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August 24, 2022

What’s Inside a Coherent DSP?

Coherent transmission has become a fundamental component of optical networks to address situations where direct…

Coherent transmission has become a fundamental component of optical networks to address situations where direct detect technology cannot provide the required capacity and reach.

While direct detect transmission only uses the amplitude of the light signal, coherent optical transmission manipulates three different light properties: amplitude, phase, and polarization. These additional degrees of modulation allow for faster optical signals without compromising the transmission distance. Furthermore, coherent technology enables capacity upgrades without replacing the expensive physical fiber infrastructure on the ground.

The digital signal processor (DSP) is the electronic heart of coherent transmission systems. The fundamental function of the DSP is encoding the electronic digital data into the amplitude, phase, and polarization of the light signal and decoding said data when the signal is received. The DSP does much more than that, though: it compensates for impairments in the fiber, performs analog-to-digital conversions (and vice versa), corrects errors, encrypts data, and monitors performance. And recently, DSPs are taking on more advanced functions such as probabilistic constellation shaping or dynamic bandwidth allocation, which enable improved reach and performance.

Given its vital role in coherent optical transmission, we at EFFECT Photonics want to provide an explainer of what goes on inside the DSP chip of our optical transceivers.

There’s More to a DSP Than You Think…

Even though we colloquially call the chip a “DSP”, it is an electronic engine that performs much more than just signal processing. Some of the different functions of this electronic engine (diagram below) are:

*Figure 1: Layout of the electronic engine used in a coherent optical transceiver, including the sequence the signal follows when processed by the engine.*

Analog Processing: This engine segment focuses on converting signals between analog and digital formats. Digital data is composed of discrete values like 0s and 1s, but transmitting it through a coherent optical system requires converting it into an analog signal with continuous values. Meanwhile, a light signal received on the opposite end requires conversion from analog into digital format.
Digital Signal Processing: This is the actual digital processing. As explained previously, this block encodes the digital data into the different properties of a light signal. It also decodes this data when the light signal is received.
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. Thanks to FEC, coherent links can handle bit error rates that are literally a million times higher than a typical direct detect link. FEC algorithms allow the electronic engine 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 2: Simplified diagram of a network link with forward error correction (FEC). The FEC encoder adds a series of redundant bits (called overhead) to the transmitted data stream. The receiver can use this overhead to check for errors without asking the transmitter to resend the data.

Framer: While a typical electric signal sent through a network uses the Ethernet frame format, the optical signal uses the Optical Transport Network (OTN) format. The framer block performs this conversion. We should note that an increasingly popular solution in communication systems is to send Ethernet frames directly over the optical signal (a solution called optical Ethernet). However, many legacy optical communication systems still use the OTN format, so electronic engines should always have the option to convert between OTN and Ethernet frames.
Glue Logic: This block consists of the electronic circuitry needed to interface all the different blocks of the electronic engine. This includes the microprocessor that drives the electronic engine and the serializer-deserializer (SERDES) circuit. Since coherent systems only have four channels, the SERDES circuit converts parallel data streams into a single serial stream that can be transmitted over one of these channels. The opposite conversion (serial-to-parallel) occurs when the signal is received.

We must highlight that each of these blocks has its own specialized circuitry and algorithms, so each is a separate piece of intellectual property. Therefore, developing the entire electronic engine requires ownership or access to each of these intellectual properties.

So What’s Inside the Actual DSP Block?

Having clarified first all the different parts of a transceiver’s electronic engine, we can now talk more specifically about the actual DSP block that encodes/decodes the data and compensates for distortions and impairments in the optical fiber. We will describe some of the critical functions of the DSP in the order in which they happen during signal transmission. Receiving the signal would require these functions to occur in the opposite order, as shown in the diagram below.

*Figure 3. Layout of the digital signal processing blocks and the sequence followed by the signal when transmitted and received.*

Signal Mapping: This is where the encoding/decoding magic happens. The DSP maps the data signal into the different phases of the light signal—the in-phase components and the quadrature components—and the two different polarizations (x- and y- polarizations). When receiving the signal, the DSP will perform the inverse process, taking the information from the phase and polarization and mapping it into a stream of bits. The whole process of encoding and decoding data into different phases of light is known as quadrature modulation. Explaining quadrature modulation in detail goes beyond the scope of this article, so if you want to know more about it, please read the following article.
Pilot Signal Insertion: The pilot signal is transmitted over the communication systems to estimate the status of the transmission path. It makes it easier (and thus more energy-efficient) for the receiver end to decode data from the phase and polarization of the light signal.
Adaptive Equalization: This function happens when receiving the signal. The fiber channel adds several distortions to the light signal (more on that later) that change the signal’s frequency spectrum from what was initially intended. Just as with an audio equalizer, the purpose of this equalizer is to change specific frequencies of the signal to compensate for the distortions and bring the signal spectrum back to what was initially intended.

*Figure 4: How the equalizer in a communication systems work. The example in this figure is an equalizer for RF coaxial transmission (not optical), but the principle is the same. Source:* *Broadband Library*

Dispersion and Nonlinear Compensation: This function happens when receiving the signal. 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.
Spectrum Shaping: Communication systems must be efficient in all senses, so they must transmit as much signal as possible within a limited number of frequencies. Spectrum shaping is a process that uses a digital filter to narrow down the signal to the smallest possible frequency bandwidth and achieve this efficiency.

When transmitting, the signal goes through the digital-to-analog conversion after this whole DSP sequence. When receiving the signal, it goes through the inverse analog-to-digital conversation and then through the DSP sequence.

Recent Advances and Challenges in DSPs

This is an oversimplification, but we can broadly classify the two critical areas of improvement for DSPs into two categories.

Transmission Reach and Efficiency

The entire field of communication technology can arguably be summarized with a single question: how can we transmit more information into a single frequency-limited signal over the longest possible distance?

DSP developers have many tools in their kit to answer this question. For example, they can transmit more data using more states in their quadrature-amplitude modulation process. The simplest kind of QAM (4-QAM) uses four different states (usually called constellation points), combining two different intensity levels and two different phases of light.

