400G DWDM: combine QSFP-DD transceiver with DWDM coherent


The combination of DWDM and routing technology is the key to the implementation of the 400G QSFP-DD DWDM optical module. In recent years, coherent DWDM products are ready to keep pace with the introduction of 400G DWDM coherent pluggable optical modules.

This paper will study the evolution of coherent DWDM technology, so as to further study the details behind the 400G DWDM coherent pluggable optical module, and explore how to realize the integration of DWDM and routing technology.

The development of DWDM coherent optics


In less than 10 years, the DWDM module has made great progress, optical devices become smaller and smaller, and the speed is higher and higher. It has increased 10 times in the same period of time: from 40G in 2011 to 400G. By 2022, 800G pluggable optical modules have already appeared on the market.



Fiber Mall QSFP-DD-400G-DCO-ZR+

The introduction of coherent optical technology is one of the most important innovations in the development of the DWDM system. Coherent optical equipment uses advanced optical devices and digital signal processor (DSP) to send and receive complex lightwave modulation, so as to realize high-speed data transmission. At a very high level, coherent modulation remains the driving force behind high-speed optical devices, including 400G and higher rate coherent transceivers.

The first commercially available coherent DWDM system is 40G, followed by 100G. These systems are based on line cards and chassis. They can support many line cards in each system and occupy the same space as 10G rate products. It is major progress. Now they can transmit 100G rate and longer transmission distance. Over time, line card speeds have increased to 200G and beyond, but the industry is approaching an inflection point with the advent of cloud providers.

Specifically, as cloud provider networks begin to grow exponentially, there is increasing pressure on manufacturers to create smaller, faster and cheaper network components. It was this inflection point that led to the emergence of optical transponder DWDM systems. The optical transponder system eliminates the chassis and line card. It is a physically small stand-alone system, a small data center switch with a height of 1 or 2RU (1.5″-3″). The key to the feasibility of the optical transponder package is the separation of the two main components in coherent optical transmission: the optical device (laser, receiver, modulator, etc.) and the DSP (digital signal processor).

400G DWDM coherent optics

And these innovations have given rise to the pluggable CFP2-ACO (Analog Coherent Optics), a pluggable DWDM module with a relatively small CFP2 size. DSP technology is also evolving so that a single DSP chip can support multiple CFP2-ACO modules. Thus, by placing multiple DSPs in the optical transponder, manufacturers produced systems capable of transmitting 2Tbps (20x100G client connections) in 2 rack units (3″). In contrast, a chassis-based system would require 12 rack units. In addition to saving space, they are more energy-efficient.

Of course, the above is a very simple explanation of coherent signal transmission. In fact, the purpose of developers is to convert digital signals into analog signals to transmit data and convert analog signals back to digital signals at the other end.

However, the CFP2-ACO can only process analog signals, but can not process digital signals. It receives the coherent analog signal to be transmitted from DSP or transmits the received coherent analog signal to DSP to convert it into a digital signal, as shown in the figure.

CFP2-ACO DWDM transmission system

CFP2-ACO DWDM transmission system

CFP2-ACO system has been widely used in the whole industry and has become the standard form of optical transmission in almost every cloud provider’s network.

At the same time, with the introduction of CFP2-DCO, pluggable coherent DWDM optics continue to develop. “D” stands for “digital” in digital coherent optics. The developers of coherent optics once again reduced the size and power consumption of components, so both optical devices and DSP are placed in CFP2. In this way, there is no need to use the rack to accommodate the DSP, so coherent DWDM transmission can be carried out directly from the router or switch, which is the turning point of the real integration of DWDM and router.

DCO DWDM transmission in router or switch

DCO DWDM transmission in router or switch

Now, coherent optical modules have evolved to 400G ZR and 400G ZR+ in QSFP-DD packages, using the same technology as CFP2-DCO but in a smaller size. Such a compact package housing 400G DWDM coherent optical devices do provide a viable solution for the convergence of routing and DWDM.

400G DWDM coherent optical transceiver standards


400G has evolved to the point where there are several standards. These include 400ZR, 400G ZR+, 400G OpenROADM, and 400G OpenZR+, which go in slightly different directions.

First was the Optical Interconnect Forum (OIF), which created the 400ZR standard. 400ZR targets edge and relatively short-range (within 120km) data center interconnect applications. Around the same time, the OpenROADM multi-source protocol also defined specifications for 400G DWDM pluggable, with specifications focused on service provider networks, such as long-haul optical transport (>120km), advanced forward error correction (oFEC) and optional data rates (100G, 200G, 300G or 400G). Although additional features can be implemented, more power is required than the 15W specified by ZR. For this reason, the OpenROADM specification is called 400G ZR+.

Eventually, between the two organizations and various optical device manufacturers, they agreed on the best standard for combining OIF and OpenROADM, and called it OpenZR+. By combining the characteristics of each device in the same package, a highly versatile coherent DWDM optical device can be provided, as shown in the figure below.

400G OpenZR+

We can see that in the continuous improvement of the packaging, function and data rate of the optical module, the optical transmission range is getting wider and wider, and the transmission distance is getting longer. Using OpenZR+ standard, 1400km transmission distance can be achieved, which is more than 10 times of 400ZR transmission distance. With the high density 400G DWDM in routers, combined with the simplicity of traffic engineering and the path redundancy of segmented routing, we can expect a major shift in transmission network architecture.


In November 2020, Fiber Mall was invited to be the first contributor member of OpenZR+ MSA. Fiber Mall officially invested in coherent optical module development in early 2018, open to strategic cooperation with upstream supply chain to optimize innovation in low-power design and signal modulation model, and achieved significant results. Now we have launched 100G CFP-DCO, 100G CFP2-DCO digital coherent optical modules and 200G DWDM coherent optical transceiver and 400G DWDM coherent optical transceiver with strict compliance to OpenZR+ standard for data center interconnection and metro networks for ultra-long distance optical transmission.


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