Cookies Use on AFL Website We use cookies to give you the best online experience. By using our website you agree to our use of cookies in accordance with our privacy policy. Agree
Home / AFL Blog / June 2017 / The Path to 100G Single Lambda in the Data Center - Part 3
Blog posts

The Path to 100G Single Lambda in the Data Center - Part 3

Maury Wood, AFL Test & Inspection Product Line Manager

Part 3 (part 1 - part 2 - part 4)

Part three of this blog series is on the technical and commercial dynamics that will lead the semiconductor and electro-optical components industries to enable ubiquitous 100G single lambda system designs over the next several years.

Just like in radio frequency spectrum (one of many areas where RF and optical communications share useful characteristics), and to put into very simple terms: the higher the channel bandwidth, the lower the channel range/reach. And the notion of spectral efficiency is equally salient for both RF spectrum and fiber "spectrum." For example, a key parameter of different multimode fiber types is effective modal bandwidth, and the units of this parameter tell us what we need to know–MHz*km, in other words, in optical communications, the product of bandwidth and reach is fixed (at a given optical carrier wavelength, such as 850 nm or 1310 nm). It’s worth a moment's appreciation for the fact that optical carrier frequencies range in the hundreds of TeraHertz, for example 850 nm is 353 THz.

So, the key takeaway is that if we want to maintain the same reach (say 100 m for "200GBASE-SR4," not-as-yet an IEEE standard or "400GBASE-SR4," also not-as-yet an IEEE standard), then we'll need to use the same effective bandwidth or symbol rate (aka baud rate). This gives a path forward, and the industry is moving ahead with a tremendous level of consensus to utilize PAM-4 modulation (2 bits per symbol) with a 25/28 GBaud symbol rate and 50/56 Gbps bit rate. More to discuss here later, but we can say with certainty that short reach 50G single lambda will use PAM-4 (Four Level Pulse Amplitude Modulation) optical modulation, and this same multi-level modulation technology is likely to be utilized for 100G single lambda systems.

Back to the signal chain. We've started on the transmit (Tx) side, as signal processing engineers know that receivers are always tougher; it’s easier to talk than to listen I guess. So for 50G single lambda, we can envision Optical Internetworking Forum CEI-56G-VSR “CAUI-2” compliant Tx SerDes channels coming out of a theoretical next generation Broadcom switch ASIC, traversing perhaps 10 cm of 75 ohm differentially balanced RF microstrip/stripline, possibly using a Teflon-based PCB substrate material into a pluggable transceiver SerDes, with sufficient channel equalization to receive the encoded bit stream with low (perhaps 10e-14) bit error rate. There may be algorithmic Forward Error Correction on this electrical link to provide coding gain and more bit error resiliency.

Once on board the theoretical 200GBASE-SR4 transceiver module, the electrical signals go into a clock/data recovery retimer chip. If the electrical lane configuration doesn't match the optical lane configuration, then a so-called gearbox function is needed, adding cost, power and area to the design, so matched electrical and optical rates are heavily preferred in data center and other cost-pressured transceiver designs.

Since this conceptual next generation transceiver is using 25 Gbaud PAM-4 to achieve single lambda 50G, we need a "mixed-signal Digital Signal Processor" (a DSP with on-chip high speed data converters) to create the four-level PAM-4 modulation analog waveform.

As mentioned earlier, short reach 25G lambda systems universally use Non-Return to Zero (NRZ) On-Off Keying (OOK) intensity modulation in which the supply current to the VCSEL (Vertical Cavity Surface-Emitting Laser) diode is switched above the lasing threshold to create the digital ones (light emitted) and zeros (essentially no light emitted). Note that LED lasers have a finite “extinction ratio” between on and off, but still plenty of Signal-to-Noise Ratio (SNR) to achieve robust 25 Gbps short range links. This on/off bit sequence is a continuous amplitude, continuous time signal, and for our purposes can be considered a binary signal when sampled by the receiver correctly. On the other hand, the PAM-4 signal, with four valid signal/switching levels and three eye openings per symbol, can only be treated (from a Tx synthesized signal and Rx sampled signal perspective) as a continuous amplitude, continuous time "analog" or "linear" signal by both the transmitter and the receiver.

PAM-4 Signal

PAM-4 Signal with Three Eye Openings per Symbol Period (image courtesy NeoPhotonics Corp)

This is important, among other reasons, because in current NRZ/OOK 25G lambda system, the electrical/optical (EO) interface between the VCSEL driver function (downstream of the Tx Clock Data Recovery function) and the VCSEL light source is conventionally defined as "limiting" (discrete levels), whereas the optical modulator driver function in the 50G lambda system is defined as "linear" (continuous levels), and as noted above, a DSP with a simple but very fast two bit Digital-to-Analog Converter is required to synthesize the PAM-4 modulation linear (i.e., analog) signals.

Stay tuned for part four of this series for a discussion on Mach-Zehnder optical modulators.