By using more intensity levels and phases, more bits can be transmitted in one go. State-of-the-art commercially available 400ZR transceivers typically use 16-QAM, with sixteen different constellation points that arise from combining four different intensity levels and four phases. However, this increased transmission capacity comes at a price: a signal with more modulation orders is more susceptible to noise and distortions. That’s why these transceivers can transmit 400Gbps over 100km but not over 1000km.

One of the most remarkable recent advances in DSPs to increase the reach of light signals is probabilistic constellation shaping (PCS). In the typical 16-QAM modulation used in coherent transceivers, each constellation point has the same probability of being used. This is inefficient since the outer constellation points that require more power have the same probability as the inner constellation points that require lower power.

*Figure 5: Comparison of conventional 16-QAM and a 16-QAM that uses probabilistic constellation shaping (PCS). Original diagram source:* *Infinera*.

PCS uses the low-power inner constellation points more frequently, and the outer constellation points less frequently, as shown in Figure 5. This feature provides many benefits, including improved tolerance to distortions and easier system optimization to specific bit transmission requirements. If you want to know more about it, please read the explainers here and here.

Energy Efficiency

Increases in transmission reach and efficiency must be balanced with power consumption and thermal management. Energy efficiency is the biggest obstacle in the roadmap to scale high-speed coherent transceivers into Terabit speeds.

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.

Figure 6: Normalized power breakdown of the internal blocks of a 400ZR pluggable module. Although this breakdown may vary a bit among different vendors, the authors find it to be generally valid for various DSP-based modules, both IM-DD and coherent. Source: *R. Nagarajan et al., IEEE JLT (2021)*

Around 50% of a coherent transceiver’s power consumption goes into the DSP chip. Scaling to higher bandwidths leads to even more losses and energy consumption from the DSP chip, and its radiofrequency (RF) interconnects with the optical engine. DSP chips must therefore be adaptable and smart, using the least amount of energy to encode/decode information. You can learn more about this subject in one of our previous articles. The interconnects with the optical engine are another area that can see further optimization, and we discuss these improvements in our article about optoelectronic co-design.

Takeaways

In summary, 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.

Tags: 100G, 5G, 6G, access, access networks, aggregation, backhaul, capacity, coherent, DWDM, fronthaul, Integrated Photonics, live events, metro, midhaul, mobile, mobile access, mobile networks, network, optical networking, optical technology, photonic integrated chip, photonic integration, Photonics, PIC, PON, programmable photonic system-on-chip, solutions, technology, VR, WDM

Insights

August 10, 2022

The Next Bright Lights of Eindhoven

Paris may be the more well-known City of Light, but we may argue that Eindhoven…

Insights

July 14, 2022

Photonic System-on-Chip is the Future

Before 2020, Apple made its computer processors with discrete components. In other words, electronic components…

Before 2020, Apple made its computer processors with discrete components. In other words, electronic components were manufactured on separate chips, and then these chips were assembled into a single package. However, the interconnections between the different chips produced losses and incompatibilities that made the device less efficient. After 2020, starting with Apple’s M1 processor, they now fully integrate all components on a single chip, avoiding losses and incompatibilities.

Apple’s fully integrated processor consumes a third of the power and lower costs than their older processors while still providing similar performance. EFFECT Photonics does something similar to what Apple did, but with optical components instead of electronic components. By integrating all the optical components (lasers, detectors, modulators, etc.) in a single system on a chip, we can minimize the losses and make the device more efficient. This approach is what we call photonic System-on-Chip (SoC).

By integrating all optical components on a single chip, we also shift the complexity from the assembly process to the much more efficient and scalable semiconductor wafer process. Assembling and packaging a device by interconnecting multiple photonic chips increases assembly complexity and costs. On the other hand, combining and aligning optical components on a wafer at a high volume is much easier, which drives down the device’s cost. Testing is another aspect that becomes more efficient and scalable when manufacturing at the wafer level.

When faults are found earlier in the testing process, fewer resources and energy are spent processing defective chips. Ideally, testing should happen not only on the final, packaged transceiver but in the earlier stages of photonic SoC fabrication, such as measuring after wafer processing or cutting the wafer into smaller dies. Full photonic integration enables earlier optical testing on the semiconductor wafer and dies. By testing the dies and wafers directly before packaging, manufacturers need only discard the bad dies rather than the whole package, which saves time, and cost and is more energy-efficient and sustainable.

Figure 2: Package vs. die level testing. Manufacturers can find faults earlier by testing at the die level, avoiding wasting packaging materials.

For example, EFFECT Photonics reaps these benefits in its production processes. 100% of electrical testing on the photonic SoCs happens at the wafer level, and our unique integration technology allows for 90% of optical testing on the wafer. The real-world applications of SoCs are practically limitless and priceless. Electronic SoCs are used in most, if not all, portable devices, such as smartphones, cameras, tablets, and other wireless technologies. SoCs are also frequently used in equipment involved in the Internet of Things, embedded systems, and, of course, photonics. Data center interconnects are an excellent example of an application that benefits from a photonic SoC approach. As DCIs demand higher performance and reach, it’s no longer sufficient to have a solution that integrates just some parts of a system.

That is why EFFECT Photonics’ business strategy aims to solve the interconnect challenges through a holistic photonic SoC approach that understands the interdependence of system elements. By combining the photonic SoC with highly optimized packaging with cost-effective electronics, we are building a high production volume platform that can meet the demands of the datacom sector.

Tags: DWDM, Integrated Photonics, network, optical networking, optical technology, photonic integrated chip, photonic integration, photonic system-on-chip, PIC, solutions, technology

Latest events that EFFECT Photonics organizes or participates in

Insights

July 7, 2022

The Growth of Business Ethernet Services

The increasing use of data and fiercely price-conscious and multimedia-hungry business subscribers can limit revenue…

The increasing use of data and fiercely price-conscious and multimedia-hungry business subscribers can limit revenue opportunities for the network provider industry. Providers must therefore look elsewhere to grow their customer base, open new revenue streams, and boost margins. So how can they adapt their business strategies and achieve growth objectives?

