With the initial adoption of 400G coherent pluggables for IP-over-DWDM networks being driven by router interconnects, these pluggable modules based on coherent technology have been referred to as router-based coherent optics. There are now more than 200 network operators that have embraced this cost-saving paradigm.

Figure 1. Router-based coherent optics provide cost savings.

400G Coherent Modules and Open Line Systems Led the Way
As previously mentioned, the introduction of 400G interoperable coherent MSA modules that plug directly into router ports helped accelerate network operator adoption of router-based coherent optics, enabling high-capacity optical connections within a metro reach network without traditional transponder hardware. Two different mechanical form-factors for these 400G modules, QSFP-DD and OSFP, were introduced to the market, with the former being the primary form-factor being shipped today for 400G, matching the widely adopted host platform QSFP-DD slots.

The disaggregation of optical line systems has also helped progress the adoption of router-based coherent optics. These open line systems enable the insertion of wavelength transmission from router-based coherent MSA pluggable modules rather than from transponders sold by the same line system vendor. Many of the recently deployed networks utilizing router-based optical modules have been over these open line systems. In fact, approximately 70% of the above mentioned 200 end-users were utilizing an open line system.

In addition, the introduction of 400G coherent modules with high transmit optical power, such as Acacia’s Bright 400ZR+ module, helped accelerate service provider adoption because higher transmit power helps to avoid performance penalties when connecting to typical brownfield ROADM architectures. Modules such as the Bright 400ZR+ also include a transmitter tunable optical filter (TOF) to minimize adjacent channel interference that could impact performance, especially if colorless ROADMs are present in the network.

An ongoing challenge that the industry is making progress with is the ability for seamless management of coherent MSA modules. Industry groups such as the Optical Internetworking Forum (OIF) have made great progress to address this challenge, with the OIF driving the Common Management Interface Specification (CMIS). This effort continues to be an area of industry focus to further lower the adoption barrier of router-based optics.

Continuing the Momentum of Router-based Coherent Optics
To continue the adoption of router-based coherent optics, expanding interoperable MSA pluggable module capabilities were required to address network operator use cases such as long haul and ultra long haul reaches as well as a migration from 400G links to 800G links. Thanks to recent advances in coherent technology, these capabilities have been recently introduced.

400G ultra-long-haul (ULH) modules leveraging Class 3 (~120+ Gbaud data) rate technology enables the reach capability of 400G to extend from metro/regional reaches to ultra long-haul reaches, reducing the barrier for network operators to deploy router-based coherent optics in virtually any network application. Arelion recently announced a successful trial using Acacia’s Delphi-DSP based 400G ULH modules over 2,253km with margin, enabling a 35% reduction in CAPEX and 84% reduction in OPEX.

To take advantage of the latest generational increase in switch/router chip capacity resulting in I/O ports transitioning from 400G to 800G speeds, the same Class 3 generation coherent technology support 800G interoperable coherent MSA modules that plug directly into host platforms. This enables network operators who have already embraced an IP-over-DWDM architecture using router-based coherent optics at 400G to now migrate to 800G. For example, Colt recently announced that it is the first provider to successfully trial enhanced performance 800G ZR+ coherent pluggable optics, in their Cisco 8000 series router ports, in its production network. These 800G router-based coherent optics provide the capability to double Colt’s packet core capacity per link while reducing power per bit by 33.3%.

While the initial adoption of router-based coherent optics for deploying an IP-over-DWDM network were from hyperscalers and service providers, the momentum of adoption has expanded to research and education networks, enterprise networks, and many other network operators looking to optimize total cost of ownership. And the application is not limited to using coherent pluggable optics in routers, but also in network switches for fabric extension requiring an interconnect to a distant site.

Acacia’s Interoperable Modules Enabling the Future
Acacia is enabling the adoption of 400G and 800G IP-over-DWDM architectures with router-based coherent optics. The latest generation of MSA pluggable modules include 800ZR and 800G ZR+ variants as well as 400G ULH for ultra-long-haul reaches. These are all powered by Acacia’s 9th generation Delphi DSP ASIC and 130+Gbaud high-speed silicon photonic PIC technology enabling a low-power industry standard based solution. The 800G ZR+ module also includes the industry’s first standardized interoperable probability constellation shaped (PCS) mode. In addition, Acacia is a leading supplier of 400ZR and OpenZR+ compliant modules including high Tx power Bright modules for 400G based metro/regional IP-over-DWDM network.

Enabled by coherent pluggable modules, the adoption of IP-over-DWDM using router-based coherent optics continues to grow, providing significant reduction in TCO for network operators.

The Path Towards Robust, Interoperable 1600ZR/ZR+ Interfaces
With 200G per lane electrical PAM4 solutions recently introduced, network operators now have a path towards supporting 1600G host router I/O ports by using eight parallel electrical lanes (Figure 1). Similar to 400G and 800G generations, this is a key motivator in developing coherent pluggable modules to be plugged into these 1600G router ports for inter-data center optical links. However, along with the progress on the host interface side, there is still much work to be done to ensure a technically feasible and robust interoperable design for 1600ZR/ZR+ coherent pluggable modules.

Figure 1. Simple illustration of how advances in achieving 200G PAM4 can be leveraged for 1600ZR/ZR+ coherent optical transmission.

OIF Defining Both 1600ZR and 1600ZR+ Standards
Unlike previous coherent standardization efforts at 400G and 800G, in which enhanced “ZR+” performance links were defined outside of OIF, the OIF has launched initiatives to define both 1600ZR and 1600ZR+. Having both efforts occurring simultaneously enables the OIF to make decisions with both 1600ZR and 1600ZR+ in the same scope of discussions. This helps keep the two implementations as aligned as possible, which is beneficial for the industry considering the large investments of technology required. The focus of these investments includes advanced CMOS nodes to maintain low power consumption within the envelope of QSFP-DD and OSFP form-factor requirements, and advanced designs in high-speed RF/mixed-signal as the modulation approaches the Class 4 240Gbaud range (Figure 2).

Figure 2. Charting the course towards Class 4 baud rate standardization efforts.

As we saw in both the 400G and 800G generations, the foundation of 16QAM (4 bits/symbol) modulation was adopted and this is likely to also happen with the 1600G generation. For 1600G transmission, 16QAM modulation implies ~236+Gbaud data rate operation.

In addition to modulation order, the type of forward error correction (FEC) has also been a key parameter that required industry agreement. At 400G, the OIF adopted concatenated FEC (CFEC) as the 400ZR FEC and OpenZR+ MSA adopted oFEC (a high-performance FEC) for 400G ZR+. At 800G, the OIF decided to adopt oFEC for ZR, aligning it with ZR+ modes. To provide an enhanced performance mode beyond 800ZR, OpenROADM MSA defined an interoperable PCS for 800G ZR+ (Figure 3). It is likely that oFEC will be similarly adopted for both 1600ZR and combined with some interoperable PCS for 1600ZR+ modes.

Figure 3. 400G to 800G evolution of ZR vs. ZR+ implementations; how will 1600G ZR vs. ZR+ implementations be different?

What Will Be the Industry Consensus for 1600ZR/ZR+?
Every new generation of speeds-and-feeds encounters challenges around industry consensus and technology achievements that push the envelope – and 1600ZR/ZR+ is no different. There is currently great momentum driving these efforts forward, especially in anticipation of advances in generative AI that are pushing optical interconnect needs to higher bandwidths. Evidence of this momentum is apparent by other industry efforts beyond the OIF that are currently active. In addition to the OIF 1600ZR/ZR+ efforts, the IEEE has also begun working on 1.6TbE electrical and optical interface standards within the IEEE 802.3dj working group, anticipated to be ready by the second half of 2026.

