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%。     

Gaussian laser beam tailoring using acoustooptic cell

Profile shaping of a Gaussian laser beam by an acoustic wave is well described using Collins integral and ABCD matrix formalism. It is shown by a numerical simulation that the relative width of the laser beam to the ultrasonic wavelength and the acoustic pressure inside the acoustooptic cell act on the light intensity diffraction pattern.Obtained results show that the output intensity profile differs from the incident Gaussian beam shape, and it is more broadened with an increase in the acoustic pressure. The intensity of a focused laser beam is transformed in a flat form in the central region if the acoustic pressure is proprely controlled.On the other hand the intensity longitudinal range (ILR) of the flat shape is discussed along the propagation axes, we have found the ILR is about 2 mm for a focal length distance f=100 mm.     

Near-diffraction-limited dark hollow beam generated by using a hybrid control way

We propose and demonstrate the generation of dark hollow beam with plane wavefront in the near field based on a hybrid control way by using dual phase only liquid crystal spatial light modulators (LC-SLMs). One phase only LC-SLM redistributes the intensity to the target distribution based on refractive beam shaping system and the other phase only LC-SLM adaptively compensates the wavefront of the output beam by using stochastic parallel gradient descent (SPGD) algorithm. Compared with conventional refractive beam shaping system, the hybrid control way can not only improve the far-field energy concentration of the output beam, but also can correct the phase distortion of the system. Influences of phase distortion on laser beam shaping are analyzed in detail. The theoretical and experimental results show that the near-diffraction-limited dark hollow beam can be successfully generated by using this hybrid control way. The power in the main lobe of the airy disk pattern area in the far field is 2.3 times larger than that without being adaptively compensated.     

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     

四瓣高斯光束的Gyrator变换性质和矩形空心光束的产生

基于Gyrator变换,推导了四瓣高斯光束场分布的解析表达式,研究了四瓣高斯光束通过Gyrator变换后的光强分布和相位分布.结果表明:在Gyrator变换过程中,四瓣高斯光束能够转换为具有光涡旋的矩形空心光束,在获得矩形空心光束时其四顶角处光束强度最强,而四条边上的光束强度分布几乎是均匀的.对影响矩形空心光束强度和相位分布的光束参数和变换角进行了详细的分析,发现光束阶数不同,产生不同类型的空心光束;Gyrator变换的变换角则影响空心光束能量分布;空心光束亮环的大小由四瓣高斯光束的束腰宽度决定,束腰宽度越大,矩形空心光束的宽度越小.     

Gyrator transform of Gaussian beams with phase difference and generation of hollow beam

The optical expression of Gaussian beams with phase difference, which is caused by gyrator transform (GT), has been obtained. The intensity and phase distribution of transform Gaussian beams are analyzed. It is found that the circular hollow vortex beam can be obtained by overlapping two GT Gaussian beams with π phase difference. The effect of parameters on the intensity and phase distributions of the hollow vortex beam are discussed. The results show that the shape of intensity distribution is significantly influenced by GT angle α and propagation distance z. The size of the hollow vortex beam can be adjusted by waist width ω₀. Compared with previously reported results, the work shows that the hollow vortex beam can be obtained without any model conversion of the light source.     

Generation of dark hollow beam by use of phase-only filtering

A simple but effective scheme to generate dark hollow beams is proposed by use of phase-only filtering and optical Fourier transform. A Gaussian beam of fundamental mode is modulated by a pre-designed phase mask, which is a piecewise modification of an axicon lens, and followed by a Fourier transform to generate an ideal dark hollow beam at the focal plane. This method has an advantage that the total energy of the beam is conserved under paraxial approximation. Numerical calculations are provided to show the validity of the proposed scheme.     

广义双曲正弦-高斯光束的Gyrator变换性质和暗空心光束产生

基于Gyrator变换,推导了广义双曲正弦-高斯光束场分布的解析表达式,研究了广义双曲正弦-高斯光束在Gyrator变换平面上的光强分布和相位特性.结果表明,在Gyrator变换过程中,具有边缘位错相位特性的双曲正弦-高斯光束能转换为具有涡旋的暗空心光束,并确定产生的暗空心光束的拓扑荷指数为一,而不具有边缘位错相位特性的双曲余弦-高斯光束则不可能出现空心结构.对影响变换场强度和相位分布的束结构参数及系统参数进行了分析讨论.     

