Showing papers on "Diffraction efficiency published in 1995"

Formulation for stable and efficient implementation of the rigorous coupled-wave analysis of binary gratings

Abstract: The rigorous coupled-wave analysis technique for describing the diffraction of electromagnetic waves by periodic grating structures is reviewed. Formulations for a stable and efficient numerical implementation of the analysis technique are presented for one-dimensional binary gratings for both TE and TM polarization and for the general case of conical diffraction. It is shown that by exploitation of the symmetry of the diffraction problem a very efficient formulation, with up to an order-of-magnitude improvement in the numerical efficiency, is produced. The rigorous coupled-wave analysis is shown to be inherently stable. The sources of potential numerical problems associated with underflow and overflow, inherent in digital calculations, are presented. A formulation that anticipates and preempts these instability problems is presented. The calculated diffraction efficiencies for dielectric gratings are shown to converge to the correct value with an increasing number of space harmonics over a wide range of parameters, including very deep gratings. The effect of the number of harmonics on the convergence of the diffraction efficiencies is investigated. More field harmonics are shown to be required for the convergence of gratings with larger grating periods, deeper gratings, TM polarization, and conical diffraction.

Stable implementation of the rigorous coupled-wave analysis for surface-relief gratings: enhanced transmittance matrix approach

Abstract: An enhanced, numerically stable transmittance matrix approach is developed and is applied to the implementation of the rigorous coupled-wave analysis for surface-relief and multilevel gratings. The enhanced approach is shown to produce numerically stable results for excessively deep multilevel surface-relief dielectric gratings. The nature of the numerical instability for the classic transmission matrix approach in the presence of evanescent fields is determined. The finite precision of the numerical representation on digital computers results in insufficient accuracy in numerically representing the elements produced by inverting an ill-conditioned transmission matrix. These inaccuracies will result in numerical instability in the calculations for successive field matching between the layers. The new technique that we present anticipates and preempts these potential numerical problems. In addition to the full-solution approach whereby all the reflected and the transmitted amplitudes are calculated, a simpler, more efficient formulation is proposed for cases in which only the reflected amplitudes (or the transmitted amplitudes) are required. Incorporating this enhanced approach into the implementation of the rigorous coupled-wave analysis, we obtain numerically stable and convergent results for excessively deep (50 wavelengths), 16-level, asymmetric binary gratings. Calculated results are presented for both TE and TM polarization and for conical diffraction.

Laser‐induced holographic surface relief gratings on nonlinear optical polymer films

Abstract: We report observation of holographic surface relief gratings with relatively large amplitude on a second order nonlinear optical polymeric material. Surface relief gratings on these polymer films were created upon exposure to polarized Ar+ laser beams at 488 nm without any subsequent processing steps. The surface structure of the relief gratings was investigated by atomic force microscopy. The depth of the surface relief in a typical case was 120 nm which is approximately 20% of the original film thickness. The diffraction efficiency of gold‐coated gratings was investigated as a function of wavelength and capability of recording orthogonal gratings on the same film was demonstrated.

Polarized Laser Induced Holographic Surface Relief Gratings on Polymer Films.

Abstract: : Large amplitude holographic surface relief gratings on azobenzene containing polymer films were optically induced without any subsequent processing steps. The surface relief structures were observed upon exposure to an interference pattern of polarized Ar+ laser beams. The surface relief structures were investigated using atomic force microscopy. Very regularly spaced surface grating with a surface modulation depth of over 1000 A was obtained. The gratings were stable at ambient conditions and the samples were reusable after erasing the gratings by heating the film above the glass transition temperature. The orientation of the azobenzene groups were induced first and followed by the surface deformation process. The polarization of the writing beam had significant effect on the surface modulation process. The relation between the surface change and optically induced orientation of the azobenzene groups was discussed. Thermal effects were also investigated by studying the dependence of writing intensity and grating spacing on the grating formation process. (MM)

