Showing papers on "Diffraction efficiency published in 1997"

Diffraction gratings and applications

Abstract: A brief history of spectral analysis fundamental properties of gratings the types of diffraction gratings efficiency behaviour of plane reflection gratings transmission gratings icily gratings concave gratings surfacewaves and grating anomalies waveguide, fibber, an acousto-optic gratings review of electromagnetic theories of grating efficiencies testing of grating performance instrumental systems grating damage and control mechanical ruling of gratings holographic recording of gratings alternative methods of grating manufacture replication of gratings.

Diffractive Optics for Industrial and Commercial Applications

Abstract: Diffraction gratings for spectroscopy diffractive and hybrid lenses bifocal intraocular lenses laser material processing diffractive resonator optics optical testing diffractive laser diode optics diffractive optics for integrated optical sensing information processing and diffractive optics photorefractive crystals for optical metrology and optical data processing zeroth order devices security applications diffractive optics and solar cells holographic microlithography.

Improved formulation of the coupled-wave method for two-dimensional gratings

Abstract: A new formulation of the coupled-wave method for two-dimensional gratings is proposed. It is based on mathematical and physical results recently obtained for one-dimensional gratings. Numerical evidence obtained for many different diffraction problems, including dielectric, metallic, volume, and surface-relief gratings, shows that the new formulation outperforms the conventional one in terms of convergence rates. The specific case of gratings with very small thickness, for which opposite conclusions on the convergence performance are obtained, is studied and explained. The methodology can be applied to other numerical techniques that rely on Fourier expansions of the electromagnetic fields and on grating parameters such as the permittivity and the permeability.

Boundary integral methods applied to the analysis of diffractive optical elements

Abstract: We apply boundary integrals to the analysis of diffraction from both conductive and dielectric diffractive optical elements. Boundary integral analysis uses the integral form of the wave equation to describe the induced surface distributions over the boundary of a diffractive element. The surface distributions are used to determine the diffracted fields anywhere in space. In contrast to other vector analysis techniques, boundary integral methods are not restricted to the analysis of infinitely periodic structures but extend to finite aperiodic structures as well. We apply the boundary element method to solve the boundary integral equations and validate its implementation by comparing with analytical solutions our results for the diffractive analysis of a circular conducting cylinder and a dielectric cylinder. We also present the diffractive analysis of a conducting plate, a conducting linear grating, an eight-level off-axis conducting lens, an eight-level on-axis dielectric lens, and a binary dielectric lens that has subwavelength features.

Holographic recording characteristics of an acrylamide-based photopolymer.

Abstract: Recent research on an acrylamide-based photopolymerizable holographic recording material is presented. The recording characteristics of the material are discussed in detail in terms of sensitivity, diffraction efficiency, recording linearity, resolution limit, and sources of noise. Although the resolution is not sufficient for reflection holography, the recording characteristics are excellent for transmission gratings. The material was found to suffer no shrinkage during recording, and high-diffraction-efficiency slanted gratings were made. Finally, the suitability of this self-developing material to both double-exposure and real-time holographic interferometry is demonstrated.

Design of high-efficiency dielectric reflection gratings

Abstract: We discuss examples of designs for all-dielectric reflection gratings that tolerate high intensity and are potentially capable of placing up to 99% of the incident light into a single diffraction order, such as are needed for contemporary high-power lasers utilizing chirped-pulse amplification. The designs are based on placing a dielectric transmission grating atop a high-reflectivity (HR) multilayer dielectric stack. We comment on the connection between transmission gratings and reflection gratings and note that the grating and the HR stack can, to a degree, be treated independently. Because many combinations of gratings and multilayer stacks offer high efficiency, it is possible to attain secondary objectives in the design. We describe examples of such designs aimed toward improving fabrication and lowering the susceptibility to laser-induced damage. We present examples of the dependence of grating efficiency on grating characteristics. We describe examples of high-efficiency (95%) gratings that we have fabricated by using hafnia and silica multilayers.

