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PROFESSOR: Now, we've
replaced the previous fiber

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00:00:23,170 --> 00:00:24,852
with another one.

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The only difference between
this fiber and the previous one

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is that the core diameter
is now a little bit bigger.

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It's about 6 microns instead of
4 microns in the previous one.

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The setup is exactly the same.

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Again, we have the
laser here getting

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reflected by this mirror and
this mirror into the lens,

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into this fiber--

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00:00:46,180 --> 00:00:47,470
this new fiber.

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And then, the output of the
fiber goes onto the screen.

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So let's now take a look at
the output of this fiber.

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00:00:56,214 --> 00:00:57,520
Aha.

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So the first thing we see, that
doesn't look like a single-mode

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00:01:00,910 --> 00:01:03,020
at all, that single low.

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00:01:03,020 --> 00:01:07,720
And if I change the adjustment--
if we can have a camera looking

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00:01:07,720 --> 00:01:09,275
at my adjustments--

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00:01:12,600 --> 00:01:20,910
then you see that I can
get a variety of shapes.

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All I'm doing is
changing the adjustment,

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and I can get a
variety of shapes.

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00:01:24,900 --> 00:01:27,420
And clearly, it's
not like what we

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had before-- the single-mode
behavior that we had before.

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00:01:33,320 --> 00:01:35,810
And this is then the
so-called multi-mode.

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00:01:35,810 --> 00:01:39,200
What you're seeing are
different transverse modes

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00:01:39,200 --> 00:01:42,350
that can propagate
in this fiber,

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00:01:42,350 --> 00:01:44,750
because the core
diameter is bigger.

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00:01:44,750 --> 00:01:49,760
But I'm going to leave it
to you then to explain why--

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00:01:49,760 --> 00:01:54,530
exactly why you get
higher transverse modes

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00:01:54,530 --> 00:01:57,920
propagated in this fiber.

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00:01:57,920 --> 00:01:59,610
Again, the wavelength
is the same.

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00:01:59,610 --> 00:02:05,490
The only difference is that
the core diameter is bigger.

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00:02:05,490 --> 00:02:08,020
You can see this one here,
you get a nice, dark line

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00:02:08,020 --> 00:02:08,919
in the middle.

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00:02:08,919 --> 00:02:13,080
And then if I align it over
here, you can see it's blobby.

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It's a mixture of modes.

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00:02:14,080 --> 00:02:17,200
That's why you don't see a
sharp, dark line in the middle

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where you get a
mixture of modes.

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00:02:19,113 --> 00:02:20,530
While, remember,
in the other one,

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00:02:20,530 --> 00:02:22,670
no matter what I did
in the alignment,

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00:02:22,670 --> 00:02:28,330
it was still single-mode coming
out-- just one low coming out.

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00:02:28,330 --> 00:02:32,800
Now, multi-mode fibers have
many applications also.

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00:02:32,800 --> 00:02:36,730
But for today, for a lot
of sophisticated sensor

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00:02:36,730 --> 00:02:40,810
applications and
communications, one generally

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00:02:40,810 --> 00:02:42,790
uses single-mode fibers.

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00:02:42,790 --> 00:02:49,070
So here a pretty mode with
a dark line down the middle.

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00:02:49,070 --> 00:02:54,670
Now, what I would like to do
is show how touchy this fiber

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00:02:54,670 --> 00:02:57,130
is to bending.

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00:02:57,130 --> 00:02:59,770
In fact, If we take a
look at the fiber here,

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00:02:59,770 --> 00:03:05,560
as soon as I just press on
it-- if we take a close look--

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00:03:05,560 --> 00:03:07,595
I can kick light kit light out.

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00:03:07,595 --> 00:03:11,110
If I just simply press on it,
I can kick some light out.

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00:03:11,110 --> 00:03:15,970
So it's very touchy to
stress and to bends.

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00:03:15,970 --> 00:03:18,160
What I would like
to do then is show

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00:03:18,160 --> 00:03:23,650
that I can kick out some
of the transverse modes

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00:03:23,650 --> 00:03:27,070
by simply bending or
stressing the fiber.

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00:03:27,070 --> 00:03:31,300
What I'm going to do is
then bend this fiber.

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00:03:31,300 --> 00:03:36,940
And if you watch in the inset,
and see that the intensity will

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00:03:36,940 --> 00:03:38,020
go down.

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00:03:38,020 --> 00:03:43,500
But if I keep
increasing the bend,

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00:03:43,500 --> 00:03:48,630
I end up with a single-mode
propagation but weak.

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00:03:48,630 --> 00:03:53,340
And this illustrates that you
can strip off the high order

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00:03:53,340 --> 00:03:57,930
modes by simply
bending the fiber.

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00:03:57,930 --> 00:04:02,520
But the penalty is you get much
less light getting propagated.

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00:04:02,520 --> 00:04:04,890
So here we have
single-mode propagation,

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00:04:04,890 --> 00:04:08,220
and here we have multi-mode or
a mixture of multi-modes that

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00:04:08,220 --> 00:04:10,890
are propagating in the fiber.

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00:04:10,890 --> 00:04:13,470
Again, you can see that
there's a lot of light that

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00:04:13,470 --> 00:04:14,550
gets kicked out--

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00:04:14,550 --> 00:04:16,200
has to be kicked
out of the fiber.

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00:04:16,200 --> 00:04:20,279
You can see the
bend absolutely low.

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00:04:20,279 --> 00:04:26,580
In fact, this bending
effect on the transmission

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00:04:26,580 --> 00:04:31,180
of light in a fiber can
be used as a sensor--

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00:04:31,180 --> 00:04:36,180
a sensor of pressure,
a sensor of stress,

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00:04:36,180 --> 00:04:37,720
bends, and what have you.

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00:04:40,680 --> 00:04:43,830
So then, in summary,
we've looked

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00:04:43,830 --> 00:04:48,840
at the propagation of laser
light in a single-mode fiber,

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00:04:48,840 --> 00:04:54,000
and we also saw what happens
when the fiber core is a little

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00:04:54,000 --> 00:04:58,320
bit big, and we saw the
propagation of multi-modes.

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00:04:58,320 --> 00:05:01,530
And, as I mentioned earlier,
the popular fiber today

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00:05:01,530 --> 00:05:03,720
is the single-mode fiber.

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00:05:03,720 --> 00:05:06,060
But, again, there
are applications

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00:05:06,060 --> 00:05:09,050
for multi-mode fibers.