Transmission of light by optical fibres is not 100% efficient. Some light is lost, causing attenuation of the signal. Several mechanisms are involved, including absorption by materials within the fibre, scattering of light out of the fibre core and leakage of light out of the core caused by environmental factors. The degree of attenuation depends on the wavelength of light transmitted. Attenuation measures the reduction in signal strength by comparing output power with the input power. Measurements are made in decibels (dB), a very useful unit. A decibel is a logarithmic unit measuring the ratio of o/p power to I/p power. Loss in decibels is defined as
dBloss = -10 X log10 (POWER OUT /POWER IN),
Thus, if O/P power is 0.001 of I/P power, the signal has experienced a 30db loss. The minus sign is added to avoid negative numbers in attenuation measurements. It is not used in systems where the sign of the logarithm indicates if the signal has decreased (minus) or increased (plus). Each optical fibre has a characteristic attenuation that is measured in decibels per unit length, normally decibels per kilometer. The total attenuation (in decibels) in the fibre equals the characteristic attenuation times the length.
The basic attenuation mechanisms in a fibre are absorption, scattering and radiative losses of the optical energy. Absorption is related to the fibre material, whereas scattering is associated both with the fibre material and with structural imperfections in the optical waveguide. Attenuation owing to radiative effects originates from perturbations (both microscopic and macroscopic) of the fibre geometry.
There are various advantages, which make Optical Fibres better than Copper systems. The crucial operating difference between a fibre optic communication system and other types is that signals are transmitted as light. Conventional electronic communication relies on electrons passing through wires. Radio frequency and microwave communications rely on radio waves and microwaves travelling through open space. The major points which distinguish it on positive side are:
The most important optical measurement for any transparent material is its refractive index (n). The refractive index is the ratio of the speed of light in a vacuum to the speed of light in the medium (n= Cvac / Cmat). The Light always travels more slowly in a material than in vaccum, so the refractive index is always greater than 1.0 in the optical part of the spectrum. In practice R.I is measured by comparing the speed of light in the material to that in the air rather than in a vaccum.
Light is bent as it is passes through a surface where the refractive index changes- for example, as it passes from air to glass. The amount of bending depends on the refractive indices of the two media and the angle at which the light strikes the surface between them. The angles of incidence and refraction are measured not from the plane of the surface but from a line normal (perpendicular) to the surface. The relationship is known as Snell's Law, which is written ni sin I = nr sin R where ni , nr are the refractive Indexes of the initial medium and the medium into which the light is refracted, and the medium into which the light is refracted, and I and R are the angles of incidence and refraction respectively.
A mode is a stable propagation state in an optical fibre. If light travels through an optical fibre along certain paths, the electromagnetic fields in the light waves reinforce each other to form a field distribution that is stable as it travels down the fibre. These stable operating points (standing waves) are modes. If the light tries to travel in other paths, a stable wave will not propagate down the fibre-thus no mode.
Cabling is the packaging of optical fibres for easier handling and protection. Uncabled fibres work fine in the laboratory and in certain applications such as sensors and the fibre optic systems for guiding missiles.However like wires, Fibres must be cabled for most communications uses.The major reasons for the same are:
The major types of environment for optical cable can be loosely classified as follows:
The wide variety of fibre optic systems that have come into use because of the advantages of optical fibres are discussed as under :
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