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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
ownstock 12/4/2012 | 8:06:07 PM
re: Physicists Find Fiber's Limit Clearly you need to read Agrawal Second Ed Ch 7, in particaluar the first paragraph on page 305 and ask yourself 1) What would the noise spectrum of AM/PM noise look like for complex modulated signals, and having figured that out, 2) could would it interfere with the detected signals, and 3) could it be filtered out 4) how much and how?
ppm 12/4/2012 | 8:06:34 PM
re: Physicists Find Fiber's Limit hahaha

Even math and physics no longer works. Great.

The proof I supplied for applying the central
limit theorem is perfectly valid at any given
time. Having different carriers makes no
difference. Adding a separate phase to each
of N random phases still leaves those phases
random. Proof: If phi_n are uniformly
i.i.d. distributed, so are phi_n+n*omega*t.

As for the NLS, it does not require
quasimonochromatic fields. The E^2 term in
the dielectric constant is the total field
intensity, not any spectral density.

Gosh, too many textbooks will now have to be
rewritten ... or maybe not (!)
ownstock 12/4/2012 | 8:06:36 PM
re: Physicists Find Fiber's Limit Exactly! That equation is only accurate under the assumption of quasi-monochromatic waves, or if you like: pair-wise between individual tones of two complex signals. Consider it this way: when the individual tone powers of the complex multi-tone signal are reduced (spread spectrum), the nonlinear tones (cross products) are reduced proportionate to the square of spread. In simple terms, cut the signal tone powers by 3db by doubling the number of tones, the power of each cross tone drops by 6db. Therefore, spectral density, not total power, is the issue.
ubwdm 12/4/2012 | 8:06:43 PM
re: Physicists Find Fiber's Limit "Even if you did, most fiber NL effects scale non-linearly with power spectral density of the source (not total optical power)..."

Where did you get that idea? Have you ever seen
n(w) = n0(w) + n1 * (E*conj(E))
?


ownstock 12/4/2012 | 8:06:58 PM
re: Physicists Find Fiber's Limit Gentlemen, a reminder:

WDM implies frequency differences in the base carriers. So you cannot make the assertion that WDM with phase modulation tends to Gaussian anything. FDM is ultra-WDM. You cannot ignore the frequency differences of the subchannels, even for a single wavelength.

Even if you did, most fiber NL effects scale non-linearly with power spectral density of the source (not total optical power)...so when the effective spectral density is greatly reduced, as is the case here, the NL effects are also significantly reduced...
ubwdm 12/4/2012 | 8:06:59 PM
re: Physicists Find Fiber's Limit Check out Hajimiri model. I was surpised that
you mentioned L.G. Kazovsky's paper and not
Hajimiri's.

The reference: The Design of CMOS radio-frequency
integrated circuits, by Thomas H. Lee of Stanford.

ppm 12/4/2012 | 8:07:02 PM
re: Physicists Find Fiber's Limit I wouldn't worry too much about Ito calculus
if I were you. Following Mark Kac in his famous
adage, "be wise and discretize", and you don't
have to worry too much about those mathematical
subtleties, it is taken care of automatically
for you ...
ppm 12/4/2012 | 8:07:03 PM
re: Physicists Find Fiber's Limit Thanks for reiterating the point about adding
phase only signals producing amplitude noise.
I would have thought that was elementary. Oh
well.

About solitons, I agree with you that spectral
efficiencies of soliton systems are actually
pretty low, significantly less than 1bit/s/Hz,
because you have to keep the solitons apart
by a sufficiently long time slot. I presume
that at least theoretically one could use so
called multi-solitons to get the spectral
efficiencies up and get closer to a bit - but
then with the complexities of a WDM system to
deal with, I have my suspicions that even
multi-solitons won't go very high with
spectral efficiencies either. I think solitons
may have other merits, but they are not the
road to very high spectral efficiencies.
ppm 12/4/2012 | 8:07:03 PM
re: Physicists Find Fiber's Limit Here's the point: I am sure if you take
a sufficiently * narrow * total bandwidth, you
can get spectral efficiencies close to
a linear channel, because it is the interplay
between nonlinearities and dispersion that
causes problems. One way of looking at
the problem is, compute the nonlinear
length and the dispersion length. If one
of these is much longer than the other
one, then you are either in the pure
SPM or pure dispersion limit: in either
of those cases, you'll be back to the linear
channel formula for spectral efficiencies.

To take an extreme example just for pedagogical
purposes, if you stuck a bunch of audio
bandwidth channels side by side, in fiber,
covering a * small * total bandwidth, say
1GHz, I have no doubt that you can get
spectral efficiencies predicted by the linear
SNR, as long as you are not killed by the
phase noise generated by nonlinear mixing
of amplifier ASE and signal (which will
happen for long enough systems and will take
away a factor of two).

* However * the spectral efficiency when a very
broad bandwidth is being used, in a WDM system,
it is a very different story. That is where the
paper applies: to consider theoretical limits
to the fiber, you really have to excite the
whole allowable bandwidth.

Of the systems you refer to, please quote:
(1) total data rate
(2) total bandwidth (*including channel spacing*)

If (1)/(2) is significantly large, my prediction
is that (2) is comparatively small to the
total optical bandwidth.

Finally, here is a simple proof for you that
what the Kahn and Ho say about getting rid of
CPM using phase modulation is not really
correct for WDM systems: ubwdm has by now
pointed this out multiple times: suppose
you * did * produce a number of phase only
signals exp[i phi_n(t)] which carry
information. Now consider any instant of time,
and consider the total complex amplitude
at a given time:

A(t) = Sum_n exp[i phi_n(t)]

Since the signals are information bearing, at
any instant of time phi_n(t) are random.
Therefore, by applying the law of large numbers,
A(t) tends to a complex Gaussian distribution
with amplitude and phase fluctuations as
the number of WDM channels grow large.

This leads to CPM, and for large enough total
bandwidths, long enough fibers, etc, will
destroy the spectral efficiency compared to
the linear channel. As far as I am concerned,
that is the message of this paper ...

Note that you do not escape the problems by
dividing a very wide band into very narrow
channels, because the CPM seen by a given
channel will not depend on its own bandwidth.
Therefore, it is sort of irrelevant whether
you have sharp filters at your output or not.
you may need that to * get * to the allowed
(nonlinear) limit because that will require
coherent detection (otherwise you lose phase).
Unless, as ubwdm points out, you use tdm.

calpole 12/4/2012 | 8:07:04 PM
re: Physicists Find Fiber's Limit Phase error variance of OPLL inpresence of
GVD and fiber-nonlinearity can not be simply
determined by the use of common theoretical techniques in wireless..like
assumimg Tikanov distrivution of phase error etc..
..Interestingly, no good dynamic theory exists for
OPLL in presence of ISI due to
GVD and non-linearity..(
I mean starting from the fundamental
assumptions of Eto calculus etc ..)except a few standard
old papers by L.G.Kazovsky..
I guess, PPM and UBWDM will do us a favor,
by providing a very good dynmic theory of
OPLL locking, in presence of phase noise
due to GVD and non-linearity..Let do
complete cooking than a half-baked potatos..
..Since, I am not that good theory,
I couldn't solve the problem, I hope
PPM will take the challenge..I swear
it's an amazingly good problem on eto calculus..