In light of this progress, the question is “how does the industry reach consensus for 1600ZR/ZR+?” We eagerly await the outcome.

The Optical Internetworking Forum (OIF) kicked off the 400G MSA pluggable generation with development of the 400ZR implementation agreement enabling point-to-point amplified links up to 120km operating at 60+Gbaud data rates. Around the same timeframe, the OpenROADM MSA defined 400G interfaces for ROADM networks and extended reaches; the OpenZR+ MSA leveraged these higher performance interfaces to enable interoperable enhanced performance links for 400G pluggable modules (Figure 1).

The introduction of high-transmit optical power (>0dBm) ZR+ modules such as Acacia’s Bright 400ZR+ module further expanded the 400G MSA pluggable space to include brownfield ROADM network architectures (with existing transponder channels ~0dBm). Driven by increasing bandwidth demands from applications such as AI, network operators are now looking towards a new generation of MSA pluggable products that further expand applicable networking scenarios that operators can leverage to scale and meet these demands.

How Industry Standards Benefit MSA Pluggable Module Adoption
The latest array of MSA pluggable products introduces a new set of capabilities that network operators can utilize to increase capacity and extend reach. These products provide the ability to deploy 800G with ZR, ZR+, and high-transmit optical power capabilities, as well as extending the capabilities of existing 400G router interfaces to support ultra-long-haul (ULH) reach capabilities. This new generation of modules continues to leverage industry standardization while also borrowing capabilities from performance-optimized coherent solutions. These capabilities include high-baud rate transmission allowing for a doubling of baud rates from the previous Class 2 (~60+Gbaud range) generation to Class 3 (~120+Gbaud range) baud rates, the use of probabilistic constellation shaping (PCS) for enhanced transmission performance, and L-band support for spectrum range expansion.

Figure 1.  Interoperability approaches at 400G vs. 800G.

Industry standardization of coherent solutions plays a key role in enabling economies of scale. Users of 400G coherent MSA pluggable modules such as 400ZR/ZR+ have benefited from the efforts of OIF, OpenZR+ MSA, and OpenROADM MSA to provide industry agreements on module specifications resulting in a diverse supply base. We have seen similar efforts to garner industry standardization as users transition to 800G MSA pluggables. There are three main elements that differentiate 800G relative to 400G and are adapted from previously developed performance-optimized solutions.

1. Interop PCS for Enhanced Performance
   A key difference between 400G and 800G interoperability approaches for an enhanced performance “ZR+” is that instead of using enhanced performance forward error correction, oFEC, to provide improved 400G performance, 800G uses industry standard interoperable probabilistic constellation shaping (PCS) for enhancing performance. PCS is a transmission shaping technique that provides additional link performance beyond traditional transmission modes such as 16QAM. Industry standardization of an interoperable PCS transmission shaping function, once relegated to proprietary performance-optimized transponder platforms including those for submarine applications, is a tremendous leap forward in the progress of MSA pluggable module capabilities. Multi-vendor 800G module supply chain diversity from a DSP ASIC perspective is possible when the 800G ZR+ performance enhancement mode utilizes the industry standard interoperable PCS mode.
2. High Baud Rate Design
   PCS is not the only technology that has been adapted from performance-optimized solutions for MSA pluggables. 800G as well as a 400G ULH pluggable solutions require a high-baud rate design operating in the Class 3 ~120+ Gbaud data rate range. Acacia’s performance-optimized CIM 8 module capable of 140Gbaud speeds has already proven that its deployed technology far exceeds the requirement for the new generation of MSA pluggables. Operation at these high baud rates benefits heavily from the advanced integration and RF signal optimization techniques that Acacia introduced in our 400G MSA pluggable product family.

Figure 2.  Tightly integrated components enable 120+Gbaud data-rate capabilities.

3. C & L Band Support
A third element of the latest 800G MSA pluggable generation that is borrowed from performance-optimized designs is the capability to transmit in the L-band wavelength range, in addition to the traditional C-band DWDM range. By adding L-band supporting infrastructure to a network, the network capacity is approximately doubled. Network operators now have an option beyond utilizing a transponder platform if they wish to use L-band expansion to increase network capacity.

Figure 3.  New generation of coherent MSA pluggable modules to take advantage of L-Band transmission window, adding to existing C-Band support.

Pluggable Interoperable Interfaces are Driving Adoption of 800G Modules
Acacia’s latest family of coherent solutions are powered by its 9th generation DSP ASIC called Delphi. These modules include support for OIF 800ZR, interoperable 800G ZR+ using the OpenROADM interop PCS mode, and 400G ULH for ultra-long-haul reaches. These modules utilize Acacia’s 3D Siliconization providing a highly integrated design enabling high-baud rate modulation. With support for QSFP-DD and OSFP form factors, as well as >+1dBm transmit optical power and L-band support, Acacia’s Delphi generation of products leverage the deployment successes of our performance-optimized CIM 8 module to provide MSA pluggable products that offer increased capacity and longer reaches.

Figure 4.  Acacia’s latest generation of MSA pluggable 800G and 400G ULH modules.

Similar to the successful path we saw 400G pluggables experience, these modules are delivering the performance and interoperability that is critical for driving economies of scale and widespread adoption. With data center bandwidth continuing to grow rapidly, fueled by emerging new applications such as AI, these high-performance pluggable modules are on track to become an important tool for network operators to cost-efficiently scale their networks to meet this surging demand.

See Us at ECOC 2024!
Acacia is excited to be participating in the OIF interoperability demo at ECOC 2024 showcasing both its 400G and 800G pluggables; demos will take place in the OIF booth #B83. Acacia will also be demonstrating the Interoperable 800G ZR+ module in our meeting room at ECOC. Click here to set up a meeting.

We hope to see you in Frankfurt!

While the near-term focus has been on how AI will affect the technology around short reach interconnects, there will certainly be an impact to interconnections beyond the AI clusters and beyond the AI data center, in hyperscaler networks.

The question is:  beyond the short-distance high-bandwidth interconnections, how would AI traffic impact the optical transport environment beyond the intra-building network and into the metro, long haul, and longer reach applications where optical coherent transmission is heavily utilized?

Figure 1.  Sales Forecast for Ethernet Optical Transceivers for AI Clusters (July 2024 LightCounting Newsletter, “A Soft Landing for AI Optics?”).

Effect of Past Applications on the Transport Network
Bandwidth-intensive computing is attributed to both AI training as well as AI inference, where inference refers to the post-training process in which the model is “ready for the world,” creating an inference-based output from input data using what it learned during the training process. In addition to AI training’s requirements for a large amount of computing power and a large number of high-bandwidth, short connections, there are also foreseen bandwidth requirements beyond the AI data center. To understand how network traffic patterns may evolve beyond the AI data center, let’s review some examples of how the wider transport network was affected by past growth of various applications. Although these applications may not strongly match the effect of AI application traffic, it can provide some insight into the effects that the growth of AI applications may have on optical transport, and thus on the growth of coherent technology.

