Hollow Gaussian beam generated by beam shaping with phase-only liquid crystal spatial light modulator

Based on the refractive beam shaping system, the transformation of a quasi-Gaussian beam into a dark hollow Gaussian beam by a phase-only liquid crystal spatial light modulator (LC-SLM) is proposed. According to the energy conservation and constant optical path principle, the phase distribution of the aspheric lens and the phase-only LC-SLM can modulate the wave-front properly to generate the hollow beam. The numerical simulation results indicate that, the dark hollow intensity distribution of the output shaped beam can be maintained well for a certain propagation distance during which the dark region will not decrease whereas the ideal hollow Gaussian beam will do. By designing the phase modulation profile, which loaded into the LC-SLM carefully, the experimental results indicate that the dark hollow intensity distribution of the output shaped beam can be maintained well even at a distance much more than 550 mm from the LC-SLM, which agree with the numerical simulation results.     

利用纯相位型液晶空间光调制器实现空心光束

为了提高激光系统的整体效率,需要将出射光强整形为空心分布。采用纯相位型液晶空间光调制器整形近高斯分布光强到空心分布。基于能量守恒定律和等光程原理,分析了纯相位型液晶空间光调制器光束整形系统,得出了整形系统所需的相位分布。采用衍射光学方法,数值模拟了整形效果,讨论了入射光束束腰半径和强度分布等因素对整形效果的影响。利用纯相位型液晶空间光调制器实验实现了空心光束,实验测得的转换效率大于99%。     

Adaptive correction of vortex laser beam in a closed-loop system with phase only liquid crystal spatial light modulator

We propose and demonstrate the wave front correction of a vortex laser beam by using dual phase only liquid crystal spatial light modulators (LC-SLMs) and a stochastic parallel gradient descent (SPGD) algorithm. One phase only LC-SLM is used to generate vortex laser beam by loading spiral phase screen onto the wave front of input quasi-Gaussian beam. The other phase only LC-SLM under SPGD controller based on the subzone control method adaptively compensates the wave front of vortex laser beam. Numerical simulation and experimental results show that after correction, vortex doughnut like beam is focused into a beam with airy disk pattern distribution in the far field. The adaptive corrections of vortex laser beam with different optical topological charges are studied. The results show that the optical topological charge has little influence on adaptive correction. The powers in the main lobe of far field intensity distributions of vortex laser beams with different optical topological charges are all greatly improved by adaptive correction. The technique proposed in this paper can be used in optical communication, relay mirror and atmospheric turbulence correction.     

基于整形光路的低阶像差校正方法

为满足激光束对远处目标辐照强度的要求,减小热效应对输出光束性能的影响,提出了一种基于整形光路的高能激光低阶像差校正方法。该方法根据波前探测器测到的光束波前畸变,利用激光器整形光路固有的光学元件和调整机构,通过高精密电动平移台和快反镜的快速动作,动态调整各镜片的间距,实时校正激光束的低阶像差,使输出光束恢复至平行出射,从而显著减小激光束的热漂移和热畸变,有效避免了长时间出光时光束的持续发散,极大改善了输出光束性能。试验结果表明,该方法具有校正量大、控制方便、结构紧凑等优点,能够很好地改善激光器输出光束质量,显著提高激光束远场作用效果。     

Light efficiency of liquid crystal spatial light modulator using,for wavefront corrector in AO

Liquid crystal spatial light modulator (LC-SLM) has a series of attractive characteristics as a wavefront corrector in adaptive optics system such as compactness, high density integration, low cost and possibility of batch production. However it also suffers from several drawbacks such as low light efficiency. We present an analysis of the light efficiency of the phase by only using LC-SLM for wavefront corrector in AO system. The factors of polarization sensitive, pixel grid structure, phase quantization and limited phase stroke are discussed in theory. Simulation results are given based on a math model of a LC-SLM. The intensity variation results from pixel structure are measured. The light efficiency of the LC-SLM is tested for generating a series of Zernike aberrations. The effect of phase wrapping method is analyzed and tested. We demonstrate that the light efficiency reduces these factors. Experiment results from the use of the phase only liquid crystal spatial light modulator confirms these conclusions.     

