# 10 Gb/s Bidirectional Transmission with an Optimized SOA and a SOA-EAM Based ONU
## 3 10 Gb/s Bidirectional Transmission for a DFB-EAM-SOA-Based ONU
### Results and Discussion  
Figure 3 shows the BER against the received power in unidirectional US and DS transmission. Figure 3a shows the BER characteristics with a signaling rate of 10 Gb/s in a unidirectional uplink. The horizontal axis shows the average received power. The amplitude shift keying (ASK) signal was generated with a peak-to-peak RF voltage of 2 V (\(V_{\rm PP}\)) for the EAM with a bias current of 80 mA (\(I_{\rm bias}\)) for the SOA. At the FEC level of BER = \(2.4\times 10^{-4}\)[20], the sensitivities reached \(-29.5\) dBm, \(-27.5\) dBm, and \(-21\) dBm for input power (Pi) = \(-10\) dBm, \(-5\) dBm, and \(-20\) dBm, respectively. There was a 2 dB power penalty between Pi = \(-10\) dBm and \(-5\) dBm, and an 8.5 dB power penalty between \(-10\) dBm and \(-20\) dBm. The eye diagrams are also shown in the insets of Figure 3a. The eye pattern at Pi = \(-20\) dBm exhibits a thicker 1-level due to the OSNR degradation and at Pi = \(-10\) dBm and \(-5\) dBm shows the typical overshoot on account of the pattern effect [24]. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Value** \\ \hline Wavelength of distributed feedback (DFB) laser in ONU1 & 1592.5 nm \\ Wavelength of DFB laser in ONU2 & 1592.1 nm \\ Wavelength of DFB laser in transceiver1 & 1550.1 nm \\ Wavelength of DFB laser in transceiver2 & 1549.7 nm \\ Bias voltage of electro-absorption modulator (EAM) & \(-2\) V \\ Length of single-mode fiber (SMF) & 20 km \\ Attenuation of SMF & 0.2 dB/km \\ Dispersion of SMF & 16.75 ps/nm/km \\ Non-linear coefficient of SMF & \(2.6\times 10^{-20}\) m\({}^{2}\)/w \\ PMD coefficient of SMF & 0.05 ps/sqrt(km) \\ Extinction ratio of Mach–Zehnder modulator (MZM) & 30 dB \\ Symmetry factor of MZM & \(-1\) \\ Bandwidth of Mux/Demux & 20 GHz \\ Responsivity of PIN & 1 A/W \\ Dark current of PIN & 10 nA \\ Transimpedance of TIA & 2000 Ohm \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Main parameters list of the simulation setting. Figure 3: _Cont._  
We then modulated the EAM with V\({}_{\rm PP}\) = 1 V, 1.5 V, and 2 V. The DFB laser provides an input power of \(-\)10 dBm, and there is I\({}_{\rm bias}\) = 80 mA, as previous. The BER against the received optical power is plotted in Figure 3b. At BER = 2.4 \(\times\) 10\({}^{-4}\), the received power sensitivities are \(-\)29.5 dBm, \(-\)28.4 dBm, and \(-\)25 dBm for V\({}_{\rm PP}\) = 2 V, 1.5 V, and 1 V, respectively. Figure 3b also shows that the power penalties are 1.1 dB and 3.4 dB at BER = 2.4 \(\times\) 10\({}^{-4}\) when V\({}_{\rm PP}\) decreases from 2 V to 1.5 V and from 1.5 V to 1 V, respectively. Figure 3c shows the measured BER with I\({}_{\rm bias}\) = 60 mA, 80 mA, and 100 mA at V\({}_{\rm PP}\) = 2 V and Pi = \(-\)10 dBm, in which the EAM works under reverse bias and the SOA operates at forward bias. The sensitivities of \(-\)27 dBm, \(-\)29.5 dBm, and \(-\)29 dBm were obtained for I\({}_{\rm bias}\) = 60 mA, 80 mA, and 100 mA at BER of 2.4 \(\times\) 10\({}^{-4}\) for a 10 Gb/s signal with PRBS. We show the BER curves of US and DS unidirectional transmissions in Figure 3d under the appropriate condition of Pi = \(-\)10 dBm, V\({}_{\rm PP}\) = 2 V, and I\({}_{\rm bias}\) = 80 mA. At BER = 2.4 \(\times\) 10\({}^{-4}\) for the unidirectional US transmission, the sensitivities reached \(-\)29.8 dBm and \(-\)29.5 dBm for back-to-back (BtB) and 20 km fibers, respectively; in the DS direction, received sensitivities were \(-\)33.6 dBm and \(-\)33.3 dBm for BtB and 20 km fibers, respectively. For bidirectional single-ONU and multi-ONU transmissions, the BER curves and eye diagrams with the optimal condition of Pi = \(-\)10 dBm, V\({}_{\rm PP}\) = 2 V, and I\({}_{\rm bias}\) = 80 mA are shown in Figure 4. Figure 4a presents the US and DS received sensitivities of \(-\)29.5 dBm and \(-\)33.5 dBm for BtB transmission, and the sensitivities reach \(-\)28.9 dBm and \(-\)33.1 dBm after the 20 km fiber transmission at BER = 2.4 \(\times\) 10\({}^{-4}\) for 10 Gb/s.