Coherent Optics: Evolving Architectures and Emerging Market Segments

Introduction

In today's rapidly evolving telecommunications landscape, network operators face a critical challenge: How can they continue to scale their networks to meet growing bandwidth demands without proportionally increasing costs and power consumption? This tutorial explores the advancements in coherent optical technology that are helping to address this challenge, focusing on changing architectures and emerging market segments.

The Evolution of Coherent Technology Classes

Coherent optical technology has progressed through several classes, each offering significant improvements in performance, efficiency, and integration. Let's examine the key characteristics of each class:

Class 1 Coherent Technology:

• Rapid improvements in fiber capacity, power efficiency, and cost per bit

• Transition from QPSK to 16QAM modulation

• Significant benefits from Moore's Law, with digital processing consuming a higher portion of module power

• Introduction of industry-first pluggable modules

• Limited adoption of interoperable interfaces

• Operated at 30-34+ Gbaud rates with a 50GHz channel grid

Class 2 Coherent Technology:

• Standardized interfaces

• Introduction of ZR/ZR+ in client form factors, enabling router-based applications

• First deployments of constellation shaped solutions

• Adaptive baud rates, allowing transmit spectrum to closely match the channel

• Wider deployment of pluggable modules

• Operated at 60-68+ Gbaud rates with a 75GHz channel grid

• Key standards: 400ZR, OpenZR+, Open ROADM

Class 3 Coherent Technology:

• Incremental (~20%) improvements in spectral efficiency

• Interoperable PCS (Physical Coding Sublayer) in MSA (Multi-Source Agreement) pluggables

• Improvements in density and cost per bit

• Introduction of performance-optimized designs in pluggable form factors

• Operated at 120-136+ Gbaud rates with a 150GHz channel grid

• Key standards: 800LR, 800ZR, Open ROADM

Class 4 Coherent Technology:

• Recently initiated 1600ZR effort in OIF (Optical Internetworking Forum)

• Preference for single carrier (~240Gbaud) to achieve power and cost objectives

• Targeting small form factor pluggables for router deployments

• Expected to operate at 240-272 Gbaud rates with a 300GHz channel grid

• Proposed standard: 1600ZR

Solving the Network Operator's Challenge

To address the core problem of scaling networks efficiently, the industry is focusing on three key strategies:

1. Aligning development to drive economies of scale

2. Implementing solutions that improve cost and power efficiency per bit

3. Simplifying architectures to reduce overall cost and power consumption

The adoption of coherent technology generations follows a typical curve, with each new generation seeing increased unit shipments over time. This adoption pattern helps drive economies of scale and accelerates technological improvements.

Evolution of 400G ZR/ZR+

The 400G ZR/ZR+ ecosystem has evolved to address a broader range of network applications:

1. 400ZR:

   • Developed by OIF

   • Narrow scope focused on 400G DCI (Data Center Interconnect) <120km

   • Uses CFEC (Concatenated Forward Error Correction)

   • 10dBm transmit power

2. 400ZR+:

   • OpenZR+ Rev 1.0: 100GHz grid

   • OpenZR+ Rev 2.0: 75GHz grid

   • Client multiplexing

   • 100G, 200G, 300G, and 400G line rates

   • Uses oFEC (Open Forward Error Correction)

   • 10dBm transmit power

3. Bright 400ZR+:

   • OpenZR+ Rev 3.0

   • High transmit power (0dBm)

   • Improved Tx OSNR (Optical Signal-to-Noise Ratio)

   • Broader support for ROADM (Reconfigurable Optical Add-Drop Multiplexer) architectures

   • Co-existence with existing higher launch power channels

4. Bright 400LH:

   • Extended performance with PCS for multiple channel plans

   • 120Gbaud+

   • QPSK interoperability defined in Open ROADM

   • PCS interoperability under discussion

This evolution has extended the network applications that can be addressed by router-based optics, providing greater flexibility and efficiency for network operators.

Extending the ZR/ZR+ Model Beyond 400G

The industry is now working on extending the ZR/ZR+ model to higher data rates:

400G:

• ZR: OIF 400ZR (CFEC, 16QAM, 400G Client/Line)

• ZR+: OpenZR+ Rev 1.0/2.0 (oFEC, QPSK/8QAM/16QAM, 100-400G Client/Line)

• Bright ZR+: OpenZR+ Rev 3.0 (oFEC, 0dBm launch power)

800G:

• ZR: OIF 800ZR (oFEC, 16QAM, 120Gbaud+)

• ZR+ and Bright ZR+: Open ROADM/OpenZR+ (oFEC + Interoperable PCS, 120Gbaud+, 0dBm launch power for Bright ZR+)

1.6T:

• ZR: OIF 1600ZR (oFEC 16QAM proposed, 240Gbaud+)

• ZR+ and Bright ZR+: Future standardization efforts

Additionally, "Bright LH" variants are being developed for extended reach applications, supporting multiple channel plans and leveraging oFEC + PCS at 120Gbaud+.

Conclusion

The continuous evolution of coherent optical technology is enabling network operators to meet the ever-increasing bandwidth demands while managing costs and power consumption. By focusing on standardization, improved efficiency, and architectural simplification, the industry is creating a diverse ecosystem of solutions that can address a wide range of network applications.

As we move towards higher data rates and more advanced coherent technologies, the collaboration between standards bodies, equipment manufacturers, and network operators will be crucial in driving innovation and ensuring interoperability. The future of coherent optics holds great promise for enabling faster, more efficient, and more flexible networks that can support the growing demands of our interconnected world.

Reference

[1] T. Williams, "Coherent Optics- Changing Architectures and Emerging Market Segments," presented at ECOC Market Focus, 2 Oct. 2023.