Modulation Transfer Function of Blazed Diffractive Optics Produced by Diamond Turning

In blazed diffractive optics produced by the diamond turning process, the finite size of the tool determines the decrease in diffraction efficiency of the desired order. Diffracted light of other orders degrades the image quality. To analyze the influence of the tool, we evaluate diffraction efficiency and phase of periodic gratings based on the electromagnetic theory and calculate modulation transfer function using a linear system approach.     

Electromagnetic effects for the resonance-domain IR diffractive optics

Binary optics technology enables the manufacture of arrays of diffractive micro-optical elements which are used in many optoelectronic devices, e.g. the focal plane collection optics. The first-order diffraction grating efficiency decreases in the best part of the resonance-domain region of diffraction, i.e. when the grating period is close to the optical wavelength in the substrate material. A large fraction of the fast binary lens surface relief is built of staircase annular structures whose width is of wavelength scale. Therefore, the rigorous electromagnetic theory of gratings has been applied in this paper to calculate and analyze the diffraction effects for the resonance-domain longwave (8–12 μm) infrared (LWIR) binary optics. It is shown that electromagnetic effects limit the speed of the LWIR first-order diffractive lenses and the optical gain achievable with the diffractive lenses used as the focal plane collection optics in the IR detection systems.     

衍射光学元件热稳定性的分析

本文对折射元件和衍射元件的温度特性进行了分析,建立了透镜焦距和衍射效率随环境温度的变化关系,通过施密特红外望远镜系统的设计,比较了折射非球面与衍射光学校正板的温度特性对系统光学传递函数的影响,结果表明衍射光学元件具有较好的热稳定性.     

线宽误差对菲涅耳透镜衍射效率的影响

基于标量衍射理论,首先得出存在线宽误差情况下4阶菲涅耳透镜衍射效率的解析式;并以 4阶、8阶和16阶菲涅耳透镜为例,进行计算机模拟计算并系统分析线宽误差对菲涅耳透镜衍射效率的影响。通过实验测量16阶菲涅耳透镜的线宽误差,分析其对16阶菲涅耳透镜衍射效率的影响。模拟计算结果对多阶菲涅耳透镜的制作提供重要的理论指导。     

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.     

用三角形闪耀光栅实现分束器

用闪耀光栅实现了光栅分束器。给出了该分束器衍射光强分布的解析表达,讨论了影响分束性能的各种因素,并把复杂的优化设计问题转变为简单的求条件极值问题,使编程简单,计算速度快。数值模拟表明,当分别扇出2束和4束光时,衍射效率可分别达到92.50%和95.75%,扇出光束能量均匀,各级衍射效率的方差分别在10-5和10-6的数量级。与其它分束器进行了比较。     

Formation of positive and negative anisotropic holographic gratings depending on recording energy in photoreactive liquid crystalline copolymer films

Anisotropic diffractive gratings formed by birefringent molecular alignment are fabricated by a holographic technique using a photo-cross-linkable liquid crystalline copolymer (PLCCP). Depending on the exposure energy used for holographic recording, bright or dark regions of the interference fringe can be chosen for the molecular-aligned region, analogous to positive- and negative-type photoresist processing in photolithography. In conjunction with simultaneously generated surface relief gratings, the anisotropic refractive index modulation inherent in the PLCCP film exhibits different diffraction properties depending on the incident beam polarization. The anisotropic refractive index grating profiles are quantitatively presented for two cases determined by the exposure energy, based on the Fourier analysis of diffractive optics. The selection of different grating formations selected by varying only the exposure energy could provide a strategy for the fabrication of functionalized diffractive optical elements.     

Optimized staircase profiles for diffractive optical devices made from absorbing materials

We report on the optimization of staircase grating profiles for the case of absorbing grating materials. Using a simple numerical algorithm, we determined the grating parameters, maximizing the first-order diffraction efficiency for different numbers of staircase steps. The results show that there is a significant difference between the staircase profiles for nonnegligible and negligible absorption. The obtained solutions are of importance for diffractive optics in the soft-x-ray and extreme-ultraviolet ranges.     