An analysis of the anisotropic and topographic gratings in a side-chain liquid crystalline azobenzene polyester

Abstract: We have examined in detail the formation of surface relief structures in azobenzene polyesters formed by polarization holography with orthogonally circularly polarized laser beams. We show that it is possible to separate the contribution to the diffraction efficiency into an anisotropic part and a surface relief part by examining the polarization content of the first order diffracted beam. By studying the dynamics of the growth of the grating, we show that the gratings due to anisotropy and surface relief appear at the same time. Atomic force microscopic investigations of the film after irradiation reveal a strongly polarization dependent surface relief pattern.

Diffractive lenses for chromatic confocal imaging.

Abstract: A diffractive zone plate provides a highly linear wavelength-to-depth coding, allowing for nonmechanical depth scanning in a confocal microscope. This chromatic confocal microscope, constructed with 40x and 60x objectives, achieves axial position changes of 55 and 25 mum, respectively, for a wavelength tuning range of 100 nm. The corresponding longitudinal point-spread functions are measured and shown to possess full-width half-maximums of 2.52 and 2.23 mum, respectively. Two-dimensional profiles of a two-phase-level grating and a four-phase-level diffractive structure are given. The performance of the chromatic confocal microscope is consistent with that of the conventional confocal operation of the microscope.

Volume shrinkage in slant fringe gratings of a cationic ring-opening holographic recording material : Holography III

Abstract: A new photopolymer holographic recording material, ULSH-500, based on cationic ring-opening polymerization, has been further optimized to achieve low transverse shrinkage without sacrificing sensitivity. The extent of transverse (z) and lateral (x) shrinkage was determined explicitly in this study for a range of slant angles in volume holograms recorded to near saturation and in holograms of low diffraction efficiency. The values ΔK x /K x and ΔK z /K z , which represent the physical material shrinkage in the grating vector plane, were ascertained by (1) direct measurement of the differential angle changes in the reference and signal beam angles necessary to achieve Bragg matching and (2) measurement of the average refractive index. The accuracy of this method was primarily limited by the exactness in determining the angle of peak efficiency in the Bragg selectivity curve. It is demonstrated that the peak angle can be established to within a small fraction of a degree. It is shown that the assumption of anchoring and, thus, uniaxial shrinkage as embodied in the conventional fringe rotation model cannot be applied for the photopolymer ULSH-500 under the recording conditions used herein. It is demonstrated that background uplift in angular selectivity profiles can be attributed to nonuniformity in the grating strength throughout the transverse direction of the recording media, and that the uplift can be reduced to negligible levels by using a low fluence pre-imaging exposure. Calorimetric analysis of reaction kinetics was performed using direct laser irradiation delivered by optical fiber.

Demonstration of a computed-tomography imaging spectrometer using a computer-generated hologram disperser.

Abstract: We have constructed a computed-tomography imaging spectrometer that uses a phase-only computer-generated hologram (CGH) array illuminator as the disperser. This imaging spectrometer collects multiplexed spatial and spectral data simultaneously and can be used for flash spectral imaging. The CGH disperser has been designed to maintain nearly equal spectral diffraction efficiency among a 5 x 5 array of diffraction orders and to minimize diffraction efficiency into higher orders. Reconstruction of the (x, y, lambda) image cube from the raw, two-dimensional data is achieved by computed-tomography techniques. The reconstructed image and spectral-signature data compare favorably with measurements by other spectrometric methods.

Switchable Diffractive Cholesteric Gratings

Abstract: Electrically switchable diffractive gratings based on cholesteric liquid crystals are suggested. An electric field switches the cholesteric cell between three states: two with a uniform in-plane director and one with a periodic in-plane director modulation. The modulated state produces a Raman–Nath diffractive effect. Characteristic time of switching is of the order of 10 ms. Diffractive properties depend on the polarization of the incident beam and the direction of surface orientation.

