Generation of radially polarized light beams of high quality using a step-index optical fiber

We present an experimental setup to generate radially polarized beams without using high-cost optical elements. In the setup a four-segment polarization converter is used in front of the fiber to produce a pseudo radially polarized beam. A traditional step-index fiber which supports only LP₀₁ and LP₁₁ modes is then used as a mode-cleaning device. A commercial mechanical fiber-squeezer polarization controller is applied to produce adequate pressure and twist onto the fiber. The four-segment polarization converter and the fiber squeezer polarization controller are adjusted by turns for improving the beam quality in intensity and polarization. Additionally, several methods of characterizing the polarization properties of radially polarized beams are reviewed. One of the latest methods is applied for characterizing the polarization properties of the radially polarized beams produced by using our technique. The results show the highquality of the obtained beams.     

Experimental verification on tightly focused radially polarized vortex beams

The theoretical and experimental results of tightly focused radially polarized vortex beams are demonstrated. An auto-focus technology is introduced into the measurement system in order to enhance the measurement precision, and the radially polarized vortex beams are generated by a liquid-crystal polarization converter and a vortex phase plate. The focused fields of radially polarized vortex beams with different topological charges at numerical apertures （NAs） of 0.65 and 0.85 are measured respectively, and the results indicate that the total intensity distribution at focus is dependent not only on the NA of the focusing objective lens and polarization pattern of the beam but also on the topological charge l of the beam. Some unique focusing properties of radially polarized vortex beams with fractional topological charges are presented based on numerical calculations. The experimental verification paves the way for some practical applications of radially polarized vortex beams, such as in optical trapping, near-field microscopy, and material processing.     

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.     

Generation of a radially polarized doughnut mode of high quality

We present an experimental set-up to generate laser beams with locally varying polarization distribution. In a linear set-up, a radially polarized beam of high quality regarding intensity distribution, polarization and phase-front distortion is generated. This beam can be used for tight focusing. Further applications are discussed.     

Generation of optical vector beams with a diffractive optical element interferometer

We present a novel approach to generating radially and azimuthally polarized vector beams that utilize an interferometer constructed from two identical diffractive optical elements. The measured polarization properties of four vector beam states and their phase relationships are in good agreement with theoretical expectations. This interferometer is passively phase stable and robust, making it suitable for linear and nonlinear optical (superresolution) microscopy.     

大数值孔径光子筛偏振特性研究

根据角谱理论建立不同偏振照明条件下的光子筛矢量衍射模型。在此基础上,对入射光分别为线偏振光、径向偏振光、切向偏振光三种特殊偏振状态下的光子筛聚焦光强分布进行了模拟分析。研究结果表明,对于大数值孔径光子筛,入射光的偏振特性将对光子筛聚焦光强分布产生巨大影响。线偏振光将使聚焦光斑沿偏振方向拉伸,切向偏振光产生的聚焦光斑具有"中空"结构,而径向偏振光所产生的聚焦光斑呈较为规则的圆形,且其焦深优于线偏照明情况。在激光直写及高分辨成像等光子筛典型应用中采用径向偏振照明将进一步提高系统分辨力。     

Simultaneous generation of helical beams with linear and radial polarization by use of a segmented half-wave plate

Simultaneous generation of helical beams with linear and radial polarization is demonstrated by use of a segmented half-wave plate in a Ti:sapphire laser cavity. A linearly polarized Laguerre-Gaussian beam is converted to a radially polarized beam with a spiral phase shift and vice versa. In addition, these two beams coexist in the cavity, and the cavity emits one of these two beams from an output coupler.     

径向偏振调制对聚焦光斑匀滑及偏振特性的影响

基于标量衍射理论建立了光谱色散平滑(SSD)、随机相位板(RPP)和偏振匀滑(PS)联用的光束匀滑方案数值仿真模型。以斑纹对比度和偏振度为主要评价参量,利用该模型研究了径向偏振调制的匀滑及消偏振特性。研究表明,径向偏振调制可以有效降低聚焦光斑斑纹对比度和偏振度。使用半波片拼接方式产生近似径向偏振光时,拼接单元数对匀滑及偏振特性影响较小,8片拼接的匀滑效果已与理想径向偏振光几乎一致。对比分析了径向偏振调制、正交偏振调制和双折射楔三种PS方式,结果表明,由于三种PS方式本质上的一致性,光束的匀滑及偏振特性差别较小。     