Compound diffractive telescope system: design, straylight analysis, and optical test

The compound diffractive telescope is a novel space optical system which combines the structure of compound eyes with diffractive optics and so it has a lighter weight, a wider field of view (FOV), a lower cost as well as looser fabrication tolerance. In this paper, the design of a compound diffractive telescope composed of one primary lens and twenty-one eyepieces is introduced. Then the influence of diffraction orders on the performance of the system is analysed. A modified phase function model of diffractive optics is proposed to analyse the modulation transfer function (MTF) curves for 0℃ FOV, which provides a more accurate prediction of the performance of the system. In addition, an optimized mechanism is also proposed to suppress stray light. The star image and resolution tests show that the system can achieve diffraction limit imaging within ± 2℃ of FOV and ±4~mm of eccentricity. Finally, a series of pictures of an object are taken from different channels, and the splicing of pictures from adjacent FOVs is demonstrated. In summary, the designed system has been proved to have great potential applications.     

折衍二元光学用金刚石车刀设计

栏光效应是影响折衍元件衍射效率的一个重要因素,栏光效应与加工刀具有着密切的关系。分析刀具圆弧半径与栏光效应的联系、刀具参数与元件加工后的表面粗糙度的关系以及刀具耐用度等要求,设计了加工折衍二元光学元件的金刚石刀具,给出刀具加工里程数计算公式和车削实验结果。实验结果证明:设计的金刚石刀具能够满足折衍二元光学元件的产业化加工要求,刀具耐用度较强化前提高2倍多。     

红外双波段双层谐衍射光学系统设计

将谐衍射透镜应用在传统红外单波段佩茨瓦尔(Petzval)物镜上,设计得到工作波段处于3.4~4.2μm和8~11μm的红外双波段单层谐衍射光学系统。但单层谐衍射元件的衍射效率只在设计波长处衍射效率最高,随着波长相对设计中心波长向两侧偏离,主衍射级次的衍射效率逐渐下降。为提高含单层谐衍射元件光学系统的衍射效率,基于双层衍射元件衍射效率表达式研究了双层谐衍射元件的结构优化,给出了优化方法。设计出佩茨瓦尔型红外双波段双层谐衍射光学系统,其在3.4~4.2μm和8~11μm两个工作波段的衍射效率均达到90%以上,相比含有单层谐衍射面的光学系统衍射效率有了很大提升,提高了像面衬比度,完善了系统成像质量。     

二元光学在激光测距中的应用

利用二元光学器件的相位补偿作用进行波面变换,将高斯光束变换为平面光束,达到减小激光测距仪光束发散角的目的。在２０ｋｍ 处对激光光束发散角采用叠加积分衍射分析进行计算,得到结果为２ ×１０ － ４ｍｒａｄ , 远小于原来的２ｍｒａｄ 。计算还表明测程没有因能量损失而降低。     

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.     

二元光学在光学观测轰炸瞄准中的应用研究

光学观测瞄准是大中型轰炸机主要的对地攻击瞄准方式,其光学系统一般采用望远式光学系统,这就要求光学系统能满足可见光和近红外光两个光谱波段。根据国内大中型轰炸机光学观测瞄准系统的主要缺陷,结合光学观测瞄准的实际,探讨了基于二元光学的光学观测系统的组成结构和瞄准原理,分析了可见光波段混合目镜的光学性能、谐衍射物镜的衍射效率以及近红外光波段光学系统的像质,推导了横偏棱镜调平误差引起的瞄准误差的数学表达式,并对设计的合理性进行了验证。     

高衍射效率衍射望远镜系统的设计/加工及成像性能测试

为提高衍射效率,设计并制作了口径为300mm的衍射成像系统.该系统的物镜是由一块四台阶位相型菲涅尔波带片通过激光直写套刻和Ar离子束物理刻蚀技术在石英玻璃基板上加工而成.测试了衍射物镜的衍射效率,实验结果表明:衍射物镜在波长632.8nm处的衍射效率为66.4%,达到理论值的82%.搭建了衍射成像系统光路,分别采用10μm星点孔与分辨率板,测试了系统的成像性能.实验测得星点像直径为44μm,分辨率板的极限分辨率达到84lp/mm,接近该系统的理论计算值,表明该衍射成像系统具有较好的成像性能.     

