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Silicon Photonics

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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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New MSA to enable four-lambda 400-gigabit modules

A new 100-gigabit single-wavelength multi-source agreement (MSA) has been created to provide the industry with 2km and 10km 100-gigabit and 400-gigabit four-wavelength interfaces.

Mark NowellThe MSA is backed by 22 founding companies including Microsoft, Alibaba and Cisco Systems.

The initiative started work two months ago and a draft specification is expected before the year end.

“Twenty-two companies is a very large MSA at this stage, which shows the strong interest in this technology,” says Mark Nowell, distinguished engineer, data centre switching at Cisco Systems and co-chair of the 100G Lambda MSA. “It is clear this is going to be the workhorse technology for the industry for quite a while.”

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Inphi unveils a second 400G PAM-4 IC family

Inphi has announced the Vega family of 4-level, pulse-amplitude modulation (PAM-4) chips for 400-gigabit interfaces.

The 16nm CMOS Vega IC family is designed for enterprise line cards and is Inphi’s second family of 400-gigabit chips that support eight lanes of 50-gigabit PAM-4.

Its first 8x50-gigabit family, dubbed Polaris, is used within 400-gigabit optical modules and was announced at the OFC show held in Los Angeles in March.

“Polaris is a stripped-down low-power DSP targeted at optical module applications,” says Siddharth Sheth, senior vice president, networking interconnect at Inphi (pictured). “Vega, also eight by 50-gigabits, is aimed at enterprise OEMs for their line-card retimer and gearbox applications.”  

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COBO targets year-end to complete specification

Part 3: 400-gigabit on-board optics

  • COBO will support 400-gigabit and 800-gigabit interfaces 
  • Three classes of module have been defined, the largest supporting at least 17.5W 

The Consortium for On-board Optics (COBO) is scheduled to complete its module specification this year.

A draft specification defining the mechanical aspects of the embedded optics - the dimensions, connector and electrical interface - is already being reviewed by the consortium’s members.

Brad Booth“The draft specification encompasses what we will do inside the data centre and what will work for the coherent market,” says Brad Booth, chair of COBO and principal network architect for Microsoft’s Azure Infrastructure.

COBO was established in 2015 to create an embedded optics multi-source agreement (MSA). On-board optics have long been available but until now these have been proprietary solutions. 

“Our goal [with COBO] was to get past that proprietary aspect,” says Booth. “That is its true value - it can be used for optical backplane or for optical interconnect and now designers will have a standard to build to.” 

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SFP-DD: Turning the SFP into a 100-gigabit module

Part 2: New optical transceiver MSA

An industry initiative has started to quadruple the data rate of the SFP, the smallest of the pluggable optical modules. The Small Form Factor Pluggable – Double Density (SFP-DD) is being designed to support 100 gigabits by doubling the SFP’s electrical lanes from one to two and doubling their speed.

Scott SommersThe new multi-source agreement (MSA), to be completed during 2018, will be rated at 3.5W; the same power envelope as the current 100-gigabit QSFP module, even though the SFP-DD is expected to be 2.5x smaller in size.

The front panel of a 1-rack-unit box will be able to support up to 96 SFP-DD modules, a total capacity of 9.6 terabits. 

The SFP-DD is adopting a similar philosophy as that being used for the 400-gigabit QSFP-DD MSA: an SFP-DD port will support legacy SFPs modules - the 25-gigabit SFP28 and 10-gigabit SFP - just as the QSFP-DD will be backward compatible with existing QSFP modules.

“Time and time again we have heard with the QSFP-DD that plugging in legacy modules is a key benefit of that technology,” says Scott Sommers, group product manager at Molex and the chair of the new SFP-DD MSA. Sommers is also a co-chair of the QSFP-DD MSA.

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Heavy Reading’s take on optical module trends  

Part 1: Optical transceiver trends 

The industry knows what the next-generation 400-gigabit client-side interfaces will look like but uncertainty remains regarding what form factors to use. So says Simon Stanley who has just authored a report entitled: From 25/100G to 400/600G: A Competitive analysis of Optical Modules and Components.

Implementing the desired 400-gigabit module designs is also technically challenging, presenting 200-gigabit modules with a market opportunity should any slip occur at 400 gigabits.


Simon StanleyStanley, analyst-at-large at Heavy Reading and principal consultant at Earlswood Marketing, points to several notable developments that have taken place in the last year. For 400 gigabits, the first CFP8 modules are now available. There are also numerous suppliers of 100-gigabit QSFP28 modules for the CWDM4 and PSM4 multi-source agreements (MSAs). He also highlights the latest 100-gigabit SFP-DD MSA, and how coherent technology for line-side transmission continues to mature.

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Cavium broadens its Xpliant switch-chip offerings 

  • Two families of Xpliant switch chips have been unveiled: the XP60 with sub-terabit switching capacities and the mid-range XP70 devices with 1 to 1.8 terabits of capacity.
  • The switch ICs broaden the datacom and telecom markets Cavium can now address. 
  • Cavium is developing a next-generation high-end switch chip but the company is not saying when it will be announced. 

Cavium has broadened its portfolio of switch chips. The two families - the XP60 and the XP70 - have smaller switch capacities than Cavium’s XP80 Xpliant family and feature architectural enhancements.

“The new chips expand Cavium’s addressable markets to include enterprise and carrier-access networks as well as mainstream cloud data centres,” says Bob Wheeler, principal analyst for networking at The Linley Group.

John Harrsen

The switch chips enable Cavium to address 25-gigabit interface switches, power-constrained enclosure designs such as blade servers, and 5G cloud radio access networks (CRAN) and GPON aggregation.

Until now Cavium has offered three XP80 Xpliant switch ICs, the largest being a 3.2-terabits switch. In contrast, the three XP70 devices have switch capacities of 1, 1.4  and 1.8 terabits while the XP60’s three chips have 280, 560 and 720 gigabits of capacity.

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