If we look at search applications, the AI training process is generally analogous to a search engine’s crawling bots combing the internet to gather data to be indexed (AI training being much more computationally intensive). The AI inference process is analogous to the search engine being queried by the end user with results made available for user retrieval with minimal latency. While the required transport bandwidth for search bots and user queries are minimal compared to higher bandwidth applications, the cumulative effect of the search-related traffic is part of the contribution to overall transport traffic, including bandwidth from regional/local caching to minimize latency, as well as usage from subsequent traffic created by acting upon search results.

Understanding how network traffic was affected by the growth of video content delivery is another example that can inform potential AI network transport traffic patterns. A main concern resulting from video content distribution was the burden imposed on the network in delivering the content (especially high-resolution video) to the end-user. To address this concern, content caching, where higher demand content was cached closer to the end-user, was implemented to reduce overall network traffic from the distribution source to the end-user, as well as reduce latency. While it is too early to predict how much network traffic would increase due to expansive queries to and responses from AI inference applications, the challenge is to ensure the latency for this access is minimal. One could see an analogy of content caching to edge computing where the AI inference model is closer to the user with increased transport bandwidth required for these edge computing sites. However, the challenge would be to understand how this would affect the efficiency of the inference function.

Turning to cloud computing for insights on traffic patterns, the rise of (multi-) cloud and computing resulted in intra and inter-datacenter traffic (a.k.a. east-west traffic) increasing as workloads traversed across the datacenter environment. There’s a similar potential rise in this type of traffic with AI as data for training could be dispersed among multiple sites of clusters as well as inference models being distributed to physically diverse sites to reduce latency to end users.

For any of these previous examples, as the demand of these applications increases, the transport bandwidth requirements would also increase from not only the target data (e.g., search results, video), but also from overhead or intra datacenter traffic to support these applications (e.g., content caching, cloud computing, backend overhead). Traffic behavior for aggregating AI training content as well as the distribution of AI inference models and its results may be similar to the traffic patterns of these previous applications, applying pressure to network operators to increase capacity for its data center interconnect, metro, and regional networks. Long haul and subsea networks may also experience a need to expand to meet the demands of AI-related traffic.

Figure 2.  A scenario in which the network fabric physically expands due to facility power constraints, requiring high-capacity optical interconnections.

The Balance of Power and Latency
While the application examples above are related to how the AI application itself may affect bandwidth growth, what is becoming apparent is the power requirements to run AI clusters and data centers are significant. In the past, as the demand for cloud services grew, the need for large-scale data centers to have access to localized inexpensive power sources helped to drive the location selection for large data centers. However, power facility/availability constraints helped drive the adoption of physically distributed architectures, which then relied on high-capacity transport interconnects between data centers to maintain the desired network architecture (Figure 2). We anticipate a similar situation with AI buildouts requiring distributed facilities to address power constraints with potential trade-offs of reduced efficiencies for both AI training and inference. The distributed network would then rely on high-capacity interconnect transport using coherent transmission to extend the AI network fabric. Unlike cloud applications, physical expansion of the network fabric for AI applications has a different set of challenges due to compute and latency requirements for both training and inference.

Figure 3. Extremely low latency is required within the AI cluster to expeditiously process incoming datasets during the training mode. Since datasets are collected before being fed into the training cluster, the process of collecting these datasets may not be as latency sensitive.

As we plan for AI buildouts, one common question is how the physical extension of an AI networking fabric may affect AI functions. While geographic distribution of AI training is not ideal, facility power constraints are certain to lead to a growing adoption of distributed AI training techniques that attempts to mitigate introduced latency effects. As part of the training process, sourcing datasets feeding into the training cluster may not be latency sensitive and would not be as impacted by physical network extension (Figure 3). After training, when the inference model is complete, the goal is to minimize the latency between the user query to the inference model and the transmitted results to the user (Figure 4). The latency is a combination of the complexity of the query as well as the number of “hops” between the inference model and the user. Latency reduction when accessing the inference model, as well as methods to effectively distribute both the training and the inference function beyond a centralized architecture to address single-site power constraints, are ongoing discussions within the industry.

Whether driven by power constraints, dataset sourcing, or inference response efficiency, the sheer growth of AI applications will drive network traffic growth beyond AI cluster sites towards the wider network requiring high-capacity interconnects.

Figure 4.  Minimizing latency for AI inference is a key objective.

Trading off power requirements versus access to inexpensive and abundant power versus latency is familiar territory when it comes to bandwidth intensive applications. The outcome that optimizes these trade-offs is application dependent and can even be deployment-by-deployment dependent. We continue to watch the evolving AI space to see how these network architecture trade-offs will play out, with the impact of how the transport network is designed. High-capacity coherent transport can certainly influence these trade-offs. And as we have already seen, by using coherent high-capacity transport cloud architectures, networks were able to physically expand to alleviate power source constraints by provided fat-pipe links between sites. We anticipate a similar scenario with expanding AI network architectures.

The Ripple Effect
While the near-term focus on high-capacity interconnects for AI applications has been on short reach connections within AI clusters, we are already seeing bandwidth requirements begin to increase, requiring additional coherent connectivity between datacenters supporting AI. And while there is general agreement that the resulting bandwidth demand from AI applications translates to increased traffic across the network, we are at the early stages in understanding how specific segments of the network are affected. Coherent optical interconnects for high-capacity transport beyond the data center already provide performance-optimized transponder solutions at 1.2T per wavelength as well as 400G router-to-router wavelengths moving to 800G using MSA pluggable modules. This technology will continue to play a role in the transport solution supporting AI applications whether the expanding traffic is in the metro portion, data center interconnects, long haul, or beyond.

Multiple standardization bodies, including the OIF, Open ROADM, and the IEEE, are working in parallel to standardize 800G optical transmission and 800 Gigabit Ethernet (GbE) protocols, helping to foster economies of scale. In parallel, module vendors are introducing the required technologies to enable low power 120+Gbaud solutions that would fit into small form-factor modules such as QSFP-DD and OSFP.

Higher Baud Rates, Standardization, and Higher Levels of Integration
With the 400G MSA pluggable generation, the industry via OIF converged onto Class 2 ~60Gbaud rate range and 4 bits/symbol 16QAM modulation order to enable 400G transmission within a 75GHz optical spectrum (Figure 1). Open ROADM requirements, targeting service providers, also included the same modulation order and similar baud rates, as well as additional transmission flexibility specifying more modulation modes, such as 2 bits/symbol QPSK to enable longer reaches at 200G.

Figure 1. 2x scaling of baud rates and channel spacing for ~60+Gbaud Class 2 to ~120+Gbaud Class 3 transition.

At 800G, the industry has converged to a 2x scaling of baud rate resulting in Class 3 ~120 Gbaud rates, as shown in Figure 1. Channel spacing requirements double to 150GHz for 800G, providing a straightforward network scaling supporting similar reaches to 400G Class 2 devices. As with 400G, this convergence is expected to drive economies of scale of the technology supporting this baud rate class.

To realize the modulation capability for Class 3 120+Gbaud operation, advancements in both high-speed optical modulation and supporting components as well as high-speed 112G-per-electrical-lane capabilities were needed to enable 800G MSA pluggables in compact form factors.  These optical and electrical high-speed capabilities have already been proven with the introduction of Acacia’s performance-optimized Coherent Interconnect Module (CIM) 8 module which utilizes silicon photonics, high speed ADC/DACs, RF components, and Acacia’s 3D Siliconization for Class 3 120+Gbaud transmission. The high-level of integration from 3D Siliconization allows this high baud rate technology to fit into QSFP-DD and OSFP pluggable form-factors, all while maintaining high signal integrity and low power consumption.

