This week, Lightwave once again recognized the most innovative products impacting the optical communications community with its annual Lightwave Innovation Reviews. Acacia is proud to have its edge and access pluggable coherent modules portfolio score a 4.5 out of 5 in the optical transceivers and transponders category.

The benefits of coherent have already been demonstrated in the metro, long-haul and submarine markets. With 5G and edge computing roll outs, the time is right for coherent optics to take the next step and migrate to edge and access networks. This market can benefit from the scalability, operational simplicity and improved total cost of ownership that coherent has to offer.  In 2020, Acacia began customer sampling of a portfolio of product solutions designed to address a wide variety of requirements in the edge and access market.  The portfolio includes:

• 100G coherent dedicated point-to-point fiber edge and access solution
• Coherent bi-directional pluggable optical module
• 100G coherent DWDM module

Examples of different connectivity solutions in the service provider edge/access portion of the network.

100G Coherent Dedicated Point-to-Point (P2P) Fiber Edge and Access Solutions
Offered in a QSFP-DD form factor widely used for client-optics, Acacia’s new 100G coherent dedicated point-to-point (P2P) fiber edge and access solutions were designed to provide network operators the ability to scale to higher data rates to meet growing bandwidth demands over some of the most challenging optical links, while also providing operational simplicity that may lead to overall network savings.  These solutions were specifically designed for optimization in service provider edge and access applications with unamplified links up to 120km.

QSFP-DD 100G Coherent Point-to-Point (P2P) pluggable module

Compared to alternative direct-detect solutions, coherent P2P solutions are extremely tolerant to fiber types, chromatic dispersion, polarization mode dispersion (PMD), and back reflections, which simplifies operational deployment. Service provider edge and access networks, particularly those offering 5G wireless and enterprise services, can benefit from coherent technology because it has ample optical margins which can easily handle the condition and reaches of the access fiber plant resulting in shorter provisioning times. Coherent P2P solutions are user friendly in deployment and provisioning due to low laser emission, wide tolerance range and rich monitoring and diagnostic features. Pluggable coherent solutions in QSFP-DD were designed to enable network scalability to ensure that data rates can scale-up by leveraging the QSFP-DD slots with 200G and 400G versions in the future.

Coherent Bi-Directional Pluggables for Cable and 5G Wireless X-Haul Applications
Acacia’s coherent bi-directional pluggable optical module solutions were designed to transmit and receive data in both directions on a single fiber for 100G and beyond. Offered in a pluggable CFP2 form factor, Acacia’s coherent bi-directional module is designed to deliver an operationally efficient and cost-effective way for telecommunications and cable operators to increase capacity in fiber-limited networks.

CFP2 Coherent Bi-Directional Pluggables

In cable networks, particularly Remote PHY and fiber deep applications, providers often run into situations where only a single fiber is available. Historically, these networks have been served by 10G bi-directional optical modules. To meet growing bandwidth demands, a coherent bi-directional solution can provide these networks with an upgrade path to 100G and beyond. When wireless networks are migrating from 4G LTE to 5G, fibers are more often constrained because of a large number of remote nodes and some fibers are utilized by existing services. In these instances, coherent bi-directional modules can alleviate both bandwidth and fiber constraints.

100G Coherent DWDM Pluggable Modules for Cable/Fiber Deep and 5G Wireless X-haul Applications
Offered in the QSFP-DD form factor, Acacia’s new 100G coherent DWDM module was designed to provide a scalable traffic aggregation solution, as well as an upgrade path to migrate from existing 10G WDM networks to higher-performing 100G DWDM coherent links for new architectures in access networks.  Acacia’s pluggable coherent solutions in QSFP-DD were designed to enable network scalability to 200G and 400G versions in the future.

QSFP-DD 100G coherent DWDM pluggable Module

3D Siliconization Plus Proven Silicon Photonics and Coherent Transmission Expertise
This portfolio leverages Acacia’s 3D Siliconization approach, which utilizes high-volume manufacturing processes and benefits from the maturity of Acacia’s silicon photonics technology as well as its 11 years of high-performance coherent transmission expertise.

 Thanks to Lightwave
From all of us at Acacia, we want to give a big thanks to the Lightwave judges for recognizing the innovation that went into the Acacia edge and access portfolio. This market has many different requirements, which required a range of solutions, and we are proud to bring these to our existing and new customers to help them meet their bandwidth demands.

If you want to learn more about Acacia’s portfolio of edge and access pluggable solutions, please contact us.

Here are a few notable accomplishments from the past year.

