The longest distance ever reported at single carrier 600G DWDM transmission on an SDM cable
The Amitié submarine cable features Space Division Multiplexing (SDM) technology with 16 fiber pairs, more than traditional subsea cables, with repeater power shared across the fiber pairs to deliver the highest cable capacity. This real-time field trial exceeded any industry trial performance to date with Dense Wavelength Division Multiplexing (DWDM) 800G in a 150GHz channel spacing, equivalent to a spectrum efficiency of 5.33bit/s/Hz and a maximum spectral efficiency of 5.6bit/s/Hz. In addition, 600G was transmitted over 12,469 kilometers for a trans-Atlantic loopback configuration. This is the first time a 140Gbaud single carrier signal was demonstrated live, and is the longest distance ever reported at single carrier 600G DWDM transmission on an SDM cable.

According to Jamie Gaudette, GM of Cloud Network Engineering, Microsoft, “The transmission of 800G over 6,234 kilometers is a milestone that demonstrates SDM cables can deliver increased capacity over traditional subsea cables. This field trial demonstrates what is now a commercial technology for subsea routes, and we can improve the network capacity to help drive digital transformation for people, organizations, and industries around the world.”

“In the era of AI, reliable and fast network connections are more important than ever,” said Bill Gartner, SVP Optical Systems and Optics, Cisco. “Working with Microsoft on the Amitié cable to demonstrate the potential for improved overall network capacity with 800G at these distances is a significant milestone for an SDM cable, and we’re proud to drive the innovations that pave the way for ever increasing network capacity needs.”

Trial Leverages the Latest in Terabit Optics
According to Microsoft and Cisco, this trial was conducted to target improvements in subsea transmission to provide increased performance and capacity. It was performed with the Cisco NCS 1014 platform enabled by Acacia’s CIM 8, which is powered by Acacia’s Jannu digital signal processor and advanced silicon photonics.

CIM 8 features 2^nd Generation 3D Shaping that provides a 20% higher spectral efficiency over the previous generation. CIM 8 also includes advanced non-linear equalization capabilities, an important feature for submarine links. Acacia has been a leader in deployed submarine-capable silicon photonics for almost a decade.

With this recent subsea trial, Microsoft is the latest provider to announce successful transmission with the CIM 8. Previously, China Mobile, NYSERNet, Verizon, and Windstream Wholesale announced their own trials.

Join the Terabit Era
If you’re ready to join the Terabit Era with CIM 8, contact us.

1.2T Delivering Maximum Capacity, Reach & Fiber Efficiency

Acacia, now part of Cisco, is once again introducing a ground-breaking coherent solution with the industry’s first 1.2 terabits per second (1.2T) faceplate pluggable coherent module, highlighting a new 8th generation Coherent Interconnect Module family, powered by Acacia’s Jannu 5nm CMOS digital signal processor (DSP) ASIC. This solution delivers industry-leading performance with single carrier 1.2T operation and combines the Jannu DSP with 3D Siliconization packaging technology which includes the silicon photonics integrated circuit (SiPh PIC), high-speed modulator driver and transimpedance amplifier (TIA) in a single optoelectronic package.

The Jannu DSP is designed to offer network operators the ability to maximize transmission data rate across a wide range of multi-haul network applications including DCI, metro, long-haul and subsea. “By leveraging the latest 5nm CMOS process node and our advanced signal processing algorithms, we’re delivering exceptional performance with less than half the power per bit of competing solutions, enabling a pluggable deployment model,” says Christian Rasmussen, Sr. Director, DSP and Optics Engineering and Founder at Acacia.

The Industry’s Next Class of Coherent Interconnect

The architecture of the new Coherent Interconnect Module 8 (CIM 8) solution builds upon the success of the Acacia AC1200 product family and aligns closely with the latest client data rates and coherent industry standardization efforts. The CIM 8 is designed to enable network operators to double their transmission capacity over even greater reaches. For links requiring maximum capacity, the new module can provide 1.2T capacity over a single wavelength.

