Optical Cross-connects Research Articles

Optimizing Spatial Channel Networks (SCNs) in Hierarchical Optical Cross-Connect (HOXC) architectures: Impact of wavelength switching granularity on performance

The increasing demand for network bandwidth highlights the critical need to enhance optical transmission systems. Utilizing the entire C-band for a single optical channel(OCh) eliminates the requirement for wavelength switching, prompting the emergence of spatial channel networks (SCNs). SCNs transform the optical layer into hierarchical spatial division multiplexing (SDM) and wavelength division multiplexing (WDM) layers through the implementation of hierarchical optical cross-connects (HOXCs). A HOXC comprises a spatial channel cross-connect (SXC) for spatial bypass switching and multiple wavelength cross-connects (WXCs) for wavelength channel switching, ensuring efficient optical transmission. Therefore, the design of the HOXC plays a pivotal role in determining both the device cost and network performance in SCNs. This study investigates the impact of wavelength switching granularity on core-selective switch (CSS)-based HOXC architectures. We propose an adaptive dynamic Routing, Spatial Channel, and Spectrum Assignment (RSCSA) algorithm to solve routing and resource allocation problems in SCNs. By examining SCN designs with varying wavelength switching granularities, we assess the overall network deployment cost, considering both network throughput and spectrum resource utilization. Our findings indicate that adjusting the granularity of wavelength switching in SCNs can significantly affect device costs and performance, highlighting the importance of identifying an optimal SCN design that strikes a balance between these factors. These insights offer valuable guidance for the practical planning and management of future SCN deployments.

NEW INSIGHTS ON SURVIVABILITY IN MULTIPLE LINK AND NODE FAILURES OF OPTICAL NETWORKS

Survivability becomes increasingly critical in managing high-speed networks as data traffic continues to grow in both size and importance. In addition, the impact of failures is exacerbated by the higher data rates available in optical networks. The purpose of this study is to optimize the survivability approach to address the problem of capacity efficiency and fast recovery time of multiple link and node failures in one technique. Most research works done had only addressed either of these constraints in a single failure. This research intends to develop and implement a new integrated approach called Multi-Suv system model to address multiple failures in links and nodes of an optical network. Experiments was conducted to demonstrate the ability of the system model to identify the nature of failure based on the intensity of importance as defined in the fundamental scale of absolute numbers (Saaty, 1980); thus, inform on the path of restoration or protection to address the link and node failures. Following experimental simulations done on the model, the results shows that there are benefits amongst are reduced network resource usage, speedy recovery time, guaranteed availability and quality of reliability. The socio-economic value of this research will reduce the CAPEX capital investment of network infrastructures and facilities and OPEX the operational costs of service delivery of the ICT industry thereby increasing the profitability of the network and service providers. 
 Keywords: WDM (Wavelength Division Multiplex), OXADM (Optical cross Add & Drop Multiplexer), Intelli-MUX (Intelligent Multiplexer), DeMux (De-multiplexer, OPSS (Optical Switching System), OPXC (Optical Cross Connect), CAPEX (Capital expenditures), OPEX (Operations expenditures, OPM (Opinion Performance Matrix).

Reconfigurable photonic integrated transmitter for metro-access networks

A reconfigurable photonic integrated transmitter, enabling dynamic resource allocation in the metro-access network, is proposed. The device consists of a multicarrier sliceable bandwidth variable transmitter (MC-SBVT) realized in indium phosphide and a silicon-nitride-based optical cross-connect (OXC). The proposed architecture delivers full flexibility in terms of the choice of data format/bandwidth, channel spacing, and wavelength assignment. The functional design of the MC-SBVT and OXC as well as their practical realization are discussed. Preliminary characterization results of the photonic-integrated-circuit-based MC-SBVT, demonstrating the reconfigurability of the device, are also presented.

