## 7 Discussion
### Dual Pump PSA with DOF  
For the case of a dual pump PSA, our focus was primarily fixated on improving the signal gain of the system. Using analytical and numerical approaches, we argued that a sinusoidally varying profile can provide the largest gain, while profiles with multiple frequencies or a uniform profile tend to flatten out the gain spectrum. Thus we considered a dual pump PSA system with a sinusoidal dispersion profile and numerically evaluated the maximum signal gain by scanning the dispersion oscillation amplitude \(\tilde{\beta}_{2}\) and the average second order dispersion \(\overline{\beta}_{2}\). The effect of dispersion oscillation amplitude was found to be pronounced, only when \(\overline{\beta}_{2}\) is close to 0. This is because only then \(\tilde{\beta_{2}}\) becomes comparable to \(\overline{\beta}_{2}\) and the dispersion oscillation effects are not averaged out over the fiber length. For the case of \(\overline{\beta}_{2}\) is close to 0, depending on the choice of \(\tilde{\beta_{2}}\), a large range of gain, gain flatness and gain bandwidth is possible.  
On the other hand, with respect to the SBS performance, although the sinusoidal profile had a higher SBS threshold than a uniform or linear profile, it was slightly outperformed by other profiles obtained through optimization techniques. Therefore, for highly specific applications with targeted gain, gain bandwidth and gain flatness requirements, an advanced nonlinear optimization approach is required to optimize the SBS as well as the PSA performance simultaneously. However, our investigation outright suggests the advantage of using a DOF as a dual pump PSA over a standard HNLF in terms of achieving a larger PSA gain, considering the same nonlinearity in both. To validate these claims, we performed two experiments, one for the SBS and the other for the PSA to compare the performance of a DOF with that of a standard HNLF. In the SBS experiment, the DOF exhibited about 3 dB improvement of the SBS threshold over the HNLF, as expected. However, the conversion efficiency (CE) of the DOF was found to be about 10 dB lower in the case of the DOF compared to the HNLF. But the CE result should be treated with a caveat, since the DOF and the HNLF used in the experiment had different lengths, dispersion values and nonlinear coefficients. In fact, a numerically calculated CE of a DOF with the same nonlinear parameters of the HNLF shows that a DOF and a HNLF can provide the same CE, which is a crucial quantifier for the PSA gain of the system. These results convincingly prove that a DOF can outperform a standard HNLF in terms of its gain performance in a dual pump PSA system.  
Another important technical question about the PSA functionality of a DOF is that how to fabricate a DOF with a large nonlinear coefficient? This question might be addressed in a few different ways. For example, the nonlinear coefficient of a nonlinear fiber can be increased by reducing further the average core radius of the DOF. However that might lead to a change in the dispersion properties of the fiber and hence additional nonlinear optimization may be required to optimize the PSA performance. Also, changing the doping concentration in the preform might be a resource to ramp up the nonlinear coefficient of the fiber. Additionally, modifying the fiber preform profile might be another approach in this regard, that can not only provide a handle over controlling the fiber nonlinearity, but also the acousto-optic overlap for SBS mitigation [55].