Showing papers on "Diffraction efficiency published in 1981"

Rigorous coupled-wave analysis of planar-grating diffraction

Abstract: A rigorous coupled-wave approach is used to analyze diffraction by general planar gratings bounded by two different media. The grating fringes may have any orientation (slanted or unslanted) with respect to the grating surfaces. The analysis is based on a state-variables representation and results in a unifying, easily computer-implementable matrix formulation of the general planar-grating diffraction problem. Accurate diffraction characteristics are presented for the first time to the authors’ knowledge for general slanted gratings. This present rigorous formulation is compared with rigorous modal theory, approximate two-wave modal theory, approximate multiwave coupled-wave theory, and approximate two-wave coupled-wave theory. Typical errors in the diffraction characteristics introduced by these various approximate theories are evaluated for transmission, slanted, and reflection gratings. Inclusion of higher-order waves in a theory is important for obtaining accurate predictions when forward-diffracted orders are dominant (transmission-grating behavior). Conversely, when backward-diffracted orders dominate (reflection-grating behavior), second derivatives of the field amplitudes and boundary diffraction need to be included to produce accurate results.

Diffraction efficiency and energy transfer in two-wave mixing experiments with Bi 12 SiO 20 crystals

Abstract: The diffraction efficiency and energy transfer are investigated in photorefractive Bi12SiO20 crystals (BSO). Dependence on fringe spacing, electric fields, light intensity and rise-time constants are reported. The optimum crystallographic orientation for each effect is determined for vertical polarization of the recording beams. It is shown that beam coupling is a very sensitive phenomenon in BSO crystals where charge transport lengths are equivalent to usual fringe spacings. Experimental results are interpreted on the basis of the nonlinear theory of self-diffraction developed by Kukhtarev et al. [10].

Highly Conducting Lamellar Diffraction Gratings

Abstract: We outline an improved root-finding algorithm necessary for the solution of the eigenvalue equation associated with the diffraction formalism for lossy lamellar gratings. A numerical example is presented, demonstrating the adequacy of this technique for a highly-conducting aluminium grating.

Multifacet holographic optical elements for wave front transformations.

Abstract: A new type of holographic optical element combines some of the flexibility of computer-generated holograms with the high light efficiency of volume phase holograms to produce optical elements capable of arbitrary illumination transformations with nearly 100% light efficiency. The optical element is recorded by subdividing a volume hologram film surface into numerous small areas (facets), each of which is individually exposed. A final optical system consisting of two dichromated gelatin holograms in series is demonstrated. The first hologram spatially redistributes the incident light, and the second hologram produces a desired phase front on the redistributed light.

Photorefractive-index gratings formed by nanosecond optical pulses in BaTiO 3

Abstract: We write and read a refractive-index grating in a photorefractive crystal BaTiO3 with a single 20-nsec laser pulse at 532 nm. The grating formed is erasable with similar pulses. Diffraction efficiency decreases exponentially with the cumulative erasing light energy for 20-nsec pulses at intensity levels of 5–30 MW/cm2. Optical energies required to write and erase gratings with 20-nsec pulses are about an order of magnitude larger than for millisecond-to-second-long pulses at 515 nm, even though the grating is still formed by a one-photon process.

Coupled-Wave Analysis of Reflection Gratings,

Abstract: Diffraction of electromagnetic waves by longitudinally periodic media (reflection gratings) is rigorously analyzed using an exact coupled-wave approach. The analysis is formulated in a simple matrix form easily implemented on a digital computer. The intensities of the two waves, the diffracted (reflected) and transmitted waves, are calculated for a wide range of parameters. These exact results are then compared to results obtained with approximate two-wave modal, and with multiwave and two-wave coupled-wave analyses. The applicability of these approximate theories is discussed. The case of other than first-order Bragg incidence is also included.

Method for making holographic optical elements with high diffraction efficiencies

Abstract: Production quantities of a multi-element holographic scanner disc are made by optically replicating a silver halide master disc one element at a time in a dichromated gelatin film. The dichromated gelatin film swells during processing. The swell is monitored during production by determining the shift in the angle of the Bragg surfaces within the gel. The angle of the replicating beam for each element is changed from that of the original reference beam to establish a Bragg angle at exposure which will be tilted to the proper angle after swelling in order to maximize the diffraction efficiency of the element at the original reference beam angle.

Nanosecond four-wave mixing and holography in BSO crystals.

Abstract: This is seen clearly in Fig. 3, which enlarges this region of the interferogram. The magnitude and the direction of splitting (a shift towards shorter wavelength) indicate a line-of-sight approach velocity of -70 km/sec. Since the rest of the fringe is more or less undistorted, this is evidence of differential mass velocity for a portion of coronal gas relative to the rest of the coronal gas. Temperature mapping of the corona obtained from the linewidths (interpreted as pure thermal broadening), measured on the 90and 30-sec exposure interferograms, is shown in Fig. 4. A temperature inversion at a distance of -0.15 solar radius from the photosphere apparently occurs along many azimuths.

