Showing papers on "Diffraction efficiency published in 1972"

Photopolymer Material for Holography

Abstract: An experimental Du Pont holographic photopolymer material produces an index modulation in excess of 10−2 utilizing a diffusion mechanism. Optimum exposure in air is typically 30 mJ/cm2, in nitrogen 3 mJ/cm2. Composition, beam ratio, and exposure power all affect the index modulation. This, combined with thickness variations, permits diffraction efficiency to be preadjusted for a variety of desired angular responses and spatial frequencies. The material can be easily overmodulated according to Kogelnik’s phase grating theory. No wet processing is required. After total polymerization, storage at 100°C, −60°C, and under water does not significantly affect the diffraction efficiency. Image–object superposition is exact for real-time holography. Excellent copies of silver halide holograms with four times the original efficiency have been made. Grating devices with tailored peak or flat wavelength response can be constructed.

Reusable optical image storage and processing device.

Abstract: The PROM (Pockels Readout Optical Modulator) described in this paper is a device for storing and processing optical images and information in recyclable, real time operating systems. The device consists of a single active layer of a cubic photosensitive, electrooptic material. ZnS, ZnSe, and Bi12SiO20 devices have been fabricated and characterized demonstrating high writein sensitivity, high resolution, high contrast, high optical and diffraction efficiency, fast recycling times, long storage times, and freedom from fatigue. Operating modes and measured performance parameters are discussed.

Power requirements and resolution of real-time holograms in saturable absorbers and absorbing liquids.

Abstract: Diffraction gratings have been induced in absorbing liquids and saturable glasses with a ruby laser. The mechanism for the production of the gratings has been discussed. The diffraction efficiency and the resolving power of the materials has been measured and found to agree with calculations based on the models of a thermal phase grating and a grating produced by saturable absorption.

Diffraction of a Parallel- and Perpendicular-Polarized Wave from an Echelette Grating*

Abstract: The boundary-value problem of diffraction of a parallel- or perpendicular-polarized wave from an echelette grating is numerically solved. Parallel and perpendicular Wood’s anomalies are investigated. A very strong anomaly for negative incident angle into a back-scattering order is found.

Rh‐doped LiNbO3 as an improved new material for reversible holographic storage

Abstract: A poled single crystal of LiNbO3 doped with Rh as a new impurity is described in terms of its storage performance in producing high‐efficiency simple phase holograms. Doping with Rh introduces a new absorption band peaking around 4880 A, and dramatically improves the recording sensitivity, diffraction efficiency, erasing sensitivity, and persistence of holograms over the undoped crystal. Anomalous optical erasure, possibly due to self‐enhancement of the hologram, is also observed in our doped crystal.

Diffraction of a Wave Beam by an Aperture

Abstract: The diffraction field of a wave beam from a circular and a rectangular aperture is obtained in the Fresnel region by using the Huygens-Kirchhoff approximation. The diffraction field in the Fraunhofer region can be obtained simply by replacing a parameter. The diffraction field is then expanded into a series of beam mode functions. From the field distributions and the expansion coefficients, which represent the coupling of the incident beam to the various modes in the diffraction field, the effects of an aperture on the incident beam can be known. With this mode expansion method, the conditions for optimum coupling between fundamental modes are obtained and solved numerically.

A method for directly determining surface strain fields using diffraction gratings

Abstract: This paper describes a new method for determining the strain distribution at the surface of solid bodies. The method is purely optical; it uses the diffraction phenomena generated by a copy of a grating that is applied to the specimen. A suitable mask performs filtering of the diffraction pattern; the image that is reconstructed from this filtered pattern shows light and dark areas; it is shown that the boundary line of those areas is the locus of points exhibiting the same value of strain, measured along a certain direction.

