Evolution of ROADM Technology and Key Technical Principles

With the rapid growth of data traffic and diversified service demands, traditional fixed optical networks are no longer sufficient to support flexible, efficient, and automated transmission. ROADM (Reconfigurable Optical Add-Drop Multiplexer) technology emerged to meet these challenges, enabling dynamic wavelength routing and automatic optical layer reconfiguration.

ROADM devices allow network operators to remotely add, drop, or pass-through wavelengths without manual intervention or service disruption, greatly enhancing the agility and automation of optical transport networks.

Development Stages of ROADM Technology
Since WSS is a new generation of ROADM technology implementation, let’s first review the development history of ROADM. From the initial proposal of the ROADM concept to its commercialization, and then to the new generation of WSS technology, the entire process can be well illustrated by Figure 1.

Figure 1: Development History of ROADM Technology

Wavelength Division Multiplexing (WDM) is currently the most common optical layer networking technology. By multiplexing different wavelengths, it enables transmission over a single optical fiber, easily achieving capacities of Gbit/s or even Tbit/s. However, current WDM systems are essentially point-to-point line systems, and most optical layer networking can only be achieved through optical line systems constructed via terminal stations (TM). The concept of Reconfigurable Optical Add-Drop Multiplexer (ROADM) was initially proposed to enhance the flexibility of WDM, enabling cross-scheduling of information between different nodes. As shown in Figure 1, from 1998 to around 2001, the ROADM concept began to take shape. The structures depicted in the figure, based on Optical Cross-Connects (OXC) and Optical-Electrical-Optical (OEO) regenerators, followed by structures based on circulators and Demultiplexer-Switch Matrix-Multiplexer (DSM), were the initial experimental models. However, these systems, built with discrete components, suffered from high insertion loss, unstable performance, and high operational costs. Consequently, these technologies were only studied and tested during the early formation of the ROADM concept and did not achieve true commercialization.

The first commercialized technology, considered the first generation of ROADM, was Wavelength Blocker (WB) technology. Its working principle is illustrated in Figure 2. This technology uses a power splitter to divide the signals of all wavelengths into two beams. One beam passes through the WB module and is transmitted to the next ROADM network element, while the other is sent to the drop branch. The WB module’s role is to block the wavelengths that need to be dropped. The most common structure for the WB module is a Demultiplexer-Variable Optical Attenuator (VOA)-Multiplexer configuration, where each wavelength is demultiplexed and connected to a programmable VOA, which attenuates the dropped wavelengths as needed. The remaining wavelengths are then multiplexed by a WDM multiplexer and transmitted to the next network element. In the drop branch shown in Figure 2, the wavelengths to be dropped are separated by a demultiplexer, and Optical Performance Monitoring (OPM) is used to ensure power balance among the different dropped wavelengths.

Figure 2: Schematic Diagram of WB-ROADM Principle

The WB technology is currently mature, offering advantages such as low cost, simple structure, good modularity, and the ability to support flexible expansion and upgrade functions when reserving upgrade ports. It is suitable for LH and ULH systems and supports broadcast services (using the concept of power splitting). However, WB technology requires operators to purchase multiple wavelengths at once. Additionally, this structure necessitates external filters for wavelength dropping. If fixed filters are used, dynamic reconfiguration of add/drop wavelengths is not possible; only pass-through wavelengths can be reconfigured, making it difficult to transition to optical cross-connect (OXC).

Figure 3: Schematic Diagram of PLC-ROADM Principle

The second-generation ROADM shown in Figure 3 is based on Planar Lightwave Circuit (PLC) technology. It is, in fact, a development and continuation of the DSM-ROADM technology shown in Figure 1. By utilizing integrated waveguide technology, components such as demultiplexers (typically AWG), 1x2 optical switches, VOAs, and multiplexers are integrated onto a single chip, significantly reducing costs through large-scale production. As a result, PLC technology is the most cost-effective ROADM implementation. The use of 1x2 or 2x2 optical switches provides two-dimensional degrees of freedom. However, PLC-ROADM and WB-ROADM share many similarities, as both have add/drop ports tied to specific wavelengths, making it impossible to reconfigure add/drop wavelengths.

