Generation of a radially polarized laser beam by use of a conical Brewster prism

To generate a radially polarized laser beam we designed and fabricated a new Brewster optical element that consists of convex and concave conical prisms. The lateral surface of the convex conical prism was coated with a dielectric multilayer (SiO2 and Ta2O5) to enhance polarization selectivity. By combining two prisms we obtained a conical Brewster prism without beam divergence owing to refraction. A radially polarized TEM01* (R-TEM01*) mode laser beam was demonstrated when this prism was used inside a Nd:YAG laser cavity.     

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.     

Nonparaxial TM and TE beams in free space

Expressions for the fields of TM and TE laser beams in free space that are rigorous solutions to Maxwell's equations are given in a closed form. The electric and the magnetic fields are both expressed in terms of nonparaxial elegant Laguerre-Gaussian beams that are exact solutions of the Helmholtz equation. These solutions involve well-known functions, such as spherical Bessel and associated Legendre functions. Radially and azimuthally polarized beams of arbitrary order are considered, and the lowest-order radially polarized beam (TM(01) beam) is investigated in detail.     

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.     

Converging-axicon-based radially polarized ytterbium fiber laser and evidence on the mode profile inside the gain fiber

The radially polarized mode is achieved from an active Yb fiber by utilizing of an intracavity converging axicon, where the axicon acts as a TM(01) mode selector based on not only its Brewster convex surface but also the distance between its vertex and plane output coupler. The polarization state of the annular laser beam is checked by using a home-made eight-hole aperture. Furthermore, an uncoated plane glass plate is inserted into the cavity, and the reflected beam points to the existence of an annular lasing mode inside the gain fiber. The issues for developing high-power radially polarized fiber lasers also are considered.     

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.     

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.     

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.     

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.     

Efficient conversion of a radially polarized beam to a nearly Gaussian beam

We demonstrate an efficient method for transformation of a radially polarized Laguerre-Gaussian beam to a nearly Gaussian beam with much higher beam quality. The method is based on separation of the radially polarized mode into two degenerate modes and coherent addition of the modes after phase flattening. We transformed a high-power Nd:YAG radially polarized (0,1)(*) Laguerre-Gaussian beam with M(2)=2.52 and power of 30 W into a nearly Gaussian beam with M(2)=1.3. As a result, the brightness increased by a factor of approximately 2.5.     

Investigation of Near-Field Imaging Characteristics of Radial Polarization for Application to Optical Data Storage

Radially polarized incident light can generate a more confined longitudinal electric field on a focal plane in near-field (NF) optics than focusing circularly polarized light. Using this phenomenon, it is feasible to reduce beam spot size on storage media to increase the areal density of optical data storage. A radially polarized beam generates a beam spot which is 20% more confined on the 1st surface of medium than that of circularly polarized light. However, the peak intensity of total electric field sharply decreases and its transverse component is much more dominant inside the media stack. This confirms that radially polarized optics can be a candidate not for an NF recording system but for an NF read-only memory (ROM) system. Potentially, the results could be useful to understand the effect of radial and circular polarizations inside and outside medium for various applications of NF optics.     

The propagation of radially polarized partially coherent beam through an optical system in turbulent atmosphere

The propagation of a radially polarized partially coherent beam through an optical system in turbulent atmosphere is investigated. Analytical propagation formulae for the radially polarized partially coherent beam through an optical system in turbulent atmosphere are derived, and some analyses are illustrated by numerical examples related to propagation properties of the radially polarized partially coherent beam through a double-lens system in turbulent atmosphere. It is shown that the optical system, structure constant of turbulent atmosphere and the initial coherent length have significant influence on intensity, degree of polarization and degree of coherence for radially polarized partially coherent beam.     