Miniature spectrometers for biochemical analysis

Abstract: Miniature spectrometers have been demonstrated by mounting micromachined diffraction gratings onto CCD imaging devices. Two implementations are tested: one for high-dispersion and -sensitivity applications, and the other for low-cost consumer applications. The first system shows a dispersion of 1.7 nm/pixel and a resolution of 74.4 for the bandwidth of interest. The free spectral range of the device is designed to be in the visible range for this particular application. The diffraction efficiency of the system is 63%. The second, low-cost system demonstrates a dispersion and resolution of 2.55 nm/pixel and 69.8, respectively. These specifications are comparable to that of a conventional, low-end commercial spectrometer. Results are shown for their applications in biochemical analysis. Further optimization is sought by adding micromachined lenses and creating specialized, computer-generated gratings to compress and shape the spectral signal.

Zone plates with high efficiency in high orders of diffraction described by dynamical theory

Abstract: The coupled wave theory is applied to calculate the diffraction efficiency of zone plates used in high orders of diffraction. In contradiction to the existing understanding about 30%–50% of the incident intensity can be diffracted by a single high order if the line-to-space ratio of the zone structures is in the range of 0.1–0.5 and the aspect ratios are larger than 30:1. Highly efficient diffractive transmission x-ray optics with high numerical apertures and resolving power can be developed without manufacturing extremely small zones which are necessary for zone plates used in the first order.

Design and fabrication of binary slanted surface-relief gratings for a planar optical interconnection

Abstract: We propose the use of binary slanted surface-relief gratings with parallel-groove walls as input and output couplers in a planar optical interconnect. Parametric optimization of cascaded output couplers is employed to design an interconnect consisting of N output couplers producing a uniform intensity distribution with a high efficiency that may be realized in one lithographic etching step. The sensitivity of a N = 4 interconnect to various fabrication errors is analyzed. We demonstrate the operation of a slanted surface-relief grating manufactured with electron-beam lithography and reactive-ion etching for an operating wavelength of λ = 0.633 μm.

Synthetic infrared spectra.

Abstract: We have designed, microfabricated, and characterized a diffractive optical element that reproduces the infrared spectrum of HF from 3600 to 4300 cm-1. The reflection-mode diffractive optic consists of 4096 lines, each 4.5 µm wide, at 16 discrete depths relative to the substrate from 0 to 1.2 µm and was fabricated upon a silicon wafer by anisotropic reactive ion-beam etching in a four-mask-level process. We envisage the use of diffractive optical elements of this type as the basis for a new class of miniaturized, remote chemical sensor systems based on correlation spectroscopy.

Synthetic spectra: a tool for correlation spectroscopy

Abstract: We show that computer-generated diffractive optical elements can be used to synthesize the infrared spectra of important compounds, and we describe a modified phase-retrieval algorithm useful for the design of elements of this type. In particular, we present the results of calculations of diffractive elements that are capable of synthesizing portions of the infrared spectra of gaseous hydrogen fluoride (HF) and trichloroethylene (TCE). Further, we propose a new type of correlation spectrometer that uses these diffractive elements rather than reference cells for the production of reference spectra. Storage of a large number of diffractive elements, each producing a synthetic spectrum corresponding to a different target compound, in compact-disk-like format will allow a spectrometer of this type to rapidly determine the composition of unknown samples. Other advantages of the proposed correlation spectrometer are also discussed.

Dye-doped liquid crystals as high-resolution recording media.

Abstract: Light-induced anchoring of the molecular director is reported to be an efficient method for writing permanent holographic gratings in dye-doped liquid crystals. We have achieved higher sensitivity and spatial resolution in these materials with other methods. An energy density as low as 10(-1) J/cm(2) was sufficient to write gratings with a resolution higher than 100 lines/mm.

High-efficiency fused-silica transmission gratings.

Abstract: We describe the design, fabrication, and performance of high-efficiency transmission gratings fabricated in bulk fused silica for use in high-power ultraviolet laser systems. The gratings exhibit a diffraction efficiency of 94% in order m=-1 and a damage threshold greater than 13>J/cm( 2) for 3-ns pulses at 351 nm. Model calculations and experimental measurements are in good agreement.