Nonlinearities of Photopolymer Holographic Recording Materials

Abstract: An analysis is presented of the nonlinear effects of holographically recording discrete image points on a phase recording material. The analysis is restricted to thin, two- and three-beam holographic gratings recorded on a material that exhibits a linear phase shift vs exposure. Harmonics, intermodulation noise, and small signal effects are considered. Experimental measurements were carried out for three-beam holographic gratings and diffuse object holograms recorded on photopolymer recording materials. Intermodulation noise is found to be a serious limitation for discrete image point holograms, because this noise cannot be spatially separated from the desired image points. Intermodulation noise can be reduced by increasing the reference-to-object beam irradiance ratio and by reducing the diffraction efficiency. Photopolymer gratings with 50-dB signal-to-intermodulation noise ratio were obtained with diffraction efficiency greater than 1/10% at beam irradiance ratios of 400:1. The image contrast of photopolymer holograms of diffuse objects is compared with the image contrast reported for bleached silver halide emulsions.

Diffraction efficiency of magnetic hologram

Abstract: MnBi thin films have been studied as a magneto-optical memory element. Ordinarily, restricted conditions are required for preparing pure MnBi film with excellent properties and homogeneity. However, in the case of Mn-Bi-Te films, it was found that adopting the proper composition made it easy to prepare homogeneous films, and the figure of merit F/α was not worse than that of pure MnBi reported by other authors. Furthermore, the Faraday rotation was controlable by varying the composition of Mn-Bi-Te. Then, magnetic holograms were formed using the Mn-Bi-Te films exhibiting magnetization in a direction perpendicular to the film plane. The dependence of the diffraction efficiency on Faraday rotation and holographic writing conditions are reported.

Effect of Emulsion Thickness on the Diffraction Efficiency of Amplitude Holograms

Abstract: The diffraction efficiency of amplitude holograms recorded on photographic emulsions is shown to be independent of the thickness of the emulsion layer. This can be explained theoretically under the assumption that the exposure modulation is not too high, i.e., for high and moderate beam ratios K. With this assumption it is possible to demonstrate the equivalence of Kogelnik’s equation using a coupled-wave theory for thick holograms and the equations derived by Kaspar and Lamberts for thin holograms. This is true even though the former contains an explicit dependence on thickness and the latter equation includes only the MTF of the emulsion and the slope of the amplitude-transmittance log-exposure characteristic of the emulsion. This conclusion is demonstrated experimentally by measuring the diffraction efficiency for two series of very-fine-grain emulsions, which differ only in the thickness of the emulsion layer.

Accuracy of holographic images.

Abstract: The purpose of the research described in this Letter is to measure the accuracy of the holographic image of a large object and determine some of the experimental factors that may cause a deterioration of this accuracy. The results apply to either the real or the virtual image, but, since the virtual image is not readily accessible for measurements, our tests were limited to the real image. I t is known that a hologram (unlike a lens) is theoretically capable of producing an exact distortionless image of a threedimensional object. In order to obtain this result, the real image must be produced by the conjugate of the original reference beam. This requirement can be most easily satisfied by using a plane wavefront reference beam. In image formation, the hologram plate itself must be considered as an optical element in the system. The hologram plate is ideally treated as a uniformly thick photographic emulsion placed on a flat plate of homogeneous glass of uniform thickness. For the plane wavefront reference and reconstruction beams the above ideal plate introduces no harmful effects on the image. However, the photographic plates-normally used for holography (649F and 10E70) are not ideal and, as a consequence, do contribute a distortion to the images. The thickness of the emulsion was found to have an average variation of 20%. In addition, the glass substrates were found to have a significant variation in thickness. The principal effect of thie thickness variations on the images can be visualized for the simple case where the emulsion thickness variation is quadratic. e.g., assume the emulsion surface has a convex spherical shape. The emulsion media then acts like a lens in the system that introduces an unwanted magnification into the image. This problem was eliminated by putting the photographic plate in a liquid gate, which closely matched its refractive index to a pair of flat glass windows (Fig. 1). These windows were 2.5 cm thick optical flats and introduced less than one wavelength distortion in a plane wavefront passing through the windows. The index matching liquid was o-xylene. The index match was not perfect (1.506 for the xylene compared to 1.532 for the emulsion) but close enough to reduce significantly the observable emulsion thickness effects on the image accuracy. The hologram accuracy test was made with a linear standard bar as the object. The bar was 5 cm thick and 15 cm high with an over-all length of 1 m. The center of the bar was positioned opposite the center of the 13-cm × 18-cm hologram plate. The angle between the reference beam and the hologram normal was approximately 35°. Two glass reticles were set in ports, 75 cm apart at opposite ends of the bar. Each reticle had a horizontal and vertical cross pattern with 25-μ graduations set 125 μ apart. The ports were tapered front and back to give an un­ obstructed line-of-sight through the reticle to the hologram plate. The precision with which the construction and reconstruction beams can be collimated to give a plane wavefront has a large effect on the hologram accuracy. The plane wavefronts were Letters to the Editor should be addressed to the Editor, APPLIED OPTICS, AFCRL, Bedford, Mass. 01730