“Historically, Acacia has been a groundbreaker in advancing coherent technology, and the company continues to demonstrate a long-term vision for how multi-haul coherent optics should evolve over time,” said Dr. Scott Wilkinson, Lead Analyst for Optical Components at Cignal AI. “Acacia’s new Coherent Interconnect Module is a great example of market leadership. This solution is a logical next step following the widely adopted Pico DSP-based products and will provide pluggability that is attractive to both traditional and cloud operators.”

The below figure illustrates how coherent technology has evolved in response to growing bandwidth demands. Different baud-rate classes are grouped based on technological capabilities, industry standardization (such as OIF, IEEE, Open ROADM, OpenZR+ MSA, CableLabs, ITU) and common industry investments. As the below figure depicts, the next class of coherent optical products, Class 3, enables a doubling of baud rates from the current Class 2 technology. Class 2 products include both multi-haul embedded modules and 400G faceplate pluggables where the latter was driven by standardization efforts that drove heavy investments into products centered around 16QAM, 60+Gbaud per 75GHz channel transmission.

Throughout this evolution, it has been critical that each successive class provide network architecture compatibility to the previous class, primarily reach and channel width. Acacia’s Class 3 product will support 150GHz channels with double the capacity per carrier and longer reach than that of the previous class, providing a simple, scalable path that is compatible with the previous network architecture generation.

The CIM 8 family is a Class 3 solution that can address transmission of multiple 400GbE client interfaces over virtually any network application, delivering 1.2T per carrier capacity for high-capacity DCI interfaces and 800G per carrier capacity over most optical links using 4 bits/symbol (~16QAM) modulation.

Leading in Performance and Fiber Utilization

The Jannu DSP features Acacia’s second-generation 3D Shaping technology, which leverages enhanced probabilistic constellation shaping (PCS) algorithms and Adaptive Baud Rate, a feature introduced in Acacia’s Pico DSP and widely embraced by network operators. The Jannu DSP provides customers with continuous baud rate adjustment up to 140Gbaud to optimize utilization of available spectrum in a single-span or in cascaded ROADM paths. These 3D Shaping features give service providers unprecedented transmission flexibility to match their network’s architecture, optimize fiber utilization, simplify deployment, and save on both CAPEX and OPEX.

As a single carrier design, the Jannu DSP also includes Acacia’s advanced line-rate processing algorithms to efficiently overcome fiber transmission impairments over greenfield or brownfield fiber infrastructures. These power-efficient algorithms are designed to compensate for linear and non-linear impairments, as well as provide state-of-polarization (SOP) tracking with industry leading response times. In addition, the Jannu DSP leverages Acacia’s new innovative soft-decision error correction (SD-FEC) to further enhance performance.

Scaling Optical Network Architectures

Acacia has been working towards a clear vision of how to cost-effectively scale network capacity for many years. The Jannu DSP builds on the performance and architectural benefits that have made the Pico DSP such a successful Class 2 coherent solution, but with double the baud rate and enhanced performance that supports transmission up to 1.2T in 150GHz channels. With adaptive baud rate capabilities, the CIM 8 also offers maximum capacity over any reach and channel plan.

“Today is an exciting new milestone in Acacia’s history as we once again introduce an exciting new product that delivers the type of disruption needed to cost-effectively scale network capacity in the future,” said Mehrdad Givehchi, Sr. Director of Engineering for Hardware and Software and Founder at Acacia. “With every generation of product, we have been able to deliver higher transmission data rates, lower power consumption and higher performance and I am proud to see this new product carry on that legacy.”

Crossing the Pluggable Terabit Threshold with High-Density Integration

Acacia’s 3D Siliconization is a major contributing factor in how the CIM 8 can cross the terabit capacity threshold and support faceplate pluggable optics that simplify network deployment and maintenance. 3D Siliconization applies integration and 3D stacking packaging techniques to enable a single device to include all the high-speed optoelectronic functions necessary for coherent transceivers. This device decreases footprint by including the DSP, SiPh PIC, drivers, TIAs, and is manufactured using standard CMOS packaging processes that leverage the same reliability, cost, and volume scaling advantages. With 3D Siliconization, the high-speed RF interfaces are tightly coupled together, resulting in improved signal integrity for high baud rate signals.