Figure 2.  3D Siliconization used in (left to right) Class 2 MSA Pluggables and Class 3 performance-optimized CIM 8. Highly integrated co-packaging is going to be important for Class 3 800G MSA pluggable modules.

Key Features Needed for 800G MSA Pluggable Modules
To meet customer needs and drive future adoption, 800G coherent MSA pluggables should have a number of key features and capabilities related to optical transmission, client traffic, low power consumption, and interoperable module management.

Figure 3. Illustrating the key features needed for 800G MSA Pluggable modules.

Optical Transmission Features

OIF 800ZR and high-performance interoperable PCS modes. OIF is defining an interoperable standard performance 800ZR variant addressing coherent line interfaces for amplified, single span, DWDM links over 80km using Class 3 optics operating at ~4 bits/symbol (equivalent to 16QAM) modulation. For a higher performance 800G solution, Open ROADM is defining enhanced performance modes that include an interoperable probability constellation shaping (PCS) implementation utilizing both Ethernet and OTN framing.

Multi-haul capable. In current Class 2 ~60Gbaud MSA pluggable solution designs capable of adjusting the modulation mode, a long reach 200G capability is available by setting the modulation order to QPSK (2 bits/symbol) rather than 16QAM (4 bits/symbol) used for 400G transmission. This capability was standardized in Open ROADM and adopted by OpenZR+. Class 3 ~120Gbaud solutions can be used for 800G metro/regional reaches, and for multiple different standard and proprietary modes at 400G and 600G to address a wide range of network requirements.

High transmit optical power capability. Like high-Tx (>0dBm) optical power capabilities introduced in Acacia’s Bright 400ZR+ QSFP-DD modules, 800G coherent MSA modules will require optical amplification to operate over traditional brownfield networks with ROADM network elements. Internal optical amplification is a key feature variant for this generation to support these types of networks.

Client Traffic Features

800GbE client traffic support. A key client traffic protocol to be supported in 800G generation coherent MSA pluggables is 800GbE. IEEE P802.3df^TM aims to define the requirements for native Ethernet traffic at 800G data rates. To support native 800G Ethernet traffic from switch/router I/O ports, 800G generation coherent MSA pluggables are required to support this protocol.

Multiplexing lower speed 100/200/400GbE electrical client traffic. It is expected that when 800G coherent MSA pluggables are introduced, 100GbE, 200GbE, and 400GbE will continue to be widely deployed client traffic speeds. Therefore, 800G pluggables need to support the ability to multiplex these lower speed protocols into 800G transmission, with requirements being established by the standardization groups. For networks utilizing OTN, support for FlexO at 800G could be possible.

Power Consumption and Management

Low power consumption. As with other MSA pluggable solutions, optimizing for low-power operation is a priority. The leveraging of Moore’s law with decreasing CMOS node sizes along with increased ASIC functionality has been a reason for the success of coherent pluggables—providing high-capacity links in a small form-factor. Power-optimized designs continue to be a priority for the new generation of Class 3 baud rate coherent MSA pluggables, not only for compliance in 800G slots, but also for backwards compatibility into lower-rate 400G legacy ports which have more restrictive power consumption requirements.

Content Management Interoperability Services (CMIS) compliance. In addition to industry standardization efforts at the optical transmission and client traffic levels, there has been a concerted effort to ensure multi-vendor interoperability of the module management interface through CMIS, defined in OIF. This ensures a common and predictable management signaling interface between the module and the host switch/router from one module vendor to another. CMIS compliance, a key requirement for 400G MSA modules, is going to extend to 800G as well.

Interoperable PCS Modes

Taking a page from the performance-optimized coherent playbook, 800G MSA pluggable solutions are expected to introduce interoperable PCS optical transmission. PCS shapes the optical transmission using an algorithm that weights the constellation to utilize the inner points more than the outer points, resulting in improved OSNR performance with a minimal increase in overhead. While previous implementations of PCS have always been proprietary, several key DSP vendors have collaborated over the last 12 months to enable the industry’s first introduction of interoperable PCS for 800G MSA pluggables.

Figure 4. High-level block diagram comparing 800G MSA pluggable solutions utilizing 800ZR standard transmission and 800ZR+ interoperable PCS mode for higher OSNR performance.

Driven by Open ROADM, the 800G interoperable PCS implementation provides a power-optimized method to boost the performance beyond 800ZR, similar to how oFEC boosted the performance beyond 400ZR in OpenZR+ modules. This additional performance allows 800G implementations to achieve similar reaches as 400G implementations based on 16QAM transmission. This Open ROADM interoperable PCS mode operates in the 130+Gbaud range, which is slightly higher than the 800ZR mode, but can still enable transmission in 150GHz channels. With interoperable PCS, network operators can benefit from a greater range of network implementations from the initial 800G MSA pluggable offerings compared to the initial 400G point-to-point offerings of the previous generation.

Following the Right Path to 800G Pluggables

In summary, operators are looking for the following key functional requirements to transition to 800G coherent MSA pluggables.

• Support for OIF 800ZR and high-performance interoperable PCS modes
• Multi-haul capable
• High Tx optical power option
• 800GbE client traffic support
• Multiplexing lower speed 100/200/400GbE electrical client traffic
• Low power consumption
• CMIS compliance

This solution can plug directly into switch/router ports to further drive adoption of IP-over-DWDM router-based optical network architectures. The first 800G pluggable deployments are expected in 2024.

To date, multiple system vendors have converged around Class 3-based solutions (Figure 1), recently announcing their next generation offerings. This industry convergence creates the benefit of economies of scale and broad industry investments into the technology used in this baud rate class, the same class being used for 800G MSA pluggable solutions.

Figure 1.  Acacia and other coherent vendors have announced Class 3 Terabit Era solutions.

Advancements Resulting in 65% Power-per-Bit Savings Over Current Competing Solutions
Doubling the baud rate from Class 2 to Class 3 in silicon was a significant engineering achievement, combining design advancements in high-speed Radio Frequency (RF) and Analog to Digital Converter (ADC) and Digital to Analog Converter (DAC) components plus well-designed co-packaging integration of silicon and silicon photonic (SiPh) components. These achievements led to Acacia’s successful 140Gbaud in-house capability that is being leveraged in the commercially available CIM 8 solution.

With high-volume shipments of multiple coherent Class 2 module products utilizing Acacia’s 3D Siliconization, this proven co-packaging integration solution provided the foundation for extending this capability to Class 3 140Gbaud implementation utilized in the CIM 8 (Figure 2). 3D Siliconization maximizes signal integrity by co-packaging all high-speed components including the coherent Digital Signal Processor (DSP) application-specific integrated circuit (ASIC), transmitter and receiver silicon photonics, and 3D stacked RF components into a single device that is manufactured in a standard electronics packaging house. Silicon technology has demonstrated cost and power advantages over alternative technologies, making it the material system of choice for these higher baud rates. These advancements enabling a doubling of the baud rate have led to a 65% power-per-bit savings of CIM 8 over current competing solutions that utilize alternative optical material systems. In addition, the size and power savings of this latest generation enabled the ability to house this 1.2T 140Gbaud solution in a pluggable form-factor.

Figure 2.  An example of 3D Siliconization used in the CIM 8 module, resulting in a volume electronics manufacturable high-speed single device larger than a quarter.

