- [Narrator] Have you ever thought about how you get emails
or any other information from any corner of the world
within a blink of an eye?
This has been made possible by a network of cables
which are laid under the ground and below the ocean.
The cables which carry most of the world's data
are optical fiber cables.
They are also used in medical equipment.
(upbeat music)
Let's learn how optical fiber cables work
and how they have revolutionized the world around us.
Optical fiber cable is made up of thousands
of fiber strands.
And a single fiber strand is as thin as a human hair.
Optical fibers carry information in the form of light.
Let's first learn some fundamental behaviors of light
to understand the workings of optical fibers.
The speed of light changes when it passes through a medium.
And this change in speed is expressed
by the refractive index.
This variation and the speed of the light
leads to another interesting phenomenon.
Refraction, to understand what it is,
let us carry out an interesting experiment.
In this experiment, light passes through a prism.
You can see that at the interface
the light gets bent instead of going straight.
This phenomenon is known as refraction.
Refraction occurs when light passes from a medium
with a one refractive index
to one with another refractive index.
The light bends towards the interface
when it goes from a medium of high
to one of low refractive indices.
Refraction is the reason why a pencil looks bent
in a glass of water.
This simple refraction technique
is effectively used in optical fibers.
Now, let's make this experiment a hypothetical one.
Using some dopants we are able to increase
the refractive index of the glass in real time.
As we increase the refractive index,
the light will bend more and more towards the surface.
After a time you can see that,
the light will pass through the surface of the glass.
If we increase the refractive index further,
the light will suddenly come back to the first medium
as a pure reflection.
This is called total internal reflection.
The total internal reflection is possible
if we increase the incident angle
rather than increasing the refractive index.
In this case at a certain angle called the critical angle,
the light will come back to the first medium.
This phenomenon of total internal reflection
is used in optical fiber cables to transmit the light.
The simplest form of optical fiber cable is shown here.
Cylindrical glass with a high refractive index.
If the laser strikes the interface
at an angle greater than the critical angle,
total internal reflection will happen
and the light will reach the other end.
This means that light can be confined
in the optical fiber over a long distance.
No matter what complex shape the fiber forms.
Remember, total internal reflection happens
between the high refractive index glass
and the low refractive index air.
However, optical fibers need a protective coating.
A protective coating is not possible
with this configuration.
The introduction of protective material
will replace the position of the air
and cease the total internal reflection phenomenon.
An easy way to overcome this issue
is to introduce a low refractive index glass
above the core glass known as cladding.
This way total internal reflection will happen
and we'll be able to use a protective layer.
Both the core and the cladding use silica
as their base material.
The difference in the refractive index
can be achieved by adding different types of dopants.
The optical fiber we have just constructed
won't be able to carry signals for more than 100 kilometers.
This is due to various losses that happen in the cable.
This loss of signal strength
is generally called attenuation.
Absorption and scattering are the main reasons
for signal attenuation.
This is why you see amplifiers and cables
after a certain distance.
They boost the signal strength and allow signals
to be transmitted over a long distance.
The power required for the amplifier
is drawn from nearby sources.
Now, back to the main topic,
how does the optical fiber transmit information
such as phone calls or internet signals?
Any information can be represented
in the form of zeros and ones.
Assume you want to send a hello text message
through your mobile.
First this word will be converted
into an equivalent binary code
as a sequence of zeros and ones.
After the conversion your mobile phone will transmit
these zeros and ones in the form of electromagnetic waves.
One is transmitted as a high frequency
and zero as low frequency wave.
Your local cell tower picks up these electromagnetic waves.
At the tower, if the electromagnetic wave
is of high frequency, a light pulse is generated.
Otherwise, no pulse is generated.
Now these light pulses can easily be transmitted
through optical fiber cables.
The light pulses which carry the information
have to travel through a complicated network of cables
to reach their destination.
For this purpose, the entire globe is covered
with optical fiber cables.
These cables are laid under the ground and below the ocean.
It is mainly the mobile service providers
that maintain these underground cables.
AT&T, Orange and Verizon are some of the few global players
who own and maintain the submarine cable network.
A detailed cross section view of an undersea cable
is shown here.
You can see that only a small portion of the cable
is used for holding the optical fiber.
The remaining area of the cable is a mechanical structure
for protection and strength.
Now the question is, where does the amplifier get power
from under these deep oceans?
Well, for this a thin copper shell is used inside the cable.
Which carries electric power along the cable
so that the amplifiers can be powered.
This whole discussion simply means
that if optical fiber cables
do not reach a part of the globe,
that part will be isolated from the internet
or mobile communications.
If we compare optical fiber cable
to traditional copper cable, the optical fiber cable
is superior in almost every way.
Fiber optic cables provide larger bandwidth
and transmit data at much higher speeds than copper cables.
This is because the speed of light
is always greater than the speed of electrons.
The flow of electrons in a copper cable
generates a magnetic field even outside of the cable
that can cause electromagnetic interference.
On the other hand, the light which travels
through the optical cable is always confined
within the fiber.
Thus the chance of interaction with an external signal
does not exist.
One more interesting feature about optical fiber cables
is that any light signal which enters from the side
has a minimal chance of traveling along the cable.
Thus the optical fiber cables provide high data security.
You might be amazed to know that optical fiber
was first used in endoscopy
even before it was used in the telecommunications field.
In telecommunications, digital pulses are transferred
through the optical fiber cable.
However, in endoscopic cables,
visual signals which are on the analog form
are transmitted to the other end.
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