Showing papers on "Diffraction efficiency published in 2005"

An intelligible explanation of highly-efficient diffraction in deep dielectric rectangular transmission gratings.

Abstract: This paper describes in a very easy and intelligible way, how the diffraction efficiencies of binary dielectric transmission gratings depend on the geometrical groove parameters and how a high efficiency can be obtained. The phenomenological explanation is based on the modal method. The mechanism of excitation of modes by the incident wave, their propagation constants and how they couple into the diffraction orders helps to understand the diffraction process of such gratings and enables a grating design without complicated numerical calculations.

High efficiency optical diffraction device

Abstract: Lightwave diffraction device formed of a dielectric layer (4), a mirror (12) arranged at the lower face (10) of said layer, a semi­reflective structure (13) arranged at the upper face (100) of said layer, and a diffractive structure (8) arranged in said layer or on its faces. The height (H) of the layer is chosen so as to substantially satisfy the resonance condition for at least one leaky mode propagating in said layer for at least one given incident wave having a determined wavelength λ and a determined incidence angle θc. Next, the diffractive structure is arranged so that there is no propagating positive diffracted order, and so that all negative orders other than the -1st propagating order have zero or a relatively small diffraction efficiency, the reflected -1st order propagating in a direction non-parallel to the incident wave. This diffraction device allows a high diffraction efficiency of up to 100% for the -1st order.

99% efficiency measured in the -1st order of a resonant grating.

Abstract: A resonant diffraction grating comprising a mirror, a dielectric layer and a high index corrugation at the layer-air interface is shown to exhibit off-Littrow the record diffraction efficiency of 99% in the -1st reflected order at 1064 nm wavelength thanks to the excitation of a leaky mode of the layer. Such high figure is obtained by a grating 5 to 10 times shallower than in current attempts to realize high efficiency all-dielectric gratings.

Switchable two-dimensional gratings based on field-induced layer undulations in cholesteric liquid crystals

Abstract: We propose switchable two-dimensional (2D) diffractive gratings with periodic refractive-index modulation arising from layer undulations in cholesteric liquid crystals. The cholesteric cell can be switched between two states: (1) flat layers of a planar cholesteric texture and (2) a square lattice of periodic director modulation associated with layer undulations that produces 2D diffraction patterns. The intensities of the diffraction maxima can be tuned by changing the applied field. The diffractive properties can be optimized for different wavelengths by appropriately choosing cholesteric pitch, cell thickness, and surface treatment.

Fast scanning transmissive delay line for optical coherence tomography

Abstract: A novel Fourier domain fast scanning optical delay line is proposed in which the walk-off is eliminated by only two passes through a diffraction grating. Working in transmission, the novel delay line is ideal for balanced optical coherence tomography configurations with recirculation of the reference beam. We evaluate theoretically and experimentally its walk-off and dispersion compensation capabilities.

Physical and effective optical thickness of holographic diffraction gratings recorded in photopolymers.

Abstract: In recent years the interest in thick holographic recording materials for storage applications has increased. In particular, photopolymers are interesting materials for obtaining inexpensive thick dry layers with low noise and high diffraction efficiencies. Nonetheless, as will be demonstrated in this work, the attenuation in depth of light during the recording limits dramatically the effective optical thickness of the material. This effect must be taken into account whenever thick diffraction gratings are recorded in photopolymer materials. In this work the differences between optical and physical thickness are analyzed, applying a method based on the Rigorous Coupled Wave Theory and taking into account the attenuation in depth of the refractive index profile. By doing this the maximum optical thickness that can be achieved can be calculated. When the effective thickness is known, then the real storage capacity of the material can be obtained.

Multiplexed holographic transmission gratings recorded in holographic polymer-dispersed liquid crystals: static and dynamic studies

Abstract: The optimization of the experimental parameters of two multiplexed holographic transmission gratings recorded in holographic polymer-dispersed liquid crystals is investigated. Two methods are used to record the holograms: simultaneous and sequential multiplexing. These two processes are optimized to produce two multiplexed Bragg gratings that have the same and the highest possible diffraction efficiencies in the first order. The two methods show similar results when suitable recording parameters are used. The parameters of the recorded gratings (mainly the refractive-index modulation) are retrieved by use of an extension of the rigorous coupled-wave theory to multiplexed gratings. Finally, the response of the holograms to an electric field is studied. We demonstrate few coupling effects between the behavior of both gratings, and we expect a possibility of switching from one grating to the other.

Modeling and design of an optimized liquid-crystal optical phased array

Abstract: In this paper, the physics that determines the performance limits of a diffractive optical element based on a liquid-crystal (LC) optical phased array (OPA) is investigated by numerical modeling. The influence of the fringing electric fields, the LC material properties, and the voltage optimization process is discussed. General design issues related to the LC OPA configuration, the diffraction angle, and the diffraction efficiency are discussed. A design for a wide-angle LC OPA is proposed for high-efficiency laser beam steering. This work provides fundamental understanding for a light beam deflected by a diffractive liquid-crystal device.

