遗传算法用于衍射光学元件的优化设计

提出了一种基于遗传算法的衍射光学元件优化设计方法;在衍射光学元件设计中遗传算法运行参数对遗传算法性能有一定的影响:采用较大的群体规模,遗传算法越容易获得最优解;交叉算子越大,遗传算法全局搜索能力越强;选择算子对遗传算法的影响不是太大;如果要进一步提高解的精度,可选取较大的终止代数。数值计算结果表明,用遗传算法优化设计的衍射光学元件,其误差小于 5.2%,衍射效率达到 91.2%。遗传算法很适合衍射光学元件的优化设计。     

Fabrication of diffractive optical elements inside polymers by femtosecond laser irradiation

Bragg-type gratings were prepared by irradiation inside a series of optical polymers with femtosecond laser pulses and the preparation conditions of the grating were examined. Repeated scanning irradiation with femtosecond laser pulses formed gratings due to refractive index changes inside polymers. Among the polymers examined in the present study, polymethylpentene (PMP) showed the highest diffraction efficiency, which was an order of magnitude higher than those of other optical polymers. The density of PMP was the lowest among the polymers evaluated in the present study, and the large volume contraction based on its low density was responsible for the larger refractive index change of PMP. Furthermore, we fabricated large-area diffractive optical elements (DOEs) in PMP measuring 15 × 25 mm² by widening the scanning area.     

衍射光学元件灰度掩模板激光制作系统研究

根据灰度掩模的制作理论,提出了由PP8000胶片输出仪、远心成像透镜组、平行准直He-Cd激光器构成的精缩投影曝光灰度掩模制作系统。通过灰度掩模平面不同位置处提供可变的透过率,经一次光刻后得到所需的衍射光学元件。该系统不仅可采用黑白胶片制作高分辨率灰度掩模板,还可根据彩色灰度等效理论,利用彩色等效胶片实现256灰度级的扩展细分,以进一步提高灰度掩模板制作的分辨率。对于16台阶灰度掩模,其分辨率可以从0~255扩展到0~1280灰度级。利用该系统给出了二元光栅精缩后的感光图片。     

衍射光学元件分析和设计中标量理论的局限性

以衍射光栅为例,用标量理论和严格耦合波理论的对比,分析衍射光学元件各参数对标量理论适用范围的影响。结果表明在光栅周期减小,刻蚀深度增加,光栅折射率增加,光束入射角度增加以及填充因子偏离 50%等情况下,标量理论的误差将逐渐增大,其中光栅周期和刻蚀深度对标量理论的影响较大,光栅周期小到 5 倍波长或者刻蚀深度大到 5 倍波长时,标量理论将不再适用。     

Asymptotic analysis and design of diffractive optical elements

The problem of light diffraction by a micro-optical diffractive element is investigated. The method of stationary phase is applied to obtain approximate values of the integrals in the Kirchhoff approximation. The accuracy of the asymptotic approximation is studied in detail. As an application, the obtained approximate formulas are used to solve a design problem of constructing a diffractive optical element with a desired intensity distribution.     

折/衍混合透视型头盔显示器光学系统设计

为满足头盔显示器(HMD)对大视场、高像质和彩色显示的要求,提出了改进型 HMD 光学系统。该系统以彩色 LCD 为图像源实现彩色显示;以折/衍混合单透镜代替双胶合,利用其负色散和波面任意整形特性消除系统色差及改善波前像差。图像源发出的光束在分束器附近成中间像,可增大系统视场;出瞳在中继光学组元中间成像,可减小系统口径,增大出瞳直径;球面耦合器将分束器反射过来的光束准直后投射到人眼,能消除人眼反复观察不同焦距的视频图像和外界图像所造成的疲劳。该系统垂轴色差小于 42μm,视场扩大为 40°(水平)×30°(垂直),分辨力接近人眼的最小分辨能力;口径小于 46mm,重量比现有 HMD 光学系统下降 47%。     

Implementation of pseudo-nondiffracting beams by use of diffractive phase elements

We report the experimental implementation of pseudo-nondiffracting beams by use of diffractive phase elements (DPE's). Based on the conjugate-gradient method presented in J. Opt. Soc. Am. A 15, 144-151 (1998), these DPE's are designed and fabricated on a flat quartz substrate. The experimental results show that the performance of the fabricated DPE's is in good agreement with the theoretical prediction.     

