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March 20, 2024

Tunable Lasers and DSPs in the Age of AI

The use of generative artificial intelligence (AI) models is transforming several industries, and data centers…

The use of generative artificial intelligence (AI) models is transforming several industries, and data centers are no exception. AI models are computationally heavy, and their increasing complexity will require faster and more efficient interconnections than ever between GPUs, nodes, server racks, and data center campuses. These interconnects will have a major impact on the ability of data center architectures to scale and handle the demands of AI models in a sustainable way.

As we discussed in a previous article, transceivers that fit this new AI era must be fast, smart, and adapt to multiple use cases and conditions. However, what impact will that have on the tunable lasers and digital signal processors (DSPs) inside these transceivers? This article will review a couple of trends in lasers and DSPs to adapt to this new era.

The Power of Laser Arrays

In 2022, 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.

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

Transceiver developers often source their DSP, laser, and optical engine from different suppliers, so all these chips are designed separately from each other. 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.

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 can constitute up to 2 Watts or about 10-15% of transceiver power consumption.

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.

Smart Devices Built In Scale

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

In conclusion, tunable lasers and DSPs are adapting to meet the rising demands of AI-driven data center infrastructure. The integration of multi-wavelength laser arrays and the co-design of DSPs and optical engines are crucial steps towards creating more efficient, scalable, and cost-effective optical transceivers. These devices will be smart and provide some telemetry data, and must shift towards volume production and the adoption of electronics industry methodologies. These innovations not only promise to enhance the capacity and efficiency of data center interconnects but also pave the way for a more sustainable growth trajectory in the face of AI’s computational demands.

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Tags: 100ZR, access network, co-design, coherent, controlled, edge, fit for platform DSP, InP, Integrated Photonics, low power, photonic integration, Photonics, pluggables, power consumption, power conversion, QSFP 28, QSFP-DD

Insights

November 15, 2023

The Highways of Light: How Optical Fiber Works

Optical fibers revolutionized how we transmit data, enabling faster long-distance connections. These slender strands of…

Optical fibers revolutionized how we transmit data, enabling faster long-distance connections. These slender strands of glass or plastic carry light pulses and serve as the backbone of modern telecommunication networks. Optical fibers have found applications beyond communications, including imaging, sensing, and medicine, further showcasing their versatility and impact in various fields.

Early optical fibers suffered significant losses during transmission, limiting their practicality for long-haul communication. In 1966, Charles Kao and George Hockham proposed that impurities in the glass were responsible for these losses, and they suggested that high-purity silica glass could achieve a much more reasonable attenuation of 20 dB per kilometer for telecommunications. This breakthrough, for which Kao received the Nobel Prize in Physics in 2009, kickstarted an era of explosive progress and growth for optical fiber.

In 1970, Corning scientists Robert Maurer, Donald Keck, and Peter Schultz successfully fabricated a glass fiber with an attenuation of 16 dB per kilometer, exceeding the performance benchmark set by Kao and Hockham. Two years later, Corning pushed the envelope further and achieved a loss of 4 dB/km, an order of magnitude improvement over their first effort. By 1979, Nippon Telegraph and Telephone (NTT) had reached a loss of 0.2 dB/km, meaning that only 5% of the light signal was lost over one kilometer. Optical fibers were ready for the world stage and deployed worldwide throughout the 1980s. The first transatlantic optical fiber link, spanning 6000 km, was established in 1988.

In this article, we will delve into the fascinating world of optical fibers, exploring how they work and what role optical transceivers play in fiber communications.

The Principle of Total Internal Reflection

Light bends when it passes from one material to another, such as from air to water. This bending occurs due to the change in the speed of light when it encounters a different material, causing the light rays to change direction. How much light changes direction depends on the angle at which it enters the new material and the factor by which the material slows down light. The latter factor is known as the refractive index of the material.

Figure 1 GREEN PENCIL IN GLASS OF WATER 1024 — Figure 1: Light refraction from a green-colored pencil inside a glass of water.

