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SHAOUL EZEKIEL:
Now we are all set

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to look at the spectrum
of laser light.

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For example, is the spectrum--

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is it a single frequency?

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Or is it multiple frequency?

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Or what have you?

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In fact, we're going to look
at the spectrum of light

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from two lasers, two
helium neon lasers.

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We have this laser here
with external mirrors

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and the one over here
with internal mirrors.

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And the way we're going
to look at the spectrum

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is by using the optical
spectrum analyzer here.

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So now let me turn on this
laser with external mirrors.

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And the light from
the laser, then,

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is reflected by this
mirror and this mirror.

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And here it is going right
into this optical spectrum

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analyzer, which is a
scanning Fabry-Perot cavity.

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The output of this
spectrum analyzer

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then goes to an
oscilloscope over here.

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As we can see on
the oscilloscope,

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we have more than one frequency.

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In fact, we have several
frequencies, sometimes three,

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and sometimes even four.

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The spacing of the modes
here is about 270 megahertz,

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which is consistent with
the length of the laser

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cavity of 56 centimeters.

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00:02:01,950 --> 00:02:03,690
Now, the first thing
I'm going to do

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is see whether the polarization
of all these laser frequencies

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is the same or not.

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So what I'm going
to do is insert

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the polarizer in the beam and
then rotate the polarizer.

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00:02:24,960 --> 00:02:28,460
In fact, let me up the
gain a little bit here.

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00:02:31,450 --> 00:02:36,130
And let me rotate the
polarizer or the transmission

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axis of the polarizer to see
whether the polarization is

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the same for all of them.

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00:02:43,810 --> 00:02:48,460
And, as you can see, I
can extinguish all of them

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when the polarization
is horizontal

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00:02:52,000 --> 00:03:03,050
and bring them all up when
the polarization is vertical.

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00:03:03,050 --> 00:03:07,750
And remember, this is the
light that was plane polarized.

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So now we've shown that,
indeed, all the frequencies that

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come out from this
laser, all of them

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have the same polarization,
the polarization

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00:03:19,060 --> 00:03:21,350
in the vertical plane.

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00:03:21,350 --> 00:03:24,970
So let me take the polarizer
out and readjust the gain

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on the scope.

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So back now to the
three frequencies--

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you can see that
they move around.

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Because the cavity
is drifting in length

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due to the air currents, or
temperature effects, or what

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have you.

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And even, in fact, as I speak,
or as I tap on the cavity,

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you can see that I can
create a mess of the spectrum

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just by simply tapping.

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Or if I lean on the
table, you can see, again,

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that I can make the
frequencies wander around.

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Now, the fact that I have
more than one frequency

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means there is enough gain
for several frequencies

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to oscillate due to the fact
that the gain medium has

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some bandwidth-- not very
huge, but some bandwidth.

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Now, I can make the
laser go at one frequency

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by introducing loss, by
simply misaligning the cavity

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to introduce loss.

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So you can see here I've
got only two frequencies.

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And, in fact, I'm going to up
the sensitivity of the scope.

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Because the power goes
down when I misalign.

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You can see here, I have,
essentially, two frequency.

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And if I add more loss, I
can have only one frequency.

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So in a way, I can run this
laser at one frequency.

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00:05:00,070 --> 00:05:04,340
But it's difficult to
keep the other one out.

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00:05:04,340 --> 00:05:11,560
And then you can see that
as I lean on the cavity,

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I can make this
frequency move around.

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Now, generally,
that's not a good way

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of getting single frequency.

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00:05:17,110 --> 00:05:20,860
And we have other techniques for
getting single frequency, which

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we'll discuss later.

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For this laser, it's best to
align it for the highest power

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00:05:27,250 --> 00:05:27,860
out.

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00:05:27,860 --> 00:05:36,250
And this way we'll automatically
get more than one frequency.

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And for a lot of
applications, this is fine.

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00:05:41,470 --> 00:05:43,630
All right, so to
summarize, then,

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00:05:43,630 --> 00:05:48,780
this laser that is 56
centimeters long, or the cavity

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00:05:48,780 --> 00:05:54,040
is 56 centimeters long, then
gives us about three modes.

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00:05:54,040 --> 00:06:00,340
And the spacing between each
mode is about 270 megahertz.

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00:06:00,340 --> 00:06:04,480
And all the modes have
the same polarization.

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00:06:04,480 --> 00:06:10,540
Now we're ready to look at the
other laser, the laser that

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has internal mirrors and
also shorter in length.

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00:06:14,710 --> 00:06:20,400
So when we come back, then
we look at that laser.

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Now we're ready to look at
the spectrum of this laser

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here with internal mirrors.

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The set up for looking at the
spectrum is the same as before.

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But let me just
remind you of it.

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Here's the output of
the laser reflected

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by this mirror, this mirror,
into the scanning Fabry-Perot

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cavity.

