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SHAOUL EZEKIEL: Even
though laser light

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has some beautiful
properties, one

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can easily ruin these properties
by mishandling the laser light.

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One simple way of
ruining these properties

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is by reflecting the laser
light back into the laser.

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That's a no-no for a lot
of laser applications.

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So in this
demonstration, I'm going

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to illustrate what
the problem is,

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reflecting laser light
back into the laser.

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We're going to look at the
effect on the laser spectrum,

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as well as on laser intensity.

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So the setup is here.

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We have our short laser
here, our 22-centimeter laser

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with internal mirrors.

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And again, let me
remind you of the setup.

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We have the light
from the laser,

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then being reflected by this
mirror and this beam splitter

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into the scanning
[INAUDIBLE] interferometer

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to show the spectrum.

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And the spectrum,
then, is displayed

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on the oscilloscope over there.

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There's the two modes and the
little fella on the scope.

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Now, we're also going to
monitor the laser intensity.

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Again, the laser
light coming in here

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gets reflected by
this mirror, this time

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passes through this beam
splitter, then reflected

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by the mirror here
into a detector.

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And the output of
this detector can

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be also displayed on the scope.

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And then I push the button
for the other channel,

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and there it is.

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It's a sort of constant output,
but let me block the laser

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beam to show where 0 is.

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So 0 is about there, and here
is the output of the detector.

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So if I block the light again,
here's the 0 of the detector.

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And here is the
output of the laser.

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So this way, we can monitor
both the spectrum and also

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the laser intensity.

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So let me go back
to the spectrum.

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So here, we have the spectrum
displayed on the scope.

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So now I'm ready to reflect
some laser light back

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into the laser,
see what happens.

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The way I'm going to
do it is as follows--

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I have a beam splitter
over here to sample

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the light, the laser light.

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Say 50% of it actually
gets reflected over here.

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And then, this mirror--

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I have a mirror here to
reflect the light back

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into the laser again
via this beam splitter.

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If I block this, then I have no
reflected light into the laser.

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If I take this card
away, then I have

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light reflected into the laser.

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So now let's bring in the
spectrum on the scope.

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So here's the laser spectrum.

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And you can see we have,
let's say, three modes or so.

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And they're reasonably stable.

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So now what I'm going to do is
take this card away and reflect

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some light into the laser.

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And

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As you can see, the modes
start to jump around,

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and sometimes others
appear, and what have you.

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And I'm also going to play
with this mirror a little bit

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here to see whether I
can even enhance it.

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So you can see that it makes
a mess with the laser modes.

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Now again, just to make
sure that this effect is

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coming from reflection,
I block it again.

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And you see it's nice and
quiet and the modes are stable.

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If I take it away, then the
frequencies are jumping around.

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So if you have an
experiment that

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depends a lot on the
spectrum of the laser,

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you can have troubles if
there's light reflected back

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into the cavity.

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Let me block it again.

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And now, I'm going to switch
to the laser intensity.

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So I'm going to go
to laser intensity,

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and let me again
check where 0 is.

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Let's see 0 again on the scope.

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Then here is the
output of the laser.

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As you can see,
it's nice and quiet.

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So now what I'm going to
do is then remove this card

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so that we reflect some
light back into the laser.

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So here we are.

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We remove the card,
and we see that--

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I have to pick up
the alignment--

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we can see that now we see
some fluctuations in the laser

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intensity, up to about
maybe 15%, 20% or so

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of the intensity.

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So now, if I block it again,
see, it's nice and quiet.

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And if I unblock
it, you can see I

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get about this 15% or more
intensity fluctuations.

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Again, let me
remind you that 0--

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again, 0 is over here.

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So there's the output
without reflection,

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and again, here is the
output with reflection.

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Now, in order to dramatize
this effect even more,

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I'm going to select only
one frequency from the laser

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by using a polarizer.

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As we know, this laser puts
out at least two modes,

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and each one is polarized
orthogonal to the other.

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So by placing a polarizer here,
I can select only one frequency

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from the laser.

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So let me adjust the polarizer.

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And here we are.

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On this scope, we
have one frequency.

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Now what I'm going to do
is take this card away

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to reflect some light
back into the laser.

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So here we are.

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Light is reflected, and you can
see that the frequency starts

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to jump around, and also,
I get another frequency

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also appearing.

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We can see there's
violent jumping around

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of this frequency.

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Now, let me block it
again, and show you

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that it's nice and quiet,
and then take it away.

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And you can see that
it can jump around.

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Now, let's look at the
intensity when the laser is

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operating a single frequency.

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So what I'll do now
is go to the detector,

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and increase my
intensity a little bit,

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because the
polarizer attenuates.

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Let me check 0, so we
can see 0 is about here.

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Let me increase intensity
some more, and check 0 again.

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And let me set 0
to where we had it

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when we had it before
over here, so we

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can have a direct comparison.

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Now, let's look at
the intensity again.

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So here we are, about
two big divisions.

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This is when there's no
reflection back into the laser.

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Now, let me take the card away,
and pick up the reflection.

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You can see now that we get
about 30% or 40% fluctuation,

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sometimes even more when the
laser is a single frequency.

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Here we are.

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Look.

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Look how noisy the laser
output is when there is

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reflection back into the laser.

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Here we are.

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Look at that.

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Looks pretty awful.

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And if I block it,
nice and quiet.

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Take it away, let
the reflection go in,

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and you can get all
kinds of fluctuations

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in the laser intensity.

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So then, in summary,
one of the no-no's,

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then, with lasers is
reflecting its laser light back

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into the laser.

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Now, some experiments
you can get away

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without reflecting the laser
light back into the laser,

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but in some other
experiments, you

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have to reflect light back
into the laser because

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of the alignment of components.

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In that case, you would
have to use an isolater.

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Otherwise, you've got frequency
fluctuations as well as

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intensity fluctuations.

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The thing that I have
not told you is why.

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When you reflect laser
light back into the laser,

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why does it have such dramatic
effect on the laser frequency

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as well as on the intensity?

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So again, just like
before, I'm going

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to leave you with
this puzzle now,

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to figure this one
out for yourself.