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Roger, Some brief comments which I hope address
some of your points below: The term Coherent is intentional, but does
not refer to coherent detection at the receiver, rather coherent addition of
the WDM channels at the transmitter. Thank you for your note regarding IP
bylaws. Coherent WDM does indeed require
multiplexing and demultiplexing optics, in common with any WDM solution.
However, within the transmitter, the WDM component is used to process continuous
wave signals, and so dispersion is less of an issue. At the receiver, standard
WDM components are used, with additional complexity compared to a traditional
WDM system. The implications of a tighter channel spacing are compensated for
the fact that one wavelength tuning operation (if necessary) gives the correct
filter position for all 10 (in a 10 x 10 Gb system) wavelengths. So, to first
order, I do not believe that the optical components for Coherent WDM would be
particularly more expensive than for a WDM solution. We have a solution for the MWS which
provides arbitrary channel uniformity with good power efficiency, based on a
single standard continuous wave laser source. An optical amplifier is indeed currently
used between the MWS and the modulator array however, this may not always be
necessary, firstly depending on reach and receiver sensitivities, and secondly
based on refinements currently under investigation which eliminate the problem. For the multiplexing, I think you already
have the bones of the idea. You split the MWS into separate channels, each individually
modulated. When recombining them however, you ensure that the optical phases
and data delays are controlled to within a reasonable accuracy. This is the key
aspect. At the receiver, spectral overlap indeed occurs, however,
considering that an interfering channel is effectively high pass filtered by
the WDM demultiplexer (only the high frequency components are passed) the
resulting interference is in the form of return to zero like pulses. Given that
the signals originated from a single transmitter, with a common clock frequency,
these RZ interference pulses may be aligned to the bit crossing of the
demultiplexed channel, where they have negligible impact. Whilst this surely
reduces the phase margin of the receiver slightly, it has enabled transmission
of WDM signals with an information spectral density of 1 b/s/Hz with a data
rate per wavelength of 42 Gbit/s. 10 Gb/s at 10 GHz should therefore be
possible. A more polished explanation of Coherent
WDM multiplexing and demultiplexing may be found in Photonics Technology
Letters, Vol 18, No 12, pp1338- In summary, I agree, cost and power budget
will be key factors, but Coherent WDM does offer the possibility of
simultaneously achieving all of the technical targets. Huge, With regard to coding, multi-wavelength
codes have indeed been investigated for use in WDM systems, including Coherent
WDM. However, the ability to minimize the crosstalk within Coherent WDM itself was
found to be so successful that minimal additional benefit from FEC was
available. Best wishes 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@xxxxxxxxxx web site: www.tyndall.ie/research/photonics-systems-group/index.htm From: Roger Merel
[mailto:roger@xxxxxxxxxxx] 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@xxxxxx] 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@xxxxxxxxxx web site:
www.tyndall.ie/research/photonics-systems-group/index.htm |