A Blazed Holographic Concave Grating for Monochromators

Abstract

The theory of blazed holographic gratings given in a previous paper is applied to the design of a blazed holographic aberration-corrected spherical concave grating. Two solution types are found which cover different ranges of blaze wavelength and aberration. The use of the refractive indices of the substrate, and the medium in front of the photoresist, as optimising parameters is demonstrated. Four ray-traced solution examples are given which include the distributions of blaze angle and wavelength across the grating surface.     

光刻胶灰化用于全息离子束刻蚀光栅制作

针对全息离子束刻蚀衍射光栅制作中,光刻胶光栅浮雕图形的制作是至关重要和困难的,引入O2反应离子刻蚀对光刻胶光栅进行灰化处理,给出光刻胶灰化技术在全息离子束刻蚀衍射光栅制作闪耀光栅、浅槽矩形位相光栅、自支撑透射光栅中的具体应用。实验结果表明,这一新工艺的突出优点是降低了苛刻的全息曝光、显影要求,使得光栅线条光滑、线空比和槽深可控。     

Apodizing holographic gratings for dual-wavelength operation of broad-area semiconductor lasers

Apodizing holographic gratings are designed to have a Gaussian reflectivity profile in the -1 order and a complementary reflectivity profile in the specular (0) order. They are obtained by the interference of two Gaussian beams on a glass plate covered with a photoresist. These gratings are intended to be used as the coupler of the external cavity of a broad-area semiconductor laser. When the grating is oriented to get the -1 order counterpropagating with respect to the incident beam, single-longitudinal- and single-lateral-mode operation is obtained. We report on the fabrication technique of an apodizing holographic grating designed to allow lasing on two wavelengths simultaneously, with a preselected wavelength separation. The results obtained with a commercial broad-area laser are presented.     

Scanning methods for production of blazed relief gratings

Abstract

New methods for the production of blazed relief grating profiles using grating masks with either binary or harmonic transmittance have been developed. These methods are based on the recording of a scanning wave, properly modulated by a mask, through a grating period into photoresist. The applicability of the gratings produced by this method would be especially in the infrared or near-infrared wavelength regions in the reflection mode. These methods have been theoretically analysed and the principal ideas of the methods were experimentally verified.     

Convex grating types for concentric imaging spectrometers

The properties of convex gratings fabricated by electron-beam lithography are investigated. Three grating types are shown. The first is a single-panel, true blazed grating in which the blaze angle stays constant relative to the local surface normal. This grating provides high peak efficiencies of approximately 88% in the first order and 85% in the second order. The second grating has two concentric panels, with each panel blazed at a different angle. This type permits flexibility in matching the grating response to a desired form. The third type has a groove shape that departs from the sawtooth blazed profile to increase the second-order bandwidth. All these types are difficult or impossible to produce with conventional techniques. The gratings compare favorably with conventional (holographic and ruled) types in terms of efficiency and scatter. Simple scalar models are shown to predict the wavelength response accurately. These gratings allow the optical designer to realize fully the considerable advantages of concentric spectrometer forms.     

改善MEMS闪耀光栅衍射效率的研究

研究了在硅材料上利用MEMS (Micro-Electro-Mechanical System)的各向异性腐蚀技术制备 闪耀光栅。采用氧化削尖工艺去除光栅制备过程中掩膜在闪耀面上留下的平台,得到一个连续的闪耀面;同时对闪耀面进行表面抛光,改善闪耀面的粗糙度,减小对入射光的散射。理论分析和实验测试证明,该工艺方法能够将MEMS闪耀光栅的衍射效率提高10%左右。     

Fabrication of three-dimensional photonic crystals by interference lithography with low light absorption

We report on the fabrication of polymer templates of photonic crystals by means of holographic (or interference) lithography. The grating is written in a SU-8 photoresist using a He-Cd laser of wavelength 442 nm. The use of the wavelength found within the photoresist low absorption band enables fabricating structures that are uniform in depth. Parameters of the photoresist exposure and development for obtaining a porous structure corresponding to an orthorhombic lattice are determined.     

基于双频光栅的微分干涉相衬方法

针对传统微分干涉相衬技术对双折射性物体成像会有伪像的问题,用双频闪耀光栅取代传统微分干涉相衬的分光元件诺曼斯基棱镜,设计了一种微分干涉相衬方法.对双频闪耀光栅设计,模拟入射平面波通过它之后的光强分布,结果表明它能产生两束剪切角很小的光.用双频光栅微分干涉相衬方法对纯相位样品模拟成像,仿真结果和物体的相位轮廓一致.分析了...     