Posted: 6/15/2017 by AFL | with 0 comment(s)
Filed under: 100G, AFL, Data Center, Single Lambda, Test and Inspection
Comments
Blog post currently doesn't have any comments.
Leave comment



 Security code

Recent Posts

New Splicers Push Limits

10/16/2018 11:00:00 AM | with 0 comments
AFL has released two new flusion splicers to the market. The Fujikura 41S Core Sensing fusion splicer, and the Fujukura 31S fusion splicer have both raised the bar of splicer technology.

Representing AFL’s Cable and Connectivity team at SCTE-ISBE Cable Tec Expo

10/11/2018 10:20:29 AM | with 0 comments
Will Miller from AFL's Cable and Connectivity team will be onsite at the SCTE-ISBE Cable Tec Expo, in Atlanta, Oct. 23-25.  

Enclosures Designed for You

9/30/2018 7:51:24 AM | with 0 comments
At AFL we put customers first. This is why we highly value customer feedback. We want to hear how our solutions are working for you, what you like about them and what we can do better!



AFL Optical Ground Wire (OPGW)

9/21/2018 8:12:36 AM | with 0 comments
OPGW's applications and what type might be best for your project. Check out the video "AFL Optical Ground Wire (OPGW)" on  AFL's new resource center to learn about Optical Ground Wire.

AFL Accessories Overview

9/14/2018 7:18:05 AM | with 0 comments
An overview of our accessories business including a brief history, product walk through as well as our engineering and testing services.

Blog Tags

100G 12R 12S 2014 2015 2016 21s 22S 31S 41S 62S 70R 70S Access Networks accessories ADSS Aerial Aeros AFL AFL. Air Blown Fiber AlumaCore Aluminum APM-101 APM-102 APTA ASIS Associates Best Practices BICSI Blog bluetooth bus Cable cable deployment Cables Cable-Tec Careers Case Case Studies Catalogs Cell Tower Demarcation Certification CGM Plus Rack Panel CI&M Innovators Award Cleaning Cleaning Supplies Cleaver CO2 Lasers Community Outreach compression Compression Accessories Conductor Connections Connectivity Connectors Co-ops Core-alignment Corporate CT-30 CT50 CTIA CWDM DAS Data Center Data Centers Dead Ends Demonstration Distribution Enclosure DT Duel Dura-Line DWDM eABF electric Enclosure end-to-end solutions Energy Enterprise Equipment ESB Expo Exterior Distribution Cabinet Facebook Fall Fall 2013 FAST SC FASTConnect FC Features Fiber Fiber Cleaver Fiber Connector Inspection Fiber Distribution fiber inspection Fiber Laser Fiber Optic Fiber Optic Cable Fiber Optic Intrusion Detection System Fiber Optic Training Fiber OpticTraining Fiber Prep Fiber Security Fiber Shaping Fiber Splicing fibre optic cable field installable Field Splicer flexscan Flickr FOCIS FOCIS Duel FOCIS Flex FOCIS WiFi2 FSM-100 FTTh FTTx Fujikura FuseConnect Fusion Fusion Splicer Fusion Splicers Fusion Splicing Gas Glass Processing Google Governor Nikki Haley Grand Canyon Greenville Grounding Hardware High-Wire Walk HiTemp IDEAA IEEE infrastructure Inspection Instagram Install Installation Installing Instructions Integrated Solution International Sales Meeting Interns Kit Laser Last Mile LAZERMaster LC Light Brigade LightLink LightWave LinkedIn live line LL-550 LL-550 & LL-580 enclosures LL580 LL-580 LMHD LZM-100 M210 M310 Maintenance Metro Ethernet Metro-E MicroCore Mining Month of Service MSOs Network Networks NFPA Nik Wallenda Noyes OCS OEM Oil One-Click OPGW Optic Optical Optical Connectivity Optical Sensing Optical Splicing OSP OSP MicroCore: LM200 & LMHD Series OTDR Outside Plant Paper PCS-100 Stripper Perimeter Security System PES Photonics Photonics West PhotonicsWest PM Splicing Poli-Mod power Procedures Product Registration Products Rail Railway Recoater Recruiting resource center Resources Rogue Rural Utilities Rusty Williams SB01 SC SCADA SCTE SCTE Cable-Tec SDN Selection Sensing Services Sidewinder Rapid Cable Deployment System Silver Level Award single circuit outage Single Lambda SkyWrap Small-Cell Smart-Grid Smartphone Snapshot Social Software Defined Networking Solutions Specialty Splicer Spider Web Ribbon SPIE Splice Enclosure Splicer Splicers Splicing ST Stockbridge Dampers Stranded Stainless Steel Cable Stream Streetcar Structured Cabling Study substation SuperMobility Support SWAGE Swaging SWAT SWFT System systems T&D T&I: Terminal Termination Instructions Test Test &Inspection Test and Inspection Testing Titan Tools trade show Trade Shows Training Transit transmission Transportation Tutorials Twitter Ultra HD Upstate Alliance Utility VFI V-Groove Video Videos Vodafone WDM WDM900 We Connect Webinar Website White White Paper White Papers White-Paper Wi-Fi Winter Wireless Wrapping Tube Cable WTC/SWR

Sign Up for eConnect!