integration Archives - EFFECT Photonics

Insights

March 22, 2023

Transceivers in the Age of AI

Artificial intelligence (AI) will have a significant role in making optical networks more scalable, affordable,…

Artificial intelligence (AI) will have a significant role in making optical networks more scalable, affordable, and sustainable. It can gather information from devices across the optical network to identify patterns and make decisions independently without human input. By synergizing with other technologies, such as network function virtualization (NFV), AI can become a centralized management and orchestration network layer. Such a setup can fully automate network provisioning, diagnostics, and management, as shown in the diagram below.

Figure 1: Example of a carrier network with different applications (mobile, home, office) automated via NFV control and AI management.

However, artificial intelligence and machine learning algorithms are data-hungry. To work optimally, they need information from all network layers and ever-faster data centers to process it quickly. Pluggable optical transceivers thus need to become smarter, relaying more information back to the AI central unit, and faster, enabling increased AI processing.

Faster Transceivers for the Age of AI

Optical transceivers are crucial in developing better AI systems by facilitating the rapid, reliable data transmission these systems need to do their jobs. High-speed, high-bandwidth connections are essential to interconnect data centers and supercomputers that host AI systems and allow them to analyze a massive volume of data.

In addition, optical transceivers are essential for facilitating the development of artificial intelligence-based edge computing, which entails relocating compute resources to the network’s periphery. This is essential for facilitating the quick processing of data from Internet-of-Things (IoT) devices like sensors and cameras, which helps minimize latency and increase reaction times.

400 Gbps links are becoming the standard across data center interconnects, but providers are already considering the next steps. LightCounting forecasts significant growth in the shipments of dense-wavelength division multiplexing (DWDM) ports with data rates of 600G, 800G, and beyond in the next five years. We discuss these solutions in greater detail in our article about the roadmap to 800G and beyond.

Figure 2: Shipments of high-speed DWDM ports by data rate (historical data and forecast). Source: LightCounting Optical Communications Market Forecast, April 2022.

Coherent Modules Need to Provide More Telemetry Data

Mobile networks now and in the future will consist of a massive number of devices, software applications, and technologies. Self-managed, zero-touch automated networks will be required to handle all these new devices and use cases. Realizing this full network automation requires two vital components.

Artificial intelligence and machine learning algorithms for comprehensive network automation: For instance, AI in network management can drastically cut the energy usage of future telecom networks.
Sensor and control data flow across all network model layers, including the physical layer: As networks grow in size and complexity, the management and orchestration (MANO) software needs more degrees of freedom and dials to turn.

These goals require smart optical equipment and components that provide comprehensive telemetry data about their status and the fiber they are connected to. The AI-controlled centralized management and orchestration layer can then use this data for remote management and diagnostics. We discuss this topic further in our previous article on remote provisioning, diagnostics, and management.

For example, a smart optical transceiver that fits this centralized AI-management model should relay data to the AI controller about fiber conditions. Such monitoring is not just limited to finding major faults or cuts in the fiber but also smaller degradations or delays in the fiber that stem from age, increased stress in the link due to increased traffic, and nonlinear optical effects. A transceiver that could relay all this data allows the AI controller to make better decisions about how to route traffic through the network.

A Smart Transceiver to Rule All Network Links

After relaying data to the AI management system, a smart pluggable transceiver must also switch parameters to adapt to different use cases and instructions given by the controller.

Let’s look at an example of forward error correction (FEC). FEC makes the coherent link much more tolerant to noise than a direct detect system and enables much longer reach and higher capacity. In other words, FEC algorithms allow the DSP to enhance the link performance without changing the hardware. This enhancement is analogous to imaging cameras: image processing algorithms allow the lenses inside your phone camera to produce a higher-quality image.

Figure 1 Simplified diagram of a network link with Forward Error Correction (FEC). — Figure 3: Simplified diagram of a network link with forward error correction (FEC). The FEC encoder adds redundant bits (overhead) to the transmitted data stream. The receiver can use this overhead to check for errors without asking the transmitter to resend the data.

A smart transceiver and DSP could switch among different FEC algorithms to adapt to network performance and use cases. Let’s look at the case of upgrading a long metro link of 650km running at 100 Gbps with open FEC. The operator needs to increase that link capacity to 400 Gbps, but open FEC could struggle to provide the necessary link performance. However, if the transceiver can be remotely reconfigured to use a proprietary FEC standard, the transceiver will be able to handle this upgraded link.

Reconfigurable transceivers can also be beneficial to auto-configure links to deal with specific network conditions, especially in brownfield links. Let’s return to the fiber monitoring subject we discussed in the previous section. A transceiver can change its modulation scheme or lower the power of its semiconductor optical amplifier (SOA) if telemetry data indicates a good quality fiber. Conversely, if the fiber quality is poor, the transceiver can transmit with a more limited modulation scheme or higher power to reduce bit errors. If the smart pluggable detects that the fiber length is relatively short, the laser transmitter power or the DSP power consumption could be scaled down to save energy.

