What Are the Top 5 Drivers Behind ROADM Evolution?

Since its introduction in the early 2000s, ROADM technology has evolved significantly. Key advancements include the adoption of WSS, increasing WSS port counts, reduced footprints, more flexible add/drop capabilities, and the transition from fixed grid to flexible grid. This passage will help you understand the top 5 drivers behind the ongoing evolution of ROADM technology.

What Are ROADMs?

Reconfigurable Optical Add-Drop Multiplexers (ROADMs) are crucial components in optical communication networks, facilitating flexible routing of optical signals without the need for conversion to electrical signals. They offer reconfigurability, allowing real-time adjustments of signal routing paths within the network. ROADMs support optical add-and-drop functionalities, enabling selective insertion or extraction of individual optical channels without disrupting other channels. Leveraging multiplexing techniques, they combine multiple optical channels onto a single fiber for transmission and demultiplex them at receiving nodes. Optical switching enables rapid signal routing within the optical domain, eliminating latency and energy consumption associated with signal conversion. Managed by network management systems, ROADMs ensure efficient resource utilization through real-time monitoring and scheduling. Their deployment enhances network agility, efficiency, and scalability, meeting the demand for high-speed, high-capacity data transmission in modern communication networks. WSS technology, amplifiers, optical channel monitor (OCM), optical supervisory channel (OSC), and optical time-domain reflectometer (OTDR), which are the 5 key enablers of ROADM evolution.

Figure1. Top 5 Drivers Behind ROADM Evolution

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.

WSS Technology Advancement

While early ROADMs utilized wavelength blocker and planar lightwave circuit (PLC) technologies, today's primary WSS technologies are microelectromechanical systems (MEMS) mirrors, which include digital light processing (DLP), liquid crystal (LC), and liquid crystal on silicon (LCoS). Roadm with lower port counts typically employ cost-effective DLP or LC technology, while those with higher port counts generally opt for the more expensive LCoS technology.

The evolution of WSS has seen a remarkable increase in port numbers, progressing from 1×2 to 1×30+ and expected to reach even higher counts such as 48 or 60 in the future. Units have advanced from single WSS to dual and quad configurations, enhancing capabilities for routing and selection. The C-band spectrum has expanded significantly from 3,200 GHz to 4,800 GHz, with recent developments extending it to 6,000 GHz in the C-band and 9,600 GHz in C+L bands, integrating separate WSS functionalities. Channel spacing has shifted from 100 GHz to 50 GHz and adopted flexible grids with granularity starting at 12.5 GHz and moving to 6.25 GHz. Performance enhancements include improved cascadeability, allowing more WSS units along the wavelength path, and squarer passband shapes that reduce filter narrowing penalties. WSS footprint has notably decreased, especially with edge-optimized (1×4) designs. Dual and quad WSS configurations have minimized space requirements for route-and-select ROADMs, enabling multiple functionalities on a single blade (referred to as "node-on-a-blade") to further reduce footprint.

Amplifier Advancement

Amplifiers have progressed significantly in their capacity to provide higher gain. One major factor contributing to this increased gain is the adoption of integrated ROADM-on-a-blade architectures, which feature internal connections to the amplifiers, thereby allowing higher power levels. Additionally, advancements have been made in minimizing the amplified spontaneous emission (ASE) noise introduced relative to the gain achieved. Another development has been the transition from fixed-gain amplifiers to variable-gain amplifiers. Variable-gain amplifiers typically operate within specific span loss ranges, with at least three types needed (e.g., 0-18 dB, 14-25 dB, 22-35 dB). This evolution has further progressed to switchable-gain amplifiers, which can cover a wide span loss range (e.g., 0-32 dB) using a single part number. There has also been a trend towards hybrid amplification, combining Erbium-doped fiber amplification (EDFA) with Raman, aimed at reducing noise.

OCM Advancement

OCMs enable real-time monitoring of wavelength power levels, facilitating their adjustment via WSS at ROADM sites or dynamic gain equalization (DGE) at ILA sites to optimize each wavelength's power. OCMs also support network troubleshooting. Recent advancements include flexible-grid OCMs and high-resolution coherent OCMs. Coherent OCMs offer precision below one gigahertz, ensuring accurate monitoring of spectral segments independent of neighboring channel power levels. They significantly reduce C-band scanning times from seconds to hundreds of milliseconds, enabling enhanced processing of spectral characteristics such as detecting valid channels, identifying center wavelengths, and assessing optical signal-to-noise ratio (OSNR).

OSC Advancement

The OSC serves as a communication pathway between adjacent nodes, facilitating various functions such as link control, in-band management, control plane operations (e.g., ASON/GMPLS), and span loss measurement. OSC data rates have advanced from around 2 Mb/s to approximately 100-155 Mb/s, and most recently to 1 Gb/s. Initially located at the shelf controller, the OSC has shifted to the ROADM card, and more recently to SFP pluggables. This evolution accommodates different types of OSC tailored to specific application needs and interoperability requirements.

OTDR Advancement

An OTDR emits light pulses into the fiber under test and analyzes the light that returns through scattering and reflections. Applications include pinpointing the location of fiber breaks, detecting increased fiber attenuation, and intrusion detection. Integrated OTDRs became available as a ROADM option around 2015. Recently, SFP-based OTDRs have emerged as a compact single-fiber alternative to larger OTDRs that support multiple fibers via an optical switch. There are now SFPs integrating both OTDR and OSC functionalities; initially serving as an OSC, they switch to an "out-of-service" OTDR upon detecting a fiber break.

Coherent OTDRs represent another recent advancement. While conventional OTDRs measure loss, coherent OTDRs can also assess parameters like chromatic dispersion, polarization mode dispersion (PMD), and changes in polarization state. They can monitor the entire span of a repeated trans-oceanic fiber, including passing through amplifiers. Coherent OTDRs have potential applications such as early warning of terrestrial fiber breaks through vibration detection from construction work, and monitoring seismic activity in submarine cables.

Conclusion

In summary, ROADM technology has evolved with enhanced WSS port counts, amplified gain, advanced OCM and OSC functionalities, and integrated OTDR capabilities. These innovations have significantly improved network flexibility, efficiency, and reliability, addressing the growing requirements for high-capacity data transmission. Ongoing advancements promise continued enhancements in optical communication networks, supporting their role in facilitating robust and scalable infrastructure for modern telecommunications.