Spectral properties of multiorder diffractive lenses

Abstract: Diffractive lenses have been traditionally designed with the first diffracted order. The spectral characteristics of diffractive lenses operating in higher diffracted orders differ significantly from the first-order case. Multiorder diffractive lenses offer a new degree of freedom in the design of broadband and multispectral optical systems that include diffractive optical elements. It is shown that blazing the surface-relief diffractive lens for higher diffraction orders enables the design of achromatic and apochromatic singlets. The wavelength-dependent optical transfer function and the associated Strehl ratio are derived for multiorder diffractive lenses. Experiments that illustrate lens performance in two spectral bands are described, and the results show excellent agreement with the theoretical predictions.

Near-field imaging of atom diffraction gratings: The atomic Talbot effect.

Abstract: We have demonstrated the Talbot effect, the self-imaging of a periodic structure, with atom waves. We have measured the successive recurrence of these self-images as a function of the distance from the imaged grating. This is a near-field interference effect, which has several possible applications that are discussed.

High-efficiency multilayer dielectric diffraction gratings.

Abstract: The design and performance of a new type of high-efficiency diffraction grating for use in either transmission or reflection are described. The gratings are produced in a multilayer dielectric coating deposited upon optically flat substrates. By proper design of the multilayer stack and grating structure, a diffraction efficiency in excess of 96% for polarized light in the m = -1 order in reflection has been achieved.

Harmonic diffractive lenses

Abstract: The harmonic diffractive lens is a diffractive imaging lens for which the optical path-length transition between adjacent facets is an integer multiple m of the design wavelength λ(o). The total lens thickness in air is mλ(o)/(n - 1), which is m times thicker than the so-called modulo 2π diffractive lens. Lenses constructed in this way have hybrid properties of both refractive and diffractive lenses. Such a lens will have a diffraction-limited, common focus for a number of discrete wavelengths across the visible spectrum. A 34.75-diopter, 6-mm-diameter lens is diamond turned in aluminum and replicated in optical materials. The sag of the lens is 23 µm. Modulation transfer function measurements in both monochromatic and white light verify the performance of the lens. The lens approaches the diffraction limit for 10 discrete wavelengths across the visible spectrum.

An electro‐optically controlled liquid crystal diffraction grating

Abstract: A structure for an electro‐optically controlled liquid crystal diffraction grating is proposed, which can dramatically simplify the fabrication process of liquid crystal optical gratings. The structure consists of two alternating stripes. Each stripe is a hybrid liquid crystal cell with adjacent stripes oriented perpendicularly. This kind of electro‐optically controlled diffraction grating in principle gives 100% diffraction efficiency and no polarization direction dependence. The detailed fabrication process is presented.

Efficient implementation of rigorous coupled-wave analysis for surface-relief gratings

Abstract: A computationally efficient implementation of rigorous coupled-wave analysis is presented The eigenvalue problem for a one-dimensional grating in a conical mounting is reduced to two eigenvalue problems in the corresponding nonconical mounting This reduction yields two n × n matrices to solve for eigenvalues and eigenvectors, where n is the number of orders retained in the computation For a two-dimensional grating, the size of the matrix in the eigenvalue problem is reduced to 2n × 2n These simplifications reduce the computation time for the eigenvalue problem by 8–32 times compared with the original computation time In addition, we show that with rigorous coupled-wave analysis one analytically satisfies reciprocity by retaining the appropriate choice of spatial harmonics in the analysis Numerical examples are given for metallic lamellar gratings, pulse-width-modulated gratings, deep continuous surface-relief gratings, and two-dimensional gratings

Multilayer dielectric diffraction gratings

Abstract: The design and fabrication of dielectric grating structures with high diffraction efficiency used in reflection or transmission is described. By forming a multilayer structure of alternating index dielectric materials and placing a grating structure on top of the multilayer, a diffraction grating of adjustable efficiency, and variable optical bandwidth can be obtained. Diffraction efficiency into the first order in reflection varying between 1 and 98 percent has been achieved by controlling the design of the multilayer and the depth, shape, and material comprising the grooves of the grating structure. Methods for fabricating these gratings without the use of ion etching techniques are described.