The high-density packaging as well as an advanced high-speed modulator design provides superior frequency response that enables the 140Gbaud performance as explained in this white paper.

Acacia Technology Leadership and Vision

Transforming industry trends into a development strategy takes the kind of vision Acacia has repeatedly shown in driving the adoption of pluggable digital coherent optical (DCO) modules based on silicon photonics. However, Acacia’s silicon photonics expertise expands beyond just pluggable coherent solutions. As shown in the above figure, Acacia has previously introduced multi-haul products in previous classes. In fact, the AC400 Class 1 product was the first commercially deployed all silicon photonics-based coherent module for submarine applications and the silicon photonics-based AC1200 was the industry’s first Class 2 multi-haul solution. The newly introduced CIM 8 continues this history of technology leadership by providing industry leading performance based on the same silicon photonics used in high volume pluggable modules. In addition, Acacia is leveraging the investments being made in next-generation standardized solutions. Aligning these developments helps Acacia to accelerate time to market for both standardized and multi-haul products and benefit from the combined scale.

Acacia was a pioneer of silicon photonics for coherent applications in 2012 when it was the first coherent module vendor to envision silicon as the platform for the integration of multiple discreet photonic functions, increasing the density and reducing cost of optical interconnect products.

“Acacia’s vision and technology leadership has enabled us to transform coherent transmission. Our pioneering role in silicon photonics has led to the introduction of industry leading coherent pluggable and multi-haul solutions,” said Benny Mikkelsen, CTO and Founder of Acacia. “With every new product introduction, we have delivered the right solution at the right time to help our customers meet their growing bandwidth demands, and this new product introduction is a culmination of this proven technology leadership.”

To learn more, visit Coherent Interconnect Module 8 page or contact us.

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.

The Pacific Telecommunications Council (PTC) annual conference is the “Pacific Rim’s premier telecommunications event,” and Acacia will have a front row seat for it this year. PTC ‘19 kicks off Sunday, January 20 and runs through Wednesday, January 23, 2019, where subsea communications will be a hot topic, as organizations are faced with the challenge to enable increased utilization of deployed fiber for improved network capacity.

Given the fact subsea is of particular importance to Acacia and its customers, we’re excited it will be a big part of the “From Pipes to Platforms” theme at the conference – and that it comes on the heels of three subsea trials Acacia collaborated on and a blog article on the topic:

• Acacia announced the first 400G Transmission Across 6,600km Trans-Atlantic Marea Cable Using the Acacia AC1200 Coherent Module
• Cisco demonstrated the NCS 1004 driving a single wavelength of 300G over a 10,000km sub-sea link
• ADVA delivered 300G over trans-Atlantic distance in FSP 3000 TeraFlex trial
• Hongbin Zhang shared Two Key Elements Needed for Future Transponders in Submarine Systems in his recent blog post which discusses power limitations of submarine systems and reducing cost per bit for both transponder and wet-plant

“Transmission of 400G over 6,600 km is a significant milestone and demonstrates what can be achieved with higher performance transmission optics combined with a well-designed line system. This field demonstration of an advanced solution that is just becoming commercially available highlights the value of Marea’s open architecture to evolve with the latest technology. By allowing early adoption of new technologies such as Acacia’s AC1200, we are able to increase the utilization of our deployed fiber, maximize our investment and improve network capacity.”

—Mark Filer, Principle Optical Engineer, Azure Networking, Microsoft Corp.

The 400G subsea field demonstration, conducted by Acacia in collaboration with Microsoft and Facebook, helped to demonstrate that improvements in capacity and spectral efficiency can enable increased utilization of deployed fiber and improved network capacity. Modulation formats greater than 4 bits/symbol were utilized to achieve a spectral efficiency of 6.41 b/s/Hz on the 6,600 km cable. This was achieved using Acacia’s suite of 3D Shaping features that enable fine-tune adjustment of the modulation characteristics to provide network operators with the flexibility to customize the transmission to their network requirements. This is especially important in a submarine network where multiple providers share the same fiber link, and are trying to optimize its capacity utilization of their assigned spectrum.

