New paper on #ArXiv!
When (coherent) light propagates in a multimode fibre, each mode accumulates a slightly different phase, so the output looks like a random speckle patter.
But it is NOT random!
This non-randomness has a number of consequences, among them the fact that the you get predictable patterns in the apparently random output. We show that a focused input will always result in a ring of excess intensity at the same radius of the input (and suggest how this might be useful).