Reconfigurable Optical Add-drop Multiplexers Research Articles

Wavelength-Division Demultiplexing Enhanced by Silicon-Photonic Tunable Filters in Ultra-Wideband Optical-Path Networks

With the recent growth of network traffic demands, a high-capacity optical-path network based on reconfigurable optical add-drop multiplexers (ROADMs) is highly desirable. Among the various technologies available, extending the useable frequency band is one of the most promising solutions since it can enlarge the network capacity with relatively small signal-quality degradation. However, increasing the number of wavelength signals detracts flexible signal-drop capabilities of the ROADM because the effective dynamic range of the digital coherent receiver is restricted by the number of wavelength signals input to the receiver. To overcome this difficulty, we adopt the ROADM structure that sets an optical wavelength-tunable filter in front of each receiver. This scheme retains the effective receiver dynamic range even if a large number of wavelength signals are multiplexed. In this paper, we demonstrate flexible signal drop in a C+L-band optical-path network by using an optical wavelength-tunable filter implemented on a single silicon-photonic chip. The prototype filter is monolithically implemented in conjunction with an input-port selector so as to reduce the total system loss. The filter comprises multiple asymmetric Mach-Zehnder interferometers for wavelength tuning and symmetric Mach-Zehnder interferometers for input-port selection. Its effectiveness is experimentally confirmed by measuring the bit-error ratios of 32-Gbaud/100-Gbps dual-polarization quadrature-phase-shift-keying signals and 32-Gbaud/200-Gbps dual-polarization 16-ary quadrature-amplitude-modulation signals. A single wavelength signal is extracted out of 285 wavelength signals multiplexed over the C+L band and successfully demodulated with negligible penalty in terms of the optical signal-to-noise ratio. The net fiber capacity reaches 57 Tbps.

Enhanced resilience towards ROADM-induced optical filtering using subcarrier multiplexing and optimized bit and power loading.

In flexible optical networks, lightpaths are routed from source to destination thanks to reconfigurable optical add-drop multiplexers (ROADMs) placed in every node connected by a line system. As ROADMs are based on wavelength selective switches (WSSs), which imply channel filtering, a non negligible performance reduction can be observed when lightpaths traverse many network nodes. Control plane software network controllers that base their decision on the quality of transmission of the physical layer must also take into account the impact of ROADMs cascading. The penalties caused by WSS cascading in single-carrier systems are mostly driven by uncompensated inter-symbol interference (ISI), and therefore their evaluation cannot be based on simple analytical solutions as required for real-time network control. In this paper, we consider a 200G optical system based on multi-subcarrier (MSC) modulation with hybrid modulation formats. Using a fully analytical framework, we show that it is possible to predict ROADM-induced penalties offline and accurately estimate the required settings for their transmitter-side mitigation through bit and/or power-loading. Moreover, resorting to both simulations and experiments, we also demonstrate that properly optimized MSC signals can provide an increased resilience to ROADM filtering with respect to standard single-carrier systems: applying analytically-driven bit-loading over 8×4 Gbaud and 16×2 Gbaud MSC signals after crossing 8 cascaded WSSs in a noise loading scenario, we experimentally found ∼2 dB gain in required SNR with respect to a traditional 32 Gbaud single-carrier signal.

A 5G C-RAN Optical Fronthaul Architecture for Hotspot Areas Using OFDM-Based Analog IFoF Waveforms

Analog fronthauling is currently promoted as a bandwidth and energy-efficient solution that can meet the requirements of the Fifth Generation (5G) vision for low latency, high data rates and energy efficiency. In this paper, we propose an analog optical fronthaul 5G architecture, fully aligned with the emerging Centralized-Radio Access Network (C-RAN) concept. The proposed architecture exploits the wavelength division multiplexing (WDM) technique and multicarrier intermediate-frequency-over-fiber (IFoF) signal generation per wavelength in order to satisfy the demanding needs of hotspot areas. Particularly, the fronthaul link employs photonic integrated circuit (PIC)-based WDM optical transmitters (Txs) at the baseband unit (BBU), while novel reconfigurable optical add-drop multiplexers (ROADMs) cascaded in an optical bus are used at the remote radio head (RRH) site, to facilitate reconfigurable wavelength switching functionalities up to 4 wavelengths. An aggregate capacity of 96 Gb/s has been reported by exploiting two WDM links carrying multi-IF band orthogonal frequency division multiplexing (OFDM) signals at a baud rate of 0.5 Gbd with sub-carrier (SC) modulation of 64-QAM. All signals exhibited error vector magnitude (EVM) values within the acceptable 3rd Generation Partnership Project (3GPP) limits of 8%. The longest reach to place the BBU away from the hotspot was also investigated, revealing acceptable EVM performance for fiber lengths up to 4.8 km.