In his recent blog post on the growth of submarine cables and how the Cisco NCS 1004 fits, Cisco’s Product Manager Optical Systems & Optics Group, Sushin Suresan, talks about how spectral efficiency is the key metric to maximizing capacity on expensive submarine cable assets.

The Cisco NCS 1004 is the latest product in the DCI form-factor NCS 1000 series. It delivers multi-haul coherent DWDM transponders that provide state of the art performance for sub-sea applications using granular baud-rate + bits per symbol tuning, time-hybrid modulation, transmit signal shaping and non-linear equalization. Each 2RU form-factor NCS 1004, powered by Acacia’s Pico Digital Signal Processor chip, provides 8 coherent DWDM ports that operate from 100G to 600G.

ADVA announced that its FSP 3000 CloudConnect TeraFlex succeeded in transporting 300Gbit/s of data per wavelength over a 6,800km fiber link. ADVA and Acacia conducted the trial, which was the first in the industry to cover the trans-Atlantic distance with a fiber link typical of a cost-optimized submarine cable using 300Gbit/s channels and a commercially available transponder with real-time digital signal processing (DSP). The demo, which also achieved the highest spectral efficiency for such a link carrying 300Gbit/s per wavelength with 70GHz channel spacing, highlights the capabilities of the ADVA FSP 3000 TeraFlex solution, which incorporates the suite of advanced performance features in Acacia’s Pico coherent DSP, to support flexible, ultra-high-capacity, long-haul and subsea data transport.

Triggered by the increasing demand for data driven by cloud-based services, we’re seeing a significant expansion in long-haul and subsea cable networks. We believe these trials give a major boost not only to long-haul and submarine network operators but also to cloud-content and digital media providers looking for ways to cost-efficiently deliver more to their customers around the globe.

Attending PTC? Alan Gibbemeyer, senior director of global business development at Acacia, will be on hand at the conference along with other members of the team to discuss these developments. To schedule a meeting at PTC ’19 contact us.

Subsea communications has become a hot topic lately as demand for data-flows across the world’s oceans continue to increase. One of the challenges is how to enable increased utilization of deployed fiber for improved network capacity. This topic as well as other challenges and the future of the global submarine industry is scheduled to be discussed at the upcoming Pacific Telecommunications Council’s annual event, PTC 2019 (January, 20–23, 2019, Honolulu, HI).

In the 1980s, the first optical fiber submarine cable, TAT-8, was installed to link the United Kingdom, France, and the United States. The TAT-8 system contained two fiber pairs, and the signal on each optical fiber was modulated at 295.6 Mbps on 1310 nm.

Fast forward 30+ years, and today, most commercial coherent transponders are able to support 100 Gbps and 150 Gbps per wavelength in C-band across the transoceanic distance¹. Acacia Communications recently announced the first 400G single carrier DWDM transmission on the 6,600 km Marea submarine cable between Virginia Beach, Virginia and Bilbao, Spain². In the field demonstration, conducted by Acacia in collaboration with Microsoft and Facebook, modulation formats greater than 4 bits/symbol were utilized to achieve a spectral efficiency of 6.41 b/s/Hz on the 6,600 km cable. This was achieved using Acacia’s 3D Shaping features that allow network operators to adapt the transmission characteristics to match filter characteristics of the network. This is especially important in a submarine network where multiple providers share the same fiber link, and are trying to optimize its capacity utilization of their assigned spectrum.

