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PROFESSOR: Now, we've replaced the previous fiber
with another one.
The only difference between this fiber and the previous one
is that the core diameter is now a little bit bigger.
It's about 6 microns instead of 4 microns in the previous one.
The setup is exactly the same.
Again, we have the laser here getting
reflected by this mirror and this mirror into the lens,
into this fiber--
this new fiber.
And then, the output of the fiber goes onto the screen.
So let's now take a look at the output of this fiber.
Aha.
So the first thing we see, that doesn't look like a single-mode
at all, that single low.
And if I change the adjustment-- if we can have a camera looking
at my adjustments--
then you see that I can get a variety of shapes.
All I'm doing is changing the adjustment,
and I can get a variety of shapes.
And clearly, it's not like what we
had before-- the single-mode behavior that we had before.
And this is then the so-called multi-mode.
What you're seeing are different transverse modes
that can propagate in this fiber,
because the core diameter is bigger.
But I'm going to leave it to you then to explain why--
exactly why you get higher transverse modes
propagated in this fiber.
Again, the wavelength is the same.
The only difference is that the core diameter is bigger.
You can see this one here, you get a nice, dark line
in the middle.
And then if I align it over here, you can see it's blobby.
It's a mixture of modes.
That's why you don't see a sharp, dark line in the middle
where you get a mixture of modes.
While, remember, in the other one,
no matter what I did in the alignment,
it was still single-mode coming out-- just one low coming out.
Now, multi-mode fibers have many applications also.
But for today, for a lot of sophisticated sensor
applications and communications, one generally
uses single-mode fibers.
So here a pretty mode with a dark line down the middle.
Now, what I would like to do is show how touchy this fiber
is to bending.
In fact, If we take a look at the fiber here,
as soon as I just press on it-- if we take a close look--
I can kick light kit light out.
If I just simply press on it, I can kick some light out.
So it's very touchy to stress and to bends.
What I would like to do then is show
that I can kick out some of the transverse modes
by simply bending or stressing the fiber.
What I'm going to do is then bend this fiber.
And if you watch in the inset, and see that the intensity will
go down.
But if I keep increasing the bend,
I end up with a single-mode propagation but weak.
And this illustrates that you can strip off the high order
modes by simply bending the fiber.
But the penalty is you get much less light getting propagated.
So here we have single-mode propagation,
and here we have multi-mode or a mixture of multi-modes that
are propagating in the fiber.
Again, you can see that there's a lot of light that
gets kicked out--
has to be kicked out of the fiber.
You can see the bend absolutely low.
In fact, this bending effect on the transmission
of light in a fiber can be used as a sensor--
a sensor of pressure, a sensor of stress,
bends, and what have you.
So then, in summary, we've looked
at the propagation of laser light in a single-mode fiber,
and we also saw what happens when the fiber core is a little
bit big, and we saw the propagation of multi-modes.
And, as I mentioned earlier, the popular fiber today
is the single-mode fiber.
But, again, there are applications
for multi-mode fibers.