Does a laser beam spread when passing through doped fiber?

[God Plays Dice]

Will a laser exposed to ions (doped fibre) spread out in the direction of polarisation as momentum sends the peaks in one direction and the troughs in another. Or, as total momentum is 0 does the wave continue on straight.

 

[SongDog]

Continue on straight? In a fibre? They normally are made with a radially graduated index of refraction to self-focus along the central axis, aren't they?