-Calpole
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
calpole 12/4/2012 | 8:07:29 PM
re: Physicists Find Fiber's Limit The result is not by me,..
standard results publsihed by
Elraife et al in JLT '91 Jan {?]
I guess on limitation of transmission distance
by GVD etc....I verified it. Also
you can verify the same from
'Optical Networking" by Bonnoni, where
there is a contribution from Enrico
Forestari, on it with a different method.
No chirp is assumed.
Also you can see "Coherent Communication"
by Betti etal, that book conatins a lot of
results proving the superiority of
IMDD system in transmission related impairment.
calpole 12/4/2012 | 8:07:29 PM
re: Physicists Find Fiber's Limit Well, in your last posting, there are many mistakes which I don't want to point out.

But if you were true [ which is not true either],
then you have used Shanon's theorem
wrongly. Your SNR has to be with respect to based-band
signal in this case. Which means,
your SNR calculation assumes amplitude
modulation if I go by your formula!!!
And you are finding amplitude fluctuation
from beating!!!! You have to calculate
its magnitude after filtering the respective
channel..

Secondly, \eta calculation assumes the separation of
dispersion and non-linearity operator.
No matter whatever be the approach,
either by analytical IST method or numerical SSF
, that is always the inherent assumption and
so is your calculation. I hope this point
is pretty clear. Non-commutation of both the
operator is the starting point..
..
..
Further, as far the paper go, I have
strong feeling that Authors may be
good theoretician but surely venturing
this field for a recent time. Otherwise,
they would have thought of more meaningful
way of getting amplitude fluctuation
to fit into the Shanon's formula.
-Calpole
ubwdm 12/4/2012 | 8:07:29 PM
re: Physicists Find Fiber's Limit To be more specific, GVD is there. But that's
not what calpole is complaining.

Back to the phase modulation simulation results,
do you include negative chirp in you simulation,
calpole? The numbers do not look realistic to me.
Could you be more specific about the conditions
you used? Any consideration as to channel bandwidth etc?

ownstock 12/4/2012 | 8:07:32 PM
re: Physicists Find Fiber's Limit Vacuum Polarization (Physics) A process in which an electromagnetic field gives rise to virtual electron-positron pairs that effectively alter the distribution of charges and currents that generated the original electromagnetic field. (Human Biology) A process in which a childishly trivial posting on an electronic bulletin board generates a virtual intellectual gradient, lowering the average IQ of the board, resulting in a waste of time for everyone.
ppm 12/4/2012 | 8:07:32 PM
re: Physicists Find Fiber's Limit Your questions have already been answered,
if you had chosen to read my post carefully.

The NLS has both dispersion and nonlinearity
in it simultaneously. I have no idea what you
mean by decoupling dispersion and nonlinearity.
If there were only nonlinearity, or only
dispersion, the evolution equations are trivial.
I would have thought that this was adequately
elementary.

I think that the basic misunderstanding that you
have stems from a fictional notion of a phase
only (you call it angle only) signal in a WDM
context. I will include below the passage from
my last post that explains why this is in error.
I suggest you perform the following exercise
for yourself. Construct your favourite signal
source (has to be WDM, and has to be
information-bearing). If one allows for
real time cooperation between channels, it
should be mathematically possible (though of
no practical interest) to construct an
*approximately* phase only (or angle only)
electric field even with many channels.
Propagate your equation beyond a dispersion
length. Lo and behold! You will find that amplitude fluctuations have appeared. For
system lengths significantly longer than the
dispersion length, which is what is treated in
the paper, one can pretty much neglect these
initial transients.

-------------------

From before:

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). I won't burden you with
the necessary math - try it our for yourself;
you'll find that trying to twist the phase
of a sine wave to make it carry information
will inevitably put power out of band.
(2) 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.
In principle, one could make all the WDM
channels cooperate so that the * joint * signal
had a constant phase (up to a small out of band
leakage) initially. However, that would require
electronic bandwidths of THz (if we had that
we wouldn't be discussing WDM in the first case.
Finally, even if you started with a phase
only signal, dispersion would eventually
convert phase fluctuations to amplitude
fluctuations. For these reasons, you cannot
escape amplitude fluctuations and hence
CPM. Therefore the comment about eliminating
CPM is rather glib. However, assuming that
they were the paper reviewers, they voted with
their feet by recommending publication of
the paper.


ubwdm 12/4/2012 | 8:07:33 PM
re: Physicists Find Fiber's Limit
Let me try to help you out a little:
You worked with four Nobel Laureates.
You have won it five times. You have started
six startups, and built seven DWDM systems
covered everything from components to costomer care. You have used eight optical PLLs with
nine commercial LOs for every ten Terabits.
It shows.

Now please get lost in your communication classes,
at least until you figure out vacuum polarization in copper.


ownstock 12/4/2012 | 8:07:34 PM
re: Physicists Find Fiber's Limit Ubwbm:

OT: First, in reality, the Nobel prize is only as good as the person that receives it...the prize by itself does not bestow nobility nor super-intelligence nor ethics. Like most awards, it is half technical and half political.

I have worked for/with several Nobel laureats (four to be exact) in my career, and they ranged from very smart and honorable (earned it and deserved it, poor politicians) to average smart and sneaky (stole their colleagues or students work and called it their own, great politicians).

Sorry, but getting the Nobel prize is not the be-all, and end-all proof of intelligence. LU is a loser company, period. If they are still at LU, they are losers...all the more because they are there with all their great credentials, etc. Probably what we call "walking dead", or "retired-in-place (RIP)". I know one Nobel prize winner (one of the ones I worked for many years ago) pimping a start-up for options and cash...letting his name (and the prize) be associated with a flakey VC-run spin-flip operation.

Now, as to nobody having done a good optical LO, you only confirm my assertion that you do not read.

Suggest you check out the following:

http://www.tuc.nrao.edu/~demer...

I can give you commercial references if you like for complete LO/detector systems you can buy off the shelf, but I will leve that to you as homework.

-Own



calpole 12/4/2012 | 8:07:50 PM
re: Physicists Find Fiber's Limit Thanks for your posting, I guess we must
continue to debate without attacking
personally..But, there are many things with
which, I can not agree on the article...



"""
First, you have this discussion about bit
rates at fixed eye opening penalties.
You probably won't take this well, but the
quantity discussed in the article is the
channel capacity. This is the maximal possible
rate of error free transmission. Using some
particular modulation and ECC scheme, one can talk
about bit rates and the corresponding BER or other
penalty measure. That is more ad hoc than the
capacity. Admittedly, the capacity is a
theoretical limit than a practical quantity;
however, that is the quantity in the paper,
not the quantities you want to discuss. It
provides a wall that the bit rates for fixed
penalties cannot cross (and if you have a good
enough coding scheme, like a turbo code, you can
make the (bit rate) vs (SNR per bit (EbNo))
curves approximate the capacity limit. Almost
by definition, the quantities you are interested
in are more derivative (and therefore need
transformation) before being compared to
channel capacity. """"


>>>>>>Your above assessment is correct. No problem
in converging in opinian.

It is true that CPM, FWM build up with system
length. This is in fact the effect that leads
to the capacity limits discussed in the paper
(specifically from CPM). It is just that the
buildup is different from that of amplifier
noise or other additive forms of noise. In
fact, the length dependence is specifically
present in the formula for the peak capacity
if you care to look (through I0).
The peak capacity goes to zero at infinite length.
However, the dependence is not a simple inverse
relation to distance, and therefore peak rate
times length is not a good metric. In fact, the
results show that the bit-rate times length
metric needs revision for the nonlinear
effects to be treated properly.

