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PROFESSOR: Previously,
we've shown several ways

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of getting low contrast
fringes in a Michelson

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two-beam interferometer.

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For example, when the
mirrors are shaking,

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you get poor contrast-- when
the intensities in the two arms

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are unequal, when we have
orthogonal polarization

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between the beams and the two
arms of the interferometer.

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In this demonstration,
we're going

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to show another way, and
probably the most common way,

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of getting low contrast fringes
in a two-beam interferometer.

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And here the setup.

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Again, we have a
helium-neon laser here.

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Here's the beam from the laser
being reflected by this mirror

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into an optical isolator made
up of a quarter-wave plate

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and a polarizer to prevent
light going back into the laser.

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The beam leaving the
isolator is here,

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getting reflected by this
mirror into the interferometer.

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Here's one arm of
the interferometer,

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and here is the other
arm of the isolator.

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And the two beams leaving the
isolator will be reflected

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by this mirror into this
lens, and then from the lens

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onto the screen.

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So, now, let's
look at the screen.

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And we see that we have fringes,
and the contrast in the fringes

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look pretty good.

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00:01:52,420 --> 00:01:56,730
And you can see, I can
adjust the alignment of one

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00:01:56,730 --> 00:01:58,050
of the arms--

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that's what I'm
doing right now--

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to separate the spots and
bring them back together again.

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And then when I
take my hand away,

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you can see I have pretty
good contrast in the fringes.

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00:02:09,750 --> 00:02:12,510
You also notice that,
in this arrangement,

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the two arms of the
interferometer are equal.

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And we've indicated
this by the 0 over here.

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00:02:21,070 --> 00:02:26,580
So the 0 indicates that this
arm is equal to this arm.

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Now, as I move one arm, let's
say by a few centimeters--

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and, also, you
want to note this,

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that the scale here
is in centimeters.

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This is 10, 30 50, and so on.

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So let's say around
5 centimeters.

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Let's look at the
fringe contrast.

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00:02:45,570 --> 00:02:50,260
And we again look
at the fringes,

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and you can see that,
indeed, the contrast

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is a little less
than what it was

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when we had equal path length.

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Let me go a little bit more,
let's say around 10 centimeters

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or so.

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Again, let me
check the alignment

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by adjusting one of the mirrors.

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And you can see that, now,
the contrast is indeed

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getting very poor.

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Let's go even further, let's
say around 30 centimeters path

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length difference.

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Again, here are the two spots.

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And the fringe
contrast is pretty bad.

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Let's go, let's say, around
45 centimeters or so.

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Again, check on the alignment
of the interferometer,

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00:03:46,530 --> 00:03:50,460
and, indeed, we don't even
see any fringes at all

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or essentially 0 contrast.

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But let's go on.

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Let's go on, let's say, to
position around 72 centimeters.

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00:04:02,220 --> 00:04:05,814
And now, let me
check the alignment.

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And we can see that now we're
beginning to see some fringes.

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00:04:10,890 --> 00:04:13,020
So the contrast is not quite 0.

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Let me go further, say,
around 80 or so centimeters.

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Again, check on the alignment.

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A little bit better.

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And we go to 95
or so centimeters.

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And, wow, we see some
very good contrast.

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The contrast has come back.

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In fact, it's just as good as
when we were at equal arms.

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00:04:43,760 --> 00:04:51,520
Now, let's go further than 95
centimeters, around maybe 105.

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00:04:51,520 --> 00:04:56,360
And let's look, again,
at the fringe contrast.

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00:04:56,360 --> 00:04:59,320
You can see that they're
getting poorer again.

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00:04:59,320 --> 00:05:06,600
Let's go even further, here,
around 115 centimeter path

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00:05:06,600 --> 00:05:09,350
length difference.

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And let me readjust, and see
that the fringes have almost

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disappeared.

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00:05:16,470 --> 00:05:19,760
So let's go back and make sure
that we didn't do anything

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00:05:19,760 --> 00:05:20,890
wrong before.