Stretching and compression of laser pulses by means of high efficiency volume diffractive gratings with variable periods in photo-thermo-refractive glass

Abstract: High efficiency reflective volume Bragg gratings with chirped gratings recorded in photo-thermo-refractive glass having an absolute diffraction efficiency exceeding 95% in transmitting and reflecting modes are used to stretch and/or compress ultrashort laser pulses with high efficiency. Robustness, compactness, thermal and laser stability along with placement of multiple elements in the same space provides femtosecond laser system with high efficiency of stretching and re-compression of femtosecond pulses.

Fabrication of high-efficiency diffraction gratings in glass

Abstract: We investigated a microfabrication process for optical gratings with periods of micrometer order that use ultrafast laser pulses in semiconductor-doped glass. ZnS- or PbS-doped SiO2–Al2O3–B2O3–CaO–ZnO–Na2O–K2O glass was prepared by a melting method. Glass transmission diffraction gratings with a high refractive-index difference were fabricated with femtosecond laser pulses. The first-order diffraction efficiencies of these gratings were approximately 80%, and the first-order diffraction angles of these gratings were 8° at telecommunication wavelengths.

Polymer crystallization/melting induced thermal switching in a series of holographically patterned Bragg reflectors

Abstract: Holographic photopolymerization (H-P) is a simple, fast and attractive means to fabricate one-, two- and three-dimensional complex structures. Liquid crystals, nanoparticles and silicate nano-plates have been patterned into submicron periodical structures. In this article, we report fabrication of a one-dimensional reflection grating structure by patterning a semicrystalline polymer, polyethylene glycol (PEG), in Norland resin (thiol-ene based UV curable resin) matrix using the H-P technique. Sharp notches observed in the reflection grating of this Norland/PEG system indicate a finite Δn present in the system due to spatial segregation of the PEG and Norland resin. The notch position red shifts upon heating and the diffraction efficiency (ratio between diffraction and incident light intensity, DE) increases from ∼20% to 60% for the Norland 65/PEG 4600 grating. This dynamic behavior of the reflection grating is also fully reversible. The unique thermal switching behavior is attributed to the melting/formation of PEG crystals during heating/cooling. By employing different molecular weight PEGs which have different melting temperatures, a series of switching temperatures have been achieved. Since PEG can be easily coupled with a variety of functional groups, this research might shed light on fabricating multifunctional Bragg gratings using the H-P technique.

Holographic volume gratings in a glass-like polymer material

Abstract: We demonstrate the possibility to write a volume holographic grating with a diffraction efficiency of almost 100% and a grating period >0.3/spl mu/m in a novel glass-like polymeric material based on PMMA. It is succeeded due to the thermal amplification of the grating during the writing process with a following fixation by an incoherent optical illumination. The grating is stable for room temperature. The different processes of generation, amplification, and fixation are discussed both for symmetrical and slanted holographic gratings.

Characterization of an acrylamide-based photopolymer for data storage utilizing holographic angular multiplexing

Abstract: An acrylamide-based photopolymer formulated in the Centre for Industrial and Engineering Optics has been investigated with a view to further optimization for holographic data storage. Series of 18–30 gratings were angularly multiplexed in a volume of photopolymer layer at a spatial frequency of 1500 lines mm−1. Since the photopolymer is a saturable material, an exposure scheduling method was used to exploit the entire dynamic range of the material and allow equal strength holographic gratings to be recorded. This investigation yielded the photopolymer M/# for moderately thin layers. Photopolymer temporal stability was also studied by measuring variations of material shrinkage, Bragg selectivity curve, and diffraction efficiency.

Design, optical characterization, and operation of large transmission gratings for the laser integration line and laser megajoule facilities

Abstract: Within the framework of the laser integration line (LIL) and the laser megajoule, we describe the design, optical characterization, mounting, alignment, and operation on the LIL of large 420 mm × 470 mm transmission gratings. Two types of grating were manufactured. The first, operating at a wavelength of 1.053 µm, was used for deviation purposes. The second, operating at a wavelength of 0.351 µm, was used for both deviation and focusing purposes. We demonstrate that these large transmission gratings are suitable for nanosecond-regime operation on high-power laser facilities.

Electrically switchable Fresnel lens using a polymer-separated composite film.

