Showing papers on "Diffraction efficiency published in 1998"

Non-volatile holographic storage in doubly doped lithium niobate crystals

Abstract: Photorefractive materials are being widely investigated for applications in holographic data storage1. Inhomogeneous illumination of these materials with an optical interference pattern redistributes charge, builds up internal electric fields and so changes the refractive index. Subsequent homogeneous illumination results in light diffraction and reconstructs the information encoded in the original interference pattern. A range of inorganic and organic photorefractive materials are known2, in which thousands of holograms of high fidelity can be efficiently stored, reconstructed and erased. But there remains a problem with volatility: the read-out process usually erases the stored information and amplifies the scattered light. Several techniques for ‘fixing’ holograms have been developed3,4,5,6, but they have practical disadvantages and only laboratory demonstrators have been built7,8,9,10. Here we describe a resolution to the problem of volatility that should lead to the realization of a more practical system. We use crystals of lithium niobate — available both in large size and with excellent homogeneity — that have been doped with two different deep electron traps (iron and manganese). Illumination of the crystals with incoherent ultraviolet light during the recording process permits the storage of data (a red-light interference pattern) that can be subsequently read, in the absence of ultraviolet light, without erasure. Our crystals show up to 32 per cent diffraction efficiency, rapid optical erasure of the stored data is possible using ultraviolet light, and light scattering is effectively prevented.

Blazed binary subwavelength gratings with efficiencies larger than those of conventional échelette gratings

Abstract: We introduce a new structural cutoff beyond which subwavelength gratings cease to behave as homogeneous media and discuss its effects on the proper selection of the sampling periods of subwavelength diffractive elements. According to this analysis, a 3?-period blazed binary grating composed of square pillars is designed for He–Ne operation and is fabricated by etching of a TiO2 layer deposited upon a glass substrate. Its first-order measured diffraction efficiency is 12% larger than the theoretical efficiency of an ideal blazed echelette grating in glass with the same period.

Optical heterodyne detection of laser-induced gratings

Abstract: A novel optical arrangement for heterodyne detection of laser-induced gratings based on the use of a phase mask for both excitation and probe beams provides phase stability and control without the need for an active stabilization scheme. The arrangement greatly simplifies the laser-induced grating experiment. The performance of the technique in both transmission and reflection geometries is illustrated through measurements of bulk and surface acoustic waves generated by picosecond laser pulses.

Diffractive optical element and optical system having the same

Abstract: A diffractive optical element includes a plurality of laminated diffraction grating surfaces. Each of the diffraction grating surfaces is formed to have a sufficiently small grating thickness as compared with a grating pitch thereof.

Infrared Photorefractive Polymers and Their Applications for Imaging

Abstract: Photorefractive polymers with high diffraction efficiency in the visible and near-infrared regions of the electromagnetic spectrum have been developed. These polymers, which have a large dynamic range because of their high orientational birefringence, incorporate a dye designed to have a large dipole moment and a high linear polarizability anisotropy. Such polymers have enabled demonstrations of imaging through scattering media, using a holographic time-gating technique at a wavelength that is compatible with the transparency of biological tissues and with the emission of low-cost semiconductor laser diodes.

Normal-incidence guided-mode resonant grating filters: design and experimental demonstration.

Abstract: Guided-mode resonant grating filters have numerous applications. However, in weakly modulated gratings designed for use at normal incidence, the filtering resonance of these subwavelength-period devices splits for angles of incidence that are even slightly off normal incidence. Strongly modulated gratings are designed that essentially overcome this practical problem near normal incidence. In addition, these gratings can have, by design, either broad or narrow spectral characteristics. An experimental demonstration (1.5-2.0-mu m wavelength range) of such a normal-incidence guided-mode resonant silicon grating upon a sapphire substrate is presented. The measured reflection resonance had a FWHM of 67-100 nm for angles of incidence of 0-8 degrees and peak efficiency of ~80% .

High-efficiency subwavelength diffractive element patterned in a high-refractive-index material for 633 nm.

