# Integrated Fiber-FSO WDM Access System with Fiber Fault Protection
## 3 Experiment and Results  
In the experiment, we use a C-band tunable laser source (TLS) to provide the different output wavelengths for the downstream (\(\lambda_{\text{IC}}\) to \(\lambda_{\text{IC}}\)) and upstream fiber links (\(\lambda_{\text{SC}}\) to \(\lambda_{\text{SC}}\)), as displayed in Figure 5a. The TLS (General Photonics, TLS-101-C) has ITU-T specification tuning range from 1528.77 to 1563.86 nm with 13 dBm largest output power and 0.4 nm tuning step. Each WDM wavelength connects to the polarization controller (PC) and 10 GHz Mach-Zehnder modulator (MZM) to carry 10 Gb/s on-off keying (OOK) modulation  
Figure 4: The new designed architectures of RN, ONU, AP and OWU in the hybrid fiber-FSO WDM-PON network. Figure 3: The occurrence of fiber fault at the (**a**) “F1” and (**b**) “F2” points in the presented WDM-PON system, respectively. signal for regarding as the symmetric downstream and upstream traffics in the CO, ONU, respectively. The PC can control the polarization state and obtain the optimal output power. Two 3-port optical circulators (OC\({}_{1}\) and OC\({}_{2}\)) are applied to isolate the downstream and upstream transmissions. The modulated signal will travel through a length of 25 km single-mode fiber (SMF) for bidirectional data connection. Furthermore, to enhance the modulation signal performance, an optical pre-amplifier is placed in front of the 10 GHz PIN photodiode (PD) for measurement. To demonstrate the FSO transmission in PON access, we can use a new setup for signal measurement, as plotted in Figure 5b. Here, we also apply 10 Gb/s OOK on the MZM for the wireless FSO traffic. To generate and collect the FSO signal, two FCs are exploited between the ONU and OWU. In the experiment, the related parameters of diameter, divergence angle and focal length of FC are 2 cm, 0.016\({}^{\circ}\) and 3.713 cm, respectively. In addition, the acceptable wavelength bandwidth of FC is from 1050 to 1620 nm. The wireless FSO length is \(\sim\)2 m between OWU and ONU for the convenience of measurement. Moreover, the SMF link is also set at 25 km for data transmission. Through the optimal alignment of FSO link, the insertion loss between two FCs is around 3 dB here. Next, to realize the 10 Gb/s OOK bit error rate (BER) performance, eight WDM wavelengths of 1535.04 (\(\lambda_{\text{1C}}\)), 1535.82 (\(\lambda_{\text{2C}}\)), 1536.61 (\(\lambda_{\text{3C}}\)), 1537.40 (\(\lambda_{\text{4C}}\)), 1538.19 (\(\lambda_{\text{5C}}\)), 1538.98 (\(\lambda_{\text{6C}}\)), 1539.77 (\(\lambda_{\text{7C}}\)), and 1540.56 nm (\(\lambda_{\text{8C}}\)) are selected as the downstream (\(\lambda_{\text{1C}}\) to \(\lambda_{\text{4C}}\)) and upstream fiber traffics (\(\lambda_{\text{5C}}\) to \(\lambda_{\text{8C}}\)), respectively. Based on the periodic feature of AWG, the wavelengths of \(\lambda_{\text{1C}}\) and \(\lambda_{\text{3C}}\), \(\lambda_{\text{2C}}\) and \(\lambda_{\text{6C}}\), \(\lambda_{\text{3C}}\) and \(\lambda_{\text{7C}}\), and \(\lambda_{\text{4C}}\) and \(\lambda_{\text{8C}}\) can be through the connected ports of 1 to 4 for bidirectional signal link, respectively. In the experiment, the output power of TLS at the port \"2\" of OC\({}_{1}\) and OC\({}_{2}\) is set at 7.5 dBm to carry the OOK modulation signal to avoid the nonlinear effect, as illustrated in Figure 5a. Here, the measured 10 Gb/s OOK BER performances of the wavelengths of \(\lambda_{\text{1C}}\) to \(\lambda_{\text{8C}}\) at the back-to-back (BTB) state are observed in Figure 6a, respectively. The measured power sensitivities of \(\lambda_{\text{1C}}\) to \(\lambda_{\text{8C}}\) are \(-29.5\), \(-28\), \(-27\), \(-26\), \(-25.5\), \(-25.5\), \(-25\) and \(-25\) dBm at the forward error correction (FEC) level (BER \(\leq 3.8\times 10^{-3}\)), respectively. Hence, the achievable power budgets of \(\lambda_{\text{1C}}\) to \(\lambda_{\text{8C}}\) are 36.5, 35.5, 34.5, 33.5, 33, 33.25 and 32.5 dB, respectively. Through 25 km SMF transmission, the 10 Gb/s BER measurements of eight wavelengths are exhibited in Figure 6b.