Space Division Multiplexing Networks Research Articles

Deep-reinforcement-learning-based RMSCA for space division multiplexing networks with multi-core fibers [Invited Tutorial

The escalating demands for network capacities catalyze the adoption of space division multiplexing (SDM) technologies. With continuous advances in multi-core fiber (MCF) fabrication, MCF-based SDM networks are positioned as a viable and promising solution to achieve higher transmission capacities in multi-dimensional optical networks. However, with the extensive network resources offered by MCF-based SDM networks comes the challenge of traditional routing, modulation, spectrum, and core allocation (RMSCA) methods to achieve appropriate performance. This paper proposes an RMSCA approach based on deep reinforcement learning (DRL) for MCF-based elastic optical networks (MCF-EONs). Within the solution, a novel state representation with essential network information and a fragmentation-aware reward function were designed to direct the agent in learning effective RMSCA policies. Additionally, we adopted a proximal policy optimization algorithm featuring an action mask to enhance the sampling efficiency of the DRL agent and speed up the training process. The performance of the proposed algorithm was evaluated with two different network topologies with varying traffic loads and fibers with different numbers of cores. The results confirmed that the proposed algorithm outperforms the heuristics and the state-of-the-art DRL-based RMSCA algorithm in reducing the service blocking probability by around 83% and 51%, respectively. Moreover, the proposed algorithm can be applied to networks with and without core switching capability and has an inference complexity compatible with real-world deployment requirements.

Heuristic solutions to optimize the traffic routing in space division multiplexing networks

This paper underscores the symbiosis between industrial engineering techniques and optical communication technologies, bringing valuable insights to the optimization of traffic routing in communication networks using constructive heuristics. Exploring and realizing the potential of optical fiber networks has received more attention due to the popularity of various network services and the ensuing increase in overall internet traffic. Space Division Multiplexing (SDM) stands out among the technologies designed to increase the functionality of core and urban mesh/ring networks. SDM's primary goal is to significantly increase transmission system bandwidths by effectively using the spatial dimension. The routing of traffic within flex-grid SDM optical networks has been optimized in this paper using two novel heuristic algorithms. The experimental results show that the proposed algorithms outperform the solutions presented in previous research from the literature.

Field Trial of SDN-Controlled Probabilistic Constellation Shaping Supporting Multiple Rates Over a Coupled-Core Multi-Core Fiber

Space division multiplexing (SDM) is being investigated in support of traffic growth. SDM networks offer multiplexing over space and spectrum dimensions. Probabilistic constellation shaping (PCS) is a solution to optimize spectral efficiency by adapting the constellation of a M-ary quadrature amplitude modulated (QAM) signal to physical layer impairments. At the control plane, in the recent years NETCONF and YANG have been identified as the protocol and the data modeling language, respectively, for the configuration and management of network devices. Significant effort has also been directed to network disaggregation and vendor neutrality: an example is the work done within the OpenConfig consortium. <p xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">In this paper, we present a SDM field trial based on a deployed coupled-core four-core fiber, where a software-defined network (SDN) controller configures probabilistic constellation shaping through NETCONF, by optimizing spectral efficiency either with respect to the path length, or in response to degradations due to soft failures. The NETCONF protocol relies on the OpenConfig YANG data model to configure and manage transceivers. In the field trial the integrated data and control planes are demonstrated with multiple rates (800-850-900-950-1000 Gb/s) for optical reach values ranging from 910 km to 2730 km. Soft failures are also introduced and a characterization of their impact on transmission performance is provided in order to design the recovery strategy deployed by SDN controller. Indeed, depending on the changes in the electrical signal-to-noise ratio, the SDN controller reconfigures the constellation shaping to restore the optical connection affected by the failure with a reduced line rate.