Effects of fabrication errors on diffraction efficiency for a diffractive membrane

The demand for space-borne telescopes with an aperture of 20 m is forcing the development of large diameter diffractive Fresnel zone lenses(FZLs) on membranes. However, due to the fabrication errors of multi-level microstructures, the real diffraction efficiency is always significantly smaller than the theoretical value. In this Letter,the effects of a set of fabrication errors on the diffraction efficiency for a diffractive membrane are studied. In order to verify the proposed models, a 4-level membrane FZL with a diameter of 320 mm is fabricated. The fabrication errors of the membrane FZL are measured, and its diffraction efficiency in the +1 order is also tested.The results show that the tested diffraction efficiency is very close to the calculated value based on the proposed models. It is expected that the present work could play a theoretical guiding role in the future development of space-borne diffractive telescopes.     

Invertible dark-center diffraction of the metal film induced by femtosecond laser

We found reversible dark-center diffraction of the transmitted probe beam passing through the chromium film, which is induced by the pump femtosecond laser. The dark-center diffraction of the transmitted probe beam appears and disappears with and without the pump beam. A view of diffractive optics with binary phase plate is put forward, which explains the reversible dark-center diffractive optical phenomenon. The pre-ablated hole on the metal film can be regarded as a uniform light filed without phase modulation, the surrounding circular part around the pre-ablated hole can be regarded as “phase modulated”. Therefore, this diffraction optic view might be helpful for us to understand the phase change of the metal film introduced by the femtosecond laser pulse.     

衍射光学元件精密模压模具设计及预补偿

衍射光学元件较球面、非球面光学元件在校正色差方面具备较大优势,尤其是在红外光学领域,应用衍射光学元件可进一步增加光学系统的设计自由度。随着红外光学市场的进一步增大,常规的衍射光学金刚石车削技术难以满足大规模需求,精密模压技术成为解决上述问题的关键技术。模具设计是实现精密模压的重点,为了缩减模具设计周期,该文采用有限元仿真方法对模具进行预先设计及补偿,并试加工。采用单站式精密模压机对设计的模具进行了精密模压试验。模压试验结果表明：采用合理的工艺参数,能够实现衍射光学元件面形精度PV达到0.56μm,位置误差<0.011 mm,环带高度误差<0.12μm,验证了仿真预先补偿在衍射光学模具设计中的有效性。     

小F数衍射微透镜阵列光学性能研究

本文基于小光点扫描,在国内首次提出了一种测试小F数衍射微透镜阵列光学性能的方法.利用光通信用半导体激光器和探测器建立了一套测试系统,并对所研制的大阵列,小单元尺寸的多相位菲涅耳衍射微透镜阵列的衍射效率和点扩散函数进行了测试.结果表明,衍射微透镜实际衍射光斑比理论衍射受限光斑扩展不大,8相位和16相位石英微透镜的衍射效率分别高达80.2%和87.5%,说明本实验室对多相位的设计理论和制作工艺已基本掌握,满足应用的要求.     

温度变化对谐衍射透镜性能的影响

分析了温度变化对谐衍射透镜的焦距、谐振波长、衍射效率等性能的影响,建立了相应的数学表达式,并对其进行了数值模拟.提出了漂移函数的概念,以表征焦距和谐振波长随温度的变化程度.另外,还给出了各谐振级次的衍射效率随温度变化的相关规律.最后比较了在相位匹配数不同、但设计波长相同时,温度变化对衍射效率的影响,发现相位匹配数越大,衍射效率对温度变化越敏感.