# What are optic fibres/cables in simple words?

1. Feb 10, 2018

### physea

I suppose there's tons of information about optic fibres, but what are they really, their functions, their limitations, pros and cons?

2. Feb 10, 2018

### Staff: Mentor

3. Feb 10, 2018

### physea

So basically optic fibers are hollow cables with mirror-like inner surfaces that reflect light so that light is transmitted?

Are there losses? Are they used for information transfer?

4. Feb 10, 2018

### NFuller

I guess you could think of it that way. The more technical description is that fiber optic cables are a type of waveguide (https://en.wikipedia.org/wiki/Waveguide) for visible light.
Yes, but they are low for good cables. I think I've seen cables where the loss is around 1dB/km.
Of course. Telecommunication companies use these for internet traffic.

5. Feb 10, 2018

### physea

Thanks
Other applications?

6. Feb 10, 2018

### Staff: Mentor

The wiki article has a long section on applications.

7. Feb 10, 2018

### Staff: Mentor

8. Feb 10, 2018

### physea

Interesting, from that value, can we save the % loss for 2500km? or per km?

9. Feb 10, 2018

### NFuller

I'm not sure what you mean by saving a percent loss. Do you mean how much light intensity will be lost over a 2500km cable?

10. Feb 11, 2018

### blue_leaf77

For the ones used in actual applications, not quite so. It's true that a hollow cable having reflective inner surface can also transmit light from one end to the other but such scheme proves to suffer from high amount of loss per unit distance (I either read it somewhere or heard it from someone else, too bad it was long time ago I can't remember the source). In reality, generally speaking optical fibers take the form of a coaxial dielectric and flexible tube tube. It is coax because it consists of at least two 'layers'. The innermost tube is called core and the outer one is called cladding. One of the condition for it to be able to guide light is to have the core having higher refractive index than the cladding. In this way, rays hitting the core-cladding interface will undergo total internal reflection provided the incoming angle is bigger than the critical angle between the two media. As the others have mentioned, there is a wealth of information on optical fiber and a forum thread is by no means sufficient to even cover the basic, you will need an introductory textbook on this topic at the very least. As subfields of optics, optical fibers and lasers pose the same level of breadth and development.

11. Feb 11, 2018

### physea

Yeah sorry typo. I mean how much % will be lost.

12. Feb 11, 2018

### NFuller

Over 2500km, using the example given, 2.5dB of the signal is lost. The fraction $f$ of power transmitted is then
$$f=10^{-2.5/10}=0.56$$
So the fraction of power lost is $44\%$.

13. Feb 11, 2018

### physea

Mmm very interesting.
Do we know how much light energy is released per gram of petrol or natural gas burned?

14. Feb 11, 2018

### NFuller

I don't know, you will probably have to do some googling to find the answer. Since this question is not directly related to the original question though, it is probably more appropriate in a separate thread.