For my research, we looked at optical vortex modulation schemes for a while. With polarisation (spin angular momentum), there are only two orthogonal states. With orbital angular momentum you can have infinite integer "charge" states that are orthogonal (yes you can have negative charge as well). So in principal by encoding each stream with one of these states, a bunch can be transmitted at the same time in the same beam.
I might venture a guess that this was done over a short distance in a closed room. Problem arrises when these beams counter strong turbulence or scattering, then the vortices are either destroyed or coupled together. So the channel leakage and signal to noise becomes a problem.
Still neat though, bundling 8 channels is impressive.
In a previous discussion here, it was figured out that "twisting" is equivalent to MIMO. Is that your understanding? If so, it will be subject the same capacity limitations, meaning that capacity is approximately linear with degrees of freedom, then tapers off to a limit, the limit being related to the volume occupied by the receiver and transmitter.
Presumably the number of "integer charge" states is in some way related to the volume of space occupied?
Yeah, those problems are part of what makes this so interesting to me, especially since they so are dramatically alleviated in space. I'm betting we'll see this technology used in future space technology, and perhaps future research into the technology will take place up there. The future beckons, and it's partially in space!
> According to Thide, OAM should allow us to twist together an "infinite number" of conventional transmission protocols without using any more spectrum. In theory,
I don't know much about the science, but anyone who claims their new breakthrough has an infinite capacity immediately sets off my skepticism.
Caveat: I don't know much about OAM. In my mind, I'm thinking of it as analoguous to CDMA for radio signals, where you can have many different signals occupy the same frequency band, but they correlate to different (orthogonal, ideally) codes, so that adding one more signal shouldn't affect any of the others. In reality, it takes a finite amount of power (energy) to transmit data, and imprecision in your equipment does create some "leaking" between channels.
If you picture a "traditional" signal as a single bar (from a bar graph), and picture OAM as any number of these bars, but splaying out from a center point radially into a circular form, you can see a theoretically infinite number of bars. 360 of them, spaced 1 degree apart, then 720 spaced at half-degree intervals, etc. (As I understand the article, they also spin these at certain speeds, which makes sense for practical reasons, but isn't necessary for the analogy) The problem becomes when your equipment can't distinguish between two neighbouring bars (or the transmitting equipment lets the bars leak, or something about the channel, eg turbulence, confuses matters).
So in mathworld, it may be theoretically infinite, but in the real world we'll hit limits. There will be a limit due to quantum mechanics because energy differences can only be measured to a certain precision. A limit due to physics of noise, and a limit due to what we can practically realize (and all the ugliness of dealing with a planet with a pesky atmosphere and temperature changes).
In a trivialized mathworld, many calculations are useless for the real world. Trivializing the same way, we might as well ignore the capacitance of wires, cross-talk, and general EM noise and claim that wires can transfer at infinite bandwidth. Or say that air has no resistance, therefore we can launch a cannonball into orbit around the earth at roughly sea-level.
Both of these analogies are wrong. CDMA is unrelated to OAM. Think of CDMA as a way to trade-off effective bit-rate with signal power, even in noisy environments and a way to minimize cross-signal noise.
What you're describing is a description of polarization.
As to OAM, it's a bit hard to explain because it's difficult to visualize. Think about an ordinary radio wave going back and forth like a sine wave in a single plane. This is linearly polarized light. Now, with the same frequency and phase another photon could also be polarized at a different angle, say 90 deg. to the other photon. However, you can essentially combine both aspects of waves in varying degrees, creating elliptical and circularly polarized waves. You can think of that as the photon "spinning". OAM is on a different scale. Normally we think of light as traveling in purely straight lines other than the wave effects. However, take that model of a rotating photon wave-packet and at a larger scale imagine the photon "in orbit" along a trajectory, taking a cork-screw path.
Circularly polarized light is like a spinning football being thrown. Light with non-zero OAM is like a curve ball (although admittedly the analogy breaks down).
Fair 'nuff, and in general I don't like using analogies, precisely because they are imprecise or break down, as mine did. My main goal was to show that just because you can, in theory, overlay an infinite number of channels, doesn't mean there won't be very real limits on performance.
At first glance, I though OAM sounded like circular polarization with different angular velocities. Now your comment makes me think there's more to it. Maybe someday I'll get back on track for a physics degree, and I'll be able to think through it properly.
If it has yet to be, I'm sure that an upper bound can and will be calculated. The SAM/OAM properties that are being exploited here are physical phenomenon that conform to quantum mechanics just like everything else. We're talking about photons here that have physical/dimensional properties -- therefore, there WILL be an upper bound. I agree with user:recursive that "infinite" is a bad choice of wording.
I'm pretty sure a lot of them would realize that 4TB and "infinite" are substantially different and argue with your severe mischaracterization of the capacity of your hard drive.
you are incredibly wrong. "More than I think you'll need" and "infinite" are not the same. Capped "unlimited" data plans are a good example. Company expense accounts are another example.
Nope. They'll start recording video from CCDs to it in the form of 30 fps sequential, uncompressed full-frame images. 40 hours of video later they'll wonder why the "infinite" drive you provided them is so small.
