Showing papers on "Diffraction efficiency published in 1991"

Efficient optical elements to generate intensity weighted spot arrays: design and fabrication

Abstract: We report on the design and fabrication of fully 2-D surface relief diffraction elements that can split a single collimated beam into many beams in an arbitrary intensity distribution. These splitters were designed by computer using simulated annealing, and made into phase gratings by electron-beam lithography followed by plasma etching into quartz glass. Both two and four phase level gratings have been fabricated, allowing a wide range of uniform and weighted spot patterns to be generated. These grating elements have a measured diffraction efficiency of over 74%, with the beam intensity ratios accurate to within 1% of their target values.

Binary Optics Technology: Theoretical Limits on the Diffraction Efficiency of Multilevel Diffractive Optical Elements

Abstract: : Diffractive optical elements are being considered as potential solutions to a number of optical design problems that are difficult or impossible to solve with conventional refractive and reflective elements. Two unique characteristics of diffractive elements can be exploited; the first is teh dispersion property. Diffractive structures bend light rays of longer wavelengths more than those of shorter wavelengths, which is the reverse of refractive materials; therefore, diffractive structures minimize or eliminate the dispersive effects of refractive materials. The second unique characteristic is the relative ease with which arbitrary phase profiles can be implemented. Advances in both diamond turning technology and the use of semiconductor fabrication equipment have made possible the construction of a variety of diffractive elements. Diamond turning technology allows fabricating diffractive surfaces over large areas in a relatively short period of time. However, there are limitations: the phase profile has to be circularly symmetric, and the accuracy with which a diffractive profile can be made is dependent on the tip size of the diamond turning tool. Using semiconductor fabrication equipment to make diffractive elements has become a powerful technique. This particular approach produces a stepped approximation, referred to as a multilevel structure, to the ideal profile.

Polarization conversion from diffraction gratings

Abstract: If a surface-modulated diffraction grating is arranged so that the scattering vector does not lie in the plane of incidence of the illuminating radiation, then polarization conversion can occur. This conversion can be strongly enhanced if a surface mode, such as a surface-plasmon-polariton or guided mode, is excited at the grating surface. In this paper we develop a mathematical description of the polarization conversion process, using a method that transforms the grating surface into a planar system. The predicted polarization conversion is compared very favorably with data for surface-mode-enhanced conversion.

Maximized photorefractive holographic storage

Abstract: Recording angularly multiplexed holograms in photorefractive media requires an understanding of both the recording and erasure dynamics. In this paper a coupled‐wave analysis is used to describe both the recording and erasure processes. This analysis has been applied to the recording of multiplexed holograms, resulting in a procedure to record maximum diffraction efficiency holograms. Experimental results confirming the theory for both the dynamics of a single exposure and the efficiencies of multiple exposures are presented. Using an uncoupled recording wave analysis, an expression for the dependence of the diffraction efficiency on the number of exposures in the case of equal record/erase time constants is presented. Approximate results are presented for the case of unequal time constants. This serves to set an upper limit on the diffraction efficiencies for a given saturation index of refraction modulation.

Large photoinduced birefringence in an optically nonlinear polyester polymer

Abstract: A nonlinear optical polyester with disperse red side groups exhibited a stable photoinduced birefringence, from 0.14 at 850 nm to 0.21 at 633 nm, after exposure to linearly polarized short‐wavelength visible or ultraviolet light. Thin‐film wave plates, birefringent diffraction gratings and waveguides were written in this polymer using photoexposure. Thin‐ film wave plates of <1 μm thickness showed ≳π/4 phase delay and good temporal stability. A 9:1 diffraction efficiency ratio of the two polarization eigenmodes was measured from thin birefringent gratings. This ratio is in good agreement with the theoretical prediction obtained using a simple three‐level model.

Upper bound of the diffraction efficiency of diffractive phase elements

Abstract: An upper bound of the diffraction efficiency of diffractive elements that only influence the phase of the illumination wave is derived. The derivation only utilizes the specification of the desired diffraction pattern. It is independent of the technique to design and fabricate the diffractive element. The theory is based on the transmittance approach to describe the effect of the element on the illumination wave.

Optical modulation element having two-dimensional phase type diffraction grating

Abstract: An optical modulation element characterized in that a two-dimensional phase type diffraction gratings, whose refractive index changes for each predetermined period along two different directions in a predetermined plane, is adopted, and a changing state of the refractive index is controlled to change characteristics of the diffraction grating, so that a diffraction phenomenon of light incident on the element is controlled.