Abstract: A Fresnel lens with electrically-tunable diffraction efficiency while possessing high image quality is demonstrated using a phase-separated composite film (PSCOF). The light scattering-free PSCOF is obtained by anisotropic phase separation between liquid crystal and polymer. Such a lens can be operated below 12 volts and its switching time is reasonably fast (~10 ms). The maximum diffraction efficiency reaches ~35% for a linearly polarized light, which is close to the theoretical limit of 41%.

A polarization insensitive 2×2 optical switch fabricated by liquid crystal–polymer composite

Abstract: A polarization insensitive 2×2 optical switch was fabricated with liquid crystal–polymer composite by means of holography. The highest diffraction efficiency achieved was 85.7%. The rise time and the decay time measured were 36 and 160μs, respectively, at an applied electric field of 18.2V∕μm. The polarization-dependent loss was 0.03dB measured for s- and p-polarized light at the wavelength of 632.8nm.

Investigation of physical properties of quartz after focused ion beam bombardment

Abstract: Optical properties (transmission and refractive index) and phase change (from amorphous to crystal) of a commonly used glass, quartz, were investigated before and after focused ion beam (FIB) bombardment with ion energy from 30 to 50 keV. We found different influences of FIB bombardment on the optical properties and chemical structure of the quartz in the wavelength region of visible and near infrared, respectively. The quartz still can be used in the infrared wavelength for conventional optical applications. As an application example, an array of diffractive optical elements (DOEs) was directly fabricated on the quartz by the FIB milling. The measured diffraction efficiency of the DOEs is 83.5%, which is acceptable for practical use.

Illumination apparatus for display device using hologram-based light guide plate (LGP)

Abstract: An illumination apparatus and method for a display device designed such that light is incident on a hologram or hologram pattern at an angle for which diffraction efficiency is the highest. The illumination apparatus includes at least one point light source which emits light and a light guide plate (LGP) which has the at least one point light source disposed on a side thereof and a hologram pattern on a top surface which permits the light incident from the point light source to exit from the top surface. The side of the LGP facing the point light source is inclined such that the light is incident obliquely on the hologram pattern at an altitude angle which provides high diffraction efficiency.

High-Performance Transmission Holographic Gratings via Different Polymerization Rates of Dipentaerythritol Acrylates and Siloxane-Containing Epoxides

Abstract: Well-defined transmission holographic gratings with high diffraction efficiency were fabricated by irradiating a mixture of photopolymerizable materials with different reaction rates consisting of ...

Femtosecond-laser induced formation of grating structures in planar polymer substrates

Abstract: Grating structures are fabricated in polymer substrates by femtosecond- (fs-) IR-laser radiation. Several sorts of polymer are employed as polymeric basic material for the grating manufacture: polymethylmethacrylate (PMMA), fluorinated PMMA, poly-N-methyl methacrylimide (PMMI), polycarbonate (PC), polyimide (PI) and polysiloxane. Surface and volume gratings are fabricated by positioning the focus of the fs-laser beam directly on the surface or inside the volume of the polymeric substrate, so by the right choice of the irradiation parameters the irradiated zone of the substrate is ablated, modified or carbonized. The sample is moved according to a scanning algorithm in order to generate the grating structure. The diffraction efficiency of the grating samples is measured by a HeNe laser. Some proposals are given for the mechanism of the grating generation.

Objective lens and optical pickup apparatus

Abstract: In an objective lens for use in an optical pickup device, when NA1 and N2 (NA2 < NA1) represent a needed numerical aperture of the objective lens on an image side, a first spot and a second spot represent a spot formed by the light flux having passed through the central region, a mth order diffracted ray and a nth diffracted ray represent a diffracted ray having the maximum diffraction efficiency among diffracted rays, the central region nearly corresponds to a region through which the light flux in the inside of the numerical aperture NA2 passes, the light amount of the nth order diffracted ray which reaches the inside of the second spot is less than that of the mth order diffracted ray which reaches the inside of the first spot, and the mth order diffracted ray and the nth order diffracted ray satisfy the relationship of m = n.

Analytical model of the optical response of periodically structured metallic films

Abstract: In this paper we investigate the optical response of periodically structured metallic films constituted of sub-wavelength apertures. Our approach consists in studying the diffraction of transverse magnetic polarized electromagnetic waves by a one-dimensional grating. The method that we use is the Rigorous Coupled Waves Analysis allowing us to obtain an analytical model to calculate the diffraction efficiencies. The zero and first order terms allow determining the transmission, reflectivity and absorption of symmetric or asymmetric nanostructures surrounded either by identical or different dielectric media. For both type of nanostructures the spectral shape of the enhanced resonant transmission associated to surface plasmons displays a Fano profile. In the case of symmetric nanostructures, we study the conditions of formation of coupled surface plasmon-polaritons as well as their effect on the optical response of the modulated structure. For asymmetric nanostructures, we discuss the non-reciprocity of the reflectivity and we investigate the spectral dependency of the enhanced resonant transmission on the refractive index of the dielectric surrounding the metal film.

