Has coherent optical transmission run its course?

Feature: Coherent's future

Three optical systems vendors share their thoughts about coherent technology and the scope for further improvement as they look two generations ahead to symbol rates approaching 100 gigabaud   

Optical transmission using coherent detection has made huge strides in the last decade. The latest coherent technology with transmitter-based digital signal processing delivers 25x the capacity-reach of 10-gigabit wavelengths using direct-detection, according to Infinera.

Since early 2016, the optical systems vendors Infinera, Ciena and Nokia have all announced new coherent digital signal processor (DSP) designs. Each new generation of coherent DSP improves the capacity that can be transmitted over an optical link. But given the effectiveness of the latest coherent systems, has most of the benefits already been achieved?

Source: Infinera

“It is getting harder and harder,” admits Kim Roberts, vice president, WaveLogic science at Ciena. “Unlike 10 years ago, there are no factors of 10 available for improvement.”

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Next-generation coherent adds sub-carriers to capabilities

Briefing: DWDM developments

Part 2: Infinera's coherent toolkit 

Source: Infinera

Infinera has detailed coherent technology enhancements implemented using its latest-generation optical transmission technology. The system vendor is still to launch its newest  photonic integrated circuit (PIC) and FlexCoherent DSP-ASIC but has detailed features the CMOS and indium phosphide ICs support.

The techniques highlight the increasing sophistication of coherent technology and an ever tighter coupling between electronics and photonics.    

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BT makes plans for continued traffic growth in its core

Briefing: DWDM developments

Part 1

Kevin Smith: “A lot of the work we are doing with the trials have demonstrated we can scale our networks gracefully rather than there being a brick wall of a problem.”

BT is confident that its core network will accommodate the expected IP traffic growth over the next decade. Traffic in BT’s core is growing at between 35 and 40 percent annually, compared to the global average growth rate of 20 to 30 percent. BT attributes its higher growth to the rollout of fibre-based broadband across the UK.

The telco is deploying 100-gigabit wavelengths in high-traffic areas of its network. “These are key sites where we're running out of wavelengths such that we need to implement higher-speed ones,” says Kevin Smith, research leader for BT’s transport networks. The operator is now trialling 200-gigabit wavelengths using polarisation multiplexing, 16-quadrature amplitude modulation (PM-16QAM).

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Data centres to give silicon photonics its chance

Part 4: A large data centre operator’s perspective

The scale of modern data centres and the volumes of transceivers they will use are going to have a significant impact on the optical industry. So claims Facebook, the social networking company.

Katharine Schmidtke

Facebook has been vocal in outlining the optical requirements it needs for its large data centres.

The company will use duplex single-mode fibre and has chosen the 2 km mid-reach 100 gigabit CWDM4 interface to connect its equipment.

But the company remains open regarding the photonics used inside transceivers. “Facebook is agnostic to technology,“ says Katharine Schmidtke, strategic sourcing manager, optical technology at Facebook. “There are multiple technologies that meet our requirements.” 

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Verizon tips silicon photonics as a key systems enabler

Verizon's director of optical transport network architecture and design, Glenn Wellbrock, shares the operator’s thoughts regarding silicon photonics.

Part 3: An operator view

Glenn Wellbrock is upbeat about silicon photonics’ prospects. Challenges remain, he says, but the industry is making progress. “Fundamentally, we believe silicon photonics is a real enabler,” he says. “It is the only way to get to the densities that we want.”

Glenn Wellbrock

Wellbrock adds that indium phosphide-based photonic integrated circuits (PICs) can also achieve such densities.

But there are many potential silicon photonics suppliers because of its relatively low barrier to entry, unlike indium phosphide. "To date, Infinera has been the only real [indium phosphide] PIC company and they build only for their own platform,” says Wellbrock.

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The quiet period of silicon photonics

Michael Hochberg discusses his book on silicon photonics and the status of the technology. Hochberg is director of R&D at Coriant's Advanced Technology Group. Previously he has been an Associate Professor at the University of Delaware and at the National University of Singapore. He was also a director at the Optoelectronic Systems Integration in Silicon (OpSIS) foundry, and was a co-founder of silicon photonics start-up, Luxtera.

Part 2: An R&D perspective

If you are going to write a book on silicon photonics, you might as well make it different. That is the goal of Michael Hochberg and co-author Lukas Chrostowski, who have published a book on the topic.

Michael HochbergHochberg says there is no shortage of excellent theoretical textbooks and titles that survey the latest silicon photonics research. Instead, the authors set themselves the goal of creating a design manual to help spur a new generation of designers.

The book aims to provide designers with all the necessary tools and know-how to develop silicon photonics circuits without needing to be specialists in optics.

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Silicon photonics: "The excitement has gone"

The opinion of industry analysts regarding silicon photonics is mixed at best. More silicon photonics products are shipping but challenges remain.

Part 1: An analyst perspective

"The excitement has gone,” says Vladimir Kozlov, CEO of LightCounting Market Research. “Now it is the long hard work to deliver products.” 

Dale Murray, LightCounting

However, he is less concerned about recent setbacks and slippages for companies such as Intel that are developing silicon photonics products. This is to be expected, he says, as happens with all emerging technologies.

Mark Lutkowitz, principal at consultancy fibeReality, is more circumspect. “As a general rule, the more that reality sets in, the less impressive silicon photonics gets to be,” he says. “The physics is just hard; light is not naturally inclined to work on the silicon the way electronics does.”

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