Evaluation of Device Cost, Power Consumption, and Network Performance in Spatially and Spectrally Flexible SDM Optical Networks

To keep pace with the rapidly increasing network traffic demand anticipated in the coming years, space-division multiplexing (SDM) has become a promising technology for overcoming the limitations of the current wavelength-division multiplexing based optical networks. The key advantage of SDM-based optical transmission is that it increases the network capacity by extending the available spatial dimensions. In this context, several innovative technologies have been proposed to support spatially and spectrally flexible all-optical transmission. One typical technology of this kind is called spatial and spectral super-channel (Spa & Spe SpCh) transmission. Newly designed Spa & Spe SpCh transceivers can support high-speed transmission with excellent spectral efficiency by allowing the optical carriers to be flexibly distributed in both the spatial and spectral domains without guardbands. In addition, SDM-based reconfigurable optical add/drop multiplexers (ROADMs) with varying spatial switching granularities and add/drop ports have been developed to support super-channel transmission (e.g., routing and add/drop). Moreover, ROADMs with spatial lane change support, which can further improve the routing flexibility in the network layer, have also been proposed in recent years. Different modes of implementation of the aforementioned technologies have distinct advantages and disadvantages. These technologies can be combined in various ways to support different types of SDM transmission, leading to variable network performance. In this paper, we evaluate the device cost, power consumption, and network performance of SDM transmission systems based on different implementations of these technologies to serve as a reference for future research.

Optical filtering penalty estimation using artificial neural network in elastic optical networks with cascaded reconfigurable optical add–drop multiplexers

For future elastic optical networks, the narrow filtering effect induced by cascaded reconfigurable optical add–drop multiplexers (ROADMs) is one of the major impairments. It is essential to accurately estimate the filtering penalty to minimize network margins and optimize resource utilization. We present a method for estimating filtering penalty using machine learning (ML). First, we investigate the impact of ROADM location distribution and bandwidth allocation on the narrow filtering effect. Afterward, an ML-aided approach is proposed to estimate the filtering penalty under various link conditions. Extensive simulations with 9600 links are implemented to demonstrate the superior performance of the proposed scheme.

Reconfigurable optical add-drop multiplexer at 1550 nm using magnetically-coupled switches based on a photonic crystal

In this paper, a design of reconfigurable optical add-drop multiplexer (ROADM) is proposed, which consists of magnetically-coupled switches, bus waveguide, drop waveguide, add waveguide, and reflection cavity based on a photonic crystal (PC). The ferrite materials are introduced to form microcavities in a two-dimensional PC. Then, the magnetic permeability of ferrite rods is changed by adjusting the magnetic field strength, and a scheme for magnetic permeability distribution of magnetically-coupled switches is proposed. The simulation results indicate that the incident wave with wavelength 1550 nm can be downloaded and uploaded successfully, and the transmissivity of wave is both above 90% when the add function and drop function are conducted. The quality (Q) factor is approximately 103. Moreover, the overall size of the device is 13.5 μm× 10.5 μm, which is highly suitable for wavelength division multiplexing (WDM) system in the photonic integrated circuits.

Adaptive Compensation of Bandwidth Narrowing Effect for Coherent In-Phase Quadrature Transponder through Finite Impulse Response Filter

Coherent in-phase quadrature (IQ) transponders are ubiquitous in the long-haul and the metro optical networks. During the transmission, the coherent signal experiences a bandwidth narrowing effect after passing through multiple reconfigurable optical add-drop multiplexers (ROADMs). The coherent signal also experiences a bandwidth narrowing effect when electrical or optical components of the coherent IQ transponder experience aging. A dynamic method to compensate the bandwidth narrowing effect is thus required. In the coherent optical receiver, signal bandwidth is estimated from the raw analog-to-digital converter (ADC) outputs. By adaptively adjusting the tap coefficients of the finite impulse response (FIR) filter, simple post-ADC FIR filters can increase the resiliency of the coherent signal to the bandwidth narrowing effect. The influence of chromatic dispersion, polarization mode dispersion, and polarization dependent loss are studied comprehensively. Furthermore, the bandwidth information of the transmitted analog signal is fed back to the coherent optical transmitter for signal optimization, and the transmitter-side FIR filter thus changes accordingly.