Takeaways

Optical networks will need artificial intelligence and machine learning to scale more efficiently and affordably to handle the increased traffic and connected devices. Conversely, AI systems will also need faster pluggables than before to acquire data and make decisions more quickly. Pluggables that fit this new AI era must be fast, smart, and adapt to multiple use cases and conditions. They will need to scale up to speeds beyond 400G and relay monitoring data back to the AI management layer in the central office. The AI management layer can then program transceiver interfaces from this telemetry data to change parameters and optimize the network.

Tags: 800G, 800G and beyond, adaptation, affordable, AI, artificial intelligence, automation, CloudComputing, data, DataCenter, EFFECT Photonics, FEC, fiber quality, innovation, integration, laser arrays, machine learning, network conditions, network optimization, Networking, optical transceivers, photonic integration, Photonics, physical layer, programmable interface, scalable, sensor data flow, technology, Telecommunications, telemetry data, terabyte, upgrade, virtualization

The Light Path to a Coherent Cloud Edge Banner

Insights

November 9, 2022

The Light Path to a Coherent Cloud Edge

Smaller data centers placed locally have the potential to minimize latency, overcome inconsistent connections, and…

Smaller data centers placed locally have the potential to minimize latency, overcome inconsistent connections, and store and compute data closer to the end user. These benefits are causing the global market for edge data centers to explode, with PWC predicting that it will nearly triple from $4 billion in 2017 to $13.5 billion in 2024.

As edge data centers become more common, the issue of interconnecting them becomes more prominent. This situation motivated the Optical Internetworking Forum (OIF) to create the 400ZR and ZR+ standards for pluggable modules. With small enough modules to pack a router faceplate densely, the datacom sector could profit from a 400ZR solution for high-capacity data center interconnects of up to 80km. Cignal AI forecasts that 400ZR shipments will dominate the edge applications, as shown in the figure below.

Figure 1: Forecast of 100G port equivalents shipped for edge applications. These shipments are overwhelmingly 400ZR standard technology. Source: Cignal AI Transport Applications Report Q42021.

The 400ZR standard has made coherent technology and dense wavelength division multiplexing (DWDM) the dominant solution in the metro data center interconnects (DCIs) space. Datacom provider operations teams found the simplicity of coherent pluggables very attractive. There was no need to install and maintain additional amplifiers and compensators as in direct detect technology. A single coherent transceiver plugged into a router could fulfill the requirements.

However, there are still obstacles that prevent coherent from becoming dominant in shorter-reach DCI links at the campus (< 10km distance) and intra-datacenter (< 2km distance) level. These spaces require more optical links and transceivers, and coherent technology is still considered too power-hungry and expensive to become the de-facto solution here.

Fortunately, there are avenues for coherent technology to overcome these barriers. By embracing multi-laser arrays, DSP co-design, and electronic ecosystems, coherent technology can mature and become a viable solution for every data center interconnect scenario.

The Promise of Multi-Laser Arrays

Earlier this year, Intel Labs demonstrated an eight-wavelength laser array fully integrated on a silicon wafer. These milestones are essential for optical transceivers because the laser arrays can allow for multi-channel transceivers that are more cost-effective when scaling up to higher speeds.

Let’s say we need an intra-DCI link with 1.6 Terabits/s of capacity. There are three ways we could implement it:

Four modules of 400G: This solution uses existing off-the-shelf modules but has the largest footprint. It requires four slots in the router faceplate and an external multiplexer to merge these into a single 1.6T channel.
One module of 1.6T: This solution will not require the external multiplexer and occupies just one plug slot on the router faceplate. However, making a single-channel 1.6T device has the highest complexity and cost.
One module with four internal channels of 400G: A module with an array of four lasers (and thus four different 400G channels) will only require one plug slot on the faceplate while avoiding the complexity and cost of the single-channel 1.6T approach.

Table 1: Comparison of different ways to implement a 1.6 Terabit/s link: four modules of 400G, a single-channel 1.6T module, and a module with four channels of 400G.

Multi-laser array and multi-channel solutions will become increasingly necessary to increase link capacity in coherent systems. They will not need more slots in the router faceplate while simultaneously avoiding the higher cost and complexity of increasing the speed with just a single channel.

Co-designing DSP and Optical Engine for Efficiency and Performance

Transceiver developers often source their DSP, laser, and optical engine from different suppliers, so all these chips are designed separately from each other. This setup reduces the time to market and simplifies the research and design processes but comes with trade-offs in performance and power consumption.

In such cases, the DSP is like a Swiss army knife: a jack of all trades designed for different kinds of optical engines but a master of none. For example, current DSPs are designed to be agnostic to the material platform of the photonic integrated circuit (PIC) they are connected to, which can be Indium Phosphide (InP) or Silicon. Thus, they do not exploit the intrinsic advantages of these material platforms. Co-designing the DSP chip alongside the PIC can lead to a much better fit between these components.

