How does Total Internal Reflection in a mm/sm fiber work?

[TinaTina]

I know Snell's Law and I do understand that there are single and multimode fibers. A fiber carries light hence to total internal reflection. so far so good. But is it only a single mode fibre that works with total internal reflection? And if so, how does the multimode fibre carry light then?

 

[jambaugh]

Here is what I understand about the general principles of fiber optics. I am no expert on the engineering aspect of them or properties of specific commercially available products.

We're talking wave-guides here and for the higher order modes the effective angle at which the plane waves (or photons if you wish) impinge upon the fiber walls is greater. (until you have 90deg to the wall for standing waves). Thus higher modes will be reflected less and escape more readily when the fiber bends. Whether you are modeling fibers as internally reflecting or whether there is axially focusing refraction due to radial difference in index of refraction this should be the same issue with higher modes. Above some order, the fiber just can't retain that mode of the light at that frequency. How high a mode order can be sustained would be a function of the fiber's material properties and diameter and I imagine that need not exclude several of the lowest order modes in some cases.

 

[jambaugh]

Here's a good page I found on the web: https://www.rp-photonics.com/waveguides.html

 

[TinaTina]

Thank you, jambaugh. That was helpful!