The Rise of 400G Pluggables
Bandwidth demands have continued to grow, putting pressure on cloud providers to increase the data center interconnects (DCI) that link their facilities around the globe. This has helped to drive the emergence of new architectures that could support coherent transport in the same form factors as client optics, to satisfy those demands in a cost-effective and operationally efficient way. 400G pluggables were designed to be plugged directly into switches and routers, offering the same density for both coherent DWDM and client optics in the same chassis.

Utilizing our 3D siliconization technology, Acacia introduced a family of 400G pluggable solutions featuring an expansive list of interoperability modes (400ZR, OpenZR+, Open ROADM MSA and CableLabs Coherent Optics Physical Layer Specification). These solutions were designed to enable DCI and service provider network operators to address increasing bandwidth demand through a simplified network architecture, helping to reduce both capital and operational expenditures.

Modules based on multi-sourced 400ZR DSPs are now being validated for readiness in DCI applications and network operators are evaluating OpenZR+ solutions with enhanced functionality.  For example, Acacia and Inphi recently demonstrated interoperability of 400ZR over 120km. In addition, Acacia and NTT Electronics announced successful interoperability testing of 400ZR and OpenZR+. At Acacia, we believe we will see system vendors and network operators announcing trials in the near future.

Coherent Moves to Edge and Access
The benefits of coherent have already been demonstrated in the metro, long-haul and submarine markets, and with the coming of 5G and edge computing, the time is right for coherent optics to take the next step and migrate to edge and access networks. We believe this market can benefit from the scalability, operational simplicity and improved total cost of ownership that coherent has to offer.

To address the wide variety of requirements in the edge and access market, Acacia recently announced a portfolio of products, including a coherent bi-directional pluggable optical module for cable and 5G wireless X-haul applications, a 100G coherent point-to-point edge and access solution for 5G Wireless X-haul and Enterprise Services, and a 100G coherent DWDM module for cable/fiber deep and 5G wireless X-haul applications.

Multi-Haul Coherent Solutions Take Off
With bandwidth demands continuing to rise, network operators have been looking for solutions that improve efficiency and maximize capacity utilizing while reducing network cost.  Multi-haul solutions have emerged to meet the needs of many markets including cloud, metro, long-haul and submarine network applications by providing the high performance and flexibility features required to address meet these demanding applications.

Acacia’s AC1200 product family offers customers a multi-haul solution designed to cost-effectively improve network utilization in a wide range of network architectures. Supporting transmission speeds of up to 1.2 Tbps, the AC1200 utilizes Acacia’s 3D shaping technology designed to optimize fiber capacity and reach by filling gaps in margin and spectrum. In addition to its higher capacity and density, Acacia’s AC1200 product family, when embedded inside a number of our network equipment manufacturer partners’ products, provides features designed to enable network operators to improve efficiency while reducing network costs.

Here are a few examples.

Long-haul Terrestrial Applications
ADVA announced that the FSP3000 TeraFlex broke multiple industry records in live network trial. ADVA also announced that FUNET trialed ADVA FSP 3000 TeraFlex to dramatically expand network capacity and Serverius, one of the Netherlands’ largest data center service suppliers, is leveraging its FSP 3000 TeraFlex terminal to massively increase the capacity of its deployed infrastructure.

Submarine Applications
Cisco is making waves in the subsea market having demonstrated the benefits of the NCS 1004 over a subsea cable in production achieving record results. Cisco and Superloop announced two deployments of up to 400G for 4600km on the INDIGO West cable from Singapore to Australia, and the INDIGO Central cable from Perth to Sydney, featuring a two-fibre pair ‘open cable’ design with new spectrum sharing technology.

Oh What a Year – But the Show Must Go On
As NGON & DCI World and ECOC go virtual this year, I am looking forward to presenting in the following two panels. I hope to see many of you online and from all of us at Acacia…stay safe and healthy and have a great rest of the year.

• NGON & DCI World on Dec 1 at 9:00 am ET: “Next Generation Optics for Increased Capacity”
• ECOC Market Focus on Dec 7 at 17:00 CET: “A Path to Next Generation Coherent”

Contact us if you would like to schedule a meeting with myself or one of my colleagues.

October is a hot month for cable technology shows with both the SCTE Cable-Tec Expo and the Light Reading Cable Next-Gen Digital Symposium taking place. While both shows are virtual, we expect there will be some conversations around the emergence of coherent technology in the edge and access market.  As Sterling Perrin, Sr. Principal Analyst at Heavy Reading commented in Acacia’s first edge and access product announcement, “Over the past decade, coherent technology has moved beyond its long-haul origins and is now firmly entrenched in the metro.  With the coming of 5G and edge computing, the time is right for coherent optics to take the next step and migrate to edge and access networks.”

