高斯光束在有外电场的中心对称光折变材料中的演化

为了研究在有外电场的中心对称光折变材料中一束高斯光束及两束高斯光束相互作用的演化,将高斯光束作为入射波,采用数值方法求解波传播方程.结果表明,当外加电场,光强峰值和束腰参量分别与晶体匹配的情况下,一束高斯光束能演化为稳定的屏蔽空间明孤子,而在不匹配时,一束高斯光束在传播过程中则呈现周期性的压缩或膨胀现象.同时也发现,在作用距离范围内.两束同相位高斯光束相互吸引,并伴随着融合现象,两束反相位高斯光束相互排斥,且排斥作用随着光束初始间距的减小而增加.     

(1+1)维光折变亮屏蔽孤子相互作用的数值分析

利用束传播法对(1 1)维光折变亮屏蔽孤子间的相互作用进行了详细地数值模拟研究.结果表明两同相孤子相互吸引,并伴有孤子融合现象,且孤子融合发生时的作用距离随着孤子初始间距的增大而增大;两反相孤子相互排斥,且排斥作用随着孤子初始间距的减小而增强;当两孤子的相位差位于区间(0,π)和(-π,0)中时,其相互作用过程将伴有能量转移,但两区间内能量转移的方向相反,并表现出反转对称性;由于孤子间的距离和相对相位对孤子间的相互作用均有很大的影响,因此多个孤子间的相互作用过程非常复杂,但可以通过分析相邻孤子间的相互作用对其进行定性的预测.     

(1+1)维光折变亮屏蔽孤子相互作用的数值分析

利用束传播法对（1+1）维光折变亮屏蔽孤子间的相互作用进行了详细地数值模拟研究.结果表明：两同相孤子相互吸引,并伴有孤子融合现象,且孤子融合发生时的作用距离随着孤子初始间距的增大而增大;两反相孤子相互排斥,且排斥作用随着孤子初始间距的减小而增强;当两孤子的相位差位于区间（0,π）和（－π,0）中时,其相互作用过程将伴有能量转移,但两区间内能量转移的方向相反,并表现出反转对称性;由于孤子间的距离和相对相位对孤子间的相互作用均有很大的影响,因此多个孤子间的相互作用过程非常复杂,但可以通过分析相邻孤子间的相互作用对其进行定性的预测.     

高斯型空间光孤子相互作用的数值研究

 通过数值模拟的方法，对高斯孤子在对数型饱和非线性介质中的相互作用进行了研究，考查了两光束间的相对振幅和相对相位对其相互作用的影响。结果表明：高斯孤子之间的相互作用敏感地依赖于两光束间的相对振幅和相对相位。在不同的振幅差异范围内，光束间的主要作用交替地表现为相互排斥和相互吸引，并由于高斯孤子的不稳定性，导致了光束在碰撞后以一种尺寸周期性变化的呼吸模式传输。随着相对相位的增大，两光束间始终持续地表现出强烈的排斥作用，直到相对相位增加到一个2π周期之后。而且碰撞之后，光束也都以呼吸模式进行传输，其分离的角度越大，呼吸就越明显。     

扩散效应对相干屏蔽光伏孤子对相互作用的影响

为了研究扩散效应对相干屏蔽光伏孤子对相互作用的影响.将两个平行传播相干屏蔽光伏孤子光束作为入射波,采用数值方法求解有扩散效应的波传播方程.结果表明,在扩散效应影响下,两相干屏蔽光伏孤子光束同相相互作用不是原来融合,而是使它们趋于分离,且光束之间存在能量的耦合;异相孤子的相互作用不再是简单的相互排斥,在一定的条件下,两束孤子光束既互相排斥,又同时向同侧偏转.     

双曲余弦高斯光束在强非局域介质中的传输

从描述强非局域介质中光束传输的非线性薛定谔方程出发,研究了(2+1)维双曲余弦高斯光束在强非局域介质中的传输性质及其相互作用,给出了双曲余弦高斯光束在强非局域非线性介质中传输的解析表达式和二阶矩束宽的解析表达式,同时对两束双曲余弦高斯光束之间的相互作用进行了解析和数值分析。结果表明单光束入射强非局域介质时,存在一个临界功率,当入射功率等于临界功率时,光束在传输过程中的二阶矩束宽可以保持不变;当入射功率不等于临界功率时,光束的二阶矩束宽呈周期性变化。两束双曲余弦高斯光束共同传输时会相互吸引,并且横向强度分布变的较为复杂,给出了两束光传输时相互作用后的强度分布和轴上光强演化等结果。     

