## 3 Performance Evaluation-Simulation Results
Leakage:  
* Lu: leaked upper-band signals to the lower section,
* L\({}_{\text{L}}\): leaked lower-band signals to the upper section. Idlers from leakage:  
* \(i_{\text{L}}\): idlers generated from leaked upper-band signals in the lower section,
* \(i_{\text{L}}\): idlers generated from leaked lower-band signals in the upper section. We focus on the two CWDM channels that are at the edges of the two bands (e.g., 1330 nm and 1350 nm). These two wavelengths represent the two worst-case scenarios, since they are closer to the OPA pump wavelength and at the roll-off of the filter transfer functions. Hence, they and their associated crosstalk terms experience the least suppression through the filters. The other channels experience isolation of \(>\)40 dB, and hence, they do not significantly contribute to any signal deterioration within the system. We must note here that in a CWDM system, the wavelength channel is defined within a \(\pm\)6 nm bandwidth [1]. Figure 7 shows the placement of 1330 nm and 1350 nm signals under the filter transfer function with their allowable bandwidth of \(\pm\)6 nm (shaded area). Therefore, the effect of the generated crosstalk terms (e.g., idlers and leakage) strongly depends on the actual  
Figure 6: (**a**) Available signal gain of the 170-nm bandwidth for the given fiber parameters and selection of pump optical powers and wavelengths; (**b**) noise figure (NF) performance. transmitter wavelength. We investigate the effect of simultaneous transmitter wavelength misalignment from the ITU defined center channel of 1330 nm and 1350 nm in the generation of crosstalk terms and the impact on the design of the HROPA. This misalignment can be up to 6 nm on either side of the designed center wavelength of the signal band [1]. We identify the worst case scenario as the one with \(S_{\mathrm{U}}\) at 1344 nm and \(S_{\mathrm{L}}\) at 1336 nm. Figure 8 shows all of the signals and crosstalk terms for the above worst-case scenario at point C1 and C2 of the HROPA. Points C1 and C2 are defined in Figure 1 and correspond to the 1350-nm port and 1330-nm port of the CWDM, respectively. As can be seen in Figure 8a, the main crosstalk term on the \(S_{\mathrm{u}}\) channel (1344 nm) is \(i_{L\mathrm{L}}\) (the idler generated from the leaked lower band signal at 1336 nm in the upper arm of the OPA section), and its crosstalk is \(\sim\)-11 dB. Similarly, at point C2, the highest crosstalk term on \(S_{\mathrm{L}}\) channel (1336 nm) is \(i_{SU}\) (\(\sim\)-6 dB), as shown in Figure 8b. In the first case, the crosstalk term \(i_{L\mathrm{L}}\) is under the pass band of the WDM MUX, whereas in the second case, the \(i_{SU}\) is under the roll-off of the filter transfer function, and hence, they are not filtered out or suppressed  
Figure 8: Amplified signals and crosstalk terms after the CWDM (**a**) at point C1 or the 1350-nm port, with a worst-case crosstalk of \(-\)11 dB from \(i_{\mathrm{L}}\), and (**b**) at point C2 or the 1330-nm port, with a worst case crosstalk of \(-\)6 dB from \(i_{\mathrm{SU}}\). Figure 7: The possible wavelength position of the 1330 nm (\(\pm\)6 nm) and 1350 nm (\(\pm\)6 nm) channels under the filter transfer function (P\({}_{\mathrm{P}}\) OPA pump). enough as they pass through the WDM MUX. Crosstalk terms at the other wavelengths do not contribute to the signal degradation, because they are suppressed by the filters and have much lower optical power. Using the Synopsis RSoft simulation tool, we evaluated the bit error rate (BER) performance for the above worst-case scenarios for standard non-return-to-zero (NRZ) 10-Gbps signals. The receiver was calibrated for a BER of 10\({}^{-12}\) at a received optical power of -28.5 dBm. The highest crosstalk term _i_su at -6 dB affects the BER performance significantly, giving a BER of \(>\)10\({}^{-2}\) at the 1330-nm port. To improve the amplifier performance so that it can be utilized in a PON requires further suppression of the crosstalk terms.