Showing papers on "Diffraction efficiency published in 1990"

Guided-mode resonances in planar dielectric-layer diffraction gratings

Abstract: The guided-mode resonance behavior of the evanescent and propagating fields associated with an unslanted, planar diffraction grating is studied by means of the rigorous coupled-wave theory. For weakly modulated gratings, the condition on the guided-mode wave number of the corresponding unmodulated dielectric-layer waveguide may be used to predict the range of the incident angle or wavelength within which the resonances can be excited. Furthermore, the locations of the resonances are predicted approximately by the eigenvalue equation of the waveguide. As the modulation amplitude increases, the location and shape of the resonances are described in detail by the rigorous coupled-wave theory. The results presented demonstrate that the resonances can cause rapid variations in the intensity of the external propagating diffracted waves.

Diffractive optical elements: iterative calculation of quantized, blazed phase structures

Abstract: A procedure to calculate a highly quantized, blazed phase structure is presented. Characteristics that are concentrated on are a high diffraction efficiency and a large signal-to-noise ratio. The calculation techniques are based on iterative Fourier-transform algorithms. Stagnation problems are discussed, and methods to overcome them are described.

Two-dimensional array of diffractive microlenses fabricated by thin film deposition

Abstract: A 2-D array of 10 x 10 diffractive lenslets was fabricated and tested Each lenslet has a rectangular aperture and a size of 15 mm x 15 mm The focal length of each lenslet is 47 mm The array was produced by depositing thin films of silicon monoxide on a quartz glass substrate and by using photolithographic techniques The performance of the lenslets is based on the diffraction of light at a Fresnel zone plate (FZP) The FZP pattern was implemented as a phase structure with eight discrete levels The diffraction efficiency was measured to be 91%

Photorefractive gratings in the organic crystal 2-cyclooctylamino-5-nitropyridine doped with 7,7,8,8-tetracyanoquinodimethane

Abstract: The photorefractive properties of single crystals of the organic material 2-cyclooctylamino-5-nitropyridine doped with 7,7,8,8-tetracyanoquinodimethane are investigated with two-wave mixing experiments. Refractive-index gratings with an amplitude of up to 1.8 × 10−6 could be recorded for grating vectors along the twofold axis. The phase gratings showed a phase shift of between 20° and 70° with respect to the intensity grating. An experimental procedure for determining phases and amplitudes of absorption and phase gratings is discussed in detail.

Aztec surface-relief volume diffractive structure

Abstract: Surface-relief diffractive optical elements made interferometrically typically are recorded with the off-axis technique, for which the fringe maxima lie on planes that are predominantly perpendicular to the recording surface and with an intensity variation that is sinusoidal. Such a structure can be readily replicated by mechanical means. Volume diffractive elements, on the other hand, which result from beams propagating in opposite directions, have fringe planes that are predominantly parallel to the surface and, as such, cannot be mechanically replicated. A new type of surface-relief diffractive structure called Aztec is discussed here; it combines features of both off-axis and volume recording geometries, with the result being a phase-quantized, or terraced, surface-relief pattern. The groove profile, instead of being sinusoidal, resembles a stepped pyramid. This structure has been replicated by metal mastering and molding into plastic in the same manner as conventional embossed surface-relief elements, but the diffraction characteristics are typical of volume phase reflective structures. Light of a given wavelength is resonantly diffracted from steps that are a half-wavelength apart and with a bandwidth that is inversely proportional to the number of steps. Color control has been achieved by overcoating the step structure with a clear dielectric that shifts the resonant wavelength to a new value, depending on the index of refraction of the dielectric. Information content is less for the single-layered, but stepped, Aztec structure than for the usual multilayered volume diffractive element. Deep Aztec stepped gratings have also been fabricated by optical lithography, using multiple-mask techniques.

Three-dimensional (vector) rigorous coupled-wave analysis of anisotropic grating diffraction

Abstract: The rigorous coupled-wave analysis of diffraction by grating(s) formed in general anisotropic media is reviewed and extended. The method is first applied to a single slanted phase and/or amplitude grating with general three-dimensional incidence of a plane wave. The regions external to the grating can be isotropic, uniaxial, or biaxial anisotropic. The cases of gratings in isotropic media and of the grating vector lying in the plane of incidence (scalar analysis) are obtained as limiting cases of this general analysis. Coupling between the two orthogonal polarizations vanishes in these limiting cases. The Bragg conditions for various combinations of ordinary (for isotropic and uniaxial) and extraordinary (for uniaxial) polarized waves are quantified. The analysis is then extended to multiple cascaded gratings and to volume-superposed gratings. Sample calculations are presented for single anisotropic gratings (a lithium niobate photorefractive hologram in air and an interdigitated-electrode-induced grating in an electro-optic crystal), for multiple cascaded gratings (a lithium niobate hologram with grating strength varying with thickness), and for superposed gratings (multiplexed hologram storage). Applications for this analysis include optical storage, switching, modulation, deflection, optical interconnects, beam splitting, beam combining, and data processing.

