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.

In a few short days several members of the Acacia Communications team are scheduled to attend the annual European Conference on Optical Communication (ECOC) Conference and Exhibition at the Fiera Roma in Rome, Europe’s largest optical communications event. On the agenda for this year’s show are topics ranging from next-generation data centers to optics as an enabler for 5G. We are bringing a full team to talk to customers and partners and to learn more about the advancements our colleagues are making in optical communication technologies.

The 600G Era

We’re looking forward to discussing the progress of coherent optical technology and the industry transition to the 600G (per-wavelength) era which where capabilities, such as high-baud-rate optics, fine-tuned coherent transmission flexibility, and enhanced forward error correction (FEC) algorithms, are raising coherent performance to new levels.

In addition, today’s technology is designed to enable common coherent optical hardware to achieve the high-performance finesse of a long-distance link, the sheer raw capacity for shorter edge links, and scales to applications in between. This is the reason why some refer to the 600G era coherent technology as multi-haul technology since the same set of hardware can address long-haul, metro, and DCI-edge networks.

Learn more about the 600G era by reading “Fill Capacity Gaps in Your Optical Network” just published in the Autumn Edition of Optical Connections Magazine.

AC1200 600Gbps per Wavelength Coherent Transmission Demonstration

Acacia plans to show its AC1200 coherent module with dual-core design enabling 1.2 Tbps error-free transmission over fiber with 600 Gbps per wavelength coherent transmission. The AC1200 also features high-baud-rate high-performance capabilities as well as fine-tuning of the transmission using 3D Shaping which optimizes reach and capacity utilization. 600 Gbps performance requires DSP and optics technology with both high bandwidth and high performance. Acacia’s AC1200 module, which is based on its internally developed silicon photonics technology, uses advanced techniques to minimize implementation penalties and high-gain soft-decision forward error correction (FEC) to improve performance margin. Not only does this allow AC1200 to achieve high capacity 600 Gbps per wavelength transmission, with up to 1.2 Tbps, but it also results in improved performance at lower data rates, such as 200 Gbps and 400 Gbps, compared to existing solutions.

Plug coherent in edge and access

Also at the show, we expect to hear a lot of discussion around advances in coherent technology to help meet growing bandwidth demand beyond the core. Today, coherent is moving from metro core to access aggregation networks. The industry is working to standardize coherent solutions for even shorter reach interfaces including two new specifications recently announced by CableLabs that are the result of a focused effort to develop coherent optics technology for the access network and bring it to market quickly.

We look forward to discussing Acacia’s participation in the industry organizations such as Optical Internetworking Forum (OIF), IEEE, and Cable Labs and the trend towards using coherent solutions such as Acacia’s award-winning CFP2 for shorter distances.

Acacia Communications’ Thought Leaders to Present

In addition, Acacia Communications experts plan to lead discussions and share knowledge covering topics such as Digital Signal Processing (DSP), subsea communications, and lab automation. See below for a complete listing of who, when, where and what each plans to share with ECOC attendees:

Sunday, September 23, 2018

• 9:00-12:30 – Timo Pfau, principal DSP engineer, will present on “What can DSP bring to optical access” in the DSP for Next Generation Optical Access Workshop.
• 9:00-12:30 – Christian Rasmussen, founder and vice president of digital signal processing and optics, will share his thoughts on the “Future scaling and capabilities of DSP algorithms” in the Coherent DSP in Optical Communications Workshop.
• 9:00-12:30 – Hongbin Zhang, principal DSP engineer, will present on the “Future of transponders for submarine transmission” in the Submarine Systems Workshop.

Tuesday, September 25, 2018

• 1:30 – 3:00 p.m. – BinBin Guan, optical engineer, will lead the Lab Automation Hackathon in which there will be eight demonstrations for various common lab automation tasks.

Wednesday, September 25, 2018

• 1:30 p.m. – Timo Pfau will present his invited paper titled, “High performance coherent ASIC” as part of the SC3 – Digital Signal Handling Techniques for Optical Communication Systems session.

Our team of experts is available to discuss Acacia Communications’ latest innovations in optical interconnect solutions. If you’d like to meet with Acacia at ECOC, contact us. We’d love your feedback!

“We are honored to accept this prestigious award. Based on our experience, we see coherent moving from metro core to access aggregation networks such as 5G backhaul and fiber deep Remote PHY for cable access. Access networks delivering high bandwidth services could dramatically increase energy requirements to power and cool the equipment providing these services. We believe Acacia is on the forefront of providing the high performance, low power, compact form factor solutions necessary to help enable data centers to run on sustainable energy.”