Polarization holographic high-density optical data storage in bacteriorhodopsin film

Abstract: Optical films containing the genetic variant bacteriorhodopsin BR-D96N were experimentally studied in view of their properties as media for holographic storage. Different polarization recording schemes were tested and compared. The influence of the polarization states of the recording and readout waves on the retrieved diffractive image’s intensity and its signal-to-noise ratio were analyzed. The experimental results showed that, compared with the other tested polarization relations during holographic recording, the discrimination between the polarization states of diffracted and scattered light is optimized with orthogonal circular polarization of the recording beams, and thus a high signal-to-noise ratio and a high diffraction efficiency are obtained. Using a He–Ne laser (633 nm, 3 mW) for recording and readout, a spatial light modulator as a data input element, and a 2D-CCD sensor for data capture in a Fourier-transform holographic setup, a storage density of 2 × 108 bits/cm was obtained on a 60 × 42 µm2 area in the BR-D96N film. The readout of encoded binary data was possible with a zero-error rate at the tested storage density.

Electron diffraction from free-standing, metal-coated transmission gratings

Abstract: Electron diffraction from a free-standing nanofabricated transmission grating was demonstrated, with energies ranging from 125 eV to 25 keV. Observation of 21 diffraction orders highlights the quality of the gratings. The image charge potential due to one electron was measured by rotating the grating. These gratings may pave the way to low-energy electron interferometry.

Thermally stable holographic surface relief gratings and switchable optical anisotropy in films of an azobenzene-containing polyelectrolyte

Abstract: In a search for effective polymer film material for holographic surface patterning, commercially available azobenzene polyelectrolyte has been employed. Films of good optical quality in a wide range of thickness were produced. Optical dichroism up to 0.19 was induced upon irradiation with linearly polarized light. Surface relief gratings with amplitudes up to 630 nm and diffraction efficiency of 37% were inscribed by holographic exposure to the light of 488 nm. Due to the ionic nature of the material, the relief was stable at least up to the temperature of decomposition (ca. 200 °C) but could be erased and inscribed again by light.

Dammann SHG-FROG for characterization of the ultrashort optical pulses

Abstract: In this paper we propose a very simple layout of multi-shot second-harmonic-generation (SHG) frequency-resolved optical gating (FROG) using three reflective Dammann gratings ( Dammann SHG-FROG) for characterization of the ultrashort optical pulses. One reflective Dammann gratings is used as the beamsplitter and the other two compensate the angular dispersion. Both theoretical and experimental results show that the distortions of the optical pulses introduced by the reflective Dammann gratings are very small. This device should be highly interesting for characterizing the ultrashort pulse. (C) 2005 Optical Society of America.

Nonlinear diffraction in gratings based on polymer–dispersed TiO2 nanoparticles

Abstract: In this letter we report a simple technique to produce volume holographic gratings based on photopolymerizable composites containing TiO2 nanoparticles. Diffraction gratings with high refractive index modulation amplitude (up to 1.25 × 10−2) have been formed due to the periodic distribution of high refractive index nanoparticles in a low refractive index polymer matrix. The diffraction efficiency increases strongly on increasing the nanoparticle concentration. Taking the mixture with 10 wt.% TiO2 nanoparticles, gratings with high diffraction efficiency, low level of scattering and high transparency in the visible-wavelength range have been obtained. This will ultimately lead to different applications of diffractive optical elements based on nanocomposites. The dependence of the gratings’ diffraction efficiency on the intensity of probe laser pulses at 1064 nm has been explored. It is shown that the nonlinear response of the gratings is attributed mainly to the nonlinear properties of the TiO2 nanoparticles embedded in the polymer matrix. The mechanism of the grating formation and the reasons for the nonlinear behavior of the diffraction efficiency are discussed.

Optical microfabrication of highly reflective volume Bragg gratings

Abstract: An approach for fabricating wide-bandwidth and highly reflective Bragg grating structures with the technique of holographic photopolymerization of a liquid crystal (LC) polymer composite is presented. The key to this fabrication method that distinguishes it from previous methods is the use of a nonreactive solvent, acetone, to dissolve the photoinitiator and coinitiator in an acrylate monomer/LC mixture. The addition of acetone results in the creation of controllable periodic voids inside the thin film after the acetone evaporates. Peak reflectivity as high as 80% and a broad reflection bandwidth of 80nm were observed in the reflection gratings formed with acetone present in the starting mixture. It was estimated from a fit to the experimental data that the resulting index mismatch was approximately 0.2; consistent with the presence of air voids. It is determined that tunable wavelength, diffraction efficiency, and bandwidth of reflection notches can be achieved by backfilling with fluids of different ref...