As an example, Acacia’s high-performance Pico DSP-based 1.2 Terabit solution is currently deployed in well over one hundred networks around the globe and has been adopted by three of the four largest hyperscalers. In 2020 alone, Acacia shipped more than 30,000 Pico-based ports as customers increasingly recognized the competitive benefits that high-performance, flexible coherent transmission solutions can provide.

Network Transmission Flexibility Benefits

Multi-haul coherent solutions like the Pico DSP-based 1.2 Terabit solution are software configurable transponder modules that provide various transmission capacities and reaches. By varying the modulation order and baud rate settings, it can provide flexible options for service providers.

A multi-haul coherent solution addresses a range of network applications.

Balancing Modulation Order and Baud Rate

A common method of increasing throughput of a coherent channel is to increase the modulation order. However, this may result in a reduction in reach due to reduced optical signal to noise ratio (OSNR) tolerance for the higher modulation orders. Alternatively, the baud rate could be increased while maintaining a lower modulation order which provides additional capacity per channel with minimal sacrifice to reach.

Maximizing the channel capacity using continuously tunable baud rate can convert unused spectrum into usable capacity, with the goal to fill up the available channel bandwidth. However, as discussed in this whitepaper, increasing baud rate provides minimal improvements in fiber capacity once the transmission is well-matched to the channel.

Increasing transmission baud rate proportionally increases the transmission spectrum and can be used to convert unused spectrum into useable extra capacity.

With transmission flexibility, service providers can configure their client traffic with the following flexible options:

• 12×100 GbE or 3×400 GbE with 64 QAM modulation for DCI edge applications
• 8×100 GbE or 2×400 GbE with 16 QAM modulation for metro/regional and long haul
• 4×100 GbE or 1×400 GbE QPSK for the most challenging terrestrial and submarine networks

To ensure a smooth migration from 100GbE to 400GbE, it’s important to have a solution that can efficiently transport either type of traffic, or a combination of both, without restrictions on performance and functionality.

Increase Performance and Reach with 400GbE Long Haul

With 400GbE becoming the “common currency” for high-capacity Ethernet transmission, it’s important to have a solution that can support this traffic over long distances. For service providers supporting 400GbE traffic they can use Acacia’s Pico DSP-based solution and choose from various configurations, as previously mentioned, including the option to combine two 400GbE client signals into an 800G 150 GHz channel for transmission over their metro and long-haul networks.

Leveraging Acacia’s Pico DSP-based solution service providers can combine two 400GbE client signals into an 800G 150GHz channel for long haul transmission.

Migrating from 100G to Higher Speeds Just Got Easier

Multi-haul coherent solutions enable network operators to easily migrate from 100G traffic toward 400G and higher speeds to deploy new and exciting applications and services. Networks utilizing flexible coherent transmission can provide support for growing client traffic across the entire network—from DCI edge, metro, long-haul, and all the way to submarine. Learn more in this video.

Acacia is gearing up for a great show at ECOC 2019 – and Dublin is a great place to host Europe’s largest optical communications event. We look forward to having another opportunity to discuss the latest trends and innovations with our customers and partners including multi-haul applications, 400GbE, 400ZR and open optical standards.

Network operators are looking to support today’s 100GbE clients, as well as emerging 400GbE clients, across key network segments such as DCI edge, metro, long-haul and submarine in an efficient, scalable, and cost-effective manner. Multi-haul coherent solutions, software configurable transponders that provide various transmission capacities and reaches by varying the modulation order and baud rate settings, can provide flexible options for network operators.

Introducing the AC1200-SC² Coherent 1.2T Single-Chip, Single-Channel Module

One such solution is the AC1200-SC² (‘SC squared’). Powered by Acacia’s 1.2T Pico DSP chip, the AC1200-SC² coherent 1.2T single-chip, single-channel module was designed to enable network operators to support today’s 100GbE clients, as well as emerging 400GbE clients.

Figure 1: Acacia’s AC1200-SC² Coherent 1.2T Single-Chip, Single-Channel Module

We are excited to demonstrate our newest AC1200 family member just announced last week. The AC1200-SC²’s high-performance and flexibility make it ideally suited for multi-haul applications ranging from high-capacity 1.2T DCI edge to the most challenging terrestrial and submarine networks that require 400G transport over QPSK modulation. The module also features Acacia’s 3D shaping technology designed to optimize fiber capacity and reach by filling gaps in margin and spectrum. Check out this video to learn how 3D shaping allows for the fine-tune adjustment of the modulation order and baud rate to provide network operators with the ability to adapt the transmission characteristics to meet the requirements of both greenfield and brownfield deployments.

Standardized Coherent Interconnects

Standardization activities have defined a variety of interoperability modes supporting operation ranging from 100G to 400G. Recently there have been many opinions on the idea of ZR+. What if a mode was created to combine the benefits of elements from 400ZR and OpenROADM, two existing standards, to define and deliver an interoperable ZR+ mode called OpenZR+?

