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PROFESSOR: The confocal
resonator that we've looked at

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has many uses, and we'll
illustrate one of them

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right now.

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So far, the laser frequency
going into the cavity

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was only one frequency.

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We only had one frequency
coming out the laser,

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because we selected it by
means of the polarizer.

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Since the laser oscillates at
several frequencies, at least

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two, at different
polarization, then, again,

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by rotating the polarizer, I can
select the other frequencies.

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So the cavity can be used
as a spectrum analyzer.

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So if we look onto the scope,
as I rotate the polarizer,

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you see that I can bring
in other resonances.

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And these are not associated
with transverse modes

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of the cavity, but they are
associated with the frequencies

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of the laser.

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So here we are.

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Here is when the laser is
operating single frequency

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going into the cavity.

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Now, I rotate the polarizer, and
I admit the other frequencies

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from the laser, which are
different polarizations.

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So in this way, the
confocal resonator

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can be used as a
spectrum analyzer.

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Now, in practice, you have
to be careful in selecting

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the free spectral
range of the resonator

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of the confocal
cavity, so that there's

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no confusion about the
separations in the frequencies

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of the light source.

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So here we are again.

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Here's when the laser
single frequency

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going to the resonator.

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Here we have many frequencies.

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And if we had selected the
free spectral range correctly,

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we would be able to
measure exactly the spacing

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between the modes of the laser.

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So then, in conclusion,
the set of demonstrations,

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we've looked at the
plane parallel cavity,

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we've looked at the
condition under which

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the modes superimpose.

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It was very touchy--
very touchy alignment

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with the plane parallel cavity.

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Then we looked at the
cavity with curved mirrors.

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And we saw lots of
transverse modes

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if we don't have the length
of the resonator adjusted

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suitably.

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We also found that when
the length of the resonator

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was equal to the radius of
curvature of the mirrors

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in this case, the
confocal condition,

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we found that the modes
coalesced into two residences.

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And, finally, we showed
how such a focal resonator

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can be used to analyze the
spectrum of a light source

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like this laser.