Most electronic communication was carried out with the help of copper cables. In recent years, a new medium called optical fibres (OF) has been introduced. In OF communication, light signals are used instead of electrical signals.
An OF is a transparent rod usually made of glass or plastic through which light can propagate. A modern fibre consists of an optical rod core coated with cladding. The refractive index of the core is higher than that of the cladding. And one more thing, the light used is coherent light not the ordinary light.
CLASSIFICATION OF OPTICAL FIBRES
OF can be classified in to three groups according to the way light propagates down the fibre core.
- Multimode step index fibre
- Single mode step index fibre
- Multimode graded-index fibre
The mode refers to the path the light will travel through the fibres.
CHARACTERISTICS OF FIBRES
The characteristics of optical fibre depend on the type of material used and the physical size and shape.
The major causes of light losses in fibre are as follows
- Material losses: it is due to the absorption of light by the fibre material.
- Light Scattering: Light scattering occurs due to the structural imperfections and impurities.
- Waveguide and bend losses: These are caused by the imperfections and deteriorations of the fibre structure which cause radiation of light away from the fibre.
OPTICAL FIBRE CABLE
Cabling improves the mechanical characteristics of a fibre without causing deterioration of its optical properties. The outside protection of the optical fibre has the following features.
- It gives mechanical strength to the optical fibre
- It protects the fibre against breakage and damage
- Protect the fibre from excessive bending
- Permits easy field installation and maintenance.
FABRICATION OF OPTICAL FIBRE
For produce fibres directly, a method called “double crucible method” is used. In this method molten core glass is placed in the inner vessel and the molten cladding –glass in the outer vessel. When two molten glass es come together at the base of the outer container they form a glass cladded core. This molten mixture is pulled in to a fibre. Both step index and graded index fibres can be produced by this method. Graded fibres can be produced by allowing the core and cladding glass to interdiffuse after they come together. The diffusion causes a gradual change of refractive index between that of the core glass and that of the cladding glass.
FIBRE OPTIC COMMUNICATION
In most fibre optic communication systems, information is carried as intensity modulated light.FO communication is mostly as digital and the transmitted light intensity is either high (ON) or low(OFF) because only two levels are recognized. Noise immunity is much higher for digital transmission than that of analog transmission. Also the systems have short response time.
ADVANTAGES OF OPTIC FIBRES
Optic fibres are made of either glass or plastic. The basic material for glass fibre is silicon dioxide (SiO2) which is cheap and plentiful. The optic fibre has greater information carrying capacities than metallic conductors. Fibers and fibre cables are very strong and flexible. The addition of a plastic sheath increases the tensile strength of a fibre transmission line. One of the most important advantages of fibres is that they can carry large amount of information in either digital or analog form. Fibre offers a degree of security and privacy. No cross talk occurs.
The negative part of the optical fibre is the optic connector, its cost is high, its loss is also high and its installation is time consuming.
APPLICATION OF FIBER OPTIC COMMUNICATION
- The small size and large information carrying capacity of optic fibers is much higher than that of copper twisted pair cables in telephone systems.
- Repeaters can be spaced very far apart in fibre systems, that means continues passive links more than 100km long have been produced.
- OF communications are very compatible with electrified railways because they are not used by EMI.
- OF can be used in applications such as broadcast television and cable TV, remote monitoring and surveillance.
- Military applications include communications, command and control links on ships and aircrafts, data links for satellite earth stations and transmission lines.