Introduction

Fibre optics is being used to transmit television, voice, and digital data signals by light waves over flexible hair like threads of glass and plastic. It has evolved into a system of great importance and use since the 1980's. The advantages of fibre optics compared to coaxial cable or twisted pair cable, are endless. Millions of dollars are being spent to put light wave communication systems into operation, as a result of its performance.

Composition of optical fibre

* Silica based glass or plastic filaments are spun and packed into bundles of several hundreds or thousands. Bundles may be put together as rods or ribbons and sheets. * These bundles are flexible and can be twisted and contorted to conduct light and images around corners

* The thin glass center of the fibre where the light travels is called the "core". * The outer optical material surrounding the core that reflects the light back into the core is called the "cladding". * In order to protect the optical surface from moisture and damage, it is coated with a layer of buffer coating.

Cross section of a bundle

Operation of the fibre optic system

* Light is ejected into the glass core at the correct angle and transmitted; it will reflect back repeatedly with internal reflections, even when the rod is curved. Light cannot escape from a fibre optics cable. A bundle of rods of fibres is capable of taking an image projected at one end of the bundle and reproducing it at the other end.

Reflected path of light in the glass rod

Operation of the fibre optic system

* In a fibre optic system, there are a few major components to perform the task of communication. * The Input Modulator is needed; this modulates the incoming signal with a light beam. * A light emitting device is used; it can be either a light emitting diode (LED) or a semiconductor laser diode. * A fibre optic cable is used as a transportation medium. * A fibre optic system converts an electrical signal to an infrared light signal, and then transmits the signal onto an optical fibre. * An Output Modulator is used to separate the signal from the light beam. * Special connectors must be used to couple the light from the source to the fibre and from the fibre to the detector.

Advantages and Disadvantages

ADVANTAGES

* Fibre optic cables have a much greater bandwidth than metal cables.

* Fibre optic cable is less susceptible to signal degradation than copper wire. * Fibre optic cables weigh less than a copper wire cable.

* Data can be transmitted digitally.

* Lower-power transmitters can be used instead of the high-voltage electrical transmitters used for copper wires.

* Unlike electrical signals in copper wires, light signals from one fibre do not interfere with those of other fibres in the same cable.

* Because no electricity is passed through optical cable it is non-flammable, and immune to lightning.

* Impossible to tap into a fibre optics cable, making it more secure

Advantages and Disadvantages

DISADVANTAGES

* Fibre optics are that the cables are expensive to install.

* The termination of a fibre optics cable is complex and requires special tools. * They are more fragile than coaxial cable.

Analog and Digital Communication

* An analog signal changes continuously, while a digital signal can be at only a certain number of discrete levels.

* Conventionally, analog is used for audio and video communication.

* Analog technology has been used because our ears detect continuous fluctuations in sound levels, not just the presence or absence of sound.

* Going through a system where the analog signal can not be successfully reproduced outputs a distorted signal which is what happens when we get a distorted voice on the telephone or radio. * Digital transmission works the best; both analog and digital signals are found in audio and telephone systems.

* Analog signals are converted to digital before transmission and then back to analog signals.

Two main types of cables

Step Index Fibre * This cable has a specific index of refraction for the core and the cladding. It causes deformations due to the various paths lengths of the light ray. This is called modal distortion. It is the cheapest type of cabling. Within the cladding and the core, the refractive index is constant. Graded Index Fibre * In graded index fibre, rays of light follow sinusoidal paths. Although the paths are different lengths, they all reach the end of the fibre at the same time. Multimode dispersion is eliminated and pulse spreading is reduced. Graded Index fibre can hold the same amount of energy as multimode fibre. The disadvantage is that this takes place at only one wavelength.

Pulse Spreading

* Optical fibres that carry data consist of pulses of light energy following each other. The fibre has a limit as to how many pulses per second can be sent to it and be expected to emerge intact at the other end. This is known as pulse spreading which limits the Bandwidth of the fibre.

* The pulse sets off down the fibre with a square wave shape. As it travels along the fibre, it progressively gets wider and the peak intensity decreases.

Transmission Loss

* The transmission loss or attenuation of an optical fibre is perhaps the most important characteristic of the fibre; this determines if a system is practical. It controls (1) spacing between repeaters and (2) the type of optical transmitter and receiver to be used.

* As light waves travel down an optical fibre, they lose part of their energy because of various imperfections in the fibre. These losses are measured in decibels per kilometers (dB/km).

Interviews

Kushner, Jeff, Fibre Solutions Specialist, CORNING * Stated fibre optics is a revolution that may affect our lives as much as computers and integrated circuits have. Fibre optics is being compared in importance with microwave and satellite transmissions in the advanced world of communications.

Ali, Zafar, Configuration Supervisor, Unisys Canada Inc.* stated that fibre optics will make using devices that use services such as two-way television that was too costly before the development of fibre optics easier to use with better quality. In addition to an incredible bandwidth, fibre optics has smaller and lighter cables than conventional copper-conductor systems, with immunity to electrical noise, and numerous other advantages.

Conclusion

* The age of optical communications is a new era. In several ways fibre optics is a pivotal breakthrough from the electric communication we have been accustomed to. Instead of electrons moving back and forth over a regular copper or metallic wire to carry signals, light waves navigate tiny fibres of glass or plastic to accomplish the same purpose. * With a bandwidth and information capacity a thousand times greater than that of copper circuits, fibre optics may soon provide us with all the communication technology we could want in a lifetime, at a cost efficient price. * Any new communication system that does not use fibre optics, or consider its use, is obsolete even before it has been built. It is apparent that the average technician may also become superseded if he or she fails to master fibre optics. After all, the technician will be responsible for repairing and maintaining fibreÂ optic systems wherever they are used, not the engineer.

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