The example consists of a link of two spans of fibers. The bi-directional fiber model includes bidirectional optical signal propagation, a main cause of MPI. Three bi-directional connectors between the transmitter, the receiver and the fiber spans create two closed loops where the optical signal is enclosed and circulates back and forth. The bidirectional connectors enable adjusting the optical insertion loss and return loss , thereby physically creating a backward propagating optical signal.
Figure 2 shows a received eye diagram for the case when return loss at bidirectional connectors is minimal.