Ladies and gentlemen, Bell Laboratories
invites you to Hear the Light. Hello, I'm Henry Feinberg at Bell Laboratories,
and what you just saw was my way of changing sound into light. And that's what we'll be talking about
today we'll investigate some of the characteristics of light and we'll see how the Bell System is
using them for telecommunications. Our new system is called Lightwave
Communications and Bell Laboratories with the
cooperation of Western Electric and the Illinois Bell Telephone Company
has installed a working Lightwave communications system beneath the streets of Chicago. It's a
full service system carrying voices, computer data, and also video signals
from picture phone meeting service. The idea of using light as
a communications medium may seem like a a very new concept. But really it's not new at all. It's quite
old as a matter-of-fact. It's very old. It's as old as - signal fires. It's as [coughing] old as smoke signals. It's as
old as a "One if by land and two if by sea," and it's even as old as Morse code blinking from ship to ship. In fact the idea is so old that back in in 1880, only four years
after Alexander Graham Bell invented the telephone, he invented the photophone to
send his voice on a beam of light. Ahead of his time? I think you'd be surprised at how far ahead of his time Mr. Bell was. Here's a drawing of Bell's photophone. Now, the most powerful source of light
that Bell had to use was of course the Sun, so he reflected sunlight from a
mirror through lenses to form a beam of light.
The beam hit the silvered surface of a membrane
stretched across the end of a speaking tube. When Bell spoke into the tube his voice vibrations vibrated the membrane and imparted those vibrations to the beam of light, which went on. Well, that process is called modulating the
beam of light. I built my own modulator - light modulator -
out of this glass funnel, which I've covered the face of with silvered plastic, and I have a speaking tube,
a piece of hose that I'll place on top of the
projector here, my light source, and I'll reflect the
projected light onto the background. When i speak into
the speaking tube we can see how my voice vibrations are
vibrating the light. Well, this is really not a perfect working system, but it gives
you the idea of how my voice can modulate a beam of light. Well, Bell had an idea that if he
was able to put his voice onto a beam of light he should also be
able to find a way of taking his voice off of the beam of light and converting the
light back into sound. And he tried various methods with very little success until he came to the idea of using
selenium. Now... selenium is one of a class of elements that is
neither a good conductor of electricity but it's not a very good insulator
either, its sort of halfway in between. We call these elements semiconductors. The property of selenium
that intrigued Bell was that when exposed to light the
conductivity of selenium increases. It allows more current to pass
through. We can see that with this selenium
photodetector that I have here. I'll connect it in series with a meter, and I'll connect the meter in series with the selenium photodetector and a battery. And we can see that the current is passing through the meter. When I place my hand in front of the
selenium photodetector we can see that less current can pass
through it and the meter goes down. It drops well here's how Bell used the
selenium photodetector to receive the light. This is a drawing
of Bell's photophone receiver. Here he has a parabolic reflector. The purpose is to
concentrate the incoming rays of light to the focal point, and that's where he
put his photodetector. Also, he placed his photodetector in
series with batteries the same way we did, but
instead of using a meter he used a pair of telephone handsets to
convert the varying current back into sound. Well, he did it a hundred
years ago. Let's do the same thing today. For our
light source we won't use the sun, but we'll use this flashlight. And as the modulator I'll use this cardboard tube which I've
cut at an angle and I've cemented a piece
of silver plastic to the top. I'll place that on top of the flashlight
and disconnect the meter and turn the
amplifier on, I have the photodetector connected to an amplifier, and I'll aim it toward my flashlight. And when I talk into the top - [Muffled voice]] when I talk into the top you can hear my voice being sent over a beam of light. It's a very crude system and I think you'll agree it's not high
fidelity, but the thing works. And with his system Bell was able to
transmit his voice over a beam of light, over a mile away. Well we might say - and excuse the pun - that because of Bell's dependence on clear, sunny days his was a bellwether system. But
seriously - It took almost a hundred years for scientists to come up with a better
set of components. Well since that time we've found many more
uses for semiconductors, perhaps the most famous of which is the
transistor. invented in 1947 at Bell Laboratories. Today transistors and integrated
circuits are made using the semiconductor silicon.