Planar Rowland spectrometer for fiber-optic wavelength demultiplexing

Abstract: A planar Rowland spectrometer was fabricated and characterized as a wavelength demultiplexer for multimode fiber-optic applications. The spectrometer consisted of a planar multimode glass waveguide with two curved end faces and a cylindrical concave attached to one of the end faces. Semiconductor lasers with wavelengths between 0.825 and 0.845 micron were used for the measurements. Cross-talk isolation between two adjacent fibers with center-to-center separation of 175 microns (100 A in wavelength difference) was measured to be 18 dB. The device's performance was limited by grating diffraction efficiency, optical aberration, waveguide dispersion, and waveguide losses.

Light diffraction from volume phase holograms in electrooptic photorefractive crystals

Abstract: Basic peculiarities of anisotropic volume phase holograms in photorefractive electrooptic crystals of LiNbO3 and Bi12SiO20 types are discussed. General methods of analysis of diffraction phenomena in these media are considered. Theoretical calculations of diffraction efficiency and polarization of the diffracted beam obtained in dynamic approximation for a strongly birefringent as well as a cubic crystal are presented. Original experimental results on the amplitude and polarization of light diffracted from holograms in LiNbO3:Fe and Bi12SiO20 crystals are in good agreement with the theoretical considerations.

X-ray monochromator designs based on extreme off-plane grating mountings.

Abstract: Several monochromator designs are presented in which the beam is incident on the grating nearly parallel to the grooves. The efficiency of such mountings, when obeying the blaze condition, is much higher than that of classical arrangements. Because only the Snell's components of the angular coordinates are varied, this high efficiency is found over the complete wavelength range. With the designs described the wavelength region from 0.6 to 100 nm can be covered without interchanging gratings. For each wavelength the spectral resolution can be increased to ~10(4). Entrance and exit slit positions and the corresponding beam orientations are fixed.

Diffraction Gratings Ruled And Holographic - A Review

Abstract: Diffraction gratings have been evolving steadily over the last ten years. Modern ruling engines have permitted classically ruled gratings to be more efficient with flatter diffracted wavefront than ever before. Examples include sophisticated laser gratings with up to 97.5% efficiency and flatness of better than λ/3 in 17th order 6328X. Holographically recorded diffraction gratings continue to evolve with the development of blazed plane gratings and with groove densities of up to 6000g/mm. Concave holographic gratings now range from toroidal aberration corrected gratings,for use in the soft x-ray, to flat field spectrographs and scanning spectrographs operating out into the infra-red.© (1981) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Controlled shifting of the spectral response of reflection holograms

Abstract: Dichromated gelatin (DCG) is regarded as one of the best holographic recording materials due to its grainless structure yielding high resolution, and its capability of reconstructing wave fronts with high diffraction efficiency (DE). This material, unfortunately, is insensitive to light of wavelength much longer than 540 nm. Much work has been done to extend the spectral sensitivity of DCG to the red wavelengths of the He-Ne and Kr lasers. Adding suitable dyes such as methylene green or methylene blue to the dichromate sensitizing solution yields dye-sensitized dichromated gelatin (DSDCG). High DE was reported for plates coated with these materials. It was found, however, that these high DE values could not be achieved in one of the construction geometries we use, namely, the back-mirror geometry, Fig. 1. The spherical wave front here represents the reference beam which passes through the photosensitized layer striking the mirror. The wave front reflected by this mirror forms the object beam. Recording in this geometry requires a double pass through the sensitized layer. Absorption at the construction (exposing) wavelength must be kept low to achieve a beam ratio compatible with needed fringe visibility and desired high DE. Simultaneously, though, high absorption of the DSDCG layer at the construction wavelength is needed for good photo-

Variation of blaze of concave diffraction gratings.

Abstract: The change in blaze wavelength as a function of position on a blazed concave diffraction grating surface is discussed. An experimental study of ruled and interferometric blazed concave gratings showed that the change in blaze across the grating surface is much less for an interferometric grating than for a ruled grating. Thus interferometric gratings can be expected to have a more uniform distribution of efficiency across their surfaces.

Spectral Characteristics of Thick Phase Holographic Gratings

Abstract: The possibility of producing thick phase holographic transmission gratings with specified spectral characteristics is investigated. On the basis of the coupled-wave theory it is shown that by changing the modulation depth of the refractive index different spectral distributions of the diffraction efficiency may result. In the description of the spectral characteristics account must be taken of the dependence of the interaction constant on the wavelength of the reconstructing light. An experimental study is made of the spectral characteristics in the zeroth and first order of holographic gratings recorded in dichromated gelatin. Using a suitably chosen grating as a correction light filter, an equalization of the spectral sensitivity of a photosensitive element in the 450–600 nm region is achieved.

Conjugate diffraction order in a volume holographic off-axis lens

Abstract: The light irradiance in the first-order diffraction of a volume holographic off-axis lens, which gives an approximate spurious focus, is investigated both experimentally (using a holographic lens produced in dichromated gelatin) and analytically. The investigation is done by two methods, first by using a simple ray-tracing technique that takes into account only the diffraction pattern of the output rays and second by solving differential equations of the Raman–Nath type. The predictions of the analyses are compared with the experimentally observed results and show good agreement.