2^nd Generation 3D Shaping Advances Coherent Performance
The CIM 8 is powered by Jannu, Acacia’s 8^th generation coherent DSP ASIC. The design greatly expands on the success of the Pico DSP ASIC predecessor used in the widely deployed performance-optimized Class 2 AC1200 module (Figure 1). The AC1200 was the first module to introduce 3D Shaping, which provided finely tunable Adaptive Baud Rate up to 70Gbaud as well as finely tunable modulation up to 6 bits/symbol. The AC1200 had achieved record breaking spectral efficiency at the time of its introduction, as evidenced by a subsea trial over the MAREA submarine cable connecting Virginia Beach, Virginia to the city of Bilbao in Spain. Finely tunable baud rate helps maximize spectral efficiency in any given passband channel, converting excess margin into additional capacity/reach, and avoids wasted bandwidth due to network fragmentation.

Figure 3.  A popular feature is the fine-tunability of baud rate introduced by Acacia with the Class 2 AC1200; CIM 8 incorporates the same Adaptive Baud feature (as part of 2^nd Generation 3D Shaping) for Class 3 baud rate tunability.

The 5nm Jannu DSP ASIC in CIM 8 intelligently optimizes optical transmission using 2^nd Generation 3D Shaping with an increased Adaptive Baud Rate tunable range up to 140Gbaud, as well as finely tunable modulation up to 6 bits/symbol using enhanced Probabilistic Constellation Shaping (PCS). With 2^nd Generation 3D Shaping, the CIM 8 module can achieve a 20% improvement in spectral efficiency.

Terabit Era Solutions Provide Full Network Coverage
Class 3 technology not only ushers in the terabit era, but also enables full multi-haul network coverage as the high baud rate capabilities transport nx400GbE client traffic across a service provider’s entire network. Full network coverage is not only enabled by adjustment of the modulation, but also implies the capability to optimize for various network conditions which include overcoming transmission impairments.

Figure 4. CIM 8 1.2T, 1T, 800G, and 400G transmission constellations operating at Class 3 baud rates providing wide network coverage addressing multiple applications.

CIM 8 offers significant power-per-bit reductions as well as cost efficiencies for various optical network transport applications.

DCI/Metro Reaches
For transporting 3x400GbE or 12x100GbE client traffic with metro reaches in a single carrier, the CIM 8 is tuned to ~6 bits/symbol (equivalent to 64QAM, example constellation on left). Data center interconnect (DCI) applications would take advantage of this high-capacity 1.2T transport capability to tie data center locations together. This amounts to 38.4T per C-band fiber capacity.

Long-Haul Reaches

For transporting 2x400GbE with long-haul reaches, the CIM 8 is tuned to ~4 bits/symbol (equivalent to 16QAM, example constellation on the right). Wide 800G network coverage is achieved with the Class 3 140Gbaud capabilities enabling service providers to provide end-to-end 2x400GbE, 8x100GbE, or native 800GbE transport across their networks, covering essentially all terrestrial applications.

Ultra-Long-Haul/Subsea Reaches

And for ultra-long-haul/subsea reaches, the CIM 8 is tuned to ~2 bits/symbol (equivalent to QPSK, example constellation on the left). As with the previous scenarios, spectral efficiency with a wavelength channel is optimized by fine-tuning of the baud rate. These high spectrally efficient modes can carry mixed 100GbE and 400GbE traffic over the longest subsea routes in the world with lowest cost per bit. It’s worth noting that almost a decade ago, Acacia demonstrated SiPh capabilities for subsea coherent deployments. CIM 8 incorporates second generation non-linear equalization (NLEQ) capabilities to mitigate the non-linear effects of optical transmission especially for these ultra-long-haul/subsea links providing additional OSNR.

In all the above scenarios, the CIM 8 utilizes advanced power-efficient algorithms to compensate for chromatic and polarization dependent dispersion. In addition, the module accounts for coverage of aerial fiber network segments that require fast state-of-polarization (SOP) tracking and recovery due to lightning strikes. The SOP tracking speed of CIM 8 is double the speed of its predecessor. This fast SOP tracking feature can also be utilized for sensing applications.

Network Operators Achieve Record Breaking Field Trials with CIM 8
CIM 8 capabilities have already been put to the test as illustrated by multiple record breaking field trials across a wide range of applications. These include >5600km 400G transmission over a mobile carrier’s backbone network, 2200km 800G transmission over a research and education network, and >540km 1T transmission over a wholesale carrier’s network.

Acacia continues to demonstrate its technology leadership by leveraging mature knowledge in proven silicon-based coherent technology, producing the first shipping coherent solution to lead the industry into the Terabit Era with the 1.2T pluggable CIM 8 module. With the breakthrough capability of 140Gbaud transmission along with the advanced Jannu DSP ASIC using 2^nd Gen 3D Shaping and leveraging 3D Siliconization, network operators can support full network coverage for multi-haul applications, especially to support growing demands for nx400GbE and upcoming 800GbE traffic.

References:
Blog: Terabit Today: Maximize Network Coverage
Blog: How Industry Trends are Driving Coherent Technology Classifications
Blog Series: The Road Ahead for Next-Generation Multi-Haul Designs Part 1, Part 2, Part 3

Class 3 Baud Rate Solutions Provide Wide Network Coverage to Support nx400GbE
As widely known in the industry, the two primary “knobs” for optimizing coherent transmission capacity and reach are the baud rate and the bits/symbol modulation order. While rapid advances to increase capacity have pushed the upper modulation order to the current 6 bits/symbol (equivalent to 64QAM), further increases in modulation order provide incremental improvements in spectral efficiency with significant reductions in network coverage. Because of this, the industry focus to reduce cost and power consumption per bit has shifted toward achieving higher baud rate modulation per wavelength. Technology advancements that resulted in achieving 120+Gbaud coherent modulation speeds, as Figure 1 illustrates, have enabled a new generation of Class 3 capabilities which have propelled the industry from gigabit to terabit transmission capacity.

Figure 1. Achieving Class 3 baud rates enabled coherent transport solutions to break through the terabit threshold, ushered in by Acacia’s CIM 8 solution.

Terabit era performance-optimized solutions such as Acacia’s shipping Coherent Interconnect Module 8 (CIM 8), powered by the Jannu DSP, as well as announced solutions from several other optical transport vendors are now leveraging this Class 3 baud rate standardization. While this operating baud rate class enables a capacity of 1.2T for metro/DCI reaches, a key benefit is the ability to provide full network coverage at 800G to support transport of 2x400GbE clients, as shown in Figure 2. In addition, subsea applications can also be supported with greater flexibility to achieve optimal spectral efficiency. The flexibility, full coverage, and availability of these terabit era solutions make them attractive for supporting evolving transport needs as native 400GbE traffic requirements are becoming more commonplace.

Figure 2. Class 3 terabit era CIM 8 from Acacia provides 800G everywhere to transport 2x400GbE client traffic. By adjusting the transmission bits/symbol, a higher or lower number of 400GbE links can be achieved with corresponding reaches.

The CIM 8 achieves Class 3 140Gbaud rate capability using proven silicon technology to successfully break through the terabit-per-wavelength threshold. Silicon technology has demonstrated cost and power advantages over alternative technologies, making it the material system of choice for these higher baud rates. This module enables a coherent transmission solution capable of providing full network coverage to support nx400GbE traffic.

Terabit Era Solutions Support 400GbE Today, Ready for 800GbE in the Future
Supporting 400GbE client traffic over a network operator’s transport infrastructure is expected to be the dominant trend for many years to come. Terabit era solutions are ideal for these providers because they can implement a straightforward 2x scaling of channel spacing to evolve from Class 2 to Class 3 technology, as illustrated in Figure 1.