Now, one of the properties of silicon is that when properly treated and exposed to light silicon actually produces electricity. In 1954 scientists at Bell Laboratories made silicon devices efficient enough to
be called solar batteries. I have three solar
batteries or solar cells here. In this box they're connected inside to
a motor and when I place them on top of the
light source we can see that they produce enough electricity to turn
the motor. Today silicon solar cells are finding many uses on Earth but they're also
finding uses in outer space where silicon solar cells power the satellites that are circling
the Earth and also the satellites going out into deep outer space Well, let's get back down to
Earth right now as we find out some more things about
light. Well, one of the things that we know about light is that light is highly unordered. White light coming from this projector - it's vibrating in all directions or
polarizations. We can start to put some order into
light by polarizing the light with a polarizing
filter. I have such a filter here which I'll
place on top of the projector. We might think of a polarizing filter
as sort of micro miniature venetian
blinds. You can see how it works when I put a second polarizing filter on
top of the first. I'll turn the lights down so we can see how this works. Now when I
turn the second filter, just the second one, we can see how the two crossed polarizers block out almost all of the light. Well, scientists use
polarized light to investigate what happens to light when
it passes through certain materials. One of those materials that it's
interesting to show is Micah. Micah is a mineral that when placed in
polarized light is normally clear, but when I cover the micah with
this second sheet of polarizing
material in there between the two we can see the colors appear.This is
because the polarized light passing through the micah is altered. And white light contains all the
frequencies and all the colors. But in a passage through the Micah some
of the frequencies are cancelled out and we see the colors that are left. How do we know that white light has all the colors of the rainbow in it? We can dissect light. And we'll dissect
light not with a scalpel but with this glass prism, a triangular shaped glass prism. To do that I'll put another
slide on top of the projector and I'll turn it on. I'll turn my lights down and since we're going to bend the light I'll pull this image down below the
screen because the white light traveling through the prism is bent. But bent by different amounts, by the different frequencies of light, or
different wavelengths which we see as different colors. And
projected on the screen we see the visible spectrum of light
going from deep red at one end, all the way through
the spectrum into deep violet. Well, there's a lot more light
outside this visible spectrum that humans can't see. The visible spectrum is only part of an even larger spectrum of
energy called the electromagnetic spectrum. Now, the
electromagnetic spectrum consists of waves starting at the
low frequency end of low-frequency radio broadcast, TV, radar, then into the infrared, past the
visible light that we just saw to ultraviolet, X-rays, gamma rays, and cosmic rays. Let's take a look
right here above the visible range into the ultraviolet. Now, our eyes can't see ultraviolet light
directly, but we can see the effect that
ultraviolet light has on certain materials. I'll turn my lights down
and turn my ultraviolet light on, and we can
see one of these materials. It's the ultraviolet makeup that I'm wearing
on my face. This is an effect called fluorescence, and the phosphors in the makeup on my
face are similar to the phosphors on the inside of your screens of your
television sets, and also coated on the inside of fluorescent lamps. Well, let's do a different experiment with a
different phosphor, and for this I'll need a volunteer. Dawn, would come up please? Thank you. Here's Dawn Franklin. Dawn, I'm going to take your picture. Okay? 'Okay." And to do this I'll lower my screen and I'd like you to stand in front of
the screen while I get my camera. Okay, stand to the side and I'll take your picture. Let me turn
lights down a little bit, and I'll take your picture. Okay Dawn, would you step away from the
screen please? Well, shades of Peter Pan. Let me turn lights on we'll see why we
see your shadow remaining on the screen. Okay? Thank you
very much Dawn. and I'll give your picture to you at the end in the show. Okay, thank you. Well this screen is coated
with a phosphor, a different type of phosphor that absorbs light and stores the energy for a while before
each of the atoms releases the energy at random intervals. Well, scientists theorize that if they
could find a material to store energy and then a way of causing or stimulating
the atoms to release the energy all at once, they can get a powerful