Increasing cable capacity in the future is a challenging task due to fiber nonlinearity and power limitations. Two key elements important for service providers to understand in regards to the future transponders of submarine systems deployments are:

• Power limitations of submarine systems
• Reducing cost per bit for both transponder and wet-plant

Power Limitations in Submarine Systems

Compared to terrestrial optical fiber systems, the electrical power of a submarine link is supplied from power feed equipment (PFE) on the landing stations as shown in Fig.1. Normally, the PFE has 12-15 kV feed voltage and 1 Amp current, due to many practical and safety aspects. Significant electrical power is dissipated over the copper, which has 0.75 Ohm per km, between land and the undersea optical amplifiers. To make things worse, the optical amplifier has only 1.2 percent power conversion efficiency from electrical power to the output optical power³. Thus, the next large-order cable capacity increase cannot rely solely on higher SE modulation formats, which require exponentially increasing OSNR, leading to higher EDFA output power requirements. Instead, to increase the cable capacity, future submarine systems will likely rely on power efficient (PE) modulation formats and higher spatial dimensions.

Fig.1. Power Feed Equipment (PFE) Configuration to deliver power to submarine repeaters

The fiber pairs are expected to increase from six or eight pairs currently to 24 and beyond. Higher spectral bandwidth, like full C-band or C+L, may not be power efficient since gain equalization filtering (GEF) rejects 50 percent power to achieve desired flatness for a bandwidth of 35nm.

Pilipetskii⁴ showed that the most power efficient modulation scheme is at ~3 bits/s/Hz SE (similar to the SE of DP-QPSK) in a case of the ideal transponder capable of operating at the Shannon limit. The PE does not increase monotonically by decreasing the SE because optical amplifiers operate in saturation, so the power of an amplifier is constant along the system and is a sum of signal and accumulated amplified spontaneous emission (ASE) noise.

Reducing Cost per Bit

Another important argument for increasing capacity in the spatial dimension is from the economic view of the submarine system. Marine costs are fixed and account for the majority of subsea operating and installation costs, while fiber is only a small percentage of the total cost. Therefore, increasing system capacity by adding more fiber is an opportunity to improve power efficiency, while reducing the overall cost per bit. Dar summarized this idea⁵ and also found that cost-optimized SE is slightly higher than the PE-optimized SE after considering the total cost, which includes deployment, amplifier, fiber, and transponder whose cost increases with more fiber pairs.

SE between 3 bits/s/Hz and 5 bits/s/Hz, similar to DP-8QAM, could be an attractive range for future coherent transponders in submarine applications, depending on the cost model and performance margin. Unfortunately, this requirement does not align well with many hero experiments, which still push the SE closer to the nonlinear Shannon limit by employing high-performance but expensive digital nonlinearity compensation techniques or Raman amplification.

Increasing Baud Rate for Greater Capacity

Then, what is needed from future submarine transponders if not SE improvements? Since increasing SE as previously discussed may not be the most beneficial option to reduce the cost per bit, we should look at alternative options such as high baud rate transmission.

Acacia’s AC1200 coherent module utilizes high baud rates of greater than 70Gbaud enabling transmission using lower order modulation for a given data rate, which results in improved OSNR performance and greater reach. Next generation coherent submarine transponders may likely have a baud rate greater than 100Gbaud to enable even higher capacity per wavelength. Higher baud rate is especially important when designing for low cost, as discussed in the previous section. By increasing baud rate, more capacity can be added to the cable using the same amount of hardware.

Increasing the baud rate from 32Gbaud in the current 100G coherent transponder to greater than 100Gbaud requires new analog-to-digital converters (ADC) and digital-to-analog converters (DAC) techniques and advances in mixed-signal system integration. For example, one approach is to use multiple complementary metal–oxide–semiconductor (CMOS) sub-DACs and an additional analog multiplexer (AMUX) to extend the bandwidth in the analog domain⁶. Another important advancement is the integration of high-speed silicon photonic integrated circuits (PICs) with CMOS driver and TIA. To date, coherent optics have been put in hermetically sealed “gold” box packaging made of high-temperature co-fired ceramics (HTCC) with wire-bonded components inside. However, this packaging is expensive, and wire bonds add inductance which limits the bandwidth.

In the future, it is likely that we will see co-packaged optics and electronics that further integrate functionality. Acacia demonstrated co-packaged optics and electronics on a ball-grid-array package (BGA) in 2017 as shown in Fig 2⁷. Thanks to low parasitic and the reduced signal path between ASICs and PICs, this approach is particularly well suited for high baud rate applications. In addition, it is cost efficient since the fabrication can be done in a CMOS manner to enable high yield and volume.