Generating inhomogeneously polarized higher-order laser beams by use of diffractive optical elements

We propose an improved version of the earlier developed optical arrangement for generating inhomogeneously polarized laser light modes with the aid of a diffractive optical element (DOE) with carrier frequency. By eliminating lenses from the optical arrangement, we achieve the miniaturization, reduced light losses, a smaller number of parameters being matched, and a simpler system adjustment procedure. Note that all the capabilities of the previous version, namely, the universality and simple readjustment to different polarization types, are fully retained. The numerical modeling of the polarization mode combiner has made it possible to analyze its performance and capabilities. In the experiments, the quality of the resulting beams is shown to be improved. For generating higher-order cylindrical beams, a lower-order mode at the output of the polarization mode combiner is additionally transformed with a DOE that operates in the zero diffraction order, introducing radial phase changes.     

Phase-only diffractive optical elements with subdiffraction-limited depth of focus

Abstract

Phase-only subdiffraction-limited depth of focus (DOF) elements are proposed and a relatively simple method is given which reduces the number of design parameters to four. A diffractive optical element with subdiffractive-limited DOF was experimentally demonstrated for the first time. Experimental results are in close agreement with numerical prediction. Several calculated examples show that the DOF for subdiffraction-limited elements can decrease by an amount, ranging from 37% to 71% compared to the DOF for a diffraction-limited element when ρ is chosen close to zero. The reduction is determined by the focal length of the element and the required distance between the primary peak (focal plane) and the secondary peak, both in units of wavelength. The latter, i.e. the peak-to-peak distance, determines the maximum applicable DOF. The influence of input laser beam parameters, such as phase distribution and beam size, on the focusing performance have also been investigated.     

Implementation of high-resolution diffractive optical elements on coupled phase and amplitude spatial light modulators

We propose the optical implementation of diffractive optical elements onto electrically addressed liquid-crystal spatial light modulators. We compare the classic implementations onto amplitude-only or phase-only domains with the implementations onto coupled phase and amplitude (spiral) domains. We demonstrate that the coupling between amplitude and phase provides a trade-off between diffraction efficiency and the signal-to-noise ratio in the reconstruction. Furthermore, when investigating the influence of the maximum dephasing on phase domains and spiral domains through the use of optimal trade-off design, we show that phase-only domains with limited maximum dephasing can provide satisfactory performance. Finally, optical implementations are provided.     

Design and fabrication of diffractive optical elements by use of gray-scale photolithography

Diffractive optical elements (DOEs) are key components in the miniaturization of optical systems because of their planarity and extreme thinness. We demonstrate the fabrication of DOEs by use of gray-scale photolithography with a high-energy-beam sensitive glass photomask. We obtained DOE lenses with continuous phase profiles as small as 800 microm in diameter and 5.9 microm in the outermost grating pitch by selecting a suitable optical density for each height level and optimizing the process variables. Microlenses patterned with eight levels and replicated by UV embossing with the polymer master mold showed a diffraction efficiency of 81.5%, which was sufficiently high for the devices to be used as optical pickups. The effects of deviations in diffraction efficiency between the DOE height and profile design were analyzed.     

Advantages of the mixed format in geometrically nonlinear analysis of beams and shells using solid finite elements

The paper deals with two main advantages in the analysis of slender elastic structures both achieved through the mixed (stress and displacement) format with respect to the more commonly used displacement one: (i) the smaller error in the extrapolations usually employed in the solution strategies of nonlinear problems and (ii) the lower polynomial dependence of the problem equations on the finite element degrees of freedom when solid finite elements are used. The smaller extrapolation error produces a lower number of iterations and larger step length in path‐following analysis and a greater accuracy in Koiter asymptotic method. To focus on the origin of the phenomenon, the two formats are derived for the same finite element interpolation. The reduced polynomial dependence improves the Koiter asymptotic strategy in terms of both computational efficiency, accuracy and simplicity. Copyright © 2016 John Wiley & Sons, Ltd.     

Completely passive method of speckle reduction utilizing static multimode optical fibre and two-dimensional diffractive optical element

ABSTRACT

In this paper, we present experimental results on speckle noise suppression using a completely passive method. The passivity of the method is achieved owing to the absence of any mechanical, electronic, or other dynamic influences on the optical scheme elements. In the experiment, a multimode semiconductor 520?±?5-nm laser with a spectral bandwidth of 2?nm, static two-dimensional (2D) and 2?×?1D diffractive optical elements (DOEs), as well as multimode single-core optical fibre and multimode optical fibre bundle were used. The dependence of the speckle reduction efficiency as a function of the optical fibre type and optical fibre length was measured for different DOEs. A speckle contrast of 0.148 and speckle reduction coefficient of 2.38 were obtained for a 2.5-m-long multimode optical fibre bundle. The experimental results confirmed that it is possible to construct completely passive optical circuits with reduced speckle noises using static multimode optical fibres and diffraction optical elements.     

