This youtube video shows a simulation of evanescent coupling between a driven optical fiber and another that is not driven. I had never heard of evanescent fields so I looked it up on Wikipedia, where I found the following:
The transmitted wave cannot, however, be a sinusoidal wave, since it would then transport energy away from the boundary, but since the incident and reflected waves have equal energy, this would violate conservation of energy.
But the simulation demonstrates that some energy is transferred into the non-driven fiber. How is this possible if the evanescent wave isn't allowed to transport energy? Of course the simulation could be wrong, but from the comments it sounds like optical fiber technicians use this phenomenon regularly to check if a fiber is transmitting.
I looked at this question but A) it mostly goes over my head and B) it doesn't have a conclusive answer due to the question being insufficiently specific.
