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PROFESSOR: The confocal resonator that we've looked at
has many uses, and we'll illustrate one of them
right now.
So far, the laser frequency going into the cavity
was only one frequency.
We only had one frequency coming out the laser,
because we selected it by means of the polarizer.
Since the laser oscillates at several frequencies, at least
two, at different polarization, then, again,
by rotating the polarizer, I can select the other frequencies.
So the cavity can be used as a spectrum analyzer.
So if we look onto the scope, as I rotate the polarizer,
you see that I can bring in other resonances.
And these are not associated with transverse modes
of the cavity, but they are associated with the frequencies
of the laser.
So here we are.
Here is when the laser is operating single frequency
going into the cavity.
Now, I rotate the polarizer, and I admit the other frequencies
from the laser, which are different polarizations.
So in this way, the confocal resonator
can be used as a spectrum analyzer.
Now, in practice, you have to be careful in selecting
the free spectral range of the resonator
of the confocal cavity, so that there's
no confusion about the separations in the frequencies
of the light source.
So here we are again.
Here's when the laser single frequency
going to the resonator.
Here we have many frequencies.
And if we had selected the free spectral range correctly,
we would be able to measure exactly the spacing
between the modes of the laser.
So then, in conclusion, the set of demonstrations,
we've looked at the plane parallel cavity,
we've looked at the condition under which
the modes superimpose.
It was very touchy-- very touchy alignment
with the plane parallel cavity.
Then we looked at the cavity with curved mirrors.
And we saw lots of transverse modes
if we don't have the length of the resonator adjusted
suitably.
We also found that when the length of the resonator
was equal to the radius of curvature of the mirrors
in this case, the confocal condition,
we found that the modes coalesced into two residences.
And, finally, we showed how such a focal resonator
can be used to analyze the spectrum of a light source
like this laser.