# Boffins shrink light-twister to silicon scale, multiply bandwidth 10x

We've known for some time that the orbital angular momentum (OAM) of light (colloquially known as “twisted light”) can be modulated to carry information, but until now, it's only been demonstrated on large-scale laboratory lasers. That's changed with work from the State University of New York at Buffalo and the Polytechnic …

#### light beam me up

"... the researchers reckon since OAM is a quantum property that can undergo “teleportation”, ..."

I assume they mean it can be quantum entangled with another light beam?

can BT hear it?

#### Re: If a fibre optic setup is an order magnitude faster

Course they can.

..but the instant it's attached to your premises it becomes V92

Grrrrrrrrrrrrrrrrrreeeeeeeechchchhchchchch....

Is that

"Like,- 'Neuro' Light... then?"

#### Vaporware

There are some problems with the "10-fold increased bandwidth". Fortunately, the boffins were not required to transmit 10x more data to publish their paper, but a bit of pixie-dust did the job all-right. Let me explain why this is vaporware (you'll need some patience to go through the following explanation).

Physics tells us that there are three spatial dimensions (for simplicity, let's call them x,y, and z). One dimension will be the propagation direction of the light beam (usually we'd call that the z axis) and we can now freely choose any polarization state and position in the x and y axis.

Any polarization state sounds like a big amount of freedom, doesn't it? Hey let's use it to transfer data! That's the underlying concept of the orbital angular momentum = increased bandwith community. Unfortunately, there are only two spatial dimensions, so the sum of two polarization states (with respective amplitude and phase properties) are sufficient to describe any possible polarization. Commonly, physicists would describe that 'any polarization' state as a sum of x- and y- polarized light, or (perfectly equivalent) as a sum of right-handed and left-handed circularly polarized light. The circularly polarized description is better, it describes the same physics but it allows us to talk about angular momentum and that does sound great! Back to that great freedom of 'any polarization' . The laws of physics boil it down to only two distinct polarization states and that is surely a bit disappointing (where is my factor 10!?).

Now a smart kid might argue that we should relax a little and look beyond x and y: why not use some +-45 (= pi/4) degree angles between x and y as a third and fourth polarization state? Bandwidth, here we come! Unfortunately, those +-45 degree photons have a big propensity to be observed as either 0 degrees (x-axis) polarized photons or 90 degrees (y-axis) polarized photons. So we'll have to collect many photons to tell the difference between our four polarization states and that will slow us down sufficiently to destroy all the extra information we wanted to send along.

How about using the light phase to transmit data? The phase of a photon can take any value between 0 and 360 degrees, so all we have to do is encode information in there. Unfortunately, the phase is only a meaningful concept if we can relate it to a common time frame. If I wait for 1/4 of the period of the light wave (some 3.3 femtoseconds for micrometer wavelength light in your glass fiber), then the phase will be shifted by 90 degrees. We have no detectors that could directly detect such tiny time/phase-shifts of an electromagnetic wave. The only practical way to determine such shifts is to overlap two beams of light and observe their interference (two identical beams will destruct one another if their phase is shifted by 180 degrees). But if we need to send two beams to make use of the phase information, then we might as well use the second beam to transmit data independently. It turns out that the maximum amount of information that can be transferred does not change if we use simple interferometric tricks.

Now you should be ready for the big question. If the simple physics just gives us a factor 2 through polarization and nothing from the phase, how do those boffins magically increase the information bandwidth of light? (They do so on paper at least.) And the answer is simple again: They spatially displace the beam along the x and y axis. Imagine you send beams to different spots -- you could use a separate detector at each target spot and, with 100 detectors, you could increase the information 100-fold! Sounds magical? Not really, it sounds a bit trivial. (Aren't those boring telecoms already using fiber bundles to send multiplexed data?) But that's just because the idea was badly presented.

Let me try again. Imagine we use some interference tricks to control the direction of the light beams. I am sure you have heard about light diffraction, holographic gratings, and other magical tricks. No need to understand them, it's just some interference tool to control the direction of light waves and a few dollars can get you yours! Use this holographic magic to control the x/y spatial direction of the propagating light and you can start aiming at your 100 detectors. Now stop talking about space, that's a bit boring. We use light-waves, and space can be perfectly described as the Fourier domain of an interference mask. Even better, let's describe the interference in terms of circularly polarized states and angular momentum. Remember, I told you the circularly polarized description sounds better! So now we use orbital angular momentum to multiplex the bandwidth of information transmission. That does sound quite nice, doesn't it? Sprinkle some math describing the light interference onto paper and, voila, you get a tenfold increase in bandwidth, together with a science paper and lots of attention.

