# Optical fibers questions

1. Jan 9, 2006

### dervast

Hi i have some questions concerning optical fibers.. I want to know how the single mode fiber works.. I know that the light in sinle mode fiber travels across the middle of the fiber.. I cant understand why if we bend a little the fiber why the light dont hit the walls of the fiber?
Thx a lot

2. Jan 9, 2006

### Integral

Staff Emeritus
The beam, will and does hit the sides. Optical fibers have a core and a cladding material which have Indexes of refraction such that when the beam hits the wall it will be totally reflected. A key word here is the Brewster Angle so some research on that. All optical fibers have a minimum bend radius, if you attempt to make a corner with a bend less then the minimum bend radius you will have high loss. As long as you stay within the specified bend radius you will maintain good power levels.

3. Jan 10, 2006

### Claude Bile

Integral, I think you mean the critical angle, Brewster angle has to do with the polarisation of reflected light.

Dervast, the conceptual picture of a single ray travelling down the centre of the fibre is an imperfect analogy as it does not consider the field nature of light. Ray analysis is really only applicable for fibres with many modes (about 8 or greater).

Claude.

4. Jan 10, 2006

### Integral

Staff Emeritus
Thanks for the correction Claude, this what happens when you dredge up 30yr old terms from your memory.

5. Jan 14, 2006

### dervast

Thx a lot but waht do u mean by "the conceptual picture of a single ray travelling down the centre of the fibre is an imperfect analogy as it does not consider the field nature of light."

Why the signle ray ignores the nature of light?? I cant understand that... When i send a ray of light in a specific direction the light will continue moving in that direction.. One guy told me as a reply to this that the signle ray is too small as we are concerning the size of earth... When we move on the surface we cant understand the curved earth because we are many times smaller than the earth .. But still i dont think that the example is good enough for that... Because the single optical ray travels to the "air" and is not using trhe ground for that..... Do u have to suggest me sth now?
Thx a lot

6. Jan 15, 2006

### dervast

Thx a lot guys but i want to learn and if u have sometime plz try to reply me... thx a lot really

7. Jan 15, 2006

### SaturnV

I'm certainly not overly knowledgable about physics (I'm a freshman engineer), but I do remember seeing something about fiber optics in a physics book at some point in time. If you're asking why the light doesn't just leave the fiber optic as it turns a corner here, I think the outer material has a very different refractive index, so it bends the light correctly to make the light follow the bend. If the radius is to low, it will exit the wall (can it be too high as well?). I at least some of this is correct, and answers your question.

8. Jan 15, 2006

### Claude Bile

Light is an electromagnetic field, so in order to obtain a complete description of light in an optic fibre, you need to obtain a description of the E and B fields in the fibre, which is done by solving Maxwell's equations. The ray analogy only considers the direction of propagation of the light.

For example, the shape of the guided mode in a single-mode fibre is a Bessel function, which is well approximated by a 2D Gaussian curve (this is easily verified through experiment). This can only be derived using Maxwell's equations, ray analysis alone cannot give this result.

P.S. I would disregard what this 'one guy' said, I don't think he really understood your question to be perfectly honest.

Claude.

9. Apr 27, 2011

### vardhan_harsh

we know that the cladding material is of lower refractive index than the inner core's refractive index.but what if instead of outer cladding glass material, air is used(which too has a lower refractive index)?would it also not cause TIR inside the core.