Impact of the Interplay Between Mode Dispersion and Kerr Effect in SDM Optical Networks

Space division multiplexing (SDM) – such as based on multi-mode fibers – is under investigation to support the growing capacity demands. In optical networks, the estimation of quality of transmission (QoT) is a fundamental control task to identify the most suitable transmission parameters (e.g., modulation format) to meet proper spectral efficiency while satisfying the required optical reach. The Gaussian Noise (GN) model is widely adopted for QoT estimation. However, the basic version of the GN model does not account for the interplay between mode dispersion and the Kerr effect. In this paper, we investigate the impact of mode dispersion on SDM networks exploiting strongly coupled modes, through an extended GN model. Network performance analysis is evaluated. First, an analysis of the achievable information rate (AIR) with Gaussian data statistics is presented. Then, a network capacity analysis is shown constrained to a set of supported modulation formats and a fixed symbol rate. Finally, blocking probability is studied with the same set of supported modulation formats with variable symbol rates. The presented network analysis shows that accounting for mode dispersion increases the throughput considering: i) Gaussian data statistics, ii) different modulation formats. Moreover, the blocking probability analysis shows a blocking reduction when mode dispersion is considered in the physical layer modeling.

Heterogeneous optical network incorporating low-loss ring-core fiber and single-mode fibers seamlessly connected by all-fiber vortex (de)multiplexer

We propose and experimentally demonstrate heterogeneous optical network to make the space-division multiplexing (SDM) network with ring-core fiber (RCF) compatible with the legacy optical networks with conventional single-mode fiber (SMF). Two homemade all-fiber mode selective couplers are fabricated by fusing pre-tapered conventional SMF with multi-mode fiber (Type: Corning OM3), serving as orbital angular momentum (OAM) (de)multiplexer to enable seamless connection between heterogeneous optical network incorporating low-loss RCF (loss: 0.26 dB/km) based SDM network and ultra-low-loss SMFs (loss: 0.154 dB/km) based networks. We transmit 30-Gbit/s 8-ary quadrature amplitude modulation signals over two OAM modes (OAM1,1 and OAM2,1) in different mode groups without multiple-input multiple-output equalization techniques through 100-km heterogeneous RCF and SMF network (25-km ultra-low-loss SMF + 50-km low-loss RCF + 25-km ultra-low-loss SMF). The measured optical signal-to-noise ratio penalties for multiplexing transmission are less than 5.73 dB at a bit-error rate of 3.8 × 10−3, indicating that the demonstrated OAM-assisted heterogeneous optical network has a favorable performance.

QoS-Aware routing, space, and spectrum assignment in space division multiplexing networks

Nowadays, Space Division Multiplexing (SDM) is one of the solutions proposed to increase the capacity and optimal allocation of resources in Elastic Optical Networks (EON). The issue of allocating resources in these networks involves the optimal allocation of route, space and spectrum for the establishment of traffic requests. The quality of service (QoS) provided for establishing traffic requests must vary according to their importance and priority, so that high quality requests require guaranteed bandwidth and higher levels of service than low quality requests. Hence, reducing the blocking probability of high-quality data traffic is one of the most important goals in networks. In this paper, three new QoS-aware algorithms are proposed for route, space and spectrum assignment of dynamic traffic in SDM-EONs that reduce the blocking probability of high-quality requests as follows: Reservation without Common Core (RwoC), Reservation with One Common Core (RwC), and Reservation and Recoring with One Common Core (RRwC). The results of utilizing these algorithms and also adding distance adaptive modulation format show that the blocking rate of high-quality requests is decreased and the general spectrum utilization is increased specially in RRwC algorithm. Of course, this decrease causes a slight increase in the blocking rate of low-quality requests. Another advantage of using the RRwC algorithm is that using the slots with the same index in the recoring method reduces costs because in this case there is no need to change the frequency of transceivers. Since, this algorithm is more cost-effective. In the first proposed algorithm, dedicated cores are designed to create high quality traffic. In the second algorithm, in addition to dedicated cores, a common core is also used between high and low-quality traffic. Finally, the recoring method has been used for high quality traffic to further reduce their blocking rates.

Wavelength Selective Devices for SDM Applications Based on SPOC Platform

The increase in communications network traffic has led to a demand for a larger transmission capacity. To meet this demand, the combination of wavelength division multiplexing (WDM) and space-division multiplexing (SDM) is being extensively investigated. Among various SDM systems, that based on multi-core fiber is attractive because network flexibility increases while it maintains the transmission capacity to be enlarged in proportion to the number of SDM channel number. However, problems arise because the number of input&#x002F;output ports increases dramatically in optical nodes. To solve these problems, there is a novel optical configuration technology, the spatial and planar optical circuit (SPOC), which makes the integration of multiple WSSs and filters possible. The SPOC platform has been used to achieve variety of devices such as optical dispersion compensation and WSSs. One of the great advantages of SPOC platform is that we can integrate multiple optics of WSSs in a module, which contribute to miniaturization of devices. In this paper, we review the devices for SDM networks, which are realized with SPOC platform including WSS arrays, wavelength cross-connects (WXCs), multi-channel optical performance monitors, and its related technology.