Abstract: We propose the use of high-index materials for the fabrication of subwavelength diffractive components operating in the visible domain. This approach yields a reduction of fabrication constraints and an improvement of theoretical performance. A blazed grating with subwavelength binary features and with a period of 5.75 wavelengths is designed and fabricated in a TiO2 layer coated upon a glass substrate. The first-order diffraction efficiency measured with a He–Ne laser beam is 83%, which is slightly larger than that achieved theoretically by the best standard (continuous profile) blazed grating fabricated in glass with the same period.

Atom Optics

Abstract: In the field of m m optics, re~earclir~s are attempting to realize the same optical elements for atomic beams ~f is M " D ~ in the convenliond optics, Many laborstories around the world have made beautiful demonstratiom ofdevices such as mirrors, lenses. diffraction gratings, wave guides, resonators and interferometers for atoms. The applications oC these new techniques mnge from new experiment.; probing the ivaw nature of matter to the possibility of wil ing nsnosfrucfvres on surfaces. This tutorial will give a general overview ofthe lield as it now Presider: F. Trager, Universilaf Kasse/, Kasie/, GERMANY 08.30 QMA1 (Invited) DOCHART

Analyses of the Diffraction Efficiencies, Birefringence, and Surface Relief Gratings on Azobenzene-Containing Polymer Films

Abstract: Dynamical experimental studies on the diffraction efficiencies and the formation of birefringence grating and surface relief grating on doped and/or covalently bonded azobenzene derivatives containing polymer films were carried out using laser beams with different polarizations. From polarization analyses of the first-order (±1) diffracted beams, the contributions to the diffraction efficiency are separated into an anisotropic (or birefringence) part and a surface relief part. During the growth of the gratings the dynamical responses of both contributions appear to be quite distinct, and estimates of the time variations of the anisotropic phase shift, Δφ, due to the induced birefringence and of the surface relief height, 2Δd, due to the polymer mass transport are obtained. Calculations and simulations of the theoretical expressions allow us to confirm the experimental findings and to reproduce all the observed polarized first-order diffraction curves with good agreement, even when the surface relief is im...

As 2 S 3 photosensitivity by two-photon absorption: holographic gratings and self-written channel waveguides

Abstract: We observed two-photon-induced refractive-index changes in As2S3 by exposure in the 800-nm region. We studied this photosensitivity by writing interference gratings on a 2-μm As2S3 thin film. This property is the key to creating self-written channel waveguides in a planar As2S3 slab.

Recording of holographic diffraction gratings in photopolymers : theoretical modelling and real-time monitoring of grating growth

Abstract: Recording of holographic volume diffraction gratings in Du Pont's photopolymer HRS-150 is studied theoretically and experimentally. Particular attention is paid to the dynamics of the recording process. The temporal evolution of holographic gratings is monitored for a range of illumination intensities in two ways: (i) by means of a multiple-exposure approach when intensity profiles of recording beams are uniform, many holographic exposures are necessary and each exposure is carried out for a particular value of the total illumination intensity; (ii) by means of a single-exposure approach when intensity profiles of recording beams are strongly Gaussian and a volume grating with spatially distributed diffraction efficiency arises, depending on a local (average) value of the total illumination intensity. The second approach proves to be a useful tool providing us very quickly with qualitative information about the dynamics of the recording process while the first one, which is much more time-consumi...

High-density disk storage by multiplexed microholograms

Abstract: We propose a concept of holographic data storage which promises storage capacities of more than 100-Gbyte on a digital versatile disk (DVD)-sized disk. The information is stored bitwise in form of microscopic reflection holograms. High storage densities can be achieved by combining multiplexing methods and multilayer storage. A theoretical model for microscopic reflection holograms generated by focused Gaussian beams is proposed. Experimental results are presented for the recording and characterization of microholograms in DuPont's HRF-800 photopolymers. The local distribution of the diffraction efficiency was investigated by applying a confocal scanning microscope setup. Single-color holograms with a radius of 1.8 and 12 /spl mu/m depth have been recorded. We observe a blue shift in the spectral response of the microholograms of less than 2% due to shrinkage of the polymer. In the case of threefold wavelength multiplexing, all wavelengths are clearly resolved in the spectral response having spectral width /spl Delta//spl lambda/ of less than 10 nm (FWHM). Baking the holograms for 1 h at 120/spl deg/C nearly doubted the diffraction efficiency while the spectral response of the microholograms broadened by a factor two. We showed that 80-ps pulses are sufficient for holographic recording in the photopolymers. An optimized pre-illumination allows a significant increase in diffraction efficiency.