Single-point diamond turning and replication of visible and near-infrared diffractive optical elements

Abstract: High-fidelity diffractive surfaces have been generated with single-point diamond-turning techniques. A key to the success of this technique is the ability to shape the diamond tool tip to provide the optimum phase-relief profile, given manufacturing constraints. Replication technology is used to transfer the phase-relief surface into a thin epoxy or photopolymer layer on a glass substrate. Diffraction efficiency results for a wide range of zone widths are presented to provide the reader with a baseline of expected performance for replicated visible and near-infrared diffractive optical elements. In addition, a new method for analyzing diffractive surface structures is presented. The ray-trace algorithm quickly provides accurate results of predicted diffraction efficiency for arbitrary zone profiles, which is extremely valuable in predicting manufacturing errors.

High resolution polarization gratings in liquid crystals

Abstract: Efficient recording of polarization gratings in dye-doped liquid crystals is reported. By exploiting the effect of light-induced anchoring of the molecular director, it has been possible to write stable holographic gratings with a diffraction efficiency of 8% at a resolution of 1000 lines/mm. The required surface energy density of 0.3 J/cm2 sets these materials among the most sensitive for optical storage.

Intensity dependence and white-light gating of two-color photorefractive gratings in LiNbO(3).

Abstract: Photorefractive gratings have been written in Pr:LiNbO3 by use of a diode laser for writing and of filtered white light (390–520 nm) as a gating source. The gating light increases the writing efficiency by more than 3 orders of magnitude, and the two-step writing process provides nondestructive readout. The material sensitivity for two-color writing rises strongly for gating wavelengths near the bandgap and approaches that of Fe-doped lithium niobate at power densities of a few watts per square centimeter. In addition, we show that the dynamic range of the recording process is strongly dependent on the writing intensity.

Blazed diffractive optics.

Abstract: Diffractive optical elements with blazed profiles can, in theory, have 100% relative efficiency We review several methods for making such elements and compare their advantages and limitations Our emphasis is on processes to produce elements other than those with approximate, stepped surfaces, such as binary optical elements For optical methods, we offer an expression relating the maximum numerical aperture of a diffractive lens with a given maximum efficiency to the numerical aperture of the recording system

Effects of Material Systems on the Polarization Behavior of Holographic Polymer Dispersed Liquid Crystal Gratings

Abstract: This study has investigated the effects of the material systems involved in the fabrication processes on the polarization behavior of a volume holographic grating. The diffraction efficiency of the gratings fabricated using prepolymer/liquid crystal mixtures shows strong dependence on the polarization of incoming light. Depending on the materials used in the formation of a grating, the diffraction properties are such that either p- or s-polarized light is strongly diffracted while the light with the other polarization is very weakly diffracted. The magnitude of the dependence on the polarization is greatly affected by the type of monomers, liquid crystals and substrates. The comparison of various types of monomers added to the base prepolymer mixtures, two distinctly different types of liquid crystals and glass slides and indium-tin oxide (ITO) coated glass as substrates was carried out using polyester-based and urethane-based oligomers.

Rigorous electromagnetic analysis of diffraction by finite-number-of-periods gratings

Abstract: The diffraction characteristics of finite-number-of-periods (FNP) gratings are determined by using rigorous electromagnetic analysis based on the two-region formulation of the exact boundary element method. Gratings with 5, 9, 13, and 17 periods are analyzed and compared with infinite-number-of-periods (INP) gratings in terms of diffraction efficiencies and field patterns. Both dielectric transmission diffractive devices and metallic reflection diffractive devices are treated for TE and TM normally incident light. Furthermore, both 2-level and 8-level grating structures are treated. The diffraction properties of FNP gratings are shown to approach smoothly those of INP gratings as the number of periods increases.

