Optical Fibre

Optical Fibre
It is a glass fibre of diameter around 125um of high refractive index in the core coated by a glass with low refractive index. Now a ray of light incident on one end will emerge out through other end suffering multiple total internal reflection without losing energy.

Types of optical fibre
1) Monomode: Optical fibre in which inner glass rod is thinner as compared to outer coating
2)Multimode: Optical fibre in which inner glass is thicker as compared to outer coating
on the basis of refractive index of glass used
1)Step index: Optical fibre which consist of glass rod of high refractive index of 1.52 in core and glass of refractive index 1.48 in coating
2)Graded Index: Optical fibre which have glass of high refractive index goes on decreasing gradually from core to surface.

Advantage of Graded index
In case of step index when a wave enters into the glass rod suffers disperson and so a wave will be total internally reflected from different point as shown in figure so that they will come out from other end in different time. But in case of graded index the wave after dispersion will be totslly internally reflected from diffrent height as shows in figure. When they suffer total internal reflection on lower side they will converge again in single point at different place so that single can be obtained at a single time from other end.
The minumum angle of incidence in air which makes total internal reflection possible in glass is called acceptance angle.
We have for total internal reflection
((meu))=1/(sinC)==>sinC=1/((meu))
This implies
sinC = ((meu1))/((meu2)) ------1
let us consider an incident ray be incidenting in glass making angle of incidence in air i such that ((meu1)) >((meu2)), total internal reflection r = 90 - 2
((meu2)) = sini/sinr = sini/sin(90-c) = sini/cosC
cosC = sinI/((meu2))--------------2
squaring and adding 1 and 2 will give i=sin(inverse)*sqrt((meu1)square - (men2)square)) This is the value of minimum angle of acceptance

Use of optical fibre
1 It is used to transfer electronic data or signal
2 It is used in telecommunication
3 It is used for direction signalling
4 It is used in medical purpose as optical pipe
5 It is used in computer networking

Advantage if optical fibre
1 It is fast and we can transfer signals without loss of much energy
2 It is cheap to be used for long distance
3 Repitors can be kept at about 35 km whereas they must be kept at about 1 km in other cables
4 It is most secured
5 It has very large bandwidth from 10^13 hz--10^16Hz

Disadvantage of optical fibre
1 It is expensive for short distance
2 Highly skilled manpower is needed for its maintenance