Reconfigurable Optical Add-drop Multiplexers Research Articles

Routing and Regenerator Planning in a Carrier’s Core Reconfigurable Optical Network

The introduction of colorless-directionless reconfigurable-optical-add-drop multiplexers (ROADMs) brings new capabilities to the physical network, including bandwidth pre-positioning, bandwidth-on-demand, optical layer mesh restoration, and wavelength defragmentation. In order to utilize these new capabilities, carriers need to pre-deploy transponders at endpoints as well as regenerators at intermediate locations when needed. Optimizing routing and regenerator planning in a carrier's core ROADM network provides significant savings. In this paper, we describe our algorithms to minimize the total number of regenerator sites and the corresponding number of regenerators needed at each chosen site. We consider both bandwidth-on-demand wavelength services and multilayer restoration needs to provide the upper layer with more efficient restoration. Portions of this work were presented at the Optical Fiber Communications Conference and Exhibition (OFC), March 2017.

Emergency optical network planning with multi-vendor interconnection and portable EDFAs

In this paper, we introduce an emergency optical network design problem for the low-cost post-disaster recovery of core optical transport networks. We take into account both the interconnection of the surviving multi-vendor resources and the utilization of portable emergency erbium-doped fiber amplifiers (EDFAs). The surviving multi-vendor optical nodes (e.g., reconfigurable optical add/drop multiplexer (ROADM) or wavelength cross-connect (WXC) nodes) and fiber links are the available resources during a disaster recovery and should be efficiently utilized first. The portable EDFAs are emergency resources added to replace the optical nodes that have either been destroyed or are down because of power outages. To solve this design problem, we propose an emergency network planning method using an integer linear programming (ILP) formulation such that the locations for the emergency interconnection of the multi-vendor networks and placement of the portable EDFAs are optimally selected. Evaluations show the benefits of the multi-vendor interconnection approach and utilization of emergency portable EDFAs.

Cost-Effective DWDM ROADM Design for Flexible Sustainable Optical Metro–Access Networks

Architectural changes are required for the underlying networks to support the expected Internet data traffic volume growth caused by the popularization of cloud services, 5G-based services, and social networks, to provide a highly dynamic connectivity. Cost-effective and energy-efficient solutions for flexible network subsystems are required to provide future sustainable networks. In this paper, we present a cost-effective dense wavelength division multiplexing (DWDM) reconfigurable optical add/drop multiplexer (ROADM) design enabling optical metro–access networks convergence. The cost-effective DWDM ROADM capabilities also have been assessed in an ultradense wavelength multiplexing (u-DWDM) ring network scenario. In particular, achievable network throughput has been considered.

Evaluation of Core-Continuity-Constrained ROADMs for Flex-Grid/MCF Optical Networks

To effectively keep pace with the global IP traffic growth forecasted in the years to come, flex-grid over multi-core fiber (MCF) networks can bring superior spectrum utilization flexibility, as well as bandwidth scalability far beyond the non-linear Shannon's limit. In such a network scenario, however, full node switching re-configurability will require enormous node complexity, pushing the limits of current optical device technologies with prohibitive capital expenditures. Therefore, cost-effective node solutions will most probably be the key enablers of flex-grid/MCF networks, at least in the short- and mid-term future. In this context, this paper proposes a cost-effective reconfigurable optical add/drop multiplexer (ROADM) architecture for flex-grid/MCF networks, called CCC-ROADM, which reduces technological requirements (and associated costs) in exchange for demanding core continuity along the end-to-end communication. To assess the performance of the proposed CCC-ROADM in comparison with a fully flexible ROADM (i.e., a fully non-blocking ROADM, called FNB-ROADM in this work) in large-scale network scenarios, a novel lightweight heuristic to solve the route, modulation, core, and spectrum assignment problem in flex-grid/MCF networks is presented in this work, whose goodness is successfully validated against optimal ILP formulations previously proposed for the same goal. The obtained numerical results in a significant number of representative network topologies with different MCF configurations of 7, 12, and 19 cores show almost identical network performance in terms of maximum network throughput when deploying CCC-ROADMs versus FNB-ROADMs, while decreasing network capital expenditures to a large extent.

