# Integrated Fiber-FSO WDM Access System with Fiber Fault Protection
## 2 Proposed PON Access Architecture
The WCP of ONU\({}_{1}\) is applied to separate the downstream fiber (\(\lambda_{\rm 1C}\)) and FSO wavelengths (\(\lambda_{\rm 1L}\)). The FC is applied to generate the FSO signal  
Figure 1: (**a**) Conventional WDM-PON system. (**b**) FSO transmission system integrated with WDM-PON. Figure 2: Proposed self-protected WDM-PON system together with fiber and FSO signals for data access against fiber fault. for wireless transmission. Initially, the SW is connected to the \"1\" point for fiber signal connection. As seen in Figure 2, the \(\lambda_{\text{1C}}\) signal of the CO will be separated two wavelengths by the 1 \(\times\) 2 CP passing through the upper (fiber path) and lower connecting fibers (FSO path) simultaneously and then into the ONU\({}_{1}\). Moreover, the separated \(\lambda_{\text{1C}}\) signal from the lower connecting fiber would be blocked by the SW to avoid the signal interference in the ONU\({}_{1}\), as exhibited in Figure 2. Therefore, the fiber signal \(\lambda_{\text{1C}}\) will be received by the photodiode (PD) of TRx\({}_{(N+1)\text{C}}\) after passing through the SW.  
For the L-band FSO transmission, the FC of AP can be combined into the ONU\({}_{1}\) for converting the optical wireless signal, as exhibited in Figure 2. Here, the FC is exploited to deliver the corresponding FSO \(\lambda_{\text{1L}}\) from the CO and then enter the OWU\({}_{1}\) through the lower connecting fiber, as shown in Figure 2. Although the C-band baseband signals will also be transmitted from the lower connecting fiber, but it will be blocked by the WCP of ONU\({}_{1}\). This means that the C-band fiber signal cannot affect the L-band FSO signal. Through a wireless FSO transmission length, the \(\lambda_{\text{1L}}\) signal is collected by the FC of the OWU\({}_{1}\) and then into the PD for decoding. In the demonstration, the upstream fiber traffic of ONU\({}_{1}\) and the FSO signal of OWU\({}_{1}\) can also go back to the CO through the same transmission path, respectively, as seen in Figure 2. As a result, the proposed WDM-PON system not only can provide the fiber access traffic, but also can support the wireless FSO signal via the same ONU. As mentioned above, the downstream wavelengths of \(\lambda_{\text{1C}}\), \(\lambda_{\text{2C}}\),... and \(\lambda_{\text{NC}}\) and \(\lambda_{\text{1L}}\), \(\lambda_{\text{2L}}\),... and \(\lambda_{\text{NL}}\) are connected via the upper and lower connecting fibers into the corresponding ONU and OWU in normal state, respectively, as shown in Figure 2.0 In addition, the upstream fiber and FSO signals of \(\lambda_{(N+1)\text{C}}\), \(\lambda_{(N+2)\text{C}}\),... and \(\lambda_{(2N)\text{C}}\), and \(\lambda_{(N+1)\text{L}}\), \(\lambda_{(N+2)\text{L}}\),... and \(\lambda_{(2N)\text{L}}\) are also through the same fiber paths back to the CO. In this demonstration, the designed PON network also can provide the self-protected function against fiber fault for C-band fiber signal link. In general, the fiber breakpoint may be occurred on the feeder fiber (between CO and RN) and distribution fiber (between RN and ONU\({}_{N}\)) to interrupt the signal connection. For example, suppose there is a fiber breakpoint at the \"F1\" point between RN and ONU\({}_{1}\), as seen in Figure 3a. The bidirectional fiber signal connection between the CO and ONU\({}_{1}\) will interrupt at this moment. To prevent the signal disconnection, the SW will switch to the \"2\" position for routing signal traffic through the lower connecting fiber, when the ONU\({}_{1}\) cannot receive the corresponding downstream signal from the CO, as illustrated in Figure 3a. Hence, the downstream and upstream fiber traffics will pass through the lower fiber path for signal reconnection. Once the fiber breakpoint is repaired, the SW switches back to the \"1\" position for normal connection. Therefore, the presented WDM-PON architecture can not only provide the fiber and FSO traffics simultaneously, but also can support the fiber fault protection in the fiber PON connection to provide the seamless signal access.