Gray-scale masks for diffractive-optics fabrication: I. Commercial slide imagers.

Abstract: Fabrication of diffractive optics with binary masks requires multiple photolithographic processes to produce high-efficiency elements. Alignment or etching errors at any stage of fabrication decrease the efficiency of the element. We developed an easily accessible procedure that reduces fabrication complexity and costs by using a single gray-scale mask. The gray-scale patterns are generated by commercial slide imagers and are then photoreduced onto low-contrast film plates. Multiple-level or continuous relief structures (kinoforms) may be constructed by use of the photoreduced gray-scale patterns as lithographic masks. Diffractive-optic lenses and blazed gratings were fabricated in photoresist with this procedure. First-order diffraction efficiencies as high as 85% were measured for the blazed gratings. The advantages and the limitations of this technique are discussed.

Holographic grating formation in dye- and fullerene C(60)-doped nematic liquid-crystal film.

Abstract: Transient and persistent holographic gratings have been observed in dye- and fullerene C60-doped nematic liquid-crystal films. The nature and time evolution of the underlying mechanisms, such as space-charge field production, flows, and dielectric- and conductivity-induced torques, and the resultant director axis reorientation and refractive-index gratings are examined.

Diffractive optical modulator and method for producing the same, infrared sensor including such a diffractive optical modulator and method for producing the same, and display device including such a diffractive optical modulator

Abstract: The diffractive optical modulator of the invention includes: a plate having a portion functioning as a first electrode; a spacer layer formed on the plate; and a grating consisting of a plurality of beams having a portion functioning as a second electrode, both ends of the beams being supported on the spacer layer. In the diffractive optical modulator, by adjusting a voltage applied between the first electrode and the second electrode, a distance between the beams and the plate is varied, thereby controlling the diffraction efficiency. An insulating layer is further provided between the plate and the plurality of beams.

Apodisation of the spectral response of fibre Bragg gratings using a phase mask with variable diffraction efficiency

Abstract: Bragg gratings with sidelobe levels 26 dB lower than the peak reflectivity have been fabricated in standard telecommunication optical fibres by exposure to ultraviolet light through a phase mask with a locally varying diffraction efficiency This represents a reduction of 14 dB in the sidelobe levels compared to uniform gratings with the same bandwidth and reflectivity

High-efficiency metallic diffraction gratings for laser applications

Abstract: The design and fabrication of large-area, high-efficiency metallic gratings for use in high-power laser systems is described. The gratings exhibit a diffraction efficiency in excess of 95% in the m = −1 order (Littrow mount) and have a high threshold for laser damage. Computations and experimental measurements are presented that illustrate the effect of grating shape and polarization on efficiency. A simple theory for optical damage to metallic diffraction gratings is developed and compared with experimental measurements of the laser-damage threshold over the pulse range from 400 fs to >1 ns.

Understanding diffractive optic design in the scalar domain

Abstract: A general procedure is presented for the design of diffractive optical elements, and scalar diffraction theory is used to apply it to the design of three common diffractive elements: a diffractive lens, an array generator, and a correlation filter. The procedure reveals that most common design techniques can be classified as either direct or indirect optimizations. The key feature of the design procedure is the specification of a suitable performance measure based on the designer’s understanding of the optical system and the fabrication technology used to realize the diffractive element. The use of complex-wave amplitude, scale, and phase freedoms in design is also emphasized.