Development of microelectromechanical variable blaze gratings

Abstract: Two types of microelectromechanical variable blaze gratings (VBGs) have been designed, modeled, fabricated, and tested. The gratings operate by adjusting the blaze angle of each slat so specular reflection of the incident light matches a particular grating diffraction order. The VBG blaze angle is adjustable with either electrostatic or thermal actuators. VBGs direct incident light in discrete directions, and are useful for steering light with beam diameters greater than 1 mm and power levels greater than 1 W. Both electrostatically and thermally actuated VBGs have been constructed from gold and polysilicon using a surface-micromachining process and tested with a 20 mW continuous wave HeNe laser operating at a wavelength of 632.8 nm. The diffraction efficiency and far-field pattern have been modeled and measured. Drive voltages for both types of gratings are measured as a function of blaze angle and selected diffraction order.

Description of Diffraction Grating Behavior in Direction Cosine Space

Abstract: It is well known that the angular separation of non-paraxial diffracted orders from a linear grating varies drastically with incident angle. Furthermore, for oblique incident angles (conical diffraction), it is rather cumbersome both analytically and graphically to describe the number and angular position of the various propagating orders. One can readily demonstrate that wide-angle diffraction phenomena (including conical diffraction from gratings) are shift-invariant with respect to incident angle in direction cosine space. Only when the grating equation is expressed in terms of the direction cosines of the propagation vectors of the incident beam and the diffracted orders can we apply the Fourier techniques resulting from linear systems theory. This formulation has proven extremely useful for small-angle diffraction phenomena and in modern, image formation theory. New insight and an intuitive understanding of diffraction grating behavior results from a simple direction cosine diagram.

Convex grating types for concentric imaging spectrometers.

Abstract: The properties of convex gratings fabricated by electron-beam lithography are investigated Three grating types are shown The first is a single-panel, true blazed grating in which the blaze angle stays constant relative to the local surface normal This grating provides high peak efficiencies of approximately 88% in the first order and 85% in the second order The second grating has two concentric panels, with each panel blazed at a different angle This type permits flexibility in matching the grating response to a desired form The third type has a groove shape that departs from the sawtooth blazed profile to increase the second-order bandwidth All these types are difficult or impossible to produce with conventional techniques The gratings compare favorably with conventional (holographic and ruled) types in terms of efficiency and scatter Simple scalar models are shown to predict the wavelength response accurately These gratings allow the optical designer to realize fully the considerable advantages of concentric spectrometer forms

Analysis and optimization of fabrication of continuous-relief diffractive optical elements

Abstract: The fabrication of continuous-relief diffractive optical elements by direct laser beam writing in photoresist is analyzed. The main limitation and tolerances are identified, and their influence on optical performance is quantified. Fabricated structures show rounded profile steps resulting from the convolution of the desired profile with the writing beam. This leads to a reduction in diffraction efficiency. Optimization techniques are presented to minimize this effect. Scaling the profile depth by a factor of μ > 1 increases the first-order diffraction efficiency for blazed elements. This method is also applied to suppress the zeroth diffraction order in computer-generated holograms. A nonlinear compensation of the exposure data for the Gaussian beam convolution results in an 18% increase of the diffraction efficiency for a blazed grating with a 10-μm period to a value of 79%.

Design of a diffractive optical element for wide spectral bandwidth.