The third-generation ROADM technology, which emerged to address these limitations, is centered on Wavelength Selective Switching (WSS). As shown in Figure 4, compared to WB, the key feature of WSS is the elimination of the WB module, enabling each wavelength to be independently switched. As illustrated in Figure 4, a multi-port WSS module can independently assign any wavelength to any path. Consequently, WSS-based networks offer multiple degrees of freedom, eliminating the need to predefine network interconnection architectures, as required by WB or PLC technologies.

Figure 4: Schematic Diagram of WSS-ROADM Principle

Figure 5: Functional Diagram of the WSS Module

As shown in Figure 5, each wavelength signal is independently and programmably controlled, directed to either the Express port or the drop port based on actual needs. The Express port then combines the new add signals and transmits them to the ROADM output module. Similar to WB technology, an Optical Performance Monitor (OPM) is used at the system’s end to ensure power balance across multiple wavelength signals.

WSS-ROADM System and Typical Applications

Wavelength Selective Switching Reconfigurable Optical Add-Drop Multiplexer (WSS-ROADM) represents a cornerstone of third-generation optical networking technology. Unlike its predecessors, such as Wavelength Blocking (WB) and Planar Lightwave Circuit (PLC)-based ROADMs, WSS-ROADM eliminates the need for fixed wavelength modules, offering unparalleled flexibility in wavelength management. This technology enables dynamic reconfiguration of wavelengths, making it a critical component in modern optical communication networks.

System Composition of WSS-ROADM
The WSS-ROADM system is designed to provide flexible wavelength routing and management. Its core components and functionality can be summarized as follows:

Wavelength Selective Switching (WSS) Module
The WSS module is the heart of the system, capable of independently controlling each wavelength signal. As illustrated in typical functional diagrams, the WSS module directs wavelengths to specific ports (e.g., Express or drop ports) based on programmable configurations.

Multi-port WSS modules allow any wavelength to be assigned to any path, providing multiple degrees of freedom. This eliminates the need for predefined network interconnection architectures, a limitation found in WB and PLC-ROADM systems.

Express and Add/Drop Ports
The Express port handles pass-through wavelengths and combines them with new add signals for transmission to the ROADM output module.

Add/drop ports facilitate the insertion or extraction of specific wavelengths, enabling localized traffic management without disrupting the entire network.

Optical Performance Monitor (OPM)
At the system's end, an OPM ensures power balance across multiple wavelength signals, maintaining signal integrity and network performance. This is a shared feature with WB-ROADM systems, ensuring consistent power levels for reliable transmission.

Control and Management Systems
WSS-ROADM systems incorporate advanced software for dynamic wavelength provisioning and network management. This allows operators to reconfigure wavelengths in real-time, adapting to changing traffic demands.

The architecture of WSS-ROADM supports colorless, directionless, and contentionless (CDC) operations, meaning wavelengths can be added or dropped without being tied to specific ports, directions, or wavelength conflicts. This flexibility is a significant advancement over earlier ROADM technologies, which were constrained by wavelength-specific ports and limited reconfiguration capabilities.

Figure 6: Diagram of a 2-Degree WSS-ROADM
   
As shown in Figure 6, the ROADM system based on WSS technology consists of a 1×N and an N×1 WSS module. The 1×N WSS can route any combination of wavelengths from the input port to any of the output ports. Conversely, the N×1 WSS can signals from any input port, combine any desired wavelength sets with those from other input ports, and output the merged signal.

This ROADM architecture exhibits wavelength-independent characteristics at both the add and drop ports—any downstream port can carry signals of any wavelength, and any wavelength can be transmitted through the upstream ports.