超短脉冲径向偏振光束的解析解及时空耦合效应

从超短脉冲光束的傍轴传输方程出发, 运用傅里叶变换和相关数学算符的对易性, 得到了超短脉冲径向偏振光束的解析表达式.该解可适用于任意脉冲驱动的径向偏振光束. 基于该解析表达式并结合具体例子,讨论了超短脉冲径向偏振光束在自由空间中的传输性质. 结果表明, 在传输过程中时空耦合主要体现在光束边沿的脉冲延迟. 这一效应导致了脉冲不同时间位置处横向光强分布随传输的变化, 以及脉冲前后沿关于束腰的不对称性. 本文的方法同样适用于得到超短脉冲方位角偏振光束的解析解和传输性质.     

Efficient extracavity generation of radially and azimuthally polarized beams

We demonstrate an efficient transformation of a linearly polarized Gaussian beam to a radially or an azimuthally polarized doughnut (0,1)* Laguerre-Gaussian beam of high purity. We use a spatially variable retardation plate, composed of eight sectors of a lambda/2 retardation plate, to transform a linear polarization distribution to radial/azimuthal distribution. We transformed an Nd:YAG Gaussian beam with M(2)=1.3 to a radially and azimuthally polarized (0,1)* Laguerre-Gaussian beams with M(2)=2.5 and degree of radial/azimuthal polarization of 96-98%.     

Production of radially or azimuthally polarized beams in solid-state lasers and the elimination of thermally induced birefringence effects

Production and amplification of radially and azimuthally (tangentially) polarized laser beams are demonstrated. Based on the different focusing between radially and tangentially polarized light in thermally stressed isotropic laser rods, Nd:YAG laser oscillators were developed to produce low-loss stable oscillation in a single polarization. Pure radially polarized light at 70 W with M2 = 2 and on-axis impure radially polarized light at 150 W with M2 = 2.5 were achieved. The radially polarized beams were then amplified while good beam quality and polarization purity were retained. Complete elimination of thermal-birefringence-induced aberrations was demonstrated. This should allow much better beam quality from rod-based high-power lasers.     

Propagation and parametric characterization of the polarization structure of paraxial radially and azimuthally polarized beams

Analytical expressions are provided for describing the free-space evolution of the polarization structure of paraxial beams whose electric-field vector at some transverse plane exhibits either a radially or an azimuthally polarized behavior. At each transverse plane, the polarization distribution across the beam profile is characterized by means of two sets of parameters, namely, the so-called (local) radial Stokes representation, and the (overall) percentage of the irradiance associated with the radial and azimuthal field components. The propagation laws for these sets of parameters are also shown. As an illustrative example, a radially polarized beam is analized whose wavefront contains a spiral phase factor.     

Generation of azimuthally and radially polarized beams by coherent polarization beam combination

A new architecture for generating pure azimuthally and radially polarized beams is presented. It involves coherent polarization beam combination of two orthogonally polarized LP(11) fiber modes. Experimental results reveal that high purely polarized (polarization purity of 95% or better) azimuthal and radial beams can be generated.     

显微物镜聚焦光场任意三维偏振方向的控制

线性偏振光在显微物镜焦斑处入射,与径向偏振光相干叠加产生三维偏振光场,通过调节两束光光强比率和入射线偏振光偏振方向实现聚焦光场任意三维偏振方向的控制.基于矢量光场衍射理论建立了仿真计算模型,对所提三维偏振方向控制方法的可行性进行理论验证和实验评估.构建实际控制光路并进行初步测试,实验结果表明了该方法的可行性,且相比于其他三维偏振控制方法,本文所提方法和其控制光路更为简单易实现.     

Bessel beams: Effects of polarization

An approximation to a Bessel beam produced by tightly focusing linearly polarized light is known to produce a smaller central lobe than focusing plane polarized light. This is because the plane polarized wave gives a broad central lobe caused mainly by a parasitic longitudinal field component. It is known that this problem can be overcome by focusing radially polarized light. Here we demonstrate that other polarization distributions based on a linear combination of transverse electric (TE1) and transverse magnetic (TM1) fields can give a beam even narrower than for the radially polarized case. Special cases of this combination are identified, corresponding to the smallest width (TE1), and the maximum peak intensity compared with the side lobes (electric dipole polarization). Axially-symmetric forms can be generated by illumination with elliptically polarized light. A particular case is azimuthal polarization with a phase singularity, which is equivalent to TE1. For a semi-angular aperture of 60°, the TE1 case gives a central lobe width 9% narrower than for radially polarized illumination, while for plane polarized illumination it is 12% wider than the radially polarized case.     