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The output of the cavity then
goes onto the oscilloscope

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over there.

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Now, as you can see on the
scope, and let me adjust the--

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center the scan.

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As you can see, we
have two big modes.

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And these are spaced
by 680 megahertz,

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which is consistent with
the length of the laser

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cavity of 22 centimeters
given by the--

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680 megahertz is
given by c over 2L,

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the spacing between
longitudinal modes.

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So these are two longitudinal
modes of the laser.

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This little fellow
here is a third mode

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that's coming in
at an odd position

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because the free spectral range
of the scanning Fabry-Perot

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cavity is not
quite large enough.

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So we're getting a wrap
around from another order.

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So let's not worry
about this one too much.

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So let's look at, essentially,
the two main longitudinal modes

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of this laser.

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Now, let's look at now the
polarization of these modes.

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So now what I'm going to
do is take a polarizer

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and, as we did with the other
laser, let's take the polarizer

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and look at the spectrum.

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Now, if we look at
the scope after I

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make a slight adjustment
of the gain because

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of the loss in the polarizer,
now what I'm going to do

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is look at the spectrum
of the laser light

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on the scope as a
function of polarization.

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So first, you can see with the
polarization set at this angle,

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you see essentially we have
predominantly one mode.

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And then if I rotate
the transmission axis

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of the polarizer, I can
extinguish this mode

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and bring up the other one.

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00:09:01,420 --> 00:09:04,390
Now, let me just center
the mode on the scope

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so I can bring up the other one.

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Now you can see, indeed, I
have single frequency operation

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on just one mode.

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So I either have this mode here
or, if I rotate the polarizer,

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I can bring up the other one.

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So again, let me go
back to the first one

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and then to the one over here.

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Now, this is
different from what we

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had with the laser with external
mirrors and the Brewster

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windows where we found
that all the modes were

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of the same polarization.

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Now here we find that one
mode is one polarization.

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And the other one is polarized
orthogonal to the first one.

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And, in fact, this explains
why the output of the laser

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wasn't plane polarized as in
the external mirror cavity.

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Now, this is very
interesting that because we

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have internal mirrors the modes
have orthogonal polarization,

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at least adjacent modes have
orthogonal polarization.

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And, in fact, this
is a very easy way

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of selecting single
frequency operation

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by simply placing a
polarizer in the beam

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and then selecting
one frequency.

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Now, let's look at this
single frequency behavior.

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If I want to tune
the laser frequency,

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I simply blow some air
onto the laser to--

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I blew too much.

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Let me do it again.

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So you can see that
I can scan the laser

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frequency by simply changing
the length of the cavity.

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In this case, I'm
cooling off the cavity.

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Now, let me also point out that
the line width that you see,

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the line width that you see
here is not the laser line width

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at all.

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The line width that
you see here is

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determined by the scanning
Fabry-Perot cavity.

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The line width, the true
line width of the laser

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is very narrow.

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In fact, in principle it's
a fraction of a millihertz.

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But because the laser
jitters and what have you,

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you'll get a little broadening,
but certainly nowhere near as

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wide as what you see over here.

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And again, let me
bring up the other mode

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or the other polarization.

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And again, this one will
also tune across the gain

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00:12:02,470 --> 00:12:06,610
curve of the cavity.

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I mean a gain curve of
the medium of the laser.

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So in summary, then, we've
seen that for this laser here

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that's about 22
centimeters apart,

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that we get longitudinal
modes of 680 megahertz apart.

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And the polarization of
each mode is different.

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In fact they have--

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adjacent modes have
orthogonal polarization.

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While with this longer laser
of length 56 centimeters,

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we found that the mode
spacing was 270 megahertz.

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Again, it's consistent with
the length of the laser--

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and that the polarization of
all the modes was identical.

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Now, I want to leave
you with this puzzle.

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Why, in this case here, for the
laser with internal mirrors,

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the polarizations of
the modes were different

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while the one here the
polarization was the same?

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Well, this is-- to answer
this one here is easy because

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of the Brewster windows.

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And the only polarization
that can laze

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is the polarization set by the
angle of the Brewster windows.

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But in this case here,
we don't have any windows

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to set the polarization.

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00:13:35,620 --> 00:13:41,560
We only have just two mirrors
sealed onto the discharge tube.

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And we saw that adjacent modes
have orthogonal polarizations.

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00:13:48,130 --> 00:13:52,900
So here's a nice little
puzzle for you to think about.

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00:13:52,900 --> 00:13:55,270
Now we're not done yet
with laser properties.

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00:13:55,270 --> 00:13:57,760
There are several
other experiments

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00:13:57,760 --> 00:13:59,240
that we have prepared for you.

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00:13:59,240 --> 00:14:02,050
So when we come back, we'll
show you other aspects

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of laser behavior.