not to be a douche on this, but what is my incentive?
The same as for not downloading music that is not being offered legitimately. (Not even for free; you'd actually be paying the pirate, and not the artist.)
Photonics to the rescue indeed; but I thought wave-synchronised light sources at this distance would be considered part of the lab-experiment grade equipment this was said to be doable without.
Right, and this was the big problem with coherent when it was first proposed for optical systems back in the 1980s.
Now, you just ensure that the local oscillator is within a few tens or hundreds of MHz of the signal carrier, which is not too difficult. A residual phase drift of several hundred Mrad/s sounds high, but compared to a few tens of Gbauds symbol rate, it is not that much and can be compensated in software.
Quick question: my understanding is that in wireless, we're at 4-5 bits/s/Hz. Why is that figure so much lower with fiber?
Because it's more complicated to reach for a high spectral efficiency. Until now, on fiber, it was possible to just increase the spectral bandwidth (increase the number of wavelengths in a single fiber, in fact). In wireless, on the contrary, the spectrum is much more regulated--if only because it is shared among everybody, whereas what happens in a fiber doesn't affect anything outside it. Thus the drive for a high spectral efficiency in radio.
Polarization in multimode fiber is out because the polarization tends to become random after it is transmitted through a long enough multimode fiber.
Oh, singlemode fiber isn't better in that regard, but yes, that's certainly SMF they're talking about, if only because that's what installed in current long-distance links. Also, you can indeed have polarization-maintaining SMF, but not over hundreds of kilometers. For what is actually done to multiplex over polarization, see my earlier post.
Come to think of it, could you then encode data in linear, elliptical, as well as circular polarization directions?
I don't see why not, though the encoding might be slightly more complicated. To answer your question about how to generate a PolMux signal, you take two lasers, which you modulate independently, then inject into a polarizing beam splitter. You can also change any polarization into another using quarter- or half-wavelength plates, or fiber-loop polarization controllers. The former use properties of certain crystals to rotate polarization axes; the latter are simply loops of fiber optics which you orient and warp (google "polarization controller").
Just some random thoughts that come up because the article isn't very technically detailed.
Indeed. I haven't even seen which principle they use for the announced system. I assume it's PolMux+DQPSK at 12.5Gbaud, like everybody else at this point. But do they actually use DSP, or did they remain analog for now?
Different wavelengths follow different paths down the fibre and will arrive with different latency and distortion; so multiple wavelengths carry concurrent frames, rather than concurrent bits;
Well, yes. There are "wavelength-striped" systems in laboratories, but only for short-distance links AFAICT.
Also, no production DSP will pull phase information out of optical frequencies; to do so reliably requires a sample rate of at least 4x the frequency, so your 1530nm signal would need to be sampled and processed at around 800,000 GHz (yes, the best part of 1 PHz. Per-channel). Good luck with that.
Electronics won't do for this, photonics to the rescue!
The article implies that it's easy to do, there was simply never a need before. I seriously doubt that it's a trivial thing to accomplish a four-fold increase in bandwidth on existing infrastructure.
It's not, as you have pointed out. My interpretation is that, on the contrary, phase and polarization diversity (which I'll lump into "coherent" optical transmissions) are hard enough to do that you'll try all the other possibilities first: DWDM, high symbol rates, differential-phase modulation... All these avenues have been exploited, now, so we have to bite the bullet and go coherent. However, on coherent systems, some problems actually become simpler.
Polarization has a habit of wandering around in fiber.
Quite so. Therefore, on a classical system, you use only polarization-independent devices. (Yes, erbium-doped amplifiers are essentially polarization-independent because you have many erbium ions in different configurations in the glass; Raman amplifiers are something else, but sending two pump beams along orthogonal polarizations should take care of it.)
For a coherent system, you want to separate polarizations whose axes have turned any which way. Have a look at Wikipedia's article on optical hybrids, especially figure1. You need four photoreceivers (two for each balanced detector), and reconstruct the actual signal by digital signal processing. And that's just for a single polarization; double this for polarization diversity and use a 2x2 MIMO technique.
That's why it's so expensive compared to a classical system: the coherent receiver is much more complex. Additionally, you need DSP and especially ADCs working at tens of gigasamples per second. This is only just now becoming possible.
Phase-encoding has similar problems. Dispersion, the fact that different frequencies travel at different velocities (this leads to prisms separating white light into rainbows), will distort the pulse shape and shift the modulation envelope with respect to the phase. You either need very low dispersion fibers, and they already need to use the best available, or have some fancy processing at a receiver or repeater.
Indeed. We are at the limit of the "best available" fibers (which are not zero-dispersion, actually, to alleviate nonlinear effects, but that's another story). Now we need the "fancy processing". And lo, when we use it, the dispersion problem becomes much more tractable! Currently, you need all these dispersion-compensating fibers every 100km, and they're not precise enough beyond 40Gbaud (thus 40Gbit/s for conventional systems). With coherent, dispersion is a purely linear channel characteristic, which you can correct straightforwardly in the spectral domain using FFTs. Then the limit becomes how much processing power you have at the receiver.
The article downplays how hard these problems are. It implies that the engineers simply didn't think it through the first time around, but that's far from the case. A huge amount of money and effort goes into more efficiently encoding information in fiber. There probably is no drop in solution, but very clever design in new repeaters and amplifiers might squeeze some bonus bandwidth into existing cable.
