This design is a simplified representation of a PON with a CO and ONT connected via 20 km of optical fiber. Optical filters model the filtering effects of AWGs in an actual PON network. In the CO, a downstream transmitter generates a 1550-nm 2.5-Gbps NRZ signal. The ONT routes the downstream signal to both a receiver and an RSOA-based upstream transmitter. A 1.25-Gbps NRZ signal modulates the RSOA, effectively overwriting the downstream signal. The transmitter then sends this new upstream signal back over the same 20 km of fiber, after which the CO routes it to a receiver. OptSim simulates the entire bidirectional design in two passes, first taking into account the downstream transmission in the fiber, and then simultaneously accounting for both the upstream and downstream signals.
The effectiveness of this design depends on the ability of the RSOA to remodulate the received downstream signal with the new upstream signal. Towards this end, the high-pass filtering effect of the RSOA, which becomes more pronounced at higher input powers, helps to suppress the downstream signal. As the power of the optical signal at the RSOA input is increased, the RSOA does a better job of eliminating any modulation on this input and superimposing the modulation of the RSOA control current.
Figures below depict the eye diagram of the received signal in the upstream RX for each of the three RSOA input powers under consideration: