Polarization-dependent gain in a Raman fiber amplifier in the depleted pump regime is investigated. Numerical investigations are realized and show that gain fluctuations are highly reduced when signal and pump interactions allow pump depletion.
This article presents an experimental investigation of polarization pulling based on stimulated Raman scattering induced by a continuous, fully polarized pump wave in an optical fibre. By means of probe wave states of polarization aggregation, degree of polarization and gain analysis, an assessment of all-optical polarization control, repolarization and amplification in a standard single mode fibre in depleted pump regime is presented. Polarization pulling is proved to be dependent on fibre length, pump power, and pump state of polarization, with respect to pump depletion effect, acting oppositely to the polarization pulling effect. Authors evaluate results aiming to obtain a configuration of stimulated Raman scattering parameters, which allows for the most effective polarization pulling, with respect to statistical stability of the process measured by standard deviation of polarization degree and polarization-dependent gain of the probe wave. The highest polarization attraction efficiency was obtained for 15 km, 0.97 W and the horizontal polarization state of the pump wave.
Polarization-dependent gain (PDG) in fiber amplifiers is studied theoretically. A mathematical model is developed by treating the dopant ions in glass host as partially anisotropic oscillators, and both the signal and the pump effects are taken into account. It is illustrated that PDG includes two components. One arises from polarization-selective excitation of the dopant ions by the pump; the other originates from polarization-selective deexcitation of the lasing ions by the saturating signal, known as polarization hole burning (PHB). PDG depends strongly on the operating state of the amplifiers such as the level of gain compression, the anisotropy of the dopant ions, and the relative polarization orientations of the signal and the pump. In a small-signal regime, PDG is induced only by the pump while in large-signal condition it is the sum of the two components. The good agreement between the theoretical calculations and the reported results confirms the validity of the model.
In fiber optical parametric amplifiers (FOPAs), the output states of polarization (SOPs) of both the signal and the idler are affected by the pump SOP through the polarization pulling effect. We proposed a normalized model to characterize the general dynamics of polarization pulling in degenerated pump FOPAs. By using this model, we determined the relationship between the output degree of polarization (DOP) and the gain of the FOPAs over three orders of magnitude changes in polarization-mode dispersion (PMD) coefficient. We demonstrated numerically that when fibers with typical PMD coefficients are used, the output signals of the FOPAs can achieve higher DOP than those of co-propagating fiber Raman amplifiers, which depends on whether the parametric gain is sufficiently large to compensate the PMD-induced phase mismatch. We also found that for FOPAs using different types of fibers with moderate PMD coefficients, this DOP-gain relationship exhibits highly similar behaviors. We thus adopted a set of modified empirical formulas to analyze this relationship. This formula can also describe the relationship between the output DOP and the conversion efficiency of the idler. The results indicate that in all cases studied, FOPAs achieve much stronger polarization pulling to the idlers than the signals.