 Fig.2 Co-packaging optics and electronics by flip-chip bonding on low-temperature co-fired ceramic (LTCC) substrate

Improving Transmission Performance with Advanced Modulation Formats

The improvement of the transmission performance of transponders enables the cost reduction of wet-plant, e.g. longer repeater span and cheaper fiber, etc. Researchers are still exploring advanced modulation schemes to improve the sensitivity performance and the tolerance to both linear and nonlinear transmission effects.

There are three categories of advanced modulation formats: (1) probabilistic shaping (PS), where the in-phase and quadrature (I-Q) points are employed with non-uniform probabilities; (2) geometric shaping (GS), in which they are positioned with non-uniform spacing; and (3) multi-dimensional coded modulation. In theory, PS has about 1 dB higher sensitivity than GS. Multi-dimensional coded modulation with iterative decoding has a performance in between the two ranges. In practice, the advantage of PS depends on the implementation of distribution matching and FEC. Multi-dimensional coded modulation has the strongest tolerance to the transmission penalty because it has a larger minimum Euclidean distance. This feature can be utilized in some nonlinear compensation schemes⁸. Interestingly, all three methods can achieve similar hero records over long-haul transmission as shown in Fig. 3.

Fig.3. Record hero experiments over transoceanic distance with PS (blue) and GS or coded modulation (red).

Future Submarine Transponders and Moore’s Law

Finally, let’s take a look at the role of future transponders from the view of Moore’s law. A decade ago, the first 40 Gbps digital coherent transponder was developed by Nortel and had about 20 million gates and 100 million transistors based on the use of 90nm CMOS technology. Fast-forward 10 years, and Acacia’s Pico digital signal processor (DSP) has about three billion transistors using 16 nm CMOS technology, a 30 times increase of transistor number. With similar power dissipation, it achieves 1.2 Tbps, also 30 times larger throughput compared with 40Gbps. In comparison, Apple’s latest iPhone has 6.9 billion transistors⁹ by using 7nm CMOS technology.

The optical coherent transponder can continue to benefit from the development of the whole semiconductor industry. Future digital coherent transponders based on 7nm and 5nm CMOS technology running even more advanced DSP algorithms and achieving higher throughput at less power consumption, may reduce the cost of both transponders and wet-plant systems.

Reference

[1] V. Kamalov et al., “Evolution from 8QAM live traffic to PS 64-QAM with Neural-Network-Based Nonlinearity Compensation on 11000 km Open Subsea Cable,” in OFC, 2018, paper Th4D.5.

[2] First 400G Transmission Across 6,600km Trans-Atlantic Marea Cable Using Acacia Communications AC1200 Coherent Module

[3] T. Frisch et al., “Electrical Power, A Potential Limit to Cable Capacity,” in SubOptics, 2013, paper Tu 1C 04.

[4 ] A. Pilipetskii et al., “The Role of SDM in Future Transoceanic Transmission System,” in ECOC, 2017, paper Tu 1E.

[5]R. Dar et al, “Cost-Optimized Submarine Cables Using Massive Spatial Parallelism,” JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 36, NO. 18, SEPTEMBER 15, 2018, PP3855-3865.

[6] H. Yamazaki et al, “160-GBd (320-Gb/s) PAM4 Transmission Using 97-GHz Bandwidth Analog Multiplexer”, IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 30, NO. 20, OCTOBER, pp1749-1751.

[7] C. Doerr et al., “Silicon Photonics Coherent Transceiver in a Ball-Grid-Array Package,” in OFC, 2017, paper Th5D.5.

[8] H. Zhang et al., “DP-16QAM Based Coded Modulation Transmission in C+L Band System at Transoceanic Distance,” in OFC, 2016, paper W1I.2.

[9] Apple A12 Bionic: 7 Billion Transistors, 5 TOPS Neural Engine & More