Propagation of a phase-locked circular dark hollow beams array in a turbulent atmosphere

The propagation of phase-locked circular dark hollow beams array in a turbulent atmosphere is studied. An analytical expression for the average intensity distribution at the receiving plane is obtained based on the extended Huygens-Fresnel principle. The effects of turbulence, dark parameter and beam order of the beams array on the intensity pattern are studied and analyzed. It is found that the intensity pattern of the phase-locked circular dark hollow beams array will evolve from a multiple-spot-pattern into a Gaussian beam spot under the isotropic influence of the turbulence. The intensity pattern of beam array with a larger dark parameter and beam order evolves into the Gaussian-shape faster with increasing propagation distance.     

非均匀关联径向偏振部分相干光的产生

从理论和实验两方面对非均匀关联径向偏振部分相干光的产生进行了研究.理论上,基于相位关联与相干度的联系,推导出了非均匀关联径向偏振部分相干光的2×2阶交叉谱密度矩阵及相干度分布.实验上,利用一个相位型液晶空间光调制器的不同区域,对入射的完全相干的径向偏振光的两个正交偏振分量分别加载随机相位调制,并实验测量了这种光束的相干度分布及其对光强分布的影响.实验结果验证了光束相干度的非均匀关联结构,并且通过改变随机相位的高斯调制半宽可以改变光束的相干性分布.研究表明,随着随机相位的高斯调制半宽的增加,光束中两点间的相干度逐渐减小,其光强分布由圆环状逐渐变化为类平顶的光强分布.这种非均匀关联的径向偏振部分相干光在激光微操纵和材料加工等领域具有一定的潜在应用价值.     

大气湍流像差散焦和像散与高斯涡旋光束焦面光强

分别研究了构成大气湍流波像差中的散焦和像散两个低阶像差对高斯涡旋激光束传输和成像的影响.采用菲涅耳-基尔霍夫衍射积分理论和大气湍流波相位结构函数的平方近似研究了聚焦高斯涡旋光束在大气湍流中散焦和像散影响下焦面光强的分布特性.导出了斜程传输条件下接收面上平均光强分布的积分表达式,并采用数值模拟方法研究湍流强度、传输距离和拓扑电荷对焦面光强的调制规律.结果表明:在弱湍流起伏区域,散焦和像散两类像差对高斯涡旋光束的光强分布影响都很小,可以忽略;在中等湍流区域,随着光束传输距离和湍流强度的增加,两类像差都导致高斯涡旋光束的光强峰值降低、束径扩展、中心暗斑扩大.当单拓扑电荷高斯涡旋光束传输时,在同等传输条件下,像散导致的光强峰值降低比散焦更严重,主亮斑区域外的次级亮环强度更大,光斑和中心暗斑扩展更明显.与单拓扑电荷光束相比较,散焦和像散导致双拓扑电荷光束的扩展更加明显,中心光斑更大,亮环区域外的次级亮环更明显;但是,由于光的相干性的降低和光束的偏折效应,像散导致光束中心的暗斑变为次级亮斑.     

贝塞尔-高斯涡旋光束相干合成研究

基于相干合成技术,提出了对特定离散空间分布的高斯光束阵列加载离散涡旋相位生成二阶贝塞尔-高斯(Bessel-Gaussian,BG)涡旋光束的方案.利用干涉法、桶中功率和相关系数对合成BG涡旋光束的拓扑荷、光束质量进行了定量评价及参数优化.结果表明:基于相干合成技术能够产生特定的目标BG涡旋光束,阵列子光束紧密排布时合成BG光束的光束质量更高.该方法的提出对于其他涡旋光束的产生或者涡旋光束功率的提高具有一定的参考意义.     

Changes in the spectral degree of polarization of a partially coherent dark hollow beam in the turbulent atmosphere for on-axis and off-axis propagation point

We report analytic formulas for the elements of the 2 × 2 cross spectral density matrix of a partially coherent dark hollow beam (DHB) with circular symmetry propagating through the turbulent atmosphere. In this paper, from these formulas changes in the spectral degree of polarization of partially coherent DHB for on-axis and off-axis propagation point and for near and far fields are determined for different source conditions.     

具有余弦-高斯关联结构函数部分相干贝塞尔-高斯光束的传输性质及四暗空心光束的产生

基于广义惠更斯-菲涅耳衍射积分公式,获得了余弦-高斯关联结构函数部分相干贝塞尔-高斯光束交叉谱密度函数通过近轴ABCD光学系统传输时的解析表达式.并因此探讨了该类光束经过自由空间传输时光强分布的演化特性.结果表明,余弦-高斯关联部分相干贝塞尔-高斯光束在合适的参数条件下能呈现自分裂等奇异传输特性.特别地,这种自分裂可实现暗空心光束的复制,即从一个暗空心光束获得四个相似的暗空心光束.并且发现这些传输特性和关联结构函数结构密切相关,因此调控关联结构函数分布以实现调制光的相干长度和空间分布性质从而可实现操控光束传输行为.由于暗空心光束在工程技术领域的重要应用价值,本文的研究结果提供了实现四暗空心光束的可能方案,从而在激光通信、微粒操控等方面具有重要的应用前景.     