Meeting Growing Bandwidth Demand
Streaming services for residential entertainment have long been drivers for bandwidth demand over service provider access networks. Applications that go beyond streaming entertainment, such as uploading 4K-resolution videos into the cloud for social-network sharing and the increased use of two-way video chats and file sharing tied to work-from-home requirements, are driving the need for increased bandwidth, especially when traffic from a large number of end users is aggregated. The ability of the legacy optical service provider edge and access infrastructure to support this growing bandwidth demand is a challenge. That’s why cable network operators (a.k.a, multi-service operators, MSOs) currently utilizing 10G optical links are seeking to upgrade to 100G links that operate over edge and access fiber routes.
Examples of different connectivity solutions in the service provider edge/access portion of the network.

In fiber-constrained access environments, it’s not unusual to have data transmission over a single fiber strand carrying both downstream and upstream traffic —a.k.a, bi-directional (BiDi) traffic. Network operators upgrading their service provider edge/access bandwidth face the challenge of overcoming limitations due to legacy optical technology not capable of supporting data rates beyond a certain speed for a given distance, while at the same time supporting BiDi transmission.

The Migration of Coherent into Edge and Access
Meanwhile, much progress has been made over the past decade to reduce the size, power and cost of coherent solutions which has enabled its evolution from supporting long-haul to metro networks. More recently, requirements from data center network operators for high-capacity, high-density, solutions in pluggable form factors widely used for client-optics slots have driven industry-wide investments into ultra-compact, low-power and economical coherent solutions that can be manufactured in high volumes. This opens up new addressable markets for coherent solutions in edge and access networks including cable that can provide scalable and operationally flexible solutions to address growing bandwidth demands.

Cable Network Applications for Coherent
The cable industry has been driving standardization of coherent for access aggregation in this segment for quite some time.  As the hybrid fiber-coax (HFC) networks evolve toward remote PHY architectures, fiber is being deployed deeper in the network, resulting in increased available bandwidth to residential end-user customers, while eliminating bottlenecks in the HFC network. 10G optical interfaces are pushed closer to the end users, resulting in aggregation points in the network where 10-20 remote PHY devices come together.

Remote PHY network

Acacia believes that coherent can be an effective way to transport these aggregated signals back to the hub. In some cases, it may only be necessary to transport a single coherent wavelength back to the hub, but since coherent solutions leverage dense wavelength division multiplexing (DWDM) technology, this approach could provide the capability to expand capacity by up to two orders of magnitude in the future.

As discussed above, in cable networks, particularly Remote PHY and fiber deep applications, providers often run into situations where only a single fiber is available. Historically, these networks have been served by 10G Bi-Di optical modules. To meet growing bandwidth demands, a coherent BiDi solution can provide these networks with an upgrade path to 100G and beyond. Also, in these situations where optical waves are transmitted in a single fiber medium from both directions, coherent receivers can efficiently help to eliminate the crosstalk from back reflections when it operates at a different wavelength than its transmitter.

Acacia’s Portfolio of Edge and Access Solutions
To serve the cable market, Acacia has developed solutions designed specifically for these applications.

• This week, Acacia announced that it was sampling a new 100G DWDM coherent pluggable solution. Offered in the QSFP-DD form factor, Acacia’s new 100G DWDM module was designed to provide a scalable traffic aggregation solution, as well as an upgrade path to migrate from existing 10G WDM networks to higher-performing 100G DWDM coherent links for new architectures in access networks, such as cable/fiber deep and 5G wireless X-haul applications.
• Also this week, Acacia announced the sampling of new coherent bi-directional pluggable optical module solutions designed to transmit and receive data in both directions on a single fiber for 100G and beyond. Offered in a pluggable CFP2 form factor, Acacia’s coherent bi-directional module is designed to deliver an operationally efficient and cost-effective way for telecommunications and cable operators to increase capacity in fiber limited networks.

Let’s Talk About Cable and Coherent
If you want to learn more about how coherent is ready to help cable providers, click here to set up a meeting.

Considered the focal point for thought leadership, engineering innovation and deal-making, Cable-Tec is the premier venue to learn how cable is improving services today and spearheading bold new approaches for the future, including 10G, IoT, artificial intelligence, machine learning, data analytics and more. This year’s Expo is even more special as it celebrates two milestones – SCTE•ISBE’s 50^th anniversary and entry into the 10G Era.

Coherent Technology for Cable Access
One major trend we expect to be discussed at this year’s Expo is how access networks represent an emerging opportunity for coherent interconnections. To support this, the cable industry has been driving standardization of coherent for access aggregation in this segment for quite some time.

