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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
http://www.lightreading.com
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ownstock 12/4/2012 | 8:07:07 PM
re: Physicists Find Fiber's Limit Since it was mentioned elsewhere, I would suggest you look into CenterPoint and Kestrel's technology. It is essentially FDM analog, with complex modulation on each channel, not as you suspect, simple plain-FM. But even plain FM is pretty good.

The channels (as seen pre-PLL) do overlap due to some spectral spreading, even after filtering, but this is perfectly allowable, so long as the data that emerges out of the subchannel in-lock has an acceptable error rate, and the time to achieve lock is also acceptable. Note the filtering is electrical, not optical, so can be very sharp and narrow.

Since each of the channels can be treated almost independently (save for some channel spacing), they can easily be tailored for the local dispersion slope. This can be done using adaptive algorithms commonly found in low cost MODEM (signal processing) chips.

Although the overall spectral density of the commercial equipment from each of these companies is not 10 bps/Hz, it can be significantly above 3 bps/Hz.

The fact that this type of system was not even discussed is the reason they had to go to Nature to get press...the general approach (to high spectral density data) is fairly common knowledge, and has been done (some might say over done!) in the relevant journals...
jayja 12/4/2012 | 8:07:09 PM
re: Physicists Find Fiber's Limit From "A Study in Scarlet" by Arthur Conan Doyle - Sherlock Holmes would not care about the theoretical bandwidth in a fiber. He would care about how to effectively bring bandwidth to users and how to put products and services on it that they want.

His ignorance was as remarkable as his knowledge. Of contemporary literature, philosophy and politics he appeared to know next to nothing. Upon my quoting Thomas Carlyle, he inquired in the naivest way who he might be and what he had done. My surprise reached a climax, however, when I found incidentally that he was ignorant of the Copernican Theory and of the composition of the Solar System. That any civilized human being in this nineteenth century should not be aware that the earth travelled round the sun appeared to me to be such an extraordinary fact that I could hardly realize it.

"You appear to be astonished," he said, smiling at my expression of surprise. "Now that I do know it I shall do my best to forget it."

"To forget it!"

"You see," he explained, I consider that a man's brain originally is like a little empty attic, and you have to stock it with such furniture as you choose. A fool takes in all the lumber of every sort that he comes across, so that the knowledge which might be useful to him gets crowded out, or at best is jumbled up with a lot of other things, so that he has a difficulty in laying his hands upon it. Now the skillful workman is very careful indeed as to what he takes into his brain-attic. He will have nothing but the tools which may help him in doing his work, but of these he has a large assortment, and all in the most perfect order. It is a mistake to think that that little room has elastic walls and can distend to any extent. Depend upon it there comes a time when for every addition of knowledge you forget something that you knew before. It is of the highest importance, therefore, not to have useless facts elbowing out the useful ones."

"But the Solar System!" I protested.

"What the deuce is it to me?" he interrupted impatiently: "you say that we go round the sun. If we went round the moon it would not make a pennyworth of difference to me or to my work."

thesource 12/4/2012 | 8:07:18 PM
re: Physicists Find Fiber's Limit When do you think the long haul market will pick up again and telecoms resume lighting dark fiber???
km
thesource 12/4/2012 | 8:07:18 PM
re: Physicists Find Fiber's Limit does anyone think this technology is dead for the foreseeable future???
what do you think of CORV products in the area??
thanks in advance.
km
ubwdm 12/4/2012 | 8:07:19 PM
re: Physicists Find Fiber's Limit Okay, it's time to finish this FM, angle modulation business.

1). FM has nothing to do with phase modulation
in question.
The phase modulation in question as raised by
reviewers is a modulation on E field, or a
phasor as called in Goodman's. The intensity of
a phasor (E*conj(E)) is then always constant.
Take a phasor into NLS, you will see SPM and
XPM does not set a limit for its propagation,
as the reviewers pointed out.
Now let's talk about more realistic multi
channel systems since capacity is the question.
This is what the reviewers should go further.
We can have two type of systems: Multiplicative or additive. DWDM is additive. So everything about "a phasor" is now totaly off, now we have a lot of cross terms, each representing the inter
channel crosstalk and the results are much worse
than OOK systems since phase is more sensitive
to everything.
For a multiplicative (modulated) system, you can have bandwidth limited multichannels. You can do
that by cascading multiple MZ modulators
(as Centerpoint, Kestrel are doing). So now
we are essentially similiar to OTDM business when capacity is the question: Will nonlinear SPM and XPM set a limit for capacity. The answer is now not so obvious but still yes! Because now the nonlinearity and dispersion will broaden or compress each channel differently also it varies along the fiber for each channel, which limit how
close the channel can be placed. But,you may ask, what about soliton? The answer is: Well, the soliton efficiency has not changed compared to DWDM case. We still cannot do better than the NON-Shannon conforming limit.