Just as operators scale up high-capacity data center interconnects to cope with these needs, providers can add Ethernet services for high-capacity services. These can offer a differentiated and competitive service to corporate customers, ranging from 1G to 100G and beyond. Not only will Ethernet services add a cost-effective alternative to existing services, but they also ensure business Ethernet offerings are set up to complement wide area networks and hybrid network services.

According to Ovum, the global enterprise Ethernet services market will grow at 10.7% CAGR, exceeding $70bn by 2020 (Ovum’s Ethernet Services Forecast, Sep 2015), and is now the de facto wide-area network data connectivity technology. Ethernet will be a significant portion of the data service market driven by enthusiasm for higher bandwidth services. This growth will continue as we adopt more cloud-based applications and enterprises embrace digital transformation. Business Ethernet solutions can be further boosted with tunable DWDM transceivers.

Towards Carrier Ethernet

With applications and data volumes exploding in organizations of all types and sizes, there is an increasing need for 1GbE+ connections, with 10GbE+ connectivity for company headquarters and even 100GbE+ connections for data center connectivity. Specifically, demand is being driven by the proliferation of bandwidth-hungry applications. An MRI scan, for example, can be a 300GB file, which would take around 7 hours to download over a 100Mbps connection. Over a 10GbE link, that time falls to just 4 minutes and 28 seconds on a 100GbE link – this can be the difference between life and death when a consultant needs to make a time-critical decision on how to treat their patient best.

Enterprises know all about cost-containment and budget constraints. Virtual private networks (VPNs) based on legacy MPLS protocols have their place when delivering wide-area connectivity. These VPNs can offer any-to-any connectivity and scale to thousands of sites – but, at the higher speeds needed for high-bandwidth applications, they can be significantly more expensive than business Ethernet to deploy and maintain.

In addition, the MPLS routers needed for IP VPN have acquired more and more protocols and complexity over the last few decades. The cost of implementing all of these protocols, and securing them against attack, has driven leading service providers to demand a radically more straightforward way of building networks.

With business Ethernet, the network infrastructure and management can be unified under Ethernet protocol, making the network easier to plan, deploy and manage at scale. This means fewer routers, more remotely programmable services, and fewer truck rolls resulting in a lower cost per bit than comparable VPN solutions. These savings scale across 1GbE, 10GbE, and 100GbE connections with less CAPEX investment, helping to increase the predictability of delivery costs over time. WDM solutions can further boost this capacity.

Towards a Coherent Upgrade with 100ZR

Almost every organization uses the cloud in some capacity, whether for development and test resources or software-as-a-service applications. While the cost and flexibility of the cloud are compelling, many IT executives overlook the importance of fast, high-bandwidth wide-area connectivity to make cloud-based applications work as they should.

These needs might require businesses with huge traffic loads to upgrade to 25G, 100G, or even 400G speeds. These capacity needs would require coherent technology. Fortunately, advances in electronic and photonic integration have miniaturized coherent line card transponders into pluggable modules the size of a large USB stick.

Many of these business applications will require links between 25 Gbps and 100 Gbps that span several tens of kilometers to connect to the network provider’s headend. For these sites, the 400ZR pluggables that have become mainstream in datacom applications are not cost-effective when utilization is so low. This is where 100ZR technology comes into play.

100ZR is currently a marketing term for a short-reach (~80 km) coherent 100Gbps in a QSFP pluggable. Targeted at the metro edge and enterprise applications that do not require 400Gbps, 100ZR provides a lower-cost, lower-power pluggable that also benefits from compatibility with the large installed base of 50 GHz and legacy 100 GHz DWDM/ROADM line systems.

Self-Tuning Makes Management Easier

Businesses that need to aggregate many sites and branches into their networks will likely require tunable transceiver solutions to interconnect them. The number of available channels in tunable modules can quickly become overwhelming for technicians in the field. There will be more records to examine, more programming for tuning equipment, more trucks to load with tuning equipment, and more verifications to do in the field. These tasks can take a couple of hours just for a single node. If there are hundreds of nodes to install or repair, the required hours of labor will quickly rack up into the thousands and the associated costs into hundreds of thousands. Self-tuning modules significantly overcome these issues and make network deployment and maintenance more straightforward and affordable.

Self-tuning allows technicians to treat DWDM tunable modules the same way they would grey transceivers. There is no need for additional training for technicians to install the tunable module. There is no need to program tuning equipment. There is no need to obsessively check the wavelength records and tables to avoid deployment errors on the field. Technicians only need to follow the typical cleaning and handling procedures, plug the transceiver, and the device will automatically scan and find the correct wavelength once plugged. This feature can save providers thousands of person-hours in their network installation and maintenance and reduce the probability of human errors, effectively reducing capital and operational expenditures (OPEX).

Takeaways

With business Ethernet, one can set up super-fast connections for customers and connect their locations and end-users with any cloud-based services they use. Business Ethernet solutions can be further boosted with tunable DWDM transceivers. If business future-proof their networks with upgrades like 100ZR transceivers, they can scale up connectivity seamlessly to ensure that applications always provide an excellent end-user experience. That connectivity is never a limiting factor for customers’ cloud strategies. As the business sector seeks to upgrade to greater capacity and easier management, tunable and coherent transceivers will be vital in addressing their needs.

Tags: 5G, access, aggregation, backhaul, capacity, DWDM, fronthaul, Integrated Photonics, LightCounting, metro, midhaul, mobile, mobile access, network, optical networking, optical technology, photonic integrated chip, photonic integration, PIC, PON, programmable photonic system-on-chip, solutions, technology

Insights

June 29, 2022

The Future of 5G Fronthaul

The 5G network revolution promises to fulfill the capacity needs that previous cellular generations could…

The 5G network revolution promises to fulfill the capacity needs that previous cellular generations could no longer provide to the ever-increasing customer demands. This network generation is expected to revolutionize the concept of telecommunication and bring the most anticipated services of larger bandwidth, higher speed, and reduced latency as part of the modern cellular network. The upgrade from 4G to 5G has shifted the radio access network (RAN) from a two-level structure with backhaul and fronthaul in 4G to a three-level structure with back-, mid-, and fronthaul:

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

5G promises to interconnect exponentially more devices than before, with higher speed and latencies. As a result, 5G edge network bandwidth requirements can reach up to 100 times more than 4G. These requirements will put more pressure on the existing fiber infrastructure, and mobile network operators are expected to deliver their 5G promises with relatively little expansion in their fiber infrastructure.