Efficient, Polarization-Independent, Reflective Liquid Crystal Phase Grating

Abstract: A reflective liquid crystal diffraction grating is proposed which makes use of the maximum available surface area. The structure consists of alternating stripes, each a twisted-nematic domain with surface orientation perpendicular to that of its neighbors. All domains have the same twist sense. This device yields, in principle, 100% diffraction efficiency independent of incident polarization.

Super-smooth x-ray reflection grating fabrication

Abstract: Blazed, grazing incidence x-ray reflection gratings are an important component of modern high resolution spectrometers and related x-ray optics. These have traditionally been fabricated by diamond scribing in a ruling engine, or more recently by interferometric lithography followed by ion etching. These traditional methods result in gratings which suffer from a number of deficiencies, including high surface roughness and poor control of the groove profile. These deficiencies lead to poor diffraction efficiency and high levels of scattered light. We have developed a novel fabrication method for fabricating blazed x-ray reflection gratings which utilizes silicon wafers that are cut 0.7° off of the (111) plane. In solutions such as potassium hydroxide (KOH), silicon is etched in 〈111〉 directions orders of magnitude slower than in other directions, resulting in extremely smooth {111} facets. The gratings are patterned using interferometric lithography with 351.1 nm wavelength and transferred into the substrat...

Optical components containing complex diffraction gratings and methods for the fabrication thereof

Abstract: Methods for forming holographically processes polymer-liquid crystal composites (HPP7LCC) films and electrically switchable gratins utilizing HPP/ICCS are disclosed. These involve imaging a complex mask (10') which may be computer generated, containing the pattern to be imaged onto a film (12) of an HPP-LCC by use of a pair of interfering wavefronts (14A, 14B) as shown in the figure. By suitably selecting the angle between the pair of interfering wavefronts (14A, 14B), a desired submicron holographic pattern can be formed in the HPP/LCC film (12) in order to achieve optimum diffraction efficiency, with the pattern being modulated by the pattern in the mask to achieve the desired optical effect. Alternatively, the imaging may use contact lithography, where the mask (10') is in direct contact with the HPP/LCC film (12) as shown in figure 2 or the mask may be a focussing Fresnel element (10) as shown in figure 3.

Replication of continuous-relief diffractive optical elements by conventional compact disc injection-molding techniques.

Abstract: Continuous-relief diffractive optical elements have been replicated by use of conventional compact disc injection-molding techniques. Two continuous-relief microstructures, a blazed grating and a fan-out element, were chosen to evaluate the replication process. Original elements were fabricated by direct-write electron-beam lithography. Optical measurements and atomic force microscopy were used for investigating the replication fidelity.

Rapid fabrication of diffractive optical elements by use of image-based excimer laser ablation

Abstract: We describe a method of fabricating multilevel diffractive optics by excimer laser ablation. A portion of a chrome mask containing many patterns is illuminated by 193-nm laser light and imaged by an objective lens onto a poly(imide) substrate. Ablation of an entire single pattern is achieved in a single laser pulse. Multiple pulses are used to vary the ablation depth, and multiple patterns are used to create a variety of multilevel optics. We have successfully fabricated arrays of eight-level diffractive microlenses with varying focal lengths and decenters. The optics performed with diffraction-limited focusing and near-theoretical diffraction efficiency (92%).

Analysis and synthesis of circular diffractive lens with local linear grating model and rigorous coupled-wave theory

Abstract: We describe a local linear grating model that permits estimation of the diffraction efficiency of high-numerical-aperture two-dimensional circular lenses with one-dimensional rigorous coupled-wave analysis. The model is self-consistent and is consistent with the scalar theory that results for low-numerical-aperture lenses. The accuracy of the model increases with the number of Fresnel zones of the lens. On the basis of this model we optimize the discrete phase profile of the lens for the maximum diffraction efficiency, using the random search method. The TE and the TM modes are optimized simultaneously. Their phase variations as functions of local grating periods are removed by cyclic shifts of the grating profiles, resulting in constructive summation at the focus and high diffraction efficiency.