Design and evaluation of a reconfigurable optical add-drop multiplexer with flexible wave-band routing in SDM networks

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.

On Throughput Optimization in Software-Defined Multi-Dimensional Space Division Multiplexing Optical Networks

The capacity expansion of a single time division multiplexing (TDM) or wavelength division multiplexing (WDM) domain is difficult to cope with long-term capacity pressure. The emergence of space division multiplexing (SDM) technology provides a new dimension for the expansion of optical network capacity, namely the spatial dimension. However, this also poses a challenge to both network control and resource optimization. Therefore, to improve the flexibility of network control, we propose an architecture of software-defined multi-dimensional SDM optical networks based on architecture on demand (AoD) nodes and multi-core fiber (MCF) links in this paper. Next, we mathematically describe the system model including a model of the SDM network, a model of multi-type service requests, and a MCF model. Then, an integer linear programming (ILP) is developed to minimize the index of multi-dimensional resources occupied by multiple types of services in the time, frequency, and space domains. To reduce inter-core crosstalk between homogeneous cores, the core classification algorithm is designed to build multi-granular network layers. Also, for adjacent heterogeneous cores, we utilize the AoD to customize different functions and use different spectrums to transmit different types of services. Finally, we propose routing and multi-dimensional resource allocation algorithms to optimize network throughput. The simulation results show that the proposed algorithms can effectively improve the throughput of the software-defined multi-dimensional SDM optical networks compared with the benchmark algorithms.

TAPI-enabled SDN control for partially disaggregated multi-domain (OLS) and multi-layer (WDM over SDM) optical networks [Invited

Network operators are facing a critical issue on their optical transport networks to deploy 5G + and Internet of Things services. They need to address the capacity increase by a factor of 10, while keeping a similar cost per user. Over the past years, network operators have been working on the optical disaggregated approach with great interest for achieving the required efficiency and cost reduction. In particular, partially disaggregated optical networks make it possible to decouple the transponders from the transport system (known as an open line system) that are provided by different vendors. On the other hand, space division multiplexing (SDM) has been proposed as the key technology to overcome the capacity crunch that the optical standard single-mode fibers are facing to support the forecasted 10 × growth. Spatial core switching is gaining interest because it makes it possible to deploy SDM networks to bypass the overloaded wavelength division multiplexing (WDM) networks, by provisioning spatial media channels between WDM nodes. This paper presents, to the best of our knowledge, the first experimental demonstration of transport-application-programming-interface-enabled software defined networking control architecture for partially disaggregated multi-domain and multi-layer (WDM over SDM) optical networks.

Evaluation of optical MIMO transmissions through multicore fiber links with an optical switch

Multiple-input and multiple-output (MIMO) signal processing technique has been introduced to eliminate the interference caused by crosstalk along Space Division Multiplexing (SDM) links. In optical networks, optical paths are set between end-to-end nodes via several switches. Optical switches connect light signals using appropriate switching rules. Consequently, different combinations of light signals transmitted through several coupled Multi-Core Fibers (MCFs) along an end-to-end optical path. In turn, this type of optical transmission results in an increased complexity with respect to the MIMO signal processing at the receiver and also in a reduced transmission capacity of the entire communication system. To analyze this issue in detail, an optical MIMO network model with MCFs and optical switch was considered, and the eigenvalue distribution of channel matrix along with average capacity of MCF links was discussed. The analysis showed that the optical switch disperses the eigenvalue distribution of channel matrix and reduces the capacity by approximately 15% for SDM network with MCFs. This degradation rate is increased noticeably for larger crosstalk in MCF links and is increased moderately with an increasing number of switches.

