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PROFESSOR: This demonstration
is for those of you

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who would like to dig a
little more deeply into laser

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behavior.

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As we know, a laser,
like this one here,

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consists of two mirrors
and an amplifier.

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And if the gain of
the amplifier is

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bigger than the
losses in the cavity,

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then we can get laser action.

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And if let's say, this mirror
has a bit of transmission,

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then we can get laser light
coming out that we can use.

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So let me turn this laser on
and show you here on this card

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the output of the laser.

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Now in this
demonstration, we'd like

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to show that the light
inside the laser cavity,

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that's going backwards and
forwards between the two

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mirrors inside the
laser cavity, is

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much bigger than the light
that's coming outside.

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Question is, how do we do this?

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Because if we try to place
a detector inside the laser

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cavity, like here, we would
interrupt the laser action.

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And the laser quits.

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So obviously, that's not
a smart way of doing this.

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So what we're going
to do is we're

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going to use a simple
method like this.

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We're going to take
a piece of glass.

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And depending on the orientation
of the piece of glass,

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we're going to sample the
light both outside the cavity

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and inside the cavity.

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And hopefully that
the last that we

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will place inside the
cavity, the laser cavity,

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is so small that we will
not interrupt the laser

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action at least by too much.

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So here we go.

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So first, we have to show that
we can sample some of the light

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with this piece of glass.

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So here we are.

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Here's this piece of glass.

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Now, let me tell you
what's happening.

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This piece of glass is
placed at this orientation.

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And then, I can rotate it like
so using this rotation stage.

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Now, if you look at the
reflection from piece of glass

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on this little
screen, you can see

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that the light gets
extinguished when

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I am at the Brewster angle.

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Remember the polarization
of this laser

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in the vertical
plane so that I can

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achieve Brewster
angle condition using

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this orientation of the glass.

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So now when I have
Brewster's angle,

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it's when these two markers--

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one on the rotating
part of the stage

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and the other one on the
fixed part of the rotation

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stage-- when they line up,
we're at Brewster's angle.

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And when I move away from
Brewster's angle, like so,

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I can increase the
amount of light

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sampled by the piece of glass.

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So let's say, if I set
it this position here,

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then I have a small
percentage of light

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that's reflected by the piece
of glass onto the screen.

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Now, what I'm going to do is
take a similar piece of glass

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and put it again in
the beam like so.

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And again, the markers are the
same as in the other stage.

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So when the markers
are lined up,

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then I get no light reflected.

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This means Bruce's condition.

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And when I'm over here, I get
pretty much the same amount

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of light as in this case.

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So now, I've calibrated
these two pieces of glass

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in terms of angle.

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Now, what I'm going to do,
I'm going to take one of them.

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And without changing
the angle, place it

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inside the laser cavity.

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And hopefully, the laser
won't go out on me.

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So as I put it in here,
without changing the angle,

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you can see I'm going to
sample some of the light.

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And you'll see it
on this screen.

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And note that the word
"inside" here, that

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means that this piece of glass
is inside the laser cavity.

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And then, the one
outside the laser

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cavity is labeled again
by the word "outside."

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And now, I would like you to
look at the spot reflected

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by the piece of glass
outside the cavity

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and the spot reflected
by the piece of glass

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inside the cavity.

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So if we have now the
split screen in position,

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you can see that the one inside
is much brighter than the one

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outside.

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In fact, it's
brighter in this case

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by about a factor of 50 or so.

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The question I want
to leave with you

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is, why the light
coming out of the laser

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is much weaker than the
light inside the laser?

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And what determines that ratio?

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So that's a nice
little puzzle for you.

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Now sometimes, one would like
to increase the light coming out

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of the laser.

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Now, how can you do it?

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Well, you notice
that since there's

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a lot of light inside the
cavity, one way of quickly

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increasing the laser
intensity, even

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though for a very short
time, is by removing

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one of the mirrors--

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like for example, if I take
this mirror out from the laser

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very quickly, then
I can essentially

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dump out all the photons that
are stored in the laser cavity.

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In fact, this is
called cavity dumping.

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And what this gives
you, this gives you

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a huge increase in the
number of photons coming out

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from the laser, but only
for a very short time.

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In fact, it just
takes all the photons

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that's stored in the laser
cavity and just dumps them out

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and, of course, for
a very short time.

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In fact, the time is just twice
the transit time of the cavity.

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So the fact that
one knows something

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about the light inside
the laser is bigger

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than the light outside
the laser, then one

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can come up with this concept
of cavity dumping which can be

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useful for some applications.