Showing papers on "Diffraction efficiency published in 1988"

Hybrid diffractive-refractive lenses and achromats

Abstract: Hybrid elements containing optical power with both diffractive (holographic) and refractive components are shown to be useful for obtaining arbitrary or, in special cases, achromatic dispersive characteristics. In one configuration a volume holographic element is coated on the surface of a crown glass lens, and by varying the power distributions among the refractive and holographic components while maintaining constant overall optical power the effective Abbe V numbers of the resultant hybrid element are shown to span all real numbers excepting a narrow interval around zero. In the achromat case (V number = ∞), both refractive and diffractive components are of the same sign resulting in much smaller glass curvatures than in all-refractive achromat doublets or apochromat triplets. The large separation between holographic partial dispersions and available glass partial dispersions is shown to lead to hybrid three-color achromats with greatly reduced glass curvatures. Applications are expected to include broadband achromatic objectives and chromatic aberration corrector plates in high performance optical systems. Such corrector plates may have any net power (including zero) while exhibiting effective V numbers that are positive or negative and that span a wide range, e.g., ±1 or ±1000. Further advantages include reducing the need for choosing high dispersion glasses, which may be costly and difficult to grind or polish. High diffraction efficiency and broad spectral bandwidths (in excess of 3000 A) are obtained in the holographic optical elements using single-element central-stop and cascaded element designs.

Coupled-Wave Analysis Of Two-Dimensional Dielectric Gratings

Abstract: Diffraction by two-dimensional surface-relief dielectric gratings are analyzed using rigorous three-dimensional vector coupled-wave approach. The method applies to arbitrary plane wave angle of incidence, wavelength, and polarization. In the resulting conical diffraction, the input TE and TM polarization are coupled and the diffracted orders are, in general, elliptically polarized. Diffraction characteristics of two-dimensional binary gratings are presented. Ultrahigh spatial-frequency gratings (grating period less light wavelength) are shown to exhibit polarization independent antireflection behavior (zero-reflectivity).

Method of fabricating high efficiency binary planar optical elements

Abstract: The high diffraction efficiency regime of binary gratings occurs at periodicities on the order of a wavelength in order to produce grating which diffract radiation with efficiencies of greater than 90%, a fabrication procedure is disclosed which uses halographic and very large scale integration techniques which allow fine control over the periodicity and depth parameters to produce binary planar optical elements having a ratio of λ/T greater than one (where λ equals the wavelength of an illuminating wavefront, and T equals the grating periodicity). Additionally, the disclosed process produces high optical quality diffractive elements with phase precision of as high as λ/100. These diffractive elements include laser beam multiplexers, beam profile shapers, and binary lenses which are lossless optical transfer functions.

Diffractive-reflective optical interconnects.

Abstract: We have demonstrated the feasibility of diffractive–reflective optical interconnects. These interconnects consist of a sandwich of a holographic plane and a reflective plane. Various possibilities like beam relaying, connection switching, and broadcasting are discussed.

Relative phases of electromagnetic waves diffracted by a perfectly conducting rectangular-grooved grating.

Abstract: A complete solution of plane-wave scattering from a groove-corrugated surface of infinite extent for arbitrary incidence is presented. The electromagnetic wave is decomposed into fast and slow modal representation, and the solution is accomplished through the use of the mode-matching method. We solved for the zero-order diffraction efficiency and the phase of each polarization component for arbitrary incident angles. Our results have verified special cases previously published by others and have illustrated the phase shift between polarizations associated with the diffraction process as a function of the incident angles.

Modulation transfer function measurements for thin layers of azo dyes in PVA matrix used as an optical recording material.

Abstract: Diffraction efficiencies and characteristic erasure times were measured for two recording materials suitable for real-time holography and four-wave mixing. A study of real-time transmission volume grating formation was done in conjunction with polarization states of the writing beams for 30-microm thick layers. Modulation transfer function curves presented were obtained using a versatile device for which the spatial frequency domain was 500-4000 cycles/mm.

Stratified volume holographic optical elements.