Now, back to the FM confusion. FM modulates carrier frequency on the RF power. The instanous intensity is not constant. In optics, the equivalent carrier frequency is the wavelength.
The confusion is actually quite consistent with other stuff being said, again due to lack of basic training.


ppm 12/4/2012 | 8:07:26 PM
re: Physicists Find Fiber's Limit I think I can safely say I have a working
as well as theoretical knowledge of FM,
having worked on wireless communication systems.
In any case, FM signals do not violate my
statement about band limitation: a pure
phase signal is guaranteed to have out of band
power. FM is no exception. One can try to suppress
out of band power but cannot get rid of it.
For a refresher, see Proakis P.215-217.

Adding together independent FM signals which
are spaced close enough together to have high
spectral efficiency (ie no interchannel
spacings) is also guaranteed to generate large
amplitude modulations.

I think I don't need to respond to the bizzare
statement about needing square law detectors to
obtain amplitude fluctuations from a beating
signal. Adding nontrivial phase only signals
together produce non-phase-only signals, namely
signals with amplitude fluctuations. As these
propagate down the fiber, these will generate
CPM in the usual way.

The coherent subband systems that I have seen
touted don't have anywhere near the spectral
efficiencies corresponding to the constellations
used.
Spectral efficiency = bit rate /total bandwidth
(*including* interchannel spacings). The large
constellations are meaningless - you simply
have to look at the delivered total data rates
and the total bandwidths counting channel
spacing. One can put arbitrary complicated
signals on channels spaced far apart - that
will do nothing to improve the true spectral efficiency.
ppm 12/4/2012 | 8:07:26 PM
re: Physicists Find Fiber's Limit At this point, your posting seems to have turned
into gobbleydegook, so I find it hard to respond.
The capacity estimate in the paper is obtained as
usual by bounding mutual entropies, following
general statement of Shannon capacity for the
channel. Strictly speaking, a bound is computed
based on a WGN process at the input, which
inherently contains amplitude and phase
fluctuations.
ubwdm 12/4/2012 | 8:07:28 PM
re: Physicists Find Fiber's Limit Some common telecom terminology for ownstock:
Phase buildout,
SPI,
Utopia,
Rammon,

Please get lost until you figure out them.
Thanks.


ubwdm 12/4/2012 | 8:07:28 PM
re: Physicists Find Fiber's Limit In other word, meaningless results. None of
sources you mentioned are updated and are
for a single channel only. Without considering chirp it is meaningless.



ownstock 12/4/2012 | 8:07:28 PM
re: Physicists Find Fiber's Limit You said: The comments in the Kahn and Ho article about phase modulation schemes getting rid
of amplitude fluctuations (and hence CPM)
are not quite accurate. The reason is somewhat
subtle: to do that with a WDM system, you
would have to do two things: (1) build
a *bandlimited* phase only signal. This cannot
be done (except for the trivial case
of a constant tone).

Answer: Wrong, it is done every day, all day long: it is called the FM band on your radio...each of the signals on the radio is band limited...wonder how they overcame your math problem? Think a bit...


You also said: Further, one * cannot * enforce the phase only behaviour by designing individual
channels to be phase only - ever hear of beating?
adding two phase only signals in general will
produce amplitude fluctuations.

Answer: Wrong again. Well, first, to be accurate, two phase only signals would generate a sum and difference signal (beating) on a square law detector, but there is no square law detector (at least in the fiber) here.

Next, even if there was, you assume a coherent relation between the modulation signal sources. Don't.

You are correct in that nonlinearity can cause PM to AM conversion...but less PM means less AM, so that is the approach: multiple subchannels: just like on the FM radio.

So in summary, multiple FDM complex modulated signals, each with independent coding, independent sources of modulation, on each of multiple wavelengths, none of which are coherent to each other, but each of which is detected coherently, through an optical filter, on a balanced detector. Subchannels filtered and processed digitally on low cost ICs.

It's in field trials with a major telecom customer today...I guess they forgot to see if it was working...

-Own
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