Crosstalk-aware routing, spectrum, and core assignment in space-division multiplexing networks with bidirectional multicore fibers

Space-division multiplexing (SDM) has been proposed to increase fiber transmission capacity to cope with the increasing bandwidth requirements in the network. The use of multicore fiber (MCF) is considered to be a simple and feasible way to implement SDM technology. The intercore crosstalk (XT) in the MCF affects the performance of network transmission. Currently, bidirectional transmission of MCF has been proven to have lower XT than traditional unidirectional transmission of MCF. But there are few research studies on routing, core, and spectrum allocation strategies with bidirectional transmission. Thus, to get more XT reduction, we propose two different routing, core, and spectrum allocation strategies according to the traffic loads in the network based on the bidirectional transmission of MCF. The simulation results show that the proposed algorithm has lower blocking rate and higher resource utilization than the benchmark algorithm in the two stages of different traffic loads in the network.

Adaptive software defined networking control of space division multiplexing super-channels exploiting the spatial-mode dimension

Space division multiplexing (SDM) super-channels are the only way to guarantee a sustainable scaling of the optical line interface rate. High-capacity terabit interfaces can be deployed by logically associating several optical sub-channels transmitted in parallel in different spatial cores or modes in order to create an (logical) optical channel with the desired interface rate. This paper is focused on the use of spatial modes for SDM super-channels. First, we present a software defined networking (SDN)-controlled disaggregated SDM network architecture that deploys the proposed sliceable-mode transceivers with multiple-input multiple-output equalization. Then, we present the adaptive SDN workflow and heuristics for the provisioning, scaling up/down, and soft-failure restoration of SDM super-channels using spatial modes. Two proofs of concept are deployed in the joint testbed between KDDI Research and CTTC.

Routing, modulation format, spectrum and core allocation in space-division-multiplexed programmable filterless networks

Abstract Elastic optical networks are considered as promising solutions to improve the spectral efficiency of optical networks and fulfill the diverse bandwidth requirements of heterogeneous network applications. Elastic filterless optical networks with passive broadcast-and-select architecture are introduced to offer network agility. However, the presence of unfiltered signals stems from the signal broadcast and the absence of filtering (so-called drop-and-waste transmission), can lead to an overhead in spectrum usage. This issue can be alleviated by combining the unprecedented flexibility of programmable architecture on demand (AoD) nodes with the extra diversity in spatial domain provided by space division multiplexing (SDM). In this paper, we propose a hardware-aware strategy based on the two-objective binary particle swarm optimization (BPSO) to solve the routing, modulation format, spectrum and core allocation (RMSCA) problem in programmable filterless SDM networks. The proposed BPSO-based hardware-aware RMSCA (BH-RMSCA) strategy aims at minimizing jointly the number of optical couplers and maximum backplane switch size inside the filterless AoD nodes. The physical layer impairment in terms of the inter-core crosstalk (XT) is taken into account in solving the resource allocation problem. Simulation results show that the proposed BH-RMSCA strategy successfully provides Pareto-optimal solutions with remarkable hardware savings compared to the benchmark methods. The cost-efficient network solutions are achieved at the expense of a favorable trade-off with the spectrum usage and the path length, especially for higher connected topology.

Scaling SDM Optical Networks Using Full-Spectrum Spatial Switching

In today's optical networks, the capacity supported by a single optical fiber is far higher than the demand between source–destination pairs. Thus, to take advantage of the installed capacity, lightpaths allocated between distinct source–destination pairs share the capacity provided by an optical fiber. This sharing is only possible if these lightpaths have different wavelengths. With the exponential evolution of traffic, the demand between source– destination pairs in a network can approach or exceed the capacity of a single fiber. In this scenario, spacedivision multiplexing (SDM) networks appear as a viable option, and new network-sharing technologies become relevant. In this paper, we study different switching approaches for SDM networks with uncoupled spatial superchannels—such as networks with multicore fibers or bundles of single-mode fibers—which result in different network-sharing strategies. We start by comparing three switching architectures considering the evolution of traffic over time: (1) full-spectrum spatial switching (full-spectrum SS); (2) wavelength switching (WS) of uncoupled spatial superchannels; and (3) independent switching (IS) of spatial and wavelength channels. IS provides high network flexibility at the cost of complex switching nodes. On the other hand, full-spectrum SS and WS simplify network nodes but reduce flexibility by switching superchannels. Simulation results indicate that WS offers poor utilization in the long term. Full-spectrum SS, in contrast, exhibits low utilization in the short term, but outperforms IS in the long term. We also propose alternative hybrid switching strategies that start with IS and change to full-spectrum SS after a certain number of spatial channels are activated. The simulations suggest that well-designed strategies for migration from IS to full-spectrum SS delay premature activation investments while saving on switching costs.