Abstract: A computational algorithm for analyzing diffraction properties of optical devices, the optical beam propagation method, has suggested a new class of devices by which Bragg regime (thick grating) response can be obtained from a spaced sequence of thin grating layers. Such stratified volume holographic optical elements (SVHQE’s) can emulate distributed volume gratings in terms of diffraction efficiency and angular selectivity and in addition possess periodic diffraction properties that might serve, for example, as interconnections for optical cellular logic arrays. SVHOE’s also offer a unique capability for altering the device diffraction response on a layer-by-layer basis, allowing for control of both the diffraction peak width and the angular separation of adjacent peaks.

Interference-term real-time measurement for self-stabilized two-wave mixing in photorefractive crystals.

Abstract: We report the real-time direct interference-term measurement for a two-wave-mixing experiment in photorefractive crystals. Knowledge of the interference term may provide information concerning diffraction efficiency, interference pattern-to-recorded hologram phase shift, and optical activity and anisotropic diffraction properties of these materials. This method comprises phase modulation of one of the interfering beams and synchronous detection of the first and second harmonics in the resulting output irradiance modulation. Simultaneous detection of both harmonics enables the measurement to be made even in strongly perturbed conditions, since one harmonic is used for measuring and the other is used for operating an active stabilization system. Experimental results for Bi12TiO20 are reported.

Optical correlators with fast updating speed using photorefractive semiconductor materials.

Abstract: An updatable optical correlator which uses a photorefractive compound semiconductor to generate real-time matched filters is proposed. Using compound semiconductors offers high speed with low optical input intensities. Here we discuss issues affecting the performance of this correlator. This includes an analysis of the Bragg diffraction and a discussion of the speed and power considerations of these materials. Experimental results obtained using photorefractive GaAs are also presented.

Infrared Applications Of Diffractive Optical Elements

Abstract: Diffractive optical elements have the potential to improve the performance of infrared optical systems. An approach to constructing high quality diffractive elements has been developed using standard integrated circuit techniques. It is possible to implement arbitrary phase profiles since the elements are computer generated.

Antireflection gold surface-relief gratings: experimental characteristics

Abstract: A systematic procedure using the effective index method and impedance matching has recently been developed Appl. Opt.26, 3123 ( 1987)] for the design of antireflection high-spatial-frequency rectangular-groove gratings on lossy materials including high conductivity metals. The design procedure in turn can be used as a starting point to design antireflection metallic gratings with lower spatial frequencies using rigorous coupled-wave analysis. These lower spatial-frequency gratings have the advantage of being easier to fabricate. In the present work, a particular antireflection gold grating design (having a period of 1.0 μm, a filling factor of 50%, and a groove depth of 147.5 nm for use at a freespace wavelength of 500 nm, normal incidence, and polarization parallel to the grooves) was fabricated and its diffraction characteristics experimentally measured. The grating indeed showed very nearly zero specular reflection in the blue region of the spectrum. Unlike previously reported antireflection anomalies, the effect is broadband occurring over a broad range of wavelengths and angles of incidence, and for both orthogonal polarizations. This work clearly shows that the systematic design of zero specular reflection grating surfaces is possible.

Photochromic gratings in photorefractive materials.

Abstract: The steady-state and dynamic behavior of absorption and phase gratings in semi-insulating GaAs and InP have been studied by four-wave mixing. These measurements permit independent study of the space-charge-field screening kinetics and free-carrier recombination kinetics in these materials. The study is important both for the design of optimized materials for photorefractive applications and for the characterization of the defect electronic structure in these materials.

Optical generation of coherent surface acoustics: an optically based probe of surface structure and dynamics.

Abstract: Above-band-gap picosecond pulses are used to generate holographically high-frequency, coherent, surface acoustic modes on semiconductor surfaces Optical diffraction from the surface acoustics is in superposition to a free-carrier phase grating that acts as an amplifying cross term in the diffraction process The detection limits are of the order of 10−4 nm for the surface displacement Frequencies up to 2 GHz have been realized, with frequencies >20 GHz possible A quantitative theory for the photothermal coupling has been developed In addition, propagation of the optically generated surface modes has revealed a solid–liquid phase transition of the water layer at TiO2–H2O interfaces

Recursive design for an efficient HOE with different recording and readout wavelengths.

Abstract: The design of a holographic optical element for focusing a collimated off-axis beam to an on-axis point is described. It is designed according to a novel recursive technique in which the recording is done at a wavelength which differs from the readout. In this recursive technique the final holographic optical element is recorded by using other holograms to provide the aspheric recording wavefronts necessary for reducing the aberrations and maximizing the diffraction efficiency. The design is illustrated with an example where an f/3.0 focusing element is recorded at 514.5 nm and read out at 1064 nm. A spot size of 15microm and a diffraction efficiency of ~60% were measured.