OSNR Penalty-Free Add/Drop Performance of DSP-Enabled ROADMs in Coherent Systems

The newly proposed digital-signal-processing (DSP)-enabled reconfigurable optical add/drop multiplexer (ROADM) without incorporating optical-electrical-optical conversions, referred to as a soft-ROADM, performs dynamic, flexible, and transparent add/drop operations at wavelength, sub-wavelength, and spectrally overlapped orthogonal sub-band levels. The cost-effective soft-ROADMs offer vital software-defined-networking-based networking functionalities for seamlessly integrating optical and mobile networks to realize elastic cloud access networks. In this paper, detailed theoretical and numerical investigations are undertaken of soft-ROADM add/drop operation performance in coherent systems. It is shown that the soft-ROADM add/drop operation performance is independent of both signal spectral location and fiber transmission distance and also shows excellent robustness to variations under system operating conditions. More importantly, under practically achievable system conditions, the soft-ROADM add/drop operations introduce negligible optical signal-to-noise ratio (OSNR) penalties. Moreover, numerical explorations of the impacts of key physical aspects associated with both the soft-ROADMs and coherent systems are also conducted, based on which optimum soft-ROADM design parameters are identified.

On-chip reconfigurable optical add-drop multiplexer for hybrid wavelength/mode-division-multiplexing systems.

A silicon-based on-chip reconfigurable optical add-drop multiplexer (ROADM) is presented for hybrid wavelength-division-multiplexing-mode-division-multiplexing systems. The present ROADM consists of a four-channel mode demultiplexer, four wavelength-selective thermo-optic switches based on microring resonators, and a four-channel mode multiplexer. With the present ROADM, one can add/drop one of wavelength channels of any mode to/from the multimode bus waveguide successfully with an excess loss of 2-5dB and an extinction ratio of ∼20 dB over a wavelength range of 1525-1555nm.

SDN control of optical nodes in metro networks for high capacity inter-datacentre links

Worldwide demand for bandwidth has been growing fast for some years and continues to do so. To cover this, mega datacentres need scalable connectivity to provide rich connectivity to handle the heavy traffic across them. Therefore, hardware infrastructures must be able to play different roles according to service and traffic requirements. In this context, software defined networking (SDN) decouples the network control and forwarding functions enabling the network control to become directly programmable and the underlying infrastructure to be abstracted for applications and network services. In addition, elastic optical networking (EON) technologies enable efficient spectrum utilization by allocating variable bandwidth to each user according to their actual needs. In particular, flexible transponders and reconfigurable optical add/drop multiplexers (ROADMs) are key elements since they can offer degrees of freedom to self adapt accordingly. Thus, it is crucial to design control methods in order to optimize the hardware utilization and offer high reconfigurability, flexibility and adaptability. In this paper, we propose and analyze, using a simulation framework, a method of capacity maximization through optical power profile manipulation for inter datacentre links that use existing metropolitan optical networks by exploiting the global network view afforded by SDN. Results show that manipulating the loss profiles of the ROADMs in the metro-network can yield optical signal-to-noise ratio (OSNR) improvements up to 10 dB leading to an increase in 112% in total capacity.

Real-Time ROADM Filtering Penalty Characterization and Generalized Precompensation for Flexible Grid Networks

Concatenated filtering is an important impairment in high-spectral density transmission systems enabled by flexible-grid reconfigurable optical add-drop multiplexers (ROADMs). In this paper, an analytical model for wavelength selective switches (WSS) modules is used to determine the statistical distribution of experimentally measured filter shapes and to obtain the cascaded filter characteristics. A generalized approach for filtering penalty assessment is proposed and verified using a commercial 200G 16-quadrature amplitude modulation (16QAM) transponder and WSS modules. A practical method for filter penalty reduction is described, whereby a single, optimized digital pre-equalization filter is used to compensate filtering effects of multiple concatenated WSSs. This pre-equalization of cascaded ROADM filters is demonstrated with a 100G quadrature phase shift keying (QPSK) C-form factor pluggable (CFP) transponder operating on the 37.5-GHz grid, resulting in a doubling of the number of WSSs, which can be supported per link from three to six.

Low-Cost CDC ROADM Architecture Based on Stacked Wavelength Selective Switches

A highly flexible, stacked, switch module is proposed, wherein multiple independent 1 × N wavelength selective switches (WSSs) can be realized on a single 4k liquid crystal on silicon device. The stacked WSS module can be configured in different ways for application at either the transit side or the add/drop side of a colorless, directionless, and contentionless (CDC) reconfigurable optical add/drop multiplexer (ROADM). TwoROADMarchitectures are proposed based on the stacked WSS modules. Their costs are analyzed for both a 4-deg network node and a larger 8-deg node. The first proposed ROADM architecture with full CDC features is shown to realize a cost reduction of at least 35% in these two test network nodes, when compared with the conventional CDC ROADMarchitecture based on the standalone WSSs and multicasting switches (MCSs). The second ROADM architecture proposed has a small probability of wavelength contention, which could be prevented by a local wavelength assignment algorithm. According to our cost estimation, we are able to aggressively reduce the number of components at the add/drop side and make an overall cost reduction of >70% and >80% in the 4- and 8-deg network nodes, respectively.

