# Hybrid Self-Protected Fiber-FSO WDM-PON System with Fiber Breakage Prevention
## 2 Proposed Hybrid Fiber-FSO WDM-PON System  
Figure 1a presents the proposed hybrid self-restored fiber-FSO WDM-PON system against fiber breakage. In the OLT location, we design two OLTs--OLT\({}_{\rm A}\) and OLT\({}_{\rm B}\)--which provide WDM downstream baseband (fiber) and FSO signals to each corresponding optical network unit (ONU) and optical wireless unit (OWU), respectively. Here, to deliver the fiber and FSO downstream and upstream wavelengths, we can separate the available channel bandwidth to the A and B bands based on the periodic output characteristic of 1 \(\times\)\(N\) arrayed waveguide grating (AWG) for wavelength arrangement, respectively, as illustrated in Figure 1b. The channel spacing of AWG (50 or 100 GHz) depends on the practical demand. This means that the wavelength bandwidth is divided into A\({}_{\rm D}\), A\({}_{\rm U}\), B\({}_{\rm A}\) and B\({}_{\rm U}\) bands, respectively. Hence, the downstream fiber signals of \(\lambda_{\rm A1}\), \(\lambda_{\rm A2}\),... and \(\lambda_{\rm A(N/2)}\) and upstream traffics of \(\lambda_{\rm A(N/2+1)}\), \(\lambda_{\rm A(N/2+2)}\),... and \(\lambda_{\rm A(N)}\) in the A-band range (A\({}_{\rm D}\) and A\({}_{\rm U}\)) can be assigned for bidirectional data access, where \(N\) = 2, 4, 8, 16, etc. Identically, the FSO downstream (\(\lambda_{\rm B1}\), \(\lambda_{\rm B2}\),... and \(\lambda_{\rm B(N/2)}\)) and upstream wavelengths (\(\lambda_{\rm B(N/2+1)}\), \(\lambda_{\rm B(N/2+2)}\),... and \(\lambda_{\rm B(N)}\)) also can be used in the B\({}_{\rm D}\) and B\({}_{\rm U}\) bands, respectively, as seen in Figure 1b. Each downstream baseband (\(\lambda_{\rm A1}\) to \(\lambda_{\rm A(N/2)}\)) and FSO channels (\(\lambda_{\rm B1}\) to \(\lambda_{\rm B(N/2)}\)) via the corresponding laser diode (LD) is connected to the polarization controller (PC), 40 GHz Mach-Zehnder modulator (MZM), optical circulator (OC), corresponding port of 1 \(\times\)\(N\) AWG, 1 \(\times\) 2 optical coupler (CP), and A-/B-band WDM filter (WF), respectively, and then  
leaves the OLT, as seen in Figure 1a. Moreover, the wavelengths of \(\lambda_{\rm A1}\), \(\lambda_{\rm B1}\), \(\lambda_{\rm A(N/2+1)}\) and \(\lambda_{\rm B(N/2+1)}\) can pass through the link port \"1\" of AWG\({}_{\rm A}\) and AWG\({}_{\rm B}\) simultaneously for connection due to its original features. Therefore, on, the \(\lambda_{\rm A(N/2)}\), \(\lambda_{\rm B(N/2)}\), \(\lambda_{\rm AN}\) and \(\lambda_{\rm BN}\) can transmit through the \"\(N\)-th\" connected port at the same time. As mentioned above, utilization of the different wavelengths as the downstream and upstream channels can avoid the RB noise when various WDM signals are applied [23]. Then, all the downstream traffics will pass through the corresponding feeder fiber, remote node (RN\({}_{\rm A}\) and RN\({}_{\rm B}\)) and distribution fiber and enter the optical network unit (ONU\({}_{\rm A}\)) and optical wireless unit (OWU\({}_{\rm B}\)), respectively, as exhibited in Figure 1a. The same characteristics of \(1\times N\) AWG\({}_{\rm A}\) and AWG\({}_{\rm B}\) are placed in the OLT and RN sides. In the designed network, the difference is that the access points (APs) are placed in the appropriate positions in the FSO access to transmit and receive the bidirectional WDM FSO signals by using the fiber-based collimator (CM) [14; 23]. The WF can separate the baseband (A band) and FSO channels (B band) for the corresponding signal transmission. To construct the ONU\({}_{\rm A}\), the corresponding upstream LD, WF, PC, OC, MZM, 40 GHz PIN photodiode (PD) and \(1\times 2\) optical switch (SW\({}_{\rm A}\)) are used. Moreover, the OWU\({}_{\rm B}\) uses the same optical components as the ONU\({}_{\rm A}\), but only adds the CM for the wireless FSO link, as seen in Figure 1a. Usually, all the SWs of ONU\({}_{\rm A}\) and OWU\({}_{\rm B}\) are linked to the \"a\" point for downstream and upstream path transmission in normal status.