Unidirectional device for a ring laser using an acousto-optic modulator.

Abstract: The behavior of the first-order diffracted beam of an acousto-optic modulator is investigated under rotation of the acousto-optic crystal about an axis perpendicular to the plane of the incident and diffracted beams. It is found that the diffraction efficiency as a function of this rotation is not symmetric about the Bragg angle of incidence. A unidirectional device for a ring dye laser that uses this effect is demonstrated.

Demonstration of guided-wave phenomena at extreme-ultraviolet and soft-x-ray wavelengths

Abstract: We report an explicit demonstration of classical guided-wave propagation at XUV and soft-x-ray wavelengths. Experiments were performed using narrow-band synchrotron radiation at 5, 20.8, 21, and 30 nm. Free-standing gold transmission gratings served as waveguide structures. These structures had a 300-nm grating period with waveguide channel widths as small as 100 nm and were as thick as 700 nm in the direction of guided-wave transmission. Guided-wave phenomena were manifest in strongly asymmetric diffraction patterns resulting from the angular tilt of the transmission-grating normal from the incident-beam direction.

High-order anisotropic diffraction in photorefractive crystals

Abstract: High-order anisotropic diffraction and anisotropic self-diffraction in barium titanate (BaTiO3), strontium barium niobate (SrxBa1−xNb2O6, or SBN), and barium strontium potassium sodium niobate (Ba2−xSrxK1−y NayNb5O15, or BSKNN) have been observed: up to fourth order in BaTiO3 and SBN and up to fifth order in BSKNN. It is shown that the scattered beams arising from anisotropic self-diffraction are composed of multiple orders. To model anisotropic diffraction, high-order terms have been included in Kukhtarev’s solution of the transport equations for diffusion-dominated transport. Light-induced grating decay rates and diffraction efficiencies were measured as a function of grating vector and modulation index for the first three orders in BaTiO3 and were found to be in good agreement with the model.

Polarization sensitivity of noise gratings recorded in silver halide volume holograms.

Abstract: The formation of holographic gratings in silver halide emulsions is limited by scattering from photosensitive grains embedded in these materials. The most serious consequence of this effect is a sharp reduction in the diffraction efficiency of a volume hologram reconstructed at the formation angle. This has been attributed to a noise grating, resulting from the interference between a beam illuminating the emulsion and scattered light. Although the presence of these gratings has previously been reported, their dependence on the relative orientation of the polarization of the construction and reconstruction beams has not been discussed. This report shows that the correct relative polarization orientation reduces the detrimental effects of these gratings and improves the efficiency of holograms formed in silver halide emulsions.

Dual wavelength polarization selective holographic optical element

Abstract: A polarization-selective holographic element having first and second holographic layers, each holographic layer including holograms comprising a plurality of fringes which are recorded with light having a first wavelength λ1. The holographic optical element transmits a first component of light having a second wavelength λ2 without diffraction and diffracts a second component of the light having the second wavelength by a selected angle. The holograms have a high diffraction efficiency and are recorded with beams making angles σ1 and σ2 with a normal to the surface of the holographic layers, where |θ2 -θ1 |=|σ2 -σ1 |=2α, λ2 /(sinθ1 +sinθ2)=λ1 /(sinθ1 +sinθ2), and θ1 and θ2 are the incident and diffracted angles of the second component of the light having the second wavelength.

Infrared recording with gelatin films.

Abstract: Gelatin films glued to an O-ring are proposed as a new procedure to record interference patterns when a CO2 laser is used as an infrared light source. With this method the unwanted effects given by a substrate, on which the thin recording film is laid, are avoided. To characterize the medium, interferometric studies including the recording of diffraction gratings have been done. Diffraction efficiencies of ~30% have been obtained when coherent red light (0.6328-μm wavelength) was sent normally to the gratings. An example of an infrared recorded hologram is shown.

