【作者】 滕晓丽；

【导师】 苗润才；

【作者基本信息】 陕西师范大学 ， 光学， 2003， 硕士

【摘要】 本文工作包括两部分内容：一、研究了横向双脉冲在各向同性正常色散介质中传输时脉冲之间的相互作用；二、首次考虑表面声波中的高次谐波，从理论上得到非线性条件下衍射强度分布的解析表达式，并与实验结果进行对比。 已有的研究表明：高斯型的单脉冲在正常色散介质中传输时，其功率若超过自聚焦的临界功率，则在色散、衍射、自聚焦和自相位调制等效应的共同作用下，脉冲纵向分裂，横向展宽或分裂。如果输入信号为横向双脉冲，且每一脉冲的入射功率都超过自聚焦的临界功率，我们的研究表明：因脉冲和脉冲之间的相互作用，双脉冲在传输过程中，每一脉冲的传输特性会发生变化。 本文针对这一问题，考虑横向双脉冲输入信号，且每一脉冲的入射功率都超过自聚焦的临界功率；假定介质为各向同性的非线性正常色散介质；数学模型中考虑群速度色散、三阶色散、衍射、三阶色散—衍射、自聚焦、自相位调制等效应。通过数值计算来获得双脉冲在各向同性的非线性正常色散介质中的传输规律。主要结论如下： 一、当入射双脉冲的功率均略高于自聚焦的临界功率时，脉冲纵向分裂，横向展宽。若两脉冲之间的位相不同，脉冲在介质中传输时出现不同的现象： a)当初始相位差为0时，传输过程中脉冲的能量不发生相互转移；两脉冲之间存在“引力”，逐渐在两脉冲之间形成一新的小脉冲；脉冲两侧强度非对称分布。 b)当初始相位差为π时，传输过程中脉冲的能量也不发生相互转移；两脉冲内侧之间存在“斥力”；脉冲两侧强度非对称分布，内侧强度高于外侧。 c)当初始相位差为π/2时，传输过程中脉冲的能量发生相互转移，其中一个脉冲功率逐渐减小，而另一脉冲功率逐渐增大，在纵向、横向都发生分裂现象。 二、当入射双脉冲的功率均远大于自聚焦的临界功率时，脉冲纵向和横向都发生分裂现象。若两脉冲之间的位相不同，脉冲在介质中传输时出现不同的现象： a)当初始相位差为0时，传输过程中脉冲的能量不发生相互转移；脉冲横向两侧出现小峰，两主脉冲各自内侧的小峰之间存在“引力”，逐渐在两脉冲之间形成一新的小脉冲；两主脉冲外侧的小峰横向发散。 b）当初始相位差为。时，传输过程中脉冲的能量也不发生相互转移：两主脉冲各自内侧的小峰之间存在“斥力”，此时内侧、外侧小峰均横向发散。 C）当两脉冲初始相位差为。／2时，传输过程中脉冲的能量发生相互转移，两主脉冲内侧的小峰之间的“相互作用力”是由初始相位差和相互间发生的能量转移共同决定。 本文另一部分内容研究了液体表面低频声波的非线性声光效应。实验上我们得到反衬度非常高的高阶衍射图样，从该衍射图出发，针对大振幅液体表面波，研究其高阶衍射效应。由于在我们的实验条件下，液体表面波的振幅比通常固体表面超声波的振幅高出三个数量级，高次谐波的影响不能忽略。因此，在这一部分里，我们考虑液体表面波的基波和二次谐波，由非线性理论来讨论衍射场的分布规律，并用非线性高次谐波理论对实验结果进行拟合，得到更符合实际的理论结果。更多还原

【Abstract】 Our research includes two parts: one part, we study the question on the two pulses in the transverse x dimension propagating in normal dispersive nonlinear media. The other part, we discuss nonlinear acoustic-optical effect from liquid surface waves at low-frequency.The propagation of high-peak-power laser pulses in isotropic dispersive nonlinear media gives rise to strong nonlinear effects such as four-wave mixing, stimulated Raman processes, self-focusing and self-phase modulation which lead to strong modifications of the pulse characteristics. Many recent studies have shown that pulse splitting occurs in the normal dispersive media, when the pulse with power beyond the threshold for self-focusing. In other hand, if two high-peak-power pulses in the transverse x dimension propagate in dispersive nonlinear media, the interactions between pulses are interest and importance.In this paper, we take two pulses in the transverse x dimension into account. We calculated numerically several important phenomena when two pulses propagate in the isotropic normal dispersive nonlinear media. The numerical method includes some of linear and nonlinear term, such as group-velocity dispersion, third-order dispersion, diffraction, mixed space-time third-order terms, self-focusing and self-phase modulation.Through our numerical calculation, we obtained several useful results in our model. First, when the power of two pulses is just now beyond the lower intensity threshold, self-phase modulation and velocity dispersion split the pulses into two pulselets in time, a) when their relative phase is zero, there are no power transfer between the pulses, the pulses appear to attract each other, the intensity in the center region between the pulses is increase, b) when their relative phase is , there are no power transfer between the pulses too, the two pulses appear to repel each other, c) when their relative phase is /2, power transfers between the two pulses. One pulse’s power is decrease. In the same time the other pulse’s power is increase, which lead to the pulses split in both time and transverse space dimensions. Second, when two pulses intensities above the higher threshold, after the temporal pulse spitting occurs, pulses split in transverse space dimension too. Thus each pulses format two additional satellite peaks centered in the space dimension, the interactions consist in the two middle satellite peaks.In this paper, the other question we discuss is nonlinear acoustic-optical effect from liquid surface waves at low-frequency. Steady and high visible diffraction patterns with higher order maxima from liquid surface acoustic waves at low-frequency were obtained experimentally. Considering the high-order harmonic acoustic waves on liquid surface, the corresponding analytic expression of diffraction intensity pattern under the nonlinear condition was derived and theoretical results were compared with the experimental data. It was shown that they are well agreement.更多还原