How to use fiber optics to illuminate glass tiles?
Consider using high intensity LEDs at the edges.  This is how they illuminate light panels in elevators - using some kind of diffusing backing behind it or properly-selected tiles that can be lit from the top or bottom.

The problem with using fiber optics is if they break, you can't service them if they're mortared along with the tile.  Light the tile from the edges and the lighting system can be serviced.