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SHAOUL EZEKIEL: As we
have already learned,

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the most important
component in a laser,

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or the heart of the
laser, is the gain medium

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or the optical amplifier.

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This is no ordinary amplifier.

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This is an amplifier for light.

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And in this
demonstration, we're going

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to show you, and hopefully
convince you, that, indeed,

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light can be amplified.

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The amount of amplification
is not so huge.

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But I'm sure we'll make it
convincing enough so that you

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get the feel that, indeed,
that light can be amplified.

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The setup we're
going to use is here.

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We're going to have
a laser, which is

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going to be our light source.

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Here's the output of the laser.

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We're going to reflect
it by this mirror here

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and then this mirror over here.

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Now, the laser beam then
enters this optical amplifier.

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Now, this optical amplifier
is essentially this.

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It's a discharge tube,
helium neon gas mixture

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that will give gain
amplification or gain at 6328

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Angstroms.

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In fact, the light enters
this window here and then

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comes out at the other end.

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And that's what we
have essentially

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mounted here right below.

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So then the output
through this amplifier--

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and the amplifier right
now is turned off--

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then goes onto a
detector over here.

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And then the output
of the detector

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then goes onto an
oscilloscope over here.

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And also we look at the
output on a digital meter.

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All right, so we have two ways
of looking at the same output.

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All right, now we are
ready to set zero.

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So first, what I'm
going to do is then

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block the beam of
light over here.

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And let's look at the zero
on the scope and the zero

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on the meter.

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Now, the meter says 008,
which is not quite zero.

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And the reason for that is
because we have room light

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hitting the detector.

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So what I'm going to
do to get rid of that,

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I'm going to put this little
hood over the detector.

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Now we see that the output of
the meter now is, indeed, zero.

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And also, hopefully, then
the output of the scope here,

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this will be our zero
on the oscilloscope.

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Now, if I take this card away,
and let the light go through,

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you can see that now the output
on the scope has changed.

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And the meter reads around
364, or 63, or thereabout,

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which is the output
of the laser.

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Now all I have to do is
block the laser beam.

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And we go back to zero on
the meter as well as zero

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on the scope.

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Here we are.

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The laser beam's back again.

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And then we get that
same value again.

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Now, we're all familiar
with absorption of light.

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And I just want to
just demonstrate it,

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just for reference.

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I'm going to put
a piece of glass

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in front of the laser beam.

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And we know that glass
at normal incidence

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has a reflectivity of
about 4% per surface.

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So I should get an
attenuation of about 8% or so

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when I put this piece of
glass in the beam of light.

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Now you can see the meter
has dropped now to 330--

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300 and, well, if I can keep
it still, 320 something.

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Now, if I take the
piece of glass away,

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now we go up to the previous
number of 360 something.

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So you can see we have
an attenuation of about 8

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or so percent.

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Now what I'm going to do now
is put this piece of glass

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before the detector also.

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And, indeed, I'm going to
hold it against the tube

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here so that I don't
shake too much.

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And again you can see that the
attenuation is also about 8%

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or so.

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Here, let me put
it in the front.

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And then also then I'll
put it in the back again.

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And hopefully I'll put
it in the same position.

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You can see this--

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I can hold it still.

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And then take it away,
and then roughly we

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get this 8% attenuation.

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So now we've demonstrated
that, indeed, light

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can be easily attenuated.

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Now we come to this crucial
demonstration of gain.

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So now I want you to then watch
both the scope and the meter

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as I turn on the amplifier.

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So here I'm going to now
turn on the amplifier.

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And remember, this number
is around 363 or so, 364.

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Now you can see on the scope
we jumped a little bit.

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And the meter has gone up
to 380 something, which

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is an increase of about 5%.

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Let me turn it off.

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Again, you can see on
the scope it went down.

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In fact, just watch the
scope for a little while.

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I'm going to turn it
on and off very fast

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so that you get a feel that--

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here it goes, up a little
bit, and down a little bit.

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Since it's only a
few percent, it's

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difficult to see it on the
scope the way I have it set up.

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But it's much easier
to see it on the meter.

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So you can see that
with amplification we

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have 380 something.

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With no amplification, then
we're back to 360 something.

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Again, roughly, it's
about 5% amplification.

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Now, you may wonder that
maybe what we're getting

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is when I turn on the amplifier
that we're getting light

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from the amplifier
that hits the detector.

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It's not really amplification.

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So to prove this,
what I'm going to do

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is block the beam of light
going into the amplifier

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so that there's no light
going on the detector.

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We're back to our
zero on the meter.

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And now what I'm
going to do, I'm

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going to turn on the
amplifier just by itself just

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to see if there's any light
from the amplifier falling

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on the detector.

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So here we go.

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Watch the meter and the scope.

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And here is the amplifier on.

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Again, you can see
that there's no change

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on the meter or the scope.

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Here's off.

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And do it again, on, and off.

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So, indeed, we've shown that
light from the amplifier

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or spontaneous emission
from the amplifier

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is not hitting the detector
and increasing the output.

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So again, let me just
redo it again for you.

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Here is the light
falling on the detector

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without amplification,
again 360 something.

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And here comes the amplification
again, yes, close to 380.

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It's a little bit less
than we had before.

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Maybe the amplifier's
getting a little old.

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Here we are without
amplification.

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We have this value.

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So in summary, we've
shown that light

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can be, indeed, amplified.

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In this case, we used
a helium neon amplifier

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to amplify light from
a helium neon laser.

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We showed you a gain or
amplification of 5% per pass.

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Now, you may think that's
a small amount of gain.

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But a lot of helium neon lasers
use gain of even less than 5%.

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And they have many,
many applications.

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Of course, if you
have even more gain,

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then you can get more
powerful laser outputs

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with even greater
number of applications.