Volume phase hologram with liquid crystal in microvoids between fringes

Abstract: Composite holograms are disclosed wherein microvoids between the holographic interference fringes are filled with a liquid crystal. The diffraction efficiency and other holographic properties of such composite holograms may be varied by external stimuli, e.g., application of an electric field of thermal energy.

High sensitivity optical image processing device based on CdZnTe/ZnTe multiple quantum well structures

Abstract: We present results on the operation of a high sensitivity semi‐insulating multiple quantum well device for optical image processing. This device operates in the visible spectrum using II‐VI CdZnTe/ZnTe multiple quantum well structures. Incident light creates charge carriers that screen an applied ac electric field modulating the absorption and refractive index of the structure through the quantum confined Stark effect. In this way, an incident intensity pattern is recorded as an absorption and refractive index variation. The semi‐insulating nature of the material eliminates the need for pixelation. In a wave‐mixing experiment, a peak diffraction efficiency of 0.25% was observed from 2.25 μm active layer of the device. Two‐beam‐coupling gain coefficients of ∼500 cm−1 at wavelengths longer than the exciton absorption peak should be possible.

Degenerate four-wave mixing measurements of high order nonlinearities in semiconductors

Abstract: Degenerate four-wave mixing experiments on ZnSe and CdTe semiconductor samples with picosecond laser pulses at wavelengths below the bandgap are described. The authors identify the dominant nonlinearities in ZnSe and CdTe. They determine these to be fast third-order nonlinearities, due to the same processes which give rise to the effects of bound-electronic refraction and two-photon absorption, while higher order effects are due to free-carrier refraction. Measurements of the absolute magnitude of the combined third order susceptibilities are described. Studies of higher order effects due to free-carrier gratings are discussed. To obtain a quantitative measurement of the carrier induced nonlinearities, an expression for the diffraction efficiency of these carrier gratings was developed, and a value for the free-carrier refractive index coefficient in ZnSe was found. By measuring the angular dependence of the grating decay, the carrier diffusion coefficient was determined as a function of carrier density. >

Holographic transmission elements using improved photopolymer films

Abstract: Volume phase holographic gratings provide the high diffraction efficiency often required from holographic optical elements. In order to provide wide angular bandwidth, high index modulation is also required. A new addition to the family of Du Pont holographic photopolymer films has significantly higher index modulation for the grating spacings typical of holographic transmission elements. In this paper, the recording of volume phase transmission gratings in this holographic photopolymer is described. The dependence of holographic properties on exposure conditions, grating characteristics, and processing are discussed. The performance characteristics of holographic optical elements produced in this material are also presented.

Holographic interferometry using anisotropic self-diffraction in Bi(12)SiO(20).

Abstract: Anisotropic self-diffraction in photorefractive Bi12SiO20 crystals is applied to the investigation of dynamic holographic recording through the diffusion process. Specific attention is paid to optimization of experimental parameters for real-time holographic interferometry. The resultant interferometer is applied to obtain both time-average and double-exposure interferograms of vibrating and displaced structures.

Diffraction efficiency of volume holograms written by coupled beams.

Abstract: We describe volume holograms written in photorefractive crystals in which the writing beams experience two-beam coupling We obtain analytic expressions for the diffraction efficiency of a weak reading beam that has a different polarization from the writing beams Good agreement is obtained between experimental and theoretical results

Design of diffractive singlets for monochromatic imaging

Abstract: The Seidel aberrations of a rotationally-symmetric diffractive lens with an arbitrary phase profile are presented. It is shown that by a proper choice of phase function and aperture stop position, third-order coma and astigmatism can be eliminated for any chosen conjugate ratio. Since a diffractive lens has an inherent zero value for the Petzval sum, the image plane is flat in both tangential and sagittal meridians. The substrate curvature of the lens may be chosen to introduce a prescribed amount of distortion to allow for use as a Fourier transform lens or a laser scan lens. Examples are given of lens performance in finite conjugate imaging and laser scanning, where the f − θ condition is satisfied.

Re-imaging optical system employing refractive and diffractive optical elements

Abstract: A re-imaging optical system (36) is provided which utilizes a combination of refractive and diffractive optical elements. The system includes an objective lens group (38) for focusing an incoming beam of light to a first image plane. A relay lens group (40) refocuses the beam of light in a second image plane after it passes through the first image plane. The relay lens group (40) includes a diffractive optical element (48) for correcting aberrations introduced by the objective and relay lens groups. The diffractive optical element (48) eliminates a number of individual optical elements which would otherwise be required to correct aberrations, thereby reducing the total number of optical elements in the system. In the preferred embodiment, the diffractive optical element (48) comprises a binary optical element. The resulting system yields better image quality, better throughput, and a lighter, less bulky, re-imaging optical system (36).

