Ciena's multi-format 400G coherent QSFP-DD pluggable

Ciena showcased a working 400-gigabit Universal coherent pluggable module at the ECOC 2022 conference and exhibition in Basel, Switzerland.

Ciena is using its WaveLogic 5 Nano coherent digital signal processor (DSP) for the Universal QSFP-DD coherent pluggable module.

"We call it universal because it supports many transmission modes - interoperable and high performance; the most in the industry," says Helen Xenos, senior director of portfolio marketing at Ciena. 

A look inside Ciena's 400-gigabit Universal coherent pluggable module. TOF is the tunable optical filter. Source: Ciena

The pluggable has custom extended-performance modes and supports three industry formats: the 400ZR interoperable standard, the 400ZR+ multi-source agreement (MSA), and the OpenROADM MSA. (See tables below).

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Hyperscalers' needs drive a new class of coherent DSP

Coherent digital signal processors (DSPs) companies have supported two families of coherent chips for some time. That's because no single coherent DSP can meet all the market's requirements.

The coherent DSP used for highest-performance optical transmissions must include advanced coding techniques, forward error correction, and a high symbol rate to send as much data as possible on a single wavelength and maximise reach.

Helen Xenos at NGON & DCI World

In contrast, a DSP for coherent pluggable modules needs to be power-efficient and compact to meet the optical module's power envelope and size constraints; a 400ZR QSFP-DD and a CFP2-DCO 400ZR+ being examples.

According to Ciena, now there is a need for a third category of coherent DSP for 1.6 terabit-per-second (Tbps) and 3.2Tbps transmissions over short distances for next-generation switch routers.

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OFC highlights a burgeoning coherent pluggable market

A trend evident at the OFC show earlier this month was the growing variety of coherent pluggable modules on display.

Whereas a coherent module maker would offer a product based on a coherent digital signal processor (DSP) and a basic design and then add a few minor tweaks, now the variety of modules offered reflects the growing needs of the network operators.

Tom WilliamsAcacia, part of Cisco, announced two coherent pluggable to coincide with OFC.

The Bright 400ZR+ QSFP-DD pluggable form factor is based on Acacia’s existing 400ZR+ offering. It has a higher transmit power of up to 5dBm and includes a tunable filter to improve the optical signal-to-noise ratio (OSNR) performance.

Acacia’s second coherent module is the fixed wavelength 400-gigabit 400G ER1 module designed for point-to-point applications.

“I can understand it being a little bit confusing,” says Tom Williams, vice president of marketing at Acacia. “We have maybe five or six configurations of modules based on the same underlying DSP and optical technology.”

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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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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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OIF addresses 800-gigabit coherent interfaces

The OIF has started work on 800-gigabit coherent interfaces, a follow-on to its 400-gigabit 400ZR specification work.

Two requirements are being addressed: an 800-gigabit dense wavelength division (DWDM) interface with a 80-120km span for data centre interconnect, and an unamplified single-channel fixed-wavelength 2-10km coherent link for campuses.

The need for 800 gigabit

Tad Hofmeister

“When we hit that 90 per cent mark on 400ZR, we had people stand up and say: ‘We are ready to start 800ZR’,” says Karl Gass, OIF, physical link layer working group – optical vice-chair.

But completing the work has taken time. “The first 90 per cent of a project takes about half the time and the last 10 per cent takes the other half,” says Gass.

So only in mid-2020 did the OIF’s attention turn to the new standard, starting with determining the use cases.

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DT chooses Nokia for a major optical network upgrade

Deutsche Telekom is redesigning its domestic optical network and has chosen Nokia as its equipment supplier.

“They are re-architecting and rolling out, in a short time, a huge portion of their optical network,” says Kyle Hollasch, (pictured) director of optical portfolio marketing, Nokia. “We are displacing in many parts of the network four different vendors.”

Network architecture

Deutsche Telekom’s legacy mesh-based wavelength-division multiplexing (WDM) network uses equipment from several vendors. In the last decade, Deutsche Telekom also added to the core an IP-optical solution from Cisco Systems.

Now, the CSP is replacing the mesh-WDM network and the Cisco IP-optical core with an OTN-WDM core from Nokia.

“They are unifying their traffic from all of their business services, government services, 5G anyhaul and the core IP network onto one core WDM network,” says Hollasch. 

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