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10.OPTICAL INSTRUMENTS eLearn.Punjab

The disadvantage of the step-index fibre (Fig. 10.18 a) can considerably be reduced by using a graded

index fibre. As shown in Fig. 10.18 (b), the different wavelengths still take different paths and are

totally internally reflected at different layers, but still

they are focussed at the same point like X and Y etc. It

is possible because the speed is inversely-proportional

to the refractive index. So the wavelength l1travels a

longer path than l2or l but at agreater speed.
3

Inspite of the different dispersion, all the wavelengths Fig.10.18
arrive at the other end of the fibre at the same time.
With a step-index fibre, the overall time difference may Light paths in (a) step-index and (b) graded-index
be about 33ns per km length of fibre. Using a graded fibre.
index fibre, the time difference is reduced to about 1
ns per km.

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