Everything You Need to Know About ROADM Types

Coherent interfaces drive optical network growth and generate significant optical hardware revenue. Advances in coherent technology have allowed optical networks to handle 30% annual traffic growth while keeping capital expenditures (capex) low. While coherent interfaces are essential, they require a photonic layer infrastructure, or line system, to transport and route wavelengths from source to destination. This is where Reconfigurable Add/Drop Multiplexers (ROADMs) come in. ROADMs have replaced fixed-wavelength filters, offering programmability and numerous benefits in the photonic layer. So, do you know what types of ROADM there are? Let's learn it together!

ROADM Types

The total ROADM can be three types according to the size, spectrum, and internal interconnect.

ROADM Size

The number of ports on a ROADM line card is crucial for determining the maximum optical degrees (lines) within a ROADM node, with any remaining ports reserved for future degrees or add/drop components. Initially designed as 2-degree devices, ROADMs have evolved to include advanced 32-port line cards. While few network architectures require such high line-facing degrees, ROADMs with higher port counts enable the design of highly interconnected networks, support for multiple parallel optical lines, and scalable add/drop capacities — often a blend of these capabilities. However, the increase in ports also introduces higher costs and complexity, highlighting the ongoing importance of lower-port-count devices in networks with less demanding connectivity needs.

ROADM line cards with 9 ports or fewer are typically classified as "Small" ROADMs, while those with more than 9 ports are categorized as "Large". Small ROADMs commonly utilize 9-port configurations, though there is increasing interest in 2-4 port ROADMs for applications in access and aggregation scenarios. Meanwhile, 32-port ROADMs are predominant in the Large category, replacing earlier 20-port models that were not universally adopted by equipment vendors.

ROADM Spectrum

A DWDM system’s effective spectrum, defining the range for transporting wavelengths, is governed by the bandwidth capabilities of its amplifiers and ROADMs. Over the last two decades, WDM networks have primarily utilized the C-band, spanning up to 4.8 THz between 1529 nm and 1567 nm. These systems can accommodate up to 96 channels spaced at 50 GHz intervals, typically carrying wavelengths from 100G to 200G, or 64 channels spaced at 75 GHz intervals, supporting wavelengths from 300G to 600G. Cutting-edge Gen90P coherent interfaces now enable capacities of up to 40 Tb/s using 800G wavelengths, particularly effective over shorter distances. As coherent technology nears the Shannon Limit in spectral efficiency, there's a growing interest in broadening the operational spectrum of WDM systems to enhance overall capacity. Key strategies include integrating support for the L-band (1575 nm to 1617 nm) through dedicated sets of ROADM and amplifier line cards, and optimizing the existing C-band within unified ROADM and amplifier setups.

Three categories of ROADMs distinguished by their spectral coverage are currently documented, with additional variants anticipated in the coming years.

4.8THz C-band – the predominant choice in deployed ROADMs. This classification encompasses legacy ROADMs supporting less than 4.8THz.

4.8THz L-band – an extra ROADM card integrated into C-band ROADMs alongside band splitter/combiners and L-band amplifiers. Occasionally, L-band ROADMs are independently deployed apart from C-band ROADMs.

6THz C-band – utilizing a single line card and associated amplifiers to facilitate wavelengths from 1524nm to 1572nm, offering 25% more capacity compared to 4.8THz C-band ROADMs.

6THz L-band – not yet available for shipping; these cards will pair with 6THz C-band ROADMs, supporting wavelengths spanning 1575 to 1626nm.

Figure1. ROADM Spectrum

ROADM Internal Interconnect

A third variable quantifies the proportion of ROADMs incorporating an Optical Cross Connect (OXC). An OXC-enabled ROADM utilizes an OXC chassis rather than a traditional "fiber shuffle" patch panel, featuring backplane connectors on the ROADM and add/drop line cards instead of faceplate LC or MPO connectors. This design eliminates the need for fiber patch cables between degrees and add/drop components. OXC chassis are equipped with an all-optical backplane and maintenance-free, dust-proof connectors, promising significant reductions in footprint and cabling complexity compared to conventional ROADM setups. Currently, OXC ROADMs are exclusively provided by three Chinese optical network equipment manufacturers. While predominantly deployed within China, they are also marketed globally (excluding North America). Most OXC ROADMs fall into the Large category, although there is a growing availability of 9-degree variants as the technology advances and gains broader acceptance.

Benefits of ROADMs

Topology Flexibility: ROADMs provide any-to-any wavelength connectivity, independent of fiber topology, allowing for A-to-Z wavelength paths that match network traffic demands without costly regeneration.

Resiliency: ROADMs, combined with a distributed control plane or centralized controller, can route wavelengths around network failures without the need for redundant stand-by transponders.

Operational Efficiency: The optical layer is fully provisioned with ROADMs, requiring no additional touches or re-fibering to add capacity. Innovations like MxN add/drop, line loading, and optical cross-connect chassis further enhance efficiency.

FS ROADM Introduction

FS D7000 series ROADM-09T is based on advanced next-generation wavelength-selective switch (WSS) technology to support full colorless, directionless, and contentionless (CDC-F) system configuration. This module is a built-in pre-amplifier and booster amplifier which can simplify the installation and scalability of your ROADM network while improving its overall flexibility and performance. ROADM-09T supports N*12.5GHz flexible grid (4≤N≤12). Besides, this module can achieve add-and-drop services or pass-through.

Figure2. FS ROADM

Conclusion

The evolution of ROADMs has brought flexibility, efficiency, and resiliency to optical networks. As the industry continues to advance, understanding the different types of ROADMs and their applications will be crucial for future-proofing network infrastructures.