[opensuse] Friday - something to wrap your head around (literally) "twisted light"
Or more formally OAM: https://www.theregister.co.uk/2018/10/25/twisted_light_networking_speeds/ apparently has the potential to increase fiber-throughput 100 fold. Now I like to think I'm roundly educated, and my undergrad did dig heavily into the wave-particle duality of photons and light, but this is the first time I've heard about OAM or its use to encode information. Future may not be all that boring after all. -- David C. Rankin, J.D.,P.E. -- To unsubscribe, e-mail: opensuse+unsubscribe@opensuse.org To contact the owner, e-mail: opensuse+owner@opensuse.org
On 10/26/2018 10:17 AM, David C. Rankin wrote:
Or more formally OAM:
https://www.theregister.co.uk/2018/10/25/twisted_light_networking_speeds/
apparently has the potential to increase fiber-throughput 100 fold.
The issue is not fibre bandwidth, but getting data on and off the fibre, with currently available tech. I read a while ago that a fibre could carry up to 2.5 petabit/s, which is far above what any equipment can handle. To make better use of the fibre, multiple wavelengths are used. Typical systems used in telecom use a dozen or so wavelengths, but over 200 are possible. Even then, available equipment still can't come close to using the available bandwidth at a given wavelength. -- To unsubscribe, e-mail: opensuse+unsubscribe@opensuse.org To contact the owner, e-mail: opensuse+owner@opensuse.org
Op vrijdag 26 oktober 2018 16:17:56 CEST schreef David C. Rankin:
Or more formally OAM:
https://www.theregister.co.uk/2018/10/25/twisted_light_networking_speeds/
apparently has the potential to increase fiber-throughput 100 fold.
Now I like to think I'm roundly educated, and my undergrad did dig heavily into the wave-particle duality of photons and light, but this is the first time I've heard about OAM or its use to encode information. Future may not be all that boring after all. An old UNIX tutor: "And even if we could get data transported through whatever medium at the speed of light, dealing with these data would require computing at light speed". This was at a conference where we just heard that 1Mbit/sec accross copper telephony cables was not technically impossible ( where the market said it was ). Like James, my tutor hit the nail: all data transfer speeds depend on getting the data on and off the transport medium.
-- Gertjan Lettink a.k.a. Knurpht openSUSE Board Member openSUSE Forums Team -- To unsubscribe, e-mail: opensuse+unsubscribe@opensuse.org To contact the owner, e-mail: opensuse+owner@opensuse.org
On 10/26/2018 01:29 PM, Knurpht-openSUSE wrote:
This was at a conference where we just heard that 1Mbit/sec accross copper telephony cables was not technically impossible ( where the market said it was ).
???? A T1 line could carry 1.544 Mb over 2 pairs, back in 1962, well before there was such a thing as Unix. https://en.wikipedia.org/wiki/T-carrier#Transmission_System_1 -- To unsubscribe, e-mail: opensuse+unsubscribe@opensuse.org To contact the owner, e-mail: opensuse+owner@opensuse.org
On 26/10/2018 19.45, James Knott wrote:
On 10/26/2018 01:29 PM, Knurpht-openSUSE wrote:
This was at a conference where we just heard that 1Mbit/sec accross copper telephony cables was not technically impossible ( where the market said it was ).
????
A T1 line could carry 1.544 Mb over 2 pairs, back in 1962, well before there was such a thing as Unix.
https://en.wikipedia.org/wiki/T-carrier#Transmission_System_1
Yes, but a T1 was a coaxial cable, designed to carry that speed, whereas a telephone copper pair was designed, if it was at all designed, to carry voice. What almost everybody thought, was that it was impossible to carry 1 Mb/s on those POT lines as they were. -- Cheers / Saludos, Carlos E. R. (from 42.3 x86_64 "Malachite" (Minas Tirith))
On Fri, 26 Oct 2018 20:42:22 +0200 "Carlos E. R." <robin.listas@telefonica.net> wrote:
On 26/10/2018 19.45, James Knott wrote:
On 10/26/2018 01:29 PM, Knurpht-openSUSE wrote:
This was at a conference where we just heard that 1Mbit/sec accross copper telephony cables was not technically impossible ( where the market said it was ).
????
A T1 line could carry 1.544 Mb over 2 pairs, back in 1962, well before there was such a thing as Unix.
https://en.wikipedia.org/wiki/T-carrier#Transmission_System_1
Yes, but a T1 was a coaxial cable, designed to carry that speed, whereas a telephone copper pair was designed, if it was at all designed, to carry voice.
