Reference: List of Optical Communication Systems
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Radio waves and light are both electromagnetic waves and they both consist of photons. The photons of radio waves have far lower energy per photon than the photons of the light that mammals can naturally see. Optical communication systems use frequencies of electromagnetic waves that are far higher than the frequencies of radio waves, but lower than the frequencies of X-rays. 

The energy of a photon is calculated by using the formula

Photon_energy_in_J = frequency_in_Hz * Planck_Constant ~
~ frequency_in_Hz * 6.626*(10^34) J/Hz

Key-phrases for Fossilrepository search engine: optical links, optical networks, optical relays, optical relay stations .