Bill Hammack, the
engineer says:
I find this a
fascinating object. Its a fiber optic cable for a stereo. If I shine this laser
pointer down the cable, it guides the light out the other end. These cables are
used to connect our world today, and are capable of transmitting information
across countries and oceans, but first, let me show you how it works. I have a
bucket that I modified with a window in front.
And on the other side, I
put a stopper in this hole right here. I have a bottle of propylene glycol with
a just a little bit of creamer in it. A ring stand. And, of course, a laser
pointer. Now keep your eye on this plug when I turn out the lights. That's
wonderful. The light follows the liquid’s flow all the way to the bucket. Amazing.
It does this because of total internal reflection.
As the light enters the
stream it is reflected as soon as it hits the interface between air and liquid.
You can see here the first reflection, and then the second and the third. This
occurs because there’s a difference between the index of refraction of the
guide material here propylene glycol and the outside air in this case. Recall
that any time light strikes a surface it can either be absorbed by the material,
reflected from it or pass into and through it the latter we call refraction. Its
easier to see from a top view.
Reflection and
refraction can happen at the same time, but if a light ray hits the surface at
an angle greater than the critical angle it will be completely reflected and
not refracted. For this propylene glycol and air system as long
as a beam hits the
surface at an angle greater than 44 point 35 degrees measured from the normal
it will propagate down the stream via total internal reflection. To create this
same effect in an optical fiber engineers create a core of glass usually pure
silicon dioxide and an outside layer called "cladding" which they
also typically make from silicon dioxide, but with bits of boron or germanium
to decrease its index of refraction.
A one percent difference
is enough to make a fiber work. To make such a long, thin piece of glass
engineers heat a large glass perform. Its center is the pure core glass and the
outside the cladding. They then draw or pull a fiber by winding the melt onto a
wheel at speeds up to 1600 meters per minute. Typically these drawing towers
are several stories tall. The height allows the fiber to cool before being
wound onto a drum. One of the greatest engineering achievements was the first
ocean-spanning fiber optic cable. Called TAT-8 it extended from Tuckerton, New
Jersey following the ocean floor over 3,500 miles until branching out to Widemouth,
England and Penmarch, France. Engineers designed the cable carefully to survive
on the ocean floor. At its center lies the core. Less that a tenth of an inch
in diameter it contained six optical fibers wrapped around a central steel
wire.
They embedded this in an
elastomer to cushion the fibers; surrounded it with steel
strands, and then sealed it inside a copper cylinder to protect it from
water. The final cable was less than an inch in
diameter, yet it could handle some 40,000 simultaneous
phone calls. The essence of how they send information through
a fiber optical cable is very simple. I could have a
pre-arranged code with someone at the end perhaps we'll use Morse
code and I just block the laser so that the person at
that end sees flashes that communicate a message. To transmit an analog signal like voice from a phone call along
the cable engineers use pulse code modulation.
We take an analog signal
and cut it up into sections and then approximate the wave's loudness or
amplitude as best we can. We want to make this a digital signal, which means discrete values of loudness and not just any value. For example, I’ll use four bits, which means I have
sixteen possible values for the loudness. So the first four
sections of the signal could be approximated by about 10, 12, 14 and 15. We then take each section and convert its amplitude to a series of
ones and zeros. The first bar of value ten when encoded becomes
1-0-1-0. We can do this for each section of the curve. Now, instead of looking at the green waveform, or even the blue bars, we can think of the
signal as a series of ones and zeros organized by time. And it is that sequence that we send through a fiber optic cable.
A flash for a one and
nothing for a zero. Of course, the exact method of encoding is known
at the receiving end, so it is a trivial matter to decipher the
message. Now, you may be wondering how a laser pulses can
travel nearly 4000 miles across the ocean. It doesn’t without some help
because the light will escape from the sides of the fibers. Look back at our propylene stream. Here's how the light
attenuates as it travels. You can see here a narrow beam in the bucket, that broadens a bit when it enters the stream. And then after the first bounce the beam leaves even broader then
it entered. That's because the interface with the air is
uneven and the rays that make up the beam strike at
slightly different angles.
When that beam makes its
second reflection those individual rays diverge even more, until by the time it reaches the third bounce many of the rays are
no longer at the critical angle and can exit from the
sides of the stream. Here it happens in a few inches, but in a cable like TAT-8 the signal travels a stunning 50 kilometers before it needs to be amplified. Absolutely amazing.
0 Komentar untuk "How Fiber optic cables work and how engineers use them to send signals"