The initial rollout of 5G has already happened in most developed countries, with operators switching from 1G SFP transceivers and grey transceivers to 10G SFP+ or wavelength division multiplexing (WDM). Mobile networks must move to the next phase of 5G deployments, which will exponentially increase the number of devices connected to the network. These more mature phases of 5G deployment will require operators to scale capacity cost-effectively. The move to 25G tunable optics in fronthaul networks will enable this expansion in capacity in an affordable way and help promote a long-awaited convergence between mobile and fixed access networks.

Enhancing Mobile Fronthaul Capacity

The move from 4G to 5G networks saw many operators upgrade their 10G grey transceivers to tunable 10G transceivers and 25G grey transceivers to make the most of their fiber infrastructure. However, as the 5G rollout moves into a more mature phase, the demands of fronthaul networks will often require even greater capacity from fiber infrastructure.

These future demands are a key reason why South Korea’s service providers decided to future-proof their fiber infrastructure and invested in 10G and 25G WDM technology since the early stages of their 5G rollout. Over time, providers in other countries will find themselves fiber-limited and turn towards tunable technologies. These trends are why LightCounting forecasts that the 25G DWDM market will provide the most significant revenue opportunity in the coming five years.

These 25G tunable transceivers support a typical reach of 10 km. It can reach up to 15 or even 20 km with extra amplification and compensation. Maximizing the capacity of fronthaul fiber will not only be beneficial for mobile network providers and telecom business developers, system architectures, equipment manufacturers, and product developers.

Graph DWDM Annual Sales — Figure 2: Fronthaul tunable optics transceiver revenues. Source: LightCouting Next-Gen Access Optics Report (2021)

DWDM Solutions For Fronthaul Aggregation

The transitions to 3G and 4G relied heavily on more efficient use of broader RF spectrum blocks. These transitions were as simple for many cell sites as changing the appropriate radio line card at a base station unit. The same cannot be said about the transition to 5G. Particularly this second phase of 5G deployment will require a more profound restructuring of mobile network architecture. These mature 5G networks will use higher frequency bands, which require the deployment of more cell sites and antennas to cover the same geographical areas as 4G. In contrast, existing antennas must upgrade to denser antenna arrays.

The characteristics of this second 5G deployment mean that operators must deploy larger-bandwidth channels and more total channels due to the additional base stations. DWDM is an excellent fit for interconnecting these new smaller cell sites since it allows operators to quickly increase their number of channels without having to lay out new and expensive fiber infrastructure. Thanks to the 25G capacity, these additional channels can be easily aggregated into the fronthaul transport network without being limited by fiber infrastructure.

Figure-5g-frount-haul-deep-fiber — Figure 3. Comparison between 4G legacy fiber deployments in access networks vs. future deep fiber deployments in 5G. The latter needs significantly more fiber capacity and infrastructure that can handle this aggregation, such as DWDM.

The Dream of Fixed-Mobile Access

Carriers that provide both fixed and mobile network services may often have to deal with situations in which their fixed and mobile access networks compete against each other. Since these networks often use different technology standards and even transmission media (e.g., legacy coaxial networks in fixed access), these carriers often have to build additional and arguably redundant optical infrastructure.

These carriers have long dreamed of merging their fixed and mobile network infrastructures under the same standards and transmission pipes. Such solutions reduce the need to build and manage redundant infrastructure. The expansion of fiber infrastructure and WDM technology might finally provide them with the opportunity to do so.

Figure-metro-core-network — Figure 4: Simplified example of some critical technologies and network architectures that could be merged in a converged fixed-mobile access network solution. These include radio access networks (RAN) with DWDM technology and Common Public Radio Interface (CPRI) protocols, as well as passive optical networks that combine time-division multiplexing (TDM) and WDM technology.

Passive optical networks (PON) have become a popular solution to implement fiber-to-the-home solutions. These networks’ bandwidth and latency requirements are leading to the standardization of 25G PONs and WDM-PON technology. Now that both mobile and fixed access are considering 25G WDM solutions, it might be a good time to revisit the subject of convergence, as together, this network will offer various quality services such as communication, entertainment, and data acquisition without any terminal, application, network, and location limitation.

Takeaways

25G tunable optics will become an industry standard for mobile fronthaul in the coming years. They allow operators to make the most of their existing fiber infrastructure, maximizing bandwidth and increasing the network’s ability to aggregate more signals from smaller and more numerous 5G base stations. For certain carriers, they could enable a future converged fixed-access network that simplifies the installation and management of their infrastructure. As shown by the example of South Korean network operators, it pays off to anticipate all these future demands and invest in a future-proof network that can scale up quickly.

Tags: 5G, access, aggregation, backhaul, capacity, DWDM, fronthaul, Integrated Photonics, LightCounting, metro, midhaul, mobile, mobile access, network, optical networking, optical technology, photonic integrated chip, photonic integration, PIC, PON, programmable photonic system-on-chip, solutions, technology

Insights

June 22, 2022

What is DWDM and Why Should You Care?

Imagine a couple of small trucks moving along a country road in opposite directions, carrying…

Imagine a couple of small trucks moving along a country road in opposite directions, carrying goods between factories and consumers. As the population grows and demand increases, the trucks grow in number, and diversity of goods and traffic increases. City planners must start adding lanes until, eventually the tiny country road has become a large multi-lane highway with 18-wheelers moving vast volumes of different types of merchandise every day. A similar rapid expansion in ‘cargo’ has happened in telecommunications.