Software-Defined Silicon-Photonics-Based Metro Node for Spatial and Wavelength Superchannel Switching

Due to the growing popularity of optical superchannels and software-defined networking, reconfigurable optical add-drop multiplexer (ROADM) architectures for superchannel switching have recently attracted significant attention. ROADMs based on micro-electro-mechanical system (MEMS) and liquid crystal-on-silicon (LCoS) technologies are predominantly used. Motivated by requirements for low power, high-speed, small area footprint, and compact switching solutions, we propose and demonstrate spatial and wavelength flexible superchannel switching using monolithically integrated silicon photonics (SiP) micro-ring resonators (MRRs). We demonstrate the MRRs’ capabilities and potential to be used as a fundamental building block in ROADMs. Unicast and multicast switching operation of an entire superchannel is demonstrated after transmission over 50 km of standard single mode fiber. The performance of each sub-channel from the 120 Gb∕s QPSK Nyquist superchannel is analyzed, and degradation in error vector magnitude performance was observed for outer sub-channels due to the 3 dB bandwidth of the MRRs, which is comparable with the superchannel bandwidth. However, all sub-channels for all switching cases (unicast, multicast, and bi-directional operation) exhibit performance far below the 7%FEClimit. The switching time of the SiPMRRchip is such that high-capacity superchannel interconnects between users can be set up and reconfigured on the microsecond time scale.

Space division multiplexing optical communication using few-mode fibers

Abstract To realize ultra-high capacity long-haul transmission, space-division multiplexing (SDM) has emerged as a promising solution, in which each data channel is modulated into an individual spatial/polarization modes in few-mode fibers (FMF) to increase the overall number of parallel channels. In this paper, we review the latest advances in SDM technology on the FMF, component, digital signal processing (DSP), as well as transmission demonstrations. First, we introduce the FMF characteristics, fabrication and manufacturing issues including modal dispersion, mode coupling, and nonlinearities. We next discuss in detail several key SDM components such as spatial multiplexers/demultiplexers (MUX/DeMUX), optical amplifiers, mode converters and SDM reconfigurable optical add-drop multiplexer (ROADM). Accordingly, we explore the DSP algorithms for SDM systems, covering least mean squares (LMS), recursive least squares (RLS), hardware complexity analysis, and mode dependent effects. Besides, a number of recent experimental validations are evaluated enabling higher transmission capacity for short, medium and long distances.

Optical back propagation for fiber optic networks with hybrid EDFA Raman amplification.

We have investigated an optical back propagation (OBP) method to compensate for propagation impairments in fiber optic networks with lumped Erbium doped fiber amplifier (EDFA) and/or distributed Raman amplification. An OBP module consists of an optical phase conjugator (OPC), optical amplifiers and dispersion varying fibers (DVFs). We derived a semi-analytical expression that calculates the dispersion profile of DVF. The OBP module acts as a nonlinear filter that fully compensates for the nonlinear distortions due to signal propagation in a transmission fiber, and is applicable for fiber optic networks with reconfigurable optical add-drop multiplexers (ROADMs). We studied a wavelength division multiplexing (WDM) network with 3000 km transmission distance and 64-quadrature amplitude modulation (QAM) modulation. OBP brings 5.8 dB, 5.9 dB and 6.1 dB Q-factor gains over linear compensation for systems with full EDFA amplification, hybrid EDFA/Raman amplification, and full Raman amplification, respectively. In contrast, digital back propagation (DBP) or OPC-only systems provide only 0.8 ~ 1.5 dB Q-factor gains.

Application of Waveguide/Free-Space Optics Hybrid to ROADM Device

As optical networks have evolved from point-to-point systems to ring or mesh networks, the optical devices that are needed to construct optical nodes have become more important and need to be more scalable. Hybridization of waveguide and free-space optics, or spatial and planar optical circuits (SPOCs), may provide the solutions for such needs. An SPOC platform is attractive because it can take advantage of both waveguide technology and free-space optics. Waveguide technology provides a high degree of integration of optical functionality for devices such as splitters and non-wavelength selective switches while free-space optics supplies a high degree of parallelism with two-dimensional spatial light modulators such as a liquid crystal on silicon (LCOS) devices. In this paper, we summarize the basics of SPOC technology and review its application to reconfigurable optical add-drop multiplexing (ROADM) devices. The key elements of a waveguide on an SPOC platform are an arrayed-waveguide grating and a spatial beam transformer. The latter functions as a microlens array and provides attractive features such as dense integration of switches. An LCOS device has numerous phase modulating pixels, enabling flexible manipulation of lightwaves. Using an SPOC platform, we constructed and demonstrated devices for ROADM applications including a wavelength filter, tunable optical dispersion compensators, and wavelength selective switches (WSSs). The WSSs range from an ultrahigh port count WSS to a single module wavelength cross connect.