Efficient multilevel phase holograms for CO 2 lasers

Abstract: Multilevel phase holograms for monochromatic radiation at a wavelength of 10.6 μm are recorded as surface relief gratings with multilevel discrete binary steps. Our experiments show that diffraction efficiencies close to 90% can be achieved both for transmissive and reflective elements. The reduction of efficiency due to errors in the depth and the width of the step levels is considered.

Transmission grating spectroscopy and the Advanced X-Ray Astrophysics Facility

Abstract: The use of transmission gratings with grazing-incidence telescopes in celestial x-ray astronomy is reviewed. The basic properties of transmission grating spectrometers and the use of "phased" gratings to enhance the diffraction efficiency are outlined. Special attention is given to the AXAF high-energy transmission grating (HETG) being fabricated at MIT. The HETG operates over the range 0.4 to 8 keV, gives resolving powers of 100 to 1000, effective areas of 10 to 300 cm2, and a minimum detectable line flux of 1 to 10 x 10-6 photons cm-2 s-1. The instrument consists of a single array with two types of membrane-supported grating facets: medium-energy gratings (0.6-μm period, 0.5-μm-thick silver) mounted behind the outer three AXAF mirrors, and high-energy gratings (0.2-μm period, 1.0-μm-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 mesh-supported grating facets of 1.0-μm period and is optimized for operation down to 80 eV. Gratings 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 line-emitting regions.

Extinction theorem analysis of diffraction anomalies in overcoated gratings

Abstract: A rigorous analysis based on the extinction theorem is presented to study anomalous resonant effects from single-layer- and multilayer-overcoated, low-efficiency diffraction gratings. Anomalously high diffraction efficiency at resonance results from the coupling of the incident beam into guided waves that can be propagated within the composite structure. This analysis incorporates a recursive or R-matrix propagation algorithm that is numerically stable, and the results agree favorably with both experimental and other theoretical research. Numerical results are presented to investigate the influence of certain parameters (i.e., groove depth and shape and the number of high- and low-index overlayers) on the diffraction efficiency at resonance. In the analysis a wavelength of 0.6328 μm and a grating period of 0.7 μm were chosen so that only a −1 diffracted order and the specular beam are reflected from the gratings. Perfect transfer of the grating relief to the film boundaries does not occur in all instances; it depends on the grating and film characteristics, together with the conditions during deposition. We investigate the effect of nonreplication of the grating profile at film interfaces on anomalous diffraction. For the cases studied, it is found that nonreplication has the effect of reducing the strength of the resonant outcoupling. Incident beams with a Gaussian intensity profile are also considered. This effect is accomplished by having multiple incident beams weighted such that their superposition forms a Gaussian envelope. Numerical results show the effect of guided-wave resonances for which distortion of the reflected envelopes is seen.

Diffraction-limited blazed reflection diffractive microlenses for oblique incidence fabricated by electron-beam lithography.

Abstract: Blazed reflection diffractive microlenses potentially have many uses. The fabrication accuracy and the optical characteristics of these microlenses can be greatly improved compared with transmission diffractive microlenses. The reflection microlenses for oblique incidence can be flexibly used without a beam splitter. The functions of the electron-beam writing system that we developed have been expanded so that the diffraction-limited microlenses for oblique incidence can be fabricated. It is demonstrated that the fabricated microlens exhibits diffraction-limited focusing characteristics with 78% high efficiency at a large oblique angle of 30°. These microlenses could be used as key devices in planar optics and optical interconnections.

Design of achromatized hybrid diffractive lens systems

Abstract: The design of two broadband hybrid diffrve-refrve optical systems a landscape type lens and a Schmidt telescope was investigated. The systems were achromatized using the large negative dispersion characteristic of kinoforms. In the scalar wave regime these strucwres can approach 100 efficiency for one object point and wavelength but efficiency inevitably decreases when these parameters change. We evaluated polychromatic image quality taking diffraction efficiency into account by constructing properly weighted geometric point spread functions from several diffracted orders and calculating modulation transfer functions. The MTFs of the hybrid achmmats were improved at high spatial frequencies but reduced at low frequencies because inefficiency caused diffrtion into non-design ders. 2.

Iterative techniques to integrate different optical functions in a diffractive phase element

Abstract: Diffractive optics allows the incorporation of several optical functions, e.g., wave shaping and focusing, in one element. A method suitable to calculate a diffractive phase element with this feature is described. Coding and quantization effects are analyzed. As an example an array generator with integrated focal power is designed.

