routing switches based on AWGs. We implemented twin 1x135 AWGs on a monolithic PLC chip.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Hardware scale analysis shows that flexible waveband routing OXCs can substantially reduce the numbers of WSSs and EDFAs. Transmission experiments on developed prototype that adopts monolithic arrayed DC type matrix switches verify their technical feasibility.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We propose highly reliable and cost-effective large-scale OXC architecture. By placing protection WSSs in each subsystem, the proposed OXC can substantially improve the total system reliability cost-effectively. The annual failure time of a network can be greatly reduced with a small number of additional WSSs.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Institute of Electrical and Electronics Engineers Inc.  

We propose efficient optical networking technology employing virtual direct links that offer coarse/fine hybrid granular routing. We compare the effectiveness of the proposal to different networking technologies that employ coarse granular routing.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Institute of Electrical and Electronics Engineers Inc.  

We propose MxM WSS based ROADM architecture that employs main/junction subsystem OXCs and OXC-assisted transponder-bank add/drop. Extensive computer simulations verify that the proposed architecture offers high routing performance regardless of the physical network topology costeffectively.

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Optica Publishing Group  

We propose a cost-effective and scalable OXC/ROADM that consists of a subsystem-modular express switch part and a transponderbank- based add/drop part. The effectiveness of the proposed architecture is verified via a hardware scale evaluation, network performance simulations, and transmission experiments. The architecture enables large throughput and offers significant hardware-scale reductions with marginal fiber-utilization penalty against the conventional architectures. A part of the OXC/ROADM designed to accommodate 35x35 express fiber ports and 2,800 transponders for add/drop is constructed. Its net throughput reaches 314 Tbps using 80 channels of 120-Gbps signal (30-Gbaud dual-polarization quadrature phase-shift-keying signals with 7% overhead are assumed). (C) 2015 Optical Society of America

DOI： 10.1364/oe.23.014796

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

The wavelength-tunable filter is a key device for creating colorless, directionless, and contentionless reconfigurable optical add-drop multiplexers. Although arrayed-waveguide gratings (AWGs) can realize compact and cost-effective tunable filters, crosstalk in wavelength-division-multiplexing systems increases as the frequency grid spacing decreases. To resolve this problem, we propose a tunable-filter architecture that can eliminate the crosstalk by utilizing two specific (commeasurable) cyclic AWGs in conjunction with multiple switches based on symmetric Mach-Zehnder interferometers and an asymmetric Mach-Zehnder interferometer (AMZI) filter. To verify the technical feasibility of the proposed architecture, we fabricate a monolithic prototype using planar-lightwave- circuit technology. We confirm, for the first time, its excellent performance by measuring insertion loss, polarization-dependent loss, and bit error ratios (BERs). Alternative tunable-filter configurations employing multiple AMZI-based filters are newly investigated for future applications.

DOI： 10.1109/jphot.2015.2400397

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

We propose a novel optical path routing mechanism that combines coarse-granularity optical multicast with fine-granularity add/drop & block. We implement the proposal in an optical cross-connect node with broadcast-and-select functionality that offers high cost-effectiveness since no addition equipment from conventional ROADMs is needed. The proposed method, called branching, enhances the routing capabilities over the original grouped routing networks by enabling wavelength paths to be established through different GRE pipes. We also present a novel path/GRE routing and wavelength/GRE index assignment algorithm that supports the new routing function. Numerical experiments using real network topologies verify the improved routing performance and the superior efficiency of the proposed control algorithm over original GRE-based networks.