Topology configuration scheme for accelerating coflows in a hyper-FleX-LION

To address the challenges that data center networks (DCNs) are currently facing, optical circuit switching in DCNs has been introduced and a number of architectures to improve the performance of DCNs in terms of port capacity, energy efficiency, and data transfer latency have been proposed. This led to a few all-optical interconnects (AOIs), among which Hyper-FleX-LION possesses the reconfigurability that can efficiently support various traffic patterns. In this work, we study the topology management of AOIs in Hyper-FleX-LION to efficiently schedule coflows in them so the average coflow completion time (CCT) can be minimized. We first propose time-efficient algorithms to address the provisioning of a single coflow and then extend the algorithms for multi-coflow scenarios. The simulation results demonstrate that AOIs in Hyper-FleX-LION can accelerate coflows better than the traditional AOIs based on optical cross-connects and achieve an acceleration of up to 5.6 × under the assumed situations. Our simulations also verify that, for coflow scheduling in Hyper-FleX-LION, our proposal can reduce the average CCT and significantly outperform benchmarks.

Femtosecond Laser Direct Writing of Optical Overpass.

With the rapid increase in information density, problems such as signal crosstalk and crossover restrict the further expansion of chip integration levels and packaging density. Based on this, a novel waveguide structure—photonic jumper wire—is proposed here to break through the technical restrictions in waveguide crossing and parallel line wrapping, which hinder the integration of photonic chips. Furthermore, we fabricated the optical overpass to realize a more complex on-chip optical cross-connection. Our method and structure promote a series of practical schemes for improving optical chip integration.

On the Bilevel Optimization for Remapping Virtual Networks in an HOE-DCN

Hybrid optical/electrical datacenter network (HOE-DCN) uses the inter-rack networks that consist of both electrical Ethernet switches and optical cross-connects (OXCs), for better cost-efficiency and scalability. Meanwhile, to provision dynamic network services well, the operator of an HOE-DCN needs to deploy virtual networks (VNTs) and remap them adaptively. Therefore, this work studies the problem of VNT remapping in an HOE-DCN from a novel perspective, i.e., the remapping schemes should be optimized for not only the network status after the remapping but also the transition to realize it. Specifically, we model this problem as a bilevel optimization, where the upper-level optimization aims at selecting proper virtual machines (VMs) to migrate such that the estimated latency of VM migration can be minimized, and the lower-level optimization determines the actual scheme of VNT remapping for minimizing the number of resource hot-spots. We first formulate a bilevel mixed integer linear programming (BMILP) model for the bilevel optimization, and then propose a polynomial time algorithm based on enumeration to solve it approximately. Extensive simulations verify the effectiveness of our proposal.

SNR Re-Verification-Based Routing, Band, Modulation, and Spectrum Assignment in Hybrid C-C+L Optical Networks

Band division multiplexing technology is a promising medium-term transitional solution to deal with the traffic flood, by leveraging the existing optical fiber transmission infrastructure. Additionally, the deployment of multi-band optical networks requires updating and upgrading key components, such as multi-band amplifiers and multi-band optical cross-connects. From cost view, it would be a promising way for operators to expand transmission capacity on some of the links with high priority, which are used heavily. Therefore, the application scenario of this paper is positioned in the C-C+L hybrid optical networks. In this scenario, the introduction of the L-band brings additional inter-band stimulated Raman scattering (ISRS) which is a non-negligible nonlinear effect, leading new constraint for service provisioning. This paper proposes an ISRS-aware routing, band, modulation, and spectrum assignment (RBMSA) scheme, considering the impact of ISRS generated by new requests on existing requests. By implementing the SNR re-verification, not only the transmission quality of the current request can be realized, but also the service quality of the existing requests will not be seriously degraded to be unacceptable. A two-dimensional model of request bandwidth and duration is established for band selection. Priority is provided for arranging the requests in the L-band when it is available. For each single request, when the least number of slots required by adopting different modulation formats is the same, lower-order modulation is preferred to improve its SNR tolerance of this request under the ISRS impact. Simulation results indicate that the proposed scheme can effectively decrease the blocking probability, and improve the channel SNR for the successfully established requests.