高斯光束在外加空间调制电场的光折变晶体中的演化

研究了高斯光束在外加空间调制电场的光折变晶体中的演化,结果表明,在不同的外加调制电场作用下高斯光束的光场分布会随着外电场变化而被调制。重点研究了同相位和反相位以及相位差为π/2的高斯光束在外加空间调制电场的光折变晶体中的相互作用,在外加空间分布线性增加电场作用下出现了许多新的实用特性,通过调整外加电场分布,可以让两同相位相干光靠得更近而不发生融合,也可以让两靠得很近的反相位光束可控偏转,还可以控制相位差为π/2的两相干光束进行能量转移。这种连续变化的电场可以用阶梯分布的电场替代,为工程实现提供了可能。     

竞争型非线性介质中艾里-高斯光束交互作用的调控

基于分步傅里叶法研究了艾里-高斯光束在三次聚焦和五次散焦竞争型非线性介质中交互作用的调控.结果表明:当入射角度为零时,同相位艾里-高斯光束相互吸引,五次散焦非线性强度较弱时,可形成周期逐渐缩短的呼吸孤子或强度不变的孤子.五次散焦非线性强度较强时,呼吸孤子的平均宽度变大甚至出现光束分叉现象,形成孤子对.反相位艾里-高斯光束相互排斥,排斥力随五次散焦非线性强度单调递增.当入射角度不为零时,通过控制光束入射角度的正负和初始间距.同相位和反相位艾里-高斯光束交互作用时可以同时表现出相互吸引和排斥的现象.     

饱和非线性介质中艾里-高斯光束的传输与交互作用

研究了艾里-高斯光束在饱和非线性介质中的传输与交互作用.结果表明,入射光为单艾里-高斯光束时,在一定功率范围内,调节初始振幅与光场分布,可形成传输方向可控的呼吸孤子.当初始振幅增加时,孤子的呼吸周期先减小后增大.入射光为两艾里-高斯光束时,同相位光束相互吸引,两光束中心位置两侧附近产生呼吸孤子和对称孤子对.反相位光束相互排斥,中心位置两侧仅产生对称的孤子对.光场分布越趋于高斯分布,两艾里-高斯光束相互作用就越强,孤子对的数目越少.     

匹配高斯光束在耗散晶体中的孤波演化及偏转特性

研究了匹配的高斯光束在加外电场的耗散型光伏光折变晶体中的动态演化及其偏转特性.结果表明，对于给定的耗散系统，若该系统能支持某一特定的耗散全息屏蔽光伏明孤子，也可能找到与之相匹配的高斯光束，则该匹配的高斯光束能在此给定的耗散系统中演化成稳定的屏蔽光伏空间孤子.同时，考察了扩散效应对此匹配的高斯光束的影响，结果显示，扩散效应将造成此匹配的高斯光束孤波在演化过程中产生偏转现象，而且其偏转过程与之相对应的明孤子的偏转过程具有极其类似的特征.通过比较研究匹配的高斯光束与耗散全息屏蔽光伏明孤子的偏转特征曲线，我们发现，对于匹配的高斯光束，其空间偏转的数值计算结果与明孤子偏转的解析计算结果非常接近.总之，在耗散系统中，匹配的高斯孤波的动态演化特性(包括偏转过程)与耗散全息屏蔽光伏明孤波的演化特性是相符合的.因此，人们往往可直接用匹配的基模高斯光束代替耗散全息屏蔽光伏明孤波，为理论分析和实验研究提供了便利.     

强外加电场与大调制度下光折变动力学光栅形成研究

强外加电场与大调制度在光折变效应的研究中已经得到了广泛应用。采用PDECOL算法,严格求解光折变带输运方程,得到外加电场时不同调制度下光折变晶体中随时间变化的空间电荷场、载流子浓度,并讨论了外加电场对它们的影响。通过将物质方程与耦合波方程联立数值求解,可得到光折变光栅形成过程中两波耦合增益系数以及光束条纹相位的变化。模拟结果表明,在强外加电场作用下,两束记录光之间的光强与相位耦合都得到了增强,而原有的解析式忽视了强外加电场与大调制度对空间电荷场相位耦合的影响,此时不再适用。同时发现折射率光栅与记录光束条纹均发生弯曲,并不再保持平行。     

Numerical simulations and experimental investigations on transverse effects of two-wave mixing in a photovoltaic photorefractive crystal

The transverse effects of two Gauss waves mixing in a photovoltaic photorefractive crystal,i.e.,the dynamical evolutions of signal beam with different parameters,are traced numerically and experimentally.Both the one-dimensional numerical simulations and two-dimensional experimental investigations show that the self-focusing effect of signal beam becomes more obvious as the intensity of pump beam and the angle between the signal and pump beams increase.In accordance with these results,the state of two Gauss...     