To illustrate the impact of co-designing PIC and DSP, let’s look at an example. A PIC and a standard platform-agnostic DSP typically operate with signals of differing intensities, so they need some RF analog electronic components to “talk” to each other. This signal power conversion overhead constitutes roughly 2-3 Watts or about 10-15% of transceiver power consumption.

Figure 2: Simplified diagram of the building blocks of a coherent transceiver in which the DSP drives the optical engine directly from its digital-to-analog converter (DAC). This setup can minimize the power conversion overhead of optical transceivers.

However, the modulator of an InP PIC can run at a lower voltage than a silicon modulator. If this InP PIC and the DSP are designed and optimized together instead of using a standard DSP, the PIC could be designed to run at a voltage compatible with the DSP’s signal output. This way, the optimized DSP could drive the PIC directly without needing the RF analog driver, doing away with most of the power conversion overhead we discussed previously.

Figure 3: Comparison between the drive power and power consumption of three different PIC + DSP pairings: standard DSP with a silicon PIC, standard DSP with an InP PIC, and an optimized DSP with an InP.

Additionally, the optimized DSP could also be programmed to do some additional signal conditioning that minimizes the nonlinear optical effects of the InP material, which can reduce noise and improve performance.

Driving Scale Through Existing Electronic Ecosystems

Making coherent optical transceivers more affordable is a matter of volume production. As discussed in a previous article, if PIC production volumes can increase from a few thousand chips per year to a few million, the price per optical chip can decrease from thousands of Euros to mere tens of Euros. Achieving this production goal requires photonics manufacturing chains to learn from electronics and leverage existing electronics manufacturing processes and ecosystems.

While vertically-integrated PIC development has its strengths, a fabless model in which developers outsource their PIC manufacturing to a large-scale foundry is the simplest way to scale to production volumes of millions of units. Fabless PIC developers can remain flexible and lean, relying on trusted large-scale manufacturing partners to guarantee a secure and high-volume supply of chips. Furthermore, the fabless model allows photonics developers to concentrate their R&D resources on their end market and designs instead of costly fabrication facilities.

Further progress must also be made in the packaging, assembly, and testing of photonic chips. While these processes are only a small part of the cost of electronic systems, the reverse happens with photonics. To become more accessible and affordable, the photonics manufacturing chain must become more automated and standardized. It must move towards proven and scalable packaging methods that are common in the electronics industry.

If you want to know more about how photonics developers can leverage electronic ecosystems and methods, we recommend you read our in-depth piece on the subject.

Takeaways

Coherent transceivers are already established as the solution for metro Data Center Interconnects (DCIs), but they need to become more affordable and less power-hungry to fit the intra- and campus DCI application cases. Fortunately, there are several avenues for coherent technology to overcome these cost and power consumption barriers.

Multi-laser arrays can avoid the higher cost and complexity of increasing capacity with just a single transceiver channel. Co-designing the optics and electronics can allow the electronic DSP to exploit the intrinsic advantages of specific photonics platforms such as indium phosphide. Finally, leveraging electronic ecosystems and processes is vital to increase the production volumes of coherent transceivers and make them more affordable.

By embracing these pathways to progress, coherent technology can mature and become a viable solution for every data center interconnect scenario.

Tags: campus, cloud, cloud edge, codesign, coherent, DCI, DSP, DSPs, DWDM, integration, intra, light sources, metro, modulator, multi laser arrays, photonic integration, PIC, power consumption, wafer testing

Insights

October 26, 2022

The Evolution to 800G and Beyond

The demand for data and other digital services is rising exponentially. From 2010 to 2020,…

The demand for data and other digital services is rising exponentially. From 2010 to 2020, the number of Internet users worldwide doubled, and global internet traffic increased 12-fold. From 2020 to 2026, internet traffic will likely increase 5-fold. To meet this demand, datacom and telecom operators need constantly upgrade their transport networks.

400 Gbps links are becoming the standard for links all across telecom transport networks and data center interconnects, but providers are already thinking about the next steps. LightCounting forecasts significant growth in shipments of dense-wavelength division multiplexing (DWDM) ports with data rates of 600G, 800G, and beyond in the next five years.

*Figure 1: Shipments of high-speed DWDM ports by data rate (historical data and forecast). Source: LightCounting Optical Communications Market Forecast, April 2022.*

The major obstacles in this roadmap remain the power consumption, thermal management, and affordability of transceivers. Over the last two decades, power ratings for pluggable modules have increased as we moved from direct detection to more power-hungry coherent transmission: from 2W for SFP modules to 3.5 W for QSFP modules and now to 14W for QSSFP-DD and 21.1W for OSFP form factors. Rockley Photonics researchers estimate that a future electronic switch filled with 800G modules would draw around 1 kW of power just for the optical modules.