Scheduling Virtual Network Reconfigurations in Parallel in Hybrid Optical/Electrical Datacenter Networks

A hybrid optical/electrical datacenter network (HOE-DCN) integrates the benefits of electrical packet switching (EPS) and optical circuit switching (OCS) for better architectural scalability. Meanwhile, as each network service usually involves multiple virtual machines (VMs) that form a virtual network (VNT), how to reconfigure the VNTs in an HOE-DCN to adapt to the dynamic network environment becomes an interesting and important problem to tackle. In this work, we optimize the procedure of VNT reconfigurations that remap VNTs to given virtual network embedding (VNE) schemes, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i.e.</i> , scheduling the required VM migrations and optical cross-connect (OXC) reconfiguration properly such that the overall VNT reconfiguration latency can be minimized. We first assume that all the VM migrations should be scheduled before the OXC reconfiguration to minimize service interruptions, and formulate a mixed integer linear programming (MILP) model to solve the scheduling problem exactly. Then, with the same assumption, we propose a time-efficient heuristic to schedule parallel VM migrations in batches such that the bandwidth competition can be significantly relieved. Finally, we divide the OXC reconfiguration into several progressive steps, and design a joint scheduling algorithm to arrange VM migrations together with the OXC reconfiguration steps.

Performance of Dynamic Optical Path Networks with Large-scale WBSS-based Optical Cross-connects

In this paper, we have proposed a generalized large-scale optical cross-connect (OXC) architecture utilizing waveband selective switches (WBSS) for realizing future cost-effective, bandwidth-abundant and flexible optical networks. The developed architecture implements multiple WBSSs for each incoming fiber and small size wavelength selective switches (WSSs) for dropping optical paths while simply deploying 1´2 WSSs or 1´2 optical couplers for realizing the adding function. Thanks to the use of WBSSs, which are more cost-effective and simpler devices, the developed architecture enables a significant hardware scale reduction. The WBSS-based OXC, however, suffers from a limited routing capability, which relies on the inner node parameter (i.e., the WBSS number per input fiber) and the waveband granularity of WBSSs. We, therefore, evaluate the hardware scale requirement of our developed architecture in comparison with that of conventional WSS-based OXC. It is verified that a substantial hardware scale reduction can be achieved by using the proposed architecture, especially for high port count OXCs or when applying coarser granular WBSSs. Moreover, we also assess the performance of dynamic optical networks based on the proposed OXC. Numerical simulations show that the network offers a substantial necessary hardware scale reduction at the cost of a small performance offset comparing to that of the network using conventional WSS-based OXC.

Technoeconomic analysis of spatial channel networks (SCNs): benefits from spatial bypass and spectral grooming [Invited

Toward the forthcoming spatial division multiplexing (SDM) era, a spatial channel network (SCN) was recently proposed in which the optical layer is explicitly decoupled from the hierarchical SDM and wavelength division multiplexing (WDM) layers, and an optical node evolves into a spatial cross-connect (SXC) and wavelength cross-connect (WXC) to achieve a hierarchical optical cross-connect (HOXC). In this paper, we perform a technoeconomic analysis of SCNs comprising different HOXC architectures employing different planning policies on how the degree of spectral grooming by WXCs and spatial bypassing by SXCs in an SCN affects the total required number of spatial lanes (SLs), whose physical entity is the core in parallel single-mode fibers or a multicore fiber and the total node cost in the SCN. Toward this end, we develop a routing and SDM/WDM multilayer resource assignment (RSWA) heuristic in which spatial bypassing and spectral grooming are performed such that the required number of SLs is minimized. Using RSWA with four planning policies, i.e., express-only, express/local-hybrid (spatial-bypass-oriented), express/local-hybrid (spectral-grooming-oriented), and local-only, we compare the performance levels of a high-port-count matrix-switch-based HOXC and core selective switch (CSS) based HOXC with those of the baseline-stacked conventional WXCs as the network traffic load increases. Here, a CSS is a new type of spatial switch, which is the counterpart to a wavelength selective switch in a current WDM network. We clearly show that hierarchical spatial bypassing and spectral grooming are beneficial in terms of the required number of SLs and network-total node cost when the required number of SLs between optical nodes in an SCN is equal to or greater than roughly 10.

