## 4 Implementation of ROA\({}_{3}\)
### Optimum Design of the Bidirectional EDFA
#### 4.2.2 Backup Optical Amplifier for ROA\({}_{2}\)  
For ROA\({}_{3}\) to serve as a backup optical amplifier for ROA\({}_{2}\), its pumping configuration must be forward pumping in order to provide the function of high gain and low NF, similar to those of a pre-amplifier (pre-amp).  
The simulation setup is shown in Figure 10. From Figure 10a, the total IL from A to B is 6.54 dB; hence, the input optical power of the EDFA is \(-9.54\) dBm. Because the minimum input optical power of commercial C-band EDFA pre-amplifiers ranges from \(-25\) to \(-30\) dBm per channel [8], the input optical power of the EDFA can be reduced to \(-25\) dBm by an attenuator.  
The NF and the gain of the forward-pumping EDFA vary with the EDF length at different pumping powers of 50, 100, and 150 mW, as shown in Figure 11. To achieve the gain requirement of 30 dB, the EDF length and pumping power must be greater than 7.5 m and 50 mW, respectively. Furthermore, the NF decreases as the pumping power increases when the EDF length is less than 9 m. For an EDF length greater than 9 m, NF increases as the pumping power increases.  
Figure 10: Simulation setup of a backup optical amplifier for ROA\({}_{2}\): (**a**) detailed setup and (**b**) simplified setup (ATT: attenuator, RBW: resolution bandwidth, OSA: optical spectrum analyzer).  
Figure 9: NF vs. EDF length and P\({}_{\text{out}}\) vs. EDF length at various pumping power values.  
Table 4 lists the parameters of the EDFA in ROA\({}_{3}\) for the simulation results. For the EDFA having a backward- or forward-pumping configuration, the pumping power is 200 mW or 50 mW, respectively. In a practical application, the pump power can be adjusted by the driving current of the pump laser for achieving energy efficiency.  
The simulated EDFA performance is experimentally verified by assembling an EDFA by a commercial highly doped EDF, a pump laser, a wavelength multiplexer, and an optical isolator into a forward- or backward-pumping configuration. The experimental setup is shown in Figure 12 for backward- and forward-pumping configurations. A 10 Gb/s MZM transmitter is used as the signal source with an output optical power of \(-3.3\) dBm. The pumping source is a 1480 nm pump laser, and a 7.5 m EDF is used as the gain medium. Since the maximum output current of the laser controller is limited at 1 A, the output power of the pump laser can only reach 178 mW.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Model Parameter** & **Value** \\ \hline Er\({}^{3+}\) Core Radius of EDF & 3 \(\mu\)m \\ Length of EDF & 7.5 m \\ Pumping Power & 50 or 200 mW \\ Pumping Wavelength & 1480 nm \\ Er\({}^{3+}\) Concentration of EDF & 1.25 \(\times\) 10\({}^{25}\) m\({}^{-3}\) \\ Pumping Configuration & Forward or Backward \\ \hline \hline \end{tabular}
\end{table}
Table 4: Parameters of EDFA in ROA\({}_{3}\).  
Figure 11: NF vs. EDF length and gain vs. EDF length at various pumping power values.  
Figure 12: Experimental setup for the EDFA in ROA\({}_{3}\): (**a**) backward-pumping configuration and (**b**) forward-pumping configuration.  
The output optical power curve under different input optical powers in the backward-pumping configuration, Figure 13a (curve) shows that an output optical power of 16 dBm can be obtained when the input power is \(-\)10.8 dBm. If the pump laser can output 200 mW of pumping power the output power is expected to reach 20 dBm. Figure 13b (curve) shows the gain curve under different input optical powers in the forward-pumping configuration. When the input power is \(-\)25 dBm, the gain can reach 31 dB, which is in line with the simulation results, and the NF value is less than 4 dB.