Thus, many incentives exist to continue improving the performance and power consumption of pluggable optical transceivers. By embracing increased photonic integration, co-designed PICs and DSPs, and multi-laser arrays, pluggables will be better able to scale in data rates while remaining affordable and at low power.

Direct Detect or Coherent for 800G and Beyond?

While coherent technology has become the dominant one in metro distances (80 km upwards), the campus (< 10 km) and intra-data center (< 2 km) distances remain in contention between direct detect technologies such as PAM 4 and coherent.

These links were originally the domain of direct detect products when the data rates were 100Gbps. However, at 400Gbps speeds, the power consumption of coherent technology is much closer to that of direct detect PAM-4 solutions. This gap in power consumption is expected to disappear at 800Gbps, as shown in the figure below.

*Figure 2. Coherent DSP power consumption is getting closer to that of PAM-4 direct detect solutions as link speed increases and will match it at 800G. Source: NeoPhotonics Blog*

A major for this decreased gap is that direct detect technology will often require additional amplifiers and compensators at these data rates, while coherent pluggables do not. This also makes coherent technology simpler to deploy and maintain. Furthermore, as the volume production of coherent transceivers increases, their price will also become competitive with direct detect solutions.

Increased Integration and Co-Design are Key to Reduce Power Consumption

Lately, we have seen many efforts to increase further the integration on a component level across the electronics industry. For example, moving towards greater integration of components in a single chip has yielded significant efficiency benefits in electronics processors. Apple’s M1 processor integrates all electronic functions in a single system-on-chip (SoC) and consumes a third of the power compared to the processors with discrete components used in their previous generations of computers. We can observe this progress in the table below.

Mac Mini Model	Power Consumption (Watts)
Mac Mini Model	Idle	Max
2020, M1	7	39
2018, Core i7	20	122
2014, Core i5	6	85
2010, Core 2 Duo	10	85
2006, Core Solo or Duo	23	110
2005, PowerPC G4	32	85

Table 1: Comparing the power consumption of a Mac Mini with an M1 SoC chip to previous generations of Mac Minis. [Source: Apple’s website]

Photonics can achieve greater efficiency gains by following a similar approach to integration. The interconnects required to couple discrete optical components result in electrical and optical losses that must be compensated with higher transmitter power and more energy consumption. In contrast, the more active and passive optical components (lasers, modulators, detectors, etc.) manufacturers can integrate on a single chip, the more energy they can save since they avoid coupling losses between discrete components.

*Figure 3: Example of integrating a laser to other photonic components through different integration approaches. The approach of full integration (right side) is the one that minimizes losses.*

Further improvements in power consumption can be achieved by co-designing electronic digital signal processors (DSPs) with the photonic integrated circuit (PIC) that constitutes the transceiver’s optical engine. Standard DSPs are like a Swiss army knife: a jack of all trades designed for different kinds of optical engines but a master of none.

A DSP that is co-designed and optimized alongside the PIC can better exploit specific advantages of the PIC. For example, if an indium phosphide PIC and a DSP are co-designed together instead of using a standard DSP, the PIC could be designed to run at a voltage compatible with the DSP’s signal output. This way, the optimized DSP could drive the PIC directly without needing the RF analog driver, drastically reducing a power conversion overhead that is often 10-15% of transceiver power consumption. To learn more about the advantages of co-design, you can read our article on the topic.

Reducing Complexity with Multi-Laser Arrays

Earlier this year, Intel Labs demonstrated an eight-wavelength laser array fully integrated on a silicon wafer. These milestones will provide more cost-effective ways for pluggables to scale to higher data rates.

Let’s say we need a data center interconnect with 1.6 Terabits/s of capacity. There are three ways we could implement it:

Four modules of 400G: This solution uses existing off-the-shelf modules but has the largest footprint. It requires four slots in the router faceplate and an external multiplexer to merge these into a single 1.6T channel.
One module of 1.6T: This solution will not require the external multiplexer and occupies just one plug slot on the router faceplate. However, making a single-channel 1.6T device has the highest complexity and cost.
One module with four internal channels of 400G: A module with an array of four lasers (and thus four different 400G channels) will only require one plug slot on the faceplate while avoiding the complexity and cost of the single-channel 1.6T approach.

*Table 2: Comparison of different ways to implement a 1.6 Terabit/s link: four modules of 400G, a single-channel 1.6T module, and a module with four channels of 400G.*

Multi-laser array and multi-channel solutions will become increasingly necessary to increase link capacity in coherent systems. They will not need more slots in the router faceplate while simultaneously avoiding the higher cost and complexity of increasing the speed with just a single channel.

Takeaways

The pace of worldwide data demand is relentless, with it the pace of link upgrades required by datacom and telecom networks. 400G transceivers are currently replacing previous 100G solutions, and in a few years, they will be replaced by transceivers with data rates of 800G or 1.6 Terabytes.

