Design of refractive/diffractive hybrid optical elements for beam shaping with large diffraction pattern

Diffractive optics is an important technique for beam shaping with high light efficiency and strong diffraction pattern flexibility. Since the diffraction angle is limited by the unit size of the diffractive optical element(DOE),the size of the required diffraction pattern is always rather small. In this Letter, refractive/diffractive hybrid optical elements(RDHOEs) consisting of a DOE and a lens are used to realize beam shaping for a large diffraction pattern. The lens, as the component of the RDHOEs, can not only be concave but also convex,and the double sampling Fresnel diffraction algorithm is developed for the design of these two types of RDHOEs.The simulation and experimental results provide solid evidence to demonstrate the proposed method with the pure phase spatial light modulator.     

Diffractive optics: Design, realization, and applications

This paper presents methods for designing and recording optimal computer-generated diffractive optical elements. The design method is based on an analytic ray-tracing procedure for minimizing aberrations. The recording involves computer-generated masks and multiple lithographic processes in order to form reflective and transmissive multilevel, surface relief-phase, diffractive elements. As a result, the elements can have high diffraction efficiencies over a broad range of incidence angles. Even generalized diffractive elements that operate with highly uniform diffraction efficiency and polychromatic radiation can be designed and recorded by optimizing the shape and height of the relief gratings. To illustrate the effectiveness of the diffractive optical elements, they have been incorporated into a number of applications, involving visible as well as infra-red radiation. Some that deal with coordinate transformation, beam shaping, and polarization control are briefly reviewed.     

Laser writing system for fabrication of diffractive optics elements

We report a laser writing system for fabrication of diffractive optical elements with He-Cd laser. The wavelength of the light source is 441.6 nm. The output beam is collimated into parallel light with uniform intensity distribution after passing through the spatial filter with a pinhole of 25μm and the collimating device. A microscopy objective lens with numerical aperture (NA) of 0.65 is used to focus the beam into a small diffraction spot. Any pattern can be written with this system. Experimental results are presented.The written gratings and the phase patterns were verified with a conventional optical microscopy and the Taylor Hobson equipment.     

衍射光学元件改善激光谐振腔输出特性的研究

运用矩阵本征值方法,对用位相型二元光学元件构成的激光谐振腔进行了数值计算,重点分析了圆形反射镜腔在不同菲涅耳数条件下的模结构参量特性.结果表明,这种谐振腔可以有效地提高基模与高阶模的模式分离度,并且可以根据需要,定制出理想的输出光束 关键词： 模式分离 衍射光学元件 激光谐振腔 矩阵本征值     

Diffractive elements based on spiral beams as devices for determining the depth of bedding of radiation sources

A modified iteration method is proposed for calculations in developing diffraction optical elements for the generation of rotating light fields. The main feature of the proposed approach is the use of the spiral light beam theory for selecting the initial phase approximation. The diffractive optical elements under study could be used in microscopic analysis of the spatial location of different objects.     

Micromechanics-based digitally controlled tunable optical beam shaper

A digitally controlled programmable optical beam shaper that uses a two-dimensional (2-D) small-tilt micromirror device is introduced. The optical intensity profile at any spatial position can be independently modified by use of a macropixel, incorporated with binary pulse-width modulation. An experimental proof-of-concept optical beam shaper with a commercial 2-D digital micromirror device has shown that the spatial profile of a He-Ne laser beam can be digitally manipulated. Investigation of the analog intensity control showed 256-level nonlinear degamma behavior with a measured 24.5-dB optical contrast ratio and a 10-bit spatial resolution. The performance of this tunable optical beam shaper is limited by the dimensions of the micromirror, the intermirror distance, the size of the optical beam, the number of bits used to control the micromirror, the diffraction effect, and the quality of the imaging optical system.     

Formation of the Intensity Distribution in Laser Beams due to Difraction on Structures of Small-Size Optical Inhomogeneities

The possibilities of formation of the intensity distribution in laser beams due to diffraction of radiation on spatial structures of small-size optical inhomogeneities are considered. Computer modeling of beam profile formation by plates-apodizers containing two-dimensional structures of discrete amplitude or phase scatterers is performed. The results presented testify to the prospects of optical elements with scattering centers for laser beam shaping in UV, visible, and IR spectral ranges.     