With full network coverage at 800G using ~4 bits/symbol transmission, the CIM 8 module can not only support today’s 2x400GbE client traffic, but it is also ready to enable 800GbE interconnections when that end-user demand materializes in the future.

With More than 65% Power-per-bit Savings, Why Delay on Achieving Power Reduction Goals?
CIM 8 takes advantage of Acacia’s most advanced silicon photonics technology that enables 140Gbaud capabilities resulting in doubling the capacity of each wavelength without doubling the component count, size, and power of the coherent device. In addition, advancements such as Acacia’s 3D Siliconization, which leverages highly integrated opto-electronic packaging, as well as the adoption of power-efficient signal processing algorithms, all contribute to a 65% power-per-bit savings of CIM 8 over current competing solutions being deployed.

With the industry striving to reduce overall power consumption, adoption of available solutions such as the Acacia CIM 8 can help towards achieving these goals sooner. Delaying the deployment of power saving technology on a network-wide scale can result in continued carbon emissions at current levels, delaying the achievement of power consumption reduction goals.

By incorporating coherent technology innovations to achieve low power Class 3 140Gbaud transmission while leveraging volume processes, CIM 8 is a result of the investments Acacia has made over generations of silicon-based products, providing a significant advantage for providers looking to drive long-term power-per-bit reductions without sacrificing performance and reach.

Maximizing Coverage for the Long-Term with Terabit Era Solutions
Network operators have multiple options to consider when migrating their infrastructures to support the growth of nx400GbE client traffic and preparing for 800GbE. While router-based optics solutions are gaining momentum, those wanting to maintain a transport optical layer are looking towards terabit era performance-optimized coherent solutions such as Acacia’s CIM 8 to support this growth. The CIM 8 module is shipping today and not only provides flexibility in supporting nx400GbE client traffic with wide network coverage, but also follows the direction of industry baud rate standards that allow for scalable network/channel-spacings as well as the utilization of mature silicon technology. This enables service providers to future proof their networks for traffic demands, while optimizing their return on investment and minimizing risk.

The 400G OpenZR+ MSA expanded 400ZR reaches far beyond 120km by utilizing higher-gain forward error-correction coding (FEC) and increased compensation for chromatic and polarization mode dispersion. This enabled the use of 400G pluggables not only in hyperscale environments, but also in metro/regional service provider network environments. Service providers have thus been focusing on these “plus” versions of 400G modules for their network needs.

Brightening ROADM Networks
The latest expansion of 400G pluggable applications is being enabled by QSFP-DD form-factor modules, such as Acacia’s Bright 400ZR+ QSFP-DD, which have an optical transmitter power at least ten times greater than 400ZR. The higher transmitter power expands the applications that can be addressed by 400G coherent pluggable modules to include brownfield and greenfield metro/regional networks with reconfigurable optical add-drop multiplexer (ROADM) nodes, shown in Figure 1.

Figure 1.  Recently introduced Bright 400ZR+ QSFP-DD modules expand applications of 400G coherent pluggables to brownfield and greenfield metro/regional networks with ROADM nodes.

ROADMs enable the ability to selectively add/drop wavelengths to/from the network, avoiding optical-electrical-optical (OEO) conversions on channels passing through the ROADM node. Enabled by wavelength selective switch (WSS) technology, ROADM-based architectures can provide operational advantages compared to fixed optical add/drop solutions that required manual intervention for reconfiguration.

These network elements operate in tandem with terminal DWDM equipment and amplified single-span or multi-span line systems. To ensure optimal aggregate optical SNR performance, a certain level of transmit optical power and uniformity is required across the entire DWDM wavelength band. Having a 400G coherent pluggable QSFP-DD solution with high transmit optical power would enable 400G router-based optics to be deployed in a ROADM network architecture with existing wavelength traffic.

Bright Applications
Figure 2 illustrates two general examples of how Bright 400ZR+ QSFP-DD modules can be used in ROADM networks for sites with or without existing DWDM transponder terminal equipment.

(a)

(b)

Figure 2. 400G QSFP-DD pluggables with high transmit optical power can be used at a network node in new deployments (a) or with (b) existing DWDM transport/transponder terminal equipment.

IP-over-DWDM over ROADM Infrastructure
While point-to-point IP-over-DWDM architectures can be fully supported using 400ZR or OpenZR+ modules, there may be instances in which IP-over-DWDM traffic may need to traverse over a legacy ROADM infrastructure (Figure 2a), perhaps as an overlay with existing sites populated with DWDM transport terminal equipment (Figure 2b). Figure 2b also illustrates an alien-wavelength example where a router-port wavelength is inserted into a line system of a different vendor.

While there are network designs that support the use of -10dBm 400ZR+ modules over a ROADM network, it becomes a challenge when the DWDM transmission from these modules are adjacent to multiple legacy higher optical power channels (Figure 3a) resulting in relatively lower received optical power and OSNR. Legacy transponder terminal equipment typically have a per-channel ingress optical power in the 0dBm range. One solution is to add an external optical amplifier per module to boost the optical transmit power to the 0dBm level into the line system ingress.

By taking advantage of highly integrated silicon photonics design and packaging technology, such as Acacia’s 3D Siliconization, it is possible to eliminate the need for an external amplifier by incorporating the optical amplification into the compact QSFP-DD form factor module. Utilizing a 400G coherent pluggable with a higher optical transmitter power of at least 0dBm ensures sufficient optical power, relative to other DWDM wavelengths, to traverse through legacy ROADM nodes, as shown in Figure 3b.
(a)
(b)

Figure 3.  (a) A standard 400ZR/ZR+ module with transmitter output power of -10dBm co-exists with legacy higher power channels, subject to lower received optical power and OSNR; (b) a Bright 400ZR+ wavelength ensures channel power uniformity in ROADM/line-system ingress when combined with other line-system traffic.

Transmitting in High Fidelity
Many ROADM architectures utilize colorless multiplexing. This type of ROADM has a passive optical combiner element (Colorless MUX in Figure 4) that takes individual fiber inputs from different transmission wavelengths and combines them together into a single fiber without any optical filtering, hence the name colorless. In wavelength-agile network designs, external control of the tunable wavelengths avoids any contention issues. To minimize wavelength crosstalk/noise through colorless ROADMs, the transmitter OSNR of each ingress wavelength needs to be maximized. One way to achieve this is to attenuate the laser amplified spontaneous emission (ASE) noise using a band-pass optical filter to minimize optical crosstalk/noise into nearby channels. And to accommodate wavelength-agile networks, a tunable optical filter (TOF) is utilized.

Figure 4.   Illustration highlighting the benefits of a TOF inside a coherent module with internal optical amplification when multiple channels pass through a colorless multiplexer; comparison between (a) modules without TOF vs. (b) modules with TOF.

Figure 4 illustrates how multiple wavelengths are combined using a colorless MUX. In figure 4a, the noise from multiple modules without a TOF impacts nearby transmission channels through a colorless MUX. By using a TOF, not only does is transmit OSNR improved, but also the noise affecting adjacent channels is minimized. The Figure 4 inset illustrates how a TOF suppresses noise resulting in an optimal situation of a module wavelength insertion surrounded by legacy wavelength transmissions, shown in Figure 4b.