The cost and power consumption of coherent technology remain barriers to more widespread capacity upgrades, but the industry is finding ways to overcome them. Tighter photonic integration can minimize the losses of optical systems and their power consumption. Further improvements in power efficiency can be achieved by co-designing DSPs alongside the optical engine. Finally, the onset of multi-laser arrays can avoid the higher cost and complexity of increasing capacity with just a single transceiver channel.

Tags: bandwidth, co-designing, coherent, DSP, full integration, integration, interface, line cards, optical engine, power consumption, RF Interconnections, Viasat

We interview Russell Fuerst, our VP of Digital Signal Processing.

Insights

September 29, 2022

The Future of Coherent DSP Design: Interview with Russell Fuerst

Digital signal processors (DSPs) are the heart of coherent communication systems. They not only encode/decode…

Digital signal processors (DSPs) are the heart of coherent communication systems. They not only encode/decode data into the three properties of a light signal (amplitude, phase, polarization) but also handle error correction, analog-digital conversation, Ethernet framing, and compensation of dispersion and nonlinear distortion. And with every passing generation, they are assigned more advanced functions such as probabilistic constellation shaping.

There are still many challenges ahead to improve DSPs and make them transmit even more bits in more energy-efficient ways. Now that EFFECT Photonics has incorporated talent and intellectual property from Viasat’s Coherent DSP team, we hope to contribute to this ongoing research and development and make transceivers faster and more sustainable than ever. We ask Russell Fuerst, our Vice-President of Digital Signal Processing, how we can achieve these goals.

What’s the most exciting thing about joining EFFECT Photonics?

Before being acquired by EFFECT Photonics, our DSP design team has been a design-for-hire house. We’ve been doing designs for other companies that have put those designs in their products. By joining EFFECT Photonics, we can now do a design and stamp our brand on it. That’s exciting.

The other exciting thing is to have all the technologies under one roof. Having everything from the DSP to the PIC to the packaging and module-level elements in one company will allow us to make our products that much better.

We also find the company culture to be very relaxed and very collaborative. Even though we’re geographically diverse, it’s been straightforward to understand what other people and groups in the company are doing. It’s easy to talk to others and find out whom you need to talk to. There’s not a whole lot of organizational structure that blocks communication, so it’s been excellent from that perspective.

People at EFFECT Photonics were welcoming from day one, making us that much more excited to join.

What key technology challenges must be solved by DSP designers to thrive in the next 5 to 10 years?

The key is to bring the power down while also increasing the performance.

In the markets where coherent has been the de-facto solution, I think it’s essential to understand how to drive cost and power down either through the DSP design itself or by integrating the DSP with other technologies within the module. That will be where the benefits come from in those markets.

Similarly, there are markets where direct detection is the current technology of choice. We must understand how to insert coherent technology into those markets while meeting the stringent requirements of those important high-volume markets. Again, this progress will be largely tied to performance within the power and cost requirements.

As DSP technology has matured, other aspects outside of performance are becoming key, and understanding how we can work that into our products will be the key to success.

How do you think the DSP can be more tightly integrated with the PIC?

This is an answer that will evolve over time. We will become more closely integrated with the team in Eindhoven and learn some of the nuances of their mature design process. And similarly, they’ll understand the nuances of our design process that have matured over the years. As we understand the PIC technology and our in-house capabilities better, that will bring additional improvements that are currently unknown.

Right now, we are primarily focused on the obvious improvements tied to the fully-integrated platform. For example, the fact that we can have the laser on the PIC because of the active InP material. We want to understand how we co-design aspects of the module and shift the complexity from one design piece or component to another, thanks to being vertically integrated.

Another closely-tied area for improvement is on the modulator side. We think that the substantially lower drive voltages required for the InP modulator give us the possibility to eliminate some components, such as RF drivers. We could potentially drive the modulator directly from that DSP without any intermediary electronics, which would reduce the cost and power consumption. That’s not only tied to the lower drive voltages but also some proprietary signal conditioning we can do to minimize some of the nonlinearities in the modulator and improve the performance.

What are the challenges and opportunities of designing DSPs for Indium phosphide instead of silicon?

So, we already mentioned two opportunities with the laser and the modulator.

I think the InP integration makes the design challenges smaller than those facing DSP design for silicon photonics. The fact is that InP can have more active integrated components and that DSPs are inherently active electronic devices, so getting the active functions tuned and matched over time will be a challenge. It motivates our EFFECT DSP team to quickly integrate with the experienced EFFECT PIC design team to understand the fundamental InP platform a bit better. Once we understand it, the DSP designs will get more manageable with improved performance, especially as we have control over the designs of both DSP and PIC. As we get to the point where co-packaging is realized, there will also be some thermal management issues to consider.