用于圆对称光束波前变换的位相型光学系统

由两个位相型衍射光学元件构成的光学变换系统可以实现一般圆对称光束的波前变换。在只需满足菲涅耳衍射近似条件下,采用稳相法与输入输出法相结合,可以设计出具有较高精度的光学变换系统。 关键词：     

晶体X射线劳厄衍射分束特性研究

本文基于X射线衍射动力学分析了劳厄晶体的分束特性,模拟了晶体吸收和入射光角发散对于透射光和衍射光摇摆曲线的影响,定量给出晶体衍射面内角调节范围和晶体加工厚度对于劳厄衍射分束比的调制.在实验中,采用分析晶体和分束晶体的消色散配置限制入射光角发散的影响,实现300μm厚Si(220)晶体面内角调节劳厄衍射分束的精确测量,并得到300μm, 400μm和500μm厚度Si晶体分束比的调节范围,实现了透射光和衍射光强度的定量调制.     

A single-mode-fiber-based point diffraction interferometer

Based on the modification of a point diffraction interferometer, a method for measuring the optical fields from remote objects and external sources is developed. In this method, the reference beam is obtained from the studied beam by filtration of its spatial spectrum with a single-mode optical fiber. To eliminate the effect of phase errors caused by the nonideality of elements of the optical scheme on the measurement results, we developed a procedure of interferometer calibration that permits one to determine these errors with the aim of subtracting them from the measured field phase distributions.     

纳米分辨相干反斯托克斯拉曼散射显微成像

采用附加探测光声子耗尽法来实现超衍射极限相干反斯托克斯拉曼散射显微成像. 此方法引入一束环形分布的附加探测光来消耗点扩展函数周边的相干声子, 实现点扩展函数的改造, 从而达到超越衍射极限的空间分辨率. 为了获得更高的空间分辨率和更佳的相位匹配条件, 通常需采用高数值孔径物镜对抽运光、斯托克斯光和探测光进行聚焦, 此时标量衍射理论不再成立. 基于矢量衍射理论, 分析了线偏振光、圆偏振光先后经过螺旋相位片和高数值孔径物镜后的光强分布, 结果表明: 圆偏振光在高数值孔径物镜后焦平面的光强分布呈中心对称状, 较线偏振环形光更适合作为附加探测光. 此外, 采用全量子理论分析了附加探测光声子耗尽法. 结果表明: 当附加探测光与探测光强度比为80时, 成像系统的横向空间分辨率可以达到45 nm; 继续提高附加探测光强度, 空间分辨将进一步提高.     

基于衍射光学元件的激光相干合成研究进展

从衍射光学元件的基本原理出发,围绕连续波和脉冲波两大应用领域,综述了国内外基于衍射光学元件实现共孔径相干合成的研究进展。在国内,上海光学精密机械研究所分别实现了连续光和脉冲光的合成,连续光实现了206 W的输出功率,光束质量1.38,合束效率29.6%;脉冲光实现了峰值功率1.02 kW,重复频率2.2 MHz的ns级脉冲相干合成光束,合束效率61%。在国外,连续光方面实现了5 kW量级的合成光输出,合束效率82%;脉冲光方面实现了平均功率150 mW,重复频率100 MHz的fs级脉冲相干合成光束,合束效率83.4%。最后对基于衍射光学元件的激光相干合成技术的未来发展做出了展望,相信在不久的将来,基于衍射光学元件的相干合成技术会不断发展,逐渐突破技术瓶颈,从而为更多的应用领域奠定坚实基础。     

Displacements and deformations of a vortex light beam produced by the diffraction grating with embedded phase singularity

Properties of vortex light beams produced by a diffraction grating with groove bifurcation (“fork” structure) are studied in the case of small diffraction angles. Analytical expressions are derived for the amplitude distribution of a diffracted beam generated from an incident Gaussian beam with arbitrary radius and wavefront curvature, transversely shifted and inclined with respect to the nominal axis (normal to the grating crossing it at the bifurcation point). In such situations, the output beam becomes asymmetric; the optical vortex core and the intensity maximum displace orthogonally to the incident beam shift direction. The nearest vicinity of the vortex core preserves its circular symmetry and the optical vortex remains locally isotropic. The effects of misalignment depend on the incident beam characteristics, the diffraction order and the propagation distance behind the grating. Experimental measurements support the results of calculations.     

全息光栅曝光系统中空间滤波器孔径与激光束腰关系的选择方法

在全息光栅制作过程中, 由空气悬浮颗粒及光学器件缺陷引起的散射光会被记录在光刻胶上, 既影响光栅衍射效率, 又会带来光栅杂散光. 为消除系统散射光, 需要对激光光束的高频成分进行滤波处理. 但是, 激光光束通过空间滤波器后发生衍射, 进而影响干涉场质量, 因此选择合适的空间滤波器孔径是搭建全息光栅曝光系统的前提条件. 根据标量衍射积分理论, 通过卷积计算滤波光束的振幅分布分析光束的相位变化, 以光束相位突变位置远离光束中心为依据确定滤波器针孔半径与光束束腰半径的比值下限; 根据滤波光束的能量守恒关系, 通过滤波光束的高斯参数变化描述光束的衍射强弱规律, 以光束发生衍射的临界条件确定滤波器针孔半径与光束束腰半径的比值上限. 结果表明, 当空间滤波器针孔半径与激光束腰半径之比满足1.52< a/ω₀<2.2时, 滤波光束曝光区域相位恒定, 光束能量通过率大于99%, 在此区间空间滤波器滤波效果最好.     

