# POL-MUX System for Noncoherent Optical Networks
## 4 Proposal of a Wide-Band Passive Optical Network (WDM-PDM-PON)
### Simulation Analysis  
It is very important to observe how the changes in the PER values set for the polarization beam splitters affect the demultiplexed signal of each ONU unit. Assuming the usage of a dynamic polarization controller (block Pol. Track in the simulation model) the quality of the signal is dependent on the PER. The influence of this parameter can be compared by eye diagrams. Figure 13 shows how the eye diagrams change for PERs of 30 dB (a) and 15 dB (b). Figure 14 shows eye diagrams for PERs of 10 dB (a) and 5 dB (b). Figure 11: Simulation model of WDM-PDM-PON. Figure 12: Spectra of other demultiplexed signals: (**a**) Spectrum of the 17 th demultiplexed signal, (**b**) Spectra of other demultiplexed signals. Eye diagrams show how decreasing the PER parameter gradually degrades the quality of the resulting signal. \(PER=30\,\mathrm{dB}\) is typical for high-quality polarization beam splitters, and as we can see in Figure 13, the output signal is not influenced (degraded) by the POL-MUX, and the eye diagram is clear, corresponding to a BER of \(10^{-25}\). For the initial simulation, \(PER=25\,\mathrm{dB}\), which is typical for commonly used polarization beam splitters. Additionally, for this setup, the signal quality is very good and corresponds to a BER of \(10^{-21}\). With a further decrease in the PER, it is possible to see changes in the signal quality. For \(PER=20\,\mathrm{dB}\), a BER of \(10^{-19}\) is observed and for \(PER=15\,\mathrm{dB}\) the BER is reduced to \(10^{-15}\). Significant signal degradation occurs for \(PER=10\,\mathrm{dB}\), as we can see from Figure 14, where the eye is closing and the BER is only \(10^{-11}\). The most significant signal deterioration occurs when the PER is reduced to \(5\,\mathrm{dB}\), when the eye is significantly closed, and the bit error rate reaches only \(10^{-4}\). The BER limit for PONs is \(10^{-12}\)[29]. Figure 15 shows Poincare spheres, where (a) corresponds to OSA1 and (b) corresponds to OSA2. The outputs from both OLT units contain the same WDM signals, both of which are linearly polarized. Before the PBS, both signals were set in terms of the polarization angle to match the polarization planes of the PBS, i.e., the signal from OLT2 was rotated approximately \(90^{\circ}\). As is obvious from Figure 15, point S (Stokes parameter) is on the \(x\) and \(y\) planes and is on the surface of the sphere, which means that the signals are fully polarized (\(DOP=1\)). Figure 14: Eye diagram for different PER settings-(a) \(10\,\mathrm{dB}\), (b) \(5\,\mathrm{dB}\). Figure 13: Eye diagrams for different PER settings-(a) \(30\,\mathrm{dB}\), (b) \(15\,\mathrm{dB}\). After the multiplexed signal passes through PBS1, the polarization partially changes from linear to elliptic, as shown in Figure 16. The use of a dynamic polarization controller can maintain the angle of polarization but not the state of polarization; nevertheless, this is not important for PON networks in which the detectors are insensitive to the state of polarization. After the demultiplexed signal passes through the next 5 km of optical fibers (ODN2 and ODN3), the state of polarization is further changed based on theoretical assumptions from linear to elliptical, as shown in Figure 17a. Most of the points are concentrated in the \(x\) and \(y\) planes, so the signal is still partially linearly polarized. On the other hand, Figure 17b shows that signals in the second arm (ODN3) are concentrated along the \(z\) axis (S3), so we can say that it is partially circularly polarized (right-hand). The signals from both arms are fully polarized because all points are on the surfaces of the spheres. Poincare spheres of the wavelength demultiplexed signals are shown in Figure 18. The spheres are formed by all points from all demultiplexed signals of the corresponding OLT units (different colors correspond to different wavelengths). As we can see, the spheres in Figures 17 and 18 are analogous, and the distributions of points are the same.