Film characteristics pertinent to coherent optical data processing systems.

Abstract: Photographic film is studied quantitatively as the input mechanism for coherent optical data recording and processing systems. The two important film characteristics are the amplitude transmission vs exposure (TA − E) curve and the film noise power spectral density. Both functions are measured as a function of the type of film, the type of developer, developer time and temperature, and the exposing and readout light wavelengths. A detailed analysis of a coherent optical spatial frequency analyzer reveals that the optimum do bias point for 649-F film is an amplitude transmission of about 70%. This operating point yields minimum harmonic and intermodulation distortion, whereas the 50% amplitude transmission bias point recommended by holographers yields maximum diffraction efficiency. It is also shown that the effective ac gain or contrast of the film is nearly independent of the development conditions for a given film. Finally, the linear dynamic range of one particular coherent optical spatial frequency analyzer is shown to be about 40–50 dB.

Possible use of phase holograms as optical elements

Abstract: A theoretical analysis is made of the possibility of using suitably prepared holograms in the formation of wave fronts of different shapes. In some cases such holograms can replace multilens objectives. The necessary condition for successful application of such holographic optical elements is a low value of the light losses. A phase hologram of diffraction efficiency of about 70% was prepared and used in holographic systems in which light transmission was comparable with that in conventional optical systems.

Apparatus for deriving diffractive rays by diffraction gratings

Abstract: Apparatus for deriving diffractive rays by diffraction gratings which enables light measurement to be effected with high accuracy in addition to the feasibility of making the whole apparatus in a compact size. These features of the apparatus are obtained by the provision of a photoelectric element on the rear side of an index grating at a position in which it faces a movable scale grating with a small distance from this latter grating.

Laser Beam Steering.

Abstract: : An electronic beam scanner for CO2 lasers was designed and constructed. Specific requirements to be met were: three degree scan range, 100 resolvable beam positions, 6000 Hz raster scan in one dimension, and a capability for 100 W of 10.6 micrometer output. The device employs acousto-optic Bragg diffraction using germanium with a longitudinal acoustic wave and laser beam polarization in the crystallographic direction. Sawtooth FM over a 27 MHz band centered near 100 MHz is used to provide the scan mechanism. A single small Ba2NaNB5O15 transducer provides sufficient acoustic beam divergence to yield a 3 deg. scan capability with a 1.4 dB falloff in output beam power at the scan limits. Direct-contact water cooling of the Ge crystal is provided to prevent thermal runaway. Cylindrical, reflective, telescopic optics are used for beam conditioning and recollimation. Computer calculations provided impedance characteristics and frequency response curves for general transducer design. Transducer bonding was done ultrasonically using thin metal films of Ag, Au, and Ag-In. The AG-In gave best overall performance. An rf-to-acoustic conversion efficiency of 34% was achieved at band center (4.7 dB effective transduction loss). Radio frequency driver power required for 50 (70)% diffraction efficiency was 31 (49) W.

Possibilities of Optical Elements Design Using Phase Holograms

Abstract: Design principles of holographic optical elements are discussed. It is shown that a phase hologram with a high efficiency can be produced that transforms the input wavefront into an output of the required directivity. Such holograms can be used in laser systems instead of complex multilens objectives. Holograms have been obtained experimentally with a diffraction efficiency of 70%.