What almost everybody thought, was that it was impossible to carry 1 Mb/s on those POT lines as they were.
With all respect to everybody, 1.544 Mbps over two pairs is less than 1 Mbps per pair, s I'm not sure where the argument is, nor why it's worth discussing, especially on the support list rather than offtopic. -- To unsubscribe, e-mail: opensuse+unsubscribe@opensuse.org To contact the owner, e-mail: opensuse+owner@opensuse.org
On 26/10/2018 20.50, Dave Howorth wrote:
On Fri, 26 Oct 2018 20:42:22 +0200 "Carlos E. R." <robin.listas@telefonica.net> wrote:
On 26/10/2018 19.45, James Knott wrote:
On 10/26/2018 01:29 PM, Knurpht-openSUSE wrote:
This was at a conference where we just heard that 1Mbit/sec accross copper telephony cables was not technically impossible ( where the market said it was ).
????
A T1 line could carry 1.544 Mb over 2 pairs, back in 1962, well before there was such a thing as Unix.
https://en.wikipedia.org/wiki/T-carrier#Transmission_System_1
Yes, but a T1 was a coaxial cable, designed to carry that speed, whereas a telephone copper pair was designed, if it was at all designed, to carry voice.
What almost everybody thought, was that it was impossible to carry 1 Mb/s on those POT lines as they were.
With all respect to everybody, 1.544 Mbps over two pairs is less than 1 Mbps per pair, s I'm not sure where the argument is, nor why it's worth discussing, especially on the support list rather than offtopic.
Two pairs? Ah, two T1 lines. Coaxial lines. But they carried 1 Mbit on each. 1.544 Mb/s, or 2 Mb/s on E1, on each direction. -- Cheers / Saludos, Carlos E. R. (from 42.3 x86_64 "Malachite" (Minas Tirith))
On 10/26/2018 02:56 PM, Carlos E. R. wrote:
Two pairs? Ah, two T1 lines. Coaxial lines. But they carried 1 Mbit on each. 1.544 Mb/s, or 2 Mb/s on E1, on each direction.
The North American T1 was over 2 twisted pairs. Living in Canada, I have no experience with E1. The international carriers, that transported telecom between countries & continenents, would have to deal with conversion between T1 & E1. -- To unsubscribe, e-mail: opensuse+unsubscribe@opensuse.org To contact the owner, e-mail: opensuse+owner@opensuse.org
On 26/10/2018 21.32, James Knott wrote:
On 10/26/2018 02:56 PM, Carlos E. R. wrote:
Two pairs? Ah, two T1 lines. Coaxial lines. But they carried 1 Mbit on each. 1.544 Mb/s, or 2 Mb/s on E1, on each direction.
The North American T1 was over 2 twisted pairs. Living in Canada, I have no experience with E1. The international carriers, that transported telecom between countries & continenents, would have to deal with conversion between T1 & E1.
They were split into channels which were sent their different ways inside the switch (exchange). The problem were the channels, because they had different capacities: 64 Kbps on the E1 and 56 kbps on the T1, IIRC. I think there was a software conversion done, but I never saw it. A further consideration was the µ-law algorithm, different each side of the Atlantic (A-law my side). I did not need to know ;-) That 56 Kbps was the reason why modems had that top speed at that side of the Atlantic. And ISDN too. Linux had to know that (lame excuse, I know). -- Cheers / Saludos, Carlos E. R. (from 42.3 x86_64 "Malachite" (Minas Tirith))
On 10/26/2018 04:00 PM, Carlos E. R. wrote:
They were split into channels which were sent their different ways inside the switch (exchange). The problem were the channels, because they had different capacities: 64 Kbps on the E1 and 56 kbps on the T1, IIRC. I think there was a software conversion done, but I never saw it. A further consideration was the µ-law algorithm, different each side of the Atlantic (A-law my side). I did not need to know ;-)
That 56 Kbps was the reason why modems had that top speed at that side of the Atlantic. And ISDN too. Linux had to know that (lame excuse, I know).