Transformation of a hollow Gaussian beam into a ring-shaped beam using a phase aperture

Propagation of a hollow Gaussian beam diffracted by a circular phase aperture is studied without making the paraxial approximation. The analytical expression of the intensity of the apertured hollow Gaussian beam is presented in the far field. The influences of the truncation parameter and the order of hollow Gaussian beam on the intensity distributions are discussed. It is shown that a circular ?-phase aperture can be used to transform a hollow Gaussian beam into a ring-shaped beam in the far field with the appropriate parameters.     

Spirally polarized axisymmetric Bessel-modulated Gaussian beam

Focusing properties of the spirally polarized axisymmetric Bessel-modulated Gaussian beam with quadratic radial dependence (QBG beam) are investigated theoretically by vector diffraction theory. Calculation results show that intensity distribution in focal region can be altered considerably by beam parameter μ and spiral parameter C that indicates polarization spiral degree. For certain real value μ, focal spot can evolve from one focal spot to one focal ring, spherical crust focal shape, then two focal rings on increasing spiral parameter C. It was found that under condition of different μ, evolution principle of focal pattern differs very remarkably on increasing C. And some novel focal shapes may appear, including rhombic shape, quadrangular shape, two-spherical crust focus shape, two-peak shape, one dark hollow focus, two dark hollow focuses pattern, triangle dark hollow focus.     

Coherent beam combination of two thulium-doped fiber laser beams with the multi-ditheringtechnique

Coherent beam combination of two thulium-doped fiber laser beams using a multi-dithering technique is presented for the first time. In the experiment, two fiber lasers centered at 1948.6 nm are coherently combined, and a phase modulator based on piezoelectric ceramics transducer is connected in one beam path to compensate for the phase errors between the two beams. When the phase control system is closed loop, the fringe contrast of the far-field intensity pattern is improved to be more than 75%, from 15% in open-loop, and the residual phase error is less than λ/20. The experimental results show that the performance of the phase control system is robust and the control bandwidth is more than 1 kHz, which indicates that the above approach can be scaled to facilitate the coherent beam combination of kilo-watt level thulium-doped fiberlaser.     

Modified hollow Gaussian beam and its paraxial propagation

A model named modified hollow Gaussian beam (HGB) is proposed to describe a dark hollow beam with adjustable beam spot size, central dark size and darkness factor. In this modified model, both the beam spot size and the central dark size will be convergent to finite constants as the beam order approaches infinity, which are much different from that of the previous unmodified model, where the beam spot size and the central dark size will not be convergent as the beam order approaches infinity. The dependences of the propagation factor of modified and unmodified HGBs on the beam order are found to be the same. Based on the Collins integral, analytical formulas for the modified HGB propagating through aligned and misaligned optical system are derived. Some numerical examples are given.     

Propagation of partially coherent controllable dark hollow beams with various symmetries in turbulent atmosphere

Normalized intensity distribution, the complex degree of coherence and power in the bucket for partially coherent controllable dark hollow beams (DHBs) with various symmetries propagating in atmospheric turbulence are derived using tensor method and investigated in detail. Analytical results show that, after sufficient propagation distance, partially coherent DHBs with various symmetries eventually become circular Gaussian beam (without dark hollow) in turbulent atmosphere, which is different from its propagation properties in free space. The partially coherent DHBs return to a circular Gaussian beam rapidly for stronger turbulence, higher coherence, lower beam order, smaller p or smaller beam waist width. Another interesting observation is that the profile of the complex degree of coherence attains a similar profile to that of the average intensity of the related beam propagating in a turbulent atmosphere. Besides the laser power focusablity of DHBs are better than that of Gaussian beam propagating in turbulent atmosphere.     

The temperature properties of matching Gaussian beam in biased two-photon centrosymmetric paraelectric photorefractive crystals

The effects of temperature on the intensity profiles and self-deflection of matching Gaussian beam in biased two-photon centrosymmetric paraelectric photorefractive crystals are investigated by numerical simulation. The results show that the matching Gaussian beam can involve into a stable solitary wave after some propagation distance. The peak intensity decreases and the width of the matching Gaussian beam increases as temperature rises. The absolute value of the bending distance decreases monotonically with increasing of temperature. The crystal proposed here is potassium lithium tantalite niobate (KTN).