Theoretical properties of electron wave diffraction due to a transversally periodic structure in semiconductors

Abstract: Electron-wave diffraction caused by a grating structure is analyzed. In the grating, potential energy varies periodically in the direction perpendicular to the electron transport. Interference between models in the grating strongly affects diffraction efficiency. As a result, both high and low efficiency, about 90% and less than 1% respectively, can be obtained depending on the electron energy. Thus, the diffraction can be switched by changing the acceleration voltage of the electron. >

Soft X-ray grating efficiencies: Reciprocity theorem, blaze maximum and isoefficiency curves

Abstract: In this article data are presented that show the validity of the reciprocity theorem for diffraction by a blazed grating in the soft X-ray region. With this theorem, a simple formula for the calculation of the diffraction efficiency in blaze maximum can intuitively be derived. Calculations with this formula will be compared to experimental data for first and second order diffraction. Isoefficiency curves for these orders can also be drawn from the experimental data. These will be used to discuss the working parameters of some monochromator designs. By combining the advantages of two well-known monochromator concepts, a new design will be proposed that will allow high resolution while it can be optimized for high transmission and good suppression of higher harmonics.

Combining low aberrations and high diffraction efficiency in holographic optical elements.

Abstract: A method for designing and recording holographic optical elements that have low aberrations and high efficiencies is presented. It is based on incorporating a holographic element with low aberration into a final element in order also to achieve high diffraction efficiency.

Optically And Acoustically Rotated Slow Shear Bragg Cells In TeO 2

Abstract: Bragg cells using the slow-shear wave propagating along the [110] direction in Te02 have become widely utilized in the optical signal processing field. Devices with aperture lengths up to 100 gsec have been fabricated using this mode. The center frequencies of such devices have ranged from 50 to 90 MHz with bandwidths ranging from 20 to 60 MHz. For many signal processing applications, however, significant improvement in the Bragg cell performance can be obtained if either an optically rotated or an acoustically rotated variation on the standard crystal cut is used instead of the typical crystal orientation. These slightly varied orientations can result in significant enhancement in diffraction efficiency and dynamic range as well as in allowing the Lre of linear polarization on the input beam. The work presented examines in detail the various physical properties of these special crystal cuts. The paper will also present guidelines as to when one of these crystallographic rotations will yield significant advantage over the standard orientation.© (1988) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Transmission Grating Spectroscopy And The Advanced X-Ray Astrophysics Facility (AXAF)

Abstract: We review the use of transmission gratings with grazing-incidence telescopes in celestial X-ray astronomy. Gratings were used on the Einstein Observatory and on EXOSAT, and they are planned for AXAF and SPECTROSAT. We outline the basic properties of transmission grating spectrometers and the use of "phased" gratings to enhance the diffraction efficiency. Gratings are fabricated in a multistep process involving generation of a mask followed by replication into a final grating of the appropriate thickness and material. Special attention is given to the AXAF High Energy Transmission Grating (HETG) being fabricated at MIT. The HETG operates over the range 0.4-8 keV, gives resolving powers of 100-1000, effective areas of 10-300 cm2, and a minimum detectable line flux of 1-10 x 10-6 photons cm -2 s'1. The instrument consists of a single array with two types of grating facets: medium-energy gratings (0.6 pm-period, 0.5 pm-thick silver) mounted behind the outer three AXAF mirrors, and high-energy gratings (0.2 pm-period, 1.0 pm-thick gold) mounted behind the inner three mirrors. The materials and thicknesses are selected to maximize efficiency throughout the energy band. The facets are fabricated at MIT using a process involving X-ray lithography. AXAF will also carry a Low Energy Transmission Grating (LETG) supplied by the Laboratory for Space Research at Utrecht. It uses self-supported grating facets of 1.0 gm period and is optimized for operation down to 80 eV. Both the HETG and LETG disperse the spectrum of a source across either of the AXAF focal-plane/imaging detectors. They are most effective for the study of point sources, but they also give moderate resolution spectra of slightly extended sources and can be used to map the spatial distribution of the line emitting regions.

Characterization Of Polaroid's DMP-128 Holographic Recording Photopolymer

Abstract: The fundamental holographic properties of Polaroid's DMP-128 have been measured. Diffraction efficiencies over 90% have been obtained with an index modulation of 0.033. It was found that the diffraction efficiency could be reduced from 90% to less than 10% by the use of an index matching fluid. When the index matching fluid was allowed to evaporate, the diffraction efficiency returned to its original value. This observation suggests that the index modulation produced in the photopolymer is due to tiny cracks or voids formed in the polymer.© (1988) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.