That would depend on how signalling was done. The old way was "robbed bit" signalling, where the 8th bit was used for signalling. With out of band signalling, as used in ISDN PRI an BRI, a dedicated channel was used for signalling, so that PRI ISDN would have 23 64 Kb "bearer" channels for data and a single 64 Kb "data" channel for signalling. BRI ISDN used 2 64 Kb bearer channels and 1 16 Kb data channel. With separate bearer and data channels, conversion between T1 & E1 was much easier, as you were dealing strictly with 64 Kb channels. -- To unsubscribe, e-mail: opensuse+unsubscribe@opensuse.org To contact the owner, e-mail: opensuse+owner@opensuse.org
On 10/26/2018 02:50 PM, Dave Howorth wrote:
With all respect to everybody, 1.544 Mbps over two pairs is less than 1 Mbps per pair
It's a full 1.544 M/b per pair. One pair for each direction. Later on, with some types of SHDSL, it was possible to send 1.544 Mb or more, in both directions over a single pair, but 2 pairs were generally used. https://en.wikipedia.org/wiki/Single-pair_high-speed_digital_subscriber_line -- To unsubscribe, e-mail: opensuse+unsubscribe@opensuse.org To contact the owner, e-mail: opensuse+owner@opensuse.org
On 10/26/2018 02:42 PM, Carlos E. R. wrote:
Yes, but a T1 was a coaxial cable, designed to carry that speed, whereas a telephone copper pair was designed, if it was at all designed, to carry voice.
Nope, two twisted pairs. The same sort of pairs as used for analog telephone carriers. BTW, my background is in telecom and I have worked a lot with them over the years. -- To unsubscribe, e-mail: opensuse+unsubscribe@opensuse.org To contact the owner, e-mail: opensuse+owner@opensuse.org
On 26/10/2018 21.26, James Knott wrote:
On 10/26/2018 02:42 PM, Carlos E. R. wrote:
Yes, but a T1 was a coaxial cable, designed to carry that speed, whereas a telephone copper pair was designed, if it was at all designed, to carry voice.
Nope, two twisted pairs. The same sort of pairs as used for analog telephone carriers.
BTW, my background is in telecom and I have worked a lot with them over the years.
Mine too, and all E1 I have installed were coax. -- Cheers / Saludos, Carlos E. R. (from 42.3 x86_64 "Malachite" (Minas Tirith))
This is relevant: https://www.ad-net.com.tw/what-is-t1-and-e1-difference-between-t1-and-e1-exp... In particular:
Copper Delivery: In the T1 signal there is a copper delivery among 4 wires. It is grouped into two pairs. One pair is the RX (1+2) and another is TX (4+5). The RX is the data that is from the network and the TX is to the network. In the E1, there are two types of physical delivery; balanced physical delivery and unbalance physical delivery. The balanced physical delivery has 4 copper wires. It is similar to that of T1. Whereas in the unbalanced physical delivery there is a coax connector which has one cable for RX and one cable for TX.
As a Stateside person my T1's were all 2 pair wires (going back to the early 90s). It's fair to note that POTs lines are also technically 2 pair... but only one pair was used (their was a push in the 70's to wire up the second line for teens). That said, T1's also historically required a regenerator/repeater every 1000 feet. This is a bigger consideration then the copper in my mind. Further complicating this, much like "ethernet" "T1" is a delivery standard. It turns out that for the better part of 30 years "T1" is actually shipped over a variant of SDSL, and only turned into classic T1 at the DMARC. On 1026, James Knott wrote:
On 10/26/2018 02:42 PM, Carlos E. R. wrote:
Yes, but a T1 was a coaxial cable, designed to carry that speed, whereas a telephone copper pair was designed, if it was at all designed, to carry voice.
Nope, two twisted pairs. The same sort of pairs as used for analog telephone carriers.
BTW, my background is in telecom and I have worked a lot with them over the years.
-- __________________________________________________________________________ Josef Fortier Systems Administrator fortier@augsburg.edu Phone: 612-330-1479 __________________________________________________________________________ -- To unsubscribe, e-mail: opensuse+unsubscribe@opensuse.org To contact the owner, e-mail: opensuse+owner@opensuse.org
On 10/26/2018 03:49 PM, Josef Fortier wrote:
It's fair to note that POTs lines are also technically 2 pair... but only one pair was used (their was a push in the 70's to wire up the second line for teens).
Don't confuse cable pairs with how many pairs are required for a phone. An analog phone line is one pair. Phone cables run from 1 pair up to hundreds. The typical drop wire, running from the pole to a home is one pair and inside would likely connect to a two (old) or three (new) pair cable, with each pair able to carry a phone line.
That said, T1's also historically required a regenerator/repeater every 1000 feet. This is a bigger consideration then the copper in my mind.