As the hybrid fiber-coax (HFC) networks evolve toward remote PHY architectures, fiber is being deployed deeper in the network, resulting in increased available bandwidth to residential end-user customers, while eliminating bottlenecks in the HFC network. 10G optical interfaces are pushed closer to the end users resulting in aggregation points in the network where 10-20 remote PHY devices come together.

Remote PHY network

Acacia believes that coherent can be an effective way to transport these aggregated signals back to the hub. In some cases, it may only be necessary to transport a single coherent wavelength back to the hub, but since coherent is inherently a DWDM technology, this approach could provide the capability to expand capacity by up to two orders of magnitude in the future.

Come See Us
If you are attending the Cable-Tec Expo 2019, we’d love to catch up with you while we’re there. Follow this link to set up a meeting and we’ll get back in touch ASAP. We hope to see you in New Orleans!

Is there a better place to spend the first week in August than nestled in the beautiful Rocky Mountains in Colorado? Acacia will be heading to the CableLabs Summer 2019 Conference in Keystone, Colorado from August 5-6^th. This year’s agenda is jam-packed with innovation showcases, powerful keynotes, expert panels, fireside chats and plenty of networking opportunities for professionals from all corners of the cable industry. According to CableLabs, this is the one place where the cable industry can go to get inspired—or inspire others—to build the future we can all be proud of faster, together.

We are excited to have Tom Williams, Acacia’s Vice President of Marketing, take the main stage on Day 1 of the show in a panel titled “Coherent Optics in the Access: Are we there yet?” Tom will be joined by executives from CableLabs and Comcast to discuss the current state of coherent optics in the industry and how they can be leveraged by cable companies to achieve faster and more scalable networks. Here’s a link to his panel, which will be at 11 am.

Coherent Technology for Cable Access
One trend that we expect to be widely discussed at this year’s Cable Labs Summer Conference is the belief that access networks are an emerging opportunity for coherent interconnections. The cable industry has been driving standardization of coherent for access aggregation in this segment.

As the hybrid fiber-coax (HFC) networks evolve toward remote PHY architectures, fiber is being deployed deeper in the network, resulting in increased available bandwidth to residential end-user customers, while eliminating bottlenecks in the HFC network. 10G optical interfaces are pushed closer to the end users resulting in aggregation points in the network where 10-20 remote PHY devices come together.

Remote PHY network

Acacia believes that coherent can be an effective way to transport these aggregated signals back to the hub. In some cases, it may only be necessary to transport a single coherent wavelength back to the hub, but since coherent is inherently a DWDM technology, this approach could provide the capability to expand capacity by up to two orders of magnitude in the future.

As the standards organization of the cable industry, we applaud CableLabs for their ongoing efforts in this area. In 2017, CableLabs kicked off a project to define coherent standards for cable access aggregation applications. In July 2018, CableLabs publicly unveiled for the first time, the P2P Coherent Optics Architecture Specification and the P2P Coherent Optics Physical Layer v1.0 Specification outlined below. These specifications are part of the Point-to-Point Coherent Optics family of specifications developed by CableLabs, which was further enhanced in March 2019, with the P2P Coherent Optics Physical Layer v2.0 Specification, and in June 2019, with the Coherent Optics Termination Device OSSI Specification. These specifications enable the development of interoperable transceivers using coherent optical technology over point-to-point links. This specification was developed by CableLabs for the benefit of the cable industry and includes contributions by operators and manufacturers from North and South America, Europe, Asia, and other regions.

• P2P Coherent Optics Architecture Specification – This specification provides background information regarding coherent optics technology, and how it can be used in cable access networks. More specifically, it was designed to accomplish the following:
  • Identify use cases of where operators can use P2P Coherent Optics in the access network;
  • Identify and document the common network requirements for the different use cases;
  • Identify and document through use cases the Hosts that could incorporate P2P; Coherent Optics components or modules
  • Identify and document where P2P Coherent Optics benefits each use case; and Communicate the architectural foundation on which the other P2P Coherent Optics specification depend.

• P2P Coherent Optics Physical Layer v1.0 Specification – This specification defines the optical physical layer requirements for coherent optical transceivers operating at 100 gigabits per second (Gbps). It was designed and optimized to support fiber links up to approximately 40 km. It will also support links of 80 km, and can support links up to 120 km in some circumstances.
• P2P Coherent Optics Physical Layer 2.0 Specification – This specification defines the optical physical layer requirements for coherent optical transceivers operating at 200 gigabits per second (Gbps). It was designed and optimized to support fiber links up to approximately 40 km. It will also support links of 80 km, and can support links up to 120 km in some circumstances.
• Coherent Optics Termination Device OSSI Specification – This specification defines the Operations Support System Interface (OSSI) requirements for the Coherent Optics Termination Device (CTD), which includes a transceiver that terminates one end of a P2P coherent optics link. The scope includes requirements for managing the Point-to-Point Coherent Optics Transceiver in the CTD independent of the pluggable form factors in which the transceiver may be deployed (which defines the mechanism by which information is shared between the Point-to-Point Coherent Optics Transceiver and the CTD).

