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.

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imec's novel ADC promises faster sampling rates

The analogue-to-digital and digital-to-analogue converters (ADCs/DACs) are like the equals sign in mathematics.

Joris Van Driessche

The equals sign is taught as showing two sides of an equation being the same. But really, it is a gateway between two worlds. The same applies to the ADC and DAC, which equate between the analogue and digital worlds.

Progress in wireline communications, whether client-side optics or coherent optical modems at 800 gigabits and soon 1.6 terabit, means converters must sample at higher rates.

In February, at the IEEE International Solid-State Circuits Conference (ISSCC) in San Francisco, imec detailed a proof-of-concept chip design that promises to advance high-speed ADCs.

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ADCs key for high baud-rate coherent systems

Increasing the baud rate of coherent modems benefits optical transport. The higher the baud rate the more data can be sent on a wavelength, reducing the cost-per-bit of traffic.

But engineers have become so good at designing coherent systems that they are now approaching the Shannon limit. 

Tomislav Drenski

At the OFC show earlier this year, Ciena showcased a coherent module operating at 107 gigabaud (GBd). And last year, Acacia, now part of Cisco, announced its next-generation 1.2 terabits-per-second (Tbps) wavelength coherent module operating at up to 140GBd. 

The industry believes that increasing the baud rate to 240+GBd is possible, but each new symbol-rate hike is challenging.

All the components in a modem - the coherent DSP and its digital-to-analogue (DAC) and analogue-to-digital (ADC) converters, the optics, and the analogue drive circuitry - must scale in lockstep.

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Huawei sets transmission record with new modulator

Coherent discourse: Part 1

A paper from Huawei and Sun Yat-Sen University in the January issue of the Optica journal describes a thin-film lithium niobate modulator. The modulator enabled a world-record coherent optical transmission, sending nearly 2 terabits of data over a single wavelength.

Maxim Kuschnerov

Much of the industry’s focus in recent years has been to fit coherent optical technology within a pluggable module.

Such pluggables allow 400-gigabit coherent interfaces to be added to IP routers and switches, serving the needs of the data centre operators and telecom operators.

But research labs of the leading optical transport vendors continue to advance high-end coherent systems beyond 800-gigabit-per-wavelength transmissions.

Optical transport systems from Ciena, Infinera and Huawei can send 800-gigabit wavelengths using a symbol rate of 96-100 gigabaud (GBd).

Acacia Communications, part of Cisco, detailed late last year the first 1.2-terabit single-wavelength coherent pluggable transceiver that will operate at 140GBd, twice the symbol rate of 400-gigabit modules such as 400ZR. 

Now Huawei has demonstrated in the lab a thin-film lithium niobate modulator that supports a symbol rate of 220GBd and beyond.

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