Abstract: A new method for eliminating the chromatic aberration of a diffractive optical element (DOE) for wideband wavelengths is presented. The wideband-wavelength diffractive optical element (WBDOE) consists of two aligned DOE's. The use of different dispersive materials for the two DOE's eliminates chromatic aberration. The design and simulation of a WBDOE for the visible spectrum are presented.

Electromagnetic analysis of diffraction gratings by the finite-difference time-domain method

Abstract: Diffraction gratings with feature sizes comparable to the wavelength are analyzed with a finite-difference time-domain method, which is a unique approach to electromagnetic problems in the time domain. The diffraction efficiencies obtained are in good agreement with other commonly used numerical methods in the frequency domain. As a further application, diffraction problems with pulsed light are also investigated.

Diffraction property of an axicon in oblique illumination

Abstract: The intensity distribution of a diffraction pattern in obliqueillumination is derived analytically. Experimental results arecompared with the results of theoretical analyses. Good agreementbetween them has been obtained.

Vector-based synthesis of finite aperiodic subwavelength diffractive optical elements

Abstract: We present an optimization-based synthesis algorithm for the design of diffractive optical elements (DOE’s) that are finite in extent, have subwavelength features, and are aperiodic. The subwavelength nature of the DOE’s precludes the use of scalar diffraction theory, and their finite extent and aperiodic nature prevents the use of coupled-wave analysis. To overcome these limitations, we apply the boundary element method (BEM) as the propagation model in the synthesis algorithm. However, the computational costs associated with the conventional implementation of the BEM prevent the design of realistic DOE’s in reasonable time frames. Consequently, an alternative formulation of the BEM that exploits DOE symmetry is developed and implemented on a parallel computer. Designs of finite extent, subwavelength, and aperiodic DOE’s, such as a lens and a focusing beam splitter, are presented.

Design of diffractive optical elements for multiple wavelengths

Abstract: A method for producing diffractive optical elements (DOE’s) for multiple wavelengths without chromatic aberration is described. These DOE’s can be designed for any distinct wavelength. The DOE’s are produced from two different optical materials, taking advantage of their different refractive indices and dispersions.

Laser direct-write gray-level mask and one-step etching for diffractive microlens fabrication

Abstract: High-efficiency diffractive optical elements can be achieved by an increase in the number of phase levels. We present a technique for laser direct-write gray-level masks on high-energy-beam-sensitive glass and one-step etching on the gray-level mask plate for the production of high-efficiency diffractive optical elements. Sixteen-phase-level diffractive microlenses and microlens arrays with a focusing efficiency of approximately 94% have been realized by use of the one-step nonphotolithographic fabrication technique.

Scalar integral diffraction methods: unification, accuracy, and comparison with a rigorous boundary element method with application to diffractive cylindrical lenses

Abstract: Various integral diffraction methods are systematically unified into a single framework, clearly illustrating the interconnections among the numerous scalar and rigorous formulations. This hierarchical depiction of the integral methods makes clear the specific approximations inherent in each integral method. The scalar methods are compared in detail with a rigorous open-region formulation of the boundary element method (BEM). The rigorous BEM provides a reference method for accurately determining the diffracted fields for both TE and TM incidence. The rigorous BEM and the various scalar methods are then applied to the case of focusing of normally incident plane waves by diffractive cylindrical lenses with f-numbers ranging from f/2 to f/0.5. From the diffracted-field calculations, a number of performance metrics are determined including focal spot size, diffraction efficiency, reflected and transmitted powers, and focal-plane sidelobe power. The quantitative evaluation of the performance of the scalar methods with these metrics allows the establishment, for the first time, of the region of validity of the various scalar methods for this application. As expected, the accuracy of the scalar methods decreases as the f-number of the diffractive lenses is reduced. Additionally, some metrics, particularly the focal-plane sidelobe power, appear to be particularly sensitive to the approximations in the scalar methods, and as a result their accuracy is significantly degraded.

