23-11-2012, 01:14 PM
Transmission characteristics of Optical fibers
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ATTENUATION
Light travelling in an optical fibre loses power over distance. The loss of power depends on the wavelength of the light and
on the propagating material. For silica glass, the shorter wavelengths are attenuated the most (see Fig.).
The lowest loss occurs at the 1550-nm wavelength, which is commonly used for long-distance transmissions.
Numericals
Calculate the maximum transmission disance for a fiber link with an attenuation of 0.5 dB/ km if the power launched is 1mW and receiver sensitivity is 50 μW. Solution: 26 km
A 15 km optical fibre link uses fiber with a loss of 1.5dB/km. The fiber is jointed every kilometer with connectors which give an attenuation of 0.8 dB each.Determine the minimum mean optical power which must be launched into the fiber inorder to maintain a mean optical power level of 0.3 micro Watt at the detector
Solution:703.
Absorption Losses in silica glass fibers
This type of loses occur due to Material composition and the fabrication process of the fiber which results in dissipation of some transmitted optical power as heat in the waveguide
Two types:
Intrinsic Absorption – Caused by interaction with one or more of the major components of glass
Extrinsic Absorption- Caused by impurities within the Glass
Intrinsic Absorption
Less significant than extrinsic absorption. For a pure (no impurities) silica fibre a low loss window exists between 800 nm and 1600
Intrinsic absorption is very low compared to other forms of loss
It is for this reason that fibres are made up of silica and optical communications systems work between about 800 to 1600 nm.
Intrinsic absorption results from electronic absorption bands in the UV region and from atomic vibration bands in near infra-red region.
Causes of Micro-bending
Micro deformation of fiber axis
Mechanical Stress applied on the fiber resulting in micro convexities during the cabling process.
As a result of this when light strikes at this deformation ,the condition of Total Internal Reflection is not met and so some portion of the beam will be leaked into the cladding.