随机并行梯度下降光束净化实验研究

利用自适应光学技术进行光束净化是高能激光系统中一项重要的研究内容.为实现光束净化系统的小型化和低成本,基于系统性能评价函数无模型最优化的波前畸变校正方法是适合的技术方案.就随机并行梯度下降(SPGD)最优化算法在光束净化系统中的应用展开研究.针对高能激光束常见的像差分布进行了SPGD波前校正的数值模拟,在此基础上构建了37单元自适应光学光束净化实验平台,讨论了双边扰动梯度估计和迭代增益系数自适应变化对算法收敛特性的影响.数值模拟与实验结果验证了SPGD算法对不同程度波前畸变的校正能力,表明了SPGD光束净化方案的可行性.     

纯相位液晶空间光调制器拟合泽尼克像差性能分析

纯相位液晶空间光调制器作为波前校正器构成的高分辨率、低能耗、价格低廉、易于控制的自适应光学系统受到越来越多的关注.作为一种新型波前校正器件,它对波前像差的校正能力是反映其在自适应光学系统中应用的一个重要的指标,因此有必要仔细地研究它对各种像差的校正能力,以确定其可能的应用范围.波前校正器对各阶泽尼克像差的拟合效果有效地反映了该器件对不同像差的校正能力.利用256×256像素的纯相位液晶空间光调制器(LC-SLM)产生不同系数的前36项泽尼克像差分析LC-SLM对不同像差的校正能力.讨论了填充因子、离散像素 关键词： 液晶空间光调制器 相位调制 自适应光学 泽尼克多项式     

激光窗口热透镜效应对光束质量的影响

 分析了高功率激光通过窗口时，引起的窗口热透镜效应及其对激光光束质量的影响。用冲量定理方法，推导了二维热传导方程的广义傅里叶级数解，然后按照平面热应力问题，运用有限元方法数值计算热变形。考虑热变形、折射率随温度变化，忽略光弹性效应，解出出光面的波前相差分布，讨论了远场光束质量。以空心环形光束为例，计算了熔融石英(SiO₂)和白宝石(Al₂O₃)两种常用窗口的温升分布、热变形、波前相差和光束质量，分析了冷却、辐照时间、光强分布的空间梯度和各种材料参数对光束质量的影响。     

LC-SLM像素型结构对4f脉冲整形系统的影响

液晶空间光调制器的离散型相位调制和像素间隙是4f脉冲整形系统的主要失真原因,基于以上因素的分析,建立了完善的4f脉冲整形系统数学模型,数值模拟了不同相位调制变化率、不同像元尺寸、不同间隙尺寸以及不同色散关系下的离散型相位调制和像素间隙导致的失真情况。模拟结果表明:离散型相位调制会导致整形脉冲中出现副本脉冲,相位调制变化率越大或像元尺寸越大均会导致这种副本脉冲越强,输出目标脉冲失真越严重;像素间隙也会导致整形脉冲中出现副本脉冲,间隙尺寸越大会导致这种副本脉冲越强,输出目标脉冲失真越严重;两种副本脉冲均呈等间隔分布,其周期由每个像元对应的频谱宽度决定,非线性光谱色散会使副本脉冲啁啾化且强度变小。     

Two-dimensional nonlinear beam shaping

We develop a technique for two-dimensional arbitrary wavefront shaping in quadratic nonlinear crystals by using binary nonlinear computer generated holograms. The method is based on transverse illumination of a binary modulated nonlinear photonic crystal, where the phase matching is partially satisfied through the nonlinear Raman-Nath process. We demonstrate the method experimentally showing a conversion of a fundamental Gaussian beam pump light into three Hermite-Gaussian and three Laguerre-Gaussian beams in the second harmonic. Two-dimensional binary nonlinear computer generated holograms open wide possibilities in the field of nonlinear beam shaping and mode conversion.     

