Theoretical and simulation analysis on potential impairments in bidirectional WDM-PONs

In this work, we analyzed and simulated the potential impairments due to the crosstalk problems that caused by the RB and FR in bidirectional single-fiber transmission for conventional (usually 20 km transmission without any amplification in remote node) and long-reach WDM-PONs (100 km by utilizing optical amplifier to enhance the power budget and prevent the fiber nonlinearities effects). By considering RB effect only, the best crosstalk-to-signal (C/S) ratio for the first case is achieved when the RN is placed either close to the CO or ONU vicinity. And in the second case, when the system has two gain sources (RN and ONU gain), the best C/S ratio is achieved when the RN gain is placed at the center of transmission link. Then, when combining the RB and FR effects, it was demonstrated that the C/S ratio gets worse by almost 1.5 dB in the presence of a strong reflection (r = 30 dB). Furthermore, even if the FR is maintained at the smaller reflectivity and the EDFA gain is increased, the C/S ratio will even get worse when EDFA is placed close to ONU vicinity. It is due to reamplification and re-modulation feedbacks of backreflections that may happen at the preceding and following of the RN gain.