A Deeper Dive into 400G QSFP-DD Transceiver with DWDM Coherent

The synergy between DWDM (Dense Wavelength Division Multiplexing) and routing technology stands as the linchpin for the realization of the 400G QSFP-DD DWDM optical module. In recent times, the advent of 400G DWDM coherent pluggable optical modules has spurred the development of coherent DWDM products. This paper aims to unravel the intricacies of 400G DWDM coherent pluggable optical modules, driving advancements in coherent DWDM products for cutting-edge optical communication solutions.

DWDM Coherent Optical Transceiver Development

In just under a decade, DWDM modules have seen remarkable progress, with optical devices becoming smaller and faster. Speeds have increased tenfold during this time, jumping from 40G in 2011 to 400G. By 2022, 800G pluggable optical modules had already hit the market. Now, coherent modules have evolved into 400G ZR and 400G ZR+ in QSFP-DD packages, utilizing the same technology as CFP2-DCO but in a more compact size. This compact packaging of 400G DWDM coherent optical devices offers a viable solution for converging routing and DWDM.

400G DWDM Coherent Optical Transceiver Standards

400G has evolved to embrace multiple standards, such as 400 ZR, 400G ZR+, 400G OpenROADM, and 400G OpenZR+. Each of these standards represents a distinct development direction, catering to specific requirements and applications in the rapidly advancing field of high-speed optical communication.

400G Communication Protocol Standards

400G ZR

The OIF released the 400ZR Implementation Agreement in 2020. 400ZR targets edge and relatively short-range (within 120km) data center interconnect applications. 400ZR defines a 400G pluggable coherent interface, which eliminates the footprint loss issue caused by the size difference between DWDM optical devices and client-side optical devices, effectively revolutionizing IP over DWDM (IPoDWDM).

The following are the main advantages of 400ZR :

• The 400ZR standard employs a double-polarization, 16-state quadrature amplitude modulation (DP-16QAM) scheme. By encoding information in two polarizations of light, it effectively doubles the transmission capacity of conventional 16-QAM.

• The 400ZR standard is commonly linked with the QSFP-DD form factor, known for its compact size. These form factors are designed to offer the faceplate density essential for the system architectures of telecom and, particularly, datacom operators.

400G ZR＋

"ZR+" refers to a series of coherent pluggable solutions capable of supporting line capacities of up to 400 Gb/s and reaching distances beyond the specified 80km range for 400ZR. These solutions are designed to meet various application requirements. The advancements in low-power pluggable coherent solutions will be leveraged by next-generation metro-regional networks, particularly in multi-port hardware interfaces where necessary. These metro networks will benefit from enhanced modularity, allowing support for multiple channel capacities based on reach requirements and compatibility with existing metro optical infrastructure.

The following are the main advantages of 400 ZR＋:

• ZR+ is a straightforward extension of 400ZR transcoded mappings of Ethernet with a higher performance FEC than 400ZR CFEC to support longer reaches.

• ZR+ coherent pluggable devices advance component design by introducing elements of high-performance solutions, achieving low power consumption, pluggability, and modularity.

400G OpenROADM

The Open ROADM project was initiated in 2016. The Open ROADM project outlined three distinct functions – pluggable optics, transponder, and ROADM – all governed by an open standards-based API accessible through an SDN controller.

The following are the main advantages of 400G OpenROADM:

• It specified interfaces ranging from 100G to 400G for both Ethernet and Optical Transport Networking (OTN) protocols, boasting a link reach of up to 500km, providing broader support and application scenarios for the application of 400G coherent DWDM QSFP-DD modules.

• The project introduced a robust Forward Error Correction (FEC) algorithm known as open FEC (oFEC) to support extended reach. The application of oFEC technology enables DWDM QSFP-DD modules to better correct errors in data transmission during long-distance transmission, improving the reliability and stability of the transmission.

400G OpenZR+

The OpenZR+ Multi-Source Agreement (MSA) was released in September 2020. Given the constraints of the 400ZR protocol in handling coverage over several hundred kilometers and providing the required flexibility for operators defining transmission rates and modulation types for the link, the necessity for the emergence of 400 OpenZR+ is evident.

The following are the advantages of 400 OpenZR+ over 400ZR:

• Utilizing the enhanced oFEC as defined by the Open ROADM standard.

• Implementing multi-rate Ethernet to multiplex 100G and 200G signals, offering increased flexibility in optimizing traffic across transport links.

• Supporting transport links of 100G, 200G, 300G, or 400G with various modulation types (QPSK, 8QAM, or 16QAM), enabling greater reach and capacity optimization for fiber links.

• Enhancing dispersion compensation to bolster the robustness of the fiber link.

Figure 2: 400G Protocol Standard Evolution

Coherent Technology in 400G DWDM QSFP-DD

Why Use DWDM?

Challenges

Currently, fiber optics have largely achieved low cost and minimal loss. However, the challenge of achieving high capacity over long distances remains unresolved.

Reasons

• Bandwidth Expansion

DWDM allows multiple optical wavelengths to be transmitted through a single optical fiber simultaneously, greatly expanding the network's bandwidth capacity.

• Infrastructure Optimization

By leveraging DWDM technology, service providers can optimize their existing optical infrastructure, effectively utilizing available fiber resources.

• Scalability

DWDM networks offer scalability, allowing for easy expansion of network capacity by simply adding additional wavelengths without the need to lay new fiber cables.

• Cost Efficiency

By consolidating multiple data streams onto a single optical fiber, DWDM reduces the need for additional physical infrastructure, thereby saving costs for network operators.

Why DWDM QSFP-DD Needs Coherent Technology when Increasing Bandwidth？

• Mitigating Fiber Optic Attenuation

As network bandwidth grows, issues such as fiber optic attenuation become increasingly noticeable. DWDM coherent technology tackles this challenge by utilizing tools like digital signal processing (DSP) and advanced forward error correction (FEC) algorithms. These technologies work together to improve the quality and stability of optical signals over long distances, effectively addressing the effects of attenuation, dispersion, and other factors.

• Improved Spectrum Efficiency

Coherent modulation formats enhance spectrum efficiency by using advanced modulation symbols and compact signal encoding, enabling the packing of more data into each wavelength channel. This strategy maximizes spectrum usage without requiring wider bandwidth. As a result, data transmission rates increase without expanding the spectrum, resulting in higher data throughput within the existing fiber bandwidth.

• Extending Transmission Range

With coherent transmission technology, advanced forward error correction (FEC) algorithms play a crucial role in overcoming signal degradation over long distances, ensuring reliable transmission across thousands of kilometers for DWDM systems. This method entails employing error detection and correction techniques, implementing forward error correction codes, and dynamically adjusting parameters to extend transmission ranges and enhance system coverage.

Conclusion

In summary, the integration of DWDM technology and coherent optics in QSFP-DD transceivers marks a significant step forward in high-speed optical communication. With standards like 400ZR, 400G ZR+, 400G OpenROADM, and 400G OpenZR+, the industry is rapidly expanding to meet diverse application needs. Coherent technology addresses challenges like fiber optic attenuation, enhancing spectrum efficiency and extending transmission ranges for scalable, reliable, and cost-effective optical networks. Continuous innovation will further enhance DWDM coherent transceivers , improving connectivity and efficiency in telecommunications and data communication infrastructures.