burst of light. And that's the principal of the laser. Conceived at Bell Laboratories by
two scientists in 1958. In fact the word "laser" means Light Amplification by Stimulated
Emission of Radiation. I have a low power continuous-wave laser here, which we'll use in just a little while. Bell's photo phone used a method of varying the amount of light in step with his voice. The Bell System's
new lightwave communication system uses a different method. It's called a digital
method, by which light is pulsed on and off very rapidly. Well, scientists at Bell
Laboratories wanted to find out just how fast can
light be pulsed on and off, and I have a photograph that they took of a
pulse of laser light passing through a small glass bottle like this. The bottle was filled with a
special liquid which made the pulse of light visible. I'll put the picture on the projector but I have to put the screen up first. Okay here's that picture. Turn it on and turn my lights down. The photo you see has the pulse of laser light right here in the center. Now light is
traveling through the liquid in this glass bottle at a speed of about 140,000 miles per
second. This pulse of light lasts 20 pico seconds, or twenty trillionths of a second, enough time for this pulse of light to travel only about ten millimeters, or half an inch. Now that is fast. Our lightwave communication system uses lasers - lasers pulsing on and off, but not really that fast. I have some
of those lasers right here. I'll turn my lights back up and I'll show them
to you. They're very small lasers. They don't pulse on and off nearly that
fast, they only pulse on and off about 44 million times per second. I have them here, I'll put them on top of the projector and we'll try to project them on the screen. Now it's going to be
difficult - (I'll turn the lights back down). it's difficult to see these because each
of these semiconductor lasers are smaller than a
grain of salt. Well, there's a reason why they're that
small. First of all, they're that small so that they can effectively couple their light energy into the very thin light guides which we'll see in
just a minute. They're also that small so that they can operate on very low
voltages, the same low voltages that power the
rest of transistor eyes equipment. And they're also that small so that they have a very long lifetime. Their projected average lifetime
is expected to be over a hundred years. And if you don't
believe me I invite you to wait. Another light source that's being used in our
lightwave communication systems is the light emitting diode. I have a
sample love one right here. This is similar to the light emitting
diodes that we have in our digital watches and also in our pocket
calculators. Also, light emitting diodes today are providing light for the dials of the
new Trimline touchtone telephones. The light emitting diode is a second cousin to the laser, by the way. And just for a size comparison this is
an antacid tablet that I just keep handy in case my
next experiment doesn't work. Sending a beam of light through the air
the way Mr Bell did is a very inefficient way of getting it from one
place to another. Too many things can get in the way. Fog, snow, rain, trees growing up in the
way, it's just too risky. There's a better way.
It's called light guides. Now, it took the very latest in science
and technology to make efficient light guides, but the principle of the light guide
is very old, even older than Bell's photophone. Let's
do a demonstration performed in Bell's time to illustrate how a light guide works. And for this I'd like another volunteer please. Here's Walt Lyman from Illinois Bell, thank you very much for helping out today, Walt. Would you step around the side and I'll
have to place this apron on you because we're going to be doing this
demonstration with water. Come here and I'll put this apron right around you. "What does that say there?" It says "For this I spent four years in college?" Question mark. Actually you're going
to be performing a very vital service Step right around here andI'll bring
this stand up so that we can all see. This is a one gallon paint can and its full of water. The top of the stand is magnetized so
it'll hold the paint can without danger of tipping. I'll just top of the
can. Now, inside the can is a bulb, a very low voltage light bulb. I'll
turn my lights down so that we can see it, and I'll put my hand on top and I think
you can see the light inside the can. Its low voltage and it runs off a little six volt battery, so
there's no danger of getting a shock. In front of the can is a small hole with a cork in it I'd like you to come over here and hold this beaker in front of the can, because I'm going to pull
the cork out and the stream of water will come out.
And I'd like you to hold it so that it catches the stream of water.
