Refractive index of optical fibre

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Discussion Overview

The discussion revolves around the refractive index of optical fibers, specifically why the core has a higher refractive index than the cladding. Participants explore the implications of this difference for total internal reflection, data transmission, and the physical properties of the materials used in optical fibers.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants assert that the core's higher refractive index is necessary for achieving total internal reflection, which is crucial for effective data transmission.
  • Others explain that different types of glass are used for the core and cladding to create the desired optical properties.
  • There is confusion among participants regarding the function of the cladding, with some questioning why it must have a lower refractive index than the core.
  • Some participants suggest that the cladding serves to prevent light from escaping the core and to maintain the integrity of the optical fiber by protecting it from external factors.
  • A few participants highlight the importance of the optical quality of the cladding for maintaining total internal reflection.
  • Concerns are raised about the implications of surrounding the core with air versus a stable medium, with some arguing that practical environments necessitate a stable cladding material.

Areas of Agreement / Disagreement

Participants express a range of views on the necessity and function of the cladding, indicating that there is no consensus on some aspects of the discussion. Confusion persists regarding the implications of refractive indices and the role of the cladding in optical fibers.

Contextual Notes

Some participants mention the critical angle and total internal reflection without fully resolving the mathematical or conceptual details. There are also references to practical considerations in optical fiber design that remain unexplored.

  • #31
@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'.
 
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  • #32
@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'.
 
  • #33
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
 
  • #34
@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'.
 
  • #35
You need--
1. TIR
2. To avoid anything touching the fibre that could stop TIR.

Achieved by a low index cladding.
 
  • #36
Misr said:
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.
Misr said:
you mean high??hmm that's confusing
Almost parallel to fibre axis. Even in multimode fibre the angle is much lower than 1°.
 
  • #37
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.
 
  • #38
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.
 
  • #39
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
 
  • #40
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?
 
  • #41
ok one more question,
can light penetrate the cladding?what happens in this case?
 
  • #42
@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.
 

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