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SHAOUL EZEKIEL: Now that we
know all about how lasers work,

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we're ready to ask some
very important questions.

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For example, what is the
nature of laser light?

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Here's some questions.

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What is the wavelength?

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Is the light tunable
in wavelength?

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Is the output or the
spectrum of the output--

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is it single frequency, is it
multiple frequencies, or what?

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Is the output columnated?

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Can it be columnated
to a diffraction limit?

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Is it focusable to a small spot
or a diffraction-limited spot?

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Is the output polarized?

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How much power do we
get out of the laser,

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and is it continuous
power or pulse power?

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Now, these are all very
important questions,

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and we should have answers
for all of these questions.

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Now we're going to illustrate
as many of these properties

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as possible.

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The only problem is that
I can't use the same laser

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to illustrate all
these properties.

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I'd like to use a
variety of lasers

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that we can lay our hands on
to illustrate these properties.

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For example, we'll use helium
neon lasers to illustrate

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some basic
properties, then we'll

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go on and use a
tunable dye laser

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to show how tunable some
lasers can be, and so on.

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So first, we're going to start
with a very simple property,

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which is, is the
laser light polarized?

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So we're going to start with
a helium neon laser over here

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with external mirrors.

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And the amplifier is a
helium neon amplifier

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here in the middle.

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So what I'll do
first is turn it on--

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turn this laser on--

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and here's the output
from the laser.

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I'm being reflect
it by this mirror.

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And then this mirror
is the output here,

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and then the output
goes onto the screen.

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So let's now look at that spot
on the output in close-up,

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and then I will put
a polarizer here

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in the way of the light beam
and see whether, indeed,

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the light is plane-polarized.

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So what I'm going to do--
here's the polarizer,

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and the arrow indicates
that the transmission

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axis of the polarizer
is along the vertical.

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Now, if we see that when
I'm along the vertical here,

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the spot has maximum intensity--

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but as I rotate the transmission
axis of the polarizer,

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you see that the light
gets extinguished.

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Here I go back to
vertical polarization.

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The light has a maximum.

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And I go to the
orthogonal position,

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and the light is extinguished.

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So all this says, very simply,
that the light from this laser

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is plane-polarized.

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Simple as that.

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Now, you may wonder, why
is it plane-polarized?

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Well the answer is that,
in this particular laser,

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we have the mirrors mounted
outside the discharge tube,

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and the discharge tube is
terminated by windows that

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are placed at Brewster's angle.

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And you may recollect
that at Brewster's angle,

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only certain polarizations
have zero loss as they

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go through the windows, but
the orthogonal polarization

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through that has a lot of
loss and, therefore, will not

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lase because we don't
have enough gain to make

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those polarizations lase.

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So in this case,
it's the polarization

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in the vertical plane that
actually suffers no loss as it

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goes through the
Brewster windows

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because the way the
tube is oriented.

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And that's why the output
is plane-polarized and

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plane-polarized in
the vertical plane.

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All right, let's now take a look
at another helium neon laser,

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the one that we have over here.

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I'll just take this
mirror mount out,

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and I'll turn this one off
and turn this little one on.

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So here we are.

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Here's the output of this laser.

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And then, again, just
like before, we put it

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onto the screen.

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Now, if we look
at it in close-up

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as I put the polarizer
here in the way,

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now we're ready to look at
the polarization of the light

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coming out from this laser.

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Now, you can see
when the transmission

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axis is in the vertical
plane, we have a lot of light.

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Now, as I rotate
the transmission

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axis of the polarizer, I find
small changes, but not a lot.

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And in fact, when I go to
the orthogonal position,

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I find that I cannot extinguish
the beam-- the light.

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Here we go back to
vertical polarization,

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and now to horizontal
polarization.

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And indeed, it's
small changes, but not

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huge like in the other
laser, where we, indeed,

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extinguished all the
light when we went

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to horizontal polarization.

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So the conclusion here
is that for this laser

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with internal mirrors, the
light is not plane-polarized.

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The question is, what is the
polarization of that light?

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Is it unpolarized, is
it circularly polarized,

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and what have you.

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And I want you to think about.

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But the only difference,
then, between this laser

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and the previous one is the
fact that the previous one had

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external mirrors and
Brewster windows,

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and this one here
has no windows,

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and the mirrors are just
attached to the discharge tube.

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Now, in the next
demonstration, we're

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going to look at the
spectrum of laser light

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from the one with
the internal mirrors

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and the one from
external mirrors.

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And we'll see what we
can learn from that.