Acacia has been proud to participate in this project, along with many other optical networking vendors and several large MSOs.

Come See Us
If you are attending the CableLabs Summer 2019 Conference, we’d love to catch up with you while we’re there. Follow this link to set up a meeting and we’ll get back in touch ASAP. We hope to see you in Colorado!

A Brief History

Coherent optical technology was first introduced in long haul applications to overcome fiber impairments that required complex compensation techniques when using direct detection receivers. Leveraging advanced CMOS processing nodes and reduction in design complexity, coherent solutions have moved from long haul to metro and even shorter reach optical interfaces.

With the introduction of Acacia’s CFP-DCO module in 2014, coherent became an even more compelling solution for metro and data center interconnect (DCI) applications due to its pluggable pay-as-you-grow benefits, and integrated DSP design. Today, coherent is moving from metro core to access aggregation networks (Figure 1). Looking forward, the industry is working to standardize coherent solutions for even shorter reach interfaces.

Figure 1. Coherent solutions transitioning to shorter reaches.

This model of new technology adoption in long haul interfaces, followed by a migration to shorter reach applications, has been demonstrated in the industry before: the copper-to-fiber optics transition followed a very similar path starting in the 1980’s. Technologies, such as dense wavelength division multiplexing (DWDM) and forward error correction (FEC), also followed this same pattern. It is anticipated that this trend of shorter reach applications benefiting from initial long-haul technology investment as it applies to coherent will be no different.

Industry organizations such as the Optical Internetworking Forum (OIF), IEEE, and Cable Labs have initiated coherent standardization activities, recognizing the trend towards using coherent for shorter distances. The OIF is defining a coherent standard for DWDM interfaces in DCI applications with reaches up to 120km; CableLabs is defining coherent standards for cable access networks; and IEEE is considering coherent for unamplified applications beyond 10km. All of this standardization activity reinforces the view of coherent moving to shorter reach, high volume applications. As the applications move from 100G to 400G and beyond, it is likely that coherent will be used in even shorter reach interfaces.

Demand for Bandwidth is Driving New Coherent Markets

Today, coherent technology is already being deployed into markets with a wide array of applications ranging from 10’s of kilometers to 1000’s of kilometers. While network operators in each of these markets need to manage network expansion at the lowest cost, differences in network architectures and demands drive a unique set of priorities for each. Some of these key market applications we’ll explore in this blog include: Long Haul, Metro, DCI, Remote PHY Cable Access, 5G, and applications with unamplified interfaces. We’ll discuss these applications and the factors that drive solutions for their optical interconnecting requirements.

100G Long Haul: Many Fiber Spans with Optical Amplifiers

The long haul market, where coherent was first widely adopted, is still a large segment of the coherent ports shipped each year.

Figure 2. Typical Long Haul network.

This market is highly sensitive to performance because longer reach interfaces eliminate the need for additional costly regeneration. A key optical interconnection performance parameter for coherent DWDM long haul networks is optical signal-to-noise ratio (OSNR).

OSNR & Optical Amplifier Refresher

OSNR describes the relative energy of the signal carrying the information to the energy of the noise from other sources. Optical receivers are specified based on their ability to detect the desired signal in the presence of noise.

So, why is OSNR so critical to long haul DWDM networks? Optical signals are amplified as they are transmitted across the network. At each amplifier, the signal is degraded slightly–the level of the noise is amplified relative to the signal. When the signal level is so close to the noise that it is nearly un-detectable, it needs to be regenerated—the optical signal is converted to the electrical domain where the data can be accurately decoded. The same data is then recoded, retimed, and converted back to the optical domain with a high signal to noise ratio capable of passing through additional amplifiers.

Higher performance coherent optical interfaces allow longer reaches without needing regeneration, which ultimately lowers the cost of deploying and managing these networks.

Currently, 100G QPSK modulation is still widely used in long haul networks because it offers exceptional performance due to achievable OSNR margins and maturity of technology based on 25G electronics. Fiber capacity can be further enhanced by reducing channel spacing from 50GHz to 37.5GHz. Alternatively, capacity can also be increased over the channel by increasing the baud rate. For example, doubling to 200G capacity can be achieved by maintaining QPSK modulation while doubling the baud rate. However, this would require an increase to the channel spacing.  To ensure the required channel spacing remains unaltered (e.g., 50GHz), the modulation order also needs to be increased (e.g., from QPSK to 8QAM) as the baud rate is increased.

