Introduction to Optical Fiber Cables

Have you ever wondered how emails or other information are transmitted
globally almost instantaneously?

This has been made possible primarily by a network of optical fiber cables laid
under the ground and below the ocean. These cables, which carry most of the
world’s data, are optical fiber cables. They’re also used in medical equipment.
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.

The speed of light changes when it passes through a medium and this change
in speed is expressed by the refractive index. This variation in the speed of light
leads to an interesting phenomenon called refraction.
To understand refraction, imagine an experiment where light passes through a
prism. At the interface,the light bends instead of going straight. This
phenomenon occurs when light passes from a medium with one refractive
index to one with another refractive index. The light bends towards the
interface when it transitions from a medium of high to low refractive index.
Refraction explains why a pencil looks bent in a glass of water.
This refraction technique is effectively used in optical fibers. Now, let’s extend
this experiment hypothetically. By using dopants, we can increase the
refractive index of glass in real-time. As the refractive index increases,the light
bends more towards the surface. Eventually,the light passes along the surface
of the glass. If the refractive index increases further,the light reflects into the
first medium, a phenomenon called total internal reflection.
Total internal reflection is also possible by increasing the incident angle rather
than the refractive index. By gradually increasing the angle at which light
strikes the interface, it reaches a critical angle where the light is completely
reflected into the first medium, ensuring no refraction occurs. This method
provides an alternative mechanism to achieve the same phenomenon. At a
certain angle, called the critical angle,the light reflects entirely into the first
medium. This phenomenon is key to light transmission in optical fiber cables.

The simplest optical fiber cable consists of cylindrical glass with a high
refractive index. If a laser strikes the interface at an angle greater than the
critical angle,total internal reflection occurs, and the light reaches the other
end. This ensures that light remains confined in the optical fiber over long
distances,regardless of the fiber’s shape.

Total internal reflection occurs between high-refractive-index glass and low-
refractive-index air. However, optical fibers require protective coatings, which
replace air and can disrupt total internal reflection. To overcome this, a low-
refractive-index glass, called cladding, is added around the core glass. This
allows total internal reflection while enabling protective layering. Both the core
and cladding use silica as their base material, with dopants added to achieve
the desired refractive index difference.
Optical fibers face signal losses, known as attenuation, due to absorption and
scattering. This limits signal transmission to about 100 kilometers. Amplifiers
are used at intervals to boost signal strength, drawing power from nearby
sources.

Transmission of Information

How do optical fibers transmit information like phone calls or Internet data?
Any information can be represented as binary code—zeros and ones. For
example, when sending a “hello” text, it is converted into a binary sequence.
Your mobile transmits this as electromagnetic waves, with 1s as high frequency
and 0s as low frequency.
Local cell towers pick up these waves. High-frequency waves generate light
pulses, while low-frequency waves do not. These light pulses are transmitted
through optical fiber cables,traveling through a network to their destination.
The globe is interconnected by optical fiber cables laid underground and
beneath oceans. Mobile service providers like AT&T, Orange, and Verizon
maintain these networks, including submarine cables.

A cross-section of an undersea cable reveals that only a small portion houses
optical fibers;the rest is a mechanical structure for protection and strength.
Amplifiers within these cables require power, which is supplied by a thin copper
shell integrated into the cable’s structure. This shell is strategically designed to
carry electricity along the length of the cable, ensuring power reaches the
amplifiers even in deep-sea environments. Without optical fiber cables,regions
would be isolated from the Internet or mobile communications.

Optical fiber cables outperform traditional copper cables in nearly every
aspect. They offer larger bandwidth and higher data transmission speeds, as
the speed of light exceeds that of electrons. Copper cables generate magnetic
fields, leading to electromagnetic interference, whereas light in optical fibers
remains confined, eliminating such risks. Optical fibers also provide high data
security, as external light signals rarely travel along the cable.

Interestingly, optical fiber was first used in endoscopy before
telecommunications because its ability to transmit light with minimal loss
made it ideal for illuminating and capturing images of internal body structures
during medical procedures. In telecommunications, digital pulses are
transmitted through optical fibers, whereas endoscopic cables transmit visual
signals in analog form. This versatility highlights the transformative role of
optical fibers in both communication and medical fields.