Array generation with multilevel phase gratings

Abstract: The efficiency of phase gratings used for array illuminators can be improved by increasing the number of phase levels in computer-generated diffraction gratings. This is of interest to provide power to two-dimensional arrays of optical logic devices that are used for optical computing purposes. The theory and fabrication techniques are described, and the experimental performance of a four-level grating that produces a 5 × 5 array is presented.

Laser beam lithographed micro-Fresnel lenses

Abstract: Laser beam lithography for micro-Fresnel lenses (MFLs) with blazed grooves is proposed and demonstrated including the system configuration and characteristics of the resulting lenses. The resolution is even better than that of electron-beam lithography in forming 1-μm deep relief gratings in resist. A laser beam lithographed MFL with a diameter as large as 9.6 mm is described as well as a compact MFL (N.A. 0.21) butt coupled to an optical waveguide. In these two distinct MFLs, nearly diffraction-limited spot sizes have been obtained with diffraction efficiencies of 50% or more. A specific MFL array used for an integrated optic laser Doppler velocimeter is also presented.

Design of binary-phase and multiphase Fourier gratings for array generation

Abstract: A review and comparison of design procedures for binary-phase and multiphase Fourier gratings used as array generators is presented. Grating structures include one- and two-dimensional binary-phase Dammann gratings, general binary-phase gratings, quarternary-phase Dammann gratings, and kinoforms for which coherent and incoherent designs are considered. One coherent method of design is that due to Dammann, which involves the solution of a set of N nonlinear equations in N unknowns. Although Dammann’s method generates little error, it does not permit the explicit maximization of diffraction efficiency. To increase diffraction efficiency, Dammann’s method is modified such that diffraction efficiency is a design parameter. To ensure the existence of a grating that has high diffraction and generates the desired source array, the number of grating parameters are increased, and an incoherent design is considered. Simulated annealing is applied to the solution of this problem. Examples of grating design using the different techniques are presented.

Grating lens and focusing grating coupler

Abstract: A grating lens is disclosed in which diffraction efficiency varies in the focusing direction of light on a surface of the lens. Also disclosed is a focusing grating coupler in which the diffraction efficiency of a grating formed on an optical waveguide, which is formed on a substrate, varies in the focusing direction of light.

Boundary-element analysis of plane-wave diffraction from groove-type dielectric and metallic gratings

Abstract: A numerical approach based on the boundary-element method (BEM) is described for the analysis of plane-wave diffraction from groove-type gratings. First, the diffraction problem is exactly analyzed as a two-medium boundary-value problem. Further, for metallic gratings, a simple method in which an approximate boundary condition using the surface impedance is combined with the BEM is proposed. Both cases of the TE- and TM-wave incidences are systematically formulated. Numerical examples are presented for dielectric holographic gratings and metallic Fourier gratings, and the validity of the BEM and the effectiveness of the surface-impedance approximation are confirmed.

Diffraction efficiency of binary optical elements

Abstract: We examine the role of processing errors on diffraction efficiency of binary optical elements and the validity of the Fourier model to predict diffraction efficiency We show that mask alignment error can significantly degrade efficiency Models based on the Fourier theory can adequately predict both the magnitude of diffraction efficiency and its sensitivity to processing errors for optically slow elements (f/b) For optically fast elements (

Diffraction efficiency of analog and quantized digital amplitude holograms: analysis and manipulation

Abstract: The diffraction efficiency η of analog and quantized amplitude holograms is treated. Possible ways to manipulate η are discussed in terms of utilization of different kinds of freedom in the reconstructed field. Iterative methods to increase η are suggested, and experimental results are presented. The methods are based on iterative Fouriertransform algorithms. Results of iterative and noniterative quantization techniques are compared with respect to their dependence on the original analog hologram.

Diffraction Efficiency Of Strong Volume Holograms

Abstract: We investigate the diffraction efficiency of strong volume holograms in which the coupling parameter is several times that needed for maximum diffraction efficiency. We discuss the implications of our findings on photorefractive implementations of various neural network systems.