Simply put, there is an interdependency among baud rate, modulation order, required OSNR, and channel spacing. Transmission capacity can be increased by: (1) increasing the baud rate without changing modulation order, requiring wider channel spacing; (2) holding baud rate constant while moving to a higher modulation order, resulting in narrower channel spacing at the cost of OSNR margin—higher modulation order requires higher OSNR margin; or (3) increasing baud rate and moving to a higher modulation order, requiring no changes to channel spacing but assumes there is sufficient OSNR margin when moving to the higher modulation order. Thus to upgrade to higher capacity with minimal changes to the line system optics, (3) is an attractive option assuming there is sufficient OSNR margin.

Metro: High Density Solutions for ROADM Networks

Coherent interconnections have also been widely adopted in metro core networks (Figure 3). The dynamics in these networks are somewhat different from long haul. Metro core networks usually consist of many reconfigurable optical add-drop multiplexer (ROADM) nodes where wavelengths are dropped, added, or routed to other destinations. This is done using wavelength selective switches (WSSs)–each WSS is effectively an optical filter that can narrow the total bandwidth of the channel. Passing through multiple ROADM nodes can result in significant narrowing of the available spectrum of the complete optical path. These WSSs can introduce additional impairments, such as polarization dependent loss. Though the reaches in these applications are less than long haul, they still require a high level of performance due to the many link impairments.

Metro networks are not entirely about optical performance, though. Central offices can often be crowded and power limited. High density solutions that are power efficient can be particularly valuable in these applications.

Figure 3. Typical Metro network.

In addition, metro networks handle a wide range of traffic. These networks quickly become complex to manage, with different equipment required depending on the requirements. Common solutions that can be leveraged across multiple platforms, supporting a wide variety of traffic types, and scale capacity in a cost effective manner allow network operators to manage their operational costs more effectively.

Data Center Interconnect: Cost and Power Optimized Coherent

In the last 10 years, the optical networking industry has been transformed by the requirements of large cloud network operators. High capacity interconnections are necessary between these hyperscale data centers to enable cloud functionality (Figure 4).

Figure 4. Typical DCI/Cloud network.

These DCI connections differ from traditional carrier networks in that they are usually point-to-point with no ROADMs in between, and can be within the same city or across oceans. Cost and power are generally the most critical parameters for these applications. Some use cases can be fiber constrained, making spectral efficiency a higher priority. Customers in these markets tend to be early adopters of new technology and have short product life cycles.

Large cloud network operators, along with some of the more traditional carriers, are driving changes in how network functions are partitioned between vendors, allowing them greater freedom to transition between vendors and product generations with minimal changes in the software used to control the network.

Remote PHY/Fiber Deeper: Coherent Technology for Cable Access

Access networks are an emerging opportunity for coherent interconnections. The cable industry is taking the lead in this segment by driving standardization of coherent for access aggregation.

Figure 5. Remote PHY network.

As the hybrid fiber-coax (HFC) networks evolve toward remote PHY architectures, fiber is being deployed deeper in the network (Figure 5), resulting in increased available bandwidth to residential end-user customers, while eliminating bottlenecks in the HFC network. 10G optical interfaces are pushed closer to the end users resulting in aggregation points in the network where 10-20 remote PHY devices come together.

Coherent can be an effective way to transport these aggregated signals back to the hub. In some cases, it may only be necessary to transport a single coherent wavelength back to the hub, but since coherent is inherently a DWDM technology, this approach provides the capability to expand capacity by up to two orders of magnitude in the future.

CableLabs is the standards organization of the cable industry and has recognized the need for this solution in the market. In 2017, they kicked off a project to define coherent standards for cable access aggregation applications. Acacia is participating in this project, along with many other leading optical networking vendors, as well as several large MSOs.

5G Drives Backhaul Growth

Another emerging access application is 5G backhaul (Figure 6). It is clear that backhaul demands in wireless networks are going to need to increase significantly. As more capacity is delivered to end users, the connections back to the core network must scale, as well.

Coherent offers several benefits in these access aggregation applications compared to traditional direct detect solutions. At higher data rates, it becomes very challenging to deploy direct detect solutions over 10’s of km’s without using dispersion compensation. Alternatively, solutions may consider many parallel optical interfaces, but that drives up the cost of the solution.

Figure 6. 5G Backhaul network.