Recording of concave diffraction gratings in counterpropagating beams using meniscus blanks

The most technological way of recording blazed concave holographic gratings is by using the direct- and back-reflected beams. Usually plano-concave blanks are used for diffraction grating fabrication. To compensate the refraction at the plane back surface of the blank, one has to use additional elements or two-step recording mountings. Another solution is to use diffraction grating blanks having zero optical power. A number of gratings with different groove frequencies have been fabricated using concave—convex blanks. The theoretical investigation of the aberration properties of the recording mountings, and the experimental study of the grating properties are discussed. Using the CODE V program it is shown that a significant reduction in the aberration size of the virtual recording source can be achieved by optimization of the radius of curvature of the blank back surface. The experimental results confirm the possibility of achieving gratings that can be used in flat-field spectrometers with a limit of resolution of about 2–3 nm even in the case where an aberrated virtual recording source is used.     

Analysis and optimization of fabrication of continuous-relief diffractive optical elements

The fabrication of continuous-relief diffractive optical elements by direct laser beam writing in photoresist is analyzed. The main limitation and tolerances are identified, and their influence on optical performance is quantified. Fabricated structures show rounded profile steps resulting from the convolution of the desired profile with the writing beam. This leads to a reduction in diffraction efficiency. Optimization techniques are presented to minimize this effect. Scaling the profile depth by a factor of mu > 1 increases the first-order diffraction efficiency for blazed elements. This method is also applied to suppress the zeroth diffraction order in computer-generated holograms. A nonlinear compensation of the exposure data for the Gaussian beam convolution results in an 18% increase of the diffraction efficiency for a blazed grating with a 10-mum period to a value of 79%.     

Interferometric fabrication of modulated submicrometer gratings in photoresist

Interferometric recording is applied to the fabrication of modulated submicrometer gratings in photoresist.High diffraction efficiency requires optimized recording conditions, which are obtained by the use of an on-axis continuous surface-relief grating for the generation of the object beam. The optimized phase function is copied into the resist layer by means of a self-aligned two-step recording process with an intermediate copy in a volume photopolymer hologram. As a result, we demonstrate high carrier frequency surface-relief off-axis fan-out gratings for illumination in transmission with visible light.     

Gray-scale masks for diffractive-optics fabrication: I. Commercial slide imagers

Fabrication of diffractive optics with binary masks requires multiple photolithographic processes to produce high-efficiency elements. Alignment or etching errors at any stage of fabrication decrease the efficiency of the element. We developed an easily accessible procedure that reduces fabrication complexity and costs by using a single gray-scale mask. The gray-scale patterns are generated by commercial slide imagers and are then photoreduced onto low-contrast film plates. Multiple-level or continuous relief structures (kinoforms) may be constructed by use of the photoreduced gray-scale patterns as lithographic masks. Diffractive-optic lenses and blazed gratings were fabricated in photoresist with this procedure. First-order diffraction efficiencies as high as 85% were measured for the blazed gratings. The advantages and the limitations of this technique are discussed.     

Apodizing holographic gratings for the modal control of semiconductor lasers

We first present the fabrication technique of apodizing holographic gratings. Gratings with a spatially variable reflectivity profile were obtained by the interference of two Gaussian beams on a glass plate covered with a photoresist. When the exposure time was short enough to avoid saturation of the photoresist, gratings with a quasi-Gaussian reflectivity profile for the beam reflected in the -1 order were produced; the reflectivity at the center could be as high as 71%, and the half-width of the reflectivity profile at the e(-1) position could be as small as 180 mum. Apodizing gratings were used as the end mirror of the external cavity of a broad-area semiconductor laser. Single longitudinal- and lateral-mode operation was observed over the full range of allowed injection currents.     

On-axis beam extinction through diffraction design and analysis

To optimize the functions of the surface-relief diffractive optical elements for optical limiters and other applications, we present a systematic design and analysis with numerical illustrations of the transmission properties of different surface-relief phase gratings and Fabry-Perot elements. The spectral response and the tolerance of fabrication errors have been included. An analysis shows that the blazed grating and the echelon grating, a multilevel element as a substitute of the blazed grating, can make the on-axis transmittance very low (less than 1% with one grating) over the broad visible band with a large tolerance of fabrication errors. The results are highly significant for the design of optical-limiting devices.     

