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PROFESSOR: In this
demo, I'd like

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to show how you can make a very
simple optical isolator based

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on polarization components
like a polarizer

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and a quarter-wave plate.

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An optical isolator
prevents a laser beam

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from re-entering
the laser cavity.

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Because if it
does, it can create

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a lot of intensity
variations in the laser

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and also a lot of frequency
variations in the laser.

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And in a lot of
experiments, this

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can be very detrimental
to the results.

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I'm going to illustrate
this in this simple setup.

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Here, we have a
helium neon laser.

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Here's the beam from the laser.

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I'm going reflect
it to this mirror

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and then reflect it again
to the second mirror,

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and let the spot
fall onto the screen.

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Then, I'm going
to take a mirror,

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and I'm going to place it here
to reflect the laser beam back

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into the laser cavity.

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And so, in order to see where
I'm reflecting the laser beam,

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I'd like to direct your
attention to the laser

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head, where you can see
the reflected beam is very

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close to the output of
the laser or the axis

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of the original laser beam.

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Now, I can actually put
the reflection right back

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into the laser, and you can
see a little bit of a smear.

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But for clarity, I'm
going to put this spot--

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reflect the spot slightly off
from the original laser beam

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so you can see what's happening.

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So right there.

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So you can see that if
I block the reflection,

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I block that spot.

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Now, I want to come
to the isolator.

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Now, this isolator is
made up of a polarizer,

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and I'm going to place
the polarizer right here.

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And I'm going to
set the transmission

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axis of the polarizer,
indicated by the white arrow,

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along the vertical.

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And again-- let's go
back and look at the spot

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again-- if I block
the laser beam,

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I can see I can switch it off.

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But in the presence
of the polarizer,

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the reflected beam is still
going back into the laser.

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Now, I'll place a
quarter-wave plate in here.

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And if I set the
polarization of the light

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along the aprinciple axis
of the quarter-wave plate,

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as indicated by
this vertical arrow,

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you can see that the
reflection is still there.

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Which means that the
quarter-wave plate essentially

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is acting like a piece of glass.

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And, again, if I go along
the other principal axis,

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the reflection is still
there, going right back

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into the laser.

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But if I orient the
quarter-wave plate

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at 45 degrees with respect
to the polarization

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of the light set
by this polarizer,

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then I can, as you can see,
extinguish the reflection.

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And so there's
always light in here

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where I can do an experiment,
there's light here,

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but there is no light
reflected back into the laser.

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Of course, the
reason why this works

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is because plane
polarized light, coming

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from this polarizer
here, is going once

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through the quarter-wave
plate and then

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reflected back, going twice
through the quarter-wave plate.

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So now, the planar
position is perpendicular

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to the original direction.

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So now, this 90 degree rotation
in the planar polarization

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then prevents the
light from getting

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past the polarizer here set
with the polarization vertical.

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And that's why this works.

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So, in summary, we
have illustrated

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that you can make yourself a
very simple optical isolator,

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which is very, very useful
in many experiments,

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using a polarizer and
a quarter-wave plate.