The telecommunications industry and service providers, in particular, have faced a dramatic and very rapid increase in the volume and type of data their systems must handle. Networks built initially to transmit soundwaves as electrical signals from one phone to another were now faced with managing data and video in real-time from many devices. Within approximately 30 years, we have moved from the introduction of the Internet and the creation of the Worldwide Web to the rollout of 5G wireless technology and the Internet-of-Things (IoT), through which virtually all devices can theoretically be interconnected.

Handling this exponentially increasing data traffic has required massive contributions from fiber optics and optical communications systems. In these systems, laser light carries much higher data transmission rates over greater distances than electrical signals. To encode the data into light, transmit it, and decode it back into electrical signals upon receipt, optical communication systems rely on optical transceivers. Dense Wavelength Division Multiplexing (DWDM) is a transceiver technology developed around 20 years ago that dramatically increases the amount of data transmitted over existing fiber networks. Data from various signals are separated, encoded on different wavelengths, and put together (multiplexed) in a single optical fiber.

The wavelengths are separated again and reconverted into the original digital signals at the receiving end. In other words, DWDM allows different data streams to be sent simultaneously over a single optical fiber without requiring new cables to be laid. In a way, it’s like adding more lanes to the information highway without having to build new roads!

Optical fiber with DWDM Channels — Figure 1: Example of a bidirectional DWDM system. At the central office, transceiver modules pick different wavelength channels for transmission, and these channels are combined into a single fiber through a device called a multiplexer. A demultiplexer splits the wavelength channels into separate fibers and the receiving modules on the remote end. Since this is a bidirectional system, transmission can go opposite too, from the remote end to the central office.

The tremendous expansion in data volume afforded with DWDM can be seen compared to other optical methods. A standard transceiver, often called a grey transceiver, is a single-channel device – each fiber has a single laser source. You can transmit 10 Gbps with grey optics. Coarse Wavelength Division Multiplexing (CWDM) has multiple channels, although far fewer than possible with DWDM. For example, with a 4-channel CWDM, you can transmit 40 Gbps. DWDM can accommodate up to 100 channels.

You can transmit 1 Tbps or one trillion bps at that capacity – 100 times more data than grey optics and 25 times more than CWDM. While the volume of data transmitted with DWDM is impressive, demand will continue to grow as we move toward IoT and 5G. Adding additional optical transceivers with different wavelengths to a fixed-wavelength DWDM system can significantly increase costs. Tunable DWDM transceivers allow you to control the wavelength (color) that the laser channel emits, adding flexibility and reducing cost. However, two obstacles prevented the broader deployment of DWDM technology.

First of all, installing and maintaining many new DWDM optical links was a time-consuming and costly process. Fortunately, the telecom industry developed a new weapon to face these challenges: self-tuning DWDM modules. Self-tuning DWDM modules minimize the network’s time-to-service by eliminating additional installation tasks such as manual tuning and record verification and reducing the potential for human error. They are host-agnostic and can plug into any third-party host equipment.

Furthermore, tunability standards allow modules from different vendors to communicate with each other, avoiding compatibility issues and simplifying upgrade choices. Self-tuning modules made the deployment and operation of DWDM links faster, simpler, and more affordable. The second issue had to do with size. DWDM modules were traditionally too large, so plugging them into a router required sacrificing roughly half of the expensive router faceplate capacity. Telecom operators could not accept such a trade-off. Advances in electronic and photonic integration overcame these trade-offs, miniaturizing coherent line card transponders into pluggable modules the size of a large USB stick.

Few companies worldwide supply DWDM technology with such compact sizes and self-tuning features. EFFECT Photonics is among them, and its tunable and cost-effective DWDM technologies will act as enablers of 5G and IoT, bringing the future to you today.

Tags: DWDM, Integrated Photonics, optical networking, optical technology, photonic integrated chip, photonic integration, PIC, programmable photonic system-on-chip

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

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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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May 25, 2022

Improving Edge Computing with Coherent Optical Systems on Chip

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. Cloud-native applications are driving the construction of edge infrastructure and services. However, they cannot distribute their processing capabilities without considerable investments in real estate, infrastructure deployment, and management.

This situation leads to hyperscalers cooperating with telecom operators to install their servers in the existing carrier infrastructure. For example, Amazon Web Services (AWS) is implementing edge technology in carrier networks and company premises (e.g., AWS Wavelength, AWS Outposts). Google and Microsoft have strategies and products that are very similar. In this context, edge computing poses a few problems for telecom providers too. They must manage hundreds or thousands of new nodes that will be hard to control and maintain.

At EFFECT Photonics, we believe that coherent pluggables with an optical System-on-Chip (SoC) can become vital in addressing these datacom and telecom sector needs and enabling a new generation of distributed data center architectures. Combining the optical SoCs with reconfigurable DSPs and modern network orchestration and automation software will be a key to deploying edge data centers.

Edge data centers are a performance and sustainability imperative

Various trends are driving the rise of the edge cloud:

5G technology and the Internet of Things (IoT): These mobile networks and sensor networks need low-cost computing resources closer to the user to reduce latency and better manage the higher density of connections and data.
Content delivery networks (CDNs): The popularity of CDN services continues to grow, and most web traffic today is served through CDNs, especially for major sites like Facebook, Netflix, and Amazon. By using content delivery servers that are more geographically distributed and closer to the edge and the end user, websites can reduce latency, load times, and bandwidth costs as well as increasing content availability and redundancy.
Software-defined networks (SDN) and Network function virtualization (NFV). The increased use of SDNs and NFV requires more cloud software processing.
Augment and virtual reality applications (AR/VR): Edge data centers can reduce the streaming latency and improve the performance of AR/VR applications.

Several of these applications require lower latencies than before, and centralized cloud computing cannot deliver those data packets quickly enough. As shown in Table 1, a data center on a town or suburb aggregation point could halve the latency compared to a centralized hyperscale data center. Enterprises with their own data center on-premises can reduce latencies by 12 to 30 times compared to hyperscale data centers.