Phase-locked sustainment of photorefractive holograms using phase conjugation

Abstract: A method for sustaining multiply exposed photorefractive holograms, in a phase-locked fashion, by using a pair of phase-conjugating mirrors is described. It is shown that a steady state exists where the overall diffraction efficiency is independent of the number of holographic exposures and the final holograms are exactly in phase with the initial ones. Both analytical and experimental results are presented.

Surface selectivity in four-wave mixing: transient gratings as a theoretical and experimental example

Abstract: A theoretical treatment of transient grating diffraction is derived for gratings that are spatially nonuniform in the direction perpendicular to the sample surface. This treatment is readily generalized to any four-wave mixing experiment. Both reflection and transmission geometries of diffraction are examined for the standard transient grating case, in which both grating excitation beams are incident upon the same side of the sample For samples in which the grating amplitude perpendicular to the sample surface varies slowly relative to the optical wavelength, the reflection geometry is shown to probe only the surface or the interface, while the transmission geometry probes the bulk of the sample. An experimental example using four transient grating geometries (two reflection, two transmission) is shown to yield significantly different temporal responses, illustrating the nature of the theoretical predictions. The sample is a thin molecular crystal upon a substrate Both electronic excitations (excitons) and wave-guided acoustic modes are generated and probed Distinct signals are obtained from the bulk, the crystal–substrate interface, and the free-crystal face. Model calculations are presented that illuminate the behavior of the experimental example.

Silicon microlenses for enhanced optical coupling to silicon focal planes

Abstract: The microlens array reported in this paper is for enhanced optical coupling of near IR photons into the edges of the Rockwell solid-state photomultiplier (SSPM). Here the edge-illumination is necessary to achieve the required quantum efficiency of near IR photons in the SSPM, a device which is capable of photon counting. SSPM detection requires a method of highly efficient optical coupling to the gain medium to concentrate light with a 100 fill factor where lenslets are centered with a precise 0.2 micrometers accuracy. The microlens array is designed for a center wavelength of 1.3 micrometers , 1975 micrometers pixel dimension and a speed of f/4, which results in the array being nearly diffraction limited. The smallest feature size is 0.9 micrometers for the 8-phase level devices. From this design, we have successfully fabricated an 8-phase- level SSPM microlens array which demonstrates a 0.2 micrometers alignment accuracy among all three mask levels. SEM studies of the microlens show a high-quality surface finish and near vertical sidewalls. Optical characterization demonstrates that the microlens array is diffraction- limited at the design speed and design wavelength, with diffraction efficiency higher than 84.

Extreme ultraviolet performance of a multilayer coated high density toroidal grating.

Abstract: The performance of a multilayer coated diffraction grating has been evaluated at EUV wavelengths both in terms of absolute efficiency and spectral resolution. The application of a ten-layer Ir/Si multilayer coating to a 3600-lines/mm blazed toroidal replica grating produced a factor of 9 enhancement in peak efficiency near the design wavelength approximately 30 nm in first order, without degrading its excellent quasistigmatic spectral resolution. The measured EUV efficiency peaked at 3.3% and was improved over the full spectral range between 25 and 35 nm compared with the premultilayer replica which had a standard gold coating. In addition, the grating's spectral resolution of >5000 was maintained.

Lippmann–Bragg broadband holographic mirrors

Abstract: Lippmann–Bragg broadband volume holographic mirrors, with chirp normal to the surface, represent an entirely new class of grating. These gratings are presented and analyzed theoretically by using a combination of the multiple-beam interference method and Kogelnik’s local solution for uniform gratings. Particularly noteworthy is the new grating’s combination of a very high Bragg diffraction efficiency (>99.5%) with a large tunable bandwidth (from 5 to >300 nm).

Multilayer coated gratings for x-ray diffraction: differential theory

Abstract: A rigorous electromagnetic analysis is developed for metallic gratings covered by a stack of pairs of dielectrics and absorbers. The thicknesses of the layers can be arbitrary. In particular, they can be much thinner than the groove depth, which permits the optimization of grating efficiencies for the soft-x-ray range. The non-scalar behavior of such gratings is established for high angles of incidence. Examples of performances under near-normal and grazing incidences are given.

Picosecond separation and measurement of coexisting photorefractive, bound-electronic, and free-carrier grating dynamics in GaAs.

Abstract: A novel transient-grating geometry, which is nondegenerate, copropagating, phase matched, and polarization sensitive, is used to isolate and measure independently the ultrafast dynamics of multiple coexisting gratings in GaAs:EL2 with a temporal resolution of <5 psec. This technique permits the measurement of the evolution of the photorefractive grating in materials with zinc blende symmetry, where the photorefractive grating is usually obscured by the stronger free-carrier and instantaneous bound-electronic gratings.