The result is an open, flexible and interoperable coherent solution in a small form factor pluggable module. By simply defining a data path that includes the appropriate functionality, an interoperable OpenZR+ mode can be established. These enhanced modes will allow an OpenZR+ module in a QSFP-DD or OSFP form factor to support reaches well beyond 400ZR. The OpenZR+ modes are supported by merchant DSP vendors who have exchanged test vectors to ensure the interoperability of these OpenZR+ implementations. The availability of merchant DSP solutions supporting OpenZR+ will further expand the ecosystem of module vendors supporting OpenZR+. We’re sure to hear more about OpenZR+ at ECOC. For a more detailed explanation of OpenZR+ and its growing momentum please refer to the Optical Connections Magazine contributed article titled “OpenZR+ Offers Performance and Interoperability.”

Acacia Thought Leaders Speaking at ECOC

We are proud to have our Acacia experts sharing their views about some of the topics mentioned above.

• Tom Williams, Acacia’s Vice President of Marketing, is speaking on a Market Focus Panel titled “Next Generation Coherent – Beyond Transport Networks.” Tom’s panel is scheduled for Tuesday, September 24^th at 13:15.
• Hongbin Zhang, Acacia’s Principal DSP Designer, is speaking on a panel titled “Real-Time Transmission of Single-Carrier 400 Gb/S And 600 Gb/S 64QAM Over 200km-Span Link, scheduled for Tuesday, September 24^th at 14:45.

If We are “Lucky,” We’ll See you there!

If you are attending ECOC 2019, we’d love to see you. To set up a meeting at the show, contact us.

Coherent transmission w/ varying baud rates and modulation modes applicable to multiple types of networks has been referred to as multi-haul, as described in the OFC 2018 Show Report. A ROADM-rich network design is typically attributed to traditional metro networks. However, with the rise of multi-haul capable coherent technology, the lines between what is considered metro and long haul are sometimes blurred. A previous blog post provided a brief tutorial of coherent optical communications and how it is applied to long-haul, DCI, and metro networks. I encourage you to read that post as it provides a good background for this blog post.

A benefit of coherent optical transmission is that the bandwidth capacity of a link can be increased by moving to a higher modulation order (e.g, from 2-bits/symbol QPSK to 4-bits/symbol 16QAM), as long as there is sufficient optical signal-to-noise ratio (OSNR) margin to overcome the resulting penalty. Acacia’s patented Fractional QAM (F-QAM) provides a higher level of granularity compared to traditional quantized integer-bits/symbol modulation orders, by enabling non-integer bits-per-symbol (e.g, 3.3 bits/symbol) modulation, to better optimize link capacity. Another adjustable “knob” to increase capacity is the transmission baud rate, which directly varies the spectral width of the signal. Similar to traditional quantized modulation orders, coherent technology has previously implemented quantized baud rates. However, these quantized baud rates may result in sub-optimal use of the available channel bandwidth—that is, the spectral width of the transmission does not fill up the channel’s available passband. Adaptive Baud Rate provides the granularity to enable increased optimization of the available passband. F-QAM and Adaptive Baud Rate are elements of the Acacia AC1200 coherent transponder module’s 3D shaping capability.

In a multi-haul network where optical transmission between end points may encounter numerous cascaded optical filters, one challenge is to spectrally optimize the optical transmission to fit within the aggregate passband of these filters from either fixed or reconfigurable add/drops of the network’s line system, as shown in Figure 1.

Figure 1. Spectrally quantized transmission may leave spectral gaps in aggregate passband.

As previously mentioned, quantized baud rates may not allow enough flexibility and granularity to fill up the passband. However, by using Adaptive Baud Rate, the capacity can be increased to more closely match the available spectrum within the aggregate passband of the cascaded filters with fine granularity, as shown in Figure 2.

Figure 2. Acacia’s Adaptive Baud Rate can optimize the spectral transmission to more closely match the available aggregate passband spectrum.

The aggregate passband of the cascaded filters contributes to the upper bound limit of capacity increase one can achieve in a multi-haul network optical link. In this case, I am not referring to the theoretical Shannon Limit. Rather, I am referring to the practical passband constraints that come from the implementation in a network of cascaded imperfect optical filter passbands due to variations of the filter conditions. Variations may become more prevalent if the optical transmission passes through a multi-vendor line system environment, a potential situation in a disaggregated network architecture. Having the ability to vary modulation and baud rate allows for maximal flexibility in optimizing the transmission to more closely match the line system’s available passband, as opposed to matching the line system to the terminal equipment’s optical characteristics.

As previously mentioned, Acacia’s 3D Shaping capability, as illustrated in Figure 3, enables the “dialing-in” of both modulation mode and baud rate.

Figure 3. Acacia’s 3D shaping capability enables optimization of link capacity and reach; shaping of spectral width is achieved using Adaptive Baud Rate. 

This capability equates to the ability of the optical transmission spectrally “molding itself” to the line system’s passband on a link-by-link basis. By using 3D Shaping, to a certain extent the coherent DWDM source can be decoupled from the line system since the optical transmission is optimized regardless of the pass band characteristics of the line system. This capability lends itself nicely to the disaggregation of terminal equipment and line systems.

Whether multi-haul networks use flexible ROADM architectures with flexible passbands or architectures with fixed grid spacing (50GHz, 75GHz, 100GHz), the AC1200 with 3D Shaping can be used to optimize capacity with any of these type of line systems.

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.