DOI： 10.1117/12.2078561

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We propose a novel optical-switch architecture that can be compactly implemented with novel functional configuration. A highly-integrated 180x180 optical switch prototype is fabricated using PLC technologies and its good performance is experimentally confirmed.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We demonstrate novel large-scale and cost-effective ROADM architecture exploiting the subsystem-modular express switches and tailored transponder banks for add/drop functionality. Simulations show that the proposed ROADM offers virtual-C/D/C performance. Good transmission characteristics are experimentally confirmed.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We introduce an asymmetric-port-count DC switch that can simplify fiber connection arrangement in a large-scale optical switch. A 24x4 DC switch is monolithically implemented with PLC technologies and its good performance is experimentally confirmed.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We propose a novel control algorithm for Grouped Routing networks that utilize a metric for residual capacity estimation. Numerical experiments under growing traffic scenarios confirm that it can maximize the fiber frequency utilization.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

A novel resilient optical routing network architecture that adopts finely granular protection and finely granular add/drop is presented. The routing scheme defines optical pipes such that multiple optical paths can be carried by each pipe and can be dropped or added at any node on the route of a pipe. The routing scheme also makes it possible to enhance frequency utilization within pipes as we recently verified. We develop a static network design algorithm that establishes coarse granular optical pipes so that working and backup optical paths are efficiently carried by these pipes. Numerical experiments elucidate that the number of fibers in a network can be reduced up to 20% for 400Gbps channels without any modification of hardware.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

The wide deployment of ROADM based photonic networks, particularly in metro areas, compels an increase in the number of WSSs traversed by the average optical path. The impairment caused by optical filtering of WSSs, called spectrum narrowing effect, can be a serious problem in transparent optical networks. To resolve this impairment and to enable higher fiber frequency utilization while achieving resiliency against a failure, we propose a coarse granular routing optical network architecture which adopts shared protection and its design algorithm that considers the impairment and applies an iterative relocation of bundles of paths. Proposed shared protection scheme successfully resolves the impairment caused by spectrum narrowing, and as a result, much dense optical path accommodation within fibers become possible. Numerical experiments shows that the spectral utilization efficiency can be substantially improved and hence needed number of fibers can be reduced over conventional path granular routing networks with path level shared protection.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We investigate the performances of Grouped Routing which greatly mitigates the filter passband narrowing effect. Numerical experiments demonstrate that the optimal modulation format and channel spacing, considering the impairment by filtering effect and fiber transmission, can reduce the necessary number of fibers by up to 21.9%.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

The multi-granular optical-path architecture is a viable approach to creating large-scale optical metro and core networks. We conduct transmission and distributed signaling experiments that emulate the proposed architecture and confirm its technical feasibility.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We propose a highly scalable and compact ROADM architecture that uses multiple MxN WSSs as subsystems. The effectiveness of our scheme is verified via a hardware-cost assessment, routing-performance simulations, and transmission experiments for the first time.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

A novel optical multicast tree design algorithm is proposed for elastic optical path networks; it considers signal regeneration and modulation format adaptation. The algorithm minimizes not only the number of costly regenerators needed, but also the total frequency slots necessary by optimizing the modulation formats applied. By formulating the problem as two-stage integer linear programming on an auxiliary graph, the proposed algorithm successfully calculates optimal trees on large networks within several tens of seconds.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We develop a novel network control algorithm that enhances fiber spectral efficiency and scalability of OXCs. Numerical experiments demonstrate that the spectral efficiency is improved more than 20% and necessity WSSs is reduced by 80%.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Hierarchical optical networks will be a viable solution in creating large-scale networks of the future. The networks control optical network resources in granularity of both wavelength and waveband that consists of a fixed number of wavelengths; thereby, they can reduce signalling processing load and efficiently create virtual private networks. Meanwhile, as the network scale increases, multi-stage optical switch architecture is effective in reducing hardware scale and cost. In this paper, we develop a distributed wavelength path control plane for large-degree, hierarchical optical networks that consist of nodes made by multi-stage wavelength selective switches (WSSs) and waveband selective switches (WBSSs). Along with the wavelength path signalling, optical switch control functions configure WBSSs as well as WSSs. Accordingly, we need to develop an optical switch control mechanism so that end-to-end wavelength path signalling can correctly configure both WSSs and WBSSs. As experimental results, we show that wavelength paths are successfully established and released.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

