# 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  
### Design and Analysis  
In order to analyze the quality of the signal transmission using DFB-EAM-SOA, we first tested the unidirectional transmission for both DS and US; then, we tested the bidirectional transmission under the optimal conditions we had gotten; finally, both ONUs were active and tested bidirectionally. In Figure 2, we propose a network schematic with 50 GHz channel spacing as in the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) G.694.1 standard for DWDM [22, 23]. The optical line terminal (OLT) is composed of several transceivers, and each transceiver corresponds to one ONU. The main parameters list of the simulation setting is shown in Table 3.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multicolumn{1}{c}{**SOA**} & \multicolumn{3}{c}{**Input Power**} \\  & & **Low** & **Medium** & **High** \\ \hline \multirow{4}{*}{Bias current} & low & low gain, & low gain, & no gain, \\  & high NF & high NF & high NF & high NF \\ \cline{2-5}  & medium & moderate gain, & moderate gain, & low gain, \\  & moderate-high NF & moderate-low NF & high NF \\ \cline{2-5}  & high & high gain, & high gain, & low gain, \\  & high & high NF, & low NF, & high NF, \\  & suitable for phase modulation & suitable for optical amplification & suitable for amplitude modulation \\ \hline \hline \end{tabular}
\end{table}
Table 2: Areas of operation for an SOA and its exploited performance for their application.  
Figure 2: The semiconductor optical amplifier electro\(-\)absorption modulator (SOA\(-\)EAM)—based optical network unit (ONU) for 10 Gb/s bidirectional transmission (ODN: optical distribution network; C1, C2: circulator; Mux/Demux: multiplexer/demultiplexer; VOA: variable optical attenuator; PIN: PIN\(-\)diode; TIA: trans\(-\)impedance amplifier).  
For DS transmission, the continuous wave light injected into the Mach-Zehnder modulator (MZM) employed at the OLT was modulated in non-return-to-zero (NRZ) format by a pseudo-random binary sequence (PRBS), which was generated by a distributed feedback (DFB) laser. The optical signals with different wavelengths were multiplexed and carried. After transmission over a 20 km standard single-mode fiber (SSMF), the signal was demultiplexed to different ONUs of corresponding wavelengths. The optical signal was converted into a low-noise electrical signal through a photoelectric detector and an equalizing filter. Finally, the bit error ratio (BER) was detected and analyzed.  
For US transmission, an SOA-EAM was selected to improve the transmission distance of data signals because of its wide modulation bandwidth and low chirp, as depicted in the schematic view of Figure 2. There was leading edge distortion (as pre-distortion) to form an undershoot mode when data passed through the EAM, shown in the inset (I) of Figure 2, while the carrier recombination limitation in the SOA saturation led to the formation of an overshoot mode, as the inset (II) of Figure 2 shows; such mutually compensated optical processing can be adjusted by the bias point, and the whole system can emit an undistorted optical waveform (see the inset (III) of Figure 2) [18]. Additionally, the chirp from the absorption processing of an EAM can be compensated for by adjusting the negative frequency chirp from the carrier depletion of the SOA, which can reduce the chirp generated by the SOA-EAM and the dispersion effect in the optical transmission system [17]. The signal was detected at the OLT after a 20 km transmission.