Good question, you can see a light flash on and off but to encode large amounts of information onto a channel is determined by the signal to noise ratio per Shannon's law https://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theore...
the information rate is proportional to the SNR. So you may be able to see a light flash on and one once per second but to put 1Gbps on it you need a pretty high SINR. Things like dispersion come into play.
Do you mean dispersion as in "a pulse gets longer in time due to different length of paths taken by different photons due to scattering"? If not, what do you mean?
My point about seeing a flash was precisely that SNR is high, because I can distinguish the flash from noise.
EDIT: SNR depends on frequency only inasmuch noise level depends on frequency.