高效产生任意矢量光场的一种方法

提出一种高效产生任意矢量光场的方法.利用两个光束偏移器分别对两个正交线偏振分量进行分束与合束,将传统激光模式转化为任意矢量光场.所产生矢量光场的偏振态和相位分布通过相位型空间光调制器(SLM)加载相应的相位实时调控.由于光路系统中不涉及任何衍射光学元件和振幅分光元件,光场转换效率高,仅取决于SLM的反射率,并且光路系统结构紧凑、稳定,同轴性易于调节.实验结果显示,采用反射率为79%的相位型SLM产生矢量光场的转换效率可达到58%.     

三环非均匀混合偏振同轴矢量光束的聚焦特性

基于Richards-Wolf矢量衍射积分公式,数值分析了同轴三环非均匀混合偏振矢量光束经过高数值孔径透镜的聚焦特性。该矢量光束由同轴三环局域线偏振矢量光束通过一个相位延迟角为δ的液晶相位延迟器产生,光束偏振变为包含线偏振、圆偏振和椭圆偏振的混合态。同轴三环局域线偏振矢量光束的偏振分布是由径向向内偏振的外环光束、径向向外偏振的内环光束和线偏振方向与径向方向夹角为φ2的中环光束构成。数值模拟结果显示该混合偏振矢量光束的聚焦强度分布与参数φ2和相位延迟角δ密切相关,当选取适当的φ2和δ时,在焦平面附近产生沿光轴方向的三维多点光俘获结构——暗光链,这在光学微操纵领域具有潜在的应用价值。     

Polarization properties of polarized and partially coherent Electromagnetic Gaussian-Schell model pulse beams on slant path in turbulent atmosphere

This paper is based on the unified theory of coherence and polarization of stochastic electromagnetic beams and the extended Huygens–Fresnel principle, combined with the quadratic approximation of Rytov’s phase structure function and the generalized Stokes parameters. We have derived the novel expressions for the cross-spectral density matrix elements and the degree of cross-polarization of a class of elliptically polarized spatially and spectrally partially coherent Electromagnetic Gaussian-Schell model pulse (EGSMP) beams propagating through atmospheric turbulence along a slant path. Additionally, we calculate and analyze the effects of the turbulent intensity, the initial pulse duration, waist width of the beam, the spatial coherence length and temporal coherence length et al. on the polarization properties of fully polarized and partially coherent EGSMP beams. Finally, a comparison of the impact of those factors on the partially polarization beams is made. The results show that the influences of the turbulent intensity, the initial pulse duration, waist width of the beam, the spatial coherence length and temporal coherence length et al. on the polarization properties of fully polarized and partially coherent EGSMP beams are larger. While the effects of those parameters on the partially polarization and partially coherent EGSMP beams are smaller. It is noted that the results of this paper have established sound theoretical basis on the topic of improving performance of the laser system propagating through the atmospheric turbulence.     

Manipulation of the overall polarization orientation in the focal volume of high numerical objectives

We propose an approach for tuning the three-dimensional polarization of a focusing subwavelength spot by a high numerical aperture objective. The incident beams are composed of a radially polarized beam, an azimuthally polarized beam, and a linearly polarized beam with three different weighting factors, respectively. A specially designed adjustable amplitude angular selector is also inserted at the back aperture of the objective for tuning the polarization azimuthally. It is shown that any desired overall polarization orientation can be obtained. We calculated the overall polarization orientation in the focal volume. It is found that the polar angle of the overall polarization orientation can be arbitrarily tuned by the combination of a radially polarized beam and a linearly polarized beam with different weighting factors, and the azimuthal angle can be tuned by rotating the orientation of the linearly polarized beam azimuthally.     

Forces in optical tweezers with radially and azimuthally polarized trapping beams

It has been suggested that radially polarized beams can be used to improve the performance of optical tweezers, with reduced scattering force resulting from both the polarization and the dark center of the beam [Opt. Lett. 32, 1839 (2007)]. We calculate the forces on particles in such traps, using rigorous electromagnetic theory, comparing the results with azimuthally polarized beam, circularly polarized LG 01 beams, and Gaussian beams. Our results agree qualitatively with Opt. Lett. 32, 1839 (2007), but differ quantitatively.