What will happen to light moving in a closed loop fiber-optic cable? Will it continue ad-infinitum?

Question

If a beam of light, for example sunlight or even a laser, is introduced into a closed loop of fiber optic cable, where there is no exit for the light, what would happen to it? Would the total length of the loop make a difference ie 3m of cable or 300km? Assuming total internal reflection would the light signal degrade and eventually stop?

Answer 1

yes the energy will eventually dissipate,it will do so in equal time in any length of loop.

Answer 2

All fiber optic cable has loss. Some of this is due to attenuation (scattering from imperfections in the glass) and some due to leakage (less than total reflection at the sides). Removing the leakage component, there would still be loss with distance and the light would continue to lose amplitude. However, FO cable comes in two types, multi-mode and single mode, which has an effect on how different types of light would behave in the loop.

Multi-mode cable tends to have higher losses. It could carry incoherent light (sunlight) in a loop. But with coherent light (laser) there is a speckle effect that will quickly destroy the coherency and reduce the amplitude as the light travels the cable. Generally, different length loops would be all the same (not exactly true, depending on the duration of the light pulse).

Single mode fiber will only carry coherent light and keeps that light coherent indefinitely. It has much lower losses with distance than multi-mode fiber (because it has much lower reflection losses). But loop length would have a major impact.

A short pulse of light (shorter than the loop length) in a single mode fiber would loop for a long time. But a long pulse (or continuous beam) of light would cause a different effect. When the light loops back onto the beam's entry point, it would interfere with itself (because it is coherent). The effect would be that the light would beat against itself, rising and falling in amplitude as the wave travels around the loop. If the loop length was EXACTLY a multiple of the light's wavelength, then the beam would continously gain in amplitude.

Answer 3

There is a small but non-zero loss in each reflection (absortion at the reflecting surface) and thus eventually the beam will die out in an exponential relation to the number of reflections. The length of the loop is not so important as the beam continues to go around the closed loop. It is the number of internal reflections and the material of the cable which decides the Number N for reaching a particular intensity level. Theoretically, N is infinite but practically our ability to detect the light limits N.

Answer 4

It will just fade away. The glass in the loop is not 100% free of impurities and will cause the pulse of light to degrade. there would also be a weld in the loop where the 2 ends have been joined together and this would also degrade the signal. The more times the signal passes through the imperfections the quicker it will disappear, you could therefore argue that it would vanish faster in the smaller loop as it will have to pass through the join more frequently.