# Hybrid Self-Protected Fiber-FSO WDM-PON System with Fiber Breakage Prevention
## 2 Proposed Hybrid Fiber-FSO WDM-PON System
Furthermore, the ONU\({}_{\rm A1}\) and OWU\({}_{\rm B1}\), ONU\({}_{\rm A2}\) and OWU\({}_{\rm B2}\) and ONU\({}_{\rm AN}\) and OWU\({}_{\rm BN}\) are linked by protection fiber and regarded as a group, respectively, for self-restored operation against fiber breakage. As illustrated in Figure 1a, all the fiber downstream signals of OLT\({}_{\rm A}\) will pass through the AWG\({}_{\rm A}\) and AWG\({}_{\rm B}\) of RN\({}_{\rm A}\) and RN\({}_{\rm B}\) via the \(1\times 2\) CP and then into the corresponding ONU\({}_{\rm A}\) and OWU\({}_{\rm B}\) simultaneously. However, all the downstream wavelengths of \(\lambda_{\rm A1}\) to \(\lambda_{\rm A(N/2)}\) will be blocked in the related OWU\({}_{\rm B}\) by the WF. So, the fiber wavelength will not interfere with the FSO signal. Similarly, the entire FSO signals of the OLT\({}_{\rm B}\) will not enter the corresponding ONU\({}_{\rm A}\) according to the proposed PON architecture. Therefore, the signal transmissions from the OLT\({}_{\rm A}\) and OLT\({}_{\rm B}\) do not interfere with each other. Figure 1: (**a**) Proposed hybrid self-restored FSO- and WDM-PON system with fault protection. (**b**) The periodic characteristic of AWG for the wavelength arrangement of fiber and FSO downstream (\(\lambda_{\rm D}\) and B\({}_{\rm D}\) bands) and upstream signals (\(\lambda_{\rm U}\) and B\({}_{\rm U}\) bands), respectively. Generally, we know that fiber breakpoints may occur on the feeder (between OLT and RN) and distribution fibers (between RN and ONU) in PON access [27]. The fiber breakpoints may be caused by nature or man-made factors in the PON network. Once a fiber breakage occurs between the OLT and the RN or RN and ONU/OWU, the corresponding signal link will be disconnected. To avoid the fault happening, a PON network with a self-healing mechanism would be required. Thus, the blended WDM fiber-FSO access architecture with fiber breakage prevention is achieved, as exhibited in Figure 1a. First, we assume that a fiber breakage occurs at the \"A1\" and \"A2\" point, respectively, as shown in Figure 2. The \"A1\" point will cause the entire baseband WDM-PON network to be interrupted. Therefore, each ONU\({}_{\text{A}}\) will not be able to connect to the OLT\({}_{\text{A}}\). At this time, all the SW\({}_{\text{A}}\) of ONU\({}_{\text{A}}\) switches the connected port to \"b\" automatically for signal reconnection through the protection fiber and FSO PON architecture, as displayed in Figure 2, while the PD of ONU\({}_{\text{A}}\) cannot detect the corresponding optical downstream signal. In previous studies [28; 29], an optical power monitor apparatus had to be included in OLT or ONU to decide whether to switch SW. However, in this design, we do not need to include an additional monitoring device to change the connected position for rerouting the fiber path, only depending on whether the PD detects the downstream signal. In the second case, when the \"A2\" breakpoint occurs, there will only be a disconnection between the ONU\({}_{\text{A}1}\) and OLT\({}_{\text{A}}\). Similarly, the SW\({}_{\text{A}}\) of ONU\({}_{\text{A}1}\) will connect to \"b\" for signal retransmission. As a result, no matter whether the fiber fault is at \"A1\" or \"A2\" points, it only needs to rely on the SW\({}_{\text{A}}\) of ONU\({}_{\text{A}}\) to switch the connecting fiber path. Next, suppose there is a breakpoint location at \"B1\" or \"B2\", which will result in disconnection of FSO traffic between the OLT and entire APs or the RN\({}_{\text{B}}\) and AP\({}_{\text{1}}\), respectively, as illustrated in Figure 3. Although the B-band FSO signal of OLT\({}_{\text{B}}\) will also be transmitted to the corresponding ONU\({}_{\text{A}}\) via the CP and WF, it will still be separated by the WF of the ONU\({}_{\text{A}}\). Such FSO signal transmission is similar to that described in Figure 2. Here, to solve the fiber fault occurrence of the presented FSO-PON architecture, the operated mechanism of fault protection is the same as the baseband WDM-PON described above.