# Polarization-Discriminated RSOA-EAM for Colorless Transmitter in WDM-PON
## 3 Simulation Results and Discussion
### Downstream Signal Suppression  
Figure 1b depicts the schematic operation diagram of the proposed device. In this subsection, the downstream signal suppression is studied during its first pass in the SOA, taking into account the gain affected by the contribution of the reflected light. The EAM section is biased at 0 V without modulation (i.e., \(V_{pp}\) = 0), and the suppressed downstream signal is extracted before entering the EAM. The SOA section is biased at a constant current of 150 mA. Figure 2a plots the static gain saturation curves of the 1550 nm downstream signal when \(\theta_{FR}\) varies from 0\({}^{\circ}\) to 90\({}^{\circ}\) (with \(\theta_{DS}\) fixed at 0\({}^{\circ}\)). The corresponding relationships between the downstream signal output and input powers are shown in Figure 2b, where the erased 10 Gbps eye- diagrams at a downstream signal input power (\(P_{in}\)) of -15 dBm are inserted to show the erasing effects. It is apparent that, as the Faraday rotation angle increases from 0\({}^{\circ}\) to 90\({}^{\circ}\), the downstream signal saturation is more pronounced as evidenced by the deeper slope of the gain saturation curve. Consequently, a cleaner seed DC light is obtainable, as shown by the drastically reduced ER in the eye-diagram at \(\theta_{FR}\) = 90\({}^{\circ}\) as compared to \(\theta_{FR}\) = 0\({}^{\circ}\) (see Figure 2b). These results indicate that a large (close to 90\({}^{\circ}\)) Faraday rotation angle is in favor of the downstream signal erasing. \begin{table}
\begin{tabular}{c c c} \hline \hline
**SOA Parameters** & **Value** & **EAM/FR Parameters** & **Value** \\ \hline SOA length (\(\mu\)m) & 1200 & EAM length (\(\mu\)m) & 100 \\ SOA internal loss (cm\({}^{-1}\)) & 10 & EAM internal loss (cm\({}^{-1}\)) & 35 \\ Confinement factor & 5.5\% [22] & EAM absorption coefficient (cm\({}^{-1}\) V\({}^{-2}\)) & 1050 \\ Effective refractive index & 3.23 & EAM saturation power (mW) & 20 \\ Group refractive index & 3.57 & EAM strength (mW) & 1000 \\ Quantum well gain coefficient \(g_{0}\) (cm\({}^{-1}\)) & 2400 & EAM saturation power (mW) & 20 \\ Transparent carrier density \(N_{tr}\) (10\({}^{18}\) cm\({}^{-3}\)) & 1 & FR length (\(\mu\)m) & 1000 \\ Linear recombination coefficient \(A\) (10\({}^{9}\) s\({}^{-1}\)) & 0.25 & FR round-trip transmission coefficient & 0.82 [29] \\ Bimolecular radiation coefficient \(B\) & 5.6 & \(T_{F}\) & 0.82 [29] \\ (10\({}^{-10}\) cm\({}^{-3}\) s\({}^{-1}\)) & 5.6 & \(T_{F}\) & 0.82 [29] \\ Auger coefficient \(C\) (10\({}^{-29}\) cm\({}^{-6}\) s\({}^{-1}\)) & 7.5 & \(T_{F}\) & 0.82 [29] \\ Linewidth enhancement factor & 3 & Power reflectivity of the rear facet \(R_{2}\) & 1 \\ Noise coupling coefficient & 0.0073 [39] & & \\ Power reflectivity of the front facet \(R_{1}\) & 0 & & \\ Fiber-device coupling loss (dB) & 3.5 & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Parameters of the Proposed PD RSOA–EAM. To understand the results in Figure 2, one should keep in mind that a higher SOA gain leads to more severe SOA saturation [40], hence one should keep the gain as high as possible during the first pass to suppress the downstream signal. Since the material gain is related to the carrier density (\(N\)) by \(g_{0}\ln(N/N_{tr})\)[37, 41] (with \(g_{0}\) and \(N_{tr}\) indicating the gain coefficient and transparent carrier density, respectively), we should preserve more carriers for the downstream signal. As indicated in Figure 3a, the averaged carrier density inside the active region of the SOA increases with an increased Faraday rotation angle (this is attributed to the decreased carrier consumption of the upstream light, since the upstream light TE mode power decreases with the Faraday rotation angle following Equation (S9a) in Supplementary Material, whereas the upstream light TM mode does not consume carriers at all [37, 42]).