Physicists Find Fiber's Limit

Scientists from Bell Labs have calculated the theoretical limits on the carrying capacity of glass optical fiber and concluded that there's still a long way to go before optical systems reach those limits (see Bell Labs Calculates Limits). The results were published yesterday in the journal Nature.

Partha Mitra, lead author of the paper, says his work will point the way for future research by showing which approaches are likely to come up against fundamental physical limits and which aren't. It could also aid engineers whose task it is to model the enormously complicated properties of DWDM (dense wavelength-division multiplexing) systems.

Unlike system vendors, physicists measure the information-carrying capacity of a fiber in bit/s per Hertz of spectral bandwidth (bit/s/Hz). To find the capacity in Gbit/s or Tbit/s, this number has to be multiplied by the available bandwidth of a system (that's bandwidth in the physics sense, in Hz, rather than its common telecom usage, which essentially means capacity).

Mitra and his co-author Jason Stark calculated that the theoretical limit imposed by the physical properties of optical fiber on a communication system is about 3 bit/s/Hz. This corresponds to a maximum payload of 150 Tbit/s on a single fiber, assuming that the fiber can carry signals across the wavelength range 1260 to 1620 nanometers.

Mitra points out that all existing optical systems have a lower limit of 1 bit/s/Hz. That's because they encode data using a simple on-off keying technique, which represents bits by the presence or absence of light. "We've shown that the theoretical limits are substantially greater than this," he says. "What this means is that by changing the modulation scheme, it's possible to get more data into a fiber than was thought possible."

The downside? While the work at Bell Labs suggests that fiber has plenty of room to grow, new technologies -- more complicated modulation schemes and coherent detectors, which measure both power and phase of the incoming signal -- will be needed to make the most of it.

Few would argue with Bell Labs' rather basic conclusion -- that fiber has more capacity than is currently being used. It's a no-brainer. What's new is that the researchers have been able to quantify how much surplus capacity there is, something that can't be deduced from existing communications theory.

The classical formula for calculating capacity, known as Shannon theory, predicts that capacity will increase indefinitely as the power of the optical signal goes up. That's because the signal keeps getter stronger relative to the noise, which is fixed.

In real life, however, strange "non-linear" phenomena come into play, and start creating more noise at high optical power. Mitra calls it the cocktail-party effect. "If everyone's talking at once, then you have to raise your voice in order to be heard, and if everyone raises their voice, then you can't hear anything." Much the same thing can occur among channels in the same fiber generated by DWDM systems.

The origin of non-linear effects is the fact that, rather unexpectedly, the speed of light inside a silica fiber does depend on its intensity, or instantaneous power. (Remember, the speed of light is only constant in a vacuum.) This is most likely to be observed in DWDM systems where lots channels of data are packed into the same fiber, creating very high total optical powers.

"People knew that non-linearities were doing something, but they couldn't quantify it precisely," says Mitra.

Mitra and Stark were able to include non-linearities in the calculations for the first time. Why hadn't this been done before? Simply because it required some creative mathematical thinking to reduce the equations to ones that could be solved analytically.

— Pauline Rigby, Senior Editor, Light Reading
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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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