## 3 Experiments and Results
The BERs were calculated using a fixed-threshold decision (FTD) under the variation of the average input powers at the SOA. The average input power at the PD was set to \(-\)3 dBm using VOA5 to reduce the PD noise. Regarding the OOK transmission, the BERs were initially enhanced in the gain saturation state of the SOA due to the scintillation effect mitigation by the nonlinear gains. However, the BERs were decreased in the deeper gain saturation state due to the ER degradation of the OOK during the process of scintillation mitigation. Regarding the 2 PS-OOK transmission, the BERs were improved with the increase of the average input powers at the SOA since the degraded ER was recovered by the filtering of the polarization of bit \"0\" using the LP. Regarding the 3 PS-OOK and 4 PS-OOK transmissions, the intensities of the various SOPs were equalized by the SOA in the deep gain saturation state, and the equalized SOPs were converted into the different intensity levels by the LP. Thus, the BERs of the 3 PS-OOK and 4 PS-OOK were improved with the increase of the average input powers at the SOA. Poor BER performances of 3 PS-OOK and 4 PS-OOK were observed for lower average input powers at the SOA due to the ineffective intensity equalization of the various SOPs. The BERs of the 3 PS-OOK and 4 PS-OOK were close to the 2 PS-OOK for the higher average input powers at the SOA due to the effective intensity equalization of the various SOPs in the deep gain saturation state. Furthermore, much higher average input powers at the SOA were required for a \(\sigma_{I}^{2}\) of 0.59 compared to a \(\sigma_{I}^{2}\) of 0.11 since a deeper gain saturation was needed for a \(\sigma_{I}^{2}\) of 0.59 in order to effectively mitigate the scintillation effect. Figure 6 shows the SEs of the proposed technique at a BER of \(1~{}\times~{}10^{-3}\). The received MPS-OOK signal had a high signal-to-noise ratio (SNR) due to the large output power at the SOA; therefore, the SE of the proposed technique was dependent on the average input powers at the SOA and the strength of the scintillation effect. The SE was calculated using \(\log_{2}^{M}\) under the effective scintillation mitigation, where \(M\) indicates the number of levels of the SOP [15; 16]. For the average input power at the SOA of less than \(-\)4 dBm, the SE was similar to the OOK and 2 PS-OOK with \(\sigma_{I}^{2}\)'s of 0.11 and 0.59. Regarding the average input power at the SOA of \(-\)2 dBm, the SE was improved by up to 2 bit/s/Hz with a \(\sigma_{I}^{2}\) of 0.11. For an average input power at the SOA of 6 dBm, the SE was enhanced by up to 2 bit/s/Hz with an \(\sigma_{I}^{2}\) of 0.59 as well. Consequently, the SE was improved by up to 2 bit/s/Hz under effective scintillation mitigation using the proposed technique with the average input power at the SOA of 6 dBm and a symbol rate of 1.25 Gsymbol/s. The SE can be improved by coherent detection instead of direct detection using an M-FTD, and it can be further enhanced by  
Figure 4: Eye diagrams of the proposed 3 PS-OOK and 4 PS-OOK transmissions without the scintillation effect. using an MZM with a larger ER, an SOA with lower amplified spontaneous emission (ASE) noises, an LD with a narrower linewidth, and a PD with a higher sensitivity [15; 17].