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-   -   Sending polarerized light through light fiber that is all jumbled up. (http://www.physicsforums.com/showthread.php?t=651292)

Spinnor Nov11-12 06:30 AM

Sending polarerized light through light fiber that is all jumbled up.
 
Say we have laser light propagating in the +z direction and polarized in the x direction that then enters a fiber optic cable. Let the fiber optic cable, of some length, be quite jumbled up so that the light photons are guided in a crazy path, kind of like a jumbled up ball of string. Let the twisting and turning of the fiber cable be gental so that the light propagates forward with little or no reflection or attenuation. Let the light from the fiber optic cable reemerge propagating in the +z direction, is the light still polarized in the x direction?

Thanks for any help!

Spinnor Nov11-12 08:55 AM

Re: Sending polarerized light through light fiber that is all jumbled up.
 
Does this reasoning answer my question in the affirmative?

Linear polarization in the x direction is equal amounts of left and right circularly polarized light with the proper phase choice? We expect the left and right components to propagate the same through the jumbled up optic fiber and thus when the laser light reemerges from the fiber it is still polarized in the x direction?

Is the above faulty reasoning? If so where did i go wrong?

Thanks for any help!

Spinnor Nov11-12 09:05 AM

Re: Sending polarerized light through light fiber that is all jumbled up.
 
Follow up question, what if the end of the fiber optic cable points on the x direction, what then is the polarization of the light?

Thanks for any help!

mfb Nov11-12 09:35 AM

Re: Sending polarerized light through light fiber that is all jumbled up.
 
I would expect that the polarization direction depends on the geometry of the fiber, based on the following reasoning:

Take a fiber with the transition +z -> +y, and light polarized in x-direction. Based on symmetry, I would expect that the polarization remains in x-direction.
Now add a second fiber with the transition +y -> +x. Again, based on symmetry, I would expect that the polarization follows the (x,y)-plane, and ends up as polarization in y-direction.
Now add a third fiber with the transition +x -> +z. Using the same argument as at the first fiber, I would expect a y-polarization at the end.

It is interesting that the result stays the same if the second process switches the polarization, as it would lead to a different third process, too.

reasonableman Nov12-12 02:00 PM

Re: Sending polarerized light through light fiber that is all jumbled up.
 
The polarisation state of a light wave passing through normal optical fibre is essentially random. While a perfectly straight optical fibre, with no stresses, will not affect the polarisation state this is very unlikely. Usually stresses in the fibre cause small changes in the refractive index leading to changes in the polarisation state. You can, however, get polarisation maintaining fibre. This has stress added on purpose and has a repeatable effect on the polarisation.


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