When we started doing coherent DSP designs for optical communication over a decade ago, we pulled many solutions from the RF wireless and satellite communications space into our initial designs. Still, we couldn’t bring all those solutions to the optical markets.

However, when you get more of the InP active components involved, some of those solutions can finally be brought over and utilized. They were not used before in our designs for silicon photonics because silicon is not an active medium and lacked the performance to exploit these advanced techniques.

For example, we have done proprietary waveforms tuned to specific satellite systems in the wireless space. Our DSP team was able to design non-standard constellations and modulation schemes that increased the capacity of the satellite link over the previous generation of satellites. Similarly, we could tune the DSP’s waveform and performance to the inherent advantages of the InP platform to improve cost, performance, bandwidth utilization, and efficiency. That’s something that we’re excited about.

Takeaways

As Russell explained, the big challenge for DSP designers continues to be increasing performance while keeping down the power consumption. Finding ways to integrate the DSP more deeply with the InP platform can overcome this challenge, such as direct control of the laser and modulator from the DSP to novel waveform shaping methods. The presence of active components in the InP platforms also gives DSP designers the opportunity to import more solutions from the RF wireless space.

We look forward to our new DSP team at EFFECT Photonics settling into the company and trying out all these solutions to make DSPs faster and more sustainable!

Tags: coherent, DSP, energy efficient, InP, integration, performance, power consumption, Sustainable, Viasat

Insights

June 15, 2022

Data Center Interconnects: Coherent or Direct Detect?

With the increasing demand for cloud-based applications, datacom providers are pushing forward with expanding their…

With the increasing demand for cloud-based applications, datacom providers are pushing forward with expanding their distributed computing networks. Therefore, they and telecom provider partners are looking for data center interconnect (DCI) solutions that are faster and more affordable than before to ensure that connectivity between metro and regional facilities does not become a bottleneck.

Energy usage, space, simplicity, and cost-effectiveness all impact the efficiency of DCI infrastructure. These solutions must consider watts per bit, rack space, and simplified provisioning and operating expenditure. Previously, direct detect technology could fulfill these requirements for short-reach DCIs inside data centers and campuses. However, achieving the reach and bandwidths required for edge and metro DCIs required external amplifiers and dispersion compensators that increased the cost and complexity of network operations.

Figure 1: Example of different data center interconnect (DCI) links. Intra-data center interconnects usually have lengths up to 2kms, campus interconnects up to 10km, and metro DCIs up to 100km, although there can be longer DCIs.

At the same time, advances in electronic and photonic integration allowed longer reach coherent technology to be miniaturized into QSFP-DD and OSFP form factors. This enabled the transport of 100G and 400G connections over a single wavelength and several hundreds of kilometers, which is ideal for edge and metro DCI networks. Provider operations teams found the simplicity of coherent pluggables very attractive. There was no need to install and maintain additional amplifiers and compensators as in direct detect: a single coherent transceiver plugged into a router could fulfill the requirements.

In the coming decade, the shorter-reach DCI links will also require upgrades to 400G, 800G, and Terabit speeds, and at those speeds, coherent technology comes close to matching the energy consumption of direct detect. This would make it competitive even for shorter links.

Coherent Dominates in Metro DCIs

The advances in electronic and photonic integration allowed coherent technology for metro DCIs to be miniaturized into QSFP-DD and OSFP form factors. This progress allowed the Optical Internetworking Forum (OIF) to create the 400ZR and ZR+ standards for 400G DWDM pluggable modules. With small enough modules to pack a router faceplate densely, the datacom sector could profit from a 400ZR solution for high-capacity data center interconnects of up to 80km. If needed, extended reach 400ZR+ pluggables can cover several hundreds of kilometers. As an example of their success, Cignal AI forecasts that 400ZR shipments will dominate in the edge applications, as shown in Figure 3.

Figure 2: Forecast of 100G port equivalents shipped for edge applications. These shipments are overwhelmingly 400ZR standard technology. Source: Cignal AI Transport Applications Report Q42021.

Further improvements in integration can further boost the reach and efficiency of coherent transceivers. For example, by integrating all photonic functions on a single chip, including lasers and optical amplifiers, EFFECT Photonics’ photonic System-On-Chip (SoC) technology can achieve higher transmit power levels and longer distances while keeping the smaller QSFP-DD form factor, power consumption, and cost.

Campus DCI Is The Battleground of Direct Detect and Coherent

The campus DCI segment, featuring distances below ten kilometers, was squarely the domain of direct detect products when the standard speed of these links was 100Gbps. No amplifiers nor compensators were needed for these shorter distances, so direct detect transceivers are as simple to deploy and maintain as coherent ones. However, at 400Gbps speeds, the power consumption of coherent technology is much closer to that of direct detect PAM-4 solutions.

