Infinera: building blocks and innovations allowing the evolution of ROADM

Since its introduction in the early 2000s, Reconfigurable Optical Insertion / Extraction Multiplexer (ROADM) technology has undergone a series of developments. These include moving to wavelength selective switches (WSS), increasing the number of WSS ports, reducing congestion, adding / removing more flexible, and moving to a fixed network. to a flexible network. In addition, 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 main catalysts for component-level innovation.

Three pilots for ROADM Evolution: Coherent, Open and TCO

Consistent advances in transceivers are a key factor in the evolution of ROADM. Coherent baud rates have increased from around 30 Gbaud with previous coherent generations 100G and 200G to over 90 Gbaud with the latest on-board optical engines at wavelength of 800 Gb / s, with corresponding increases in spectral width of wavelength. This has led to the requirement for flexible grid ROADMs with high bandwidths and low filter shrinkage, as discussed in a previous blog post, “Is Your ROADM Ready for the Next Generation Coherent?”. A second driving force is the move to an open optical network, which at a minimum requires ROADMs to support the wavelengths of third-party transceivers. And a third factor is the reduction in the total cost of ownership (TCO) of the optical network, which includes the footprint related to operations, power consumption, installation, commissioning, maintenance and operation. troubleshooting, as well as the capital expenditure associated with both the optical line system and the coherent transceivers.

Key innovations allowing the evolution of ROADM

Figure 1 shows the key building blocks of a generic ROADM degree: WSS, Amplifiers, Optical Channel Monitor (OCM), Optical Supervisory Channel (OSC), and Optical Time Domain Reflectometer (OTDR). Innovation in these five components is a key factor in the evolution of ROADM.

Figure 1: Main components of the ROADM diploma: WSS, amplifiers, OCM, OSC, OTDR

WSS innovations

While early ROADMs used wavelength blocking and planar light wave circuit (PLC) technologies, today’s main WSS technologies are mirrors of microelectromechanical systems (MEMS), including digital processing. light (DLP), liquid crystal (LC) and liquid crystal on silicon (LCoS). Lower port numbers typically take advantage of the lower cost DLP or LC technology, while higher port numbers typically use the more expensive LCoS technology.

Figure 2: Chronology of the evolution of WSS.

WSS has also evolved in terms of the number of ports, from 1 × 2 to 1 × 30 +, evolving to an even higher number of ports (48, 60, etc.) in the future. The number of individual WSSs in a single unit has grown from one to two with twin WSSs, typically used for road and selection, and more recently to four with quadruple WSSs. The amount of C-band spectrum has increased from 3200 GHz to 4800 GHz. Recent improvements include 6000 GHz in the C band and 9600 GHz with the C and L bands (i.e. C + L), which itself has moved from separate WSS to a single C + L WSS. . The channel spacing has also changed from 100 GHz to 50 GHz to a flexible grid with first a granularity of 12.5 GHz, then a granularity of 6.25 GHz.

Performance also improved with a better cascade, allowing more WSS in the wavelength path, as filter shrinkage penalties were reduced with a more square bandwidth shape. The WSS footprint has shrunk significantly, especially with the advent of edge-optimized (i.e. 1 × 4) WSSs. Twin WSSs have reduced the footprint required for routing and selection ROADMs, while twin WSSs and quadruple WSSs provide options to further reduce the footprint with multiple degrees on a single blade (i.e. say “knot on blade”).

Amplifier Innovations

Amplifiers have evolved in terms of the amount of gain they can deliver. One of the main factors contributing to this higher gain was the adoption of integrated ROADM-on-a-blade architectures with internal connections to amplifiers, allowing for higher power levels. They also improved in terms of the amplified spontaneous emission noise (ASE) added for a given gain. Another evolution has moved from fixed gain amplifiers to variable gain amplifiers. Variable gain amplifiers typically cover a specific range loss range, with at least three types required (e.g. 0-18dB, 14-25dB, 22-35dB). This later evolved into switchable gain amplifiers with a single part number capable of covering a very wide range of range loss (i.e. 0-32dB). There has also been a trend towards hybrid amplification combining erbium doped fiber amplification (EDFA) with Raman in order to reduce noise.

OCM innovations

OCMs provide the ability to monitor the power level of each wavelength. This information can then be used by link control to attenuate each wavelength with WSS at ROADM sites or Dynamic Gain Equalization (DGE) at ILA sites to optimize the power level of each length. wave. OCMs can also be used to troubleshoot the network. Recent innovations include flexible grid CMOs and higher resolution coherent CMOs. Coherent CMOs provide sub-GHz precision and highly accurate power monitoring of fine spectral slices independent of adjacent channel power. They reduce the C-band scan time from seconds to several hundred milliseconds. And they provide advanced processing of spectral characteristics, such as valid channel detection, center wavelength, and optical signal-to-noise ratio (OSNR).

OSC innovations

The OSC provides a communication channel between adjacent nodes which can be used for functions such as link control, in-band management, control plane (i.e. ASON / GMPLS) and loss of range measurement. OSC data rates have evolved from ~ 2 Mb / s to ~ 100-155 Mb / s, and more recently to 1 Gb / s. The OSC location has shifted from the shelf controller to the ROADM board, and more recently to SFP plug-in modules which also allow different flavors of OSC that meet specific application and interoperability requirements.

OTDR innovations

An OTDR transmits pulses of light through the fiber under test, then analyzes the returned light by scattering and reflection. Use cases include identifying the location of fiber breaks, detecting increased fiber loss, and detecting intrusion. Integrated OTDR began to emerge 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 through an optical switch. . There are also SFPs that integrate OSC and OTDR, with the SFP acting as an OSC until there is a fiber cut and then switching to an “out of service” OTDR.

Another recent innovation is coherent OTDRs. While traditional OTDRs can measure loss, coherent OTDRs can also measure parameters such as chromatic dispersion, polarization mode dispersion (PMD), and changes in polarization state. And with the ability to traverse amplifiers, they can be used to monitor the entire length of a repeated transoceanic fiber. Other potential applications for coherent OTDRs include advance warning of terrestrial fiber cuts based on construction activity vibrations and underwater monitoring of seismic activity.

Additional innovations

Additional component innovations are for multicast switches that offer an option for add / remove colorless, directionless, contention-free (CDC), fixed high-bandwidth filters, built-in ASE users that enable fast recovery in the C + L networks, coherent and DGE probes to allow optimization of the power per channel on the ILA sites. System level innovations include ROADM form factors (individual modules, ROADM-on-a-blade, pluggable optical modules, node-on-a-blade, etc.), shelving form factors (compact module, depth 600 mm, front-to-back airflow, etc.) and the link control software which defines the power levels of the amplifier and by channel.

Together these innovations enable the evolution of ROADM along seven vectors related to the coherent range of transceiver wavelength capability, fiber capacitance, add / drop flexibility and degree, footprint, openness, operations and manageability, and availability of the network. To learn more about this important topic, download Infinera’s new white paper “The Seven Vectors of ROADM Evolution”.