>>>>>>> Well, here is the problem. The formula
of \eta that you have used to assess the
SNR due to CPM either in IMDD or coherent
system defies the reality. If you talk about
angle-modulated systems, CPM affects via
following way
[1] PM-AM conversion of Modulated Phase generates RIN

[2] This RIN is transferred as CPM to other channel and cause SNR degradation of
base-band signal in phase.

Problem with the treatment is that if
I consider IMDD system then also your
\eta is wrong and if I consider
angle-modulated system then also the assessment of
\eta is wrong even analytically.

>>>>>>>>>>>>>>>>>>>>>..
>>>>>>>>>>>>>>>>>>.....


I don't know where you got the idea that
dispersion is being neglected -
the NLS has both dispersion and nonlinearity
in it. The dispersion coefficient appears
explicitly in the different formulae. The
*relevant* bandwidth is in the GHz rather than
the THz range, since the discussion is about
WDM systems. For the same reason, higher
order dispersion can be neglected. Dispersion
between channels leads to suppression of distant
channels, as also discussed.

>>>>>>>>>>.Here is the problem. You have decoupled
Dispersion and Non-linearity .
\Detlta \E_{field} term in your
expression should be term that depends on
dispersion. I don't see it.

Also, even if individual channels are in GHz, since
you are considering so many of them,
third order dispersion is definietly the most important parameters as \beta_2 will
vary accross the channel depending on
\beta_3. You have assumed all the channels
have equal \beta_2
parameters.

*********************

Also, I don't see where you get the bit about
intensity noise: the paper deals explicitly
with the E field, which means it deals with
coherent communication (amplitude and phase).

>>>>>>>If there is no-Intensity noise
how could get an effect of SNR degradation in
baseband due to CPM in Angle-modulated systems???





A discussion of Q values in this context is
misleading because the noise in question is not
Gaussian. If it were, there would not be any
discussion. We'd be back to the linear channel
case.
>>>> OK, though there are some points,
I am not diverting the attension ..>>>

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). I won't burden you with
the necessary math - try it our for yourself;
you'll find that trying to twist the phase
of a sine wave to make it carry information
will inevitably put power out of band.
(2) 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.
In principle, one could make all the WDM
channels cooperate so that the * joint * signal
had a constant phase (up to a small out of band
leakage) initially. However, that would require
electronic bandwidths of THz (if we had that
we wouldn't be discussing WDM in the first case.
Finally, even if you started with a phase
only signal, dispersion would eventually
convert phase fluctuations to amplitude
fluctuations. For these reasons, you cannot
escape amplitude fluctuations and hence
CPM. Therefore the comment about eliminating
CPM is rather glib. However, assuming that
they were the paper reviewers, they voted with
their feet by recommending publication of
the paper.

As for estimating the quantity \eta - (I presume
that is that you mean by \eta_c) -
analytical estimates are always different
from numerical ones. They are approximate,
but they are in closed form and give insight
that is difficult and inefficient to derive
from numerics.

ppm.

"He who knows that he knows ... " etc.

calpole 12/4/2012 | 8:07:51 PM
re: Physicists Find Fiber's Limit Thanks for your posting, I guess we must
continue to debate without attacking
personally..But, there are many things with
which, I can not agree on the article...



"""
First, you have this discussion about bit
rates at fixed eye opening penalties.
You probably won't take this well, but the
quantity discussed in the article is the
channel capacity. This is the maximal possible
rate of error free transmission. Using some
particular modulation and ECC scheme, one can talk
about bit rates and the corresponding BER or other
penalty measure. That is more ad hoc than the
capacity. Admittedly, the capacity is a
theoretical limit than a practical quantity;
however, that is the quantity in the paper,
not the quantities you want to discuss. It
provides a wall that the bit rates for fixed
penalties cannot cross (and if you have a good
enough coding scheme, like a turbo code, you can
make the (bit rate) vs (SNR per bit (EbNo))
curves approximate the capacity limit. Almost
by definition, the quantities you are interested
in are more derivative (and therefore need
transformation) before being compared to
channel capacity. """"


>>>>>>Your above assessment is correct. No problem
in converging in opinian.

It is true that CPM, FWM build up with system
length. This is in fact the effect that leads
to the capacity limits discussed in the paper
(specifically from CPM). It is just that the
buildup is different from that of amplifier
noise or other additive forms of noise. In
fact, the length dependence is specifically
present in the formula for the peak capacity
if you care to look (through I0).
The peak capacity goes to zero at infinite length.
However, the dependence is not a simple inverse
relation to distance, and therefore peak rate
times length is not a good metric. In fact, the
results show that the bit-rate times length
metric needs revision for the nonlinear
effects to be treated properly.

>>>>>>> Well, here is the problem. The formula
of \eta that you have used to assess the
SNR due to CPM either in IMDD or coherent
system defies the reality. If you talk about
angle-modulated systems, CPM affects via
following way
[1] PM-AM conversion of Modulated Phase generates RIN

[2] This RIN is transferred as CPM to other channel and cause SNR degradation of
base-band signal in phase.

Problem with the treatment is that if
I consider IMDD system then also your
\eta is wrong and if I consider
angle-modulated system then also the assessment of
\eta is wrong even analytically.

>>>>>>>>>>>>>>>>>>>>>..
>>>>>>>>>>>>>>>>>>.....


I don't know where you got the idea that
dispersion is being neglected -
the NLS has both dispersion and nonlinearity
in it. The dispersion coefficient appears
explicitly in the different formulae. The
*relevant* bandwidth is in the GHz rather than
the THz range, since the discussion is about
WDM systems. For the same reason, higher
order dispersion can be neglected. Dispersion
between channels leads to suppression of distant
channels, as also discussed.

>>>>>>>>>>.Here is the problem. You have decoupled
Dispersion and Non-linearity .
\Detlta \E_{field} term in your
expression should be term that depends on
dispersion. I don't see it.

Also, even if individual channels are in GHz, since
you are considering so many of them,
third order dispersion is definietly the most important parameters as \beta_2 will
vary accross the channel depending on
\beta_3. You have assumed all the channels
have equal \beta_2
parameters.

*********************

Also, I don't see where you get the bit about
intensity noise: the paper deals explicitly
with the E field, which means it deals with
coherent communication (amplitude and phase).

>>>>>>>If there is no-Intensity noise
how could get an effect of SNR degradation in
baseband due to CPM in Angle-modulated systems???





A discussion of Q values in this context is
misleading because the noise in question is not
Gaussian. If it were, there would not be any
discussion. We'd be back to the linear channel
case.
>>>> OK, though there are some points,
I am not diverting the attension ..>>>

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). I won't burden you with
the necessary math - try it our for yourself;
you'll find that trying to twist the phase
of a sine wave to make it carry information
will inevitably put power out of band.
(2) 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.
In principle, one could make all the WDM
channels cooperate so that the * joint * signal
had a constant phase (up to a small out of band
leakage) initially. However, that would require
electronic bandwidths of THz (if we had that
we wouldn't be discussing WDM in the first case.
Finally, even if you started with a phase
only signal, dispersion would eventually
convert phase fluctuations to amplitude
fluctuations. For these reasons, you cannot
escape amplitude fluctuations and hence
CPM. Therefore the comment about eliminating
CPM is rather glib. However, assuming that
they were the paper reviewers, they voted with
their feet by recommending publication of
the paper.