To be fair, the authors never talk about a tenfold increase in information bandwidth in their paper. So maybe we should assume that the general press (including TheRegister) mis-interpreted the word "potential in the manuscript. Or maybe nobody bothered to read the article, after all there is a press release, which is "able to carry 10 times or more the amount of information than that of conventional l̶a̶s̶e̶r̶s̶ " scientific publications.

It's not the first time I wrote about OAM information transfer magic in the comments. Hello, Richard Chirgwin, are you reading this? It's almost as much fun as the information teleportation magic. But now I ask TheRegister to cease and desist that nonsense for at least 2 weeks, or else I'll spam your comment section with 100 pages from the Messiah. The real Messiah.

#### Re: Vaporware

So TL;DR, meh, they need to invent a better polarisation receiver.

#### Re: Vaporware

Um, how, does that surprisingly long splaff contemplating the utility (or otherwise) of polarit(y/ies) of polar light pertain to the utility (or otherwise) of chiralit(y/ies) of spiral light, which would appear to be something else entirely?

#### Re: Vaporware

It's one of those "science that doesn't produce a viable product after the first round of research is useless and a waste of taxpayer dollars EH TUK MUH JERB" zombies. Remove the head or destroy what's left of the brain. It's the only way.

#### How does polarity pertain to chirality of spiral light?

Simple: Spiral light == circularly polarized light.

There is no separate chiral/spiral property of light and you have been suckered by the use of fancy new language to describe boring old concepts (TM).

#### Light is the exact same thing as radio waves (just shorter)

Has anyone translated this "new technology" into radio waves and actually demonstrated it?

We live in the age of B.S. hype. I don't know what to believe anymore.

#### Re: Light is the exact same thing as radio waves (just shorter)

I've always wondered why no one has done the reverse and started implementing various radio technologies to optical communications. Implementing a modulation technique like QAM seems like it'd be a fairly trivial way to greatly increase the bandwidth of an optical fiber rather than the current technique of 'turn the laser on and off really fast'.

#### Re: Light is the exact same thing as radio waves (just shorter)

Apparently, yes, they did try. The whole concept is flying way over my head, but I do get the very distinct impression that our significantly spirited friend a bit higher up is in that exact same boat.

#### Re: Light is the exact same thing as radio waves (just shorter)

One possible issue with trying to use actual phase modulation of the light (like QAM) inside a fibre optic strand is that the mechanism of total internal reflection in the fibre over (for example) a 40km path, would seem to cause a form of multipath that may (?) smear out the phase. I've no idea how much. Can't be too much else it would invoke a comb filter effect even today. Hmmm... Uncertain now. :-)

Larger issue is that they don't really know how to phase modulate light. They could easily do so on a subcarrier carried on the light.

#### Re: Light is the exact same thing as radio waves (just shorter)

Thank you Mr Bear.

Wiki: "The hype seems to have cooled down lately. Even the original promoters of OAM based communication at radio frequencies have realized that there is no real gain beyond traditional MIMO."

#### Re: Light is the exact same thing as radio waves (just shorter)

the mechanism of total internal reflection in the fibre over (for example) a 40km path, would seem to cause a form of multipath that may (?) smear out the phase

No it doesn't.

Such an installation will always use a mode-scrambling mandrel to ensure a single mode of operation. That's pretty much the first thing you learn in fibre optics...

Vic.

#### Re: Light is the exact same thing as radio waves (just shorter)

I did equipment design and R&D for fiber optic systems in the 80s and optical fibers were available then as either multi mode (multiple light paths) or single mode (light can only follow one path directly down the center). Single mode fibers eliminate multi path affects allowing much higher data rates but coupling light into the fiber and doing fiber to fiber connections was difficult. The systems described in the article could use single-mode fibers.

#### gate tech

Density along the ring will increase over time with technology.

As it approaches infinity and thus continuous, you create a solid tunnel.

Good job humans, you're on your way to star gate's.

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