Performance Analysis of WSS-Based ROADM With Shared-Per-Group Tunable Wavelength Converters

We propose a flexible shared-per-group (SPG) architecture for wavelength selective switch (WSS)-based reconfigurable optical add/drop multiplexers (ROADMs) to support dynamic datacenter wavelength-convertible networks. The key components of this module are grouping installed tunable wavelength converter (TWCs) and tunable filter (TFs). Using the SPG architecture, the demands that collide among different outputs will be all-optically resolved by a combination of TWCs and TFs without installing any switch matrix devices. Thus, the proposed SPG architecture may produce better blocking performance than the conventional shared-per-link, shared-per-wavelength architectures consuming lower costs. Meanwhile, analytical model is proposed to evaluate the SPG blocking probability by means of the self-constrained iteration method. Accurate results showed that SPG architecture requires the number of groups larger than the number of ports to produce the optimal blocking performance, saving 50% of TWCs under high and moderate traffic loads.

A Learning Living Network With Open ROADMs

We propose a “living network” as a key enabler for open reconfigurable optical add/drop multiplexer (ROADM) networks that, unlike today's use of static planning tools, adapts to varying network conditions, and allows network operation close to actual performance. We experimentally demonstrate the living network in various network conditions that autonomously keeps record of its path-level performance. The more services are added, the more accurate the performance of a newly to be established service is estimated through a learning process, which shows the feasibility of living network with an allocated margin of only 1.2 dB.

基于空分复用的可重构光分插复用器

基于空分复用的可重构光分插复用器

Polarization insensitive silicon photonic ROADM with selectable communication direction for radio access networks.

We report on an experimental prototype of a low-cost silicon photonic reconfigurable optical add/drop multiplexer (ROADM). The device is able to operate with up to 12 wavelength division multiplexing channels. In order to control the polarization of the multi-wavelength signal at the input of the device, an integrated polarization controller is investigated as an alternative to the polarization diversity device architecture. The integrated ROADM is equipped with optical switches for the selection of the path direction and variable optical attenuators for optical power control. We demonstrate the polarization insensitive routing of 10 Gb/s channels between two ROADM nodes with error-free transmission.

Monolithically integrated reconfigurable add-drop multiplexer for mode-division-multiplexing systems.

An integrated reconfigurable optical add-drop multiplexer (ROADM) for mode-division-multiplexing systems is proposed and demonstrated for the first time, to the best of our knowledge. The present ROADM with four mode-channels is composed of a four-channel mode demultiplexer, four identical 2×2 thermo-optic Mach-Zehnder switches (MZSs), and a four-channel mode multiplexer, which are integrated monolithically on silicon. All the devices are designed for operation with TM polarization. The ROADM can add/drop any one of the mode channels freely by thermally turning on/off the corresponding MZS. For the added/dropped mode-channels, the excess loss is 1-5 dB, and the extinction ratio is 15-20 dB in the wavelength range of 1535-1565 nm.

Optical Switches Based on Silicon Photonics for ROADM Application

We demonstrate compact and low-loss 8 × 8 silicon photonic switch modules, which are applicable to transponder aggregators (TPAs) in colorless, directionless, and contentionless reconfigurable optical add-drop multiplexers. Newly designed silicon optical switch chips incorporating spot size converters with polarization insensitive and wavelength insensitive properties over C/L bands are packaged. The developed module shows about 6-dB average excess optical loss, including optical coupling loss, on all 64 optical paths with low-polarization-dependent loss and low crosstalk. Using these compact optical switch modules, we construct a TPA prototype featuring over 100-port optical switch subsystem densely mounted on one board and confirm its feasibility.

Optical back propagation for compensating nonlinear impairments in fiber optic links with ROADMs.

An optical back propagation (OBP) technique is investigated to compensate for nonlinear impairments in fiber optic communication systems with reconfigurable optical add-drop multiplexers (ROADMs). An OBP module consisting of an optical phase conjugator (OPC), amplifiers and dispersion-decreasing fibers (DDFs) fully compensates for the nonlinear impairments of a transmission fiber. The OBP module can be placed after each transmission fiber (inline OBP case) or at each network node (node OBP case). For a wavelength division multiplexing (WDM) system with 2400 km transmission distance and 32-quadrature amplitude modulation (QAM) format, inline OBP and node OBP bring Q-factor improvements of 4.9 dB and 5.6 dB as compared with linear compensation, respectively. In contrast, receiver-side digital back propagation (DBP) only provides 1.3 dB Q-factor gain, due to its incapability of mitigating inter-channel nonlinear effects in fiber optic networks.