Optics: Propagation in Optical Fibers
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License | CC Attribution - NonCommercial - ShareAlike 4.0 International: You are free to use, adapt and copy, distribute and transmit the work or content in adapted or unchanged form for any legal and non-commercial purpose as long as the work is attributed to the author in the manner specified by the author or licensor and the work or content is shared also in adapted form only under the conditions of this | |
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LaserVideoOpticsMonomodefaserFiberQuality (business)Ground stationMaterial
00:20
GlasfaserkabelGlasfaserLaserOrder and disorder (physics)SensorLightFernsehsatellitCommunications satelliteTiefdruckgebietLecture/Conference
00:53
Camera lensGlasfaserkabelRear-view mirrorKopfstützeLight
01:21
GlasfaserkabelFACTS (newspaper)Lecture/Conference
01:27
GlasfaserkabelKopfstützeNuclear fuelMagnetic coreLecture/Conference
01:44
DreibackenfutterGlasfaserkabelLocherLecture/Conference
01:55
GlasfaserkabelLocherScreen printing
02:06
Glasfaserkabel
02:16
GlasfaserkabelNuclear fuelFACTS (newspaper)
02:43
GlasfaserkabelNuclear fuelLecture/Conference
02:56
Nuclear fuelWavelengthHose couplingMagnetic coreLightGlasfaserkabelMode of transportTransverse modeIntensity (physics)Video
03:58
Spare partMagnetic coreRefractive indexTransverse modeGlasfaserkabel
04:21
LightGlasfaserkabelAngle of attackSharpeningIntensity (physics)Bending (metalworking)RulerRegentropfenTransverse modeTotalreflexionFACTS (newspaper)Band gap
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GlasfaserkabelSizingTransverse modeMagnetic coreLecture/Conference
Transcript: English(auto-generated)
00:00
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00:21
In this demonstration, we're going to illustrate the propagation of light in a fiber, a glass fiber. As we know today, optical fibers have very low loss of the order of one or two dBs per kilometer. And of course, they're being used for communication as well
00:42
as other applications like sensors. So the setup we have is a helium-neon laser over here. Here's the output from the laser. We're going to reflect it by this mirror and this mirror, and then pass it through a lens.
01:02
This lens over here focuses the light into the fiber end. And if we can take a close-up of what's going on over here, what you would see is then a lens, this lens then focusing the light and fiber. And the fiber is very close to the lens. And then the rest of the fiber is here.
01:21
All right, so here is the rest of the fiber. Now this fiber, in fact, what you're seeing over here is essentially the plastic jacket. The fiber core is about four microns in diameter. And the cladding is 125 microns.
01:41
And the rest that you see here is the plastic jacket. That's why it looks so visible because it's so thick. The other end of the fiber then goes into this holder and the chuck here. It's in a fiber hole in a chuck. And the output of the fiber then is over here
02:03
onto this little screen. Now, if we can, maybe we can take a close look at the fiber end here, which shows that what you see over here, in fact, let me point to it.
02:23
What you see over here is the cladding. Essentially, we strip the jacket. And what you see here is just the cladding. And this is the 125 microns. While over here, over here is the fiber with the plastic, with the plastic jacket.
02:41
So when you remove the plastic jacket, then you have, essentially what you're seeing is just the 125 micron cladding. All right, so this is then the fiber. And there's the output of the fiber. Now, what we see, if we can then enhance this and bring it in, what we see is the single mode
03:02
behavior of a fiber. And it looks almost like a Gaussian kind of spot. Not quite Gaussian, but it looks like a Gaussian kind of spot. Now, what I'm doing now is just adjusting the coupling
03:23
into the fiber. And it's very touchy because, as I said, the core is only about four microns. All right, so this is what then a single mode fiber, the output from a single mode fiber looks like.
03:41
And as I misalign here, it doesn't make any difference. All you get is just a loss in intensity. The shape of the mode stays the same. So remember, the core is four microns. Cladding is 125. The wavelength of the light is 6328 angstroms. And the core to index difference
04:03
is about one part in 10 to the 3. So this way, you can show that, indeed, you get single mode propagation. Now, I would like to illustrate some interesting phenomena about fiber. So if we get the camera to look over here, I want to illustrate how touchy is
04:24
the propagation of light in a single mode fiber. Now, here is a piece of fiber. And you can see that there's no light scattered from the fiber. Now, all I have to do is bend the fiber. Bend the fiber, and you're beginning to see light
04:42
that gets transmitted out of the fiber, gets essentially kicked out the fiber because of the bend. And the reason for that, because you start going against the rules of propagation of light in a fiber.
05:03
For example, if you take the ray explanation, is that what you're doing, you are exceeding or you're changing the angle of light with respect to the fiber, which means that if you're
05:23
below the critical angle, then the light is no longer totally internally reflected and therefore gets kicked out. So here it is. It's very dramatic. As soon as you put a little bend in this fiber, you can kick out a lot of light. In fact, this is the glow right here. Now, if we can bring in the output of the fiber
05:43
into the inset over here, now you can see that as I increase the bend, you can see that the intensity drops quite a bit, which means I've kicked out almost all the light by simply putting a bend into the fiber.
06:03
So the illustration here then shows that if you leave the fiber alone without sharp bends, everything is fine. If you put in a bend, then you can kick out a lot of light, and then not much will be transmitted.
06:20
So you have to be careful. You don't put it too tight a bend. Otherwise, your fiber is brittle and you break the fiber. So you have to be careful how you do this. All right? So in this demonstration, we've seen how single-mode fiber behaves. In the next demonstration, we're going to bring another fiber with a different size core,
06:44
and we're going to see what comes out from that fiber.