# Integrated Fiber-FSO WDM Access System with Fiber Fault Protection
## 2 Proposed PON Access Architecture  
The original WDM-PON architecture is shown in Figure 1a. Each WDM transceiver (TRx) is placed in the central office (CO) to transmit the corresponding downstream traffic and detect the upstream signal through the \(1\times N\) arrayed waveguide grating (AWG) device for WDM access, respectively. Each connected port of AWG has 3 dB bandwidth of 0.4 nm. In addition, the related upstream traffic of the optical network unit (ONU) can transmit via the same fiber path. As we know, the feeder and distribution fibers are between the CO and remote node (RN) and RN and ONUs in PON system, respectively. Furthermore, to deliver the FSO signal by the current WDM-PON architecture, the access points (APs) and optical wireless units (OWUs) are needed to build for FSO connection [24; 25], as illustrated in Figure 1b. Hence, the FSO traffic can be integrated in PON for optical wireless data access. To provide the fiber and FSO data links in WDM access network simultaneously, a simple PON architecture is proposed as shown in Figure 2. In the CO, the C- and L-band WDM laser sources can be applied to regard as the baseband and FSO signals, respectively. Thus, all the WDM TRx of C- (TRx\({}_{\text{1C}}\), TRx\({}_{\text{2C}}\),... and TRx\({}_{\text{NC}}\)) and L-bands (TRx\({}_{\text{1L}}\), TRx\({}_{\text{2L}}\),... and TRx\({}_{\text{NL}}\)) are connected to the corresponding port of the \(1\times N\) arrayed waveguide grating (AWG\({}_{\text{1}}\) and AWG\({}_{\text{2}}\)) for the downstream fiber and FSO wavelength connections, respectively. In addition, the corresponding TRx\({}_{\text{(N+1)C}}\), TRx\({}_{\text{(N+2)C}}\),... and TRx\({}_{\text{(2)NC}}\), and TRx\({}_{\text{(N+1)L}}\), TRx\({}_{\text{(N+2)L}}\),... and TRx\({}_{\text{(2)L}}\) are placed in the corresponding ONU and OWU for fiber and FSO upstream traffics, respectively. However, in the PON access, the fiber fault also an important issue for high-speed fiber data link. Thus, the self-protected mechanism against fiber breakpoint will be significant consideration. In this investigation, to provide the fault protection of C-band fiber signal link simultaneously, the wideband 1 \(\times\) 2 and 50:50 optical coupler (CP) and C/L band WDM coupler (WCP) are also required to exploit at the output port of AWG\({}_{1}\) and AWG\({}_{2}\), respectively, as seen in Figure 2. The corresponding downstream fiber (\(\lambda_{\rm IC}\), \(\lambda_{\rm 2C}\),... and \(\lambda_{\rm NC}\)) and FSO signals (\(\lambda_{\rm 1L}\), \(\lambda_{\rm 2L}\),... and \(\lambda_{\rm NL}\)) will be separated at the RN by the AWG\({}_{3}\) and AWG\({}_{4}\), respectively, and then enter the same corresponding ONU, as illustrated in Figure 2. Here, the AWG\({}_{1}\) to AWG\({}_{4}\) are the same components in the proposed PON network. A subscript forth from 1 to 4 is used to distinguish where the AWG is placed. As we know, due to the periodic characteristic of AWG, the downstream wavelengths of \(\lambda_{\rm IC}\) and \(\lambda_{\rm 1L}\), \(\lambda_{\rm 2C}\) and \(\lambda_{\rm 2L}\),... and \(\lambda_{\rm NC}\) and \(\lambda_{\rm NL}\) can enter the connected port of \"1\", \"2\",... and \"N\", respectively. Hence, the downstream fiber and FSO signals of \(\lambda_{\rm IC}\) and \(\lambda_{\rm 1L}\), \(\lambda_{\rm 2C}\) and \(\lambda_{\rm 2L}\),... and \(\lambda_{\rm NC}\) and \(\lambda_{\rm NL}\) are sent from the CO via the AWG\({}_{1}\), AWG\({}_{2}\), AWG\({}_{3}\), and AWG\({}_{4}\), respectively, and arrive the corresponding ONU\({}_{1}\), ONU\({}_{2}\),... and ONU\({}_{N}\). Similarly, the upstream fiber and FSO signals of \(\lambda_{(N+1)\rm C}\) and \(\lambda_{(N+1)\rm L}\), \(\lambda_{(N+2)\rm C}\) and \(\lambda_{(N+2)\rm L}\),... and \(\lambda_{(2N)\rm C}\) and \(\lambda_{(2N)\rm L}\) are also through the same connected ports for data transmission via the AWGs, respectively, as seen in Figure 2.  
As plotted in Figure 2, the ONU\({}_{1}\) includes a WCP, a 1 \(\times\) 2 optical switch (SW), a fiber collimator (FC) and a TR\(\chi_{(N+1)\rm C}\).