This gap in power consumption is expected to disappear at 800Gbps, as shown in the figure below. For Terabit speeds, the prediction is that coherent transceivers will be more efficient. Furthermore, as the volume production of coherent transceivers increases, their price will also become competitive with direct detect solutions. Overall, coherent transceivers are expected to scale up better in future upgrades.

Figure 3. Coherent DSP power consumption is getting closer to that of PAM-4 direct detect solutions as link speed increases and will match it at 800G. Source: NeoPhotonics Blog

Direct Detect Dominates Intra Data Center Interconnects (For Now…)

Below Terabit speeds, direct detect technology (both NRZ and PAM-4) will likely dominate the intra-DCI space (also called data center fabric) in the coming years. In this space, links span less than two kilometers, and for particularly short links (< 300 meters), affordable multimode fiber (MMF) is frequently used.

Nevertheless, moving to larger, more centralized data centers (such as hyperscale data centers) is lengthening intra-DCI links. Instead of transferring data directly from one data center building to another, new data centers first move data to a central hub. So even if the building you want to connect to might be 200 meters away, the fiber runs to a hub that might be one or two kilometers away. In other words, intra-DCI links are becoming campus DCI links, which requires their single-mode fiber solutions.

On top of these changes, the upgrades to Terabit speeds in the coming decade will also see coherent solutions challenge the power consumption of direct detect transceivers. PAM-4 direct detect transceivers that fulfill the speed requirements require digital signal processors (DSPs) and more complex lasers that will be less efficient and affordable than previous generations of direct detect technology. With coherent technology scaling up in volume and having greater flexibility and performance, one can make the argument that it will reach cost-competitiveness in this space, too.

Takeaways

Unsurprisingly, the decision of using coherent or direct detect technology for DCIs boils down to the reach and capacity needs. Coherent is already established as the solution for metro DCIs and is already gaining ground in the campus DCI segment for 800G and Terabit speeds. With the move to Terabit speeds and scaling production volumes, it could also become cost-competitive inside the data center too. Overall, the datacom sector is moving towards coherent technology, and it pays off to have this in mind when upgrading data center links.

Tags: 800G, access networks, coherent, cost, cost-effective, Data center, distributed computing, edge and metro DCIs, integration, Intra DCI, license, metro, miniaturized, photonic integration, Photonics, pluggable, power consumption, power consumption SFP, reach, Terabit

Coherent technology can give free-space optical systems greater reach and capacity, enabling these new applications in space and terrestrial links.

Insights

June 2, 2022

Free Space Optics for Access Networks

Optical signals are moving deeper and deeper into access networks. Achieving the ambitious performance goals…

Optical signals are moving deeper and deeper into access networks. Achieving the ambitious performance goals of 5G architectures requires more optics than ever between small cell sites. As stated in a recent report by Deloitte, “extending fiber optics deeper into remote communities is a critical economic driver, promoting competition, increasing connectivity for the rural and underserved, and supporting densification for wireless.”

However, there are cases in which fiber is not cost-effective to deploy. For example, a network carrier might need to quickly increase their access network capacity for a big festival, and there is no point in deploying extra fiber. In many remote areas, the customer base is so small that the costly deployment of fiber will not produce a return on investment. These situations must be addressed with some kind of wireless access solution. Carriers have used fixed microwave links for the longest time to handle these situations.

However, radio microwave frequencies might not be enough as the world demands greater internet speeds and simply changing over to higher carrier frequencies will limit the reach of microwave links. On top of that, the radio spectrum is quite crowded, and a carrier might not have the available licensed spectrum to deploy this wireless link. Besides, microwave point-to-point links produce plenty of heat while struggling to deliver capacity beyond a few Gbps. This is where free-space optics (FSO) comes into play.

FSO is a relatively straightforward technology to explain. A high-power laser source converts data into laser pulses and sends them through a lens system and into the atmosphere. The laser travels to the other side of the link and goes through a receiver lens system and a high-sensitivity photodetector converts those laser pulses back into electronic data that can be processed. In other words, instead of using an optical fiber as a medium to transmit the laser pulses, FSO uses air as a medium. The laser typically operates at an infrared wavelength of 1550nm that is safer on the eye.

Figure 1: Free space optical system diagram

FSO has often been talked about as some futuristic technology to be used in space applications, but it can be used more than that, including ground-to-ground links in access networks.FSO can deliver a wireless access solution that can be deployed quickly and with more bandwidth capacity, security features, and less power consumption than traditional point-to-point microwave links. Furthermore, since it does not use the RF spectrum, there is no need to secure spectrum licenses.

Overcoming the challenges of alignment, and atmospheric turbulence

FSO struggled to break through into practical applications despite these benefits because of certain technical challenges. Communications infrastructure, therefore, focused on more stable transmission alternatives such as optical fiber and RF signals. However, research and innovation over the last few decades are removing these technical barriers. One obstacle to achieving longer distances with FSO had to do with the quality of the laser signal.

