Before my 2013 stroke I enjoyed doing experiments at 481THz (red light). Although my main interest was "over the horizon" digital communication, I had one line of sight AM QSO over about 11km.
On YouTube is a video of a German optical communicator that was used in WW2. As the beam was very narrow it was ideal for secret communication up to 5km. Mechanically it was a work of art.
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