Self-bending of photorefractive solitons

Self-bending of photorefractive solitons is caused by diffusion in photorefractive crystals and becomes an important effect when the beam size is in the range of the charge carriers diffusion length. In this paper we present an experimental and numerical examination of the beam bending dependence on relevant parameters such as the applied electric field and the beam intensity. We demonstrate that the bending dependence on the electric field in the low saturation regime has the form of a square function at low values of the field and becomes linear for higher values. For stronger saturation the curve gets the form of a square root function. The bending dependence on the beam intensity has a maximum at defined intensity. The experimental data are compared with numerical simulations, giving a good qualitative agreement.     

利用光折变晶体的电光、压电效应实现2×2光开关的原理

讨论了在光折变LiNbO3晶体中建立电场控制的 2× 2直通交换开关的原理设计方案。通过控制读出光栅时外加电场的大小使读出光满足或偏离布拉格条件 ,从而控制衍射效率为 1或 0 ,实现开关的交换和直通操作。全面考虑了电场作用下晶体的压电和电光效应 ,基于衍射效率公式和光栅的布拉格条件 ,给出了为获得最佳衍射效率比时外加电场及写入光束比与记录角度间的关系曲线     

Transmission behavior in a photorefractive polymer film

When a p-polarized beam propagates through a high-performance photorefractive polymer composite, poly dimethyl-4-(4^′-nitrophenylazo)benzene, its transmission behavior is influenced by three effects: the electroabsorption, the photorefractive coupling with the reflected beam from the rear surface, and the amplified scattering. From the measurements on the incidence angle dependence as well as the applied-electric-field dependence of the three effects, some conclusions are obtained. At a small incidence angle with a low applied electric field, both the amplified scattering and the electroabsorption are small whereas the coupling between the incident beam and the reflected beam plays a principal role. At a large incidence angle or with a high poling electric field, the transmission is influenced mainly by the amplified scattering and the electroabsorption. A poling electric field asymmetric loss to the amplification scattering is also observed. Received: 18 November 1998 / Revised version: 10 February 1999 / Published online: 12 April 1999     

Light-induced scattering in SBN：Cr crystal under external electric fields and its suppression

In this paper, the enhancement of light-induced scattering in congruent SBN:Cr (Sr_{0.61}Ba_{0.39}Nb_2O_6:Cr) crystals in the presence of an externally applied electric field and its suppression are studied. If a coherent image is focalized in SBN:Cr crystal without applying external electric field, the output image will remain clear, because of the weak photorefractive effect in the crystal. When a field is applied properly along the crystal axis, markedly enhanced scattering from the signal beam and the output image dispersion can be observed due to the increase of the photorefractive two-beam coupling gain and the light-induced index change in SBN:Cr crystals. By introducing a coherent or incoherent beam with higher intensity the light-induced scattering can be suppressed through the erasure of scattering gratings. The difference between coherent and incoherent beam is that the former can also amplify the signal beam as the scattered light is removed, whereas the latter can only make the signal beam revert to its initial state. The results obtained under different experimental conditions are consistent with theoretical analysis.     

Enhancement of the diffraction efficiency of the hologram recorded in photorefractive media under the action of external electric field

Diffraction efficiency as a function of the applied electric field for the non-linear regime has been calculated by solving numerically the beam coupling equations. The refractive index variation used in the beam coupling equations was calculated directly from the material rate equations via the total space charge field. The diffraction efficiency of the holograms recorded in photorefractive media is not only a function of the applied external electric field but also a function of crystal thickness, diffusion field, reduced fringe contrast modulation ratio and absorption coefficient of the materials. The effects of these parameters on the efficiency of the holograms have been studied in details. In the absence of the external applied field, it is found that the diffraction efficiency of the holograms could be maximized for a thinner photorefractive crystal having lower absorption coefficient and higher value of diffusion field, which could exist at a much lower value of reduced fringe contrast modulation ratio of the index grating. More efficient holograms can be recorded in the presence of the externally applied electric field as compared to the case of no external field.     

有偏压光折变晶体中的高斯孤子

利用变分方法求解了小光强光折变非线性薛定谔方程，得到了高斯光束在外加正偏压光折变晶体中的演化特性，以及高斯光束宽度压缩与展宽的动态振荡规律，给出了高斯光束的宽度、振幅和波前曲率的表达式，研究了势函数与光束宽度的关系.结果表明，小光强高斯光束在正偏压光折变晶体中传播时，其宽度和振幅将经历周期地变化或者展宽发散，在一定条件下高斯光束在晶体中可以形状不变地传播，形成“高斯孤子”，这些特性在工程实践中具有很高的实用价值. 关键词： 高斯孤子 高斯光束 变分方法 光折变非线性方程