Okay? Here goes... Alright. Now, we can see that the light inside the can
gets trapped inside of the stream of water and
travels to the into the stream where it breaks up on the side the beaker and that is the principal of the light
guide. Very simple. You know, if we were able to
freeze the stream of water we could make a light guide that we can
use. Well, I won't wait until the beaker fills up like I can (laughs), and the big part of this trick is to get the cork back into the hole. Let me turn the light back on so that we can do that, and see what kind
of a team we make. Here goes. Okay, that's fantastic. Well, thank you very
much Walt. I'll bring this down so that we don't tip it
over later, and turn the light out. I'll take the apron from you, and thank you very much for your help. "You're welcome." Well, that is the principal the light
guide and it's a very simple one. Luckily we have a material that's transparent, that's a solid at room temperature, and
of course i'm talking about, glass. Light guides, I have a sample of a
glass light guide here. Its a thin, hair-thin fiber of glass. Now, this is made of some of
the most transparent glass ever produced. They say that if the water
of the ocean where as transparent as the glass in this light guide we'd be able to see the bottom of the
deepest part of the ocean from the surface. Now, that is transparent. Because this glass light guide is so thin
it will be difficult to see light traveling through it. So for
demonstration purposes let's use this low-powered laser and a much thicker light guide made out of
plastic. I'll turn the laser on, put the light guide into the front of the laser and turn my lights down. And you can see how the light goes in one end of the light guide, gets trapped
inside the coil of wire, and can't get out until it reaches the other end. Now, you can't see this on your TV screens
but here in the studio we can see that the entire coil of light guide is glowing very dimly. All of the light that
is coming out of the sides of the coil would
be wasted for use in a communications system To see what happens inside of a light guide let's use a larger, much thicker tube. This is a glass tube which I filled
with water. And I'll turn my lights down a little more so that we can see this. Now, light from the laser is streaming into the bottom with the tube. When I
start to tilt the tube we can see the laser light hitting the
side of the tube. But instead of balancing out, it's
reflected back into the tube. This is called total
internal reflection and you can see it zig-zagging back and
forth inside of the tube. Well, actually the light is not
being totally internally reflected or we wouldn't see any. I put a drop of milk in the water so
that some of the light is reflected toward our eyes. It took the very latest in technology and a lot of work to make efficient light guides. The light guides that are used in our lightwave communication system consist of a central core of the ultra
transparent glass we spoke of earlier. The composition of this glass
changes gradually away from the center so that any rays have like this try to
move off-center are bent gently back so that they stay
in the center. They never even touched the outer wall of the central core. Around the central core is a cladding of a different type of glass and around that a protective layer of
plastic to prevent abrasions. The use of glass light guides in our lightwave communications system is even more efficient than using copper
wires. In fact, using a digital copper wire transmission system it will be
necessary to have an amplifier, or we call them repeaters, every mile along
the way. Using light guides, glass light guides, we
only need repeaters every four miles. And most telephone buildings in
cities where these systems are being used, will be used, have telephone buildings located less than four miles apart. in our system we take 12 glass fibers and place them side by side in a ribbon.
I'll place this ribbon on top of the projector stage so that we can see the 12 glass fibers Now by electrically bundling telephone calls together we can place
over 600 simultaneous phone calls on each pair of light guide strands. So we can place 12 of these ribbons into a protective outer covering like
this, making a light guide cable of up to 144 fibers. Now, using 600 for each light guide, or each pair of light guides, this half-inch light guide cable can carry over 48,000 simultaneous
telephone calls. Let's contrast this with a digital system. Using copper wires to carry 48,000 thousand simultaneous telephone calls, it would take two of these 1800 pair copper wire cables. Thats 7,200 wires compared to 144 light guide cables. It gets pretty crowded under those
city streets and light guide communications will help.
Well, now that we found out about light guide communications, lightwave communications, let's put
together a working Lightwave Communications link. For the transmitter we'll use this small box, it has a circuit
inside which produces pulses of electricity. The pulses of electricity are changing to
pulses of light in this light-emitting diode. For the
receiver I'll get rid of the selenium photodetector
that Bell used, and in its place I'll put a much
more sophisticated lightwave receiver. The photodetector
in this light wave receiver is hundreds of times more sensitive than the selenium photodetector
I've turned it on. For the modulating source, this transmitter is connected to a cassette tape player which I'll turn on, and now for
the link between them we'll use this plastic light guide. I'll connect one end to a hole drilled
into the light emitting diode and the other end goes over here to the receiver. [music starts playing] and you see the system works. I'll
disconnect this end and see how [music stops] sensitive [music resumes] this system is. [music plays on] Well, it works very well. So, as we've seen today [shuts music off] the
investigation of light in all its aspects has occupied the interest's of
Bell System scientists and engineers even before the official beginnings a
Bell Laboratories over fifty years ago. Today we took a
look at some of these areas and I hope we had some fun in the
process but most important we've seen that advancements like light wave
communications couldn't happen in isolation. It takes the combined
efforts of all these Bell System partners to make a
modern efficient lightwave communication system
possible. A vital partnership consisting of AT&T Bell Laboratories, Western Electric, Long
Lines and your local Bell System telephone
company, working together to help you to Hear the Light. Thank you very much.