Type of Edge		Data center	Location	Number of DCs per 10M people	Average Latency	Size
On-premises edge		Enterprise site	Businesses	NA	2-5 ms	1 rack max
Network (Mobile)	Tower edge	Tower	Nationwide	3000	10 ms	2 rack max
	Outer edge	Aggrega- tion points	Town	150	30 ms	2-6 rack max
	Inner edge	Core	Major city	10	40 ms	10+ rack max
Regional edge		Regional	Major city	100	50 ms	100+ racks
Not edge		Hyperscale	State/ national	1	60+ ms	5000+ racks

Table 1: Types of edge data centers and their characteristics. Source: STL Partners

Cisco estimates that 85 zettabytes of useful raw data were created in 2021, but only 21 zettabytes were stored and processed in data centers. Edge data centers can help close this gap. For example, industries or cities can use edge data centers to aggregate all the data from their sensors. Instead of sending all this raw sensor data to the core cloud, the edge cloud can process it locally and turn it into a handful of performance indicators. The edge cloud can then relay these indicators to the core, which requires a much lower bandwidth than sending the raw data.

Figure 1 Global data center traffic vs. useable data created per year. Graphic sourced from Cisco Global Cloud Index.

Edge data centers therefore allow more sensor data to be aggregated and processed to make systems worldwide smarter and more efficient. The ultimate goal is to create entire “smart cities” that use this sensor data to benefit their inhabitants, businesses, and the environment. Everything from transport networks to water supply and lightning could be improved if we have more sensor data available in the cloud to optimize these processes. Distributing data centers is also vital for future data center architectures. While centralizing processing in hyper-scale data centers made them more energy-efficient, the power grid often limits the potential location of new hyperscale data centers. Thus, the industry may have to take a few steps back and decentralize data processing capacity to cope with the strain of data center clusters on power grids. For example, data centers can be relocated to areas where spare power capacity is available, preferably from nearby renewable energy sources. EFFECT Photonics envisions a system of datacentres with branches in different geographical areas, where data storage and processing are assigned based on local and temporal availability of renewable (wind-, solar-) energy and total energy demand in the area.

Figure 2: High-speed fiber-optic connections allow data processing and storage to move to locations where excess (green) energy is available. If power is needed for other purposes, such as charging electric vehicles, data can be moved elsewhere

Coherent technology simplifies the scaling of edge data center interconnects

As edge data center interconnects became more common, the issue of how to interconnect them became more prominent. Direct detect technology had been the standard in the short-reach data center interconnects. However, reaching the distances greater than 50km and bandwidths over 100Gbps required for modern edge data center interconnects required external amplifiers and dispersion compensators that increased the complexity of network operations. 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 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. Cignal AI forecasts that 400ZR shipments will dominate in the edge applications, as shown in Figure 3.

Figure 3: 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’ optical 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.

Maximizing Edge Computing with Automation

With the rise of edge data centers, telecom providers must manage hundreds or thousands of new nodes that will be hard to control and maintain. Furthermore, providers also need a flexible network with pay-as-you-go scalability that can handle future capacity needs. Fortunately, several new technologies are enabling this scalable and automated network management.

First of all, the rise of self-tuning algorithms has made the installation of new pluggables easier than ever. They eliminate additional installation tasks such as manual tuning and record verification. They are host-agnostic, can plug into any third-party host equipment, and scale as you grow. Standardization also allows modules from different vendors to communicate with each other, avoiding compatibility issues and simplifying upgrade choices. The communication channels used for self-tuning algorithms can also be used for remote diagnostics and management, such as the case of EFFECT Photonics NarroWave technology.

Automation potential improves further by combining artificial intelligence with the software-defined networks (SDNs) framework that virtualizes and centralizes network functions. This creates an automated and centralized management layer that can allocate resources efficiently and dynamically. For example, AI in network management will become a significant factor in reducing the energy consumption of future telecom networks.

Figure 2: Comparing a traditional network approach (left) with an SDN/NFC approach (right).

Future smart transceivers with reconfigurable digital signal processors (DSPs) can give the AI-controlled management layer even more degrees of freedom to optimize the network. These smart transceivers will relay more device information for diagnosis, and depending on the management layer instructions, they can change their coding schemes to adapt to different network requirements

Takeaways

Cloud-native applications require edge data centers with lower latency, and that better fit the existing power grid. However, their implementation came with the challenges of more data center interconnects and a massive increase in nodes to manage. Fortunately, coherent pluggables with self-tuning can play a vital role in addressing these datacom and telecom sector challenges and enabling a new generation of distributed data center architectures. Combining these pluggables with modern network orchestration and automation software will boost the deployment of edge data centers. EFFECT Photonics believes that with these automation technologies (self-tuning, SDNs, AI), we can reach the goal of a self-managed, zero-touch automated network that can handle the massive scale-up required for 5G networks and edge computing.

Tags: 400ZR, artificial intelligence, cloud, coherent, computing, data centers, DSP, edge, edge data centers, infrastructure, latency, network, network edge, operators, optical system-on-chip, pluggables, self-tuning, services

Insights

May 4, 2022

The Growing Market for Tunable Lasers

The world is moving towards tunability. The combination of tunable lasers and dense wavelength division…

The world is moving towards tunability. The combination of tunable lasers and dense wavelength division multiplexing (DWDM) allows datacom and telecom industries to expand their network capacity without increasing their existing fiber infrastructure. Furthermore, the miniaturization of coherent technology into pluggable transceiver modules has finally enabled the widespread implementation of IP over DWDM solutions.

Self-tuning algorithms have also made DWDM solutions more widespread by simplifying their installation and maintenance. Hence, many application cases—metro transport, data center interconnects, and even future access networks—are moving towards coherent tunable pluggables. The market for coherent tunable transceivers will explode in the coming years, with LightCounting estimating that annual sales will double by 2026. Telecom carriers and especially data center providers will drive the market demand, upgrading their optical networks with 400G, 600G, and 800G pluggable transceiver modules that will become the new industry standards.

Figure 1: Market forecast for coherent DWDM modules by data rate. Source: LightCounting Market Forecast Report (April 2021)

Same Laser Performance, Smaller Package

As the industry moves towards packing more and more transceivers on a single router faceplate, tunable lasers need to maintain performance and power while moving to smaller footprints and lower power consumption and cost. Due to the faceplate density requirements for data center applications, transceiver power consumption is arguably the most critical factor in this use case.