Photorefractive Bragg diffraction in high- and low-molar-mass liquid crystal mixtures

Abstract: Orientational photorefractive Bragg diffraction is observed in high- and low-molar-mass liquid crystal mixtures doped with fullerene (C60). These novel materials are in a nematic phase without phase separating. In the functionalized materials, we observe a high two-beam coupling gain coefficient (Γ=75 cm−1) with a low applied field of 4 V/μm, low total losses (32 cm−1 including scattering, reflection, and absorption), a high diffraction efficiency (9%), and response time of 200 ms.

Phase mask with spatially variable diffraction efficiency

Abstract: A phase mask (15) for modulating a collimated light beam passing therethrough, the light beam being diffracted to photoinduce a refractive index profile in a photosensitive optical medium, the phase mask (15) comprises a substrate (17) having an outer surface provided with a plurality of parallel grating corrugations (19). The grating corrugations (19) have a non-uniform relief depth across the outer surface for photoinducing a non-uniform refractive index profile in the photosensitive optical medium. The non-uniform relief depth is defined by a variable thin film layer (21) of variable thickness overlaying the substrate (17). The grating corrugations (19) can either be etched into the variable thin film layer (21) itself or be etched into the substrate (17), with the variable thin film layer (21) being deposited on it after etching. Methods to make such phase masks are also provided.

Kinoforms designed to produce different fan-out patterns for two wavelengths

Abstract: Kinoforms (diffractive optical elements) were designed to produce different fan-out (i.e., spot) patterns when illuminated with green (543-nm wavelength) and red (633-nm) light. Three design examples are presented, each using one of three different techniques for this wavelength discrimination. If the fan-out pattern is to be produced in the near field (Fresnel region) of the kinoform, focusing–defocusing distinguishes between the two colors. For a far-field pattern the color distinction can be obtained either by active suppression of unwanted spots, which also decreases the diffraction efficiency, or, preferably, by an increase in the maximum phase modulation of the kinoform (to more than 2π rad). All three examples were designed with a method based on the full scalar wave equation and optimal-rotation-angle optimization. The designed kinoforms were manufactured and performed, at least qualitatively, as predicted by the design.

Rotating optical fields: experimental demonstration with diffractive optics

Abstract: Transversally sharply structured optical fields are discussed, which rotate upon propagation without any lateral expansion of the intensity profile. Finite-aperture approximations of such fields, realizable with phase-only and complex-amplitude recording, are demonstrated. Recording of at least some of the amplitude information (rather than neglecting it completely) is shown to improve the field quality considerably, in particular close to the element. Lohmann-coded binary-phase diffractive elements with restricted amplitude recording are fabricated by electron beam lithography and reactive ion etching. The experimental results are in good agreement with theory.

Orientational gratings in dye-doped polymer-dispersed liquid crystals induced by the photorefractive effect.

Abstract: We report the observation of orientational gratings induced by the photorefractive effect in dye-doped polymer-dispersed liquid crystals. The photorefractive origin of the grating-induction effect is supported by the results of two-beam coupling experiments and by the possibility of erasing the grating by uniform illumination. For all the samples a stable memory effect was observed.

Highly efficient photorefractive polymer-dispersed liquid crystals

Abstract: We report on high-resolution photorefractive polymer-dispersed liquid crystals that lead to high diffraction efficiency at lower applied fields compared with photorefractive polymers. The diffraction efficiency reaches 100% internally for 105-μm-thick samples at a field of 8 V/μm. Net optical gain and diffraction efficiencies of 56% are demonstrated in 53-μm-thick devices. Subsecond response times have also been obtained using polymeric matrices with good transport properties.

Nanosecond response of Bragg deflectors in periodically poled LiNbO 3

Abstract: For the first time, transients are measured for a bulk-optical Bragg-deflector based on an electrooptically (EO) induced grating in periodically poled LiNbO/sub 3/ (PPLN). Optical pulse risetimes of 1.2 ns are reported with a first-order diffraction efficiency of 76% for circular Gaussian beams. The fast risetime is accounted for by a substantially suppressed excitation of acoustic thickness modes and careful electrode design. For many applications, the device could replace both acoustooptical Bragg-deflectors and conventional EO amplitude modulators due to a fivefold reduction in risetime and fourfold reduction in drive power, respectively.