Higher Density DWDM Footprint
For network operators continuing to leverage an optical transport layer, the higher optical power QSFP-DD modules enable a higher density design for DWDM transponder terminal equipment. While high transmit optical power has been available in the CFP2 form factor for 400G, advanced integration and design has resulted in this capability in the smaller QSFP-DD housing.

The Acacia Bright Solution
Acacia recently introduced its Bright 400ZR+ coherent pluggable QSFP-DD module that transmits at least +1dBm of optical power to expand 400G coherent pluggable applications with brownfield and greenfield ROADM network architectures. This module leverages 3D siliconization to provide a highly integrated design enabling the silicon photonics integrated circuit (including TOF), coherent DSP, high-speed components, and optical amplifier to fit entirely into a compact QSFP-DD pluggable form-factor module. In addition to supporting interoperable OpenZR+ and Open ROADM modes, the Bright 400ZR+ QSFP-DD module can aggregate 4x100GbE client traffic and also utilize FlexO mapping for OTN transport applications.

Bright 400ZR+ Expands Applications for 400G Pluggables
High transmit power 400G pluggables, such as Acacia’s Bright 400ZR+ module, surpass the market applications served by 400ZR module by enabling an optical transmit power that is at least ten times greater (i.e., brighter). This is another significant milestone because it enables service providers and hyperscalers to take advantage of 400G pluggables in brownfield and greenfield ROADM based optical network architectures.

Additional Resources:

• Bright 400ZR+ coherent pluggable QSFP-DD module Product Page
• Arelion completes first multi-vendor, multi-layer field trial using Acacia Bright 400ZR+ modules and Cisco Routers

At OFC 2022, there were several presenters discussing a new effort dubbed “coherent lite.” Roughly speaking, this term is used to convey a simplified implementation of coherent transmission for use in short reach campus and intra-DC applications. Compared to traditional transport DWDM applications, coherent lite removes unneeded features such as laser tunability and reduces complexity of impairment mitigation features such as dispersion compensation. Compared to alternative 800G and beyond solutions, coherent lite offers higher speed per wavelength, lower laser or fiber count, and better receiver sensitivity. By leveraging the continued trends of CMOS, the key components to build coherent-lite modules in a pluggable format are on par with alternative solutions with regards to power and size. The additional link budget available with a coherent implementation can be utilized to optimize the design for cost and power. These reasons make coherent lite a compelling solution for pluggable 800G and beyond in campus and intra-DC applications.

Figure 1.  Coherent solutions moving to shorter reaches as application data rates increase; pluggable modules leading the charge towards shorter reaches.

Coherent lite has been proposed for 800G campus network applications in both IEEE and the Optical Internetworking Forum (OIF), while some in the industry are already talking about coherent interfaces inside the data center at 1.6 Tbps. The OIF was the first to take action in this space, kicking off the 800LR project in late 2020 to pursue a solution for unamplified reaches of 2-10km using fixed wavelength coherent transmission. Hyperscale network operators are expected to be the first to adopt this technology due to their high bandwidth campus and intra-DC interconnect requirements. And as bandwidth demands increase to 1.6 Tbps intra-DC links, coherent lite solutions are expected to competitively address sub-2km reaches inside the data center.

Intra-Data Center Optical Interconnect Requirements

12.8Tbps Ethernet switches required 400G pluggable modules in QSFP-DD and OSFP form factors. 400ZR coherent optical transceivers, supporting these form factors, were developed for DCI edge network applications up to 120 km in reach. With switch capacity increasing to 25.6 Tbps, followed by 51.2 Tbps, optical transceivers are expected to migrate from 400 Gbps to 800 Gbps and then towards 1.6 Tbps speeds. Scaling the optical interconnect solutions to match these increasing port speeds can be challenging. While legacy-based intensity-modulated-direct-detect (IM-DD) solutions may continue to have a role for shorter intra-DC links at 800G, longer reach intra-DC applications benefit from coherent solutions. And at 1.6Tbps port speeds, coherent can become the preferred solution even for short intra-DC links.

Besides supporting growing port speeds, intra-DC optical interconnects at 800G and beyond are required to have low power consumption, high density, and support of high-volume deployments. To meet these requirements, modules can leverage advancements in low-power CMOS technology (which follows Moore’s Law), silicon photonics, and innovative packaging and integration solutions.

Another key intra-DC requirement is interoperability, which helps to drive broad industry adoption and higher volumes, thereby lowering supply chain risks for network operators. The importance of a robust supply chain has never been more important, and the higher volumes required for intra-DC applications make interoperability critical for coherent-lite adoption.

Coherent solutions have already proven capable of meeting these data center requirements of low power, high volume, and interoperability at 400G with the successful introduction of 400ZR modules. Coherent lite solutions are anticipated to chart a similar path for 800G and beyond.

Coherent Solutions for Intra-DC Applications

400G coherent pluggable transceiver solutions have proven that high-density, low-power coherent technology tailored to inter-data center switch/router interconnection applications are achievable. The industry has now embarked on a similar effort to bring to market cost-effective, high-volume coherent solutions optimized for campus and intra-DC applications.

Traditional intra-DC optical interconnects utilize IM-DD transmit/receive technology. Generational increases in link speed have required parallelization of fibers (e.g. 400G-DR4 using four fibers) or wavelengths (e.g., FR4 CWDM), as well advanced amplitude modulation schemes such as PAM4. While this aggregate approach has been successful to date, chromatic dispersion (CD) impairments begin to impact performance as the link speed requirement increases. Due to the square relationship between CD tolerance and the modulation baud rate, as you double each wavelength’s baud rate the CD tolerance is reduced by a factor of four. Alternatively, increasing the number of wavelengths pushes outer channels further from the fiber zero dispersion point resulting in having to mitigate this entire wavelength range to meet the link budget. Thus, even though the intra-DC distances are short, traditional IM-DD methods of increasing link capacity are expected to encounter challenges as intra-DC applications move to 800G and beyond.

Figure 2. IM-DD and Coherent proposed solutions to address 800LR applications.

Figure 2a illustrates a traditional IM-DD approach to address intra-DC and campus applications over a single-mode fiber pair, which for the 800G case multiple WDM lasers would be utilized to ensure sufficient link budget at this data rate.

IM-DD solutions are expected to be utilized for 800G intra-DC application reaches in the 2km range, while coherent technology would support 800G from 2km to 10km reaches. For optical platforms such as silicon photonics where a single laser’s power can be shared across multiple fibers, we expect the IM-DD solutions to remain attractive in those short, parallel fiber applications.

Coherent Lite Offers Cost-Optimized, Low Power Solution

In comparison to the IM-DD implementation, a coherent solution (Figure 2b) addressing the same 800LR link would achieve the target link budgets using one laser, and the improved sensitivity that is achieved using coherent detection. The laser capacity can be increased four-fold by utilizing the phase and polarization dimensions of light (I/Q modulation and polarization multiplexing). Using a single laser compared to four can result in cost and power improvements. In addition, the DSP in the coherent solution can mitigate dispersion effects as it would in a traditional transport solution but with a simpler implementation for a 2 to 10km reach.

Since intra-DC architectures do not need dense wavelength transmission in fiber, grey (fixed wavelength) lasers can be used, which greatly simplifies the design and reduces module cost.  Also, the extra available link budget due to higher receiver sensitivity can be used to lower the required laser power to reduce module power dissipation. In addition, coherent technology for high-capacity transport has traditionally required higher supplier capital expenditures on a per unit basis because of lower volume, more expensive test equipment required for the stringent specification requirements that drive the need for more comprehensive test coverage. Coherent lite intra-DC modules would be tested more like IM-DD client optics, resulting in substantial reduction in manufacturing capex with higher capacity.