Joint Assignment of Spatial Granularity, Routing, Modulation, and Spectrum in SDM-EONs: Minimizing the Network CAPEX Considering Spectrum, WSS, and Laser Resources

In space division multiplexing (SDM) systems, the spatial dimensions can be divided into one or several group(s). Spatial dimensions in the same group can be jointly switched based on joint switching (J-Sw)/fractional joint switching (FrJ-Sw), resulting in less wavelength selective switch (WSS) usage. Additionally, optical carriers on the same frequency of these spatial dimensions in the same group can share a single laser source. Therefore, a coarser spatial (switching) granularity can reduce the usage of both WSSs and lasers. However, compared to the case of a finer spatial granularity, this scenario leads to more guard bands (GBs), i.e., more spectrum resources, being required. Consequently, spatial granularity is a key factor of minimizing the cost in SDM network design. In this paper, we present a problem called routing, modulation, spatial granularity, and spectrum assignment (RMGSA). Our objective is to minimize the network CAPEX, i.e., minimize the total cost related to the network resources containing the spectrum, WSS, and laser. We first present a joint ILP-RMGSA formulation to solve this problem. Next, we present a decomposition method that divides RMGSA into two substituent subproblems, i.e., first, routing, modulation, and spatial granularity and, second, spectrum assignment, and then solve them sequentially. Because of the limitation of ILP formulation in larger scale problem instances, a rerouting-based heuristic algorithm is proposed. We examine the performances of the proposed approaches via simulation experiments and find that the best spatial granularity is related to the network resources to which network operators attached more importance. Moreover, as another contribution of this paper, we analyze the impact of GB width and the size of the connection requests on the decision regarding the best spatial granularity.

Space-Division Multiplexing in Data Center Networks: On Multi-Core Fiber Solutions and Crosstalk-Suppressed Resource Allocation

The rapid growth of traffic inside data centers caused by the increasing adoption of cloud services necessitates a scalable and cost-efficient networking infrastructure. Space-division multiplexing (SDM) is considered as a promising solution to overcome the optical network capacity crunch and support cost-effective network capacity scaling. Multi-core fiber (MCF) is regarded as the most feasible and efficient way to realize SDM networks, and its deployment inside data centers seems very likely as the issue of inter-core crosstalk (XT) is not severe over short link spans (<1 km) compared to that in long-haul transmission. However, XT can still have a considerable effect in MCF over short distances,which can limit the transmission reach and in turn the data center’s size. XT can be further reduced by bi-directional transmission of optical signals in adjacent MCF cores. This paper evaluates the benefits of MCF-based SDM solutions in terms of maximizing the capacity and spatial efficiency of data center networks. To this end, we present an analytical model for XT in bi-directional normal step-index and trench-assisted MCFs and propose corresponding XT-aware core prioritization schemes. We further develop XT-aware spectrum resource allocation strategies aimed at relieving the complexity of online XT computation. These strategies divide the available spectrum into disjoint bands and incrementally add them to the pool of accessible resources based on the network conditions. Several combinations of core mapping and spectrum resource allocation algorithms are investigated for eight types of homogeneous MCFs comprising 7–61 cores, three different multiplexing schemes, and three data center network topologies with two traffic scenarios. Extensive simulation results showthat combining bi-directional transmission in dense core fibers with tailored resource allocation schemes significantly increases the network capacity. Moreover, a multiplexing scheme that combines SDM and WDM can achieve up to 33 times higher link spatial efficiency and up to 300 times greater capacity compared to a WDM solution.

