Hi Mohammad,
In a general sense, when the amplifier is used before the receiver (after the fiber line), it is called “preamplifier” and is used to amplify the signal so as to overcome the receiver noise. When it’s used after the transmitter (before the fiber), it is called “power booster” and is used to amplify the power which is not yet mixed with any noise from fiber (source: Mr Keiser’s book on optical communications). It can be used for RoF systems in the same sense.