The Bi-Di coherent transmission system is configured with at least one pair of modules coupled to one another via a single fiber. The modules each are configured with a pair of laser outputting two reference signals at respective different wavelengths λ1o and λ2o, photonic transceiver and a wavelength division multiplexer (WDM) coupler. The photonic transceivers each have transmitter and receiver branches integrated in a photonic circuit and receiving the reference signals. The transmitter is configured to modulate the received reference signals λ1oT and λ2oT which are further coupled into the WDM coupler. The WDM couplers each sort out one of the modulated signals and transmit the other modulated signal such that the transmitted modulated signal at different wavelengths λ1oT and λ2oT are coupled into respective opposite ends of the fiber and propagate towards one another in opposite directions. The transmitted modulated signals are coupled into respective branches through the WDM couplers with each transmitted modulated signal interfering with the reference signals at wavelengths λ1o and λ2o. The photodiodes of respective receiving branches are configured to detect a beat frequency of the interfering signals at the same wavelength.