When light travels from a material of a higher refractive index to one of a lower index at a specific critical angle, the light will be entirely reflected into the high-index material. This phenomenon is called total internal reflection and is the fundamental principle behind the operation of optical fibers.

Optical Fibers and Total Internal Reflection

Optical fibers consist of a high-refractive-index core surrounded by a low-refractive-index cladding layer. Light entering the fiber core through one end at the correct critical angle will bounce back whenever it reaches the boundary between the core and the cladding. This behavior effectively traps light inside the core, allowing the light pulses to propagate through the fiber with minimal loss over long distances.

Figure 2: Diagram of light trapped inside an optical fiber by total internal reflection.

Optical fibers can achieve single-mode or multi-mode operation by carefully engineering the refractive indices of the core and cladding. Single-mode fibers have a small core diameter to transmit only a single optical mode or path. In contrast, multi-mode fibers have a larger core diameter, enabling the propagation of multiple modes simultaneously.

Figure 3: Comparing light propagation in multimode fiber and single-mode fiber. While the claddings are the same size, the single-mode fiber core is much smaller than the multimode one.

The quality of the light signal degrades when traveling through an optical fiber by a process called dispersion. The same phenomenon happens when a prism splits white light into several colors. The single-mode fiber design minimizes dispersion, making single-mode fibers ideal for long-distance communication. Since they are more susceptible to dispersion, multi-mode fibers are well-suited for short-distance applications such as local area networks (LANs).

The Role of Optical Transceivers in Fiber

Optical transceivers play a crucial role in fiber communications by converting electrical signals into optical signals for fiber transmission and vice versa when the optical signal is received. They act as the interface between fiber optical networks and electronic computing devices such as computers, routers, and switches.

As the name implies, an optical transceiver contains both a transmitter and a receiver within a single module. The transmitter has a laser or LED that generates light pulses representing the electronic data signal. On the receiving end, the receiver detects the optical signals and converts them back into electrical signals, which electronic devices can further process.

Figure 4: How transceivers convert electrical data signals into light pulses and vice-versa.

There are many different approaches to encode electrical data into light pulses. Two key approaches are intensity modulation/direct detection (IM-DD) and coherent transmission. IM-DD transmission only uses the amplitude of the light signal to encode data, while coherent transmission manipulates three different light properties to encode data: amplitude, phase, and polarization. These additional degrees of modulation allow for faster optical signals without compromising the transmission distance.

Direct Detection or Coherent? EFFECT Photonics explains

Takeaways

Optical fibers have transformed telecommunications in the last 50 years, enabling the rapid and efficient transmission of data over vast distances. By exploiting the principles of total internal reflection, these slender strands of glass or plastic carry pulses of light with minimal loss, ensuring high-speed communication worldwide. Optical transceivers act as the vital link between optical fiber and electronic networking devices, facilitating the conversion of electrical signals to optical signals and vice versa. Optical fibers and transceivers are at the forefront of our interconnected world, serving as the highways of light the digital age drives on.

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Tags: amplifiers, cladding, coherent, conversion, core, Corning, detector, Direct Detect, dispersion, EFFECT Photonics, electrical, fiber, laser, Networks, Nobel Prize, noise, NTT, optical, refraction, refractive index, Telecommunications, total internal reflections, total internal relection, Transceiver

Insights

September 29, 2023

Introducing Our New Coherent Product Manager: Charlie Fu

EFFECT Photonics’ coherent technology portfolio has grown in the last two years, including coherent transceivers,…

EFFECT Photonics’ coherent technology portfolio has grown in the last two years, including coherent transceivers, laser sources, and digital signal processors. To lead this portfolio, EFFECT Photonics has hired Charlie Fu as our new Coherent Product Manager. To give you more insight into our new colleague and what drives him, we asked him a few questions.

Charlie Fu our new Coherent Product Manager

Tell us a little more about yourself and your background.

So, my whole life is actually devoted to photonics. When I was a student in college, I studied optoelectronics, and my undergraduate project was doing a diode laser response curve. I also worked hard in graduate school with my supervisor in the lab to test coherent lasers back in the early 1990s when this technology was in its early stages. Back then, the laser was based on bulk micro-optics and could transmit for maybe 10 kilometers of distance. It was a significantly bigger package than now.

