In telecommunication, the loading and unloading of signals at both transmitter and receiver is usually managed by a device called circulator. But since circulators for plastic fiber are very expensive, a cheaper alternative is being used in this case. It involves using two optical couplers but the trade offs include increase losses and cross talk.

                 An optical coupler is a four port device with the parts usually denoted alphabetically from A to D. A signal is injected into one port is literally split and becomes available on the two ports at the opposite end of the coupler (though one port's output is significantly stronger than the other). For example in the figure below, a signal injected in to port A is split between port D(the strong path) and port C (the weak path). Importantly, the optical coupler is a bi-directional device. So, a signal injected in to ports C and D is split between ports A and B and this is true even if a signal is connected to ports A and/or B at the same time. Its this property that allows us to use optical couplers to implement bi-directional fiber optic communications.

                Figure below shows the basic implementation of bi-directional fiber optic communications between two stations using optical couplers.

               In the figure, the transmitter (Tx) of station 1 is connected to the channel via the weak path of its optical coupler (that is, from port B to D). This transmitted signal is connected to the receiver (Rx) at sxtation 2 via the strong Path of its optical coupler (that is, from port B to C). At the same time, the transmitter of station 2 is connected to the channel via the weak path of the optical coupler (that is, from port D to B) and this signal is directed to the station 1 receiver via its optical coupler's its strong path (that is from port D to A).

              The fact that the two signals travelthrough a weak path of one of the optical couplers is responsible for the higher losses involved in this method of loading and unloading the signals (compared with using a circulator).