In fact, power consumption is the main obstacle preventing pluggables from becoming a viable solution for a future upgrade to Terabit speeds. Since lasers are the second biggest power consumers in the transceiver module, laser manufacturers faced a paradoxical task. They must manufacture laser units that are small and energy-efficient enough to fit QSFP-DD and OSFP pluggable form factors while maintaining the laser performance. Fortunately, these ambitious spec targets became possible thanks to improved photonic integration technology.

The original 2011 ITLA standard from the Optical Internetworking Forum (OIF) was 74mm long by 30.5mm wide. By 2015, most tunable lasers shipped in a micro-ITLA form factor that cut the original ITLA footprint in half. In 2021, the nano-ITLA form factor designed for QSFP-DD and OSFP modules has once again cut the micro-ITLA footprint almost in half. The QSFP-DD modules that house the full transceiver are smaller (78mm by 20mm) than the original ITLA form factor. Stunningly, tunable laser manufacturers achieved this size reduction without impacting laser purity and power.

Figure 2: Evolution of tunable laser form factors for coherent optics (2011-2021). Figure from Laser Focus World.

Versatile Laser Developers for Different Use Cases

The different telecom and datacom applications will have different requirements for their tunable lasers. Premium coherent systems used for submarine and ultra-long-haul require best-in-class lasers with the highest power output and purity. On the other hand, metro transport and data center interconnect applications do not need the highest possible laser quality, but they need small devices with lower power consumption to fit router faceplates. Meanwhile, the access network space looks for lower-cost components that are also temperature hardened.

Table 1: Summary of different use cases for high-performance tunable lasers.

These varied use cases provide laser developers with ample opportunities and market niches to provide fit-for-purpose solutions for. For example, a laser module can be set to run at a higher voltage to provide higher output power and reach for premium long-haul applications. On the other hand, tuning the laser to a lower voltage would enable a more energy-efficient operation that could serve more lenient, shorter-reach use cases (links < 250km), such as data center interconnects.

An Independent Player in Times of Consolidation

With the increasing demand for coherent transceivers, many companies have performed acquisitions and mergers that allow them to develop transceiver components internally and thus secure their supply. LightCounting forecasts show that while this consolidation will decrease the sales of modulator and receiver components, the demand for tunable lasers will continue to grow. The forecast expects the tunable laser market for the transceiver to reach a size of $400M in 2026.

Figure 3: Global Market sales for high-performance modulators and receivers as well as tunable lasers. (Historical Data and Forecast). Source: LightCounting Market Forecast Report (April 2021)

We can dive deeper into the data to find the forces that drive the steady growth of the laser market. As shown in Figure 4, the next five years will likely see explosive growth in the demand for high-purity, high-power lasers. The forecast predicts that the shipments of such laser units will increase from roughly half a million in 2022 to 1.4 million in 2026 due to the growth of 400G and 800G transceiver upgrades. However, the industry consolidation will make it harder for component and equipment manufacturers to source lasers from independent vendors for their transceivers.

Figure 4: Units sold in the global market for Tunable Lasers components: legacy, high-purity low-power, and high-purity high-power (Historical Data and Forecast). Source: LightCounting Market Forecast Report (April 2021)

This data indicates that the market needs more independent vendors to provide high-performance ITLA components that adapt to different datacom or telecom provider needs. Following these trends, at EFFECT Photonics, we are not only developing the capabilities to provide a complete coherent transceiver solution but also the nano-ITLA units needed by other vendors.

Takeaways

The world is moving towards tunability. As telecom and datacom industries seek to expand their network capacity without increasing their fiber infrastructure, the sales tunable transceivers will explode in the coming years. These transceivers need tunable lasers with smaller sizes and lower power consumption than ever. Fortunately, the advances in photonic integration are managing to fulfill these laser requirements, leading to the new nano-ITLA module standards. However, even though component and equipment vendors need these tunable lasers for their next-gen transceivers, the industry consolidation can affect their supply. This situation presents an opportunity for new independent vendors to supply nano-ITLA units to this growing market.

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Tags: acquisition, coherent, coherent communication systems, coherent optical module vendor, coherent technology stack, datacenters, datacom, DWDM, high-performance, hyperscalers, independent, Integrated Photonics, lasers, noise, OEM, optical engine, optical transceivers, performance, photonic integration, Photonics, pluggables, power consumption, reach, self-tuning, Telecom, telecom carriers, Transceivers, tunable, tunable laser, tuneability, VARs, versatile

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April 28, 2022

Co-Designing Optics and Electronics for Versatile and Green Transceivers

Network and data center operators need fast and affordable pluggable transceivers that perform well enough…

Network and data center operators need fast and affordable pluggable transceivers that perform well enough to cover a wide range of link lengths. However, power consumption and thermal management are the big obstacles in the roadmap to scale high-speed transceivers into Terabit speeds.

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.

Around 50% of a coherent transceiver’s power consumption goes into the digital signal processing (DSP) chip that also performs the functions of clock data recovery (CDR), optical-electrical gear-boxing, and lane switching. Scaling to higher bandwidths leads to even more losses and energy consumption from the DSP chip and its radiofrequency (RF) interconnects with the optical engine.

Figure 1: Normalized power breakdown of the internal blocks of a 400ZR pluggable module. Although this breakdown may vary a bit among different vendors, the authors find it to be generally true for various DSP-based modules, both IM-DD and coherent. Source: R. Nagarajan et al., IEEE JLT (2021)

Thus, a great incentive exists to optimize the interface between the module’s DSP chip and the optical engine to make the transceiver more energy efficient. This need for optimization and efficiency makes co-designing the optical and electronic systems of the transceiver more important than ever.

