Optical Fibres: Definition, Principle, Types, Uses

Optical Fibre: An optical fibre is a flexible and transparent fibre made by drawing glass (silica) or plastic to a diameter slightly thicker than that of human hair. The significance of optical fibers lies in the fact that they are able to transmit data signals over large distances and with higher speed and bandwidth than other mediums. Hence they are used to provide internet, telephone and TV services, being made up of glass and plastic, optical fibers are not affected by electromagnetic interference when they transmit data.

Let’s understand everything about optical fibre including the definition, design, working principle of optical fibre, types, and their advantages over traditional wires. It is important that you read thoroughly to have a clear understanding of the basic concepts. For easier understanding of concepts, we have explained everything with illustrations. In this article, we have discussed in detail what is optical fibre.

What is Optical Fibre?

Optical fibre is the technology associated with data transmission using light pulses travelling along with a long fibre which is usually made of plastic or glass.

Design Of An Optical Fibre

Optical fiber is made of a thin glass core (diameter 10 to 100µm) surrounded by a glass coating called cladding, protected by a jacket of plastic.

A Fibre Optic Relay System consists of the following components:

1. The Transmitter – It produces the light signals and encodes them to fit to transmit.
2. The Optical Fibre – The medium for transmitting the light pulse (signal).
3. The Optical Receiver – It receives the transmitted light pulse (signal) and decodes them to be fit to use.
4. The Optical Regenerator – Necessary for long-distance data transmission.

Working of Optical Fibre

Optical fiber works on the principle of total internal reflection. When light travelling in an optically dense medium hits a boundary at a steep angle (larger than the critical angle for the boundary), the light is completely reflected. This is called total internal reflection.

This effect is used in optical fibres to confine light in the core. Light travels through the fibre core, bouncing back and forth off the boundary between the core and cladding. Because the light must strike the boundary with an angle greater than the critical angle, the only light that enters the fibre within a certain range of angles can travel down the fibre without leaking out.

This range of angles is called the acceptance cone of the fibre. The size of this acceptance cone is a function of the refractive index difference between the fibre’s core and cladding.

Key Terms Used In Optical Fibre

The definition and understanding of an optical fibre are incomplete without the key terms that are used to describe its design and working. All the important terms used in an optical fibre are described below:

1. Critical Angle (θ_c)

At core-cladding interface, if θ_c = θ, then

$$\cos {\theta _c} = \sqrt {{{{\mu _1}^2 – {\mu _2}^2} \over {{\mu _1}}}} \Rightarrow {\theta _c} = {\cos ^{ – 1}}\left( {{{\sqrt {{\mu _1}^2 – {\mu _2}^2} } \over {{\mu _1}}}} \right)$$

2. Acceptance Angle (θ_a)

The value of maximum angle of incidence with the axis of fibre in the air for which all the incident light is totally reflected is known as acceptance angle.

If θ_a = Acceptance angle, ?₁= refractive index of core and ?₂=refractive index of cladding, then

$$\sin {\theta _a} = {{\sqrt {{\mu _1}^2 – {\mu _2}^2} } \over {{\mu _0}}} \Rightarrow {\theta _a} = {\sin ^{ – 1}}\sqrt {{\mu _1}^2 – {\mu _2}^2} $$

3. Numerical Aperture

Light gathering capability of an optical fibre is related to its numerical aperture. This is defined as the sine of its acceptance angle. That is,

$$NA = \sin i = \sqrt {{\mu _1}^2 – {\mu _2}^2} $$

The numerical aperture can also be given in terms of relative core, cladding index difference (Δ), where

$$\Delta = {{{\mu _1}^2 – {\mu _2}^2} \over {2{\mu _1}^2}}$$

Thus,

$$NA = \sqrt {{\mu _1}^2 – {\mu _2}^2} = {\mu _1}\sqrt {2\Delta } $$

4. Fibre Attenuation

In practice, a very small part of light energy is lost from an optical fibre. This reduction in the light energy is called attenuation and is described by

$$I = {I_0}{e^{ – {a \over x}}}$$

where,

I = Intensity of light when it enters the fibre
I₀= Intensity of light as a distance x along the fibre
a= absorption coefficient or attenuation coefficient

Types Of Optical Fibre

Optical fibres are classified on different parameters such as refractive index, materials used and mode of propagation of light. Let’s see each classification in detail.

Based on Refractive Index

• 1. Mono Mode Optical Fibre: It has a very narrow core of diameter about 5?m or less, cladding is relatively big.

• 2. Multi-mode Optical Fibre: It is again of two types:
  • • (i) Step Index Multi Mode Fibre: The diameter of the core is about 50?m. The core has a constant R. The refractive index then changes to a lower value of ? which remains constant through the cladding.
  