Opening up ROADMs: Let Us Build a Disaggregated Open Optical Line System

At the lowest layer of today's communication networks is an optical line system (OLS), a physical network of equipment, which carries high-frequency analog light signals over thousands of kilometers. Traditionally, an OLS was delivered as a turn-key solution by a single vendor. Within an OLS, reconfigurable optical add/drop multiplexers (ROADMs) are active devices responsible for routing spectral chunks between input and output ports. ROADMs are arguably the most complex physical component of an OLS. In this paper, we describe an open design of a Czech Light ROADM, including the optical hardware, electronics, software, and the northbound communication interface. The performance of the ROADMs is evaluated in two test scenarios.

Optimization of Optical Networks Based on CDC-ROADM Technology

New generation of optical nodes in dense wavelength division multiplexed networks enables operators to improve service flexibility and make significant savings, both in operational and capital expenditures. Thus the main objective of the study is to minimize optical node resources, such as transponders, multiplexers and wavelength selective switches, needed to provide and maintain high quality dense wavelength division multiplexed network services using new generation of optical nodes. A model based on integer programming is proposed, which includes a detailed description of an optical network node. The impact on the network performance of conventional reconfigurable optical add drop multiplexer technology is compared with colorless, directionless and contentionless approaches. The main focus of the presented study is the analysis of the network congestion problem arising in the context of both reconfigurable optical add drop multiplexer technologies. The analysis is supported by results of numerical experiments carried out for realistic networks of different dimensions and traffic demand sets.

CO-OFDM System with 16-QAM Subcarrier Modulation Using Reconfigurable Optical Add Drop Multiplexer

Abstract In this paper, sixteen 50 GHz spaced orthogonal frequency division multiplexing channels at 10 Gbps has been investigated with reconfigurable optical add drop multiplexer based on digital optical switch. The effects of fiber link length and input optical signal power on bit error rate, quality factor, output signal power, optical signal to noise ratio at the receiving side are observed. It is observed that maximal transmission distance of 2,100 km is achieved with an input optical power of −8 dBm.

Design of full-bandwidth ROADM based on MMI and MRRs

In this paper, we propose a full-bandwidth reconfigurable optical add/drop multiplexer (ROADM) equipped with multiplexer/demultiplexer (MUX/DEMUX) and switch array. Based on the pairwise multimode interference (MMI) and side-ported MMI, the full-bandwidth MUX/DEMUX is achieved. The switch array based on microring resonators (MRRs) can achieve the function of the wavelength selective switches. By designing the second-order series-coupled MRR structure, using thermo-optic effect, the switch is able to get a switching ratio above 16 dB. The proposed ROADM system shows good performance for application in dynamic optical networks.

Compensation of filter cascading effects and non‐linearities in flexible multi‐carrier‐based optical networks using a complex‐kernel‐based support vector machine

Filter cascading effects are identified and compensated in flexible coherent multi-carrier optical networks using, for the first time, a complex-valued support vector machine (C-SVM) non-linear equaliser which is compared to phase-conjugated subcarrier-coding (PC-SC)-based non-linearity compensation. The transmission performance of super-channel-based dual-polarisation coherent multi-band optical OFDM (DP-MB-OFDM) is analysed in a flexible network with two-stage wavelength selective-switch reconfigurable optical add-drop multiplexer for coarse and fine switching granularity. It is shown that filter cascading effects have significant impact only on edge OFDM sub-bands. On the other hand, C-SVM outperforms PC-SC resulting in an extension of the transmission-reach, for instance, by 420 km when employing 32-quadrature amplitude modulation at a targeted bit-error-rate of 10−3. This occurs since C-SVM can compensate more effectively stochastic-induced nonlinear cross-talk effects. Moreover, the interplay between polarisation-mode dispersion (PMD) and fibre non-linearity is also investigated in DP-MB-OFDM, revealing that a high PMD causes time fluctuation on the non-linear interaction affecting the balance of the ‘twin-subcarriers’ in PS-SC.

