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.
]]>
Acacia is excited to be participating once again in person! We expect a number of hot topics to be front and center at the show – from the path to coherent lite, interoperable 400ZR/ZR+ pluggables ramping, and of course high-performance multi-haul solutions.
Coherent Lite
Over the last year, the industry has been abuzz around a new effort called “coherent lite,” which is a simpler implementation of coherent transmission that is designed for use in short reach campus and intra-DC applications. It essentially removes the features required for traditional transport DWDM applications.
Coherent lite pluggables can provide a competitive cost structure at 800G and beyond, utilizing fewer lasers, while meeting campus and intra-DC requirements. This solution can be a candidate for applications typically addressed by IM-DD solutions. We believe this will be a hot topic at the show.
The Rise of Standardized 400G Pluggables
The 400G coherent pluggable market is one of the most important developments in optical networking today. According to Cignal AI’s 4Q21 Transport Applications Report, shipments of 400ZR/ZR+ modules are expected to triple in 2022 as cloud operators such as Microsoft, Amazon, and Google are joined by traditional network operators such as AT&T, Windstream, and COLT rolling out high volume deployments. Similarly, LightCounting’s Market Forecast from April 2022 estimates that the opportunity for pluggables based on 400ZR and 800ZR technology will surpass $2.8 billion by 2027.
Clearly, interoperable standards have played a key role in the rapid growth of pluggables. Standardization helps carriers by providing simplicity, interoperability and volume-driven cost. This has helped increase demand. The aggressive ramp of these products has been faster than any previous coherent technology generation. Several OIF members including Acacia are set to showcase multivendor interoperability for critical networking technologies at ECOC 2022.
OIF’s 400ZR project is proving successful in facilitating new and simplified architectures for high bandwidth inter-data center interconnects and promoting interoperability among coherent optical module manufacturers including Acacia’s 400ZR QSFP-DD. This 400ZR interop demo shows a full implementation across an 80km DWDM ecosystem using multiple form-factor pluggable modules, 400GbE routers, a 75 GHz C-band open line system, and test equipment solutions from multiple vendors. The demo provides evidence of widescale 400ZR deployment readiness based on a broad ecosystem of interoperable solutions.
Acacia’s newest entrant to the pluggable module category is the high transmit power 400G pluggable, which we are calling our “Bright” module. We recently introduced the Bright 400ZR+ QSFP-DD pluggable coherent optical module with high optical performance designed for seamless deployment in ROADM line systems. These modules are designed to expand the applications for this technology by addressing a variety of ROADM architectures such as those with colorless multiplexing. With greater than +1 dBm transmit optical power and high transmit OSNR, Acacia’s Bright 400ZR+ gives network operators more flexibility to address a range of brownfield and greenfield applications utilizing the high-density QSFP-DD form factor.
High-Performance Multi-Haul Solutions
While pluggables are becoming extremely popular for router-based solutions in DCI, metro and some long-haul applications, transponder-based solutions are still required for long-haul, ultra-long haul and subsea. Transponders that include multi-haul high-performance modules can help network operators maximize their fiber investment, reduce power, simplify operations, and decrease costs.
The industry has shifted its focus to increasing the baud rate per carrier, while also introducing performance-improving innovations, to enable end-users to achieve cost-efficiencies as bandwidth demand continues to grow. As an example, newly introduced Class 3 120+Gbaud solutions based on silicon photonics offer a path to cost-effectively double capacity over a single carrier compared to earlier implementations.
Acacia’s performance optimized multi-haul coherent solutions have proven to be extremely effective at helping service providers meet these challenges. Earlier this year, Acacia announced it had shipped more than 100,000 ports of its Pico DSP which powers the company’s highly successful AC1200 1.2TB module.
Building on this success, Acacia introduced its Coherent Interconnect Module 8 (CIM8), which is based on the company’s newest DSP technology, the 8th generation Jannu 5nm DSP, and includes second generation 3D Shaping and continuous baud rate adjustment of up to 140Gbaud. This new module achieves several industry firsts:
- The first single-carrier 1.2TB multi-haul pluggable module
- The first coherent module to support 140Gbaud
- The first 5nm multi-haul DSP
Acacia successfully demonstrated 140Gbaud performance with our high-performance silicon photonics at this year’s OFC 2022.
Speaking Sessions
To hear more about the topics discussed in this article, don’t miss our speaking sessions.
- Workshop: Adaptive everything! Do optical networks really need more flexibility?
- Presentation title: 400ZR and OpenZR+: Enabling New Network Architecture Optimization
- Speaker: Lorenzo Ghioni
- Date/time: Sunday September 18that 9:15-9:25 am
- Market Focus Panel: Modules/Sub-Systems/Connectors/Line-Cards
- Presentation title: Next Generation Coherent Interfaces: Standardized Pluggable vs Proprietary Multi-haul
- Speaker: Tom Williams
- Date/time: Tuesday September 20that 10:00-10:25 am
Come See Us!
If you are attending the show and want to connect, we’d welcome the opportunity to meet with you. Click here to set up a meeting. We hope to see you in Switzerland!