# Polarization-Discriminated RSOA-EAM for Colorless Transmitter in WDM-PON
## 3 Simulation Results and Discussion
### Upstream Signal Integrity and Power
Meanwhile, for high ERs (e.g., 5 dB) of the downstream input signal, the PD RSOA-EAM (at 84\({}^{\circ}\)) can significantly reduce the XGS effect when compared to the horn-waveguide RSOA-EAM, as evidenced by its much thinner \"1\" level. In practical applications, the downstream signal input power may vary among different ONUs at different locations in the WDM-PON. Its impact on the optimum rotation angle therefore needs to be studied. Figure 7a plots the normalized _FOM_ as a function of the rotation angle at different downstream signal input powers. As the downstream signal input power increases from \(-25\) dBm to \(-5\) dBm, the optimum rotation angle decreases from \(88^{\circ}\) to \(73^{\circ}\) correspondingly, which is further plotted in Figure 7b. This effect indicates that for an ONU placed closer to the remote node (RN), the rotation angle of the proposed device should be set at a smaller value for achieving its best performance (we define it as \"adaptive \(\theta_{FR}\)\"). Figure 7a also indicates that the full width at half- maximum (FWHM) of the _FOM_ can be broadened with an increased input power, which means that higher input powers are beneficial to relax the fabrication requirements as the optimum rotation angle is easier to hit. To show the device performance differences with an adaptive \(\theta_{FR}\) and a constant \(\theta_{FR}\) (i.e., \(\theta_{FR}\) irrespective to the downstream signal input power), their Q-factors are plotted as a function of the downstream signal input power in Figure 7c. It is found that the adaptive \(\theta_{FR}\) in general gives the highest Q-factors, the constant \(\theta_{FR}\) of \(87^{\circ}\) favors the relatively low input powers (around \(-20\) dBm), and the constant \(\theta_{FR}\) of \(80^{\circ}\) favors the relatively high input powers (around \(-10\) dBm). Nonetheless, they all give higher Q-factors than the constant \(\theta_{FR}\) of \(0^{\circ}\), i.e., the design without polarization rotation. Figure 5: Dependence of (**a**) total gain \(P_{us}/P_{in}\) and \(Q^{2}\), and (**b**) figure of merit (_FOM_) on the rotation angle when the downstream input signal power is \(-10\) dBm. (**c**) One-way static gain characteristics along the downstream and upstream paths, respectively, with and without considering the XGS effect. Figure 6: _FOM_ versus the downstream input signal ER of the horn-waveguide RSOA–EAM [22], and of the proposed PD RSOA–EAM at a rotation angle of \(0^{\circ}\) and \(84^{\circ}\). The inlets are the upstream signal eye-diagrams (data rates of the down- and upstream signals: 10 Gbps and 5 Gbps, respectively). Finally, to verify the colorless operation potential of the proposed device, Figure S4 (see the Supplementary Material) plots the wavelength dependence of the output power and Q-factor of the upstream signal when the downstream signal input power is \(-15\) dBm and the rotation angle is \(84^{\circ}\). We can find that the full width half-maximum (FWHM) optical bandwidth of both the integrity (the Q-factor) and the power of the upstream signal are larger than \(50\) nm, sufficient to cover the \(20\) nm wideband operation requirement as specified by NG-PON2 [44].