Global optical networks continue to evolve, necessitating new and innovative solutions to meet the requirements of network operators to maximize fiber utilization and reduce the cost of data transport. Coherent optical transmission has been the key enabling technology supporting both requirements over the last decade—so much so that operators will continue to make use of its benefits over the next stages of network evolution.

Depending on the requirements of different parts of the network, two primary types of coherent solutions are emerging. One will support performance-optimized systems, which maximize fiber capacity and programmability. The other will support footprint-optimized systems, where performance trade-offs are made to support low power and small space requirements for high-density modular systems.

For some data center operators, single-span connectivity at 400G capacity between data centers is an example where footprint-optimized, coherent solutions - having tailor-made capabilities for reach, network compatibility, and spectral efficiency - will be used.  These relatively high-capacity, modular 400ZR solutions will be based on coherent technology implemented in QSFP-DD and OSFP—small, pluggable form factors that were defined with this use case in mind. 

Figure 1: Quad Small Form-factor Pluggable – Double Density (QSFP-DD) and Octal Small Form-factor Pluggable (OSFP)

Next-generation metro-regional networks will be able to take advantage of advances made in relatively lower-power pluggable coherent solutions in multi-port hardware interfaces where required . In these metro networks, modularity will be further enhanced by supporting multiple different channel capacities depending upon the reach requirements and compatibility with the installed metro optical infrastructure. These pluggable coherent modules—with line capacities up to 400 Gb/s and reaches well beyond the 80km specification for 400ZR—fit the general profile for ZR+.

ZR+ vs. 400ZR
To date, there are no industry specifications or standards that explicitly outline which applications will be supported by ZR+. Rather, the term is a general reference to a potential range of solutions, or operating modes, for coherent pluggable solutions. In contrast, for the 400ZR IA, collaborations between the OIF and MSA organizations are ongoing to ensure the specification is compatible with the desired mechanical form factors and management interfaces.

Although the 400ZR IA does not place any constraints on the physical form factor, it does stipulate a maximum power dissipation of 15W for any implementations. This places constraints on the design and selection of 400ZR DSP, electro-optical, and peripheral components, but allows for support of the smallest form factors (QSFP-DD and OSFP).

These very compact form factors have limits on maximum power dissipation to allow adequate thermal management for use in high-density, multi-port sockets. Given that capacities and reaches for ZR+ are not standards-driven at this stage, it’s not surprising that there are different interpretations of what mechanical form factors the products will utilize and which features and capabilities they will support.

Another comparison between 400ZR and ZR+ is with respect to interoperability. In the OIF 400ZR IA, the line signaling rate (baud) and modulation format, among other parameters, are specified to arrive at a standardized, interoperable specification. As there is a specific, constrained application for 400ZR (point-to-point, single-span DCI links), it has been possible to generate a specification addressing interoperability. ZR+ applications, being broader in scope, will be less amenable to interoperable standards unless broken down into a series of specific applications or application subsets. Despite the lack of direct connectivity between the ZR+ concepts and standards development, there are two areas where international standards organizations are exploring applications that could potentially fall into the definition of ZR+. One is the ITU-T Study Group 15, where a metro ‘black link’ application for less than 450km is being considered.  The other is Open ROADM.

ZR+ definitions
One definition of ZR+ is a straightforward extension of 400ZR transcoded mappings of Ethernet with a higher performance FEC to support longer reaches. In this interpretation, ZR+ modules would be narrowly defined as supporting a single-carrier 400 Gb/s optical line rate and transporting 400GbE (or 4x100GbE) client signals at metro reaches (up to around 500km).

Another definition is a pluggable coherent module, or set of module products, that supports a range of different line rates, client types and reaches, and which are compatible with metro photonic line systems. This could include modules supporting 200 to 400 Gb/s capacity at metro reaches or even metro/regional reaches, as well as support for OTN in addition to Ethernet.

For extended reaches, in addition to higher performance FEC, these modules will require tunable optical filters and amplifiers. Compared to the performance of a 400ZR pluggable module, or an extended reach ZR+ version of 400ZR, these solutions are sometimes referred to as ‘ZR++’ to indicate the greatly extended capabilities for transporting up to 400 Gb/s line rates.

ZR+ use cases
Looking at use cases for potential ZR+ pluggable solutions, it’s expected that they will be deployed across a range of products including switches, routers, and optical transport equipment. The applicable product will depend on customer operational requirements, network architecture, and traffic growth rates.

400ZR and ZR+ applications

Figure 2: 400ZR and ZR+ applications

Considerations for implementation
In addition to standardization and interoperability, there are other considerations for the implementation of 400ZR and ZR+. 400ZR solutions will not be easy to design and deliver in the target power and area constraints. They require realization of 60 GBaud, 16-QAM 400 Gb/s coherent transmission in a form factor previously used for very short-reach Ethernet data communications applications.

400ZR performance, while not comparable to the capacity and reach of coherent solutions for long-haul terrestrial and submarine network, is still a leap forward in the application of coherent technology. It requires skill sets in foundational technologies like digital and analog design in CMOS for the coherent DSP, and photonic integrated circuit design in silicon photonics for the optical front-end. These elements must be co-designed for optimal implementation.

ZR+ will take implementation challenges to the next level by adding some of the elements for high-performance solutions while pushing component design for low-power, pluggability, and modularity. Depending on the application requirements for reach, propagation in real metro networking scenarios, number of transmissions modes, and client interface support, this will have an impact on the power dissipation of the implementation, which will impact the choice of mechanical form factor. The combination of system and component design experience will pave the way for solutions supporting the next generation of DCI and metro networks