光纤中的瞬态SBS过程的数值分析与探讨

 通过数值方法得到了光纤中的瞬态SBS过程中耦合波和声场的强度分布图，从而分析了光纤中瞬态SBS的发生过程以及光纤长度、泵浦脉宽等因素对SBS反射率及阈值的影响，据此提出了一种新的光纤相位共轭器。     

Phase conjugating mirror combining a fused silica rod and a fiber

A phase conjugating mirror combining a fused silica rod and an optical fiber is presented. This configuration combines a large diameter fused silica rod with a high laser damage threshold and an optical fiber with a low stimulated Brillouin scattering (SBS) threshold. The composed SBS generator-amplifier construction, similar to the two-cell type makes a forced SBS process, in which the Stokes beam reflected by the fiber is injected into the fused silica rod. Using this combined phase conjugating mirror in a laser-diode-pumped laser master oscillator power amplifier system, we obtained 42% SBS reflectivity at most at 100 Hz repetition rate. At the same time, the inverse beam quality parameter M(2) is reduced from 3 to 1.6, while at 400 Hz repetition rate, M(2) is reduced from 4 to 1.7.     

光-力耦合受激布里渊散射方程组

 采用位移形式表征各向同性介质的运动，从而对传统的SBS耦合波方程组进行改造，使之适用于多维的情况并体现光学理论和力学理论的耦合。采用有限差分方法数值求解了光学-力学耦合型后向SBS方程组，得到了瞬态的位移场、速度场和应力场。计算结果表明，当泵浦光光强为2.0×10 ¹⁵W/m ²时，SBS可激发厚度为1cm的K9玻璃样品的表面产生出0.1nm量级的位移和10m/s量级的速度，对应的应力幅度达到10MPa量级。     

Form Invariance and Conserved Quantity for Non-holonomic Systems with Variable Mass and Unilateral Constraints

The paper studies the form invariance and a type of non-Noether conserved quantity called Mei conserved quantity for non-holonomic systems with variable mass and unilateral constraints.Acoording to the invariance of the form of differential equations of motion under infinitesimal transformations,this paper gives the definition and criterion of the form invariance for non-holonomic systems with variable mass and unilateral constraints.The condition under which a form invariance can lead to Mei conservation quantity and the form of the conservation quantity are deduced.An example is given to illustrate the application of the results.     

Conformal Invariance and Conserved Quantity for?the?Nonholonomic System of Chetaev’s Type

In this paper the definition of conformal invariance and determining equation for the holonomic system which correspond to a nonholonomic system of Chetaev’s type are provided. Conformal factor expression is deduced through relationship between a system’s conformal invariance and Lie symmetry. The necessary and sufficient condition that the system’s conformal invariance would be Lie symmetry under transformations by the infinitesimal one-parameter transformation group is obtained. The conformal invariance of weak and strong Lie symmetry for the nonholonomic system of Chetaev’s type is given using restriction equations and additional restriction equations. And the system’s corresponding conserved quantity is derived with the aid of a structure equation that gauge function satisfied. Lastly, an example is taken to illustrate the application of the result.     

组合型光纤相位共轭器的数值模拟

 组合光纤是一种结合了小口径光纤SBS阈值低和大口径光纤损伤阈值高的特点而形成的新型光纤相位共轭器。通过建立数学模型，数值计算了组合光纤中SBS形成的过程，并对其特性参数进行了一定的理论分析。结果表明组合光纤不但可以大幅度降低SBS阈值、获得高反射率和扩大动态范围，而且通过优化其几何尺寸可以抑制Stokes波形的波动现象并显著压缩泵浦光脉冲。     

Conformal Invariance and a New Type of Conserved Quantities of MechanicalSystems with Variable Mass in Phase Space

Conformal invariance and a new type of conserved quantities of mechanical systems with variable mass in phase space are studied. Firstly, the definition and determining equation of conformal invariance are presented. The relationship between the conformal invariance and the Lie symmetry is given, and the necessary and sufficient condition that the conformal invariance would be the Lie symmetry under the infinitesimal transformations is provided.Secondly, a new type of conserved quantities of the conformal invariance are obtained by using the Lie symmetry of the system. Lastly, an example is given to illustrate the application of the results.     

Effect of water temperature on pulse duration and energy of stimulated Brillouin scattering

The water temperature has a strong effect on the kinematic viscosity, which is inversely proportional to the phonon lifetime and the gain coefficient. The higher the temperature is, the smaller the kinematic viscosity is, and the larger the phonon lifetime is. At a low pump power and a short focal length, we can derive a single-peak stimulated Brillouin scattering (SBS) pulse. The duration of this single-peak SBS pulse depends mainly on the phonon lifetime of water. With the increase of the water temperature, the duration of such a single-peak SBS pulse will become longer, and the SBS energy will become higher for the gain coefficient, which is related to the phonon lifetime. Therefore, varying the medium temperature can lead to the changes of SBS pulse duration and SBS energy.     