Multichannel high extinction ratio polarized beam splitters based on metasurfaces

Separating lights into different paths according to the polarization states while keeping their respective path's polarizations with high purification is keen for polarization multiplex in optical communications. Metallic nanowire gratings with multi-slits in a period are proposed to achieve polarized beam splitters(PBSs) in reflection and diffraction. The setting of multi-slits largely reduces the reflection of photons with a transverse magnetific field via the plasmonic waveguiding effect, which leads to highly polarized output lights with extinction ratio larger than 20 dB in each channel. The proposed reflection/diffraction PBSs enrich the approaches to control the polarization states with the advantages of wide incident angles and flexible beam splitting angles.     

强非局域介质中高斯型损耗空间光孤子

 利用变分法研究了1+1维傍轴高斯光束在含有小损耗的强非局域非线性介质中的传输特性，得到了光束各参量的演化方程、束宽的演化规律和一个临界功率。在介质损耗足够小的前提下，当光束初始功率等于临界功率时，得到了一个束宽随传输距离缓慢展宽的准空间光孤子——损耗空间光孤子；当光束初始功率小于临界功率时，光束束宽则按雅可比椭圆正弦函数和椭圆余弦函数作准周期展宽变化；当光束初始功率大于临界功率时，光束束宽将从按雅可比椭圆正弦函数和椭圆余弦函数作准周期压缩变化过渡到按雅可比椭圆正弦函数和椭圆余弦函数准周期展宽变化。     

孔径光阑输出对太赫兹激光器模式特性的影响

孔径光阑耦合技术是大功率光泵气体太赫兹激光器的一种常用谐振腔设计方法。在不考虑波导对腔内模式形成的影响下,采用传输矩阵的特征向量法,针对孔径光阑耦合太赫兹谐振腔的腔内本征模式以及输出光束特性进行了数值模拟,得到了该种谐振腔在不同g参数下损耗最低的几种本征模式及其相应的衍射损耗;采用衍射积分与矩阵光学结合的方法计算了孔径光阑输出太赫兹激光的光束特性,得到不同本征模式的远场衍射光斑及发散角。研究工作为孔径光阑耦合的气体太赫兹激光器设计提供了理论参考。     

Thin element approximation for the analysis of blazed gratings: simplified model and validity limits

The thin element approximation is widely used to predict the diffraction efficiency of thin periodic diffractive optical elements (DOEs). However, as the period-to-wavelength ratio is reduced, the approximation becomes inaccurate. A model based on a “shadow concept” can be used to predict the diffraction efficiency with high accuracy. Hereby we extend the model to include the effect of multi-level staircase structures and non-perpendicular incident angles. We also present an error map and define regions of validity for the thin element approximation (TEA) and the shadow model.     

Design and verification of diffractive optical elements for speckle generation of 3-D range sensors

The optical projection using speckles is one of the structured light methods that have been applied to three-dimensional (3-D) range sensors. This paper investigates the design and fabrication of diffractive optical elements (DOEs) for generating the light field with uniformly distributed speckles. Based on the principles of computer generated holograms, the iterative Fourier transform algorithm was adopted for the DOE design. It was used to calculate the phase map for diffracting the incident laser beam into a goal pattern with distributed speckles. Four patterns were designed in the study. Their phase maps were first examined by a spatial light modulator and then fabricated on glass substrates by microfabrication processes. Finally, the diffraction characteristics of the fabricated devices were verified. The experimental results show that the proposed methods are applicable to the DOE design of 3-D range sensors. Furthermore, any expected diffraction area and speckle density could be possibly achieved according to the relations presented in the paper.     

Measuring the topological charge of optical vortices with an axicon

We analyzed the spatial spectrum of the diffraction intensity pattern of an ideal Bessel beam and found an implicit rule that the number of the bright rings in the spatial spectrum is equal to the topological charge of the Bessel beam. The radius of the bright and dark rings has some relation with the topological charge and can be determined accurately. It provides us with a new way for measuring the topological charge of an optical vortex through its diffraction intensity pattern after an axicon. The results of simulation coincide with the theory.