Optical carriers are in the region of Hz to Hz, i.e. three to six orders of magnitude higher than microwave frequencies. However, the atmosphere is a poor transmission medium for light waves. Optical communication only became a widespread option with the development of low-loss dielectric waveguide.
Optical fibres have very small diameters.
A very large number of fibres can be carried in a cable the thickness of a coaxial cable. Optical fibres are almost completely immune from external fields. They do not suffer from cross-talk, radio interference, etc. It is difficult to tap into an optical line. It is extremely difficult to tap into an optical line unnoticed.