Hierarchical Routing and Resource Assignment in Spatial Channel Networks (SCNs): Oriented Toward the Massive SDM Era

In the past few decades, the architecture of optical networks has undergone significant evolution, from the earliest wavelength-division multiplexing (WDM) optical networks to elastic optical networks (EONs) and later to space-division multiplexing (SDM) EONs, to address the continuous growth of Internet traffic. By 2024, Pbps-level optical networks are expected, far exceeding the capacity limit of single-mode fibers. The massive SDM era is on the horizon. In this context, a newly designed architecture for optical networks called the spatial channel network (SCN) architecture, which achieves high cost efficiency by means of practical hierarchical optical cross-connects, has recently been proposed. However, the evolution of optical network architectures will simultaneously present challenges related to network optimization. For instance, with the evolution from WDM optical networks to EONs, the kernel network optimization problem was transformed from the routing and wavelength assignment (RWA) problem into the routing and spectrum assignment (RSA) problem due to the additionally introduced constraint of spectrum contiguity. Similarly, specially designed algorithms are also expected to be essential for addressing the network optimization problem in SCNs. In this paper, we define this new problem as the routing, spatial channel, and spectrum assignment (RSCSA) problem. We propose an integer linear programming (ILP) model and a heuristic algorithm to solve the RSCSA problem. We examine the performance of the proposed approaches via simulation experiments. The results show that both proposed approaches are effective in finding the optimal solutions or solutions close to the lower bounds. To the best of our knowledge, this is the first work to focus on the network optimization problem in SCNs.

Elliptical-Core Highly Nonlinear Few-Mode Fiber Based OXC for WDM-MDM Networks

In order to realize an optical cross-connect (OXC) converting wavelengths and spatial modes into one-dimensional switching ports, we propose an active mode selective conversion without parasitic wavelength conversion, based on the intermodal four-wave mixing (FWM) arising in a few-mode fiber (FMF). First, we design a dispersion-engineered elliptical-core highly nonlinear FMF (e-HNL-FMF) with a graded refractive index (RI) profile, which can independently guide 3 linearly polarized (LP) spatial modes. Meanwhile, a high doping concentration of germanium in the core leads to relatively high intermodal nonlinear coefficients of 3.23 (W·km) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> between LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> and LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11a</sub> modes and 3.14 (W·km) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> between LP01 and LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11b</sub> modes. Next, we propose an e-HNL-FMF based OXC scheme for wavelength division multiplexing-mode division multiplexing (WDM-MDM) networks. After optimizing both the e-HNL-FMF length and pump power, we can realize either active mode selective conversion over the designated wavelength-band or three-wavelength to three-mode superchannel conversion for 100 Gbaud 16-quadratic-amplitude modulation (16-QAM) signals over the C-band. Due to excellent characteristics of the e-HNL-FMF, both cost and configuration complexity of the OXC can be reduced, showing great potentials for all-optical signal processing in the future WDM-MDM networks.

