部分相干平顶光束在线性增益(损耗)介质中的光谱特性

根据Li提出的平顶光束模型和部分相干光理论，推导出部分相干平顶光束在线性增益(损耗)介质中传输的光谱分布解析表达式，定量研究了部分相干平顶光束在线性增益(损耗)介质中传输的光谱特性，分析了近场和远场轴上光谱的变化特点，以及介质的增益(损耗)特性、光源的中心频率、空间相干参数以及谱宽等因素对光谱变化的影响. 研究结果表明，部分相干平顶光束在线性增益(损耗)介质中传输时，光源的中心频率、光谱宽度、空间相干参数对相对谱移量影响较大，但对光谱分布随传输距离的总体变化趋势影响较小，而光谱分布随传输距离的总体变化趋势则主要由平顶光束的阶数所决定. 关键词： 部分相干平顶光束 光谱特性 谱移 线性增益(损耗)介质     

高功率激光放大器中光束的成丝和B积分

研究了高功率激光放大器中光束的成丝问题,利用弗兰兹－诺德威克（Frantz-Nodvik)模型将准稳态波动议程推广到介质有饱和增益的情形,进而得到高功率激光放大器中小尺度调制的传输方程,在小信号增益和饱和增益情形下,分别得到了小尺度调制方程的精确解析解和数值解。结果表明,对于相同的输入光强,增益使小尺度调制增益谱的范围加宽,最快增长频率增高,最大增长率提高;同时,在放大介质中,小尺度调制的最快增长频率和最大增长率还随传输距离变化,增益饱和效应使小尺度调制的增长速度以及B积分的增长低于无饱和放大情形。     

Small-scale self-focusing of divergent beam in nonlinear media with loss

<正>Bespalov-Talanov theory on small-scale self-focusing is extended to include medium loss for a divergent beam.Gain spectrum of small-scale perturbation is presented in integral form,and based on the derived equations we find that the cutoff spatial frequency for perturbation keeps a constant value.The larger the medium loss is,the smaller the fastest growing frequency and the maximum gain of perturbation with defined propagation distance are.For a given medium loss the maximum gain of perturbation becomes larger,while the fastest growing frequency becomes smaller as the propagation distance becomes longer. Furthermore,physical explanations for the appearance of these features are given.     

Self-deflection of steady-state bright spatial solitons in biased photorefractive crystals

The self-bending process of steady-state bright spatial solitons in biased photorefractive media is investigated by taking into account diffusion effects. By integrating numerically the nonlinear propagation equation, it is found that the soliton beam evolution is approximately adiabatic. The self-deflection process is further studied using perturbation analysis, which predicts that the center of the optical beam moves on a parabolic trajectory and, moreover, that the central spatial frequency component shifts linearly with the propagation distance. Relevant examples are provided.     

小宽带光谱色散匀滑光束传输特性研究

利用激光聚变研究中心自主开发的小宽带传输放大软件对小宽带光谱色散匀滑(SSD)光束进行具有"时间、空间、光谱分辨"的传输模拟,研究了SSD光束通过空间滤波器和自由空间的传输特性.给出了时间、空间、光谱的对应分布图,对比分析了SSD光束与非SSD光束的传输效应.研究结果对于合理设计SSD光束的发散角与空间滤波器小孔之间的匹配关系、选择适宜的调制频率,以及选择束匀滑色循环数等都有重要意义,同时对于光路设计特别是细光束自由传输距离的设计具有指导意义.     

时空非相干光孤子的传播和相干特性

研究了时空非相干椭圆形高斯光束在饱和对数型非线性介质中的自陷行为,得到了孤子的形成条件并分析了其相干特性,讨论了光束自陷的变化形式,发现光束的初态和材料非线性对其有决定性的影响, 并进一步获得了光束半径和光束相干半径的解析表达式,发现光束的相干特性不但会随传播距离产生周期性的变化,且随组份频率的增大而减弱.     

