Transmission And Excitation Of Quadratic Spatial Soliton

It has been known since the early days of nonlinear optics that the cascading of second-order nonlinearity processes generally leads to nonlinear phase shifts.If the wave-vector mismatch of all waves involved is large,the phase shift resembles that achieved with an effective third-order nonlinearity.During the past few years,intense research activity has focused on both the experimental and theoretical investigations of phenomena related to these cascaded second-order processes,for example, in the second-order nonlinearity media quadratic soliton can be formed by use of cascading of second-order nonlinearity.By learning existing research achievement,this dissertation is dedicated to investigating the excitation and transmission of quadratic spatial soliton.Some valuable research results have been obtained,and their potential use is also discussed.The major work is as follows.Firstly we have got second-order nonlinear coupled equations from Maxwell equations.After some change on the coupled equation,for the fundamental beam we get the additional phase change being proportional to the fundamental intensity under small-signal similarity,which has an analogy with self phase modulation in the process of third-order nonlinearity.We also find that strong pump beam can modulate the weaker signal beam in the up-conversion process.This makes it possible to control one light with another light.Secondly we consider the propagation of continuous wave in the slab waveguide under type-ⅠSHG conditions,getting the coupled equations which govern the evolution of quadratic spatial solitons with the slowly varying envelope approximation.We numerically investigate the propagation of beams with various profiles in slab waveguide by split-step integral approach.We find if the launched beams are appropriate,the periodic oscillation quadratic spatial solitons will be excitated.The value of normalized wave-vector mismatch parameterβhas a great impact on power threshold value which soliton formation need.At last we investigate the propagation of beams in the a lossless quasi-phase-matched(QPM) grating under two different input conditions.It indicates that effective third-order nonlinearity,similar to Kerr effect,is induced by modulating refraction index and second-order susceptibility of QPM grating.Numerical calculations indicate that fundamental soliton is rapidly excited when fundamental wave(FW) is only launched,and it presents oscillation which is inherent for quadratic solitons;at this time, second harmonic wave(SHW) also excites soliton,and the corresponding dispersive wave is more visible.However,while both FW and SHW are launched and their amplitudes and beam widths are appropriate,the solitons excited are stable,and the periodicity of oscillation of fundamental wave is clearly evidence,and the dispersive wave of SHW is extremely weak.Moreover,there is a critical value of the amplitude of SHW to excite well-defined soliton.