The Future of Coherent Transceivers is Here

The latest developments in coherent optics have solidified two key trends in the development of optical transceivers that will guide the industry for the foreseeable future. The first is an industry-wide bifurcation in how coherent transceivers are designed and optimized, while the second is the convergence of coherent optics into integrated digital coherent optic (DCO) transceivers.

The first development recognizes that there is no “one size fits all” optimization for coherent transceivers. On one hand, the traditional industry push for faster baud rates, ever-higher headline capacities, and maximizing capacity-reach will continue in the form of performance-optimized optics that seek to eke out performance as close to the Shannon Limit as is possible. On the other hand, there has emerged a consensus that another important design priority for coherent transceivers focuses on optimizing for low power and packaging into industry-standardized pluggable form factors such as QSFP-DD and CFP2. This optimization enables cost-effective, modular deployment for a wide range of use cases and networking platforms. This comes with a tradeoff however, as optimization for low power and small size reduces performance metrics such as maximum capacity per wavelength and optical reach. The second development is a result of the ever-increasing speeds of coherent transceivers. These will henceforth require ever-tighter integration of all their constituent parts-- the DSP, the high-speed opto-electronics such as coherent modulator and receiver, and the driver and amplification electronics. These can no longer connect between distinct modules mounted across a circuit-board and must now be carefully co-designed and miniaturized into a DCO transceiver to maintain signal integrity at ever-higher operating speeds.This article will explore how these characteristics are finding reality in the fifth-generation photonic service engines (PSEs).

Pluggable DCO transceivers optimized for metro-regional

In the camp of power and form-factor optimized coherent transceivers are the compact PSE transceivers. Implemented into a 400G multihaul DCO, the compact transceivers provide 400 Gbps of capacity in a CFP2 pluggable form factor, for use in routers and optical transport systems. Using the latest generation of 7-nm silicon CMOS node technology to reduce power per bit while simultaneously scaling the baud rate up 67 Gbaud, 400G multihaul transceivers typically support high-order modulations such as 16QAM to enable transmission of 400 Gbps per carrier over several hundreds of kilometers.
With an output launch power of 0 dBm and compatibility with 75-GHz WDM channel spacings, 400G multihaul transceivers also support transmission across multiple reconfigurable optical add/drop multiplexer (ROADM) nodes. This ability enables optical bypass of wavelengths across intermediate nodes as well as source-to-destination transport between any two nodes in complex fiber network topologies.
Combined with support for both Ethernet and OTN client inputs and support for interoperable standards such as OpenZR+ and OpenROADM, the use of 400G multihaul transceivers is an ideal choice for a wide range of use cases in access, metro aggregation, and metro-regional core networks that require capacity upgrades, all the while leveraging a pay-as-you-go deployment model enabled by a pluggable form factor and the flexibility to operate in both optical transport and router platforms.

Super-coherent optics take DCOs to the Shannon Limit

At the other end of the spectrum is the super coherent PSE transceiver optimized to provide the maximum capacity-reach performance over challenging long-haul and subsea applications. Operating at 90 Gaud, super-coherent optics implement advanced features such as probabilistic constellation shaping (PCS), high-gain/low overhead forward error correction (FEC), and continuous baud rate adjustment. Their ability to operate in 100-GHz WDM channel spacing enables seamless upgrades over existing ROADM networks and channel plans.
These features enable performance to with 1-2 dB of the Shannon Limit and power wavelength speeds up to 600 Gbps over distances of 1500 km or more. Speeds of 400 Gbps over multiple thousands of kilometers are possible as well over highly flexible CDC-F wavelength-switched networks.
This level of performance is only achievable by integrating the entire photonic service engine into tightly optimized DCO. This approach enables super-coherent optics to be utilized as an embedded or pluggable DCO, enabling implementation across a range of optical transport platforms including compact modular, disaggregated transponders, integrated WDM/transponder platforms, and P-OTN switches with integrated WDM optics, while enabling all of these to support the highest capacities at the longest distances.

Optimizing coherent solutions for the application

The latest trends in coherent transceivers recognize the need of network operators to scale the capacity and performance of their optical transport networks, while at the same time optimizing these for the lowest total cost of ownership (TCO) across the wide range of network use cases from access and metro networks all the way to the most challenging long-haul and subsea links.

By leveraging DCO transceivers optimized for both low power and industry-standard form factors, complemented by super-coherent DCOs implementing the latest advances in coherent modem technology, operators can choose the most efficient approach for scaling each part of their networks.Serge Melle is director, IP-optical product marketing, at Nokia.