I was very lucky to start my career during an optical communications boom, working on fiber optic devices and modules with JDS Uniphase. I built my career there, starting with optical design and a lot of learning.

I then moved to a network company, Nortel Networks, designing links for long-haul transmission systems. We ensured the performance of the optical system and how to specify all the optical components modules to ensure the required performance. Nortel had a very ambitious project at the time. It was a technology challenge of transmitting 40G long haul transmission. Perhaps too ambitious, the year 2000 was perhaps too early still to go coherent.

So yeah, my whole career has been devoted to optics. From working as a hardware design engineer and learning all the optical transponder optical module design. Working with a few well-known brands such as Oclaro.

What did you find exciting about working for EFFECT Photonics?

So I think EFFECT Photonics has a good combination of people and technology, with many interesting technology innovations. The entrepreneurial drive to achieve success.

What attracted me the most was the core technology message of where light meets digital. If we look around, quite a lot of companies have photonics technology OR digital signal processing (DSP) technology. But almost no one has both IP for DSP and photonic technology. Having those IPs puts EFFECT Photonics in a very unique, prestigious position.

What do you find exciting about coherent technology and what has drawn you to it over your career?

I still believe coherent technology is in its infant stage in its application to optical communication. There are still a lot of things to do. For example, moving it to access network communications instead of just long haul. That’s something EFFECT Photonics wants to do and why I’m very excited about the future of our coherent optics.

The coherent system implementation may have changed a lot and gotten smaller, but the technology, the core concept, it’s still exactly the same as 30 years ago. I want to help develop these new systems with the new technology available to optics in the semiconductor sector.

I’m very excited to develop products that use these new technologies, hence why I am now in a Product Manager position.

I’m very excited to leverage my experience and knowledge and I’m very confident I can make a positive contribution to EFFECT Photonics in product design and development.

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Tags: Access Network Communication, Charlie Fu, coherent, Coherent Product Manager, Core Technology Message, digital signal processing (DSP), EFFECT Photonics, Entrepreneurial Drive, Fiber Optic Devices, Innovative Technology, Integrated Photonics, IP for DSP and Photonic Technology, Long-Haul Transmission, Optical Communication, Optical Design, Optical Module Design, Optical Transponder, Photonic Technology, Photonics, Photonics Technology, Product Design, Product Development, Semiconductor Sector

Insights

July 11, 2023

Where is 100ZR Needed?

Simply relying on traditional direct detect technologies will not meet the growing bandwidth and service…

Simply relying on traditional direct detect technologies will not meet the growing bandwidth and service requirements of mobile, cable, and business access networks, particularly regarding long-distance transmission. In many instances, deploying 100G coherent dense wavelength division multiplexing (DWDM) technology becomes essential to transmit larger volumes of data over extended distances.

Several applications in the optical network edge could benefit from upgrading from 10G DWDM or 100G grey aggregation uplinks to 100G DWDM optics:

Mobile Mid-haul: Seamless upgrade of existing uplinks from 10G to 100G DWDM.
Mobile Backhaul: Upgrading links to 100G IPoDWDM.
Cable Access: Upgrading uplinks of termination devices like optical line terminals (OLTs) and Converged Cable Access Platforms (CCAPs) from 10G to 100G DWDM.
Business Services: Scaling enterprise bandwidth beyond single-channel 100G grey links.

However, network providers have often been reluctant to abandon their 10G DWDM or 100G grey links because existing 100G DWDM solutions did not fulfill all the requirements. Although “scaled-down” coherent 400ZR solutions offered the desired reach and tunability, they proved too expensive and power-intensive for many access network applications. Moreover, the ports in small to medium IP routers used in most edge deployments do not support the commonly used QSFP-DD form factor of 400ZR modules but rather the QSFP28 form factor.

