Study On Stimulated Brillouin Scattering And Its Suppression Techniques In Fiber Raman Amplification Systems

In this thesis,Raman and Brillouin scatterings,the nonlinear processes,in fiber are first introduced.Besides their general properties,particular attention is attributed to their polarization states and the consequent effects.The polarization mixing efficiency,describing the alignment between the polarization states of a pump and a signal both for the Raman and Brillouin scattering processes,in fiber Raman amplification systems are systematically discussed.A key theoretical analysis on Raman and Brillouin gains,when differently polarized pump is launched,is conducted,after which a pair of form-unified and approximate equations for the thresholds measured in power of both stimulated Raman and Brillouin scatterings with arbitrary polarization states is deduced.Emphasis has been paid to the fiber Raman amplification systems,for which it is assumed that the pump power is launched forwardly resulting in a forwardly propagating Raman signal and a backwardly propagating Brillouin scattering process.In fiber Raman amplification systems,it is proposed that Faraday rotation is introduced to reinforce the suppression on Brillouin scattering process.An externally applied magnetic field can lead to the rotation effect,in which the polarization states of the linearly polarized pump and Raman signal are designed to keep aligned and a resulted high Raman gain.While the linearly polarized pump and Brillouin scattering process periodically misalign because of the rotation,leading to a reduction on the Brillouin gain effectively.Based on the misalignment,the polarization mixing efficiencies are normalized and represented,combining both polarization overlapping and pump depletion.The polarization mixing efficiency for Raman remains unity while that for Brillouin decreases from its asymptote of 1 to 0.5.The Brillouin scattering can be suppressed by changing the pump depletion or the polarization misalignment by means of reducing the pump attenuation coefficient of fiber,enlarging the Verdet constant of material or strengthening the external magnetic field.Utilizing the described theory,a novel Brillouin scattering suppression scheme for fiber Raman amplification systems with an applied magnetic field was designed and proposed.It is found that the rotation effect induced by the applied magnetic field can extra suppress the Brillouin gain by nearly 3 dB,additionally,without sacrificing the Raman efficiency.