Today’s coherent implementations are generally based on tunable laser technology. While fixed laser implementations are a consideration in these applications, tunable solutions can offer operational benefits by significantly reducing the number of spares that need to be stocked. Tunable solutions also tend to support shorter lead times, accelerating the ability to turn up new services. Lastly, coherent solutions are future proof with the ability to scale capacity by increasing data rate or adding additional wavelengths.

Adoption of coherent technology in access networks could offer an additional benefit that may not be obvious at first. Since the same solutions can address a wide range of network interfaces (e.g., access aggregation, metro, and regional), it may be possible to collapse the supply chain for multiple applications into a single solution. This could offer significant operational efficiencies for network operators.

Unamplified Point-to-Point Interfaces

Unamplified point-to-point interfaces are essentially client optical interfaces for connections between buildings (Figure 7). As data rates have increased, it has been more and more challenging for direct detect solutions to address these kinds of applications. At 100G and 200G, proprietary coherent solutions are already used for links in the 40-80km range.

Figure 7. Point-to-point.

Looking forward to 400G, this application is within the scope of the OIF 400ZR project. In addition, the IEEE study group that is considering solutions beyond 10km for data rates of 50G, 100G, 200G, and 400G is evaluating coherent alternatives for these applications.

Since these interconnections are not amplified, they are not characterized by their tolerance to low OSNR. In these applications, transmitter power and receiver sensitivity are the key parameters that define the usable link budget.

Coherent detection offers the same increase in performance for power limited sensitivity as it does for noise limited applications. Volumes for these applications can be larger than transport applications and coherent implementations will need to be cost effective and extremely power efficient.

Coherent is moving to shorter reach as data rates increase

As we’ve outlined in this blog, there are a number of applications for which coherent is well suited. DCI, metro, and long haul are existing markets that have benefited from coherent for several years now. Emerging applications such as Remote PHY for cable access, 5G backhaul, and unamplified “ZR” interfaces are evolving as standards efforts and deployment strategies are still in the early stages. What is clear is that operators struggling to meet the growing demands of bandwidth are motivated to optimize their optical networks for capacity and reach in order to minimize cost. And space and power restrictions continue to be a challenge as additional hardware is deployed in constrained environments. Standardization will help advance the coherent evolution underway.

Stay tuned for upcoming blog posts in which we will focus on how advanced 3D shaping of coherent modulation can optimize various types of coherent networks.

Silicon photonics can enable reduced development time, higher levels of integration, and fewer manual assembly steps than more traditional optics. The result? A powerful, easier-to-manage product that empowers cloud, content, and service providers to stay ahead of increases in network capacity demand.

Proven technology

Silicon-based photonic integrated circuits (PICs), which integrate all the high-speed optics necessary for both transmit and receive functionality, enable the density required for pluggable coherent modules. These PICs include the optical polarization-controlling functions that may require external components when integrating using Indium Phosphide (InP). By reducing the number of active alignment steps, SiPh-based products improve yield and ramp more efficiently.

While metro applications served as the primary market for SiPh’s initial service provider network implementations, SiPh is used in long-haul — even submarine — applications today. The submarine market has always required high-performance technology, even if it came at higher price points. Combining SiPh with high-performance digital signal processing (DSP) technology enables submarine-network performance equal to or better than the more expensive, discrete component-based approaches (Figure 1).

Figure 1. The coherent SiPh PIC reduces cost and size versus the use of more expensive, discrete components.

That said, InP technology remains important for laser functionality. But separating that laser function from the high-speed optics can produce several benefits. For example, InP generally requires thermo-electric coolers (TECs) to maintain tight temperature stability. SiPh, on the other hand, can operate over a wide temperature range with no impact on performance. The high-speed interface is optimized by putting the optics close to the DSP and moving the laser further from the DSP, where the TEC doesn’t have to work as hard to maintain constant chip temperature.

Silicon photonics provides further benefits

Using SiPh in coherent applications creates products with rich feature sets that offer high density and pluggability.