How Coherent 100ZR Can Move into Mobile X-haul

Figure-5gc-dwdm-midhaul — Figure 1: Simplified diagram of a DWDM link in the midhaul and backhaul of a mobile network.

The transition from 4G to 5G has transformed the radio access network (RAN) structure, evolving it from a two-level system (backhaul and fronthaul) in 4G to a three-level system (backhaul, midhaul, and fronthaul) in 5G:

Fronthaul: The segment between the active antenna unit (AAU) and the distributed unit (DU).
Midhaul: The segment from DU to the centralized unit (CU).
Backhaul: The segment from CU to the core network.

Figure-backhaul-fronthaul — Figure 2: Simplified diagram that shows and compares the building blocks of 4G (left) and 5G (right) access networks and the links between these blocks.

Most developed countries have already initiated the rollout of 5G, with many operators upgrading their 1G SFP transceivers to 10G SFP+ devices. Some of these 10G solutions incorporated DWDM technology, but many were single-channel grey transceivers. However, to advance to the next phase of 5G deployments, mobile networks must install and aggregate a greater number of smaller base stations to accommodate the exponential increase in connected devices.

These advanced stages of 5G deployment will necessitate operators to cost-effectively scale fiber capacity using more prevalent 10G DWDM SFP+ solutions and 25G SFP28 transceivers. This upgrade will pressure the aggregation segments of mobile backhaul and midhaul, which typically rely on link aggregation of multiple 10G DWDM links into a higher bandwidth group (e.g., 4x10G).

Figure 4x10G link aggregation — Figure 3: Comparison between 4x10G link aggregation and using a single coherent 100ZR link.

However, this type of link aggregation involves splitting larger traffic streams and can be intricate to integrate within an access ring. Adopting a single 100G uplink diminishes the need for such link aggregation, simplifying network configuration and operations. To gain further insight into the potential market and reach of this link aggregation upgrade, it is recommended to consult the recent Cignal AI report on 100ZR technologies.

Coherent 100ZR Uplinks Driven by Cable Migration to 10G PON

Figure-Fixed-Access-Uplinks-DWDM — Figure 4: Simplified diagram of a DWDM link between the coherent termination device (CTD) and the hub in a cable access network.

Cignal AI’s 100ZR report also states that the primary catalyst for 100ZR adoption will be the multiplexing of fixed access network links transitioning from 1G to 10G. This trend will be evident in the long-awaited shift of cable networks from Gigabit Passive Optical Networks (GPON) to 10G PON, driven by the new DOCSIS 4.0 standard. This standard promises 10Gbps download speeds for customers and necessitates several hardware upgrades in cable networks.

To multiplex these larger 10Gbps customer links, cable providers and network operators must upgrade their optical line terminals (OLTs) and Converged Cable Access Platforms (CCAPs) with 100G DWDM uplinks. Additionally, many of these new optical hubs will support up to 40 or 80 optical distribution networks (ODNs), making the previous approach of aggregating multiple 10G DWDM uplinks insufficient for handling the increased capacity and higher number of channels.

Anticipating these needs, the non-profit R&D organization CableLabs has recently spearheaded the development of a 100G Coherent PON (C-PON) standard. This proposal offers 100 Gbps per wavelength with a maximum reach of 80 km and a split ratio of up to 1:512. CableLabs envisions that C-PON, with its 100G capabilities, will play a significant role not only in cable optical network aggregation but also in other scenarios such as mobile x-haul, fiber-to-the-building (FTTB), long-reach rural areas, and distributed access networks.

Advancements in Business Services with 100ZR Coherent and QSFP28

Figure-cloud-DWDM — Figure 5: Simplified diagram of a DWDM link between a business and a cloud provider.

Nearly every organization utilizes the cloud in some capacity, whether for resource development and testing or software-as-a-service applications. However, leveraging the cloud effectively requires fast, high-bandwidth wide-area connectivity to ensure optimal performance of cloud-based applications.

Like cable networks, enterprises will need to upgrade their existing 1G Ethernet private lines to 10G Ethernet to meet these requirements, consequently driving the demand for 100G coherent uplinks. Cable providers and operators will also seek to capitalize on their upgraded 10G PON networks by expanding the reach and capacity of their business services.