High-frequency acousto-optic mode locker for picosecond pulse generation

Abstract: We modeled, designed, and built a 500-MHz acousto-optic mode locker with a diffraction efficiency of 28% per 1 W drive power. The transducer is zinc oxide sputtered onto a sapphire substrate. A new figure of merit is defined for the mode-locker design, which indicates that sapphire is a good substrate material. Pulse widths of less than 10 psec with an average power of 150 mW were achieved from a 500-MHz pulse-rate, diode-pumped, cw mode-locked Nd:YLF laser using a pump power of 700 mW.

Holographic image storage in iron-doped lithium niobate fibers

Abstract: Holographic recording on a real‐time basis of two‐dimensional images in an iron‐doped lithium niobate fiber with a 200 μm diameter and 3 mm length is demonstrated for the first time. The erasure time constant as a function of readout beam power, the diffraction efficiency as a function of incident reference angle, and the results of angular‐multiple recording of two images are shown. These results suggest that the LN fiber could replace bulk materials for holographic recordings in optical parallel processing.

X-ray Bragg diffraction on periodic surface gratings

Abstract: We report on the X-ray Bragg diffraction analysis of periodic surface gratings on (100) GaAs crystals. We show that the X-ray Bragg diffraction is a very powerful method to investigate ultrafine surface gratings and allow us to determine not only the grating period but, in particular, the width of the grooves and to characterize the quality of the sidewalls, which is not easily possible with optical methods. The X-ray diffraction on surface crystal gratings is analogue to the Fraunhofer diffraction of multiple slits or reflection gratings.

Theory of complementary holograms arising from electron–hole transport in photorefractive media

Abstract: The time evolution of space-charge field gratings in photorefractive media is derived for a simple two-level electron–hole-transport model. At one level, charge transfer occurs by electron transport, and at the other, by hole transport, thus leading to the formation of complementary gratings. If the characteristic lifetimes of the gratings differ significantly, extended-lifetime gratings are possible. The role of the applied electric field in the evolution of such gratings is examined, and limiting cases are considered. The results of the derivation are compared with examples of complementary gratings in the literature.

Analysis of dielectric lamellar gratings coated with anisotropic layers

Abstract: The properties of a dielectric grating coated with an anisotropic layer are studied by using our previous analyses [ J. Opt. Soc. Am.71, 811– 818 ( 1981); IEEE Trans. Microwave Theory Tech.MTT-35, 937– 945 ( 1987)] for anisotropic dielectric gratings. Two peculiar properties that are due to the anisotropy of the material are shown. One is the enhancement of the change in polarization of the incident wave by the Bragg conditions for anisotropic gratings, which occurs in relatively deep gratings. The other is the anisotropic grating anomaly that is due to the excitation of the surface waves, which occurs in shallow gratings.

Diffraction properties of fixed gratings in photorefractive media

Abstract: The diffraction properties of fixed volume gratings, including the effect of additional photorefractive energy coupling between the incident wave and the diffracted wave, are considered. Both transmission gratings and reflection gratings are treated. Analytical solutions for phase gratings are derived, and numerical solutions for absorption gratings are also obtained. The results are presented and discussed.

Achromatic waveguide lenses

Abstract: A lens in a waveguide of an integrated optical circuit is achromatized (corrected for chromatic dispersion which varies the focal length of the lens with wavelength) by forming the lens as a hybrid mode-index/diffractive doublet. The chromatic dispersion of the diffractive element cancels the dispersion of the refractive element (the mode-index lens) such that the derivative of the focal length goes to zero at the design wavelength, and significantly reduces chromatic dispersion over a substantial wavelength range. For example, a 10 mm focal length, F/5 hybrid mode-index/diffractive lens may be provided with a useable wavelength range of approximately 80 mm for typical waveguide materials.

Effects of beam focusing on the efficiency of planar waveguide grating couplers.

Abstract: Results of a theoretical and experimental investigation into the variation of coupling efficiency with coupling angle are presented for various beam focusing conditions for an integrated optical grating coupler. This investigation shows that the acceptance angle of the grating coupler can be broadened, within a relatively large range and with a relatively small loss of coupling efficiency, by focusing the incident laser beam.