Electron-Beam Directly Written Micro Gratings For Integrated Optical Circuits

Abstract: The principles and techniques of writing various types of micro gratings directly on chalcogenide (As2S3) film waveguides using the electron beam are reviewed. A computer-controlled apparatus has been developed so that we can fabricate any gratings of uniform/ chirped periods with slant lines which have the area of the order of 1 mm2, the diffraction efficiency of nearly 100 %, and the minimum grating period of 0.2 µm. Combination of those micro gratings results in integrated grating circuits (IGC). An IGC for guided-beam multiple division has been successfully fabricated.© (1981) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Wideband Acousto-Optic Bragg Cells

Abstract: Wideband acousto-opt ic Bragg cell s play a key role in the real-time optical signal processors; e.g., the acousto-optic spectrm analyzers. The critical issue has been the relatively low diffraction efficiency of present state-of-the-art GHz bandwidth Bragg cells. This paper presents the recent progress of efficient wideband Bragg cell developments. Experimental results were obtained with a shear wave LiNb03 device and a longitudinal GaP device. Experimental results demonstrated at 0.633 micrometer include 3 dB bandwidth of 1 to 1.3 GHz and diffraction efficiencies of 3 to 20 percent per RF watt.

In-Line Holographic Lenses of High Numerical Aperture

Abstract: In-line holographic lenses of numerical aperture of 0.4 and 0.55 were successfully fabricated for the purpose of the application to an optical disk player with a He-Ne laser. They were recorded at 6471 A in the methylene-blue sensitized dichromated gelatin film of 15 µm in thickness. The special design of the optical recording system and the careful processing of the recording materials were required. The evaluations of the modulation transfer function, the focused beam profile, the average diffraction efficiency and the stability test were performed. Bragg and Raman Nath diffractions occurred in the hologram were also investigated to explain the observed results.

Generalized ray-tracing for diffraction gratings of arbitrary form

Abstract: The technique of generalized ray-tracing, which retains information on the principal directions and principal curvatures of the wavefront through the operations of transfer, refraction or reflection, is extended to the case of diffraction gratings of arbitrary form. The specific application to ruled gratings or holographic gratings produced by the interference of light from two collimated beams or two point sources is also discussed, as is the case in which a plane of symmetry is present in the complete system.

Reconstruction of binary-data-page thick holograms for an arbitrarily oriented reference beam

Abstract: The readout of a page of binary data from a holographic memory utilizing a thick recording material is treated for arbitrary angular positioning of the reference beam The diffraction efficiency associated with each bit in the data page is calculated Histograms showing the distribution of the diffracted powers and thus the fidelity of reconstruction for example cases are presented

Optical recording of periodic structures in photopolymer layers

Abstract: The optimal conditions were determined for recording thin phase gratings in photopolymer materials based on the oligourethanacrylates. A substantial increase in the diffraction efficiency was obtained by selecting a preexposure with the optimal amount of energy. On the basis of the experimental material presented, a hypothesis is put forward of the existence of two processes which take place during recording periodic structures in photopolymers. These are the diffusion of the photosensitive initiator into the region of the photopolymer where the interference field is strongest and the preferential growth of radical chains in the direction of the photopolymer regions coinciding with the field minima.

High Efficiency Acousto-Optic Diffraction Into the Second Bragg Order

Abstract: Conventional Bragg diffraction devices operate in the first order with the light incident at the Bragg angle. Certain advantages would accrue, such as larger deflection angles and frequency shifts, if reasonably efficient operation c ould be obtained in the second order. In this comnunication we present both theoretical and experimental details of such a mode of operation with 70% efficiency and > 20% bandwidth. In 1976 Alferness’ analyzed the diffraction efficiency of thick holograms operating in the second order Bragg regime, with incidence angle equal to twice the Bragg angle. He concluded that 100% efficiency was possible for certain values of the hologram thickness. In so far as diffraction by a thick hologram is similar to acousto-optic diffraction by a rectangular sound column, a similar mode of operation ought to be possible for acousto-optic devices. Certain advantages would accrue from this such as a doubled frequency in frequency shifters and a doubled deflection angle in beam deflectors. this comnunication we report on such a mode of operation.

Investigation of anomalous acoustooptic interaction in lithium niobate crystal

Abstract: Expressions are derived for the calculation of the central frequencies f 1,2 0 , and the frequency band δf of wide-band anomalous acoustooptic interaction in crystals of arbitrary symmetry class. The effect of the difference between the ray and phase velocities of the diffracted light on the magnitude of f 1,2 0 and δf is pointed out. Results are presented of the computed acoustooptic quality factor M2, the frequencies f 1,2 0 , δf and the electromechanical coupling constant for the tangential field kD for cuts of LiNbO3 crystals ensuring maximum diffraction efficiency. It is shown that the maximum M2=22.10−18 cgs units in this crystal is attained in the case of anomalous light diffraction by the slow shear wave propagating in the Y-120‡ direction. To the same cut there corresponds the maximum KD=0.685. The design of anomalous acoustooptic deflectors developed is described. Experimental results are presented.