Evanescent Bessel beam generation through filtering highly focused cylindrical vector beams with a defect mode one-dimensional photonic crystal

We propose a simple setup for generating evanescent Bessel beams using the defect mode of one-dimensional (1D) photonic crystal. The angular selectivity provided by the defect mode mimics the role of an axicon for Bessel beam generation. When an azimuthally polarized beam is strongly focused onto a 1D defect mode photonic crystal interface, an evanescent Bessel beam of the first-order is produced, while an evanescent Bessel beam of the zeroth-order will be created under a radially polarized beam illumination. Switching between a donut shape and a solid focal distribution can be easily realized by controlling the polarization of the illumination. Such a versatile evanescent Bessel beam generation may find potential applications in optical trapping.     

Experimental Research on Focusing Property of Radially Polarized Beam Based on Solid Immersion Lens

Realization of a near-field optical virtual probe based on an evanescent Bessel beam is strongly dependent on a radially polarized beam; this makes it essential to study the focusing property of the beam. In this paper, two experimental setups based on a fiber device and a liquid crystal device, respectively, are built to generate a radially polarized beam. This beam and an annular radially polarized beam are focused by means of a high numerical aperture objective and a solid immersion lens (SIL). Near-field distribution of the focus spot, the evanescent Bessel field, is experimentally measured with a scanning near-field optical microscope (SNOM). The full width at half maximum (FWHM) of the central peak of the evanescent Bessel field is about 200 nm in the close vicinity of the bottom surface of SIL. This has potential for use as a near-field optical virtual probe.     

Achievement of longitudinally polarized focusing with long focal depth by amplitude modulation

The longitudinal component of a focused beam is split into two parts along the optical axis to obtain a longitudinally polarized long focal depth using amplitude filtering based on Euler transformation and a radially polarized Bessel-Gaussian beam. Numerical results indicate that long focal depth and FWHM can be easily achieved with 9λ and 0.8λ, respectively. A radially polarized beam can be converted into a longitudinally polarized beam with a conversion efficiency of 51.0%. It can therefore be believed that the proposed scheme can be widely used to generate a longitudinally polarized beam for particle acceleration, laser cutting, and optical trapping.     

Propagation of radially polarized beams in the oceanic turbulence

Based on the extended Huygens–Fresnel principle and the unified theory of coherence and polarization of light, we investigate the propagation properties of a radially polarized beam through turbulent ocean. Analytic formulae for the spectral density, the spectral degree of polarization, and the beam characteristics of such a beam on propagation are discussed. It is shown that under the influence of oceanic turbulence, the radially polarized beam will change to a partially polarized one and the beam profile will approach to a Gaussian distribution.     

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.     

Characterizing polarization properties of radially polarized beams

In the present work two methods of characterizing polarization properties of well-known radially polarized beams are discussed in both theoretical aspect and experimental aspect. A rotating linear polarizer used behind the beam is known to be used to qualitatively characterize the polarization properties of a radially polarized beam. In this paper for the first time we give the mathematic model of this characterization process. The proposed model helps to analyze the known experimental results. On the other hand two global parameters have been previously proved to be used to characterize the linear or circular polarization content of the radially polarized beams. In this paper for the first time we propose the theoretical model of determining the two parameters in experiments. Some experimental results on characterizing the polarization properties of the real radially polarized beam produced by using different approaches are shown.     

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。     

Three-dimensional multiple optical cages formed by focusing double-ring shaped radially and azimuthally polarized beams

We propose and simulate a method for generating a three-dimensional(3D) optical cage in the vicinity of focus by focusing a double-ring shaped radially and azimuthally polarized beam. Our study shows that the combination of an inner ring with an azimuthally polarized field and an outer ring with a radially polarized field and a phase factor can produce an optical cage with a dark region enclosed by higher intensity. The shape of the cage can be tailored by appropriately adjusting the parameters of double-mode beams. Furthermore, multiple 3D optical cages can be realized by applying the shift theorem of the Fourier transform and macro-pixel sampling algorithm to a double-ring shaped radially and azimuthally polarized beam.