Spectral hole burning and holography. II, Diffraction properties of two spectrally adjacent holograms

Abstract: The first paper in this series presented a detailed analysis of the diffraction efficiency of a single holographic grating obtained by spectral hole burning. This paper covers the influence of phase changes which occur during the burning of a single hologram, and the interaction of spectrally adjacent two laser induced gratings. A theoretical description of the holographic efficiencies which takes into account the influence of the relative phases between the two gratings. Experimental results are presented and discussed in terms of the theory.

Designing holographic lenses with different recording and readout wavelengths

Abstract: A method for designing and recording transmission holographic lenses, having low aberrations and high diffraction efficiencies, in the presence of a recording–readout wavelength shift, is presented. The method is based on a recursive design technique, in which the final hologram is recorded with complex wave fronts that are derived from intermediate holograms. The design is illustrated for a lens with an f-number of 2.5 and a large offset angle, recorded at 488 nm and read out at 633 nm. A nearly diffraction-limited spot size and an efficiency of >80% are measured.

Photorefractive integrated-optical switch arrays in LiNbO 3

Abstract: A new optical switch design is reported that consists of two sets of parallel channel waveguides intersecting at right angles. Arrays based on Ti:LiNbO3 technology, with waveguide intersections iron doped to provide photorefractive sensitivity, have been fabricated. Interfering guided beams are used to write index gratings at the waveguide intersections, which in turn diffract a portion of an incident signal beam into the crossing waveguide. Arrays ranging from 15 × 15 up to 50 × 50 waveguides are constructed in an active area of 5 mm × 5 mm. Diffraction efficiency and writing-time measurements are reported along with a proposal for a practical architecture.

Monolithic diffraction spectrometer

Abstract: A monolithic diffraction spectrometer having a diffraction grating formed over a light sensing array is provided. The diffraction grating serves to diffract wavelengths of interest to an underlying photosensitive device while diffracting other wavelengths away from the photosensing element. By forming a diffraction grating with a variable pitch, or multiple diffraction gratings having various pitches, any number of specific light wavelengths can be detected with a high degree of precision. When a diffraction grating having a pitch which is in the order of the incident wavelength of light is used, improved sensitivity is achieved.

High-speed, low-power optical phase conjugation using a hybrid amorphous silicon/ferroelectric-liquid-crystal device

Abstract: Optical phase conjungation using an amorphous silicon/ferroelectric-liquid-crystal light modulator shows an optical response of 100 microsec with a diffraction efficiency of 8.5% for an incident optical intensity of 1 mW/cm(2).

Multiple-scattering theory of wave diffraction by superposed volume gratings

Abstract: We present a powerful multiple-scattering approach to wave diffraction by media whose permittivity contains overlapping periodic modulations, such as those found in thick holograms or in other applications involving superposed volume gratings. For this purpose we consider sequential wave scattering in a planar model having two periodic variations that are inclined at an arbitrary angle Δϕ with respect to each other. We thus show that all the diffracted fields can be described by flow diagrams that provide physical insight into the wave-scattering process. We then examine the particularly relevant case of small angular separations Δϕ and find that the individual diffracted orders can be evaluated by applying simple flow-graph considerations to the mathematical formulation. This procedure readily provides accurate numerical results, together with an estimate of the errors incurred if simplifying approximations are introduced. Furthermore, we show that the two-grating model can be readily extended to situations having any number of superposed periodicities.

Multiplexed substrate-mode holograms

Abstract: We present a simplified model and experimental verification for multiplexed substrate-mode volume holograms. The analysis is based on coupled-wave theory for uncoupled S- and P-polarization states of the incident beam. The theory is used to investigate a practical situation with the reconstruction beam normally incident upon the substrate plane and the diffracted multimode beams propagating in orthogonal directions within the substrate. Model results are then used to optimize the performance of experimental holograms formed in dichromated gelatin. The resultant multiplexed grating splits the incident beam into four orthogonal beams with nearly equal intensities, for a total efficiency of 80%.

Intraplane guided wave massive fanout optical interconnections

Abstract: One‐to‐30 guided wave optical interconnections are demonstrated at 632.8 nm using a highly multiplexed waveguide volume hologram. This technology is capable of providing intrachip and intrawafer optical interconnections. The theoretical limit of the fanout number is addressed and experimentally confirmed. The measured data show that the diffracted beams have an average diffraction efficiency of 2.3% with ±0.2% variation. The demonstrated results can save surface space of electronic chips and also provide a large fanout capability due to the high index modulation of the volume hologram. Further applications based on this technology are very promising. Head‐up display, high‐speed optical data bus, surface enhanced Raman spectrometer, and optical sensors are some of the attractive ones.