I am not an expert but it seems to me that the bandwidth deoends on the
distance, at least for optical fibers whose diameters are significantly
larger than the wavelength. In the context of geometrical optics rays
which travel along the axis will arrive sooner than those which arrive
after numerous internal reflections. You do not want this when you transmit
bits over long distances; "yes" sent ot t=t1 should not overlap with "no"
sent at t=t1+dt, where dt is the duration of your "yes" and "no" pulses.
I would like to know what limits the bandwidth of monomode fibers (those
whose diameters are less than lambda). Does it also depend on the distance?