# Polarization-Discriminated RSOA-EAM for Colorless Transmitter in WDM-PON
## 3 Simulation Results and Discussion
### Upstream Signal Integrity and Power
To be able to find an optimum rotation angle that makes a compromise between the upstream signal integrity and power, we introduce the following dimensionless figure of merit (_FOM_):  
\[FOM=Q^{2}\times\frac{P_{us}(\mathrm{mW})}{P_{int}(\mathrm{mW})}, \tag{1}\]  
The dependence of the total gain (\(P_{us}/P_{in}\)) and \(Q^{2}\) are plotted in Figure 5a, and the _FOM_ is plotted in Figure 5b, respectively, both as a function of the Faraday rotation angle. As a _FOM_ peak appears at 80\({}^{\circ}\), we know that the device will offer an overall best performance at this optimum Faraday rotation angle. At this angle (80\({}^{\circ}\)), the output power and dynamic ER of the upstream signal are 11 dBm and 8.3 dB, respectively, exceeding the minimum required value at 4 dBm and 6 dB, respectively, as specified by NG-PON2. To further clarify the effectiveness of the device, in Figure 5c we plot the one-way static gain for signals going downstream (at 0\({}^{\circ}\) or TE mode, from left to right) and upstream (at 80\({}^{\circ}\), from right to left), respectively, when only the down- or upstream signal is inside the cavity (i.e., without XGS) and when they both exist inside the cavity (i.e., with XGS). For the case without XGS, we find that the SOA is well-saturated for an input power higher than \(-\)30 dBm along the downstream path, whereas the SOA is in its linear amplification region for an input power lower than \(-\)20 dBm along the upstream path, which indicates that the proposed PD RSOA-EAM can indeed erase the downstream signal and linearly amplify the upstream signal simultaneously. The device's fairly wide working range (from \(-\)30 dBm to \(-\)20 dBm, i.e., where the saturation discrimination can be achieved) facilitates the power management in operation, hence is superior to the conventional design without utilizing the polarization dimension, not only in terms of the gain discrimination scale, but also in terms of the power operation range. When both the down- and upstream signals exist inside the cavity, the XGS will bring in extra saturation compared to that without XGS, as shown by the lower output powers in Figure 5c. By utilization of polarization discrimination, however, even under XGS we still manage to have the downstream signal saturated and the upstream signal linearly amplified. Although the saturation of the downstream signal is not as good and the linear gain of the upstream signal is lower, the gain difference between the down- and upstream signals brought in by the polarization discrimination still exists nonetheless, which is not achievable in a straight SOA design without using the polarization dimension [22]. Figure 6 gives a comparison of the _FOM_ calculated from the proposed device (when the rotation angle is set at 0\({}^{\circ}\) and 84\({}^{\circ}\), respectively) and measured with the horn-waveguide RSOA-EAM [22] as referred to in the introduction. The inlets are the corresponding upstream signal eye-diagrams at output. The downstream input signal power is \(-\)14 dBm, and the EAM is modulated at 5 Gbps as performed in [22] for a fair comparison. In addition, note that the rotation angle is changed to 84\({}^{\circ}\) instead of 80\({}^{\circ}\) because the optimum rotation angle varies with the input power, as will be discussed later. The ER of the downstream input signal is increased from 0 dB (i.e., downstream CW) to 5 dB. It is seen from Figure 6 that both the horn structure and the polarization rotation design can improve the _FOM_, while the polarization rotation design is more advantageous. It is also observed that the two schemes can both improve the quality of the upstream signal eye-diagram. Measured in Q-factor, the improvements are 3.7 dB and 10 dB, respectively, for the horn RSOA-EAM and the PD RSOA-EAM with \(\theta_{FR}\) set at 84\({}^{\circ}\).