# Optical fiber cables, how do they work?

September 3, 2019

Have you ever thought how can you get mail or any information from any corner of the world within a blink of eye. This became possible through the network of cables which are laid across the ground and ocean. The cable; which carry worlds most of the data is the optical fiber cable. They are also used in medical equipment as well.

## Working of optical fiber cables

Let’s understand the working of optical fiber cables and how it revolutionized the world around us. Optical fiber cable is made up of a thousands of fiber strands and single fiber strand is as thin as a human hair (Fig: 1A). Optical fibers carry the information in the form of light, as shown in Fig:1B. A single fiber can carry almost 25,000 calls simultaneously. Let’s learn some fundamental behaviour of light to understand the working of optical fiber.

## Understand refraction and refractive index through an experiment

The speed of light is changes when it passes through a medium and this change in speed is expressed by the refractive index (Fig:2). Refractive index is define as follows.

Refractive index(n) =

The speed of light in vacuum

The speed of light in medium

This variation in the speed of the light leads to another interesting phenomenon, refraction. To understand what it is let’s do an interesting experiment. In this experiment, the light is incident on the prism with the help of laser. You can see that at interface the light gets bend instead of going straight. This phenomenon is known refraction. Refraction occurs when light passes from a medium of one refractive index to another refractive index. The light bends towards the interface when it goes from a medium of high to low refractive index, this is illustrated in Fig:3A. Refraction is the reason why a pencil looks bent in a glass of water (Fig:3B). This simple refraction technique is effectively used in optical fibers.

nAir < nGlass

## Physics of Total Internal Reflection (TIR)

Now, let’s make this experiment a hypothetical one. Using some dopants you are able in 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 stage you can see that, the light will pass through the surface of the glass (Fig: 4B). If we increase refractive index further, the light will suddenly come back to the first medium as a pure case of reflection. This is called Total Internal Reflection (TIR).

The light would also bend if we increase the incident angle rather than increasing refractive index. In this case, after a certain angle, called critical angle the light will come back to the first medium (Fig: 4C).

The total internal reflection phenomenon is used in optical fiber cables to transmit the light.

A simplest form of optical fiber cable is shown here, a cylindrical glass with high refractive index. If the laser strikes the interface at an angle greater than critical angle, the total internal reflection will happen and light will reach the other end (Fig: 5A). This means that light can confine in optical fiber over a long distance no matter what complex shape the fiber forms as shown in Fig: 5B.

Remember, here total internal reflection happens between high refractive index glass and low refractive index air. However, the optical fiber needs a protective coating (Fig:6). A protective coating is not possible for this configuration. The introduction of protective material will replace the position of air and cease the total internal reflection phenomenon.

An easy way to overcome this issue is introduce a low refractive index glass above the core glass known as cladding (Fig:7A). This way the total internal reflection will happen and we will be able to use protective layer. Both core and cladding use Silica as base material, the difference in the refractive index can be achieved by adding different type of dopants. Core and cladding of optical fibers should made of extremely pure silica. Because the impurity present in the glass absorb the light.

## Optical fiber amplifier

The optical fiber we just have constructed won’t be able to carry signals for more than 100 kms. This is due to various losses happening in the cable. This loss of signal strength is generally called attenuation. Absorption and scattering are the main reasons for signal attenuation. Absorption occurs due to the presence of impurities. Another cause of attenuation is scattering. Scattering of light caused by molecular level irregularities in the glass structure. When the scattering happens, the light scatter in all directions. Non-uniformities in the manufacturing of the fiber is also one of the causes of attenuation. When light strikes these imperfections in the fiber, the condition of total internal reflection is not attained and the light leaks out of the core. This is why you see amplifiers after a certain distance. They boost the signal strength and allows signal to transmit over a long distance. The power required for amplifier is drawn from nearby sources(Fig: 8).

## How does optical fiber involved in communication?

Now, back to the main topic. How does the optical fiber transmits information such as phone call or internet signal. Any information can be represented in the form of 1 and 0’s. Assume you want to send ‘HELLO’ through your mobile(Fig: 9A). First, this word will be converted in the sequence of 1s and 0s and your mobile phone will transmit these 1s and 0s in the form of electromagnetic waves. 1 is transmitted as high frequency and 0 as low frequency wave (Fig: 9B).

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 (Fig: 9C), otherwise no pulse is generated. Now, these light pulses can easily be transmitted through optical fiber cables.

## Optical fiber cable under the sea

The light pulses which carry the information, have to travel through a complicated network of cables to reach the destination. For this purpose the entire globe is covered with optical fiber cables. These cables are laid under the ground and ocean. The underground cables are maintained by the mobile service provider mainly. AT&T, Orange and Verizon are the few global players who owns and maintains the under ocean optical fiber cables. The undersea cables are specially designed to protect from water damaged and sea animals. As of 2018, there are over 1.2 million kilometers of submarine cables in service globally.

Detailed structure of undersea cable
A detailed cross section view of the undersea cable is shown in Fig: 10A. The undersea cables are specially designed to sustain under the water. The bunch of the fibers are placed in a small plastic tube wrapped with steel wires. These steel wires are used for strength. Then this complete structure inserted in a copper tube. Here copper tube is used to arries electric power along the cable so that the amplifiers can be powered. This copper tube needs insulator as voltages is applied on it that's why it is covered in plastic tube. After that one or two more layers of galvanised steel are used to protect fibers from human activities such as fishing and anchoring. And finally polyethylene sheath is used to make it water resistant. The whole discussion simply means if the optical fiber cables do not reach a part of the globe, that part will be isolated from the internet or mobile communication (Fig:10B).

## Advantages of optical fiber cable over copper cable

If we compare optical fiber cable to traditional copper cable, the optical fiber cable is superior in almost every way as shwon in Fig:11.

### 1. Higher bandwidth

Optical fiber cable provides larger bandwidth. WDM technology is used to increase the bandwidth of the single strand of fiber cable.

#### What is Wavelength Division Multiplexing (WDM)?

Lets try to understand WDM technology with the help of an experiment. As we all know that when a white light hits the surface of prism it separates into various coloured beams, this is illustrated in Fig: 12A. Similarly a vice vera can also be achieved if prism is used in the reverse direction. Using the same phenomenon, WDM technology is able to send multiple signals simultaneously over a single optical fiber. At the transmitter side, all the signals having different wavelengths are combined by a multiplexer whereas at the receiver side all the signals are separated with the help of demultiplexer, this is illustrated in Fig:12B

### 2. Avoid electromagnetic interference

The flow of electrons in copper cable generates a magnetic field even outside the cable that can causes electromagnetic interference. On the other hand, the light which travels through the optical cable always confine within the fiber thus the chance of interaction with an external signal does not exist (Fig:13).

### 3. More Secure

One more interesting feature about optical fiber cable is that the light signals which enters from the side has a minimal chance of traveling along the cable. Thus the optical fiber cables provide high data security (Fig:14).

## Optical fiber cable used in medicine

You might be wondered to know that optical fiber was first used in endoscopy(Fig:15) even before using it in telecommunication field. In telecommunication digital pulses are transferred through the optical fiber cable. However in endoscopic cables visual signals which are in the analog form are transmitted to the other end.