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geekster 12/4/2012 | 10:17:51 PM
re: Physicists Find Fiber's Limit ferroelectric molecular photon induced
electric field poling for rewrtiable
interferance patterns for data storage.

dwdm2 12/4/2012 | 10:17:47 PM
re: Physicists Find Fiber's Limit What is a "molecular photon?" Is it something similar to excimer (perhaps keeping the molecule intact yet giving out photon)? What wavelength range are you talking here?

ppm 12/4/2012 | 8:09:30 PM
re: Physicists Find Fiber's Limit Capacity of a channel is not set by the modem:
the modulater/demodulator is useful *upto* the
basic channel capacity. A 10bits/s/Hz (or for
that matter 10bits/s/Hz) modem is fine over a
a linear channel like copper wire, say - since
those channels can support high bit rates. The
point is that nonlinearities limit the capacity
of optical fiber to significantly smaller values
than the linear channels traditionally used in
telecommunications. The capacity of a 10 bit
modem attached to a 3 bit channel is still
3 bits ...

I don't post here, just thought I should point
out the conceptual error in the last message.

ownstock 12/4/2012 | 8:09:30 PM
re: Physicists Find Fiber's Limit LR: Far from calculating a fundamantal limit, as you imply in your banner, these guys calculate a limiting case...and not well at that. Consider their assumption of an information density of only 3 bits/Hz...a common analog modem can achieve 10 bits/Hz...

Bottom line: There are much more efficient ways of transmitting information over fiber than commonly employed today. DWDM/OOK is just plain wasteful of bandwidth...

I would however be concerned about the need for more (unlit) fiber in the future...demand will stabilize in the face of high bandwidth growth, as people figure out how to stuff more into a single fiber...

ppm 12/4/2012 | 8:09:29 PM
re: Physicists Find Fiber's Limit typo in message just posted: bracketed text
should have read 16bits/s/Hz.
nanodelta 12/4/2012 | 8:09:28 PM
re: Physicists Find Fiber's Limit >"There are a couple of companies out there that are utilizing just this kind of technology"

Would it be ok for you to name them?

ownstock 12/4/2012 | 8:09:28 PM
re: Physicists Find Fiber's Limit ppm:

Let me teach you something...a Copper wire system is far, far from linear...to get that kind of density (10bps/Hz) requires some real hard work in equalization and FEC...and it is all done with less than 20 dB SNR and in the presence of echo and crosstalk...makes the non-linearities in fiber look like child play...

Just shows that you should not go to wideband digital OOK signals...

There are a couple of companies out there that are utilizing just this kind of technology (analog optical carrier and exotic modulation), so that if scaled, it would far surpass the figures sited in the article...and doing it in fiber...with greatly REDUCED nonlinear effects...

At least one is in field trial with a major carrier.


ownstock 12/4/2012 | 8:09:28 PM
re: Physicists Find Fiber's Limit I think anyone with FDM analog technology could do it, Kestrel comes to mind...there are a couple of others...don't know if Harmonic is doing it today, but they do have the technology...

Basically anyone putting RF carriers down fiber...that is the whole HFC market...

ppm 12/4/2012 | 8:09:27 PM
re: Physicists Find Fiber's Limit All the effects you mention (Echoes, crosstalk,
need to equalize) are present in linear systems.
The fact that it is difficult to achieve the
capacity of a linear channel does not speak to
the capacity itself.

FEC does not distinguish linear from nonlinear
channels. To get close to capacity you certainly
need error correction in any system.

All communication systems have
nonlinearities. Even vacuum has some,
theoretically. The real question is, to take
the nonlinear effects into account in a given
system with given parameters. *Long haul*
optical communications unfortunately suffer
from severe nonlinear effects. These are well
known; what has not been understood before is
how to estimate the capacity limitations from
the nonlinear effects.

Of course, if you take a short piece of
optical fibre, you don't see the nonlinear
effects at issue here: so you have to be
specific about system length, etc. If you take
a few meters of fibre, nonlinear effects are
pretty much completely negligible. If you were
to intersperse repeaters with very short
spans of fibre, the capacity limits would
approach that of a linear system. Of course,
this is not economically feasible.

If you read the original article carefully,
which is in the context of long haul optical
communications, the relevant questions are discussed.


sanddune 12/4/2012 | 8:09:26 PM
re: Physicists Find Fiber's Limit ownstock,
why don't you write a paper in a peer reviewed
trade publication negating their findings.
Hiding behind LR's message board to confuse
the readers doesnot demonstrate your true
knowledge. It only justifies your vision to
confuse everyone.
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