Beam propagation factor of partially coherent beams in gain or absorbing media

The definition of second-order intensity moments in the spatial domain and spatial frequency domain has been generalized for the case that the linear gain or absorbing media are included, where the wave number is generally complex. The formula for beam propagation M²-factor of partially coherent beams in linear gain or absorbing media has been given. The partially coherent flat-topped Schell-model beam is taken as an example. The closed-form expression for the beam propagation M²-factor of partially coherent flat-topped beam in gain or absorbing media has been derived. The changes of the M²-factor in media have been discussed with numerical examples. It can be shown that the M²-factor of flat-topped Schell-model beams in gain or absorbing media depends on the coherent parameter β, the coherent length σ₀, the beam order M, the propagation distance B, the imaginary part of the wave number K_i, as well as the real part of the wave number K_r.     

开放的V型三能级系统中原子注入和退出速率对位相相关的无反转激光增益和强度空间演化的调制作用

利用数值计算结果, 研究了具有自发辐射诱导相干的开放V 型三能级原子系统中的传播效应. 研究表明: 改变探测场和驱动场之间的相对位相对无反转激光(LWI)增益和 强度随传播距离的变化即空间演化具有重要的影响; 而原子的注入速率比(S)和退出速率(r₀)的改变将对位相相关的LWI增益和强度的空间演化产生明显的调制作用. 在S(r₀)的一定取值范围内, S(r₀)的值越大, LWI增益和强度越大且强度达到极大值需要的传播距离越长; 相应封闭系统中LWI增益和强度的最大值及能产生增益的传播距离都小于开放系统. Doppler效应对LWI增益和强度的空间演化也具有明显的影响, 存在Doppler效应时得到的LWI增益和强度明显小于无Doppler效应时的值.     

Small-scale Self-focusing of Intense Laser Beams in Nonlinear Media with Loss

1　Ｉｎｔｒｏｄｕｃｔｉｏｎ　　Ｉｔｉｓｗｅｌｌｋｎｏｗｎｔｈａｔｓｍａｌｌ ｓｃａｌｅｓｅｌｆ ｆｏｃｕｓｉｎｇ (ＳＳＳＦ)ｃａｎｃａｕｓｅｔｈｅｄｅｇｒａｄａｔｉｏｎａｎｄｂｒｅａｋｕｐｏｆｈｉｇｈｐｏｗｅｒｌａｓｅｒｂｅａｍｓｂｙａｍｐｌｉｆｙｉｎｇｓｍａｌｌ ｓｃａｌｅｂｅａｍｉｍｐｅｒｆｅｃｔｉｏｎｓ.Ｔｈｉｓｐｒｏｂｌｅｍｉｓｏｆｐｒａｃｔｉｃａｌｉｍｐｏｒｔａｎｃｅｂｅｃａｕｓｅｉｔｉｓｔｈｅｍａｉｎｆａｃｔｏｒｌｉｍｉｔｉｎｇｔｈｅｍａｘｉｍｕｍ ｐｏｗｅｒａｖａｉｌａｂｌｅｆｒｏｍａ…     

固体板条MOPA激光光束质量主动控制

详细介绍了固体板条MOPA激光系统输出光束特性,包括光束近场强度分布、波前畸变空间特性和时间频率特性等,并以此为设计输入,研制了大动态范围高空间分辨率变形镜。研制的自适应光学闭环校正系统在强光条件下,实现了全程闭环控制,光束质量因子平均值从开环7.4提高到4.06。根据当前波前闭环校正残差,提出了进一步提高MOPA系统输出光束质量的方法。     

TRANSVERSE MODULATION OF BEAM PROFILES IN PUMP-PROBE CONFIGURATIONS WITH BaTiO3:Ce

Transverse spatial modulation of beam profiles for a probe beam counterpropagating to a pump beam in a 45°-cut BaTiO₃:Ce crystal with an externally applied electric field is studied numerically studied. For the case that the direct ion of the external field makes an angle of 45° with the c-axis of the crystal, the probe beam, which propagates in the direction perpendicular to the external field and has a waist wider than that of the pump beam, can acquire deep spatial modulation. In contrast, the pump beam shows no spatial modulation but diffracts only. For the case the external field is directed opposite to the direction mentioned above, the probe beam shows much stronger spatial modulation as compared to the pump.     

Propagation of flexural wave in periodic beam on elastic foundations

The propagation properties of flexural wave in the periodic beam on elastic foundations are studied theoretically. The wavenumbers and traveling wave characteristics in the beam on elastic foundations are analyzed. Basing on the equations of motion, the complex band structures and frequency response function are calculated by the transfer matrix method. And the Bragg and locally resonant gaps properties and the effects are researched. A gap with low frequency and wide range can exist in a beam on elastic foundations.     

