Digital twin (DT) has revolutionized optical communication networks by enabling their full life-cycle management, including planning, prediction, optimization, upgrade, and troubleshooting. The rapid development of DT applications for optical networks, from concepts and architecture to simulation and experimental validation, has been exceptional. As a real-time virtual replica of physical optical networks, the DT of optical networks can be divided into three parts: real-time monitoring, mirror modeling, and automatic control. Recent advancements in each part have individually propelled the network towards intelligence and automation. In this paper, we make a comprehensive survey on this timely topic and illustrate the principal evolutions within these three parts. This will be followed by an overview of ongoing DT research in optical communications and networking, as well as an outlook on potential future directions for further investigation.

Following successful adoption of cloud computing, many service providers (SPs) are now using high-performance Virtual Machines (VMs) located in large datacenters owned by public cloud infrastructure providers to deploy their virtual network functions (VNFs). Since using these VMs has a cost depending on utilization time, a complex problem of VNF placement and scheduling (VPS) must be addressed to achieve satisfactory network performance (e.g., latency) while minimizing the cost paid to lease VMs. In this study, a cost-efficient VPS scheme (CEVPS) is proposed to address the VPS problem in public cloud networks considering dynamic requests of ordered sequences of VNFs. Our CE-VPS scheme goes beyond existing solutions as it models some important practical aspects such as an additional latency incurred by booting a VM and installing a VNF instance. Also, CE-VPS considers that VNFs can be multi-threaded or single-threaded, and that their throughput as a function of allocated computing resources must be modeled differently. CEVPS is formulated as a mixed inter linear program (MILP) and also as an efficient heuristic algorithm. CE-VPS achieves lower cost and latency than conventional Best-Availability and Cost- Efficient Proactive VNF Placement schemes, and a better tradeoff between resource consumption and latency performance than a conventional Low-Latency scheme.

An indirect approach based on phase measurement is proposed to measure the rotational Doppler frequency shift, which takes full advantage of the phase structure of orbital angular momentum (OAM) beams in radio domain, using a vector network analyzer (VNA) as a phase discriminator. A proof-of-concept experiment is established by an optical-controlled system with the OAM state of 1. By analyzing the experiment’s results, the rotational Doppler frequency shift is measured as 24.83 Hz (max error rate 0.67%) at $50\pi$ rad/s rotational velocity, deducing the rotational velocity as $50.18\pi$ (average error rate 0.36%).

A magnetically tunable metamaterial perfect absorber (MPA) based on ferromagnetic resonance is experimentally and numerically demonstrated. The ferrite-based MPA is composed of an array of ferrite rods and a metallic ground plane. Frequency dependent absorption of the ferrite-based MPA under a series of applied magnetic fields is discussed. An absorption peak induced by ferromagnetic resonance appears in the range of 8–12 GHz under a certain magnetic field. Both the simulated and experimental results demonstrate that the absorption frequency of the ferrite-based MPA can be tuned by the applied magnetic field. This work provides an effective way to fabricate the magnetically tunable metamaterial perfect absorber.

The Multi-access Edge Computing (MEC) system interconnected by metro optical network is gaining much attention recently. The proximity of computing resources in MEC provides the possibility of low-latency access for user equipment (UE). However, since the computing capacity of edge server is limited, the task failure may occur due to the unacceptable computing latency if the corresponding server is overloaded. Note that there is more than one MEC server at the network edge and the computationally intensive tasks on the overloaded server can be sent to nearby light-loaded servers for processing via peer offloading. In this article, we design a latency-aware task peer offloading (LA-TPO) scheme, which is an economic solution for the overload problem of MEC. We formulate the LA-TPO into a mixed integer non-linear programming (MINLP) model and develop a GA-based heuristic algorithm to jointly optimize the offloading decisions and the routing and computing resource allocation. We compare the MINLP and proposed algorithm under various scenarios, and the results show that the GA-based LA-TPO can achieve low latency, high success ratio of tasks as well as the optimization of resource utilization.

The rapid growth in Internet traffic has contributed to the need to expand network transmission capacity. Multi-band (MB) using existing standard single-mode fibers (SMFs) in the free band is an ideal way to increase the capacity of the fiber. However, the introduction of MB has transferred space division multiplexing - elastic optical networks (SDM-EONs) to multi band-SDM-EONs (MB-SDM - EONs), and changed routing, spectrum and core assignment (RSCA) problem to the routing, spectrum, core and band assignment (RSCBA) problem. At the same time, it introduces a new issue named stimulated Raman scattering (SRS), which affects transmission quality. Although an expanded fiber can carry more services, a large number of services will be interrupted if a link fails, which will bring huge losses. Therefore, it is necessary to propose a reasonable protection strategy to ensure reliable transmission quality. In this paper, the survivable RSCBA problem in MB-SDM-EONs is studied, which copes with link failures and considers SRS in signal-to-noise ratio (SNR) analysis. In order to solve the problem, we propose a band partition protection scheme: working and protection resources are allocated in different frequency bands for increasing SNR in a fault-free network. We formulate this problem and band partition protection scheme as an integer linear programming (ILP) model, which takes into account inter-core crosstalk and SNR analysis. Furthermore, a heuristic algorithm based on genetic algorithm is proposed for large-scale networks. We evaluate the performance of the proposed approaches through experimental simulation. The results demonstrate that both proposed approaches are effective in finding the optimal solutions.