Optical fibres are those very fine long glass fibres which allow light signals to travel through. They are made of silica or silicon dioxide. To produce the fibres, molted solution of silica or silicon dioxide with other materials such as arsenic, qurtz etc., at 1900 degree Celsius are drawn into a cylindrical shape which have an inner diameter in the range of 10 to 50 micrometre (1 micrometre is one millionth of a metre) and outer diameter of 100-120 micrometre. Thus its geometry is that of a long cylinder but looks like a solid glass wire to unaided eyes.
Optical fibre consists of an inner cylinder of glass, called the core surrounded by a cylindrical shell of glass or plastic of lower refractive index called cladding. Fibres may be classified in terms of refractive Index profile of core and whether a single mode or many modes are propagating in it. If the core has uniform refractive Index, it is called a step index fibre and if the core has a non-uniform refractive index, gradually decreasing from the centre towards the outer region, it is called a graded index fibre.
When light enters one end of the fibre under right conditions most of the light will propagate or move down the length of fibre and exit from the far end while a small part of light will be absorbed or scattered by the cladding. Light enters one end of the fibre at a slight angle to the axis. As it encounters a surface with a different refractive index it suffers total internal reflection. For total internal reflection to occur at the fibre walls, two conditions must be met. The first is that the core must have a slightly higher refractive index than the cladding. Second, light must have an angle of incidence greater then the critical angle. The light so reflected bounces off the walls (repeated total internal reflections) and proceeds in a zig-zag path.
Now a days these fibres are playing a very important role in wire (line) communication systems. Such optical fibers offer some important advantages over wires. Light travels faster in glass than electric signals in the conventional metallic wires. Further on using monochromatic light (lasers) the signal distortion is low. Optical fibres have a high bandwidth (data carrying capacity) of the order of GHz. Consequently, they carry 100 million times more information and 100 times faster than telephone lines.
These optical fibres are very light in weight, easily twistable and have a low attenuation (powerloss and hence information loss) i.e. 0.5 decibels/kilometre (dB/km) which is approximate 10 times less than telephone cables. Since it is made up of cylindrical silica which is non conductive and non radiative, there is no possibilities of cross talk. Fibers are resistive to high temperature as the melting point of silica is very high i.e 1900 degree Celsius. Besides, the transmitted signals are optical and problems associated with sparking at the ends of cable are not encountered in this case.
Single mode fibres have bandwidth of more than 3GHz and mostly are use in submarine cable network. These fibres are less expensive. Graded index fibres have bandwidth of 200 MHz to 3 GHz and are used in telephone trunk. These are most expensive. Step index fibre has a bandwidth of less then 200 MHz and is mostly used in data link for computer network. These fibres are least expensive.
As mentioned above the following attractive features of optical fibres have changed the shape of communication systems:
In telecommunication systems, information, which can be either in the analog or digital form reaches the transmitter through a coder. The coder converts information into a sequence of pulses (bits). The transmitter is usually a semiconductor laser or LED which is modulated by an information bearing signal and converts electrical signals into light, usually in the near infrared (0.6 to 1.6 micrometre) region of electromagnetic spectrum. After passing through the fibre, this light reaches the receiver that converts optical signal into the information bearing signal. This electrical signal after demodulation in decoder produces the audio signal.
The various advantages of optical fibres over co-axial cables have led to their extensive use in high-speed communications. These fibres were introduced in the 1970s when global communication needs were increasing at a steady rate necessitating better and faster communication systems.
Optical fibres have also become indispensable in the medical field where they are used as viewing systems to see regions well within the body. Small bundles of fibres, not much larger than a hypodermic needle can easily be inserted into the spot of interest. This technique has led to key hole surgery which eliminates major incisions. Once a device of this potential is available, one can think of very many applications.