On 10/27/2018 05:30 PM, David C. Rankin wrote:
That was the fascinating part. The:
twirl-encode->transmit->decode
So you have your fiber and you introduce up to 100 or so beams of light simultaneously with a slight twist to it wavefront. All of the distinct wavefronts propagate through the fiber and the limited interference between any of the "channels" is negligible enough it allows encoding and transmission of data on each channel. Providing not only transmission, but a complete asynchronous send-receive over each channel and the only thing they are scrambling to scale down is the technology to allow fetching the information from each channel while also handling the massive amounts of data?
The proverbial traffic-cop at the end of the fiber directing beams of light based on the amount of twist.
So the traffic-cop is directing the light at an intersection with 100 roads leading away, and each road leading away must be the same "super-highway" that is currently required to handle a single car (beam). And it all has to be small enough to segregate the channels from an end the size of a single fiber optic strand -- and the fiber optic cables are hundreds of strands bundled together.
That's just cool. Cool enough to bump Schrödinger's cat to the back burner...
Hi David, I'm certainly no physicist, but I did forward the original note to a friend who is. He's currently working on a new theory for a certain kind of non-linear wave and is trying to get published in Physical Review Letters. He's been working non-stop on his paper, but the twisted light thing seems to have give him pause. ----These are his first comments: I have briefly scanned a few articles on light with "orbital angular momentum." I probably do not understand the effect. The words do not have a concrete meaning for me. It may also be that Physical Review and the peer-review optics journals have few or no articles on light with orbital angular momentum. There is some chance nothing new is going on. Maybe this is just another way to talk about polarization. I say that because I am fairly sure circularly polarized light has angular momentum. Individual photons carry angular momentum. They spin. I ought to know about that but unfortunately I probably do not. My guess is that you can create a light beam in which the spin or angular momentum of its photons points parallel to the direction of propagation or antiparallel to it, and this would be called clockwise or counterclockwise circular polarization. Every now and then, you see a helical antenna for radio waves. I don't know whether linearly polarized light produced by a polarizing filter, like the plastic in sunglasses lenses, has zero angular momentum. I suspect symmetry dictates that it has zero angular momentum. There is a precise meaning of light "interference." It is the same thing as diffraction and simply means that two or more beams of light, when they cross, produce a total electric and magnetic field that is equal to the sum of the fields that would be produced separately by each beam. It means they pass cleanly through each other. They add up "linearly," in the parlance. You can say the same thing about water waves or waves on a string. When two separate waves meet, the total amplitude is the (linear) sum of the two separately. They stand on each other's shoulders. Interference is how diffraction gratings work. It is how a line array of hydrophones can form a narrow beam. It is the reason that a distant source of light, like a street light, will appear broken up into multiple sources if it is viewed from a large distance through a screen or mesh with regularly spaced openings. When a beam of light of one color goes through a circular aperture and falls on a screen, the intensity does not decay smoothly, but has alternating light and dark annular zones. That too is a result of diffraction or interference. Feynman wrote that, as far as he could tell, diffraction and interference have the same meaning. That is in part why I am fairly confident about what I am saying. You cannot avoid linear superposition or "interference" in this sense. So when the article says "without interfering," I suspect it means something different. It probably means that the two or more modes of vibration of the electric and magnetic field can be resolved at the receiver. They don't get mixed together in the sense of swapping energy. They propagate independently. Such non-mixing and separability would be a consequence of the principle of linear superposition that I described above. It would not be a new phenomenon. Nonlinear waves are what I study. In that field, the amplitude of the fields affects the medium, which in turn affects the way the light propagates through it. The principle of linear superposition does not apply. Strong beams will behave differently from very weak beams. ----Three hours later he wrote Here is a dumb comment on the idea of OAM of light. Linear momentum is straightforward. Multiply the mass of a body by its speed. Bingo. You've just computed its momentum. There is no common, familiar unit for momentum that I know of. Mass comes in gram, speed in cm/s, energy in ergs or joules, power in Watts, intensity in Watts/m^2, and so on. There is no familiar unit for momentum. There is no "mom," for example, defined as 1 gram-cm/s. But in any case, linear momentum is comprehensible. And, by definition, if the momentum of an object happens to be changing, then its rate of change is equal to the force on that body. Angular momentum is not so simple. It is like torque. Torque is a more subtle idea than force. You can apply exactly the same force with very different outcomes in different cases. In one case, the car won't rise. In another, with the same force, you can raise the car above the deck. That's because of where you applied the force. A longer level-arm or moment-arm means greater torque. A tiny force applied at a great distance will let me raise Bldg. 1 from the deck. Angular momentum goes with torque in the same way momentum goes with force. The rate of change of angular momentum equals the torque on a body. You need to state where you are standing relative to where the force is applied in order to know the torque you feel. The bottom line: the orbital angular momentum of a light beam is not a unique number, so it's a funny idea from the start. The orbital angular momentum probably depends on where you happen to be standing relative to the axis of the beam of light. I have no idea what, if anything, this implies. I just mention it as one aspect of optical OAM. There are articles on OAM in peer-review journals, and one in Physical Review Letters, but maybe it makes sense to question whether it's a great breakthrough in terms of bits/second. Maybe it is. But I would think that we already understood how information rates are fundamentally limited: Shannon's theorem. Probably I'm wrong. But it smells like magic if someone says they have a new way to manipulate light so as to increase the data-rate by orders of magnitude in a fiber optic. I'd think the power has to increase by the same factor, for one thing. ----Then, half a day latter he wrote: It seems that orbital angular momentum of light is a distinct phenomenon, not the same as polarization itself, and that Physical Review has at least one article on it. Maybe the phase fronts of a light beam with nonzero orbital angular momentum look like a corkscrew, and the beam spirals in some sense. But mostly I don't understand the effect. ----Then, a couple of days later: The OAM field configurations are identical, I think, to familiar, well known mode(s) of optical waveguides. I saw the name and more or less recognized it on one of the websites I briefly looked at. This is along the lines of "nothing new here." But I cannot say that honestly. I'm just ignorant. Worse, I am biased owing to a chat months ago with a mathematician on OAM. Probably someone asked him to look into the literature. Another case of optical beams surprised me when I first ran across them. These are so-called "Bessel beams," or "focused wave modes." Contrary to intuition, beams exist that do not bloom, spread, or diffract, but instead remains collimated. There are papers on these. I would have bet my life that any beam must diffract (spread and become broader as it travels). Wrong. I simply had not bumped into Bessel beams until then. They're not tricky. They are based on the usual linear theory, not some esoteric nonlinear effect. They are created by passing a plane wave through an annular aperture (like a washer or doughnut or inner tube) in an opaque plane. Regards, Lew -- To unsubscribe, e-mail: opensuse+unsubscribe@opensuse.org To contact the owner, e-mail: opensuse+owner@opensuse.org