Method and apparatus for projection exposing

Abstract: PURPOSE: To simultaneously satisfy the large exposure field and the high resolution by diffracting the light passed through the pattern of a mask via a projection optical system, and reproducing a mask pattern on a sample from the diffracted light to be exposed. CONSTITUTION: A diffraction grating A is inserted between a mask 1 and a projection optical system 2, and a diffraction grating B is inserted between the system 2 and a wafer 4. The mask is obliquely emitted from a different direction. In this case, both the gratings A and B are assumed to be phase gratings and that the illuminated lights R, L from the two directions are coherent to each other. A zero order light R0 reaches a point A0 on the grating A, where the light diffracted in + primary direction is passed through the left end of a pupil 3, and arrived at a point B0 on the grating B. Thereafter, it is diffracted by the grating B in + primary and - primary directions, and arrived at Q and P on image surfaces. The + primary diffracted light R1 arrives at the point A1 on the grating A, where the light diffracted in the + primary direction is passed through the right end of the pupil, and arrived at the point B1 on the grating B.

Diffractive phase elements based on two-dimensional artificial dielectrics

Abstract: We have designed, fabricated, and tested a blazed artificial dielectric transmission grating in fused quartz for use at the 633-nm wavelength. By locally varying the index, using fabricated two-dimensional arrays of dielectric cylinders, we can achieve a desired phase modulation to produce a diffractive phase element in one processing step. The effective index depends on the fill fraction of the cylinders.

Highly efficient photorefractive polymers for dynamic holography

Abstract: The performance of recently developed highly efficient photoref_ ract1ve polyme_rs .base~ on the photoconducting polymer host poly( N-vinylcarbazole) 1s 1nvest1gated, and the use of these materials as recording media in dynamic holography and other applications is evaluated.- A diffraction efficiency 'rJ = 86% (limited only by absorption and reflection losses), a two-beam coupling net gain coefficient r = 200 cm - 1 and light-induced refractive index modulations as high as lin=7X 10-3 are demonstrated. Hologram growth rates of the order of 500 ms are observed with. recording light intensities <10 mW /cm2 , using either lowpower laser diodes (675 nm) or a HeNe laser (633 nm). The materials show good sensitivity in this part of the spectrum. Applications such as dynamic time-average interferometry are demonstrated.

Polarization-selective computer-generated holograms: design, fabrication, and applications.

Abstract: We constructed polarization-selective computer-generated holograms that apply an independent phase profile during readout by horizontal and vertical light polarizations. These elements are composed of two surface-relief-etched birefringent substrates joined face to face. We describe the design methodology for arbitrary birefringent substrate and gap materials. We show how these holograms are fabricated with standard microelectronics technology and discuss the effects of etching and alignment errors on performance. We demonstrated a diffraction efficiency of 60% with a polarization contrast ratio of >100:1 using a multilevel phase hologram made from two birefringent lithium niobate substrates. We also showed that a single-layer SiO(2) thin-film antireflection coating on all surfaces can reduce reflections from the high-index substrates without significant effect on hologram performance. We also consider some possible applications of this technology and demonstrate experimentally a dual focal-length lens and a self-interconnecting binary 2 × 2 polarization switch.

Refractive and diffractive properties of planar micro-optical elements.

Abstract: The refractive and the diffractive properties of planar micro-optical elements are investigated. The transition between purely refractive and purely diffractive planar microlenses is numerically simulated for the example of differently designed phase-matched Fresnel elements. Results obtained from numerical simulations and experiments show that the refractive and diffractive types exhibit a distinctly different behavior in the presence of small fabrication errors or wavelength deviations. Based on these results, design rules for various applications, including low- and high-numerical-aperture lenses and hybrid refractive–diffractive elements, are derived. For a high-numerical-aperture (f/# = 1.0) lens the experimental characterization of the irradiance distribution in the image space is presented and shown to agree well with theoretical predictions.

