## 3 Performance Improvement of Three-Segment Raman Amplified Link
These two results strongly indicate the advantage of the span with the three-segment hybrid fiber core structure compared to the span with the conventional single fiber core structure. Figure 3: Pump power distribution of a bidirectional pumped distributed Raman amplification span for the three-segment hybrid fiber core structure compared to the conventional single fiber core structure over a propagation distance of 390 km. The nonlinear Kerr effect in the form of self-phase modulation (SPM) induces a nonlinear phase shift that indicates the degree of signal distortion and the reduction of quality factor [28]. In a single-channel optical transmission system, the amount of nonlinear phase shift (\(\Phi_{NL}\)) is proportional to the integral of signal power density, as shown in Equation (8). Figure 5 shows a comparison of the nonlinear phase shift between the three-segment hybrid fiber effective core area structure and the conventional single-segment single fiber core structure along the 390-km fiber transmission span. The span with the three-segment hybrid fiber core structure has a lower accumulated nonlinear phase shift compared to the conventional single fiber core structure. The decrease of 1.29 radian is due to the lower signal power density of the three-segment hybrid fiber core structure at the first segment (0-90 km). Figure 6 illustrates a comparison of the OSNR profile of a 1 mW signal between the three-segment hybrid fiber effective core area structure and the conventional single-segment single fiber core structure along a 390-km fiber transmission span. The span with the three-segment hybrid fiber core structure achieved a 0.31 dB higher OSNR compared to the conventional single fiber core structure. Figure 4: Signal power density of the three-segment hybrid fiber core structure compared to the conventional single fiber core structure. The hybrid fiber core structure has a lower signal power density on the first segment and a higher power density on the third segment, which indicates two advantages for the system performance. Figure 5: Nonlinear phase shift of the three-segment hybrid fiber core structure compared to the conventional single fiber core structure. The hybrid fiber core structure has a lower SPM-induced nonlinear phase shift, which helps to reduce nonlinear signal distortion. This improvement is due to the higher minimum value of the signal power density at the third segment (290-390 km). The OSNR calculation is based on the single-polarization of the single-sided-passband power spectrum density of the optical signal and the dual-polarization of the Raman ASE noise. Table 4 provides a summary of the numerical analysis results for the nonlinear phase shift (NLPS) and OSNR for the conventional single fiber core structure and three-segment hybrid fiber core structure, respectively. The numerical analysis results show that improvements to the NLPS and OSNR can be achieved simultaneously. This achievement would be impossible with the sole use of a conventional single fiber core structure. The realization of a modulated signal in the transmission simulation, which is expected to improve transmission performance, will be explained in the next section.