As for estimating the quantity \eta - (I presume
that is that you mean by \eta_c) -
analytical estimates are always different
from numerical ones. They are approximate,
but they are in closed form and give insight
that is difficult and inefficient to derive
from numerics.

ppm.

"He who knows that he knows ... " etc.

ppm 12/4/2012 | 8:07:57 PM
re: Physicists Find Fiber's Limit I have read the article already, thank you.
I have not read your thesis, however, I have this
strange feeling that it wouldn't help me
advance my knowledge about the issues at hand.

About your post: let me point out a few things.

First, you have this discussion about bit
rates at fixed eye opening penalties.
You probably won't take this well, but the
quantity discussed in the article is the
channel capacity. This is the maximal possible
rate of error free transmission. Using some
particular modulation and ECC scheme, one can talk
about bit rates and the corresponding BER or other
penalty measure. That is more ad hoc than the
capacity. Admittedly, the capacity is a
theoretical limit than a practical quantity;
however, that is the quantity in the paper,
not the quantities you want to discuss. It
provides a wall that the bit rates for fixed
penalties cannot cross (and if you have a good
enough coding scheme, like a turbo code, you can
make the (bit rate) vs (SNR per bit (EbNo))
curves approximate the capacity limit. Almost
by definition, the quantities you are interested
in are more derivative (and therefore need
transformation) before being compared to
channel capacity.

It is true that CPM, FWM build up with system
length. This is in fact the effect that leads
to the capacity limits discussed in the paper
(specifically from CPM). It is just that the
buildup is different from that of amplifier
noise or other additive forms of noise. In
fact, the length dependence is specifically
present in the formula for the peak capacity
if you care to look (through I0).
The peak capacity goes to zero at infinite length.
However, the dependence is not a simple inverse
relation to distance, and therefore peak rate
times length is not a good metric. In fact, the
results show that the bit-rate times length
metric needs revision for the nonlinear
effects to be treated properly.

I don't know where you got the idea that
dispersion is being neglected -
the NLS has both dispersion and nonlinearity
in it. The dispersion coefficient appears
explicitly in the different formulae. The
*relevant* bandwidth is in the GHz rather than
the THz range, since the discussion is about
WDM systems. For the same reason, higher
order dispersion can be neglected. Dispersion
between channels leads to suppression of distant
channels, as also discussed.

Also, I don't see where you get the bit about
intensity noise: the paper deals explicitly
with the E field, which means it deals with
coherent communication (amplitude and phase).

A discussion of Q values in this context is
misleading because the noise in question is not
Gaussian. If it were, there would not be any
discussion. We'd be back to the linear channel
case.

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). I won't burden you with
the necessary math - try it our for yourself;
you'll find that trying to twist the phase
of a sine wave to make it carry information
will inevitably put power out of band.
(2) 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.
In principle, one could make all the WDM
channels cooperate so that the * joint * signal
had a constant phase (up to a small out of band
leakage) initially. However, that would require
electronic bandwidths of THz (if we had that
we wouldn't be discussing WDM in the first case.
Finally, even if you started with a phase
only signal, dispersion would eventually
convert phase fluctuations to amplitude
fluctuations. For these reasons, you cannot
escape amplitude fluctuations and hence
CPM. Therefore the comment about eliminating
CPM is rather glib. However, assuming that
they were the paper reviewers, they voted with
their feet by recommending publication of
the paper.

As for estimating the quantity \eta - (I presume
that is that you mean by \eta_c) -
analytical estimates are always different
from numerical ones. They are approximate,
but they are in closed form and give insight
that is difficult and inefficient to derive
from numerics.

ppm.

"He who knows that he knows ... " etc.

calpole 12/4/2012 | 8:07:58 PM
re: Physicists Find Fiber's Limit Well, I have decided to stop my attack on
the article. Let switch over to the
new topic as pointed out by ubwdm: Wheather
coherent systems hold the key to enhance the capacity in future..

Actually, I started my research on coherent systems as it was very hot till the middle of
the last decade. As far as my simulation results
reveal, angle-modulated systems suffer much
more severely from dispersion and
non-linearity. So, bitrate-distance product is
always much less for angle-modulated systems.
Here is a table of maximum transmission distance
at 10Gb/s over SMF-28 with only dispersion.
by taking 1dB eye-penalty as the requirement.

NRZ-IMDD----67 Km
PSK--------53 Km
MSK------47 Km
CPFSK---- 40 Km

So, definitely for long-haul and even for medium
haul, angle-modulated systems don't seem to have
a future simply because of its poorer performance
against dispersion and non-linearity.

Howerever, in Fiber-wireless systems,
in future, Coherent systems have a bright
future for a seamless communication.


ubwdm 12/4/2012 | 8:07:59 PM
re: Physicists Find Fiber's Limit 1). I mentioned Nobel prize winner because the other guy claims that only losers are left in Bell Labs. Decent people don't call them losers.
Many losers are left in Lucent,true,but it takes
a true loser to make a statement such as
"only people left there are losers".
Please read it again.

2). Look, I am not here to defend the paper.
There are actually a few loose connections
and I agree with you that some aspects are treated not well in the paper, but the merit of the paper is clear to me. It shows from NLS that nonlinearity sets a capacity limit for a typical fiber(as we knew it for a while) and it actually shows relationship between many variables, some are system metric and some are not. It does not mean you cannot work around the limitations. For example, what if one builds
new fibers with alternating linearity coefficient
(similiar to alternating dispersion in truewave fiber) or new hollow fibers with exotic cladding? No, the paper does not build real systems for you, nor does it claim to.

[1] How valid is the estimation of
\eta_c?
It is not as accurate as some of simulations I've
seen (maybe you did some of them?) for a given
problem, but what's important here are the concepts and qualitative behaviors.

[2]How wise it is to calculate the RIN
due to non-linearity
without considering dispersion [ Mind it
they have used it only for
useless pupose ] when
author is talking about Tb/s???

The authors assume that dispersion won't help
the fiber capacity, which sounds reasonable
to me. I will address the phase modulation separately bellow. In case of soliton, it is possible to balance them out, but as I said before, the result is lower than their cases. No, they have not mathematically proven it is true.

[3] @50 Tb/s, third and fourth order dispersion
will be very crucial. How can they neglect
it in their calculation???

Again, they are under same assumption.

Now, points raised by reviewers:
1) Will coherent phase modulation (I could tell everything about the original concept and the patent, but it is a different story) improve the
capacity? Maybe, I am not sure. Would it change
discussions in the paper? It centainly
would. Why it is ignored? Because coherent modulation is not practical in at least
forseable future, not until optical PLL is
practical. Lasers work because of coherency, holographic-anything does not work because of
the required coherence.

So, the conclusion: the paper presented one of
better analysis that relates to today's understanding. It proves one thing for sure:
The capacity of a fiber as we know it today
has a capacity limit that's much lower than that
of Shannon's theory and increasing power would
not help.
You may not appreciate it because you are just out of school or whatever, but don't attack Nature because it published it.
And I don't think you have any evidence that it is pure political.







calpole 12/4/2012 | 8:08:00 PM
re: Physicists Find Fiber's Limit I guess we are fighting over an article
and even if it is written by a Nobel Lauriet
does not gurantee the fact that he will talk
sense all time.

