A monomode fiber acts as a pinhole tunnel, a waveguide for light. In
principle, it has no badwidth limit, I think, for any distance. My definition
of the bandwidth is 1/dt where dt is the duration of bits which are being
transmitted as square pulses ("yes" or "no"). I remember reading that bits
can be transmitted at near 1 Ghz. Is this the limit at which light can be
modulated by electric pulses in an electrooptical cell?