When they start to seize up a lot ofnon-lineareffects-processes inwhich small causes havedisproportionateeffects-come into play, making it much harder to see what is going tohappennext.
This could be important in future high-speed links, because when rates of data transfer approach 100 gigabits per second, non-linear effects begin to mess up the signals.
With a different hole design, crystal fibres can do the opposite trick as well, spreading the light much more evenly across the width of the fibre and thereby reducing non-linear effects.