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Optical Fibre Cladding Question

Why is the critical angle for the cladding much larger than in the optical fibre? Wouldn't that just mean light is refracted out of the fibre?

Reply 1

Andrew28913

The cladding has a greater density.

Reply 2

Qos

OP

so how does that keep light from refracting out?

Reply 3

teachercol

9

The core has a higher density. Otherwise you dont get TIR.

sinc = 1 / n

so the higher the vaue of n, the smaller the critical angle

Reply 4

Andrew28913

My god, he's right! :eek:

Guess I should do some more revision :p:

Reply 5

Qos

OP

teachercol

The core has a higher density. Otherwise you dont get TIR.

sinc = 1 / n

so the higher the vaue of n, the smaller the critical angle

Yes, I understand why the core has to have a high density. But why does the cladding that protects it have a LOWER refractive index? (because this would gave a larger critical angle, so surely the light would refract out the optical fibre?)

Reply 6

Hasan

2

You are right, the critical angle is increased with cladding.

If it wasn't cladded, then more rays would go through the fibre. However, because the critical angle is lower, some rays pass right through but some reflect a lot. This means that some rays reach the other end quicker and some slower, which makes the signal worse.

Therefore, with cladding, the critical angle is increased so fewer rays pass through, but they reach the end at the same time, so the signal is clearer.

I hope that makes sense, its really hard to explain :smile:

Reply 7

F1 fanatic

16

Qos

Yes, I understand why the core has to have a high density. But why does the cladding that protects it have a LOWER refractive index? (because this would gave a larger critical angle, so surely the light would refract out the optical fibre?)

think of it as lesser of two evils. Yes your critical angle is higher than if it was to air directly, but you keep the surface clean and don't have problems of light leaking out through scratches or damage. Remember most these fibres are run underground.

Reply 8

badeninc

2

The cladding gives it another chance at refraction and also helps to protect the optical fibre against scratching

Reply 9

NavalChicken

As Hasan said, it prevents many of the rays passing through at a lower angle, which are ultimately slower (due to greater reflection). Giving a lower output signal due to "smearing".

A standard text book should give you a full explanation. Look under "multipath dispersion"

Reply 10

Mehh

8

Perhaps one issue noone seems to have addressed is that without the cladding fibre can not be handled. Any point of the fibre which is in contact with anything absorbative of light would simply bleed off the light at the point it made contact with the fibre.
The fibre would have to be free standing and with the length of several kilometers that is a complete utter impossibility.
You could also not pack the fibres as they do so often together as any point where two fibres touched would leak into each other.

You should also know that Hasan is right. The highest quality fibres have the smallest possible critical angle, these are called single mode fibres, (the higher angles take up the higher spatial modes).

Optical Fibre Cladding Question

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