The business and enterprise services sector was an early adopter of 100G coherent uplinks, deploying “scaled-down” 400ZR transceivers in the QSFP-DD form factor when they were the available solution. However, since QSFP-DD slots also support QSFP28 form factors, the emergence of QSFP 100ZR solutions presents a more appealing upgrade for these enterprise applications, offering reduced cost and power consumption.

While QSFP28 solutions had struggled to gain widespread acceptance due to the requirement for new, low-power digital signal processors (DSPs), DSP developers and vendors are now actively involved in 100ZR development projects: Acacia, Coherent/ADVA, Marvell/InnoLight, and Marvell/OE Solutions. This is also why EFFECT Photonics has announced its plans to co-develop a 100G DSP with Credo Semiconductor that best fits 100ZR solutions in the QSFP28 form factor.

Table Comparison QSFP-DD and QSFP-28. — Figure 6: Comparison between QSFP-DD and QSFP-28 form factors for 100G applications.

Takeaways

In the coming years, deploying and applying 100G coherent uplinks will witness increasing prevalence across the network edge. Specific use cases in mobile access networks will require transitioning from existing 10G DWDM link aggregation to a single coherent 100G DWDM uplink.

Simultaneously, the migration of cable networks and business services from 1Gbps to 10Gbps customer links will be the primary driver for the demand for coherent 100G uplinks. For carriers providing converged cable/mobile access, these uplink upgrades will create opportunities to integrate additional business services and mobile traffic into their existing cable networks.

As the ecosystem for QSFP28 100ZR solutions expands, production will scale up, making these solutions more widely accessible and affordable. This, in turn, will unlock new use cases within access networks.

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Tags: 100G, 100G ZR, 100ZR, 10G, 10G PON, 5G, 5G deployment, aggregation, backhaul, bandwidth, business service, Business services, Cable access, cable networks, CCAP, Cloud connectivity, coherent, Coherent DWDM, Converged Cable Access Platforms (CCAPs), edge, Ethernet private lines, Fiber capacity, fronthaul, FTTH, IoT, Link aggregation, midhaul, mobile, mobile access network, mobile networks, Network providers, OLT, Optical line terminals (OLTs), PON, power, QSFP-DD, QSFP28, revenue, traffic, upgrade, uplink, Wide-area connectivity

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July 5, 2023

The Evolution to 800G and Beyond

Article first published 26th October 2022, updated 5th July 2023. The demand for data and…

Article first published 26^th October 2022, updated 5^thJuly 2023.

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.

shipments-dwdm — 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, as we move into Terabit speeds, the power consumption of coherent technology is much closer to that of direct detect PAM-4 solutions.

A major reason 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
2023, M2	7	5
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.

integrated-laser — Figure 2: 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.

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.

comparison-module — 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. Finally, the onset of multi-laser arrays can avoid the higher cost and complexity of increasing capacity with just a single transceiver channel.

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

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June 28, 2023

Coherent Lite and The Future Inside the Data Center

In the dynamic landscape of data centers, the demand for greater bandwidth and extended reach…

In the dynamic landscape of data centers, the demand for greater bandwidth and extended reach is rapidly increasing. As shown in the figure below, we can think about three categories of data center interconnects based on their reach

Intra-data center interconnects (< 2km)
Campus data center interconnects (<10km)
Metro data center interconnects (<100km)

Asset 6 data center interconnect — 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.

Coherent optical technology has already established itself as the go-to solution for interconnecting data centers over long distances in metro areas.

However, within the confines of data centers themselves, intensity-modulated direct detect (IM-DD) technology remains dominant. Recognizing the limitations of IM-DD in meeting evolving requirements, the industry is exploring “Coherent Lite” solutions—a simplified implementation of coherent technology designed specifically for shorter-reach data center connections.

This article delves into the concept of coherent lite technology and its potential to address the escalating bandwidth demands within data centers.