  • (ii) Graded Index Multi Mode Fibre: Refractive index decreases smoothly from its center to the outer surface of the fibre (cladding). There is no noticeable boundary between core and cladding.

Based on Materials Used

The classification based on the materials used is as follows:

1. Plastic Optical Fibers: Polymethylmethacrylate is used as a core material for the transmission of light.
2. Glass Fibers: It consists of extremely fine glass fibers.

Based on Mode of Propagation of Light

The classification based on the mode of propagation of light is as follows:

1. Single-Mode Fibers: These fibers are used for the long-distance transmission of signals.
2. Multimode Fibers: These fibers are used for the short-distance transmission of signals.

Advantages of Optical Fibres Over Wires

There are many advantages of using optical fibres over traditional wires. Some of the advantages are listed below:

1. Lower cost in the long run
2. Lower loss of signal, typically less than 0.3 db/km). So, repeater-less transmission over a long distance is possible
3. Large data-carrying capacity (thousands of times greater, reaching a speed of up to 1.6 Tb/s in the field-deployed system and up to 10 Tb/s in lab systems)
4. No electromagnetic radiation; difficult to eavesdrop
5. High electrical resistance. So, safe to use near high-voltage equipment or between areas with different earth potentials
6. Low weight
7. Signals contain very little power
8. No cross-talk between cables
9. No sparks (e.g. in automobile applications)
10. Difficult to place a tap or listening device on the line, providing better physical network security

Now that you got all the information on optical fibres, watch the following video to understand the concept better.

FAQs on Optical Fibre Communication

Q.1: What are some disadvantages of optical fibres?
Ans: Some of the disadvantages of optical fibres are:
(i) The optical fibre cables are very difficult to merge into each other and there might be a loss of the light beam within the cable while scattering.
(ii) The installation of optical cables is very expensive. Special test equipment is needed to install the optical fibres.
(iii) Fiber optic cables are compact and are very highly vulnerable while fitting.
(iv) The optical fibre cables are more delicate as compared to copper wires.
(v) Special devices are required to analyze the transmission of light through the fibre cables.

Q.2: What are the uses of optical fibres?
Ans: There are multiple usages of fibre cables. Computer networking is one of the major uses of fibre optics in which data is transmitted at a higher bandwidth. It finds application in long-distance connections of a computer network to different locations. Apart from that, fibre optics can also be used in space and military industries, cables used under-sea and medical purposes such as in MRI scans, endoscopy, surgical microscopy and other tests and in medical instruments to provide precise illumination.

Q.3: What are the advantages of optical fibres?
Ans: Some of the major advantages of optical fibres are:
(i) Economical and cost-effective
(ii) Less power consumption
(iii) Thin and non-flammable
(iv) Less signal degradation
(v) Flexible and lightweight
(vi) Excellent data security
(vii) Cost-effective
(viii) Unaffected by interference

Q.4: Name the factors that are responsible for generating attenuation of optical power in fibre.
Ans: Following are the factors that are responsible for generating attenuation of optical power in fibre:
(i) Absorption
(ii) Scattering
(iii) Waveguide effect

Q.5: What is Raman Effect?
Ans: The Raman Effect is the change in the wavelength of light that occurs when the molecules deflect the light beam.

Q.6: Why are plastic-clad silica fibre optic cables not user-friendly?
Ans: Following are the reasons why plastic-clad silica fibre optic cables are not user-friendly:
(i) The fibres are insoluble in organic solvents
(ii) Bonding becomes difficult
(iii) Connector application becomes difficult as there is excessive plasticity in the cladding

Q.7: What is the principle of fibre optical communication?
Ans: The basic principle of optical fibre is Total Internal Reflection.

Q.8: Why do we use Silica for the fabrication of optical fibres?
Ans: Silica has a perfect elasticity until it reaches the breaking point, which makes it best for fabrication.

Q.9: What is the bandwidth of optical fiber?
Ans: The bandwidth of optical fiber is 900 THz.

Q.10: How many types of Optical fibers are there based on the refractive index?
Ans: There are two types of Optical fibers based on the refractive index: Step Index Fibers and Graded Index Fibers.

Q.11: How many types can Optical fibers be classified into based on the materials used?
Ans: Optical fibers can be made up of Glass or Plastic.

Q.12: On the basis of the mode of propagation of light, in how many types can Optical Fibers be classified?
Ans: On the basis of the mode of propagation of light, Optical Fibers can be classified into two kinds. The fibers used to transmit signals to a short length of distance are called Multimode Fibers. Single-Mode Fibers, on the other hand, are the ones required to transmit signals over long distances.