Yeah, this came up the first time we wrote about OAM [0]. There's a pretty informative (and heated) discussion about it in the comment thread -- Bo Thide defending his work from attacks made by other scientists.
Call me naive, but the comments of Bo Thide in that link have a very inconsistent level. One comment he sounds like a physics professor, another one like a dude that just read "Maxwell's equation for dummies", making leaky analogies between OAM EM waves and spin-stabilized gun projectiles. I guess the analogies are a habit of teaching.
It's dealing with values of orbital momentum. If you need more capacity, you can add a new "channel" that has more momentum, and run it right next to all the others. Rinse repeat, unbounded capacity without increasing spectrum usage. I suppose eventually the amount of energy you have to put into it will cause relativistic effects :)
Given unbounded transmission power, just about any transmission method has unbounded capacity, no? Of course, assuming unbounded transmission power is stupidly unrealistic.
It is not infinite capacity, in the same way that MIMO is not infinite capacity, IE you need phisically separated antennas for MIMO to work, and you need the same with OAM, the signal needs a specific amount of physical space to rotate. AFAIK OAM do not work in a 1-dimensional copper wire or optical fibre.
There are no natural limits (aside from quantum ones), there will of course be limits imposed due to the precision and capabilities of equipment built by mere mortals.
This test showed 95 bps/hz of spectrum. For cpmparison LTE is 16 bps/hz. They allude to problems with turbulence, that may make long range terrestrial use problematic.
But I remember 1 bps/hz being a limit, we've come a long way.
If this works as well as the hype, it'll still be highly directional, right? So it'll be great for things like backhaul networks, satellite linkups etc, but it won't solve the problem of mobile wireless access (WiFi/LTE/etc). Or am I missing something?
No, you're right. From everything I've read on this, it is essentially limited to line of sight as you need the "middle" reference for the other "orbits", or whatever the heck they're called. (Assuming this is the same technique as the experiment the Italians displayed in Venice a few months ago.)
bps/hz measurements in TFA are bullshit without noise figures. Per Shannon-Hartley theorem, spectral density depends on the Bandwidth of the channel (assume infinite for free air) and noise. So what is the noise? of course DVB-T (Digital TV) has low bps/hz, the antenna is tens of KM away and there is a lot of noise! the experiment used two antennas located 1 meter away, almost zero noise. EDIT: You might as well cite the spectral density of the voyager II spacecraft onboard antenna. Hint: It's very very low.
Stuff like this is why current SETI efforts seem at best provincial.
We aren't even beginning to understand how intelligent extra-solar beings would communicate with each other. Current efforts are like a colony of ants all by themselves on an island somewhere in the middle of the Pacific building a giant super-pheromone nose on top of a 3-inch anthill.
SETI is not aimed at listening in on communications between aliens, which would be impractical for the reason you mention and more. SETI's goal is to listen for deliberate "anybody out there?" transmissions, which one can expect to be much simpler, since they would be deliberately designed to be universal and easily understood.
We already know that we aren't going to accidentally eavesdrop on extraterrestrial communications. Those are almost certain to be very tightly focused and using some sort of transmission and modulation scheme that will make them impossible to decode or even perhaps to recognize as a signal with our technology (much as, say, modern DSSS technology would look like noise to humans even a few decades ago).
SETI has always been about the attempts to discover signals that are intentionally broadcast to civilizations with our level of technology.
I really want to see a technical paper on this. the Nature article is pay walled... also, the image on the bottom right kinda looks like it's spinning when looked at out of the corner of your eye (ie, when reading the article)
In a discussion on Slashdot, I was linked to an IEEE article that claims that all OAM type transmissions are subsets of MIMO. I haven't read this in detail yet, but so far it seems interesting:
It seems like turbulence is an issue when the beam is sent through the air, but could it be transmitted through a cable over longer distances? I certainly wouldn't mind upgrading my internet speed from 25 Mb/s to 2500000 Mb/s.
Some day phone makers will subsidize carriers. All towers will have been built, bandwidth will be extreme (note I did not say "infinite" although I wouldn't object if that happened!), prices will collapse. Buy a phone, get included service free.
That looks like a template you could fill in with any emerging-but-not-yet available technology, and the resulting strategy will fail the vast, vast majority of the time, since most exciting new tech developments never come to fruition. Plus, what would "software for OAM" even mean? It's so vague as to not even constitute advice.
Given that OAM is a modulation technique, which is a physical-layer tech, nothing above firmware will even know about it. At most software will be able to read/set a flag enabling use, similar to FX/DX on Ethernet.
I think its more than that. When you exceed by an order of magnitude the bitrate, different APIs will be needed to take advantage. Sure you can just send on a socket, but it will max out at the rate you can call that API (including kernel switch, copying data, waiting for a completion interrupt etc).
Some kind of virtual hardware or mapped memory will be desirable to achieve great leaps in bandwidth usage.
I might venture a guess that this was done over a short distance in a closed room. Problem arrises when these beams counter strong turbulence or scattering, then the vortices are either destroyed or coupled together. So the channel leakage and signal to noise becomes a problem.
Still neat though, bundling 8 channels is impressive.