These are some of the compelling reasons why the industry is looking toward coherent technology for shorter connections at 800G and beyond. Coherent lite 800LR pluggables can provide a competitive cost structure, while meeting campus and intra-DC requirements. This makes these solutions clear candidates for applications typically addressed by IM-DD solutions.

Opening the Doors to Coherent

400G coherent pluggable solutions have driven the momentum towards interoperable, high-volume solutions that are enabling coherent in campus and intra-DC applications at higher data rates, especially when utilizing a cost and power optimized coherent implementation. Compared to IM-DD, coherent offers a scalable path towards higher intra-DC data rates with more capacity per laser wavelength, higher receiver sensitivity, and digital equalization of impairments. In addition, the coherent lite solution may offer less technical risk and earlier market availability.

As requirements move beyond 800G towards 1.6T for intra-DC connections, dispersion impairments and link budget requirements are expected to be even more challenging for IM-DD solutions. Because of this, coherent-lite solutions are expected to be a strong contender for high-volume 1.6T intra-DC interconnect applications.

Introduction
In Part 2 of this blog series, I discussed how the approach of common development based on silicon photonics (SiPh) benefits both performance-optimized multi-haul and MSA pluggable coherent products. In this blog, I expand on why the adoption of standardized baud rates to support Nx400GbE client traffic has been advantageous for MSA pluggable adoption as well as for maximizing multi-haul 400G network coverage in a service provider network. I also discuss how the growth in coherent pluggable shipments impacts the development of performance-optimized multi-haul coherent solutions.

Intuitive Coherent Classifications
With 400GbE becoming today’s unit of traffic currency, it is important to understand what coherent operating conditions are required to efficiently transport 400GbE client traffic. Prior generation leading-edge modulation technology for commercialization was at ~60-68Gbaud (Class 2), which utilized ~4 bits/symbol (~16QAM) modulation to support 1x400GbE client traffic over a 400G wavelength. We saw OIF 400ZR, OpenZR+ MSA, and Open ROADM adopt this transmission scheme for high-volume MSA pluggable applications. Performance-optimized multi-haul coherent solutions also leveraged Class 2 baud rates with dynamic transmission shaping that was centered at ~4 bits/symbol. In addition, multi-haul solutions supported higher capacity with 6 bits/symbol (~64QAM) for shorter reach DCI/Edge applications as well as maximum reach using 2 bits/symbol (~QPSK) for subsea applications. Multi-haul coherent DSPs can dial up or down the desired bits/symbol transmission within this dynamic range.

Moving to the next generation of coherent solutions, a similar approach is being followed to support Nx400GbE traffic per wavelength. In this case, leading edge ~120-136Gbaud Class 3 modulation technology can double the transport capacity compared to Class 2. With Class 3 technology, and using ~4 bits/symbol range, near complete coverage of a service provider’s network with 2x400GbE traffic (800G per wavelength line rate transmission) is possible, as discussed in a previous blog. By dialing the modulation bits/symbol down to ~2 bits/symbol, 1x400GbE subsea applications are possible, while dialing up to ~6 bits/symbol, 3x400GbE client traffic can be transported over a 1.2T wavelength for shorter reach DCI/Edge applications.

Figure 1 shows the number of supported 400GbE clients that can be transported over a single coherent wavelength as a function of baud rate class and ~2, ~4, and ~6 bits/symbol values.

Figure 1. Matrix showing the number of supported 400GbE clients that can be transported over a single coherent wavelength as a function of bits/symbol and baud rate class. Broad Nx400GbE network coverage becomes possible as we move to Class 3 baud rate implementations.

Figure 1 also illustrates how doubling the baud rate maintains the same network coverage (vis-à -vis reach) while doubling the capacity. With performance innovations, the coverage at this doubled capacity may even be greater than the previous class.

Implications of Shipment Trends
The good news is that the technology investments that went into supporting Class 2 coherent pluggable MSA solutions (green-shaded cell in Figure 1) are coming to fruition with high-volume shipments already occurring. In fact, Acacia recently announced over 50k port shipments of 400G coherent pluggable solutions. This is contributing to a notable transition: the number of pluggable coherent modules shipments is forecasted to exceed that of performance-optimized (embedded) coherent module shipments (Figure 2).

What are the implications of the growing adoption of MSA pluggable solutions? The overarching implication for coherent design evolution is that standardization/industry consensus (e.g., baud rates), design elements (e.g., high-speed electrical components), as well as materials and processes (e.g., silicon, SiPh, CMOS processes, co-packaging) that support high-volume MSA pluggable solutions all have a favorable impact on performance-optimized multi-haul solutions. In Part 2, I went into detail about how a common silicon platform enables a cycle of coherent development. The Figure 2 data below indicates that given this growth in MSA pluggable ports, we should see a greater beneficial impact to performance-optimized multi-haul designs because higher volume pluggable solutions can lead to better cost efficiencies, assuming both the multi-haul and MSA pluggable designs leverage a common technology platform.

Figure 2. LightCounting data showing the number of globally shipped MSA pluggable coherent ports exceeding the number of proprietary form-factor ports.

The Road Ahead for Multi-Haul Solutions—CIM 8
As the industry moves towards supporting higher transmission line rates, a natural path to higher steps in baud rate are becoming clearer. Doubling the Class 2 baud rates aligns with the Class 3 120Gbaud+ rates that are being standardized for transport of 800G client traffic. By leveraging common silicon processes and technology, performance-optimized solutions can benefit from the economies of scale.

The recently announced Acacia Coherent Interconnect Module 8 (CIM 8) powered by Acacia’s Jannu DSP are in line with this approach. The CIM 8 is a performance-optimized multi-haul solution that delivers industry-leading performance with single carrier 1.2T operation using 3D Siliconization packaging technology that includes the silicon photonics integrated circuit (SiPh PIC), high-speed modulator driver and transimpedance amplifier (TIA) in a single opto-electronic package. The miniaturization of the module components has resulted in a 140Gbaud multi-haul module design capable of faceplate pluggability.

Figure 3. Acacia’s Coherent Interconnect Module 8 is designed to incorporate many aspects of technology leveraged from higher-volume products (e.g., SiPh, processes, components, packaging).

The CIM 8 can provide efficient transport of 400GbE client traffic across the entire network, including 90 percent coverage using 800G (2x400GbE client traffic), corresponding to the Class 3 row in Figure 1.

All Roads Lead to Multi-Haul

Figure 4. By utilizing common silicon technology, Acacia can leverage the advantages of volume and high-performance designs creating a generational development cycle, with advancements over time, that can result in cost efficiencies as well as time-to-market advantages.

Coherent solutions have evolved from long-distance applications at relatively moderate volumes and are now at a point in the road where shorter distance high-volume applications are driving a demand exceeding their long-distance counterparts. By leveraging standardized baud rates aligned with the corresponding bits/symbol modulation optimized for Nx400GbE, we can increase the overlap of silicon-based design and investments between high-volume MSA pluggable and performance-optimized multi-haul coherent solutions. This enhances the coherent cycle of development, resulting in multi-haul solutions benefiting from volume manufacturable designs while leveraging common technology, which are important for maintaining cost-efficiencies for network operators as bandwidth demand continues to grow.

Learn More About the Considerations Driving Next Generation Multi-Haul Solutions:

Part 2 of 3
Part 1 of 3