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Andrew, 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@UCC.IE] 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 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 Phone: +353 21 490
4858 Fax: +353 21
490 4880 e-mail: andrew.ellis@tyndall.ie web site:
www.tyndall.ie/research/photonics-systems-group/index.htm |