Refractive index of optical fibre

[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!

 

[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.

 

[Misr]

I still don't understand..

 

[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?

 

[Misr]

What is not understandable for you?

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

 

[Vanadium 50]

Draw the picture above assuming the cladding has higher n.

 

[Misr]

Draw the picture above assuming the cladding has higher n.

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

 

[xts]

I don't understand the function of the cladding

It is needed to reflect the beam of light back to core - as it is shown on the picture you posted.
See: http://en.wikipedia.org/wiki/Total_internal_reflection

 

[Vanadium 50]

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

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.

 

[Misr]

It is needed to reflect the beam of light back to core - as it is shown on the picture you posted.

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?

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.

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

 

[Misr]

and more optical density

 

[sankalpmittal]

[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!

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.

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?

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.

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

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 !

:)

 

[xts]

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?

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.

 

[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.

 

[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 traveled by it. Paradox?

 

[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'.

 

[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?

 

[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 :(

 

[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.

 

[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.

 

[Misr]

mechanically and optically stable

what do u mean by "mechanically and optically stable"?

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.

what do u mean by a drop of condensation?

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.

I guess so Sophie, I already fixed this misconception but anyways tell me the no. of post you are talking about

 

[xts]

what do u mean by "mechanically and optically stable"?

I mean that it is mechanically impossible to suspend the fibre in the air such, that it doesn't touch anything.

what do u mean by a drop of condensation?

Drop of water condensating on the surface. Or drop of rain. Or a grain of dust. Or bird poop. Or any other dirt, which would stick to unprotected fibre hanging in the air.

 

[sophiecentaur]

I guess so Sophie, I already fixed this misconception but anyways tell me the no. of post you are talking about

Post No. 18
"so what is the need of a cladding of lower refractive index than the that of the core?"

 

[Misr]

According to this ,the function of cladding is NOT
to prevent some of the light which falls at an angle less than the critical angle from refracting to air out of the core because the critical angle is already the lowest with air,but it gives the fibre protection against the surrounding conditions
Have I got it?right?

 

[Misr]

According to this ,the function of cladding is NOT
to prevent some of the light which falls at an angle less than the critical angle from refracting to air out of the core because the critical angle is already the lowest with air,but it gives the fibre protection against the surrounding conditions
Have I got it?right?

 

[Misr]

According to this ,the function of cladding is NOT
to prevent some of the light which falls at an angle less than the critical angle from refracting to air out of the core because the critical angle is already the lowest with air,but it gives the fibre protection against the surrounding conditions
Have I got it?right?

 

[xts]

1. The role IS to prevent the light from falling out - ensuring that total internal reflection may occur (although if the core would be surrounded by other medium than glass cladding the critical angle could be bigger) and occurs predictably. The light travels through a fiber at very low angle to its axis - so a bit smaller or bigger critical angle is not an issue. If the light travels 100km along the fiber surrounded by cladding of refractive index = 0.95 that of the core, TIR is still assured - no light at all is loss due to leaking through the surface. Exactly the same, as if the core would be hung, without use of any suspenders, in the perfectly clean air.
But contrary - even a little bit of any kind of dirt stick to the core would cause the light leaking (or being absorbed)
2. Stabilize the core mechanically (and also environmentally - protecting its surface from corrosion, scratches and dirt) and separate it from other objects by distance many times bigger than wavelength (which is needed for lossless TIR)

 

[Misr]

But contrary - even a little bit of any kind of dirt stick to the core would cause the light leaking (or being absorbed)

How could this happen?does the refractive index of the dirt higher than that of the core?

 

[Misr]

The light travels through a fiber at very low angle

you mean high??hmm that's confusing

 

[Misr]

The light travels through a fiber at very low angle

you mean high??hmm that's confusing

 

[sophiecentaur]

@Misr.
Forget optical fibres. Just look up Smell's law, refraction and TIR. I think you need to think a bit harder about this and not just to argue that you are 'not wrong'.

 

[sophiecentaur]

@Misr.
Forget optical fibres. Just look up Smell's law, refraction and TIR. I think you need to think a bit harder about this and not just to argue that you are 'not wrong'.

 

[Misr]

I just meant that the angle should be great(more than the critical angle)to reflect :(
I know about Snell's law,refraction and TIR

 

[sophiecentaur]

@Misr.
Forget optical fibres. Just look up Smell's law, refraction and TIR. I think you need to think a bit harder about this and not just to argue that you are 'not wrong'.

 

[sophiecentaur]

You need--
1. TIR
2. To avoid anything touching the fibre that could stop TIR.

Achieved by a low index cladding.

 

[xts]

How could this happen?does the refractive index of the dirt higher than that of the core?

No. Read (e.g. wiki: http://en.wikipedia.org/wiki/Evanescent_wave) about TIR mechanism or very good explanation in Feynman Lectures on Physics.
The light is not reflected "sharp" on the boundary between media. The light enters to the lower index medium on some depth (order of magnitude its wavelength). If the lower index medium is not transparent, the light gets absorbed or scattered, rather than reflected.

It is a perfect demonstration: you send a laser beam through a glass prism such, that it is reflected by TIR. Then you blow some cigarette smokeinto the area behind the "reflecting" surface. You may see the smoke glowing, and if you have precise photometer, you may also notice that reflected beam gets a bit dimmer.
Similar demonstration is with total reflection between glass and water - then you add some milk or ink to the water: in both cases the reflected beam gets dimmer to such extend you may see it by naked eye.

you mean high??hmm that's confusing

Almost parallel to fibre axis. Even in multimode fibre the angle is much lower than 1°.

 

[xts]

And one more school demonstration: take a glass prism (45°/45°/90°) and look through shorter side - then you see reflection by TIR of the object behind other short side.
Then touch the long side (reflecting) with a finger - you now may see very clearly the fingerprint mixed with the reflection of what you were watching. That visible darkening of the reflected light in some places is caused by light loss on so small amount of dirt behind reflecting surface, as a grease left by fingerprint.

 

[sophiecentaur]

There is energy 'in' the low index medium. But it's in the form of an evanescent wave, which doesn't propagate. A lossy medium will absorb this energy, right in close to the interface.

 

[Misr]

Almost parallel to fibre axis. Even in multimode fibre the angle is much lower than 1°.

I think you mean that the angle of incidence is very big

 

[Misr]

A read another explanation for the function of the cladding:
The cladding is of higher refractive index than air and lower than that of the core
so if some light rays fell at an angle less than the critical angle,it will refract to the cladding and the cladding will reflect them back to the core
Is that true?

 

[Misr]

ok one more question,
can light penetrate the cladding?what happens in this case?

 

[xts]

@Misr,
it would be really great, if you read answers you already got to your questions before asking the same question again.
Be so nice and read answers #36 and #38, then follow a link to wiki you got.