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PROFESSOR: In this
demonstration,

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we're going to illustrate
the propagation of light

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in a fiber--

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a glass fiber.

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As we know, today, optical
fibers have a lot of low loss--

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very low loss--

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of the order of 1 to
2 dBs per kilometer.

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And, of course,
they're being used

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for communication, as well
as other applications,

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like sensors.

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So the setup we have is a is a
laser-- a helium-neon laser--

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over here.

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Here's the output
from the laser.

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We're going to reflect it by
this mirror and this mirror

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and then pass it through a lens.

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This lens over here focuses
the light into the fiber end.

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And if we can take a close-up
of what's going on over here,

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what you would see
is then a lens--

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this lens-- then
focusing the light.

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And the fiber is very
close to the lens.

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And then the rest of
the fiber is here.

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So here is the
rest of the fiber.

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Now, this fiber-- in fact,
what you're seeing over here

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is essentially the
plastic jacket.

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The fiber core is about
four microns in diameter,

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and the cladding is 125 microns.

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And the rest that you see
here is the plastic jacket.

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That's why it looks so
visible, because it's so thick.

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The other end of the fiber
then goes into this holder

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in the chuck here.

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It's a fiber hold in a chuck.

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And the output of the
fiber then is over here

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onto this little screen.

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Now, if we can, maybe
we can take a close look

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at the fiber end here.

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What shows that-- what
you see over here--

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in fact, let me point to it--

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is the cladding.

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Essentially, we've stripped the
jacket, and what you see here

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is just the cladding.

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And this is the 125 microns.

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While over here is the fiber
with the plastic jacket.

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00:02:40,970 --> 00:02:42,720
So when you remove
the plastic jacket,

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then you have essentially
what you're seeing

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is just 125 micron cladding.

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So this is then the fiber.

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And there's the
output of the fiber.

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Now, what we see, if we can then
enhance this and bring it in,

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is the single-mode
behavior of a fiber.

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And it looks almost like
a Gaussian kind of spot.

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Not quite Gaussian, but looks
like a Gaussian kind of spot.

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Now, what I'm doing now is
just adjusting the coupling

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into the fiber.

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And it's very touchy,
because, as I said,

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the core is only
about 4 microns.

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So this is what then
a single-mode fiber--

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the output from a
single-mode fiber--

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looks like.

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00:03:40,540 --> 00:03:43,410
And as I misalign a line here,
it doesn't make any difference.

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All you get is just
a loss in intensity.

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The shape of the
mode stays the same.

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So, remember, the core is
4 microns, cladding is 125,

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the wavelength of the
light is 6328 angstroms,

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and the core to index difference
is about 1 part in 10 to the 3.

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So this way you can
show that, indeed, you

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get single-mode propagation.

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00:04:10,950 --> 00:04:14,310
Now, I would like to illustrate
some interesting phenomena

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about fiber.

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So if we get the camera
to look over here,

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I want to illustrate how touchy
is the propagation of light

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in a single-mode fiber.

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Now, here is a piece
of fiber, and you

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00:04:29,190 --> 00:04:34,230
can see that there's no light
scattered from the fiber.

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00:04:34,230 --> 00:04:39,950
Now, all I have to
do is bend the fiber,

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and you're beginning
to see light

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that gets transmitted
out of the fiber--

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00:04:45,990 --> 00:04:53,190
gets essentially kicked out
the fiber because of the bend.

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00:04:53,190 --> 00:04:55,710
And the reason for
that, because you

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start going against the
rules of propagation of light

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00:05:01,050 --> 00:05:03,300
in a fiber.

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00:05:03,300 --> 00:05:07,290
For example, if you take
the ray explanation,

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00:05:07,290 --> 00:05:11,130
is that what you're
doing, you are exceeding

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00:05:11,130 --> 00:05:16,590
or you're changing
the angle of light

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with respect to the fiber.

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00:05:20,820 --> 00:05:24,630
Which means that if you are
below the critical angle,

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then the light is no longer
totally internally deflected

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00:05:28,260 --> 00:05:30,540
and therefore gets kicked out.

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00:05:30,540 --> 00:05:31,230
So here it is.

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00:05:31,230 --> 00:05:32,070
It's very dramatic.

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00:05:32,070 --> 00:05:35,717
As soon as you put this little
bend in this fiber, you can

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00:05:35,717 --> 00:05:37,050
you can kick out a lot of light.

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00:05:37,050 --> 00:05:39,090
In fact, there's
the glow right here.

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00:05:39,090 --> 00:05:43,290
Now, if we can bring in
the output of the fiber

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00:05:43,290 --> 00:05:45,720
into the inset
over here, now you

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can see that, as I
increase the bend,

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00:05:51,840 --> 00:05:53,890
you can see that the intensity--

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00:05:53,890 --> 00:05:55,860
here, I'll do it even more--

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00:05:55,860 --> 00:05:56,880
then drops quite a bit.

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Which means I've kicked
out almost all the light

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00:05:59,730 --> 00:06:03,300
by simply putting a
bend into the fiber.

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00:06:03,300 --> 00:06:06,120
So the illustration
here then shows

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that if you leave the fiber
alone without the sharp bends,

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everything is fine.

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00:06:10,530 --> 00:06:15,510
If you put in a bend, then you
can kick out a lot of light,

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00:06:15,510 --> 00:06:19,842
and then not much
will be transmitted.

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00:06:19,842 --> 00:06:21,300
So you have to be
careful you don't

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put it too tight of a bend,
otherwise you fiber is brittle,

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00:06:23,780 --> 00:06:24,780
and you break the fiber.

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00:06:24,780 --> 00:06:29,700
So you have to be
careful how you do this.

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So in this demonstration, we've
seen how single-mode by fiber

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behaves.

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In the next
demonstration, we're going

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to bring a another fiber
with a different-sized core,

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and we going to see what
comes out from that fiber.