相位共轭谐振腔改善激光器波前像差特性研究

在高功率激光器中采用相位共轭谐振腔技术可以补偿动态波前像差,在校正畸变的输出光束方面有极其重要的应用。分析了相位共轭腔的模式结构特性及稳定性,在实际高功率激光器中采用相位共轭谐振腔,使得激光系统的光束质量得到了极大改善。通过实验数据分析,提出在激光器系统中描述光束质量只用PV值和RMS值来量度是不足的,并利用结合光束环围能量分布图像和平均离焦系数来评价系统光束质量。实验结果表明,采用相位共轭谐振腔使得焦平面环围能量更为集中,同时平均离焦系数更接近零,结合上述方法评价系统光束质量更具实际意义。     

基于多光子脉冲内干涉相位扫描法对飞秒激光脉冲进行相位测量和补偿的研究

研制了一套基于多光子脉冲内干涉相位扫描方法的可以同时对飞秒激光脉冲进行相位测量和补偿的实验系统装置.实验中,通过自主研发的LabVIEW程序控制液晶空间光调制器和光纤光谱仪,对待测飞秒激光脉冲施加相位扫描,并同时记录受到调制的飞秒激光脉冲的倍频光谱,得到了多光子脉冲内干涉相位扫描(MIIPS)轨迹图.通过MIIPS轨迹图的三次测量和迭代运算,还原出了经过预先啁啾调制的中心波长约为810 nm、重复频率为1 kHz的飞秒激光脉冲的光谱相位,测量精度在0.1 rad以内.根据测量结果,利用液晶空间光调制器对该飞秒激光脉冲进行相位补偿,得到了近似傅里叶变换极限的飞秒激光脉冲.这一装置将在多光子显微成像、脉冲整形、飞秒激光光谱学等众多领域发挥重要作用.     

包含波面校正的四程放大系统的准直问题研究

为了解决高功率激光系统中, 尤其是多程放大系统中波前畸变校正和光路准直之间的相互干扰问题, 针对包含自适应光学系统的四程放大系统, 利用菲涅耳衍射积分理论, 建立了310 mm×310 mm大口径光束的四程传输模型, 采用坐标扩展变换的方法解决了传统计算中同时需求近、远场高分辨率带来的计算量过大问题; 基于此模型, 比较了质心法、圆拟合法和纯相位匹配滤波等三种图像处理算法对存在波面差的光束的远场中心的处理精度; 分析了波面校正对焦斑位置的影响; 本文内容为四程放大系统准直调整方案的优化提供了重要依据. 关键词： 光束准直 坐标扩展变换 自适应光学 四程放大     

Wavefront sensing with random amplitude mask and phase retrieval

A light beam with an ideal wavefront that is transmitted or reflected from an object is modified by different characteristics of the object such as shape, refractive index, density, or temperature. Wavefront sensing therefore yields valuable information about the system or the changes happening to the system. A new method for wavefront sensing using a random amplitude mask and a phase retrieval method based on the Rayleigh-Sommerfeld wave propagation equation is described. The proposed method has many potential applications ranging from phase contrast imaging and measurement of lens aberration to shape measurement of three-dimensional objects.     