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Let me go back to where we had
good fringe contrast, around 95

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centimeters.

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00:05:25,820 --> 00:05:33,470
And you can see that we
have excellent contrast.

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00:05:33,470 --> 00:05:40,430
And we go to, say,
70 or so centimeters.

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Fringe contrast is pretty bad.

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Go to about 45 or
so centimeters.

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Wow, that's really awful.

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Can't see anything.

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Now, just let me
take this opportunity

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to show you that, indeed,
I do have two beams.

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And I can block, in fact, one
beam, or block the other beam.

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Indeed, I do have two beams
that are superimposed,

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but yet absolutely no
fringes can be seen.

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And then, let me go all
the way to equal arms,

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to the 0 position.

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We adjust the two beams.

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Now, we can see we have good
fringes again, good fringe

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contrast.

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But let me go and make
this arm here even

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shorter than the other arm.

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Let me make it shorter by
about 2 or 3 centimeters.

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You can see that the fringe
contrast is not so wonderful

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as it was at the 0 position.

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Let me go to a path
length difference

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of minus 10 centimeters and
readjust the other mirror.

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And we see that fringe
contrast is very poor.

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So let me go back to
the zero position.

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and look at the fringes.

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Very good.

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And about 45 centimeters, and
we don't have any fringes.

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Now, what could the reason be
for the fringes disappearing?

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This setup is a good setup.

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I think the alignment
is pretty good.

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We're paying careful
attention to the alignment.

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And so, what can it be?

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Certainly not-- it's not
in the interferometer.

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It must be in the light source.

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Now, the light source in this
case is a helium-neon laser,

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and the separation
between the mirrors

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or the length of the cavity
is about 95 centimeters.

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The laser certainly
puts out a lot of light.

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The only thing we don't
know about the laser

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is the spectrum of the light.

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For example, is it
single frequency,

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or is it multiple
frequency, or what?

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Now, what I'm going
to do, I'm going

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to set up an arrangement
that will tell us

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what the spectrum of
the laser light is.

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And when we come back,
we'll have it already,

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and we can show you what the
spectrum of the laser light is.

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Now we've set up the arrangement
for observing the spectrum

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of the laser light.

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The setup is essentially
what we had before.

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Here is the interferometer--

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the two arms of the
interferometer--

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and the fringes, again,
we can see on the screen.

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What we've done to observe the
spectrum of the laser light,

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00:09:03,030 --> 00:09:07,440
we've added this beam splitter
over here to reflect some

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of the laser light before
it enters the interferometer

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and reflects it into
this scanning Fabry-Pérot

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interferometer.

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The output of the
interferometer is displayed

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on an oscilloscope over here.

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What we have displayed on the
face of the oscilloscope is one

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complete free spectral range
of the scanning Fabry-Pérot,

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which is about 1
and 1/2 gigahertz,

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corresponding to the
10 centimeter length

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of the scanning Fabry-Pérot.

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Now, if the laser were indeed
oscillating at one frequency,

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we would only see just one peak.

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But we see about
eight or nine peaks,

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which means that the
laser must be oscillating

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at several frequencies.

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00:10:01,070 --> 00:10:05,240
Again, from the 1 and 1/2
gigahertz free spectral range,

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we see that the separation
between the frequencies

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of the laser is
about 160 megahertz.

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And this is what we
expect, because this

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corresponds to the 95 centimeter
length of the laser cavity.

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So as you see that the laser is
not a single frequency laser,

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it has many frequencies.

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So maybe the fact that the
contrast in the fringes

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varies so dramatically
with path length difference

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00:10:41,010 --> 00:10:44,480
may have something to do with
the spectrum of the laser

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00:10:44,480 --> 00:10:44,980
light.

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00:10:47,510 --> 00:10:50,500
And I would like you
to think about that.

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But in another
demonstration, we're

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going to bring in a
single frequency laser.

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And, indeed, we're going
to study this problem

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very carefully.