# Hybrid Self-Protected Fiber-FSO WDM-PON System with Fiber Breakage Prevention
## 4 Result and Discussion  
Figure 4a,b indicate the measured 24.3 Gbit/s BER behaviors of \(\lambda_{\rm A1D}\) to \(\lambda_{\rm A4U}\) and \(\lambda_{\rm B1D}\) to \(\lambda_{\rm B4U}\) at the back to back (BB) condition, respectively. To stay below the forward error correction (FEC) limit (BER of \(3.8\times 10^{-3}\)), the detected power sensitivities of \(\lambda_{\rm A1D}\) to \(\lambda_{\rm A4U}\) and \(\lambda_{\rm B1D}\) to \(\lambda_{\rm B4U}\) are \(-33.5\), \(-33.5\), \(-33.5\) and \(-34.0\) dBm, and \(-34.0\), \(-33.5\), \(-33.5\) and \(-32.5\) dBm, respectively, as displayed in Figure 4a,b. The received powers of the fiber and FSO signals are between \(-24.0\) and \(-24.5\) dBm under the BER of \(10^{-8}\) to \(10^{-9}\). The measured eye diagrams of \(\lambda_{\rm A1D}\) to \(\lambda_{\rm A4U}\) and \(\lambda_{\rm B1D}\) to \(\lambda_{\rm B4U}\) are drawn in the insets of Figure 4 at the received powers of \(-24.0\) to \(-24.5\) dBm, respectively. These eyes are clear and open. The total length of feeder and distribution fibers is 24 km long, and the protection fiber is 1 km in the next experiment. When the \"A1\" breakpoint occurs in Figure 2, all the SW\({}_{\rm A}\) in the ONU\({}_{\rm A}\) will switch to \"b\" point to retransmit the bidirectional access signal via the protection fiber path. Then, the 24.3 Gbit/s OOK baseband signals of \(\lambda_{\rm A1D}\) to \(\lambda_{\rm A4U}\) will reroute through the FSO path connection for bidirectional transmission, as seen in \begin{table}
\begin{tabular}{c c} \hline Optical Modulator & 40 GHz MZM \\ \hline Optical Receiver & 40 GHz PIN PD \\ \hline Modulation Format & OOK \\ \hline Data Rate & 24.3 Gbit/s \\ \hline Pattern Length & \(2^{15}-1\) \\ \hline Tx Output Power & 7.5 dBm \\ \hline FSO Length & 2 m \\ \hline SMF Length & 24–25 km \\ \hline FEC Limit & BER \(\leq 3.8\times 10^{-3}\) \\ \hline \end{tabular}
\end{table} 
Table 1: Corresponding parameters of experimental setup. Figure 4: Measured 24.3 Gbit/s BER behaviors of (**a**) \(\lambda_{\rm A1D}\) to \(\lambda_{\rm A4U}\) and (**b**) \(\lambda_{\rm B1D}\) to \(\lambda_{\rm B4U}\) at the BbB status, respectively. Insets are the measured eye diagrams of eight wavelengths at the received powers of \(-24.0\) to \(-24.5\) dBm, respectively. Figure 2. The wireless FSO length is set to 2 m here for convenient measurement. Thus, Figure 5a show the 24.3 Gbit/s BER performances of \(\lambda_{\text{A1D}}\) to \(\lambda_{\text{A4U}}\) after passing through a 25 km fiber link and 2 m free space link without using dispersion compensation. Figure 5b presents the BER measurements of \(\lambda_{\text{B1D}}\) to \(\lambda_{\text{B4U}}\) FSO signals in the original FSO path (without fiber fault) after 24 km fiber and 2 m wireless FSO connections. All the insets of Figure 5 are the corresponding eye diagrams of \(\lambda_{\text{A1D}}\) to \(\lambda_{\text{A4U}}\) and \(\lambda_{\text{B1D}}\) to \(\lambda_{\text{B4U}}\) under the BER of \(10^{-8}\) to \(10^{-9}\). The eyes are also clear. The corresponding power sensitivities of the eight wavelengths are \(-27.0\), \(-27.5\), \(-29.0\), \(-29.0\), \(-27.0\), \(-26.5\), \(-27.0\) and \(-26.0\) dBm within the FEC target, respectively, as shown in Figure 5a,b. In general, the 24 and 25 km SMF transmission makes no difference to BER performance through 2 m wireless FSO link. This is because the received sensitivity decreases due to the dispersion effect as the modulation signal gradually moves to longer wavelengths. Due to the 1 km protection fiber, the WDM baseband signals will increase 0.2 dB insertion loss. The obtained power budgets of Figure 5a,b will be 34.5, 35, 36.5, 36.5, 34.5, 34, 34.5 and 33.5 dB, respectively, at the wavelengths of 1528.77, 1529.55, 1530.33, 1531.12, 1531.90, 1532.68, 1533.47 and 1534.25 nm. So, the redundant powers of the eight wavelengths are 14.7, 15.2, 16.7, 16.7, 14.9, 14.4, 14.9 and 13.9 dB, respectively.