With the continuous increase in Internet traffic, reconfigurable optical add-drop multiplexers (ROADMs) have been widely adopted in the core and metro core networks. Current ROADMs, however, allow only static operation. To realize future dynamic optical-network services, and to minimize any human intervention in network operation, the optical signal add/drop part should have colorless/directionless/contentionless (C/D/C) capabilities. This is possible with matrix switches or a combination of splitter-switches and optical tunable filters. The scale of the matrix switch increases with the square of the number of supported channels, and hence, the matrix-switch-based architecture is not suitable for creating future large-scale ROADMs. In contrast, the numbers of splitter ports, switches, and tunable filters increase linearly with the number of supported channels, and hence the tunable-filter-based architecture will support all future traffic. So far, we have succeeded in fabricating a compact tunable filter that consists of multi-stage cyclic arrayed-waveguide gratings (AWGs) and switches by using planar-lightwave-circuit (PLC) technologies. However, this multi-stage configuration suffers from large insertion loss and filter narrowing. Moreover, power-consuming temperature control is necessary since it is difficult to make cyclic AWGs athermal. We propose here novel tunable-filter architecture that sandwiches a single-stage non-cyclic athermal AWG having flatter-topped passbands between small-scale switches. With this configuration, the optical tunable filter attains low insertion loss, large passband bandwidths, low power consumption, compactness, and high cost-effectiveness. A prototype is monolithically fabricated with PLC technologies and its excellent performance is experimentally confirmed utilizing 80-channel 30-GBaud dual-polarization quadrature phase-shift-keying (QPSK) signals.

DOI： 10.1117/12.2078522

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

We propose a novel method of in-band estimation of optical signal-to-noise ratio (OSNR) using a digital coherent receiver, where OSNR is determined from second-and fourth-order statistical moments of equalized signals in any modulation format. Our proposed method is especially important in recently-developed Nyquist wavelength-division multiplexed (WDM) systems and/or reconfigurable optical-add/drop-multiplexed (ROADM) networks, because in these systems and networks, we cannot apply the conventional OSNR estimation method based on optical-spectrum measurements of the in-band signal and the out-of-band noise. Effectiveness of the proposed method is validated with computer simulations of Nyquist-WDM systems and ROADM networks using 25-Gbaud quadrature phase-shift keying (QPSK) and 16 quadrature-amplitude modulation (16-QAM) formats.

DOI： 10.1109/jphot.2014.2304557

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We fabricate a subsystem modular 64x64 OXC with C/D/C add/drop capabilities. Its throughput reaches 204.8 Tbps at the channel speed of 40 Gbps. Transmission experiments verify the performance of the prototype.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We propose a novel AWG-based wavelength tunable filter architecture that utilizes symmetric and asymmetric Mach-Zehnder interferometers for switching and filtering functions. A prototype is fabricated as a PLC and its good performance is experimentally confirmed.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We propose a novel tunable-filter configuration for optical C/D/C drop in ROADMs, which achieves low insertion loss and low power consumption. A prototype is monolithically fabricated with PLC technologies and its good filtering performance is experimentally confirmed by BER measurement.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Optical path grouped routing not only simplifies the node's architecture, but also mitigates the signal degradation due to optical filtering at each node. It can improve frequency utilization of fibers or mitigate WSS requirements.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We propose a novel method of OSNR estimation in Nyquist-WDM transmission systems based on the measurement of statistical moments of equalized signals in the digital coherent receiver. Its effectiveness is verified with computer simulations.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We investigate the maximum transmission distance of the multilevel-modulation formats for ultra-dense gridless photonic networks based on distance-adaptive modulation. Simulation results show the relationships among the transmission distance, the guard band, and the optical-filter design.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

The emergence of digital coherent optical transmission technologies is being eagerly awaited by the world. This enables us to develop spectrally-efficient transmission systems by means of polarization-division multiplexing and multilevel-modulation formats such as quadrature-phase-shift keying (QPSK) and higher-order quadrature-amplitude modulation (QAM). Thanks to recent rapid advances in the research and development of electronics, demodulation of such signals can be realized effectively by utilizing sophisticated digital signal processors (DSPs). Such digital coherent technologies have successfully been implemented in commercial systems. However, the transmission performance of photonic networks is limited by system impairments that include crosstalk and spectrum narrowing caused at reconfigurable optical add/drop multiplexers (ROADMs) and the nonlinearity of optical fibers. Current digital coherent technologies do not resolve these problems comprehensively necessitating further research. In this paper, we investigate the impacts of the system impairments through intensive computer simulations and show the maximum transmission distances of multilevel-modulation signals. Various transmission schemes for gridless networks including Nyquist wavelength-division-multiplexing (WDM) networks, which need digital coherent technologies, are evaluated. We also discuss DSP algorithms that suit photonic networks and permit digital coherent technologies to become more effective in realizing future networks.

