[1] Heterodyne and homodyne systems are of interest because they are expected to produce an increase in sensitivity of up to 20 dB[2] allowing longer hops between islands for instance.
Normal WDM using direct detection does not achieve anything like the close channel spacing of radio frequency FDM.
It involves modulating the optical power of the carrier signal to represent the transmitted data.
The intensity-modulated optical signal is generated by modulating the amplitude or the current of the light source, typically a laser diode with one or two cavity designs such as Fabry-Perot or distributed feedback (DFB).
[5] At the receiver end, direct detection (DD) is used to recover the modulated signal.