Over time, FSO developers have found a solution to this issue in adaptive optics systems. These systems compensate for distortions in the beam by using an active optical element—such as a deformable mirror or liquid crystal—that dynamically changes its structure depending on the shape of the laser beam. Dutch startup Aircision uses this kind of technology in its FSO systems to increase their tolerance to atmospheric disruptions.

Figure 2: How an adaptive optical system works.

Another drawback of FSO is aligning the transmitter and receiver units. Laser beams are extremely narrow, and if the beam doesn’t hit the receiver lens at just the right angle, the information may be lost. The system requires almost perfect alignment, and it must maintain this alignment even when there are small changes in the beam trajectory due to wind or atmospheric disturbances.

FSO systems can handle these alignment issues with fast steering mirror (FSM) technology. These mirrors are driven with electrical signals and are fast, compact, and accurate enough to compensate the disturbances in the beam trajectory. However, even if the system can maintain the beam trajectory and shape, atmospheric turbulence can still degrade the message and cause interference in the data. Fortunately, FSO developers also use sophisticated digital signal processing techniques (DSP) to compensate for these impairments.

These DSP techniques allow for reliable, high-capacity, quick deployments even through thick clouds and fog. FSO links can now handle Gbps capacity over several kilometers thanks to all these technological advances. For example, a collaboration between Aircision and TNO demonstrated in 2021 that their FSO systems could reliably transmit 10 Gbps over 2.5 km. Aircision’s Scientific Director John Reid explained, “it’s an important milestone to show we can outperform microwave E-band antennas and provide a realistic solution for the upcoming 5G system.”

An alternative for safe, private networks

An understated benefit of FSO is that, from a physics perspective, they are arguably the most secure form of wireless communication available today. Point-to-point microwave links transmit a far more directional beam than mobile antennas or WiFi systems, which reduces the potential for security breaches. However, even these narrower microwave beams are still spread out enough to cover a wide footprint vulnerable to eavesdropping and jamming.

At a 1km distance, the beam can spread out enough to cover roughly the length of a building, and at 5km, it could cover an entire city block. Furthermore, microwave systems have side- and back lobes radiating away from the intended direction of transmission that can be intercepted too. Finally, if an attacker is close enough to the source, even the reflected energy from buildings can be used to intercept the signal.

Figure 3: Geometry of an FSO system vs. a microwave link. The microwave link has a much wider spread than FSO, which makes eavesdropping and jamming easier.

Laser beams in FSO are so narrow and focused that they do not have to deal with these issues. At 1km, a typical laser beam only spreads out about 2 meters, and at 5km, only about 5 meters. There are no side and back lobes to worry about and no near-zone reflections. The beam is so narrow that intercepting the transmission becomes an enormous challenge. An intruder would have to get within inches of a terminal or the line of sight, making it easier to get discovered. To complicate things further, the intruder’s terminal would also need to be very well aligned to pick up enough of a signal.

Using Highly-Integrated Transceivers in Free Space Optics

Even though fiber optical communications drove the push for smaller and more efficient optical transceivers, this progress also has a beneficial impact on FSO. As we have explained in previous articles, optical transmission systems have been miniaturized from big, expensive line cards to small, affordable pluggables the size of a large USB stick. These compact transceivers with highly integrated optics and electronics have shorter interconnections, fewer losses, and more elements per chip area. These features all led to a reduced power consumption over the last decade. At EFFECT Photonics, we achieve even further efficiency gains by an optical System-On-Chip (SoC) that integrates all photonic functions on a single chip, including lasers and amplifiers.

Figure 4: Diagram of an FSO system connected to an EFFECT Photonics 10G SFP+ pluggable transceiver via optical fiber.

FSO systems can now take advantage of affordable, low-power transceivers to transmit and receive laser signals in the air. For example, a transceiver based on an optical SoC can output a higher power into the FSO system. By using this higher laser power, the FSO does not need to amplify the signal so much before transmitting it, improving its noise profile. Furthermore, this benefit happens with both direct detect and coherent transceivers. This is a key reason why Aircision has partnered up with EFFECT Photonics to create both direct detect and coherent free-space optical systems, since the startup ultimately aims to reach transmission speeds of 100 Gbps over the air.

Takeaways

FSO has moved from the domain of science fiction to a practical technology that now deserves a place in access networks. FSO can deliver a wireless access solution that can be deployed quickly and with more bandwidth capacity, security features, and less power consumption than traditional point-to-point microwave links. Furthermore, since it does not use the RF spectrum, it is unnecessary to secure spectrum licenses. Affordable direct detect and coherent transceivers based on SoC can further improve the quality and affordability of FSO transmission.

Tags: access networks, adaptive optics, affordable, capacity, coherent, cost-effective, deployments, free space optics, integration, license, miniaturized, photonic integration, Photonics, pluggable, power consumption, private network links, quick deployments, radio spectrum, remote communities, security, SFP, signal processing, turbulence

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