There are a number of reasons why SiPh has proven well suited for many applications, including coherent optics:

• Yield: Beyond improving manufacturing costs, high yield reduces development time by limiting the number of variables during prototyping. When working with low-yield optics technologies, it is difficult to determine if performance limitations derive from design defects or process variation. At higher levels of integration, this uncertainty is compounded. When developing complex PICs with many integrated functions, it is imperative to know that the individual building blocks are well understood and repeatable. These attributes enable the designers to focus on optimizing the interfaces between each function.
• Polarization Control: Coherent transmission increases the data rate via polarization multiplexing – two orthogonal polarizations are transmitted simultaneously at the same wavelength. This approach requires transmit and receive components that can manipulate the polarization state of an optical signal. When working with InP, polarization control is usually done using external components. Not only do these extra components increase material cost, they also add extra alignment steps that integration of these polarization control functions in the SiPh PIC can eliminate.
• Thermal Operating Range: As mentioned previously, InP components are sensitive to temperature variation and must be mounted on a TEC. Since TECs have a limited control range, they fundamentally limit the operating temperature range of InP components. In addition, TECs consume significant power in cooling mode, where the thermal design is most challenging. By comparison, the optical characteristics of SiPh vary little over temperature. SiPh doesn’t require a TEC and supports a wide operating temperature range.
• Humidity: Traditional optics degrade in high-moisture environments. For this reason, optics are packaged in vacuum-sealed gold boxes. These hermetic gold boxes contribute significantly to the cost of optical interconnects, particularly when they require high-speed interfaces. In addition, hermetic seals are historically one of the most common sources of failure for optics. Silicon is well known to be insensitive to humidity. Millions of silicon electronic components are shipped every year in non-hermetic plastic packages; moving optics to non-hermetic packaging is an important step for the industry.
• Wafer Level Testing: In addition to having higher yields than traditional optics materials, SiPh can also be tested at the wafer level. Good die can be identified early in the process, and there is no labor wasted on material that will ultimately fail. Wafer-level test is commonplace in high-volume electronics applications, but new to the world of optics.
• Wafer Size: Leveraging mature silicon process technologies means that much larger wafers can be made in silicon than traditional optics materials. Three-inch wafers are state of the art for InP fabs. Today’s SiPh runs on lines that accommodate 8-inch wafers or larger. These larger wafers result in an order of magnitude more die per wafer, which lowers cost.
• Package Level Integration: As the industry continues to move toward higher data rates and lower power, the interface between the DSP and optics is quickly becoming a bottleneck. Every time a high-speed signal needs to transition across an additional interface (IC package or pluggable connector) there is loss and distortion. Compensating for this additional loss adds power dissipation, and distortion limits performance. Using SiPh enables package-level integration that can better optimize these high-speed interfaces and accelerate the realization of higher data rates at lower power.

Leveraging silicon photonics

Understanding SiPh’s benefits, how do we best use them to drive innovation? Today’s optics architecture is optimized for client interfaces in which the laser is directly modulated. This model is easily extrapolated to external modulation when the modulator technology has the same thermal and packaging limitations as the laser. Thermally sensitive components that need a TEC to maintain a constant temperature are unlikely to be integrated with a DSP chip that also dissipates power.

On the other hand, when working with SiPh, designers can optimize the high-speed interface and separate the thermally sensitive laser. For example, the laser can be placed on another part of the line card and connected to the high-speed optics through an optical fiber. This architecture enables greater thermal flexibility, a high-speed signal path with superior signal integrity, and elimination of costly hermetic packages with high-speed interfaces (see Figure 2).

Figure 2. High-speed electro-optical package integration.

SiPh and coherent are two technologies shifting the landscape of optical communications in parallel. By moving to architectures that can optimize the benefits of each, it can be possible to have the same kind of impact on access networks as we have already seen in applications from the metro core through to submarine. Using a toolbox that includes SiPh and coherent DSP technology, designers can leverage complicated modulation formats, high baud rate, and highly integrated parallel optics to optimize designs for a wide range of applications.

Ball Grid Array Packaging Technology

The transition to low-cost packaging and standard interfacing is an important next step to further the benefits of SiPh technology. As the industry moves toward 600-Gbps capacity per wavelength using higher baud rate and higher order modulation formats, traditional packaging technology can limit performance of the interface between the DSP and optics. Ball grid array (BGA) packages address this challenge by eliminating additional connectors and optical package leads, improving bandwidth and signal integrity.

Here and now

SiPh is no longer a technology of the future. Coherent modules based on highly integrated SiPh PIC technology have been deployed in applications ranging from data center interconnects to submarines. In the next phase of maturity, the industry is learning to understand how to best leverage the benefits of SiPh to achieve the pace of innovation necessary for optical networking to meet the worldwide data traffic demands that such applications as cloud computing, 5G, and the Internet of Things will drive.

Tom Williams is Senior Director of marketing at Acacia Communications. Before joining Acacia, Williams spent 14 years at Finisar Corp. (initially with Optium, which Finisar acquired in 2008), where he was director of product line management for coherent and direct detect transport products above 100 Gbps. He has also held positions at Lucent Technologies and Northrop Grumman Corp. He has an MS in electrical engineering from Johns Hopkins University and BS degrees in electrical engineering and physics from Widener University.