# Refractive index of optical fibre

1. Sep 5, 2011

### Misr

[URL]http://www.fiberoptics4sale.com/Merchant2/fofs_img/how-optical-fiber-works.gif[/URL]

The refractive index of the core is higher than that of the cladding,
I don't understand why!

Last edited by a moderator: Apr 26, 2017
2. Sep 5, 2011

### xts

Because those are purposely made of different kinds of glass.
Purposely == to achieve such internal reflections like shown on your picture, which is desirable from data transmission point of view.

3. Sep 5, 2011

### Misr

I still don't understand..

4. Sep 5, 2011

### xts

What is not understandable for you?
Total internal reflection?
Fact, that different kinds of glass have different refractive indices?
The purpose to utilise total internal reflection to keep light within the fibre?

5. Sep 5, 2011

### Misr

I don't understand why the refractive index of the cladding must be lower than that of the core

6. Sep 5, 2011

Staff Emeritus
Draw the picture above assuming the cladding has higher n.

7. Sep 6, 2011

### Misr

I don't know how to do this because I don't understand the function of the cladding

8. Sep 6, 2011

### xts

9. Sep 6, 2011

Staff Emeritus
You don't need to know the function. All you need to know is what happens when light passes from a medium of one index to a medium with different index.

10. Sep 6, 2011

### Misr

The core alone can do that without the need to have this cladding I suppose
I think It just prevents some of the light which falls at an angle less than the critical angle from refracting to air out of the core right?

a medium of higher refractive index is the one of less critical angle and less light speed I think so how could this be helpful??

Thanks very much

11. Sep 6, 2011

### Misr

and more optical density

12. Sep 6, 2011

### sankalpmittal

Short answer : This is because cladding is made of thick plastic or glass which has low refractive index whereas core in center is made of material with high refractive index .
This explains the reason why there(core-cladding boundary ) is more deviation and total internal refraction taking place . Realize that material core has greater critical angle too.

Another reason : Realize that cladding surrounds core is made such that the optical density in uniformly distributed throughout whereas the optical density of core is concentrated at center . So When light enters core at the boundary of separation it deviates more and realize internal reflection take place from dense to rare medium and the angle which it makes is critical angle.

Yes , because at critical angle it will turn 90 degrees . Cladding also helps in preventing light to be internally reflected as some of deviates at air-cladding boundary too , I think. Realize cladding has low critical angle.

Yes , see
critical angle is
1/2(mew)1
so ,
if refractive index is high then critical angle is that less .
This is helpful because less critical angle is more refractive index , right ? So if refractive index will be more speed in medium relative to light will be that less , and hence more will it deviate towards the normal . This will further help to control internal reflection and light re-propagation to air.

Cheers !

:)

Last edited by a moderator: Apr 26, 2017
13. Sep 6, 2011

### xts

More or less, but not quite. It defines the critical angle. If the core would not be surrounded by cladding, then it would have to be surrounded by something else: at worst cheap PVC protecting the fibre mechanically. Refractive index of something like that is not well defined, thus the optical transmission properties of such fibre would not be stable. If the surrounding material (cheap PVC) would be not transparent, but even slightly opaque, you'll still experience some loss of power on every reflection. Thus it is crucial to surround the core fibre by a cladding made of high quality glass: nearly perfectly transparent (not much less than core fibre itself), uniform, and having stable refractive index.

Other issue is that glass cladding around glass core (made of different kind of glass, but of similar mechanicaal and chemical properties) is one of the most efficient possible protective layers for the core - protecting it from mechanical stress and chemical deterioration.

Your drawing is not to the scale. Cladding is much thicker - mostly because of mechanical protective reasons. For typical telecommunication fibres core has diameter 8μm, while cladding has 125μm. For multimode fibres used for computer transmissions on short distances core has 50μm, and cladding 125μm.

14. Sep 6, 2011

### sophiecentaur

If you didn't have the cladding then dirt and other stuff could get onto the surface of the inner fibre and allow light to escape through the side, if its refractive index happened to be higher than the fibre. Also, as stated earlier, it keeps the surface 'shiny' (producing specular reflections) by keeping it corrosion free.
The "function" of the cladding is to provide a well defined refractive index outside the whole of the length of the fibre.

15. Sep 6, 2011

### xts

Optical quality of the low-refractive medium (cladding - the one from which the light is reflected) is paradoxically very important for total internal reflection.
You may make two simple experiments:
1. take a glass prism, and use it to watch some object reflected by total internal reflection (outer medium is air). Touch external surface of the prism with your finger. As watching reflected light you'll see very clear fingerprint.
2. use the glass prism and a bowl of water to see the total reflection on a boundary glass-water. Now add some ink to the water and watch how it affects the reflection of the light.

The light do not leave the glass prism. It is totally reflected on its surface. But it is visibly affected by opacity or a little grease in the area which (as you think) is never travelled by it. Paradox?

Last edited: Sep 6, 2011
16. Sep 6, 2011

### sophiecentaur

There is no "paradox" here. Snells law accurately predicts the critical angle. The Abbee Refractometer works on this principle and can be used for detecting subtle changes in refractive index. Rather than being surprised by what happens with the added ink, you should see that this is a more sensitive test of concentration than the apparent 'inkiness'.

17. Sep 7, 2011

### Misr

the refractive index of air is nearly the smallest of all,so what is the use of the cladding of lower refractive index than the core???

18. Sep 7, 2011

### Misr

the refractive index of air is the least of all,so what is the need of a cladding of lower refractive index than the that of the core?I'm still very very confused :(

19. Sep 7, 2011

### xts

As we already told you: The reason is that you can't have your core fibre surrounded by air in any practical environment. You must surround it with something mechanically and optically stable.

20. Sep 7, 2011

### sophiecentaur

@Misr
It is true that the critical angle is lowest with air on the outside but using a glass cladding still gives a usable value of critical angle. You still get TOTAL reflection beyond that. Just a drop of condensation on the outside of an unclad fibre could stop it working.

btw, do you fully appreciate the phenomenon of TIR? Your last post suggests that you may have got things round the wrong way???? The cladding has to have a lower refractive index or it won't work at all.