• Raj Shanmugaraj, president and chief executive officer of Acacia Communications

We also marked our attendance at the show with the announcement of our Coherent ZR Optics Solutions for 100G CFP-DCO ZR and 100G/200G CFP2 ZR pluggable modules. Small coherent modules for very compact systems, they are designed to address 5G and fiber deep applications, as well as data center edge and enterprise campus applications.

In talking with customers about the CFP2 module, we heard that they like the capacity granularity for metro with 100G/200G per wave. We also had a lot of good discussion on the following key features:

• Support for 200G/ lambda – at a significant low cost/bit
• Simplified system integration with DCO interface
• Integrated functionality eliminating the need for additional ASICs: OTU framer function, Layer 1 Encryption
• Easy network maintenance with rich performance monitoring such as link dispersion/PMD/PDL/OSNR

To learn more about how the CFP2 pluggable coherent module can help network operators reduce CapEX and OpEx, increase reach, and prepare for future growth, watch this video.

We look forward to seeing everyone at ECOC, September 23 – 27 in Rome, Italy.

Last week, Acacia introduced coherent ZR Optics solutions for CFP and CFP2 Pluggable Modules. Let’s take a look at why these solutions have evolved to address the need for growing capacity in network access and edge applications.

It should come as no surprise that rising commercial and consumer bandwidth demands linking the end user to core networks have created the need to increase the capacity within the access portion of the network infrastructure. Today, deployed fiber-optic access networks typically utilize 10Gbps optical links, transporting either Ethernet or SONET/SDH protocol traffic with Ethernet being a dominant protocol. Direct-detect optical transceivers adhering to IEEE 10GBASE-ER Ethernet specifications enabled 40km link distances without requiring any optical amplification. To fulfill network demands that required reaches beyond 40km in the access network, transceivers meeting 10Gbps “ZR” requirements entered the market. IEEE actually never formally created a 10GBASE-ZR specification. Instead, ZR became a market de facto standard, typically understood to meet 80km reach requirements based on SONET/SDH optical specifications.

In today’s environment, cloud services, broadband and 5G applications are imposing significant bandwidth demand in access/edge networks. And the need to upgrade from 10Gbps to 100Gbps and beyond over the same distance is becoming increasingly important. We have, in a previous blog post, discussed how coherent technology has progressed from long-haul network applications to shorter reach applications (access, DCI/edge, campus). Because of its inherent intradyne detection and digital signal processing, a coherent transceiver solution fits well in these access/edge network environments that require ≥100G links to overcome high dispersion and high loss conditions. Also, designing coherent ZR transceivers with the same form factors as similar low-power consumption pluggables (such as CFP and CFP2) enable the access equipment to support different distances by simply swapping transceivers.

Figure 1. Acacia’s CFP-DCO ZR and CFP2 ZR pluggable modules

With this as a backdrop, to meet the need for growing capacity in network access and edge apps, Acacia introduced ZR coherent optics solutions to support unamplified applications beyond 40km reaches, extending to 80km and beyond. The introduced CFP-DCO ZR and CFP2 ZR coherent optical interconnect products are capable of supporting the 18W power class that was widely deployed for 10km client applications.

Both Acacia’s CFP-DCO ZR and CFP2 ZR modules have the ability to provide greater than 80km of equivalent loss budget and dispersion budget, over single mode fiber. Acacia’s CFP2 can also support 200G over these types of links. We also expect to see industry support for 400G in the CFP2 form-factor as well. These coherent ZR plug-and-play modules do not require external dispersion compensation or external optical amplification for 80km point-to-point distances.

Evolving Nomenclature

Some may recall during the rise of 10G Ethernet, there was a period of time when terms like “SR” and “LR” meant different reaches whether one came from the “telecom world” versus the “datacom world”. To a telecom person, “SR” could mean SONET OC-192 “SR-1”, a ~10km single mode reach. Whereas, to a datacom person “SR” meant IEEE Ethernet 10GBASE-SR, a 300m multimode reach. “LR” to the telecom person could mean “LR-2”, an 80km single mode link. Whereas, to the datacom person, “LR” referred to a 10GBASE-LR 10km single mode link.

As stated in the beginning of this blog, “ZR” at 10Gbps refers to an 80km-reach optical link. The good news is that the current standardization efforts to define coherent “ZR”, primarily within Optical Internetworking Forum’s 400ZR implementation agreement efforts, also include 80km reach, with the current understanding is that the “ZR” generally refers to a coherent DWDM optical reach from 40km to 80km and beyond. Stay tuned to this blog for future posts on 400ZR developments.