Laser conferral of a directed character to near-field energy transfer

With sufficient intensity, an off-resonant laser beam can modify the character of electromagnetic near-field radiation produced by a nanoscale point source, narrowing its spatial distribution. The mechanism, for which the laser frequency is significantly off-resonant from the emission radiation, is detailed and analyzed through a quantum electrodynamical analysis. Results are calculated for various positions of a polarization-sensitive probe relative to the point source, sited within the throughput beam, and contour maps exhibit variations in the registered signal over a range of input intensities. A key feature is the clear exhibition of directed propagation features, usually emergent only in the wave zone, within the near-field region of the source.     

Modulational instability of a strip beam in a bulk type I quadratic medium

The evolution of a strip (one-dimensional) fundamental beam with propagation distance owing to spatial modulational instabilities was analyzed in a quadratic medium near type I phase matching. We obtained the gain coefficient for the modulational instability and showed that the wave evolves into a clean periodic sequence of solitary waves and does not reproduce the incident beam.     

固体板条MOPA激光光束质量主动控制

详细介绍了固体板条MOPA激光系统输出光束特性,包括光束近场强度分布、波前畸变空间特性和时间频率特性等,并以此为设计输入,研制了大动态范围高空间分辨率变形镜。研制的自适应光学闭环校正系统在强光条件下,实现了全程闭环控制,光束质量因子平均值从开环7.4提高到4.06。根据当前波前闭环校正残差,提出了进一步提高MOPA系统输出光束质量的方法。     

TRANSVERSE MODULATION OF BEAM PROFILES IN PUMP-PROBE CONFIGURATIONS WITH BaTiO3:Ce

Transverse spatial modulation of beam profiles for a probe beam counterpropagating to a pump beam in a 45°-cut BaTiO₃:Ce crystal with an externally applied electric field is studied numerically studied. For the case that the direct ion of the external field makes an angle of 45° with the c-axis of the crystal, the probe beam, which propagates in the direction perpendicular to the external field and has a waist wider than that of the pump beam, can acquire deep spatial modulation. In contrast, the pump beam shows no spatial modulation but diffracts only. For the case the external field is directed opposite to the direction mentioned above, the probe beam shows much stronger spatial modulation as compared to the pump.     

Relay propagation of partially coherent flattened Gaussian beam in turbulent atmosphere

Relay propagation of partially coherent flattened Gaussian–Schell beam in turbulent atmosphere has been studied. The analytical expresses of average intensity distribution at target are derived. The effects of spatial correlation length of initial and relay beam on the average intensity are analyzed in details. Study shows that the effects of the variation of spatial correlation length of relay beam are much larger than that of initial beam. The effects of spatial correlation length on relay propagation become smaller and smaller with the increase of structure constant. When the spatial correlation length is large and turbulence is strong, the effects of the variation of spatial correlation length on relay propagation are so small that can be neglected.     

Investigation of the onset and development of forward scattering in an underdense plasma

We investigate the distortion of a spatial intensity modulation imposed on a 527 nm f/10 probe beam as it transmits through an underdense plasma characterized with Thomson scattering. Combining the measurements with full wave simulations of beam propagation through the entire plasma show that the key features of the data can be reproduced using the Kaiser thermal transport model.     

Observation of specific features of laser beam propagation in air with standing acoustic waves

The process of laser beam propagation in air with a standing acoustic wave is studied. The uncertainty σ in the spatial localization of the laser beam is found to decrease in this process. In the studied case of laser beam propagation in a tube at atmospheric air pressure, the observed effect is manifested as a decrease in σ alongside the increasing resonator (tube) quality factor.     

Distortion-free gain and noise correlation in sodium vapor with four-wave mixing and coherent population trapping

We observed optical gain as great as 30 with nearly distortion-free beam propagation in optically dense sodium vapor, using four-wave mixing. Moreover, 15-dB classical noise correlations were seen in the amplified probe and conjugate beams. To achieve this performance in such a strongly absorbing medium, one must suppress unwanted absorption and self-focusing effects. This is accomplished with coherent population trapping.