Selectable-FSR 10-GHz Granularity WDM Superchannel Filter in a Reconfigurable Photonic Integrated Circuit

We present a 10-GHz granularity reconfigurable WDM superchannel filter in a photonic integrated circuit. The filter uses a ring-resonator-assisted tapped-delay-line topology. An example configuration with two ring resonators and four tapped delay lines provides a spectral filter with a flat-top passband, a roll-off factor of 8%, a tunable central wavelength, and a selectable free spectral range, i.e., 2× or 4× the 3-dB passband bandwidth. The device was fabricated in a silicon nitride (Si3N4/SiO2) waveguide with a chip area of 0.36 cm2 and has a fiber-to-fiber insertion loss of 5 dB. We experimentally demonstrate the filter for selecting a subchannel from a WDM superchannel comprising 10-GHz subchannels with 10% and zero guard bands. Owing to the infinite impulse responses of ring resonators, this filter requires a significantly smaller area and fewer tuning elements compared with designs based on only tapped-delay-line finite-impulse-response filters. This paper contributes to the creation of chip-scale all-optical WDM superchannel transceivers and reconfigurable optical add/drop multiplexers (ROADMs).

Two Use Cases of Machine Learning for SDN-Enabled IP/Optical Networks: Traffic Matrix Prediction and Optical Path Performance Prediction [Invited

We describe two applications ofmachine learning in the context of internet protocol (IP)/Optical networks. The first one allows agilemanagement of resources in a core IP/Optical network by using machine learning for shorttermand long-term prediction of traffic flows. It also allows joint global optimization of IP and optical layers using colorless/ directionless (CD) reconfigurable optical add-drop multiplexers (ROADMs). Multilayer coordination allows for significant cost savings, flexible new services to meet dynamic capacity needs, and improved robustness by being able to proactively adapt to new traffic patterns and network conditions. The second application is important as we migrate our networks to Open ROADM networks to allow physical routing without the need for detailed knowledge of optical parameters. We discuss a proof-of-concept study, where detailed performance data for established wavelengths in an existing ROADM network is used for machine learning to predict the optical performance of each wavelength. Both applications can be efficiently implemented by using a software-defined network controller.

1 × 25 LCOS-based wavelength selective switch with flexible passbands and channel selection

Abstract Wavelength-selective-switching based reconfigurable optical add-drop multiplexers are the essential components of current and next generation dynamic optical networks. A proof-of-concept 1 × 25 wavelength selective switch is demonstrated experimentally based on liquid crystal on silicon (LCOS) technology and 2-foptical system. Each of the input C-band wavelength channel can be independently selected and switched into arbitrary output ports. The channel passband and channel spacing can be adjusted flexibly by applying holograms onto an LCOS chip. The insertion loss for polarized beam ranges from 5.4 to 7.5 dB for 1–20 output ports and increases up to 9.6 dB at side ports for 100 GHz spaced ITU-T channels over the C-band. The general channel crosstalk is below −20 dB. The minimum resolution of center frequency for each channel is 5 GHz (0.04 nm). The performance can be further improved by using an LCOS chip with smaller pixel pitch, higher-quality optical components and accurate alignment of the fiber-lenslet array.

Design and testing of an optical routing device with reconfigurable optical add/drop multiplexer and interleaver functions

A multifunction all-optical routing device for a dynamic network is presented. The proposed routing device consists of two arrayed waveguide gratings (AWGs), 12 optical circulators, and six tunable fiber Bragg gratings (TFBGs) to realize the functions of all-optical multichannel reconfigurable optical add/drop multiplexer (ROADM) and optical wavelength interleaver. The AWGs have the characteristics of wavelength cycling of light. So even if there is no additional optical architecture in the proposed optical device, it can have the function of an optical interleaver. Both ROADM and optical interleaver are the keys to enable the modern optical communication services and support the remote configuration of optical paths and the reconfiguration of network topologies. The experimental data show that the functions of add/drop and wavelength interleaver can be implemented. Three dense wavelength division multiplexing (DWDM) channels are used to evaluate the performance of ROADM of the proposed routing device. The results of this study demonstrated the feasibility of this routing device for the DWDM networks.

Deep-Neural-Network-Based Wavelength Selection and Switching in ROADM Systems

Recent advances in software and hardware greatly improve the multi-layer control and management of reconfigurable optical add-drop multiplexer (ROADM) systems facilitating wavelength switching. However, ensuring stable performance and reliable quality of transmission (QoT) remain difficult problems for dynamic operation. Optical power dynamics that arise from a variety of physical effects in the amplifiers and transmission fiber complicate the control and performance predictions in these systems.We present a deep-neural-network-based machine learning method to predict the power dynamics of a 90-channel ROADM system from data collection and training. We further show that the trained deep neural network can recommend wavelength assignments for wavelength switching with minimal power excursions.