Now, look at this: I have pointed out only
a few crucial mistakes in the paper.
So, far none of you has been able to defend
that article, because it has been a sloppy
article and now you are defending saying that
it has been written by a Nobel Lauriet????
One of the authors of the article should be more ashamed because Nobel Commitee trusted
him and now he is out to encash it
by throwing any trash that he likes?

Defend the arcticle technically ubwdm.
I have just finished my PhD in this field
and I know no matter whether that crab
is written by a Nobel Lauriet or
a big 'Ass', can not be defended
technically. If you have courage, answer following
questions:
[1] How valid is the estimation of
\eta_c?
[2]How wise it is to calculate the RIN
due to non-linearity
without considering dispersion [ Mind it
they have used it only for
useless pupose ] when
author is talking about Tb/s???
[3] @50 Tb/s, third and fourth order dispersion
will be very crucial. How can they neglect
it in their calculation???

Please, don't defend it by saying it is from
a Nobel Lauriet. Defend it technically so
that I can learn something from the erudite
person like you. Again, I will be happy
to find that I am mistaken if you can prove
it.
ubwdm 12/4/2012 | 8:08:02 PM
re: Physicists Find Fiber's Limit "At least you had the good sense to leave Bell Labs...how long did it take to figure out the only people left there are losers?"

Quite a few Nobel prize winners would be losers
because they would all lose to nameless on the net... Just like the authors of this paper.

"If that article was in your honest technical opinion "one of the best" on WDM you have ever read, I can only conclude that you must not read very much!"

That's hardly surprising to anyone here. My statement is "The paper is one of best among tens of thousands published every year".

" Apparently you have not published much either. Kahn and Ho's review called it: "a contribution", after they completely trashed their assumptions and therefore the conclusions based on them. "

And the paper gets published on Nature.

" Oh, by the way...the nonlinearity in Copper comes from several sources..."

As ppm pointed out, you forgot about vacuum polarization, if you ever learned it in your
communication classes.

"Guess none of those successful start-ups paid for you...I can guess why..."

You must be really interested in that... Sorry,
I won't tell you anything. It is not part of
this discussion...



















ownstock 12/4/2012 | 8:08:04 PM
re: Physicists Find Fiber's Limit At least you had the good sense to leave Bell Labs...how long did it take to figure out the only people left there are losers?

If that article was in your honest technical opinion "one of the best" on WDM you have ever read, I can only conclude that you must not read very much!

Apparently you have not published much either. Kahn and Ho's review called it: "a contribution", after they completely trashed their assumptions and therefore the conclusions based on them.

Hint: That's what we call damnation by feigned praise!

Oh, by the way...the nonlinearity in Copper comes from several sources...point diodes due from poor contacts and twisted wires...line amplifiers...inadequate power supplies...but we managed to find ways around the problems...one at a time...

Finally, most of what we are discussing is not taught in EM class, it is taught in Communication Theory, or Communications Systems Analysis, which you would realize if you had taken any of the courses.

Hint: That's what we call: inadequate education!

Guess none of those successful start-ups paid for you...I can guess why...
calpole 12/4/2012 | 8:08:05 PM
re: Physicists Find Fiber's Limit >>>>>>>>>>>>>>
2. " They even don't know, without dispersion,
non-linearity can do no harm to either
IMDD or Angle modulated system!!!!!"

Comment: Huh, graduated from a graduate
course, but need to improve problem solving skills. Why?
a). Fiber or waveguide without dispersion for
whole spectrum, when the capacity is
the question?
b). The phase shift of the second order
SPM/XPM indeed "does no harm" to an angle
or pure phase modulated SIGNAL. But it
will HARM an angle modulated SYSTEM
because the SYSTEM in question has
>>>>>>>>>>>>>
Well, I have commented purely on fiber
transmission assuming it to be infinite
bandwidth system to treat and simplify the
problem of non-linearity.
Most of the finite-bandwidth system can be
modelded as a filter and
filters can also cause PM-IM
or IM-PM conversion and that way non-linearity
can affect the signal quality even
if dispersion is absent. But I have made
comment pertaining to the assumptions
used in the 'Nature's paper. There is
no point bringing an well known obvious issue.

3. "With proper dispersion compensation,
we can reduce the first order RIN due to
non-linear phase , ..."

Comment: Get a clue.

>>>>>Well, this has been estblished in
a paper in IEEE-JLT, April-2000 issue..
..
I guess that was a paper on CPFSK system and
the impact of fiber non-linearity on it.

>>>>>>>>
Sad part of the story about the article in Nature
is that its treatment of RIN through
\eta parameter is extremely vague. It's
too crude to be even 1% truth, and I know
it from my past long experince of simulation of
NLS and other RIN related stuff.
So, I can never digest that story.
It was published because it was from Bel-lab,
and I can ensure you without the kind
politics from the reviewer, such kind of
crab can never be published. Since,
I had a good respect for Nature, I was
disheartened to see a paper which even does
not fit into the standard of
3rd graded journal like 'Journal of
Optical Communication" from Germany.







ubwdm 12/4/2012 | 8:08:09 PM
re: Physicists Find Fiber's Limit Okey, Okey, you got to love all these half-baked potatoes on the net.

It is impossible to argue on this board.
I am not going to correct every statements,
but a couple are enough to show their
true colors...

1) "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..."

Comment: Need better basic training in graduate level EM theory.

2. " They even don't know, without dispersion,
non-linearity can do no harm to either
IMDD or Angle modulated system!!!!!"

Comment: Huh, graduated from a graduate
course, but need to improve problem solving skills. Why?
a). Fiber or waveguide without dispersion for
whole spectrum, when the capacity is
the question?
b). The phase shift of the second order
SPM/XPM indeed "does no harm" to an angle
or pure phase modulated SIGNAL. But it
will HARM an angle modulated SYSTEM
because the SYSTEM in question has
finite bandwidth and has multiple channels.
3. "With proper dispersion compensation,
we can reduce the first order RIN due to
non-linear phase , ..."

Comment: Get a clue.

It is apparent that many are confused by the
paper. The paper is one of best among tens of
thousands published every year, most of which
indeed are useless. To attack the quality of
"Nature" sounds more like sour grape than
anything else. The paper gives a good analysis
about nonlinear nature of fiber, but not for transmission with electrical or optical regeneration. It is very valuable esp. to those of us who are building long haul, ultra long haul/submarine systems who learned many issues with expensive and time consuming experiments. Those who have done it understand that they
are not "fundamental limits" for the SYSTEM,
as only a few clueless here claim so.

It's obvious that the paper does not cover the
case when DCM used along the fiber reconditions the signal just like it does not cover systems with 2R/3R every 100KM. In real systems signals could be demuxed and optically reshaped to overcome some nonlinear effects. But again, the paper helps us to analyse these issues.

