A Guide to ROADM Compatibility: Critical Factors to Consider

The evolution of ROADM networks has advanced from 2.5G/10G to 40G, 100G, and now to coherent digital signal processing using 7-nm CMOS technology. This enables higher baud rates and 400 Gb/s pluggable modules like 400ZR and 400ZR+, supporting QSFP-DD, OSFP, or CFP2 form factors. Ensuring ROADM compatibility is essential to leverage these advancements. So, what are the key considerations of ROADM compatibility? Let's learn it together!

What Are ROADMs?

Reconfigurable Optical Add-Drop Multiplexers (ROADMs) are vital to optical communication networks, enabling flexible and real-time routing of optical signals without converting them to electrical signals. They support the selective insertion or extraction of individual optical channels, allowing multiplexing and demultiplexing of multiple channels on a single fiber. By leveraging optical switching, ROADMs facilitate rapid signal routing and reduce latency and energy consumption. Managed by network management systems, they optimize resource utilization and enhance network agility, efficiency, and scalability to meet high-speed, high-capacity data transmission demands.

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.

3 Top Factors of ROADM Compatibility

Wavelength

The spectral width of a wavelength primarily depends on its baud rate and, to a lesser extent, its roll-off. Higher baud rates mean broader spectrum usage. For example, a 60 Gbaud 400 Gb/s wavelength needs a little over 60 GHz, while a 96 Gbaud 800 Gb/s wavelength requires slightly more than 96 GHz. A 60 Gbaud wavelength with a 10% roll-off would have a spectral width of 66 GHz.

Figure1. Spectral Width in GHz

The evolution of ROADM networks has advanced from 2.5G/10G to 40G, 100G, and now to coherent digital signal processing using 7-nm CMOS technology. This enables higher baud rates and 400 Gb/s pluggable modules like 400ZR and

The next question is how much spectrum the ROADM can provide for this wavelength. It's crucial to differentiate between the grid and the actual passband of the filter/ROADM. A legacy 100 GHz grid WSS typically has a passband of 40-50 GHz, while a 100 GHz mux/demux filter has a passband of 50-70 GHz.

Figure2. Grid Spacing vs. Actual Passband (100 GHz Example)

Filter narrowing is another factor. A typical 50 GHz WSS has a 46 GHz passband per channel, but differences in width, center wavelength, or shape can reduce the effective passband. After 10 WSSs, the passband can shrink to around 36 GHz, limiting the baud rate to about 33 Gbaud.

Figure3. Filter Narrowing and WSS Cascades (50 GHz WSS Example)

For 400 Gb/s using PM-16QAM modulation, a symbol rate of at least 60 Gbaud is needed, requiring a passband of over 60 GHz. Open ROADM MSA-compatible 400G pluggables operating at 63.1 Gbaud need at least 87.5 GHz in a mesh ROADM network. For compatibility with 100 GHz/50 GHz fixed grids, XR optics can adjust the number of subcarriers to fit the passband, and some 400ZR+ pluggables can lower the baud rate to around 30 Gbaud, delivering 100 Gb/s (PM-QPSK) or 200 Gb/s (PM-16QAM).Even with higher-order modulation (PM-64QAM), 400 Gb/s requires at least 42 Gbaud, making it incompatible with legacy fixed-grid ROADMs after filter narrowing.

Supporting higher baud rates necessitates flexible-grid ROADMs with 12.5 GHz or 6.25 GHz granularity, ideally with control over both channel width and center frequency. Add/drop options include gridless/colorless (colorless-directional, colorless-directionless, or CDC) or fixed (colored-directional) with wide passband filters. The latest wide passband filters optimized for high-baud-rate wavelengths can support an overlapping passband 10% larger than the channel spacing.

Transceiver

400G coherent QSFP-DD transceivers, particularly those based on silicon photonics, generally exhibit low TX power levels, ranging from -10 to -5 dBm. In contrast, the larger CFP2 form factor can accommodate a micro erbium-doped fiber amplifier (EDFA), allowing for higher TX power between -5 and 0 dBm.

Coherent pluggable transceivers that leverage indium phosphide technology can incorporate a semiconductor optical amplifier (SOA), enabling these transceivers to achieve TX power levels up to 0 dBm, even within the compact QSFP-DD form factor. This higher TX power is particularly advantageous because reconfigurable optical add-drop multiplexers (ROADMs) typically require per-wavelength receive power around 0 dBm. As a result, many QSFP-DDs and even some CFP2s with lower TX power might necessitate additional amplification to ensure compatibility with ROADM systems.

Embedded optical engines often provide even higher TX power levels, supporting TX power up to 9 dBm and ensuring compatibility with a broader range of long-haul ROADM systems. This higher TX power mitigates the need for extra amplification, thereby simplifying network design and deployment. The combination of high TX power and advanced optical technologies in these embedded solutions offers significant advantages for modern optical networks, ensuring robust performance and extensive reach.

Colorless Add/Drop Capabilities

The various implementations of colorless add/drop functionality offer several benefits, including simplified installation, faster provisioning, optical restoration, network defragmentation, and enhanced service assurance. To maintain cost-effectiveness, modern ROADMs capitalize on coherent receivers' capability to receive multiple channels and selectively tune to specific channels. This tuning is facilitated by a tunable optical filter (TOF) integrated within coherent transceivers, which crucially prevents the accumulation of out-of-band noise. In the context of smaller form factors like QSFP-DD, the inclusion of a TOF becomes challenging due to space constraints. Without a TOF, the effectiveness of colorless add/drop functionality diminishes, potentially limiting the number of coherent transceivers that can be supported per add/drop unit. However, emerging technologies such as XR optics pluggables, leveraging indium phosphide photonics, offer out-of-band noise performance comparable to larger CFP2 modules even within the QSFP-DD form factor. This advancement ensures that QSFP-DD-based systems can achieve robust colorless add/drop capabilities, maintaining efficiency and performance in modern optical networks.

Conclusion

In summary, the evolution of ROADM networks and the advancements in transceiver technology, including QSFP-DD and XR optics, have significantly enhanced the performance, flexibility, and efficiency of modern optical networks. Key considerations like wavelength compatibility, transceiver power levels, and colorless add/drop capabilities are crucial for maximizing the benefits of these technologies, ensuring robust and scalable high-speed data transmission.