DOI： 10.1117/12.2038657

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Optical Society of America (OSA)  

This paper elucidates the optimum power-budget allocation for large-scale optical switches based on cyclic AWGs. Simulation results show that a 1,000x1,000 switch scale can be attained with 23-dB optical-power margin. © 2014 OSA.

DOI： 10.1364/ps.2014.jt5c.3

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We demonstrate that fiber capacity can be substantially increased by utilizing grouped optical path routing in a traffic growth scenario. The developed algorithm is shown to substantially mitigate filter pass band narrowing at ROADMs.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We propose a novel optical-switch configuration for intra-datacenter interconnection that consists of tunable lasers, non-cyclic AWGs, and combinations of small-size optical switches and couplers. We develop an 800x800 switch prototype and its good performance and scalability are verified by experiments.

Language：English   Publishing type：Research paper (other academic)  

Publishing type：Research paper (international conference proceedings)  

We propose a novel method of fiber-nonlinearity mitigation for ultra-long optical transmission systems, which employs two polarization modes mutually phase-conjugated. Simulations and experiments show that in the nonlinear region, this diversity method significantly outperforms both of the single- and dual-polarization schemes at the same bit rate.

DOI： 10.1049/cp.2013.1447

Publishing type：Research paper (international conference proceedings)  

We propose a novel wavelength-demultiplexing algorithm applicable to Nyquist WDM systems. The proposed scheme features the ability of perfect wavelength demultiplexing even under the frequency offset half as large as the symbol rate. Through simulations and experiments, effectiveness of our scheme is demonstrated in 16-QAM Nyquist WDM systems.

DOI： 10.1049/cp.2013.1313

Language：English   Publishing type：Research paper (international conference proceedings)  

We propose a novel Kerr-effect compensator in digital coherent receivers, in which optimized single split-steps are aligned in parallel in the frequency domain. Computer simulations show that our scheme can drastically elongate the transmission distance. © 2012 IEEE.

DOI： 10.1109/OECC.2012.6276512

Other Link： http://orcid.org/0000-0002-6115-1406

Publishing type：Research paper (international conference proceedings)   Publisher：OSA  

DOI： 10.1364/ofc.2012.oth4c.4

Language：English   Publishing type：Research paper (international conference proceedings)  

We propose a novel carrier-phase-insensitive FIR filtering scheme in digital coherent receivers using the decision-directed LMS algorithm. Our proposed scheme is more tolerant to the phase noise and the frequency offset than the conventional one. © 2011 National Sun Yat-Sen Univ.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：OPTICAL SOC AMERICA  

We measure bit-error rates of polarization-multiplexed Nyquist-WDM QPSK signals, which have rectangular spectral shapes and channel spacing equal to the symbol rate. The spectral efficiency of 4 bit/s/Hz is achieved with negligible power penalty.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：OPTICAL SOC AMERICA  

We propose a straightforward algorithm to estimate second-order PMD from adaptive FIR-filter tap coefficients in a digital coherent optical receiver. The novel algorithm is verified with 100-Gbit/s dual-polarization QPSK experiments.

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Optica Publishing Group  

We propose a novel and unified algorithm that estimates linear impairments in optical transmission systems from tap coefficients of an adaptive finite-impulse response (FIR) filter in a coherent optical receiver. Measurable impairments include chromatic dispersion (CD), differential group delay (DGD) between two principal states of polarization, second-order polarization-mode dispersion (second-order PMD), and polarization-dependent loss (PDL). We validate our multi-impairment monitoring algorithm by dual-polarization quadrature phase-shift keying (QPSK) transmission experiments. (C)2010 Optical Society of America