Co-Designing the Optimal DSP

Coherent DSPs are already application-specific integrated circuits (ASICs), but they could fit their respective optical engines and use cases even more tightly. 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 PIC but a master of none. For example, many 400ZR+ transceivers used for telecom metro and long-haul applications are using the same DSPs as 400ZR transceivers used for much shorter data center interconnects. Given the ever-increasing demand for capacity and the need for sustainability both as financial and social responsibility, transceiver developers are increasingly in need of a steak knife rather than a Swiss army knife.

Co-designing the DSP chip alongside the photonic integrated circuit (PIC) can lead to a much better fit between these components. A co-design approach helps identify in greater detail the trade-offs between various parameters in the DSP and PIC and thus improve system-level performance optimization. A DSP optimized for a specific optical engine and application could save up to a couple of Watts of power compared to the usual transceiver and DSP designs.

Figure 2: Simplified diagram of the building blocks of a coherent QSFP-DD transceiver. Original diagram source: Infinera

Co-designing DSP Interfaces for Power Efficiency

Since the optical engine and DSP operate with signals of differing intensities, they need some analog electronic components to “talk” to each other. On the transmit side, the electronic driver block takes signals from the DSP, converts them to a higher voltage, and drives the optical engine. On the receive side, a trans-impedance amplifier (TIA) block will boost the weak signal captured by the optical detector so that the DSP can more easily process it. This signal power conversion overhead constitutes roughly 10-15% of transceiver power consumption, as shown in Figure 1.

Co-designing the DSP and PIC could enable ways to decrease this power conversion overhead. For example, the modulator of the optical engine could be designed to run at a lower voltage that is more compatible with the signal output of the DSP. This way, the DSP could drive the optical engine directly without the need for the analog electronic driver. Such a setup could save roughly two watts of power consumption!

Figure-dac grey blocks — Figure 3: Simplified diagram of the building blocks of a coherent QSFP-DD transceiver in which the DSP drives the optical engine directly from its digital-to-analog converter (DAC). This setup can achieve a power consumption improvement of 10-15%.

Co-design is also vital to optimize the transceiver layout floorplan. This plan must consider the power dissipation of all transceiver building blocks to avoid hot spots and thermal interference from the DSP to the highly thermally sensitive PIC. The positioning of all bond pads and interfaces is also very important for signal and power integrity, requiring a co-design with the package and substrate.

During this floorplan development, the RF interconnections between the DSP and PIC can be made as short as possible. These optimized RF interconnects reduce the optical and thermal losses in the transceiver package and will reduce the power consumption of the analog electronic driver and amplifier.

Co-Designing Fit-For-Purpose DSPs and PICs

As shown in Figure 4, a DSP chip contains a sequence of processing blocks that compensate for different transmission issues in the fiber and then recover, decode, and error-correct the data streams. Different applications might require slightly different layouts of the DSP or might not need some processing blocks.

Figure 4: Simplified diagram of the processing blocks of a DSP chip.

For example, full DSP compensation might be required for long links that span several hundreds of kilometers, but a shorter link might not require all the DSP functions. In these cases, a transceiver could turn off or reduce certain DSP functions—such as chromatic dispersion compensation—to save power. These power-saving features could be particularly useful for the cases of shorter data center interconnect links (DCI). On the optical engine side, the laser might not require a high power to transmit over this shorter DCI link so the amplifier functions could shut down. Co-designing the DSP and PIC allows a transceiver developer to mix and match these different energy-saving features to achieve the lowest possible power for a specific application.

Takeaways

Power consumption has become the big barrier that prevents pluggable transceivers from scaling up to 800G and Terabit speeds. Overcoming this barrier requires a tighter fit between the optics and electronics of the transceiver, especially when it comes to the interface between the optical engine and the electronic DSP. By co-designing the optical engine and the electronic DSP, transceiver developers could avoid the need for an external electrical driver and reduce transceiver power consumption by 10-15%. A co-design approach can also make it easier to design fit-for-purpose transceivers that implement power-saving features tailored to specific application cases.

The benefits of this co-design approach led EFFECT Photonics to incorporate talent and intellectual property from Viasat’s Coherent DSP team. With this merger, EFFECT Photonics aims to co-design our Optical System-On-Chip with the DSP to develop fit-for-purpose transceivers that are more energy-efficient than ever before.

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Tags: acquisition, coherent, coherent communication systems, coherent optical module vendor, DSP, FEC, forward error correction, green, green transceivers, high vertical integration, independent coherent optical module vendor, Integrated Photonics, optical digital signal processing, optical engine, optical transceivers, photonic integration, Photonics, pluggables, Transceivers, tunable laser, tuneability

Telecom/Datacom

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There’s More to a DSP Than You Think…

So What’s Inside the Actual DSP Block?

Recent Advances and Challenges in DSPs

Transmission Reach and Efficiency

Energy Efficiency

Takeaways

From Matches to the Light Bulb Factory

From ASML and Photonics Research to Philips

Bringing Photonics into the Real World from the Lab

Towards Carrier Ethernet

Towards a Coherent Upgrade with 100ZR

Self-Tuning Makes Management Easier

Takeaways

Enhancing Mobile Fronthaul Capacity

DWDM Solutions For Fronthaul Aggregation

The Dream of Fixed-Mobile Access

Takeaways

Overcoming the challenges of alignment, and atmospheric turbulence

An alternative for safe, private networks

Using Highly-Integrated Transceivers in Free Space Optics

Takeaways

Edge data centers are a performance and sustainability imperative

Coherent technology simplifies the scaling of edge data center interconnects

Maximizing Edge Computing with Automation

Takeaways

Same Laser Performance, Smaller Package

Versatile Laser Developers for Different Use Cases

An Independent Player in Times of Consolidation

Takeaways

Co-Designing the Optimal DSP

Co-designing DSP Interfaces for Power Efficiency

Co-Designing Fit-For-Purpose DSPs and PICs

Takeaways

Telecom/Datacom

Emerging Industries

DSP & FEC

Ultra Pure Light Sources

Ecosystems

Pluggable Modules – Direct Detect

Pluggable Modules – Coherent

Sub-Assemblies

ASICs

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Insights

Press Releases

In the News

Events

Careers

Knowledge Center

Company Info

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