BTW, I left Bell Labs long time ago and have been with a few successful startups. I probably know
more about Lucent's failure. I have to say it is
absurd and childish to correlate this paper
with the fact that Lucent is falling.









calpole 12/4/2012 | 8:08:10 PM
re: Physicists Find Fiber's Limit I agree that I have pointed out only the effect of
SPM and XPM. With proper dispersion compensation,
we can reduce the first order RIN due to
non-linear phase , however second-order RIN
can not be compensated due to simple nature
of sinusoidal addition( a mathematical
artifact of RIN expression due to PM-AM).

Finally, any merit can be attached to that paper?
ownstock 12/4/2012 | 8:08:11 PM
re: Physicists Find Fiber's Limit Direct quote from Natures reviewers, which you can access online at www.nature.com and registering for free...

"When calculating the effects of CPM, Mitra and Stark have implicitly assumed a modulation technique involving time-varying intensity. It is already known that using a constant-intensity modulation technique, such as phase or frequency modulation, can eliminate CPM. The fibre's refractive index varies slightly with the wavelength of light, which tends to convert phase or frequency modulation to intensity modulation. This wavelength dispersion must be carefully compensated for to maintain constant intensity. If we follow the same calculations as Mitra and Stark, and constant intensity is maintained, then the spectral-efficiency limit should increase with transmitted power, in contrast with their results. In reality, as the power increases, spectral efficiency would eventually be limited by other nonlinear effects, such as four-wave mixing.

As Mitra and Stark point out, nonlinearities such as CPM can be cancelled out, in principle, by using a number of clever tricks. Better optical fibres can also help GÇö for example, hollow fibres with air cores have a reduced nonlinear response. In ordinary optical fibres, light can propagate in two orthogonal polarizations GÇö that is, with electric field lines along two perpendicular directions. A simple way to double spectral efficiency is to send two independent signals with these different polarizations and use polarization-resolved detection. Even without polarization-resolved detection, sending neighbouring signals with perpendicular polarizations is a well-known method to reduce nonlinearities."

One of those "clever tricks" is to use balanced coherent detection...which also allows you to use full IQ modulation...and it cancels first order RIN down to shot noise. OK, so it will not cancel noise cross signal terms...

People have locked external cavity semiconductor lasers for many years...I believe you can buy half way decent equipment off the shelf....data I have seen approaches the best analog RF PLO results...

Anyway, last word is: Mitra and Stark are wrong, CPM is not a fundamental limit to the bandwidth of fiber, and people have done much, much better than 3 bits/hz over fiber...and routinely achieve much better than 20 dB SNR (on analog subchannels), and do so over long distances, and through EDFAs...in the field, today.

With the misguided technical analysis they are getting from the likes of Mitra and Stark, who probably represent the best they have, no wonder LU is headed for the dumpster...

-Own
abarbier 12/4/2012 | 8:08:15 PM
re: Physicists Find Fiber's Limit "They even don't know, without dispersion,
non-linearity can do no harm to either
IMDD or Angle modulated system!!!!!"

I think your statement is only true for SPM/XPM
but FWM and the scattering effects can deplete the
signal.
Moreover with no dispersion the signals do not "walk away" from each other and FWM effects (for example) are amplified.

DSF fiber are not good for WDM...
calpole 12/4/2012 | 8:08:15 PM
re: Physicists Find Fiber's Limit Now, I have read the Nature' arcticle.

Ya, now I am even more convinced that it's
a school physics calculation with a bunch of
school
physics approximations.

They talk about RIN (relative intensity noise)
with a vaguely defined term \eta_c.
Putting that value in Shanon's capacity theorem without reading any paper on RIN due to fiber non-linearity is quite stupid thing, and they did it.
They even don't know, without dispersion,
non-linearity can do no harm to either
IMDD or Angle modulated system!!!!!
It all happens because of PM-IM or IM-PM
conversion!!! Where is this in their calculation!!! Then they would have arrived
at the right equation.

I don't want to point out other mistakes, which are useless. If Nature continues to publish
such articles, soon or later it will be another
light-reading magazine.
calpole 12/4/2012 | 8:08:25 PM
re: Physicists Find Fiber's Limit First of all, we are talking about theoretical
limit. So, at the begining I have told that
I assume receiver is ready for 100Tb/s .
200 photon per bit is nearly equal to
15dBm @100 Tb/s for a receiver sensitivity of
10^-9 PE.

Quantum limit of receiver may not be the problem
for 100 Tb/s transmission question is
whether we can have that broad-band receiver and
sunch a modulator or any OTDM technique that can
give 100Tb/s transmission.

I cited the example of 1 micro-meter to illustrate the theoretical fact that
at that small distance dispersion and
non-linear effect will be negligible.
Please, go by its intension.
calpole 12/4/2012 | 8:08:26 PM
re: Physicists Find Fiber's Limit The word 'peak capacity' can arise if there
is some optimization curve in multidimensional
plot where fiber length and physical parameters
are treated as independent parameters and we
agree that accepted received signal performance
follows certain requirements like
1 dB eye-closure or below 1dB receiver sensitivity penalty @10^-12 BER or any other
SONET SR or IR requirement. Thanks to the
strong interplay between signal format, dispersion and non-linearity, we can always
get a value of peak bitrate but only for
a certain distance. If distance is also plotted
as an independent parameter and someone says
that he has achieved a peak value for
bit-rate from the plot, the very concept of
it is quite unacceptable because of the nature
of Non-linear Schodinger Euqation where
the space variable 'z' (the length)
appear as a first-order derivative
as a consequence of paraxial approximation.
Variance SPM, XPM and FWM power are all propotional to the distance and when any non-liear
phase affects on IM systems through
PM-IM conversion, varinace of Relative Intensity Noise
becomes propotional to distance for a smaller value of total-dispersion and varies as a sine of distance for more accurate approximation.
In contrast to that , Fiber filter function
has an exponent that is always propotional to
'z'. Since, 'Q' value is inversely dependent on
the variance , Log(BER) plot for a given
receiver power is inversely propotional to the
distance if the RIN due to fiber impairment
is more dominant than the receiver noise.

Now, if we search for a bit-rate optimization
keeping the constraint of 1 dB eye-closure
or 14dB 'Q' as accepted signal quality,
optimization of bit-rate with respect to
distance is meaningless because of very mathematical nature of
NLS. That's why it is
more important to talk about Bit-rate distance
product than bit-rate only for capacity.
This has been well known fact since the days of
electrical transmission theory.



ppm 12/4/2012 | 8:08:37 PM
re: Physicists Find Fiber's Limit I see - you think that the nonlinear crosstalk
is the limiting factor for holographic storage
as well (as opposed to simple linear crosstalk
which comes from say finite thickness of the
medium and therefore finite width of the
diffraction peak). That's quite interesting.

In terms of index saturation, you are right that
the output SNR can be improved by increasing the
playback power - what I was assuming is that
due to the noisiness of the * writing * process,
a minimum index change is required to write
an image, so if the total index change was
finite that would limit the number of images.
However, I can imagine that the inter image
nonlinear crosstalk is a dominant effect ...

ppm

Petabit 12/4/2012 | 8:08:38 PM
re: Physicists Find Fiber's Limit OK, we've all had a good go at this so let's wrap it up.

Firstly let's look at photons. Optical receivers need to detect the incoming photons and turn them into electrons to allow us to use the data. One extreme is that you could use a single photon to represent a bit - this could work, but in the presence of noise the number of photons that you need for each bit rises.

I've seen receivers designed that operate at 20 photons per bit, but more realistic designs operate at 200 photons per bit.