熔融石英玻璃受激布里渊散射效应实验研究

以自行研制的大能量单纵单横电光调Q激光系统作为抽运源，在小尺寸(2 cm×2 cm×4 cm)的熔融石英玻璃样品中实现了强烈发展的受激布里渊散射(SBS)过程，在工作介质无损伤的情况下，相位共轭波能量高达110 mJ，反射率约为60％.还研究了SBS能量反射率与入射角之间的关系，证实了此前提出的“自供种子光(self-Stokes-seeding)”效应的存在.提出了双棒串接的构型，得到了更为高效的SBS过程. 关键词： 受激布里渊散射 熔融石英玻璃 激光损伤 反射率     

激光与材料参数对瞬态SBS 诱导的激光超声破坏效果的影响

 通过数值求解一维瞬态受激布里渊散射（SBS）声光耦合波方程，从理论上分析了泵浦激光参数及光学材料参数对SBS过程发生阈值的影响。以SBS过程中建立起来的应力场抗拉（压）强度和散射场的反射率为判据，分析了激光脉宽及作用区域长度对激光超声破坏材料效果的影响，探讨了短脉冲激光（约ns）引起的激光超声对材料的破坏机理；讨论了通过参数配置有效遏制SBS过程激光超声对光学材料的破坏问题。     

Confronting Finsler space–time with experiment

Within all approaches to quantum gravity small violations of the Einstein Equivalence Principle are expected. This includes violations of Lorentz invariance. While usually violations of Lorentz invariance are introduced through the coupling to additional tensor fields, here a Finslerian approach is employed where violations of Lorentz invariance are incorporated as an integral part of the space–time metrics. Within such a Finslerian framework a modified dispersion relation is derived which is confronted with current high precision experiments. As a result, Finsler type deviations from the Minkowskian metric are excluded with an accuracy of 10⁻¹⁶.     

Adiabatic invariants induced by perturbation to form invariance of mechanico-electrical systems

This Letter focuses on perturbation to form invariance of mechanico-electrical systems. Criteria and structural equations for form invariance after disturbance are proposed. Considering perturbation of all the infinitesimal generators, a new type of adiabatic invariant induced by perturbation to form invariance of the system is reported.     

Lie-form invariance of nonholonomic mechanical systems

The Lie-form invariance of a nonholonomic mechanical system is studied. The definition and criterion of the Lie-form invariance of the nonholonomic mechanical system are given. The Hojman conserved quantity and a new type of conserved quantity are obtained from the Lie-form invariance. An example is given to illustrate the application of the results.     

Hojman conserved quantity for nonholonomic systems of unilateral non-Chetaev type in the event space

Hojman conserved quantities deduced from the special Lie symmetry, the Noether symmetry and the form invariance for a nonholonomic system of the unilateral non-Chetaev type in the event space are investigated. The differential equations of motion of the system above are established. The criteria of the Lie symmetry, the Noether symmetry and the form invariance are given and the relations between them are obtained. The Hojman conserved quantities are gained by which the Hojman theorem is extended and applied to the nonholonomic system of the unilateral non-Chetaev type in the event space. An example is given to illustrate the application of the results.     

变质量Chetaev型非完整系统Appell方程Mei对称性的共形不变性与守恒量

研究了变质量Chetaev型非完整系统Appell方程Mei对称性的共形不变性和守恒量.在群的无限小变换下,定义了变质量Chetaev型非完整系统Appell方程Mei对称性和共形不变性,给出了该系统Mei对称性的共形不变性确定方程,并推导出系统相应的守恒量表达式.最后,给出了应用算例.     

Lie-form invariance of non-holonomic systems with unilateral constraints

In this paper the Lie-form invariance of the non-holonomic systems with unilateral constraints is studied. The definition and the criterion of the Lie-form invariance of the system are given. The generalized Hojman conserved quantity and a new type of conserved quantity deduced from the Lie-form invariance are obtained. Finally, an example is presented to illustrate the application of the results.     

Lagrange系统的Lie-形式不变性

本文研究Lagrange系统的Lie-形式不变性。给出系统Lie-形式不变性的定义和判据。导出由Lie-形式不变性导致的Hojman守恒量和一类新型守恒量。最后，举例说明结果的应用.     

Chetaev型非完整系统Mei对称性的共形不变性与守恒量

研究Chetaev型非完整系统Mei对称性的共形不变性与守恒量.引入无限小单参数变换群及其生成元向量,给出与Chetaev型非完整系统相应的完整系统的Mei对称性共形不变性定义和确定方程.讨论系统共形不变性与Mei对称性的关系.利用限制方程和附加限制方程得到非完整系统弱Mei对称性和强Mei对称性的共形不变性.借助规范函数满足的结构方程导出系统相应的守恒量,并举例说明结果的应用.     

Lie-Form Invariance of the Nonholonomic System of Relative Motion in Event Space

In this paper, the Lie-form invariance of a nonholonomic system of relative motion in event space is studied. Firstly, the definition and the criterion of the Lie-form invariance of the nonholonomic system of relative motion in event space is given. Secondly, the Hojman conserved quantity and a new type of conserved quantity deduced from the Lie-form invariance are obtained. An example is given to illustrate the application of the results.     

Determination of parameters of the effective particle-hole interaction in the lead region from consistency relations

The implications of translational invariance and gauge invariance for an effective particle-hole interaction of the Migdal type are investigated for²⁰⁸Pb. It is shown that translational invariance is very well satisfied while the gauge invariance requirement is met rather poorly. Four force parameters are determined from the consistency relations for the two conservation laws.