Well, yes, much effort has been devoted to the problem. After all, how many laboratories are competing for breaking transmission speed records and be rewarded by the prestige of a postdeadline paper at conferences such as OFC and ECOC
As for how much bandwidth can be squeezed into fibers, keep in mind that current systems have an efficiency around 0.2bit/s/Hz. There's at least an order of magnitude left for improvement; I don't have Essiambre's paper handy, but according to his simulations, I think the minimum bound for capacity is around 7-8bit/s/Hz.
Seems like a reference phase would be by far the easiest and most robust solution.
It has been proposed for some modulation types. However, this halves the efficiency: you use one wavelength for each reference beam, but you can't use the same reference for all the other wavelengths, due to the fact that these wavelengths travel down the fiber at different speeds. (This is called "chromatic dispersion" and can be a major pain in the neck at high bit rates.) So there would be a delay between reference and data beams, thus a phase shift, which would have to be measured and compensated for.
In practice, I believe that a known data sequence is transmitted at regular intervals so the receiver can detect it and synchronize to the emitter.
Polarization, you mean? (As in the direction along which the electrical field vibrates?)
For phase modulation, try Wikipedia. I like the diagram.
The problem in optical transmissions, unlike radio or electricity, is that you can't directly access the phase of the light. All you can do is to have two beams of light interfere together (just like with sound: if you hear two tones, very closely spaced, you will hear a low-frequency "beat" which pulses at a frequency equal to the difference between the frequencies of the original tones). That gives you access to the phase, but you need to have the vibration frequencies of your beams very close together, which is not simple. Recent advances (in DSP processors, paradoxically) are making it possible, especially for high-speed modulations.
Won't increasing the number of bits per symbol as you suggest require a higher SNR, thus meaning amplifiers have to be more closely spaced?
Good point. And even having closely-spaced amplifiers may not work, as optical amplifiers have fundamental limitations in terms of noise added (OSNR actually decreases by at least 3dB for each high-gain amplifier).
At least, that's for classical on-off keying (1 bit per symbol, using light intensity only). Coherent transmission might not have the same limit; I'd have to check the calculation to be sure. And you might be able to do something with "distributed" amplification, where instead of having localized amplifiers, you pump energy into the fiber so that it attenuates less. (E.g. use the Raman effect: light at a wavelength of 1550nm can be amplified in a silica fiber by sending another, stronger, beam of light at about 1450nm. But there's still some noise added.)
10 gbps is enough that you can do uncompressed video if you like. [...] If we get gig to the house, I mean truly have that kind of bandwidth available, I don't think we'll see a need for much more for a long, long time.
Yes and no; I'm sure we'll invent new ways of wast^Wusing bandwidth. (3DTV, telepresence, video editing on remote storage, cloud computing... What's next?)
But the problem no longer lies in the house. Not everybody has a 100Mbit/s Internet access yet, but that's coming in the next few years. Now think 1billion people simultaneously trying to access YouTube... The problem is the core network, where you must aggregate all these users' traffic. Current DWDM links in the Tbit/s range are not enough.
(Or you could try to be smart, cache some data, use multicasting... In practice, people can't be bothered not to waste bandwidth, and you can't cache everything.)
Radio and DSL already do this, but only at a few tens of Gbit/s
add color, use 256 identifiable colors then send those, send bytes instead of bits.
Already being done; TFA mentions this (for "wavelength" read "color", as the light that is being used is in the infrared).
What limits the number of wavelengths in a single fiber is the bandwidth of the amplifiers: optical fibers slightly absorb light, and current long-haul links require reamplification ca. every 100km. This is done using EDFAs (erbium-doped fiber amplifiers), which work for wavelengths in the 1530-1560nm range (the "C band"; visible light is in the 400-800nm range). Adding wavelengths outside this band would require redeploying new amplifiers along the fiber, which would be expensive; besides, other types of amplifiers aren't quite as mature as EDFAs, and you would need more of them because the fiber attenuates more outside this window.
You could also try to squeeze these wavelengths tighter, to put more of them within the C band, but they are already packed at 0.4-nm intervals, corresponding to a 50-GHz frequency interval, which holds a 10- or 12.5-Gbit/s signal with little margin, as long as conventional optical techniques are used--that is, switching the light on or off for each bit.
There remains the possibility of using smarter ways of modulating the light, using its phase and polarization, to pack e.g. 100Gbit/s in a 50-GHz bandwidth; and that's what Alcatel are doing. They are not the only ones, of course, the field of "coherent optical transmissions" has been a hot topic in the past couple of years. Now commercial solutions are getting into the field.
Note that these techniques are already widely used in radio and DSL systems, and had been proposed for optical systems back in the 1980s, before EDFAs essentially solved the attenuation problem. Now, however, we have again reached a bandwidth limit and have to turn back to coherent transmission. In the 1980s, that meant complicated hardware at the receivers, impossible to deploy outside the labs; now all the complicated stuff can be done with DSP in software. Radio and DSL already do this, but only at a few tens of Gbit/s; doing it at 100Gbit/s for optics is more challenging, and is just now becoming possible.
MECO is around 185,000 feet (35 miles).
No, that's more like the altitude at which the external tank breaks up after falling back down, according to this Shuttle reference page: Second Stage.
The same site doesn't give a clear answer to where MECO occurs, but the example given in Orbit Insertion is 80 nautical miles (148 km, 92 miles), which is definitely in space.
Aren't Arsenic compounds toxic? And aren't we (globally) trying to move away from exposing the environment to yet more toxins?
Compounds, no. Virtually all the lasers you are using are made of gallium arsenide.
"When the going gets tough, the tough get empirical." -- Jon Carroll