Machine Learning assisted aggregation schemes for optical cross-connect in hybrid electrical/optical data center networks

Making optical circuit switching suitable for handling highly dynamic and profoundly changing traffic is a considerable challenge. This motivation drives the development of a hybrid electrical/optical network towards high bandwidth and low latency. Compared with the traditional non-aggregation scheme, we provide two machine learning assisted aggregation schemes. The first one is to design optical cross-connect switches to increase the throughput of the circuit-switched network. In this solution, the optical cross-connect serves both delay-sensitive traffic flows and delay-tolerant traffic flows. As the network throughput rises rapidly, the number of ports of the optical switch remains unchanged. The second scheme is to add small port counts, which maximizes throughput while relaxing the requirements for accurate machine learning algorithms. In this paper, we have a set of four machine learning algorithms, and only the most suitable one is selected at a time. We deploy a machine learning algorithm at edge nodes instead of a central network management system. Therefore, we can simultaneously reduce network overhead and latency. Both aggregation schemes outperform the traditional non-aggregation scheme in terms of throughput, delay, and flow completion time.

On Virtual Network Reconfiguration in Hybrid Optical/Electrical Datacenter Networks

Hybrid optical/electrical datacenter networks (HOE-DCNs) build inter-rack networks with both electrical Ethernet switches and optical cross-connects (OXCs), and have been considered as a promising DCN architecture. However, to adapt to the dynamic network environment, the reconfiguration of virtual networks (VNTs) in an HOE-DCN still faces the unique difficulty that the HOE-DCN's topology can change because of the one-to-one connectivity of OXCs. To the best of our knowledge, this problem still has not been fully explored. In this article, we address this problem, and consider how to achieve effective VNT reconfiguration in an HOE-DCN such that the IT resource usages in racks can be re-balanced with the migration of virtual machines (VMs). We first formulate a mixed integer linear programming (MILP) to describe the VNT reconfiguration. Then, we solve the problem with two steps, 1) obtaining the VM migration schemes to balance the loads on racks and 2) determining the reconfiguration schemes of related virtual links (VLs) and the OXC. For the first step, we propose a polynomial-time approximation algorithm by leveraging linear relaxation. Then, we tackle the optimization of the second step by developing an algorithm that involves a linear-time dynamic programming and an integer linear programming (ILP). To solve the ILP time-efficiently, we propose another polynomial-time approximation algorithm based on Lagrangian relaxation. Our simulations confirm the effectiveness of the proposed approximation algorithms and verify that the overall procedure including them outperforms the existing approach.

Dual MAC Based Hierarchical Optical Access Network for Hyperscale Data Centers

A dual media access control (DMAC) based hierarchical optical access network (HOAN) is proposed for data center networks, consisting of passive optical distribution modules (PODMs) at passive optical access layer and top-of-cluster (ToC) switches at electronic access layer. A PODM organizes a group of servers into a computing cluster and forms the intra-cluster channel. Each ToC switch connected with several PODMs forms the inter-cluster channel based on time and wavelength division multiplexing (TWDM). A load-sensing dual-mode (LSDM) protocol is presented for the intra-cluster channel to realize distributed low-load competition and high-load polling operation; the inter-cluster channel applies any polling based TWDM protocol to offer flexible bandwidth allocation. Simulations show that the LSDM can reach 0.95 throughputs at high traffic load, less than 32 μs frame latency, 128 μs frame latency on the 99.9th percentile of the simulated results at 25 Gbps with high priority, verifying the LSDM protocol can realize high throughput, low latency, and the measured latency variation. Compared the performances of DMAC based HOAN with those of passive optical cross-connection network (POXN) and passive optical top-of-rack interconnect (POTORI) architecture, when the normalized intra-rack traffic load reaches 0.8, average frame latencies in one computing cluster are 70.4, 184, and 2480 μs at 10 Gbps, and normalized available bandwidths for inter-cluster channels are 1, 0.2, and 0.2, respectively. Furthermore, the DMAC based HOAN is characterized with the scalability capable of organizing up to 320000 servers with only 500 ToC switches.

