The long game: Acacia's coherent vision

In 2007, Christian Rasmussen made a career-defining gamble. After attending a conference featuring presentations on coherent optical transmission, he returned home, consulted his family, and quit his job at Mintera, then an optical networking equipment maker.

Christian Rasmussen

The technology he'd seen discussed promised to solve the transmission impairments associated with direct-detection-based optical transmission – chromatic dispersion and polarisation mode dispersion - that had stymied optical transport to go beyond 40 gigabits-per-second (Gbps).

"We came back and were completely excited that there was a technology that addressed all the problems that we had experienced firsthand," says Rasmussen, now Chief Technology Officer at Acacia.

His bet paid off. Acacia which he helped co-found in 2009, had a successful IPO in 2016 and would later be acquired by Cisco Systems for $4.5 billion in 2021.

Unfolding coherent optics

Increasing the baud rate has proved spectacularly successful in accommodating traffic growth in the network and reducing transport costs measured in dollar-per-bit.

In 2009, coherent modems operated around 32 gigabaud (GBd) for 100 gigabit-per-second (Gbps) wavelength transmissions. By 2024, the symbol rate has reached 200GBd, enabling 1.6 terabit-per-second (Tbps) wavelengths.

Is the priority still to keep upping the symbol rate of a single carrier when designing next-generation coherent modems?

"We are not just saying that increasing baud rate is right," says Rasmussen. The fundamental goal is reducing optical transport's cost and power consumption. "Increasing the baud rate is generally the right approach to achieve that goal but it's always to a certain degree."

Acacia's focus from the beginning has been on integrating the components that make up the coherent modem. The resulting modem need not be expensive and can deliver higher speed and extra bandwidth economically while meeting the power consumption target, he says.

"Until now, we feel that increasing the baud rate has been the right approach," says Rasmussen. "The question will be how frequently you can go up in baud rate, now that developments are expensive.”

Given the rising cost of developing coherent modems, upping the baud rate only makes sense if designers can double it with each new design, he says. Increasing the baud rate by 30 or 40 percent is too small a return, given the development effort and the costs involved.

That implies Acacia's follow-on high-end coherent modem will have a symbol rate of around 280GBd.

Acacia's coherent modules

Acacia's Coherent Interconnect Module 8 (CIM 8), launched in 2021, was the industry's first single-carrier 1.2Tbps pluggable module. The module operates at a 140GBd symbol rate.

At ECOC 2024, the company showcased its 800 gigabit ZR+ OSFP pluggable modules, featuring the Delphi coherent DSP implemented in 4nm CMOS process.

The module supports up to 131GBd and implements interoperable probabilistic constellation shaping. The Acacia module has C-band and L-band variants and supports ultra-long-haul distances when sending 400Gbps over a single carrier (see Table).

Source: Acacia

Challenges and opportunities

The path forward presents challenges and opportunities. There are several design considerations when developing a coherent DSP ASIC.

One is choosing what CMOS process to use. Considerations include cost - the smaller the geometry the more expensive the design, the transistors’ switching speed, whether the chip’s resulting power consumption is acceptable, and the CMOS process’s maturity. If the process is under development, what confidence is there that it will deliver the promised performance once the ASIC design is completed and ready for manufacturing?

The state-of-the-art CMOS process used for coherent DSPs is 3nm. Ciena’s 200GBd WaveLogic 6e is the first coherent DSP to ship using a 3nm CMOS process. Rasmussen is confident that a 3nm CMOS process can achieve at least a 250GBd symbol rate.

Another consideration is to ensure that the DSP’s analogue-to-digital converters (ADCs) and digital-to-analogue converters (DACs) can achieve the required sampling speed and quality. Typically, the ADC sample at 1.1x-1.2x the baud rate, which, for a 250GBd symbol rate, equates to the order of 300 giga-samples a second (GS/s). Achieving such speeds is exceptionally challenging.

Some research is exploring other ways to keep boosting converter sampling speed. One idea is to split the converter’s design between the DSP and a higher-bandwidth III-V material used for the driver or receiver circuitry.

Rasmussen stresses that the key is to keep the ADCs and DACs in CMOS as a part of the DSP. "Once you start going there [splitting the DAC and ADC designs], you start risking your cost and power advantage of the single-carrier approach," he says.

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Acacia timeline  

• 2007: Rasmussen attends pivotal conference on coherent transmission 
• 2009: Acacia founded; 32GBd coherent modems achieve 100Gbps 
• 2014: Acacia is first to ship samples of a coherent pluggable 100G CFP module and announced the industry’s first 100G coherent transceiver in a single silicon photonics integrated circuit package 
• 2021: Cisco acquires Acacia for $4.5 billion 
• 2021: Launch of CIM 8 (140GBd, 1.2Tbps) 
• 2024: Acacia showcases its 800ZR+ OSFP module

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Team-oriented approach

As CTO, Rasmussen emphasises the importance of working with colleagues to make decisions. "I'm very passionate about this: team-oriented decision-making," he says. His role involves extensive conversations with product managers and colleagues that interact with customers to understand market needs, alongside technical discussions and conference attendance to guide technology development.

This collaborative approach has shaped Acacia's integration strategy as well as the company becoming more vertically integrated. "Owning the whole stack so you always have everything in control," as Rasmussen puts it, has proven crucial to their success.

From Denmark to Cisco

Rasmussen's journey began in Denmark, where he completed his electrical engineering degree and doctorate in optical communications before moving to Boston. There, he joined Benny Mikkelsen, now Acacia's senior vice president and general manager, at Mintera, where they grappled with the limitations of pre-coherent optical systems.

The struggle with 40Gbps direct-detect optical transport systems ultimately led to that pivotal moment in 2007. "It did not make much commercial sense to struggle so much to get to 40 gigabits," Rasmussen recalls. When coherent transmission emerged as a solution, he and his colleagues seized the opportunity, despite the industry's post-dot-com bubble and the 2008 financial crisis.

He began working with Mikkelsen and Mehrdad Givehchi on business plans and developing the technology. “Digital signal processing was new to us, so there was a lot of stuff to learn,” he says.

After being turned down by numerous venture capital firms, one – Matrix Parners- backed the Acacia team, which also received corporate funding from OFS, part of Furukawa Electric. 

Beyond Technology

Outside the lab, Rasmussen finds balance in gardening, appreciating its immediate rewards compared to the years-long cycle of DSP design. "It's nice to do something where you can see the immediate result of your work," he says.

His interests also extend to reading. He recommends "Right Hand, Left Hand" by Chris McManus, praising its exploration of symmetry in nature, and "The Magic of Silence" by Florian Illies, which examines the enduring relevance of painter Caspar David Friedrich.

Looking ahead, Rasmussen remains optimistic about the industry's innovative capacity.

He says that semiconductor foundries do not tend to publicise their CMOS transistors’ switching frequency, but it is already above 500GHz and approaching 1,000GHz. This suggests that a DSP supporting a baud rate of 400GBd will be possible. And four to five years hence, two more generations of CMOS after 3nm are likely. This all suggests that a further doubling of baud rate to 500GBd is feasible.

"Just look at the record of innovation at Acacia and other companies in the industry; people keep coming up with solutions," says Rasmussen.