That relates to the optical power, and so drives the minimum power that you need in a fibre. The example given earlier of sending 100 Tbit/s over 1 um is a little spurious, given the enourmous optical power you would need to detect the bits at the receiver.

This is only lightly related to the information density that you can achieve. For a long haul fibre optic system, that has some optical amplifiers and some spans of fibre, then I believe the crew from Bell Labs. Some of the limits are really fundamental - like photons, and some are engineering limits to a particular system design.

I think the real fundamental limit is how many publications like lightreading will publish press releases, without reading or understanding the material. Until lightreading changes its business plan and stops needing to get as many eyeballs, and therefore stops publishing the most controversial stories that it can get away with, then we will continue to have debates on these boards.

C'mon lightreading, time to scape up a story about something really heretical, and get more people reading your site...

P.
ppm 12/4/2012 | 8:08:38 PM
re: Physicists Find Fiber's Limit Read the article in Nature first. A formula
is given for the peak capacity value, depending
on the system parameters. The dependence on
system length is logarithmic (a reasonably
weak dependence). It also tells you that bit rate
times distance is not a good metric.

BTW, the article is entitled "nonlinear limits"
not "fundamental limits".

ppm
random-reading 12/4/2012 | 8:08:40 PM
re: Physicists Find Fiber's Limit
I have read all the messages, but not the original article on Nature. Really impressed
about the knowledge of everyone.

IMHO, whatever claimed as the fundamental limit
of the fiber is a mistake. Certainly ways can be
invented to mitigate the nonlinear effect on the
fiber, using different modulation scheme at the
source and a more sensitive receiver coupled with
processing gain can reduce the SNR required for
the channel.

The article in Nature might just be addressing
the current fiber with its optical properties and
probably assumed some kind of modulation scheme
will be used, and the sensitivity of the receiver
diode. Some clever math tricks then derived the
3b/Hz number. Definitely a valid exercise.

To claim that as the fundamental limit of fiber
capacity is just wrong.

random thought
ownstock 12/4/2012 | 8:08:40 PM
re: Physicists Find Fiber's Limit Calpole:

A man with a good mind, who is not afraid to use it, is a rare thing...

Apparently you are rare...

Thank you for showing up here...

-Own
calpole 12/4/2012 | 8:08:41 PM
re: Physicists Find Fiber's Limit Well, talking about the capacity without distance
has no meaning. We can always transmit
100Tb/s over a micro-meter if transmitter and
receiver is available for 100Tb/s.

All capacity limiting factors like optical
noise, dispersion and non-linearity are propotional [ or exponentially varies] with the
transmission distance if dispersion and
non-linearity has weak coupling. Hence,
such school physics calculation is useless
without a bit-rate-distance product.
ubwdm 12/4/2012 | 8:08:49 PM
re: Physicists Find Fiber's Limit "One very effective way to combat fiber nonlinearities is to break the channel into n-subchannels".

Huh, a real copper comm guy. Why don't you
just spell out DMT? That way you can maybe
tell us a thing or two about ADSL?

Let me just say that if you get a textbook,
you will find out NLS is well, nonlinear.
The Maxwell's equation for a conductor or
the ABCD parameters for a twisted pair is
however linear. Surprised?

DMT won't help fiber much. SPM does decrease
as "sub-channel" gets smaller. But XPM is
increased. In case of DWDM, it is hard to
conclude as a general rule (Depending on optical power density, etc), but DMT won't increase capacity. Believe it or not, the Bell Labs
authors do know a few things about fiber communications... enough to publish it
on "Nature".








ownstock 12/4/2012 | 8:08:51 PM
re: Physicists Find Fiber's Limit Well, it is sweltering hot here in SilliValley, and it must be affecting my mind...so rather than just sweat and waste time, I am going to type and waste time.

I will give some hints:

1) One very effective way to combat fiber nonlinearities is to break the channel into n-subchannels...now in general, for each subchannel in the channel, there is an optimal bandwidth...which among other things is a direct function of the modulation and the detection method. Each of these subchannels is then made piecewise (much more) linear...not perfectly linear mind you, but significantly better...

2) Generalized optical IQ modulation has been done quite nicely for many years...in fact so has general IQ modulation and balanced coherent detection. At least three companies I am aware of field such equipment today and have for many years (only one has not passed field trials yet): Harmonic, which uses complex modulation, Synchronous which uses complex modulation and balanced detection, and the third of which uses all of the above and coherent detection.

The article may be correct based on the assumptions it makes, and how they choose to attack the problem, but as my mentor told me, always remember how you spell assume...

-Own

gigsonetic 12/4/2012 | 8:08:53 PM
re: Physicists Find Fiber's Limit In the recent announcements at OFC 2001 (see http://www.lightreading.com/do... ), Alcatel and NEC reached 0.8b/s/Hz. (40 Gb/s signals, 50 GHz spacing)
ubwdm 12/4/2012 | 8:08:56 PM
re: Physicists Find Fiber's Limit A hologram - spatial fringes with certain
frequency. You can think it as ITU channel in DWDM. A nonlinear media acts like RF mixer.
As the number of holograms/ITU channels
in a fiber increases, signals mixe
with themselves (SPM) and with each other (XPM).
The result is that noise will increase squarely with frequency range, or number of channels (and distance). So noise density (/per Hz) will increase with formation (bit per Hz). Initially, system with finite power budget will be limited by linear noise (transmitter/receiver noise). When more information or optical power is pumped into system, this cocktail party noise is the limit. Amplifier noise is also partly nonlinear.

And the most significant of all, it is this cocktail party noise that violates Shannan theory, other noise don't. The (Bits/per Hz)
is not really a complete measure for a nonlinear
media.

Likewise, in case of holographic memory, index saturation is not the killer since you could always increase playback power, at least theoretically. "Cocktail party Noise" is.

BTW, ture, there are ways to combat nonlinear effect. But how many bit you can get per soliton?

As to 3bps/Hz, I think it comes from
OTDM - record is about 15 fs pulse in 200 nm. about 2.5 bps/Hz.

But, if you add bi-di and polarization, you can get 10 bps/Hz per link for a short fiber.




fk 12/4/2012 | 8:08:56 PM
re: Physicists Find Fiber's Limit The examples you cite are not so much evidence that "we knew it all along," rather, they offer evidence to indicate that false conclusions can be reached by misapplying laws of physics. After it can be shown that the conclusions are false, it is actually a frequent occurrence that rethinking and reapplying physical laws and theories proves consistency after all, which is a far cry from claiming that "we knew it all the time." People who make such claims are typically suffering from bruised egos, and are looking for a fig leaf of credibility to hide behind.
realguy 12/4/2012 | 8:08:57 PM
re: Physicists Find Fiber's Limit After carefully reading all the postings, I have to say ownstock has a valid point. Some years ago a very respected physicist got a Nobel Prize for the theory of superconductivity. The theory predicted that it is impossible to have superconductivity at room temperature. Then again a few years ago, it was discovered otherwise. The same Nobel laureate now claims it is indeed very natural to have superconduction at room temperature. In fact he insists his original theory predicted it if one would have just digged little deeper. Optical gyroscope was initially were believed to be impossible because it violated the theory of general relativity. After it worked, they claim it works actually in support of general relativity.