It is important to note that in the configuration shown in Figure 6, the data flow is bidirectional (East-West), thus representing a 2-degree ROADM. However, it is also evident that in this structure, aside from the express (pass-through) port, each output port of the WSS only outputs a single wavelength. Therefore, a 2-degree ROADM can be easily upgraded to support up to N-1 degrees, as illustrated in Figure 7.

Figure 7: Schematic Diagrams of Multi-Degree WSS-ROADM Architectures
(a) Architecture based on multiple WSS modules
(b) Architecture with downlink based on optical splitters and uplink based on WSS modules
(c) Architecture with downlink based on WSS modules and uplink based on optical combiners

Figure 8: Typical Application Example of a WSS-ROADM

Typical Applications of WSS-ROADM

WSS-ROADM technology is widely deployed in various scenarios due to its flexibility and robustness. Below are some typical application examples:

1. Metropolitan and Core Optical Networks
In metropolitan area networks (MANs) and core backbone networks, WSS-ROADM systems are used to manage high volumes of data traffic. For instance, in a dense wavelength-division multiplexing (DWDM) network, WSS-ROADM enables operators to dynamically allocate wavelengths to different cities or data centers based on real-time demand. This is particularly valuable in scenarios with fluctuating traffic patterns, such as during peak business hours or major events. The colorless and directionless features ensure that wavelengths can be rerouted without physical reconfiguration, reducing operational costs and downtime.

2. Data Center Interconnect (DCI)
Data centers require high-capacity, low-latency connections to support cloud computing, big data, and content delivery networks. WSS-ROADM systems are deployed to interconnect data centers, allowing seamless wavelength provisioning to handle massive data flows. For example, a cloud service provider might use WSS-ROADM to allocate additional wavelengths between data centers during a surge in user demand, ensuring uninterrupted service. The ability to independently switch wavelengths also supports "" balancing and fault recovery, enhancing network reliability.

3. Long-Haul and Ultra-Long-Haul Transmission
In long-haul and ultra-long-haul optical networks, WSS-ROADM systems provide efficient wavelength management over vast distances. By integrating OPMs, these systems maintain signal quality across thousands of kilometers, making them ideal for transcontinental or submarine cable networks. For instance, a trans-Pacific optical network might employ WSS-ROADM to dynamically route wavelengths between Asia and North America, optimizing bandwidth usage and minimizing latency.

4. 5G and Future Network Infrastructure
The rollout of 5G networks and the anticipated development of 6G require robust optical backhaul and fronthaul solutions. WSS-ROADM systems support the high-bandwidth, low-latency requirements of 5G by enabling flexible wavelength al for fronthaul connections between base stations and core networks. Additionally, the scalability of WSS-ROADM makes it a future-proof solution for emerging technologies, such as network slicing and edge computing.

5. Disaster Recovery and Network Resilience
WSS-ROADM’s dynamic reconfiguration capabilities are critical for disaster recovery. In the event of a fiber cut or node failure, operators can quickly reroute wavelengths to alternative paths without physical intervention. For example, a European telecom operator might use WSS-ROADM to redirect traffic around a damaged cable segment, ensuring continuous service for critical applications like emergency communications or financial transactions.

ROADM technology is a cornerstone in building next-generation, intelligent optical transport networks. From the early days of WB-ROADM to the highly flexible WSS-ROADM systems, the evolution reflects the growing demand for agility, automation, and programmability.

WSS-ROADM represents a significant leap forward in optical networking, offering unmatched flexibility, scalability, and efficiency. Its core component, the WSS module, enables independent wavelength switching, freeing networks from the constraints of earlier technologies like WB and PLC-ROADM. With applications ranging from metropolitan networks to long-haul transmission and 5G infrastructure, WSS-ROADM is a cornerstone of modern and future optical communication systems. As global data demands continue to grow, WSS-ROADM will play a pivotal role in building resilient, high-capacity, and adaptive networks.