拉曼增益对高双折射光纤中暗孤子俘获的影响

Raman effect on dark soliton trapping in high birefringence fiber

从高双折射光纤中含有拉曼增益的耦合非线性薛定谔方程出发,利用拉格朗日方法,推导出了暗孤子俘获的阈值,并利用快速分步傅里叶变换,模拟了孤子的两个正交偏振分量的演化,对比了两种方法得到的阈值,探究了暗孤子俘获受拉曼增益的影响.研究发现解析解所得阈值比数值解偏小,且群速度失配越小时,二者符合得越好;并且拉曼增益减小了暗孤子的俘获阈值,当平行拉曼增益增大时,俘获阈值减小加快.

Not only the interaction between optical pulse and orbital electron but also the interaction between optical pulse and optical phonon needs to be considered when input pulse energy is large.The latter induces the simulated Raman scattering,thus generating the Raman gain.We analyze the effect of Raman gain,especially parallel Raman gain,on dark soliton trapping in high birefringence fiber by analytical method and numerical method.In the first part,we introduce some research results of soliton trapping obtained in recent years.In the second part,the coupled nonlinear Schrodinger equation including Raman gain is utilized for high birefringence fiber.The trapping threshold of dark soliton with considering the Raman gain is deduced by the Lagrangian approach when input pulse is the dark soliton pulse that the amplitude of two polarized components of the dark soliton are the same（see formula（26））.Pig.1.shows the relation between threshold and parallel Raman gain according to formula（26） when group velocity mismatching coefficient values are 0.15,0.3,and 0.5（vertical Raman gains are all 0.1）.In the third part,the propagation of the two orthogonal polarization components of dark soliton is simulated by the fractional Fourier transform method.Figures2-4 show respectively dark soliton trapping with group velocity mismatching coefficient values of 0.15,0.3 and 0.5.We consider three situations in which Raman gain is not included and the parallel Raman gains are 0.4 and 0.6 when vertical Raman gains are both 0.1 in different group velocity mismatching coefficient values.We obtain the threshold of dark soliton by numerical method under different conditions and analyze the figures.At the same time,we compare the analytical solution with the numerical solution and discuss the effect of Raman gain on dark soliton trapping.The last part focuses on our conclusion.It is found that the threshold which is obtained by analytical method is smaller than that from the numerical solution.The difference between the analytical and numerical dependences decreases with group velocity mismatching coefficient decreases.As a result,formula（26） is in good agreement with numerical data for small group velocity mismatching.The larger the group velocity mismatching,the larger the amplitude threshold of dark soliton is.It also shows that the amplitude threshold of dark soliton can be reduced due to Raman gain and the threshold is reduced faster with the increasing of Raman gain.