On the Efficiency of Survivable Flex-Grid SDM Networks

In this paper, we focus on the performance of a survivable flex-grid network with space division multiplexing (SDM). To provide network survivability, we use dedicated path protection and shared backup path protection. For both schemes, we also consider squeezed protection that allocates backup resources only for a part of the primary flow. The squeezed protection is considered for the protection level changing from 0% (no flows are protected) up to 100% (full network protection) with a step of 10%. Moreover, we consider two SDM switching policies—independent ( SDM-indsw ) and joint ( SDM-jsw ). In this paper, we define all considered problem versions using integer linear programming technique as well as propose a dedicated heuristic algorithm. Then, we perform extensive simulations focused on the performance of a survivable SDM network in terms of the spectrum usage. The results show that the path protection in an SDM network brings additional spectrum consumption; however, the amount of additional resources depends on various factors such as required protection level, applied protection method, switching policy, and traffic pattern. Moreover, the results also reveal that SDM-indsw is much more spectrum-efficient than SDM-jsw considering all protection methods and levels. It uses, on average, half of the spectrum required for SDM-jsw . Additionally, the investigation shows that SDM-jsw is especially spectrum-inefficient when realizing variable-size demands (sets including both small and large demands).

Core, Mode, and Spectrum Assignment Based on Machine Learning in Space Division Multiplexing Elastic Optical Networks

Recently, network traffic has been growing exponentially and almost reached the physical capacity limit of single mode fibers. Space division multiplexing (SDM) is a promising technology to overcome the looming fiber capacity crunch. Especially, few-mode multi-core fibers (FM-MCFs) can aggregate multiple cores into one fiber and two or more modes can be transmitted in one core, which can greatly increase the capacity yet introduce crosstalk constraints including inter- and intra-core crosstalk. To our best knowledge, there is no accurate crosstalk calculation model study in SDM optical networks with FM-MCFs. To address this issue, we first introduce the machine learning into the crosstalk prediction phase and propose a novel crosstalk estimation model (CEM) exploiting the beam propagation method called CEM-beam propagation method (BPM)-machine learning (ML), which can be used to evaluate the crosstalk during the design for the resource allocation scheme. Then, a crosstalk aware core, mode, and spectrum assignment (CA-CMSA) strategy is presented. The simulation results for crosstalk estimation at the wavelength level indicate that the crosstalk at lower frequencies is less than that at higher frequencies. Thus, the lower frequencies are always the first choice in the spectrum resource assignment phase. In addition, for our specific training set, the Levenberg-Marquardt (LM) algorithm based on machine learning performs better on the training, including regression values measurement and time consumption. The simulation results of the proposed CA-CMSA scheme also show that the resource allocation algorithm based on LM can improve resource utilization without increasing total connection set-up time. Thus, it will be the best choice for the resource assignment in SDM networks with FM-MCFs.

Fast Parallel Lightpath Re-Optimization for Space-Division Multiplexing Optical Networks Based on Time Synchronization

In recent years, driven by the rapid growth of global Internet traffic, optical networks are evolving to provide higher capacities. To enlarge transmission capacity, researchers focused on the dimension of space and proposed the concept of space-division multiplexing (SDM) by introducing multicore fiber and few-mode fiber into optical networks. In SDM networks, one of the main challenges is the crosstalk among SDM (core and mode) channels, which degrades signal quality. This degradation has a serious impact on routing and resource allocation. Previous studies have mostly been confined to the routing algorithms for crosstalk reduction but few focus on the re-optimization mechanisms in SDM networks, defined as re-routing the existing lightpaths to minimize the overall crosstalk without traffic disruption. In this paper, by analyzing the re-routing signaling processes of two straightforward serial re-optimization mechanisms, we find that the signaling interaction between node controllers and serial re-routing execution leads to long re-optimization times, which makes lightpath re-optimization impossible for practical use. In order to achieve fast lightpath re-optimization for crosstalk reduction in SDM networks, we propose a novel software-defined networking (SDN) based parallel lightpath re-optimization mechanism, enabled by high-precision time synchronization. The basic idea of our proposed parallel lightpath re-optimization mechanism is to make all the nodes execute the switching operations co-ordinately with the help of network time synchronization. Besides, we propose two key algorithms to achieve parallel re-optimization: a simulated annealing algorithm to determine the re-routing sequence of the lightpaths and a time arrangement algorithm to determine the exact time points of the switching operations at each node. Finally, we conduct a prototype experiment and large-scale network simulations to evaluate the performance of the proposed mechanism. The results demonstrate that our parallel re-optimization mechanism can reduce the overall crosstalk in a much shorter time by up to three orders of magnitude compared with the conventional serial mechanisms and thus is more practical for real applications.