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Friday, November 16, 2012

Fibre optics

Since optical frequencies are extremely large in comparison to radio waves and microwaves, signals transmitted through a light wave is capable of carrying more information than conventional radio-waves and microwaves.In the near future, the demand for information will be so high that only a technology based on data transmission vide light waves will be able to meet the demand.Indeed, British Telecom in 1984 stopped laying copper cables and started using Fiber optic cables on its long distance routes.
Advantages of fiber optic cables over metallic cables
1. Higher Bandwidth
2. Less susceptible to interference
3. Much thinner and lighter
4. Allows digital transmission
5. Low attenuation(0.2 dB/km) , as a consequence of which distance between two consecutive repeaters could be as large as 250 km.In comparision , copper cables have repeater stations every few kilometers apart.
Fiber optic cable types :
1. Single mode step index fiber
3. Multimode step index fiber
3. Multimode graded index fiber
Difference between fiber cable types
In single mode step index optical fibers, core dia is ~ 10um, and the refractive index chnages from low RI of the core  to high RI of the cladding at the core cladding interface.These cables allow only single mode of the signal to propagate through them.In addition these cables are non dispersive,  and offer low loss and higher BW.They are suitable for long distance transmissions, such as in WANs.
On the other hand, multimode cables have five times larger diameter,  starting at 50um and they allow over 100 modes to propagate through them. These cables are dispersive, and lower in BW offer high loss. So they are suitable for data sharing in SAN or LANs.The only construction difference between multimode step index fiber and single mode step index fiber is the size of the core diameter.But in the multimode graded index fiber, RI index profile changes in a parabolic manner from the centre to the periphery of the core.This compensates the dispersion effects in MMSI fibers and enhances their BW.
Nature has already used this technology
It is interesting to note that the retina of human eye consists of a large number of rods and cons which have the same kind of structure as the optical fiber. They consists of dielectric cylindrical rods sourrended by another dielectric of slightly lower refractive index. The core diameters are in the range of a few microns. However, the technology is little more advanced, as the light absorbed in these light guides generates electrical signal, which are then transmitted to the brain through various nerves.[1]

reference :
1.Optics(2nd Ed), AjoyGhatak, P655, Tata McGraw Hill (2003)

1 comment:

  1. Hey thanks for sharing those advantages of fiber optic cables. It was nice reading this post.


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