A fiber optic transmitter is a device which includes a LED or LaserSource and signal conditioning electronics that is used to inject a signal into fiber.
Fiber optic receivers capture the Light from a Fiber optic cable, decode the binary Data it is sending and then convert into an electrical signal.
Information is sent from a source to a Transmitter by means of an electrical signal. The transmitter then takes that binary data and transfers it to a light signal. The light signal is passed through fiber optic Cables and connectors until it reaches the receiver. The Receiver then takes that light signal, translates it back to an electrical signal allowing the binary data to be read by the user.
Loopbacks are used for testing and some other applications. They Feed data back to the SAME unit SO that the original transmitted data can be compared to that received, thus identifiying Transmission errors.