DOI： 10.1364/oe.18.026929

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We propose a novel digital coherent receiver, where frequency subbands of an ultrafast optical signal are detected in parallel and synthesized with DSP. A proof-of-concept experiment performing two-subband decomposition and reconstruction for PSK signals proves effectiveness of the proposed scheme.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEICE-INST ELECTRONICS INFORMATION & COMMUNICATION ENGINEERS  

Constant modulus algorithm (CMA) with the training mode is demonstrated for accurate polarization demultiplexing and equalization in a digital coherent optical receiver. Proper polarization demultiplexing with the proposed method is experimentally verified.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We propose a simple and precise algorithm to monitor multiple impairments such as CD, PMD, and PDL, which are jointly mitigated in the adaptive equalizer of a digital coherent optical receiver. We validate our algorithm by dual-polarization QPSK transmission experiments.

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

The digital coherent receiver, which is a combination of a phase-diversity optical homodyne receiver and digital signal processing (DSP), can demodulate any multilevel coded optical Signals without relying upon an optical phase-locked loop. However, the maximum symbol rate processed by such a receiver is limited by the speed of electric analog-to-digital converters and digital signal processors. Although real-time operation at 10 Gsymbol/s using an application-specific integrated circuit has recently been demonstrated, it is still difficult to increase the symbol rate beyond 40 Gsymbol/s. In order to cope with this difficulty, we propose a novel scheme, which employs a local oscillator (LO) pulsed at the subharmonic frequency of the symbol rate, enabling time-division demultiplexing of the signal at the digital coherent receiver. We demonstrate that the new type of digital coherent receiver operating at 10 Gsymbol/s can demodulate the aggregate symbol rate of 160 Gsymbol/s. From these results, we can expect ultrafast co-herent optical fiber communication in the future.

DOI： 10.1109/jlt.2008.2010333

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：VDE VERLAG GMBH  

We demonstrate transmission of a 100-Gbit/s 32-QAM dual-polarization signal over a 200-km dispersion-managed fiber link using a digital coherent receiver. The receiver sensitivity of -20 dBm at BER = 10(-3) is obtained after 200-km transmission.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

A 1.28-Tbit/s signal on a single carrier is generated by the combination of 16 time-division multiplexing, 16-QAM, and polarization multiplexing. Such a signal is demodulated with a digital coherent receiver having the time-division demultiplexing function. (C) 2009 Optical Society of America

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Phase-noise tolerance of 10-Gsymbol/s 16-QAM signals is investigated both theoretically and experimentally. Optimizing the decision-directed phase-locked loop for carrier-phase estimation, we can obtain acceptable BER performance when the laser linewidth is in the 100-kHz range. (C) 2009 Optical Society of America

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Optical Society of America  

Using a digital coherent receiver, we demodulate a 640-Gbit/s QPSK signal polarization-multiplexed and 16-fold time-division-multiplexed. In this receiver, an LO pulsed at the 10-GHz base-clock frequency demultiplexes the 160-Gsymbol/s QPSK signal into 10-Gsymbol/s tributaries. © 2008 Optical Society of America.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We propose a novel method of carrier-phase estimation for digital coherent optical receivers, which is based on the decision-feedback loop and the adaptive-equalization algorithm. 10-Gsymbol/s QPSK and 16-QAM signals are demodulated by the proposed method.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：OPTICAL SOC AMERICA  

We demodulate a 320-Gbit/s optical quadrature phase-shift keying (QPSK) signal using a novel digital coherent receiver, which features a function of time-division demultiplexing with a local oscillator pulsed at the 10-GHz base-clock frequency. (C)2007 Optical Society of America.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We demodulate a 480-Gbit/s 8PSK signal with a digital coherent receiver having time-division demultiplexing function. The receiver sensitivity is -20 dBm at BER of 10(-5).

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We demonstrate 100-km transmission of a 40-Gbit/s 16-QAM signal using a digital coherent receiver. The receiver sensitivity of -30 dBm is obtained at BER=10(-5).

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We demonstrate unrepeated 200-km transmission of 40-Gbit/s 16-QAM signals. In spite of the launched power limitation due to SPM, the acceptable BER performance is obtained owing to high sensitivity of a digital coherent optical receiver.