The assumption of the paper published was nonlinearity is a fact of life in optical fiber. Wrong! What if through some clever scheme (digital or analog) nonlinratities could be suppressed. Then again if that happens, the physicists would say, we knew it all along.
eewhiz 12/4/2012 | 8:09:09 PM
re: Physicists Find Fiber's Limit After reading all the posts, (but have not read the Nature article), it seems that todays advanced DWDM based systems are operating at best 0.4b/s/Hz...(calculated from a 10Gb/s OOK modulated signal operating in a 25GHz channel spacing using optimal matched filtering for baseband filtering). This is still a far cry from the 3b/s/Hz mentioned in the Nature article. But to go to higher throughputs will require somehting extraordinary...

Lets say we start with a 16-ary multi level PAM instead OOK (this would be difficult with current DWDM lasers). This would assign a 4 bits/symbol, this would increase the throughput to 1.6b/s/Hz, but this type of signalling is poor because all the quantization levels are in the 'real' amplitude plane; you could not transmit this signal too far before AWGN and any optical ASE would start intefering with decision thresholds. We need to use the phase plane to spread out the quantizing radii so that better Eb/No at the receiver, this is how the V.22 on up to V.90 modems were able to transmit greater BW's over 4 kHz of copper BW. The same holds true for HFC cable systems sending 256-QAM in each 6 MHz channel space.

The problem is that no-one (that I know of) manufactures or has an equivalent light based IQ modulator (that would operate at 193.1THz for example).

Even if you could build something like this, then the non-linear effects (PMD, chromatic and time disersion) will start to interfere with demodulation, so then you would need to adaptively equalize these non-linearities with technology that so far does not exist...not to mention we'll probably need FEC over all of this!!
This is very hard to do at 100Mb/sec let alone over 10Gb/s...only time and technology will tell.

At this point I'll settle for 0.4b/s/Hz...
Cheers
vaporware 12/4/2012 | 8:09:11 PM
re: Physicists Find Fiber's Limit 20 years ago John Pierce published some papers on the photon counting channel and derived the quantum limit. I think, if my memory serves me, it's about 0.8 photons per bit. The 3 bits/Hz seems odd, unless it is based on a certain modulation scheme. We'll know the real answer if someone builds a good optical phase locked loop...
Petabit 12/4/2012 | 8:09:17 PM
re: Physicists Find Fiber's Limit Ownstock,

talking of peer-reviewed papers, you might want to look one up. About a year ago, Desurvire published a nice paper in an IEEE journal. It talked about the ultimate limit of an optically amplified system - it compared and contrasted lumped and distributed amplified systems.

So just looking at the noise from amplifiers in long haul systems, the limits he calculated were 3 bit/s/Hz for distributed amplifier systems and 6 bit/s/Hz for lumped systems. Which comes very close to the Bell Labs results, without including non-linearities.

All of which is a very long way from the 0.1 bit/s/Hz we are using today.

P.
ppm 12/4/2012 | 8:09:20 PM
re: Physicists Find Fiber's Limit That's interesting - I've thought a little
bit about that problem ... I presume you are
referring to the nonlinearity having to do
with the refractive index change saturating
after writing a certain number of holograms.
I have a vague recollection that the maximum
delta(n)'s are of the order of a fraction of
a percent.

Best
ppm.
ubwdm 12/4/2012 | 8:09:23 PM
re: Physicists Find Fiber's Limit Well, ownstock is completely wrong. His
reaction is typical of many DWDM beginers
in some startups I met last years though.

To achieve 3 bps/Hz for a sigle 50Ghz channel
is not difficult. Even 10 bps/Hz is possible
in the lab for a few spool of fibers. But try it for 100+ channels for a few thousand KM transmissions, then you will start to appeciate the laws of physics.

This reminds me that when I did my disertation
many years ago, I analysed the capacity of holographical memory with similiar mathematical method. The established theory at time predicted that a 1cmX1cmX1cm LiNbO3 could hold the library of Congress, but reality was millions times under.
The cause: nonlinearity.






ppm 12/4/2012 | 8:09:24 PM
re: Physicists Find Fiber's Limit There are plenty of IEEE papers with 30 pages
of unnecessary theorems and lemmas followed by
a non-result. While Nature is not a traditional
communication theory journal, it is as valid
a forum as any to report a new way of
computing information capacities in the
presence of nonlinearities. Information theory
was to some extent born whole, there are few fundamentally new results in any case after Shannon's original publications.

I have failed to see a single valid technical point that you have raised. I sympathise with
your suspicion of the ivory tower, but I am
afraid that the IEEE and other journals are
as full of unnecessarily narrow publications,
whether from academics or not. Apart from
Turbo and LDPC codes (the latter being already
a rediscovery), there has been little over
the last couple decades that constitutes
basic advances in understanding.

Let me point out something simple. You brought
up a 10bit/s/Hz as a number. At 20dB SNR, which
you also mention, the Shannon formula for a
linear channel only gives you 7bits or so
(log2(100)=7 approx). There are inconsistencies
in your thinking even for a linear channel.

The limits in question are quite real. As for
specifying parameter values, that is clearly
inescapeable; this is not the speed of light
in vacuum or the gravitational constant that
is being determined. However, an estimate
for realistic parameter values is adequately
valuable. The same issues arise in predicting
limits for semiconductor memory chips, for
example. The world is imperfect: one has to
start somewhere.

Best,
ppm.
ownstock 12/4/2012 | 8:09:25 PM
re: Physicists Find Fiber's Limit Sanddune:

First, there is a BIG difference between writing for a peer reviewed journal, and a trade publication...they are two very different things.

Something like an IEEE or OSA technical journal is the first, and LR or Time is the latter (for example).

JMHO, but Nature is NOT the first place anyone of caliber in communications would think to publish. OTOH it would be an ideal place to publish a supposedly "fundamental limit" in fiber optics.

Precisely because they knew by going there it would not go through a tough and thorough peer review, and/or the editors would tolerate their combination of hype-title and "let's pretend it is this way" problem constraints. Sophistry.

My effort is not intended to confuse, but rather to expose the effort on the part of others to confuse / spin / hype / etc.

I have given (more than) enough information for those with technical expertise in the field of communication theory and/or fiber optics to understand...

-Own
sanddune 12/4/2012 | 8:09:25 PM
re: Physicists Find Fiber's Limit ownstock,

From my limited knowledge of comm sys
what you say is true for a "memoryless,
gaussian channel" for a copper local loop.
This efficiency of modulation is
fundamentally correlated with the channel
noise characteristics, which in case of
fiber are very different from the copper.




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.
ownstock 12/4/2012 | 8:09:26 PM
re: Physicists Find Fiber's Limit Reiterate: If you set up the wrong constraints, you will get a wrong answer...because of that, the claim of deriving fundamental limit is just plain WRONG!

The real issue is there are efficient brute force (aka easy and well known) ways to minimize the nonlinearities in fiber that they did not even consider...and to achieve well over 3 bps/Hz...so their paper is not worth the pulp it was printed on!

Sorry to say this is a typical ivory tower, tunnel vision paper...hyped to the max...designed to derive a clever (but very wrong) answer...

Enough time wasted on this....

-Own
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.

Best,
ppm.

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?

Thanks
nano
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.

-Own

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...

-Own
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.
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.

Best,
ppm.
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...

-Own
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?

Curious
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.

http://colossalstorage.net/col...
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