基于液晶空间光调制器的激光束整形

采用相位混合算法(PMA)与平滑修正法相结合的混合算法,对激光发出的高斯光束进行整形,得到了均方误差和顶部不均匀度均明显降低的等光强分布。利用液晶空间光调制器(LCSLM)的相位调制特性,实现了对高斯光束的光束整形,获得了光强均匀分布的圆光束和矩形光束输出。得到的输出光束顶部不均匀度和均方误差都低于5%,能量集中度在90%以上。表明此方法是一种实时、可控和高效的激光束整形方法。     

超衍射极限相干反斯托克斯拉曼散射显微成像技术中空心光束的形成

空心光束的质量是超衍射极限相干反斯托克斯拉曼散射显微成像技术中决定成像质量的一个至关重要的因素. 本文基于菲涅耳衍射理论,分析了螺旋相位片法产生空心光束的物理机理,并且模拟了不同的入射条件对产生的空心光束的影响. 模拟结果表明:波长与相位片中心波长匹配且光强呈圆对称分布的高斯光垂直入射到相位片上,当高斯光束中心与相位片中心完全对准时,可获得较理想的空心光束;入射光光强分布的圆对称性以及入射光中心与相位片中心的对准程度都会影响产生的空心光束的强度分布;同时,高斯光束小角度倾斜入射时,空心光的强度分布仍呈圆对称,却在观察面发生一定的位移;此外,入射光中心波长偏离相位片中心波长不大时,对产生的空心光束的强度分布几乎没有影响. 上述分析结果对用于超衍射相干反斯托克斯拉曼散射显微成像技术中理想空心光束的获取具有重要的指导意义. 关键词： 空心光束 超衍射极限 相干反斯托克斯拉曼散射 螺旋相位片     

Analytical solution for an anomalous hollow beam in a fractional Fourier transforming optical system with a hard aperture

Based on the fact that a hard aperture function can be expanded into a finite sum of complex Gaussian functions, the approximate analytical expressions for the output field distribution of an anomalous hollow beam (AHB) passing through an apertured fractional Fourier transform (FRT) system are derived. By using the approximate analytical formulae and diffraction integral formulae, the propagation properties of an AHB in circular and rectangular apertured FRT system are studied numerically. The results show that this method provides a convenient tool for studying the propagation properties of an AHB passing through apertured FRT system, and the apertured FRT system can be applied to laser beam shaping conveniently.     

采用阵列波带片波前传感的激光波前重构

 在高功率激光系统，激光束小尺度相位畸变波前探测是实现激光束中频段波前补偿与控制的重要前提。采用阵列菲涅耳波带片作为激光束波前传感的子孔径分割器，利用区域重构的Southwell模型进行波前重构。并针对波前传感器子孔径数目、小尺度相位畸变波前的参数对波前重构精度影响进行分析与模拟计算，给出了阵列菲涅耳波带片合理的子孔径数目，为20×20～30×30；对小尺度相位畸变，激光束波前的重构误差小于十分之一波长。     

一种新空心光束的理论及实验研究

提出了一种用来描述空心光束的新理论模型——空心高斯光束模型，通过控制光束参数可以方便地调制空心光束黑斑的大小.导出了空心高斯光束经傍轴光学系统的解析传输变换公式.利用导出的公式计算分析了空心高斯光束在自由空间的传输特性.在实验上利用矩阵本征值方法设计了能输出空心高斯光束的光学谐振腔，对实验上得到的空心高斯光束进行了传输特性的研究.结果表明，理论和实验相吻合.该空心高斯光束产生的光势阱为控制冷原子提供了一条新途径. 关键词： 空心高斯光束 光学谐振腔 传输特性     

用双光收缩式径向剪切干涉仪测量气体温度场

为了适应大测试区的相位物体的测试研究和控制光学系统的成本,设计出双光收缩式径向剪切干涉光学系统。通过实验得到干涉条纹,用CCD摄入计算机,经Matlab编制傅里叶变换、相位去包裹和其他数值计算程序,解调干涉条纹的相位,计算出温度扰动产生的波面畸变量,它等于折射率变化引起的相位偏移值,用轴对称气体温度场重建方法,再现了气体温度场一个层面上的温度分布。实验结果与铜一康铜微型热电偶单点测温相比较,两者相差不大。实验表明该系统具有较高的精度和测试灵敏度,适于环境温差较小的相位物体热物理量的测量。