A Novel Architecture of N × N Bidirectional Reconfigurable Multiwavelength Optical Cross Connect

AbstractA novel architecture of N×N bidirectional-reconfigurable multiwavelength optical cross connect (B-RMOXC) based on tunable Fiber Bragg grating and optical circulator is proposed. B-RMOXC network is one of crucial network element for wavelength routing in dense wavelength division multiplexing system. This paper presents a high speed, power compensated bidirectional optical cross connect and verified the performance of bidirectionality with 0.8-nm channel spacing having bit rate of 10 Gbps at different transmission distances. Optical signal-to-noise ratio of 36.62 dB and 35.55 dB is achieved with acceptable Quality factor of 6.26 and 6.99 respectively for upstream and downstream at 60 km having input transmission power of −12 dBm. It is found that the data can be communicated bidirectionally to a distance of 60 km in the presence of fiber nonlinearities without optical amplifier.

Spatial Channel Cross-Connect Architectures for Spatial Channel Networks

A spatial channel network (SCN) is a novel optical network architecture targeting the space division multiplexing (SDM) era where the optical layer is explicitly evolved into the hierarchical SDM layer and wavelength division multiplexing layer, and an optical node is decoupled into a spatial channel cross-connect (SXC) and a wavelength cross-connect to form a hierarchical optical cross-connect. In this article, we discuss a wide variety of SXC architectures based on optical matrix switches (MSs) and newly proposed core selective switches (CSSs) from the viewpoints of connection flexibility and architectural complexity. After briefly reviewing the architecture and functionalities of an SCN and its building-block optical devices, novel SXC architectures based on sub-MSs and CSSs are described and formulas are derived for each SXC architecture that give the required number of optical devices, switching mirrors, and internal fiber connections. Based on the formulations, these architectures are compared with traditional SXC architectures based on a high-port count full-MS pair and a Clos network from the viewpoints of the freedom of connection functionalities, operational benefits, and potential node costs associated with the architectural complexity in order to obtain insights regarding which network application is suited to each SXC architecture.

Feasibility Demonstration of Spatial Channel Networking Using SDM/WDM Hierarchical Approach for Peta-b/s Optical Transport

This article reports on a proof of concept demonstration of a recently proposed spatial channel network (SCN) that was conducted over an SCN testbed that comprises low-loss hierarchical optical cross-connect (HOXC) prototypes and four-core multicore fiber links. The HOXC prototypes used in the testbed are based on sub-matrix-switches and core selective switches both implemented with commercially available discrete optical switches. Using these two types of HOXC prototypes, the spatial channel networking including spatial bypassing, spatial add/drop and spectral grooming, spatial-lane change, and spatial-channel (SCh) protection is successfully demonstrated for SChs carrying 100-Gb/s–900-Gb/s optical channels.

Indium Phosphide Membrane Nanophotonic Integrated Circuits on Silicon

Photonic integration in a micrometer‐thick indium phosphide (InP) membrane on silicon (IMOS) offers intrinsic and high‐performance optoelectronic functions together with high‐index‐contrast nanophotonic circuitries. Recently demonstrated devices have shown competitive performances, including high side‐mode‐suppression ratio (SMSR) lasers, ultrafast photodiodes, and significant improvement in critical dimensions. Applications of the IMOS devices and circuits in optical wireless, quantum photonics, and optical cross‐connects have proven their performances and high potential.

Design of Novel Fiber Optical Flexible Routing System

In this paper, we propose a new versatile routing device that utilizes arrayed waveguide gratings (AWGs), optical switches, and optical circulators to implement reconfigurable optical add/drop multiplexers (ROADMs), optical interleavers, and optical cross-connect (OXC). With the development of dense wavelength division multiplexing (DWDM) technology, ROADM and OXC technologies have also been put into practical use. Thus, the optical signal can be routed directly in the optical network according to its wavelength without the need for optical-electrical-optical (OEO) conversion. Although different optical network units (ONUs) have different bandwidth requirements, the use of optical interleavers has successfully solved the connection problem between old and new systems. According to the numerical experiments of static characteristics, the proposed routing device can effectively implement three different functionalities, thereby providing greater flexibility for fiber optic network applications.