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10 GbE PMD

Rich, Ron, et. al., 

No doubt there has been considerable development
in the technology of multi-amplitude-signalling
since '76.  However, the fundamental and practical
issues of laser physics trump some of the signal
coding theories.

First, one must observe a population of laser diodes
and their Power .vs. Current (P-I) plots.  In doing so,
it will be seen there are significant differences in the
threshold current, slope efficiencies, linearity, 
saturated power, LED transition region, etc.  This is
why AM schemes in the past have always been baseband, 
bi-level modulation.  It allows the designer to make a simple
offset correction for the lasing threshold current.  One can
then pick two levels, a "1" and a "0" light level which can 
be set via controlling the peak modulation current.

In the present GbE specification, there was not even a consensus
to hold the mean power output of the laser transmitter tighter
than a 6dB range.  This does not bode well for any type of MAS
laser transmitter.  The case of an 850nm laser and mulimode fiber 
creates an additional degradation mechanism.  VCSELs transmit an
optical spatial mode which is dependent on forward current.  At
lower current levels near threshold ("0"), a smaller number of 
spatial modes are excited.  At higher drive current levels ("1"),
the laser may be fully moded and will begin to saturate in it's
P-I performance.  These modes may also be slow rise time modes or
fast rise time modes depending on the polarization (see "High
Speed Characteristics of VCSELs" by Jim Tatum, David Smith, Jim
Guenter, and Ralph Johnson).  The coupling lens will have some 
mode dependent characteristics as will the fiber.

Therefore, the use of multimode lasers such as VCSELs becomes very
difficult in a MAS encoding scheme.  This is due to the differential
delays experienced by the central axial modes and the higher order
modes at full stimulation when coupled into a multimode fiber.  This
type of behavior also is well known in simple Fabry-Perot edge emitting
lasers except for the polarization issue.  I do not see a way to 
mitigate these characteristics of the multimode laser or inexpensive
FP laser when coupled into multimode fiber using MAS modulation.
Despite the many advantages of bandwidth reduction which we all agree
would be desirable, I do not see it as a practical solution on multi-
mode fiber.  

I believe Ron has made a significant point with regard to
real world transmitter Signal to Noise Ratios (SNR).
My initial estimates for transmitter S/N ratio show something
on the order of a 7dB degradation for a PAM5 system.  If the 
transmitter S/N is sufficient, the dominant factors could be
elsewhere in the link.  However, this is not usually the case
with a multimode VCSEL.  Transmitter SNRs of 13dB-15dB and lower
are common with many real world VCSEL multimode devices.  If MAS
were to be workable I would certainly suggest single mode fiber and
Distributed Feedback Lasers (DFBs) as a starting point.

MAS laser encoding does sound like an interesting research program 
which may bear some fruit and I would be willing to contribute in
any small way I might to developing the concept further.  Perhaps what
is needed is a working demonstration of the technology.  Any volunteers?

Pat Gilliland


At 11:19 AM 11/17/99 -0800, you wrote:
>Ron, Pat,
>Virtually all communications links have made great strides to ensure
reliable high
>speed information over all media including optics, copper and wireless. If
>optical systems are in their infancy with respect to cost effectively
transport huge
>amount of data reliably. The principal thrust behind multilevel optics is
to cut the
>cost of an optical transport which has very high data rate requirements.
>802.3 HSSG has now heard of at lease 4 general options to go this:
>1) Reduce to cost of serial binary signaled systems by several orders of
>(e.g. 1 bit/baud such as OC-192);
>2) Demultiplex the data by a factor of n for transport across n individual
>links (e.g. parallel optics);
>3) Demultiplex the data by a factor of n for transport across n
wavelengths using n
>O/E sets and a single optical links (e.g. WWDM);
>4) Encode the data into multiple bits/baud or multiple subcarriers for
>across a single serial link (e.g. PAM, QAM, FSK, etc.).
>I have proposed what I believe to be the simplest multilevel modulation
scheme for
>the 10 Gbps data transport. The tradeoffs are simple: Cut the line rate by
1/2 and
>use 5 levels. Of course this results in SNR penalty of 4.1 dB optical, but
>penalty can be easily recovered by other tradeoffs in a multilevel optical
>including intelligent closed-loop full-duplex link design, Forward Error
>FEC code, careful engineering to minimize laser noise (e.g. RIN, etc.).
All of these
>techniques are well known and/or straightforward and used in most existing
>communications links.
>I'd like to get a bit more specific as to exactly where the "walls" are
with respect
>to a multi-level optical link. I do see plenty of hurdles, but I also see
>potential benefits from overcoming those hurdles and the means to do it. I'll
>intersperse some more comments in the notes below.