In FSO communication system, the major sources of noise in the detection process are shot noise and johnson noise. While for RF link, the major source of noise is the johnson noise. What is the scientific interpretation?
I have to believe that the electronics behind the optical and RF sensors would experience both types of noise, so they must only be talking about the sensor itself.
Free space optical might use a phototransistor to sense the light level and convert that to an electric current. That device itself will experience shot noise, since you have DC current (individual charges) flowing through a PN junction, in the device that gathers the photons. Whereas in RF reception, the device which gathers those photons is an antenna, which converts the EM wave to AC current flow, thereby experiencing no shot noise.
So, the way I would say this, both types of communications receivers will experience Johnson noise, which is resistor thermal noise, and shot noise, which will occur in conductors and PN junctions. But the photo-detector itself, in FSO, will be an additional source of shot noise.
http://123.physics.ucdavis.edu/shot_files/ShotNoise.pdf
