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SHAOUL EZEKIEL: We're ready to
take a close look at a laser

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and see how it ticks
and why it ticks.

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We've picked on a
helium-neon laser

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because a helium-neon
laser is a very simple one,

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and also it's one
of the first lasers.

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In fact, the first laser was
the ruby laser and helium-neon

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was right after it.

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So it was the second laser
action that was observed.

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Also, the red light from
the helium-neon laser

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is familiar to almost
every one of you,

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especially when you
go to the supermarket

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and see it at the
checkout counter.

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Usually, a red
helium-neon laser is used.

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So let me start by showing you
what a helium-neon laser looks

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like.

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Here is a helium-neon laser,
at least a helium neon laser.

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They're all different.

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Now what I would
like you to see here

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is that it's made up of
a discharge tube, which

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is this, which is
the amplifier that

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is necessary for laser action.

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And then the discharge tube
is terminated by two mirrors.

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Here's a mirror on this side.

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You can see it.

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Here's one mirror, and
here is the other mirror.

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So it's a small
laser, and the mirrors

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are sealed right onto
the discharge, too.

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Now let's see this
laser in action.

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And we have it already
set up for you over here.

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And then all I have to
do is to turn the power

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supply on and count to five,
and the laser will come on,

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and here it is.

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Laser's on.

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And see, here is the
glow in the discharge.

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And you see a pink
streak, and that's

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where the light goes
backwards and forwards

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and gets amplified.

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And here is the output mirror.

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The other mirror is a sealed,
and the output from the mirror

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here then goes onto the screen.

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And that's it.

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As long as we
power the discharge

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and have enough gain
to overcome the losses

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and we choose the mirror
transmission appropriately,

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you'll get laser action.

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Now with this kind
of a laser, it's

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very difficult to adjust
anything and to play with it.

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So what I'm going to
do, just so that you

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can see a little bit more
of how a laser works,

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I'm going to go on
and have a laser

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that the mirrors, in
which the mirrors are

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external to the two.

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Now in order to separate the
mirrors out from the amplifier,

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from the discharge
tube, in this case,

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we have to seal
the discharge tube.

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Now here is-- you can see here,
if we take a close-up of this--

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here is a sealed discharge tube.

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It doesn't have mirrors
on it but it has windows.

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Let's let me show
it to you again.

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Here is the amplifier section,
and here is the cathode.

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And the anode is over here.

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So this charge, then,
runs along this line

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over here, this
capillary tube over here.

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Now let's focus at the ends.

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We don't have, as you can see,
we don't have square mirrors.

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You can see that we
don't have windows,

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I should say, not
mirrors, because we're

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going to have the mirrors
external to the discharge tube.

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But you can see that the windows
are sealed but they're not

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square to the tube.

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And the angle, in fact, is
called the Brewster angle

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because at that angle there is
no reflection from the glass

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surfaces for a
sudden polarization.

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So that's why one uses
windows at the Brewster angle,

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so that there is no
reflection in the windows.

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And you can see both ends.

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Now if you take a look at
the other end over here,

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close look at the other end, you
see that, again, both windows

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are at a Brewster's angle.

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So here is, then,
the amplifier tube.

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And now we have another one, a
similar one, that is placed--

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and I'll position
it in the same way--

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that is placed over here,
held in place over here

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in this setup.

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And there's wires running
to run the discharge.

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So here is, then, the amplifier
tube inside this laser.

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And let me again point
to the mirrors now.

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Here is one mirror, and
here is the other mirror.

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And each mirror is held
in a hefty mirror mounts,

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and the adjustments
are over here.

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We have two
adjustments over here.

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And then we have similar
adjustments over here.

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All right.

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So here is, then, the
tube, the amplifier tube.

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The windows are terminated with
windows at Brewster's angle,

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and then we have
the two mirrors.

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So now I'm going to
turn the discharge on.

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And here it is.

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And let's see if
the laser is lasing.

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So if I put a card here,
see that, indeed, the laser

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is lasing.

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So what I'm doing
here is reflecting it

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by this mirror here
and this mirror here--

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here is the laser beam--

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and, again, onto the screen.

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So on the screen now, we
have essentially two spots.

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One is coming from the laser
with the fixed mirrors.

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That's this one.

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And then the other
one, the other spot,

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is coming from the laser with
the adjustable mirrors here.

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So what I'm going to do is first
block the laser with the fixed

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mirrors so that we don't
have any confusion,

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and the only spot, then, is
from the one from the laser

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with the adjustable mirrors.

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So now here we are.

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Here's the laser,
then, all opened up.

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And now I'm going to show you
how touchy the alignment is.

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So now, if you watch the
intensity on the screen as I

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slightly misalign, you can
see the light is out already,

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just by very small misalliance.

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So here we are.

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It's at peak value.

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And then I go the other
way, and it's gone.

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So the alignment has to
be very, very stable.

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Now I can also adjust the
horizontal alignment again.

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Show you how touchy
everything is.

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So here we are.

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Here is, then, in a nutshell,
here's the amplify section.

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The longer it is, of course,
the more gain we have,

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and the mirrors are
placed, in this case,

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about 50 centimeters apart.

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This one is a flat mirror, and
this one is a spherical mirror.

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And the alignment
has to be very stable

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because a small misalignment
would create a lot of loss,

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and then we just don't have
enough gain in the amplifier

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to overcome these losses
and the laser quits.

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Now later on,
we're going to have

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other demonstrations
that illustrate

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the properties of the laser.

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So when we come back, then
we're ready to show you

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a variety of effects
associated with the laser.