Diffraction efficiency of 200-nm-period critical-angle transmission gratings in the soft x-ray and extreme ultraviolet wavelength bands

We report on measurements of the diffraction efficiency of 200-nm-period freestanding blazed transmission gratings for wavelengths in the 0.96 to 19.4 nm range. These critical-angle transmission (CAT) gratings achieve highly efficient blazing over a broad band via total external reflection off the sidewalls of smooth, tens of nanometer thin ultrahigh aspect-ratio silicon grating bars and thus combine the advantages of blazed x-ray reflection gratings with those of more conventional x-ray transmission gratings. Prototype gratings with maximum depths of 3.2 and 6 μm were investigated at two different blaze angles. In these initial CAT gratings the grating bars are monolithically connected to a cross support mesh that only leaves less than half of the grating area unobstructed. Because of our initial fabrication approach, the support mesh bars feature a strongly trapezoidal cross section that leads to varying CAT grating depths and partial absorption of diffracted orders. While theory predicts broadband absolute diffraction efficiencies as high as 60% for ideal CAT gratings without a support mesh, experimental results show efficiencies in the range of ～50-100% of theoretical predictions when taking the effects of the support mesh into account. Future minimization of the support mesh therefore promises broadband CAT grating absolute diffraction efficiencies of 50% or higher.     

Dynamic holographic gratings with photoresist

The theoretical development of dynamic holographic gratings with piezoelectric crystals is presented. A thin layer of photoresist is applied over the surface of the piezoelectric crystal. A holographic grating was recorded over the photoresist-piezo device in such a way that when it is activated the changes in its dimensions produce a geometrical change in the grooves of the grating. Hence the initial diffraction pattern created may be dynamically modified.     

Technique for detecting flaws in metallic surfaces using an optical system with phase-type blazed gratings

In this paper, we evaluate a novel flaw-detection technique for metallic surfaces based on the use of phase-type blazed gratings. Transparent blazed gratings were prepared by the soft nanoimprint method involving the transfer of a template (a reflective grating structure used for spectroscopy) onto silicone rubber. The blazed gratings were then integrated into an imaging system to observe the reflective metal sample. Due to the low-pass-filtering properties of the gratings, the captured image was notably blurred. This characteristic aids in flaw detection on metallic surfaces because the captured image is adequate to distinguish flaws in the targeted area on the basis of the texture of the rough surface, including any other structures that were unintended. The use of double-sided gratings with crossing grating vectors was found to be efficient for homogenous low-pass filtering. Such flaw-detection techniques are expected to be useful for conducting quality inspections of rolled steel plates since the surface contains both a rough surface and undesirable flaws.     

Near-normal-incidence extreme-ultraviolet efficiency of a flat crystalline anisotropically etched blazed grating

We have measured the extreme-ultraviolet (EUV) efficiency at an angle of incidence of 10 degrees of a flat crystalline anisotropically etched blazed grating. The measured efficiencies are high for uncoated gratings and agree well with a calculated model derived from a reasonable estimate of the groove profile. The highest groove efficiencies derived from the measurements are 48.8% at 19.07 nm and 64.1% at 16.53 nm for the -2 and -3 orders, respectively, which are comparable to the best values obtained yet from a holographic ion-etched blazed grating. This presents opportunities to instrument designs for high-resolution EUV spectroscopy in astrophysics where high efficiency in high orders is desirable.     

溶胶凝胶法制备闪耀光栅及特性研究

采用一步催化法，制备ZrO2光敏溶胶，用提拉法形成了单层光敏凝胶膜。采用紫外光刻技术，将掩摸图形转移到光敏凝胶膜上，通过曝光角度的控制，实现具有一定倾角的闪耀光栅制备，以紫外分光光度计，红外分光光度计，原子力显微镜等方法对光敏凝胶膜及光栅图形进行了性能测试及表征。并用355nm激光系统进行衍射效率及激光损伤阈值测定。获得了周期为2μm，高度为60nm，闪耀角为3°的闪耀光栅。     

Metallized photoresist grating as a polarizing beam splitter

We propose and demonstrate the use of a simple holographic relief photoresist grating covered with an aluminum film as a reflecting polarizing beam splitter. The polarizing effects were achieved as a result of the nonsinusoidal profile of the grating. The best parameters of the gratings for optimizing the polarizing-beam-splitting properties were found by the introduction of the experimental profiles in diffraction calculation software. Theoretical and experimental results are presented, confirming the feasibility of the element.