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SHAOUL EZEKIEL: This
demonstration, we're

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going to show the influence
of an optical fiber

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on the polarization
properties of light.

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In this case, we're going to
use a normal single mode fiber.

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The setup is as shown here.

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We have a helium neon laser.

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

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here, then by
another mirror here,

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into a quarter-wave
plate and a polarizer.

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And, in this way, by
rotating the polarizer here,

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I can select any
linear polarization

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I want in this region.

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Then what we do is we
take the light from here,

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we pass it through a
short focal length lens,

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focus it into the fiber to the
single mode fiber over here,

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and then the output of
the fiber is, again,

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[INAUDIBLE] by another
short focal length lens,

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and then the output
of this lens,

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and we let go onto the screen.

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So this is the basic
setup, now we're

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ready to look at polarization
effects in fibers.

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Now, first, we have to establish
the state of polarization

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of the light going
into the fiber.

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What I'm going to do,
take this polarizer

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and place it over here.

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And then I'll take a
card, place it here.

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And now I want to rotate
the polarizer and the axis.

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The little white arrow here
indicates the transmission axis

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of the polarizer.

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So I can rotate
polarizer now, and then

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in this region over
here I have now,

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which means that the state
of polarization of light

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is orthogonal to the
direction of this arrow.

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So indeed we have
plane-polarized light as

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indicated by the
arrow over here, which

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is orthogonal to the one here.

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All right, so now
we've established the--

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that the state of polarization
is linear going into the fiber.

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Now, let's look at the
polarization of the light

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coming out of the fiber.

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Again, I'm going to
take the polarizer

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and place it over
here, the output.

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And now let's look at
the spot in the inset.

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Here we have the
spot in the inset.

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Now, I'm going to
rotate the transmission

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axis of the polarizer
to see what we have.

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Gee, we have indeed
plane-polarized light

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also coming out of the fiber.

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So we have plane-polarized
light going in

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and plane-polarized
light coming out.

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Now, to demonstrate
what the fiber does

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to the polarization in a very
dramatic way, all I have to do

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is twist the fiber by
just pushing on it.

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And you can see that now, just
by applying a little stress

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to the fiber--

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now I have changed
the polarization

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because now I have light
coming out of the polarizer

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while before it was not.

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Now, again, I can push the
fiber around some more.

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As you can see, I
can change the amount

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of light coming through
the polarizer, which

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means that this
state of polarization

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is changing just by simply
applying simple stress.

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I can apply mechanical
stress like I'm doing now.

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I can get the similar effect
by heating the fiber and so on

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so that the state
of polarization

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then can be influenced quite
a bit by the environment--

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the effect of the
environment on the fiber.

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You can see here I
can make it come out

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very bright through the
polarizer when originally it

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was not.

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Here we are
[INAUDIBLE] now zero,

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which it means plane polarized.

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Now, it's some state
of polarization.

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In fact, I'm going to hold it--

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hold the fiber in this position,
and then now I'm going to go

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and I'm going to turn the
polarizer to see indeed

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do I have plane-polarized
light or elliptically

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polarized light.

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Well, it's elliptically
polarized light

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because I can't [INAUDIBLE].

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So now I have changed
this state of polarization

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then from linear to elliptical.

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And, again, if I put
more stress on the fiber,

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let's see now what state
polarization I get now?

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Again, you can see it
is elliptical because I

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can't [INAUDIBLE].

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So in conclusion then,
the-- a single mode--

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irregular single mode
fiber can change the state

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the polarization of light going
into it into almost anything,

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to plane polarized,
circular polarized,

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elliptically polarized.

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And this is influenced
greatly by the environment

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that the fiber is in.

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So it's very difficult
to maintain then

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a good state of polarization or
a known state of polarization

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in a single mode fiber
because of these environmental

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disturbances.

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If one wants to maintain,
let's say, linear polarization,

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one would have to use a
polarization maintaining fiber.