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SHAOUL EZEKIEL: In this
next demonstration,

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we're going to show
how one can get a laser

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to oscillate at only one
frequency when normally

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the laser oscillates at
many frequencies associated

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with the longitudinal modes.

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In this case, we're going to
look at an argon laser, which

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is over here.

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And we're going to do the single
frequency selection by placing

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an etalon inside the cavity.

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Now, an etalon is a
short Fabry-Perot cavity

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which you then will place
inside the argon laser cavity.

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And we'll be able to
select single frequency.

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Now, we're ready to look at
the spectrum of the light

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from an argon ion laser.

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Here is the argon ion laser.

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And the output is coming
from the other end

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of the laser over here.

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We're going to reflect it by
mirrors onto this mirror here.

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And then reflected
light from this mirror

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will go onto this
mirror and is then

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reflected into this scanning
Fabry-Perot interferometer.

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The free spectral range of
this cavity is 15 gigahertz.

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And here is the detector.

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And we also put a hood over
the path between the cavity

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and detector to
prevent room light

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from reaching the detector.

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At the same time, we have
a beam splitter here.

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We're going to reflect a
little bit of the light

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into another scanning
Fabry-Perot interferometer

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over here.

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This is a longer cavity.

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It has a free spectral
range of 1 1/2 gigahertz.

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Again, the detector for
this cavity is over here.

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First, we're going to look at
the spectrum of the laser light

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with the 15 gigahertz free
spectral range cavity.

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So now, let's go
over to the scope

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and look at the output spectrum.

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Now we can see the output of
the 15 gigahertz free spectral

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range scanning Fabry-Pérot
cavity on the oscilloscope.

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What we see here is the
free spectral range,

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which is, again, 15 gigahertz.

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That's the separation
between these two peaks.

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And the spectrum of the
laser looks like it's

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about a few gigahertz wide.

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The reason for this is that
the gain curve in argon laser

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is pretty broad.

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It's of the order of
10 to 15 gigahertz.

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And lots of longitudinal
modes oscillate.

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And they compete
with each other.

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And right now, they're blending
to give you this big blob

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of several gigahertz spectrum.

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Now, for many applications,
this broad spectrum

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is not of much use, for
example, in applications

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using interferometry.

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For such applications, you need
to make the laser oscillate

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at a single frequency.

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The popular way of
generating single frequency

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in these big lasers
is to use an etalon

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and put it inside
the laser cavity

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to select out a
single frequency.

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So when we come
back, we will put

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in an etalon inside the laser
and observe single frequency

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output behavior.

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Now, I'm going to put an
etalon inside the laser cavity.

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Here is the etalon in a holder.

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It's a very simple thing.

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It's a piece of glass, parallel
piece of glass one centimeter

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thick.

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And it has a reflectivity of
about 35% on each surface.

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And it's held in this mount
here so I can then adjust it

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within the laser cavity.

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So now I'm going to place the
etalon inside the laser cavity.

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There's a little space here
between the Brewster window

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and one of the mirrors.

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So here we are.

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Here's the etalon in place.

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Now all I have to do now
is adjust the alignment.

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And here we are, now.

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We get lasing.

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We have lasing now.

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And now we're ready,
then, to go and look

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at the output of the
spectrum analyzer

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with this etalon in place.

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Since the etalon is a
solid piece of glass,

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I cannot change its
length very easily.

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But I can effectively change
its length by misaligning it.

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In this way, then, I can
get a tuning of the etalon

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by simply rotating the etalon.

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Now that we have the
etalon inside the cavity,

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the spectrum is
single frequency.

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And again, the free spectral
range is 15 gigahertz.

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But the output now
is single frequency.

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And, in fact, by
adjusting the etalon,

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I can tune this frequency across
the gain curve of the argon

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laser, which is right now
about a few gigahertz.

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So let me do it again--

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over here.

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The finesse of the cavity,
this 15 gigahertz cavity,

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is not very high.

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So what we'll do, we'll
switch to the other cavity,

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the one that has a 1 1/2
gigahertz free spectral range.

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It has a much higher finesse.

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And we'll be able to see
some interesting behavior

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of this single frequency
operation of the laser.

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On the scope, now, we have the
output of the 1 1/2 gigahertz

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Fabry-Pérot cavity.

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As you can see, the spacing
between the modes here is 1 1/2

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gigahertz.

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And the finesse is
pretty high, probably

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of the order of 300 or so.

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As we can see, the output of
the laser is a single frequency.

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

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going to misalign the
etalon and see what happens.

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Now, what you notice is that,
because I'm tuning the etalon,

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I'm also going to be
tuning the laser frequency.

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But the laser frequency
is not tuning smoothly.

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It's tuning in jumps.

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So let me do it again.

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Here we are, jumps
or so-called mode

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hops of the order of the free
spectral range of the laser

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cavity, which is of the
order of 150 or so megahertz,

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because the laser cavity
is about a meter long.

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So again, let me show
you the hops again.

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Now, let me try to do it with
the other knob on the etalon,

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since they're less
sensitive [INAUDIBLE]..

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Now you can see the
hops much better.

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Here we are.

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There's two hops
there, one, two.

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Here we are.

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In fact, we expand
this scale, and we

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can make the hops even larger.

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All right, here the
scale's expanded.

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Let me try again.

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You can see the hops now
much more dramatically.

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Again, it's about 1 1/2--

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150 megahertz or
so per mode hop.

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In fact, the intensity
is supposed to--

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as I tune the
etalon, the intensity

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is supposed to go down, and
then hop, and then go up,

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down, and then hop.

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Let's do it one last time, one
hop, another hop, another hop,

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and so on.

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So when we use the
etalon inside the cavity,

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then we should be
expecting these mode hops

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when the etalon gets
misaligned or its length

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changes by a small amount.

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Now, when one uses an etalon
like this inside a laser

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cavity, there is one thing
that one should never do,

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and that align the etalon
perfectly normal with respect

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to the axis of the cavity or the
laser beam inside the cavity.

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Because if you do,
then you're going

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to get all sorts of multiple
cavities taking place.

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And the spectrum goes
absolutely haywire.

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So now I'm going
demonstrate that.

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I'm going to now
align the Fabry--

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the etalon to be normal
to the laser cavity.

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And we see that the spectrum
goes absolutely haywire.

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And all we have to
do is just tilt away.

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And here we get quiet
single frequency behavior.

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And we go back, and bring
it to normal alignment.

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And here we see the spectrum
going absolutely haywire.

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So again, the no-no
with etalons is not

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to place them
normal to the beam.

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You just have to tilt them
away just a little bit.

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And you get nice, quiet,
single frequency behavior.