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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Infinera’s ICE6 crosses the 100-gigabaud threshold

Coherent discourse 3

• The ICE6 Turbo can send two 800-gigabit wavelengths over network spans of 1,100-1,200km using a 100.4 gigabaud (GBd) symbol rate.
• The enhanced reach can reduce the optical transport equipment needed in a network by 25 to 30 per cent.

Infinera has enhanced the optical performance of its ICE6 coherent engine, increasing by up to 30 per cent the reach of its highest-capacity wavelength transmissions.

Ron JohnsonThe ICE6 Turbo coherent optical engine can send 800-gigabit optical wavelengths over 1,100-1,200km compared to the ICE6’s reach of 700-800km.

ICE6 Turbo uses the same coherent digital signal processor (DSP) and optics as the ICE6 but operates at a higher symbol rate of 100.4GBd.

“This is the first time 800 gigabits can hit long-haul distances,” says Ron Johnson, general manager of Infinera’s optical systems & network solutions group.

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Building the data rate out of smaller baud rates

In the second article addressing the challenges of increasing the symbol rate of coherent optical transport systems, Professor Andrew Lord, BT’s head of optical network research, argues that the time is fast approaching to consider alternative approaches.

Coherent discourse 2

Coherent optical transport systems have advanced considerably in the last decade to cope with the relentless growth of internet traffic.

Professor Andrew Lord

One-hundred-gigabit wavelengths, long the networking standard, have been replaced by 400-gigabit ones while state-of-the-art networks now use 800 gigabits.

Increasing the data carried by a single wavelength requires advancing the coherent digital signal processor (DSP), electronics and optics.

It also requires faster symbol rates.

Moving from 32 to 64 to 96 gigabaud (GBd) has increased the capacity of coherent transceivers from 100 to 800 gigabits.

Last year, Acacia, now part of Cisco, announced the first 1-terabit-plus wavelength coherent modem that uses a 128GBd symbol rate.

Other vendors will also be detailing their terabit coherent designs, perhaps as soon as the OFC show, to be held in San Diego in March.

The industry consensus is that 240GBd systems will be possible towards the end of this decade although all admit that achieving this target is a huge challenge.

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OIF moves to raise coherent transmission baud rate

The Optical Internetworking Forum (OIF) has started modulator and receiver specification work to enhance coherent optical transmission performance. The OIF initiative aims to optimise modulator and receiver photonics operating at a higher baud rate than the current 32 Gigabaud (Gbaud).

"We want the two projects to look at those trade-offs and look at how we could build the particular components that could support higher individual channel rates,” says Karl Gass of Qorvo and the OIF physical and link layer working group vice chair, optical.  

Karl Gass

The OIF members, which include operators, internet content providers, equipment makers, and optical component and chip players, want components that work over a wide bandwidth, says Gass. This will allow the modulator and receiver to be optimised for the new higher baud rate.

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