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Re: [HSSG] Higher speed trade offs



Welcome to the conversation!!!  You make some excellent points about the benefits of tighter wavelength packing which can be highly beneficial.  If I understand what you are proposing correctly, it is not truly Coherent but rather simply a technique to generate a comb of tightly spaced wavelengths.  If there is indeed a different form of modulation proposed, please clarify that.


Also note that if there is intellectual property involved regarding this technique, then you should familiarize yourself with the intellectual property bylaws of the IEEE.


Further, in your 2nd paragraph, you suggest the importance of making choices which reduce electronics cost (which is also desirable); however, then you suggest an optical solution which would superficially seem to consume (and possibly then some) the savings from this electronics with complex optics.


Such multi-wavelength sources (MWS) usually seem like an excellent idea (I’ve fallen for them too), but usually they do not reliably provide sufficient power in each of the small number of desired wavelengths (and often lack uniform power per channel).  The result is that to make them useful in a communication system, they require significant optical amplification.  For a longer distance telecom system (which tend to have greater tolerance to higher cost), this may be a viable option, but it is not clear how it would be viable in the relatively lower-cost application space that we’d be defining here.  Even if viable, it would have to be superior relative to the other alternatives available to us.


I’d also appreciate if you would suggest how to modulate and receive the individual carriers.  Presumeably, some form of demux is required before modulation, and multiplex them back together.  Multiplexing and demultiplexing with such tight spacing is certainly non trivial; however once the carriers have a modulated data signal (which for 10Gbps) is certainly on the order of 10GHz ; it suggests that, at a minimum, the demultiplexing at the receiver is likely to have data energy overlapping into adjacent channels even with perfect demultiplexing (but in reality optical filters will not have infinite slope roll-off and require some guard band).  As such, it seems challenging to carry 10Gbps of data in channels spaced ~10Ghz apart.  Historically, there have been extremely fine WDM proposals; however, these are usually limited to ~1Gbps in a channel (and they have not been successful to my knowledge).


I look forward to your further thoughts on this subject.


Best Regards,


Roger Merel




From: Ellis, Andrew [mailto:Andrew.Ellis@xxxxxx]
Sent: Thursday, September 14, 2006 6:41 AM
To: STDS-802-3-HSSG@xxxxxxxxxxxxxxxxx
Subject: [HSSG] Higher speed trade offs


Dear all,


I have recently joined this reflector, and I would like to make one observation which may be of benefit to the group. However, please forgive me if I speak out of turn, am confused about some of the acronyms or cover material already agreed.


There appears to be some considerable debate relating to the trade-off between reach, data rate, buffer requirements and the capabilities of electronics with manufacturability at a reasonable cost. This has lead to the M lanes at N Gbit/s per lane discussion, with M varying between 1 and 10 and N taking values between 100 and 10 Gbit/s respectively. Higher numbers of lanes appears good for electronics cost and reach, whilst a higher serial data rate appears good for minimizing buffer requirements and maintains a narrow occupancy of the optical spectrum (to allow operation in a WDM environment, in turn allowing multiple 100 Gbps Ethernet links to operate over the same fibre).


 I would like to propose the HSSG to consider the use of a new modulation format, currently known as “Coherent WDM”, in order to simultaneously meet all of these requirements. In Coherent WDM, we use a single laser source, minimizing inventory. This source is either a mode locked source, producing multiple carriers, or a standard cw source followed by at least one sine wave driven modulator (10 GHz in this example) in order to generate an optical carrier for each lane. These carriers are then modulated using an array of modulators (one for each lane, and each driven at 10 Gbit/s in this example), with, for example, a PIC similar to the one proposed by Drew Perkins. This produces a single 100 Gbit/s (in this example) signal, occupying a small spectral width of very close to 110 GHz which is transmitted as a single entity over a link (either point-to-point or a WDM network). The compact spectrum and careful design of the PIC and drive circuits combine to give negligible skew between the lanes, minimizing buffer requirements. You thus obtain the key features of the high serial data rates. It has been demonstrated that the reach of a Coherent WDM system is dominated by effects proportional to the data rate of each lane rather than the total data rate, and 10 Gbit/s electronics may be used. You thus also obtain the key features of a high lane count, low serial data rate link.


 I would be very happy to provide further details of Coherent WDM should anybody reading this contribution feel that it is appropriate.


 Thank you for your attention




Andrew Ellis



Senior Research Fellow

Photonic Systems Group

Tyndall National Institute and Department of Physics

University College Cork



Phone: +353 21 490 4858

Fax: +353 21 490 4880

e-mail: andrew.ellis@xxxxxxxxxx

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