Reducing Dispersion Compensation

The quality of the light signal degrades when traveling through an optical fiber by a process called dispersion. The same phenomenon happens when a prism splits white light into several colors. The fiber also adds other distortions due to nonlinear optical effects. These effects get worse as the input power of the light signal increases, leading to a trade-off. You might want more power to transmit over longer distances, but the nonlinear distortions also become larger, which beats the point of using more power. The DSP performs several operations on the light signal that try to offset these dispersion and nonlinear distortions.

Asset 7 dispersion in optical fibers — Figure 2: Explaining dispersion compensation in optical fibers. A pulse of light will spread out and distort due to dispersion effects.

However, shorter-reach connections require less dispersion compensation, presenting an opportunity to streamline the implementation of coherent solutions. Coherent lite implementations can reduce the use of dispersion compensation blocks. This significantly lowers system power consumption.

The Trade-Offs Between Fixed and Tunable Lasers

Coherent lite solutions also aim to replace tunable lasers with fixed lasers to reduce costs. The use of fixed lasers eliminates the need for wavelength tuning and associated control circuitry and algorithms, simplifying the implementation and reducing operational complexities.

While fixed lasers offer significant advantages, tunable lasers will push to remain competitive. As we described in a previous article, advances in tunable laser technology aim to further reduce package footprints and leverage electronic ecosystems to reduce cost. Such developments will allow tunable lasers to keep pace with the demands of coherent lite solutions, ensuring a viable alternative for shorter-reach data center connections.

Scaling Data Center Links

The increasing length and bandwidth of links within data centers is increasingly calling for the use of coherent technology. As bandwidth scales to 1.6 and 3.2 terabits, traditional direct detect technology faces challenges in keeping up with the growing distances. Intra data center links that were previously limited to 2 kilometers are now extending to 5 or even 10 kilometers, demanding more robust and efficient transmission technologies.

In this context, coherent lite technology provides an attractive middle ground for enabling extended-reach connections within data centers. By leveraging some aspects of coherent solutions, coherent lite technologies facilitate the reliable and efficient transport of data over longer distances.

Takeaways

As data centers evolve to accommodate escalating bandwidth demands, coherent lite technology emerges as a promising solution for communication links within these facilities. By reducing dispersion compensation, simplifying their laser setups, and enabling extended-reach transmission, coherent lite solutions address the limitations of traditional direct detect technology. These advancements pave the way for enhanced performance, and seamless scalability within data center environments.

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Tags: 1.6T, 3.2T, bandwidth, coherent, Coherent Lite, Coherent Transmissions, Complexity, CWDM, Data center, datacom, dispersion compensation, Fixed Lasers, IM-DD (Intensity-Modulated Direct Detect), Interconnects, Intra DC, optical technology, power, reach, Scaling Data Center Links, Simplification, Single Wavelength

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coherent

What to Expect in 2024:

Key Highlights:

The Power of Laser Arrays

Co-Designing DSP and Optical Engine

Smart Devices Built In Scale

Takeaways

Eindhoven, The Netherlands

About EFFECT Photonics

ECOC Celebrating 50 years:

Key Highlights:

Premium Sponsors:

Media Partners:

The Principle of Total Internal Reflection

Optical Fibers and Total Internal Reflection

The Role of Optical Transceivers in Fiber

Takeaways

Tell us a little more about yourself and your background.

What did you find exciting about working for EFFECT Photonics?

What do you find exciting about coherent technology and what has drawn you to it over your career?

Explore Our ECOC2023 Demos:

How Coherent 100ZR Can Move into Mobile X-haul

Coherent 100ZR Uplinks Driven by Cable Migration to 10G PON

Advancements in Business Services with 100ZR Coherent and QSFP28

Takeaways

Direct Detect or Coherent for 800G and Beyond?

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

Reducing Complexity with Multi-Laser Arrays

Takeaways

Reducing Dispersion Compensation

The Trade-Offs Between Fixed and Tunable Lasers

Scaling Data Center Links

Takeaways

Core Industries

Emerging Industries

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Coherent Pluggable Modules

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DSP

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