Due to the presence of stimulated Raman Scattering (SRS) in the HNLF, it is possible that the optimal power distribution between the pumps may be obtained when their power is not equal.
4 THz pump more powerful the power distribution is approximately equal over a longer span of HNLF.
It was found that the minimal spacing from the pumps to the ZDF of the HNLF that could ensure that the signal is placed in the flat part of the gain spectrum with as high gain as possible was 1.
Even when the pumps are placed symmetrically with respect to the ZDF, this mismatch increases along the HNLF due to higher order dispersion and polarization mode dispersion (PMD).
This can be explained by the influence of self-phase modulation (SPM) nonlinearity that occurred during the process of amplification, and by the presence polarization mode dispersion (PMD) in the HNLF.
This is why for implementing FOPAs polarization maintaining HNLF fibers should be used, where the value of PMD does not change under the influence of external factors.
Thus this has helped to obtain configuration of the FOPA, for which phase mismatch that occurred due to linear and nonlinear effects in the HNLF was minimized.
Of course the usage of a phase modulator has lowered the pump power level at the input of the HNLF by about 3 dB, but this has significantly raised the total level of pumping power that could be launched into the HNLF without scattering a significant part of it on generation of SBS parasitic harmonics.
2 dB in comparison with the system with no SBS mitigation for the same level of pump power at the input of the HNLF.
The typical HNLF length we use in our experiments is several hundred meters, the precise value depending on the necessary gain and the pump power.
Utilizing a system with a single FOPA amplification stage we found a relationship between the length of HNLF and the pump power.
3 shows the minimal values of HNLF length and the pump power, i.