An optical fibre is a cylindrical waveguide that is generally used for both long-distance or wideband communication as well as short-distance communication. Optical fibres are generally made up of drawing glass, silica, or plastic. The diameter of a single strand of optical fibre is slightly thicker than the diameter of human hair. A number of such strands arranged in the form of a bundle are known as optical fibre cables. An optical communication model typically makes use of a light source and a network of optical fibres to transmit data from one place to the other. The optical fibre cable serves to be a medium that carries the data contained by the light ray and transmits the information to the receiver. An optical fibre communication system is a reliable mode of communication as there are minimum chances of electromagnetic interference. Optical fibres are flexible, lightweight, slim, and non-flammable in nature. Optical communication is generally preferred over other modes of communication due to fewer chances of signal degradation, high data security, cost-effectiveness, and low power consumption. The signal bandwidth of an optical fibre is generally equal to 900 THz. Optical fibres are widely used in various medical equipment and research laboratory machines. Fairy lights and other decorative equipment also make use of optical fibres.
Construction of Optical Fibre
An optical fibre typically consists of three parts, namely core, cladding, and outer jacket. The material used to make the core and cladding of an optical fibre is generally silica, plastic, or glass. The outer jacket of the fibre is made up of PVC. The size of the core typically varies between 50 µm to 500 µm. The size of the cladding varies as per the requirement. To achieve total internal reflection, the refractive index of the material should always be slightly higher than the refractive index of the material or the air present around it. This is the reason why the refractive index of the core is always maintained higher than the refractive index of the cladding. The cladding prohibits the data from being lost or distorted. The purpose of the cladding is not only to support the total internal reflection phenomenon but also to provide a protective coating to the fibre. The main purpose of a jacket layer wrapped around the fibre is to provide protection against the rough and uneven surface. It shields the core of the fibre from getting damaged and from suffering deformations at a micro-level. The optical fibre cables that are laid under the seas and oceans are more prone to suffer data losses, hence they contain more protective coatings around the core than the cables that are present simply beneath the ground.
Types of Optical Fibre
On the basis of the refractive index, type of construction material used, and the mode of propagation of light, there exist a number of optical fibres in the market. Broadly the optical fibres can be classified into three categories, given below:
On the Basis of the Refractive Index
1. Step Index Optical Fibre
The refractive index of the core and cladding of a step-index optical fibre is uniform throughout the length. If you cut a step-index optical fibre and notice its cross-sectional view, you can easily observe the step change in the refractive index of the core and the cladding.
2. Graded Index Optical Fibre
The refractive index of the core and cladding of a graded-index optical fibre is not uniform throughout the length. If you cut a graded-index optical fibre and notice its cross-sectional view, you can easily observe that the change in the refractive index of the core and the cladding decreases as the radial distance increases. The refractive index profile of the core and cladding of a graded-index optical fibre plotted on graph paper gives a parabolic shape.
On the Basis of Material Used
1. Plastic Optical Fibre
As the name itself suggests, plastic optical fibre is made up of plastic. The core element of such fibres is typically constructed with the help of chemical substances such as polymethylmethacrylate.
2. Glass Optical Fibre
The strands of glass optical fibres are made up of fine glass.
On the Basis of the Mode of Propagation of Light
1. Single-mode Optical Fibre
Single-mode optical fibre is specifically designed to carry a single monochromatic ray of light. The diameter of the core of a single-mode optical fibre is approximately equal to 9 µm. Single-mode optical fibres are generally preferred for long-distance communication.
2. Multi-mode Optical Fibre
Multi-mode optical fibres consist of a core that is slightly wide in diameter as compared to single-mode optical fibres. The broad diameter of the core helps a multi-mode optical fibre to allow the propagation of more than one ray of light from the transmitter end to the receiver end. The diameter of the core of the multi-mode optical fibre is approximately equal to 50 µm or above. Multi-mode optical fibres are usually preferred for the short-distance transmission of signals.
Working Principle of Optical Fibre
Optical fibres typically work on the principle of total internal reflection of light. The term total internal reflection indicates that no part of the signal gets refracted or transmitted to the second medium, but instead, all of the signals is retained inside the fibre. The natural tendency of a ray of light, when it encounters a change in medium, is to slow down and get refracted. The change in the speed of the propagation of light before and after entering the medium can be represented easily with the help of the refractive index of the medium. The phenomenon of refraction of light is defined as the process of a slight bending of light at the interface of two different mediums having distinct values of refractive indexes. It should be noted that when the light travels from a medium of high refractive index towards a medium of low refractive index, the ray of light tends to get bend towards the interface. The bending of light towards the interface can be increased by increasing the refractive index of the second medium. This can be done easily by introducing dopants or selective impurities into the medium. If the refractive index of the medium is increased up to a value after which the refracted light does not go out, but instead, gets diverted towards the first medium, then one can say that total internal reflection of light has been achieved. The process of achieving the total internal reflection of light by increasing the refractive index of a medium is a tedious task. An alternate method of attaining TIR or total internal reflection is to increase the incident angle of light. The incidence angle at which the light gets reflected back inside the first medium is known as the critical angle. The signal incident at an angle greater than the critical angle tends to propagate from one end to the other by undergoing multiple reflections along the curved surface of the fibre.
Working of Optical Fibre
To communicate a piece of information between the transmitter and the receiver via optical means, a network of optical fibre cables is laid beneath the surface of the earth. The data that is required to be communicated is first translated into binary format. This binary coded data is then transmitted by the transmitter to the cellular base station in the form of electromagnetic radiations. The ‘ones’ contained by the data are represented by a high-frequency signal; whereas, the ‘zeroes’ are represented with the help of a comparatively low-frequency signal. The cell tower picks up the electromagnetic radiations transmitted by a cellular device or a transmitter. When a high-frequency signal is traced, a light pulse gets generated. Similarly, when a low-frequency signal is detected, a light pulse is not generated. The light pulses are transmitted to the receiver through the optical fibre cables. The light used in optical fibre communication is highly directional in nature. When such a ray of light is made to strike the inside of the fibre at an angle that is greater than the critical angle, then the light tends to undergo multiple reflections along the surface of the fibre. This allows the signal to propagate from one end of the fibre to the other end, thereby communicating information between the transmitter and the receiver end. The phenomenon of total internal reflection does not lead to degradation of the light quality, hence the signal can travel long distances without losing the data or the information. Also, the twisted shape of the cable has no effect on the quality of data contained by the ray of light that travels through it. The signal travelling through the fibre tends to undergo various losses and a certain amount of attenuation due to absorption and scattering of light radiation. The reconstruction of the lost signal is performed by adding amplifiers to the circuit. The amplifiers help boost the signal strength and allow long-distance communication of data.