Building Blocks and Innovations Enabling ROADM Evolution
December 7, 2021
By Paul Momtahan
Director, Solutions Marketing
Since its introduction in the early 2000s, reconfigurable optical add/drop multiplexer (ROADM) technology has undergone a series of evolutions. These include the shift to wavelength-selective switches (WSS), increasing WSS port counts, reductions in footprint, more flexible add/drop, and the move from fixed grid to flexible grid. Furthermore, ROADM technology continues to evolve. In this blog, the first in a series, I will describe what is driving this evolution and some of the key component-level innovation enablers.
Three Drivers for ROADM Evolution: Coherent, Open, and TCO
Coherent transceiver advances are a key driver for ROADM evolution. Coherent baud rates have evolved from ~30 Gbaud with earlier 100G and 200G coherent generations to 90+ Gbaud with the latest 800 Gb/s-wavelength embedded optical engines, with corresponding increases in the spectral width of the wavelength. This has driven the requirement for flexible-grid ROADMs with wide passbands and low filter narrowing, as discussed in a previous blog, “Is Your ROADM Ready for Next-generation Coherent?”. A second driver is the move to open optical networking, which at a minimum requires that ROADMs support wavelengths from third-party transceivers. And a third driver is reducing optical network total cost of ownership (TCO), which includes OpEx-related footprint, power consumption, installation, commissioning, maintenance, and troubleshooting, and CapEx related to both the optical line system and the coherent transceivers.
Key Innovations Enabling ROADM Evolution
Figure 1 shows the key building blocks of a generic ROADM degree: WSSs, amplifiers, optical channel monitor (OCM), optical supervisory channel (OSC), and optical time-domain reflectometer (OTDR). Innovation in these five components is a key enabler of ROADM evolution.
Figure 1: Key ROADM degree building blocks: WSSs, amplifiers, OCM, OSC, OTDR
While early ROADMs leveraged wavelength blocker and planar lightwave circuit (PLC) technologies, the main WSS technologies today are microelectromechanical systems (MEMS) mirrors, including digital light processing (DLP), liquid crystal (LC), and liquid crystal on silicon (LCoS). Lower port counts typically leverage lower-cost DLP or LC technology, while higher port counts typically use the more expensive LCoS technology.
Figure 2: WSS evolution timeline.
WSSs have also evolved in terms of number of ports, from 1×2 to 1×30+, evolving to even higher port counts (48, 60, etc.) in the future. The number of individual WSSs in a single unit has increased from one to two with twin WSSs, typically used for route and select, and more recently to four with quad WSSs. The amount of C-band spectrum has increased from 3,200 GHz to 4,800 GHz. Recent enhancements include 6,000 GHz in the C-band and 9,600 GHz with C- and L-bands (i.e., C+L), which itself has evolved from separate WSSs to a single C+L WSS. Channel spacing has also moved from 100 GHz to 50 GHz to flexible grid with first 12.5 GHz granularity and then 6.25 GHz granularity.
Performance has also improved with better cascadeability, enabling more WSSs in the wavelength path, as filter narrowing penalties have been reduced with a squarer passband shape. WSS footprint has shrunk dramatically, especially with the advent of edge-optimized (i.e., 1×4) WSSs. Twin WSSs have shrunk the required footprint for route-and-select ROADMs, while twin WSSs and quad WSSs provide options to further shrink footprint with multiple degrees on a single blade (i.e., “node-on-a-blade”).
Amplifiers have evolved in terms of the amount of gain they can deliver. One key contributing factor to this higher gain has been the adoption of integrated ROADM-on-a-blade architectures with internal connections to the amplifiers, allowing higher power levels. They have also improved in terms of the amplified spontaneous emission (ASE) noise added for a given gain. Another evolution has been from fixed-gain amplifiers to variable-gain amplifiers. Variable-gain amplifiers typically cover a specific span loss range, with at least three types required (e.g., 0-18 dB, 14-25 dB, 22-35 dB). This later evolved to switchable-gain amplifiers with a single part number able to cover a very wide span loss range (i.e., 0-32 dB). There has also been a trend toward hybrid amplification combining Erbium-doped fiber amplification (EDFA) with Raman in order to reduce noise.
OCMs provide the ability to monitor the power level of each wavelength. This information can then be used by the link control to attenuate each wavelength with the WSS at ROADM sites or dynamic gain equalization (DGE) at ILA sites in order to optimize the power level of each wavelength. OCMs can also be used to troubleshoot the network. Recent innovations include flexible-grid OCMs and higher-resolution coherent OCMs. Coherent OCMs offer sub-GHz accuracy and highly accurate power monitoring of fine spectral slices independent of adjacent channel power. They reduce the C-band scanning time from seconds to hundreds of milliseconds. And they provide advanced processing of spectral characteristics, such as valid channel detection, center wavelength, and optical signal-to-noise ratio (OSNR).
The OSC provides a communication channel between adjacent nodes that can be used for functions including link control, in-band management, control plane (i.e., ASON/GMPLS), and span loss measurement. The OSC data rates have evolved from ~2 Mb/s to ~100-155 Mb/s, and more recently to 1 Gb/s. The location of the OSC has moved from the shelf controller to the ROADM card, and more recently to SFP pluggables that also enable different flavors of OSC that meet specific application and interoperability requirements.
An OTDR transmits pulses of light into the fiber under testing and then analyzes the light that is returned through scattering and reflections. Use cases include identifying the location of fiber cuts, detecting increased fiber loss, and intrusion detection. Integrated OTDR started to appear as a ROADM option around 2015. More recently, SFP-based OTDRs have provided a more compact but single-fiber alternative to higher-performance OTDR form factors that support multiple fibers via an optical switch. SFPs are also now available that integrate the OSC and OTDR, with the SFP acting as an OSC until there is a fiber cut then switching to an “out-of-service” OTDR.
Coherent OTDRs are another recent innovation. While traditional OTDRs can measure loss, coherent OTDRs can also measure parameters such as chromatic dispersion, polarization mode dispersion (PMD), and state-of-polarization changes. And with the ability to pass through amplifiers, they can be used to monitor the entire length of a repeatered trans-oceanic fiber. Other potential applications for coherent OTDRs include pre-warning of terrestrial fiber cuts based on vibrations from construction activity and submarine monitoring of seismic activity.
Additional component innovations relate to the multicast switches that provide one option for colorless-directionless-contentionless (CDC) add/drop, wide-passband fixed filters, integrated ASE idlers that enable fast recovery in C+L networks, coherent probes, and DGE to enable per-channel power optimization at ILA sites. System-level innovations relate to ROADM form factors (individual modules, ROADM-on-a-blade, optical layer pluggables, node-on-a-blade, etc.), shelf form factors (compact modular, 600-mm depth, front-to-back airflow, etc.) and the link control software that sets amplifier and per-channel power levels.
Together these innovations are enabling ROADM evolution along seven vectors related to coherent transceiver wavelength capacity-reach, fiber capacity, add/drop and degree flexibility, footprint, openness, operations and manageability, and network availability. To learn more about this important topic, download the new Infinera white paper “The Seven Vectors of ROADM Evolution”.