Generation of four-atom entangled decoherence-free states by interference of polarized photons

We propose a protocol to generate four-atom entangled decoherence-free states trapped in distant cavities by using the interference of polarized photons. The protocol uses the effects of quantum statistics of indistinguishable photons emitted by the atoms inside optical cavities. In the Lamb–Dicke limit, the protocol does not require the simultaneous click of detectors. This makes the protocol more realizable in experiments.     

Theoretical study on the spectral shifts of partially coherent array beams passing through ABCD optical systems

According to the extended Huygens–Fresnel integral, the expressions for the on-axis spectrum of partially coherent Gaussian Schell-model (GSM) rectangular array beams passing through ABCD optical systems have been derived. The generalized Fresnel number of the system, the spatial coherent parameter of array beamlets and the array beam parameters including the number of beamlets and the separation distance between beamlets have been taken as the characteristic parameters to compare the spectral shifts of GSM array beams for the two types of the superposition, i.e. the correlated superposition and the uncorrelated superposition. In particular, the effect of characteristic parameters on the on-axis relative spectral shifts has been discussed in detail. The results show that the spectral intensity of GSM array beams for the two types of the superposition passing through ABCD systems depends on the source spectral density S ⁰(ω), the spatial coherent parameter of array beamlets β, the generalized Fresnel number of the system F and the array beam parameters. Furthermore, for the uncorrelated superposition, the spectrum of GSM array beams only exhibit the blue-shift, whereas for the correlated superposition, GSM array beams exhibit the spectral switch and the number of spectral switches increases with the increase of array beam parameters in the near field due to the interference between beamlets. In particular, the effect of the array beam parameters on the on-axis relative spectral shift is more obvious.     

不同籽晶DKDP晶体生长和光学性能研究

籽晶是影响DKDP晶体生长和光学性能的一个重要因素.通过传统降温法,分别利用Z片和[101]晶片作为籽晶,从氘化程度为85%的溶液中生长DKDP晶体并对加工样品进行了相关测试.研究了不同籽晶对DKDP晶体的生长和光学性能的影响.实验表明,[101]晶片籽晶所得DKDP晶体能有效缩短生长周期,晶体损伤阈值提高明显,但光学均匀性和透过性能有所下降.     

A conforming unified finite element formulation for the vibration of thick beams and frames

Beams and frames are common features in many engineering structures and in this paper an approach is given to model their dynamic behaviour adequately. Whilst the eigen‐frequencies of continuous systems comprising of slender beams can be identified, in most cases of practical interest, by means of Euler or Timoshenko beam theory, for structures comprising of thick beam models this is not necessarily true since such idealizations constrain the cross‐sections to remain planar. This paper suggests an alternative approach by means of a unified fully conforming plane stress rectangular finite element which is believed to allow for more realistic representation of the shear effects and hence the strain field around the joints of such structures. The usefulness and functionality of this improved numerical approach is explored via comparison against a non‐conforming two‐dimensional plate as well as one‐dimensional Euler–Bernoulli and Timoshenko finite element formulations corresponding to a variety of beam aspect ratios representing the structures of a rotor and a portal frame. The idealization is shown to be particularly advantageous for simulating the effects of shear distortion where beams join at right angles and the transverse forces in one member interact with the extensional forces of the adjoining structure. Copyright © 2004 John Wiley & Sons, Ltd.     

蓝紫光倍频晶体ZCTC晶体生长与透过性质研究

硫氰酸锌镉（ZnCd(SCN）4），简称ZCTC，是自行研发的一种蓝紫光倍频晶体。通过恒温蒸发溶液来生长ZCTC晶体，测量了KSCN和ZCTC溶液的透过率曲线，发现SCN^-基因是影响ZCTC晶体紫外截止波长的主要因素。研究还发现ZCTC晶体（100）面的透过度更高于（001）面，通过化学镀膜可以减少晶体表面反射损耗。ZCTC晶体经180℃，24h烘烤后，在300-800nm之间透过率明显下降。     

Temperature and voltage sensings by mode‐mode interference in polarization‐maintaining optical fibers

Abstract

Two optic fiber sensing systems for temperature and voltage have been developed which utilize the mode‐mode interference of the two orthogonally polarized modes, HE^x _II and HE^y _II, in two commencal polarization‐maintaining fibers (bow‐tie and elliptical core fibers). A package of controlled programs in a Macintosh computer, which can record and process all related data automatically, is established for temperature sensing. The signal drifting problem in voltage sensings has been investigated, and the elimination of signal drifting is obtained by the phase tracking with direct current technology The agreement between the sensing results for temperature and dynamic voltage and those predicted by experimental principles is satisfactory, which confirms the validity of the developed sensing systems.