Actually over 6000'. Read the T1 article I sent earlier. A repeater (often seen as a large can on the side of a telephone pole) would typically handle several circuits, with 1 repeater card for each pair. There was fairly high voltage (+-130V IIRC) to power the cards, with the power passing through each card and onto the next down the line. There would often be a spare circuit and a switch to automagically switch a failed circuit to the spare. -- To unsubscribe, e-mail: opensuse+unsubscribe@opensuse.org To contact the owner, e-mail: opensuse+owner@opensuse.org
On 26/10/2018 19.29, Knurpht-openSUSE wrote:
Op vrijdag 26 oktober 2018 16:17:56 CEST schreef David C. Rankin:
Or more formally OAM:
https://www.theregister.co.uk/2018/10/25/twisted_light_networking_speeds/
apparently has the potential to increase fiber-throughput 100 fold.
Now I like to think I'm roundly educated, and my undergrad did dig heavily into the wave-particle duality of photons and light, but this is the first time I've heard about OAM or its use to encode information. Future may not be all that boring after all. An old UNIX tutor: "And even if we could get data transported through whatever medium at the speed of light, dealing with these data would require computing at light speed". This was at a conference where we just heard that 1Mbit/sec accross copper telephony cables was not technically impossible ( where the market said it was ). Like James, my tutor hit the nail: all data transfer speeds depend on getting the data on and off the transport medium.
Maybe, maybe not. The data can be extracted without computers, using plain old "digital electronics" and converted into several independent and slower flows (think time multiplexing) which would be handled by separate computers. Similarly as the differentiation on the telephone industry of old between "transmission" and "switching". Transmission could handle faster speeds that what switching could handle. A single long distance fibre could transmit data at high speed that on arrival would get separated into several slower fibres, for instance. What would not be that easy, maybe not possible, would be dynamic switching (because it needs computing at that speed). The fibre would use fixed slots. -- Cheers / Saludos, Carlos E. R. (from 42.3 x86_64 "Malachite" (Minas Tirith))
On 10/26/2018 12:29 PM, Knurpht-openSUSE wrote:
Op vrijdag 26 oktober 2018 16:17:56 CEST schreef David C. Rankin:
Or more formally OAM:
https://www.theregister.co.uk/2018/10/25/twisted_light_networking_speeds/
apparently has the potential to increase fiber-throughput 100 fold.
Now I like to think I'm roundly educated, and my undergrad did dig heavily into the wave-particle duality of photons and light, but this is the first time I've heard about OAM or its use to encode information. Future may not be all that boring after all. An old UNIX tutor: "And even if we could get data transported through whatever medium at the speed of light, dealing with these data would require computing at light speed". This was at a conference where we just heard that 1Mbit/sec accross copper telephony cables was not technically impossible ( where the market said it was ). Like James, my tutor hit the nail: all data transfer speeds depend on getting the data on and off the transport medium.
Yes, 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... -- David C. Rankin, J.D.,P.E. -- To unsubscribe, e-mail: opensuse+unsubscribe@opensuse.org To contact the owner, e-mail: opensuse+owner@opensuse.org
On 10/27/2018 08:30 PM, David C. Rankin wrote:
On 10/26/2018 12:29 PM, Knurpht-openSUSE wrote:
Op vrijdag 26 oktober 2018 16:17:56 CEST schreef David C. Rankin:
Or more formally OAM:
https://www.theregister.co.uk/2018/10/25/twisted_light_networking_speeds/
apparently has the potential to increase fiber-throughput 100 fold.
/snip/ 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...
I love it! Thank You!
--doug -- To unsubscribe, e-mail: opensuse+unsubscribe@opensuse.org To contact the owner, e-mail: opensuse+owner@opensuse.org
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
On 29/10/18 9:08 am, Lew Wolfgang wrote:
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
I knew that. BC -- We may have democracy, or we may have wealth concentrated in the hands of a few, but we can't have both. Justice Louis Brandeis, 1910 -- To unsubscribe, e-mail: opensuse+unsubscribe@opensuse.org To contact the owner, e-mail: opensuse+owner@opensuse.org
... this is the first time I've heard about OAM [of light] Probably because you studied it by its correct terminology, elliptical
On 2018-10-26 08:17 AM, David C. Rankin wrote: polarization. Certainly there is an angular momentum association with it, but I'm rather dubious that it would be proper to call it "orbital". BTW, I don't think quantum physics has much to do with it either -- elliptical polarization is included in any second-year physics course on classical electromagnetism. All in all, though, fascinating stuff -- good to see it entering the realm of data communications in a practical way. -- To unsubscribe, e-mail: opensuse+unsubscribe@opensuse.org To contact the owner, e-mail: opensuse+owner@opensuse.org
participants (10)
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Basil Chupin -
Carlos E. R. -
Darryl Gregorash -
Dave Howorth -
David C. Rankin -
Doug -
James Knott -
Josef Fortier -
Knurpht-openSUSE -
Lew Wolfgang