Apparatus and method for storing and/or reading data on an optical disk

Abstract: An optical memory for storing and/or reading data on an optical disk. The optical disk incorporates a material in which holographic gratings can be created at plural locations within the disk. An electro-optical head which is capable of creating these holographic gratings at any one of the plural locations is employed. The head is additionally capable of detecting the presence or absence of a holographic grating at any one of these plural locations. The presence of the holographic grating could indicate a first binary state and the absence of the holographic grating indicate a second state. Alternately, the electro-optical head could be used to vary the diffraction efficiency of the holographic gratings during their creation. In that case, the head would also be capable of detecting this variation in efficiency and produce a proportional detection signal. This signal would be used as an indicator of the value of the stored data element. The material of the optical disk is further capable of having multiple holographic gratings created at each one of the plural locations. These multiple holographic gratings are created via a beam of light from the electro-optical head which has a different wavelength or point of focus. In reading these holographic gratings, a beam of light generated by the electro-optical head is sequentially varied in wavelength or point of focus to correspond to the wavelength or point of focus and the sequence of wavelengths or points of focus employed to record each one of the data elements recorded.

Optimized binary, phase-only, diffractive optical element with subwavelength features for 1.55 μm

Abstract: We report the use of rigorous coupled-wave diffraction analysis in conjunction with the simulated annealing optimization method for performing optimal design of binary-level surface-relief diffractive elements with subwavelength, submicrometer features that exhibit quasi-linear-phase transmittance. An element designed for operation at 1.55 μm has been fabricated and is characterized.

Diffraction optical element

Abstract: PROBLEM TO BE SOLVED: To provide the diffraction optical element which is easily manufacture and reduced in the wavelength dependency of diffraction efficiency and can effectively prevent a flare, a ghost, etc., from being generated. SOLUTION: This element has 1st, 2nd, 3rd, and 4th areas 11, 12, 13, and 14 which are laminated sequentially in contact or closely, a 1st relief pattern 21 which is formed on the boundary surface between the 1st and 2nd areas 11 and 12, and a 2nd relief pattern 22 which is formed on the boundary surface between the 3rd and 4th areas 13 and 14; and the 1st, 2nd, 3rd, and 4th areas 11, 12, 13, and 14 are formed of at least three kinds of material which are at least substantially transparent to the wavelength of light in use while the 1st area 11 and 2nd area 12, and 3rd area 13 and 4th area 14 are formed of mutually different materials. The 1st and 2nd relief patterns 21 and 22 have substantially equal pitch distributions and mutually different groove depths, and their corresponding parts are arranged close to each other. COPYRIGHT: (C)1997,JPO

Investigation of computer-generated diffractive beam shapers for flattening of single-modal CO 2 laser beams

Abstract: A full cycle was realized of the photolithographic development and detailed testing of a diffractive optical element that transforms the diverging Gaussian beams of CO2 lasers into a uniformly filled-in rectangle. The zone feature size of the beam shaper, the diffractive efficiency and accuracy, the focus depth, and the stability with respect to the size and the divergence of incident Gaussian beams are studied by computer modeling. Calculated flattop intensity distributions are presented in the same form of gray-level pictures and three-dimensional plots as the corresponding results measured by an IR camera.

Fabrication of monolithic diffractive optical elements by the use of e-beam direct write on an analog resist and a single chemically assisted ion-beam-etching step

Abstract: We present a method to fabricate high-quality and environmentally rugged monolithic diffractive optical elements (DOE's). Analog direct-write e-beam lithography was used to produce analog resist profiles that were transferred into their substrates by the use of chemically assisted ion-beam etching (CAIBE) in one single etching step. An iterative method was used to compensate for the proximity effect caused by electron scattering in the resist and from the substrate during the e-beam exposure. Slope-dependent differential etch rates that occur during the transfer process were characterized and compensated for. Finally, the DOE was divided into regions with different period ranges, and the exposure dosages were set to achieve even and accurate etch depths in the final element. The presented fabrication method will increase manufacturability and reduce processing time, which will result in a general cost reduction per element.

Developed profile of holographically exposed photoresist gratings.

Abstract: A simulation of the profile of holographically recorded structures in photoresists is performed. In addition to its simplicity this simulation can be used to take into account the effects that arise from exposure, photosensitization, development, and resolution of positive photoresists. We analyzed the effects of isotropy of wet development, nonlinearity of the photoresist response curve, background light, and standing waves produced by reflection at the film–substrate interface by using this simulation, and the results agree with the experimentally recorded profiles.