De-multiplexing the required spectrum in a traffic demand into multiple non-adjacent granular spectrums for dynamic traffic grooming in EON

An elastic light-trail, the advanced version of a traditional light-trail, is one of the most recent dynamic traffic grooming technologies in an elastic optical network. In such networks, Orthogonal Frequency Division Multiplexing (OFDM) is a promising technology to execute the RSA (routing and spectrum assignment) algorithm and implement an elastic light-trail as a result. However, the reconfigurable optical add-drop multiplexers (ROADMs) and bandwidth variable Wavelength Selective Switches (WSSs) are required to implement an optically switched and spectrum varying elastic lightpath. In addition to the hardware equipment mentioned above, Add Drop Couplers (ADC) and optical ON/OFF switches (OS) are required to facilitate elastic light-trail in an elastic optical network. Hereafter, deployment of Add Drop Couplers (ADC) and optical ON/OFF switches incurs a significant network cost to facilitate elastic light-trail implementation. Toward the approach for eliminating this costly hardware setup, we have highlighted the fragmentation problem that occurs in spectrum domain in an optical fiber link. In this problem, the entire spectrum range in an elastic optical link is fragmented into several non-contiguous free spectrum slots at an instance during a data transportation session. In this regard, we have proposed “Multiple multi-hop non-contiguous elastic lightpath - MMNCEL” algorithm, where the required spectrum B of a traffic demand r(s, d, B, T) is sliced into n distinct granular spectrums {b1, b2, … bn} according to the current availability and status of fragmented spectrum slots in an elastic optical link. Moreover, to setup an elastic lightpath, the algorithm allocates the same range of spectrum from every optical link in a route to preserve Spectrum Continuity Constraint (SCC), whereas, at each hop in a multi-hop elastic lightpath the algorithm performs opto-electrical conversion. In this paper, our objective is to achieve maximum network throughput in an elastic optical network without deploying ADC and optical ON/OFF switches. However, in this work, we have investigated optical spectrum utilization efficiency for the proposed MMNCEL, existing elastic light-trail (Multi-hop Elastic Light-trail algorithm - MELT) and elastic lightpath (Multi-hop elastic Lightpath algorithm - MEL), and performed comparison measurements preciously.

YANG/NETCONF ROADM: Evolving Open DWDM Toward SDN Applications

Evolution in the field of optical networking in National Research and Education Networks led to utilization of Alien Wavelengths (AWs) and disaggregated Open Line Systems. Dynamic provisioning of AWs, in general spectrum services, is achieved via Reconfigurable Optical Add Drop Multiplexers (ROADMs) operating with flexible spectrum allocation. Northbound interface of these is usually implemented via the NETCONF protocol and through an appropriate Yet Another Next Generation (YANG) model. This paper presents a self-contained flexible spectral grid YANG model for low-level control and monitoring of a ROADM. The presented YANG model offers access to all functional components of a modern flexgrid ROADM and enables SDN applications to access and manipulate the media layer with no required external validation. The model is verified by an SDN application which implements optical path protection of AWs. A physical demonstration with model railroad intuitively shows the accomplished functionality.

Demonstration of Single-Mode Multicore Fiber Transport Network With Crosstalk-Aware In-Service Optical Path Control

Multicore fiber (MCF) transmission is considered as one of the promising technologies for breaking the capacity limit of traditional single mode fibers. Managing the crosstalk (XT) and configuring optical paths adaptively based on the XT as well as achieving longer distance and larger capacity transmission are important, because intercore XT could be the main limiting factor for MCF transmission. In a real MCF network, the intercore XT in a particular core is likely to change continuously as the optical paths in the adjacent cores are dynamically assigned to match the dynamic nature of the data traffic. If we configure the optical paths while ignoring the intercore XT value, the Q-factors may become excessive. Therefore, monitoring the intercore XT value continuously and configuring optical path parameters adaptively and flexibly are essential. To address these challenges, we develop an MCF transport network testbed and demonstrate an XT-aware traffic engineering scenario. With the help of a software-defined network controller, the modulation format and optical path route are adaptively changed based on the monitored XT values by using programmable devices such as a real-time transponder and a reconfigurable optical add–drop multiplexer.