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SHAOUL EZEKIEL: In
this demonstration,

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we're going to determine what
the state of polarization

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of this laser is.

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This laser is a
helium neon laser.

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It's a little bit more
powerful than the normal ones

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we see, for example, in
supermarket checkout counters.

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The light coming out
is about 35 milliwatts.

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And you can see the beam
here along this path.

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We're going to analyze the state
of polarization of the laser

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with this polarizer.

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And the light transmitted
through the polarizer,

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as you can see here, will
be reflected by this mirror

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and then will be reflected
again by this mirror.

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And then we let the
spot hit the screen.

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So then the purpose
of this test is

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to rotate the plane
of polarization

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or the transmission
axis of the polarizer

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to find out whether the
polarization of the laser light

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is indeed
plane-polarized or not.

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So now, while we look at
the spot on the screen,

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let me start rotating
the plane of polarization

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of the polarizer or the
transmission axis, if you want,

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of the polarizer.

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And we start in the
vertical position.

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And then we'll
start rotating as we

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observe the spot on the screen.

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As you can see, the intensity
gets weaker and weaker

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until it is--

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if you watch carefully, until
it's completely extinguished.

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And here, you notice that the
arrow is about 90 degrees away

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from where we started out.

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And this shows--
let's do it again.

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We can go here to the
vertical position--

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lot of light coming out.

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And now we go to the
horizontal position

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and extinguish the light.

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At least to the eye, it
looks pretty much complete,

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which shows that the state
of polarization of this laser

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is plane-polarized.

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And the axis or the
plane of polarization

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is indeed established
by the transmission axis

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of the polarizer, which is,
in this case-- is shown to be

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vertical by the vertical arrow.

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Some of you may have
noticed on the screen

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a secondary weak spot
near the main one

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and probably wondered
where it's coming from.

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It's not coming from the laser.

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And you can show that by
rotating the polarizer.

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And you see that
indeed, the spot rotates

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with the polarizer, which is--

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which means that it's probably
coming from internal reflection

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within the polarizer.

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So it's nothing to
be concerned about.

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Spot on the screen is
not very clean-looking.

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And there's a lot
of fuzz and fringes

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and what have you around--

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surrounding the spot.

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Again, this is not
inherent to the laser.

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This is due to the fact
that the laser beam

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had to go through some
optics, like the output

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mirror of the laser, the
polarizer, the two mirrors.

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And that's where you're
getting the corruption.

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We can easily clean
that up with a technique

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called spatial filtering, which
we will demonstrate later.

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Now, I'd like to
show how easily one

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can corrupt the plane of
polarization of laser light.

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And what I'm going to do here
is take this polarizer out

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from here, let the laser
beam hit this mirror

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and hit the second
mirror, and let

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the spot go onto the screen.

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And now I would like to analyze
the state of polarization

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not as I did before, close
to the laser, but over here

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after the two reflections.

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And again, if the
state of polarization

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has not been corrupted
by the reflection

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through these two
mirrors, then we

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would be able to extinguish
the laser beam as it

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goes through the polarizer.

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So let's see now as we look
at the spot on the screen

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and then I rotate
the transmission

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axis of the polarizer--
let's see if we

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can extinguish the laser beam.

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So I keep turning
now, getting close

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to the orthogonal position.

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And you'll see that I
go through a minimum.

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And I cannot extinguish it the
way I could in the previous--

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when the polarizer was
close to the laser.

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So what this shows--

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that it's very easy to corrupt
the polarization of the laser.

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So it means that if we go
through many components,

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we have to then place a
polarizer, like this one,

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to establish the
state of polarization

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after going through
all these components.

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And the state of polarization
will be established then

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by this polarizer.

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Now, just to make
sure that indeed we

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do have perfectly polarized
light, plane-polarized light,

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what I'm going to
do is analyze now

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the polarization of
the light coming out

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of this polarizer by means
of a-- yet another polarizer,

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which, again, used as a
polarization analyzer--

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and you can see the
arrow here, again,

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is along the transmission
axis of the polarizer.

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And if I now rotate
the transmission

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axis of the polarizer--

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and then we'll see whether I
can extinguish the light coming

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through this polarizer.

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And as you can see, I
can extinguish it now

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because it's been established
by this polarizer.

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So the conclusion of all
this is that if laser light,

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plane-polarized laser
light, is passed

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through many optical
components, the polarization

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or the state of polarization
will be corrupted.

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And therefore, we have
to use another polarizer

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to re-establish the
state of polarization.