Volume Gratings Research Articles

Photosensitivity and volume gratings in phosphate glasses for rare-earth-doped ion-exchanged optical waveguide lasers

Phosphate glasses containing enough sodium to allow the fabrication of optical channel waveguides by ion exchange are photosensitive to intense ultraviolet irradiation from excimer lasers at 193 and 248nm. Permanent submicrometer-period volume index gratings with modulation amplitudes larger than 10−4 were obtained even after annealing the irradiated samples for 2h at 230°C. Ultraviolet-visible absorption changes were measured in the irradiated areas, as well as structural modifications observed by surface profiling. Comparative studies were carried out for glasses that were also codoped with erbium and ytterbium to provide high gain at wavelengths near 1.5μm. Finally, we demonstrate narrowband gratings with reflectivities larger than 80% in silver ion-exchanged channel waveguides made in these phosphate glass substrates.

Fs-laser-induced Fabrication of Polymeric Optical and Fluidic Microstructures

In this paper first experimental results are presented on the fabrication of integrated-optical microstructures by femtosecond-laser irradiation at the surface or in the volume of a polymeric substrate. Thereby the refractive index or the optical absorption coefficient of the polymer material is locally modified in a controllable way due to nonlinear optical effects like multi-photon absorption. By this way surface relief grating structures, volume grating structures and linear waveguiding structures are generated. The surface relief gratings have diffraction efficiencies about 0.2 (first diffraction order), the diffraction efficiencies of the volume gratings are approximately smaller by one order of magnitude. Besides experiments have been performed on the modification of the polymeric surface by fs-laser irradiation followed by subsequent development of the irradiated area by an etching liquid generating linear groove structures. By this way simple linear microstructures are generated into the polymer substrate. The results of these experiments are relevant for the fabrication of three-dimensional structures in the polymeric substrate in the future. All the gratings and the linear microstructures can be considered as basic components for the fabrication of complex micro-optical and fluidic systems in a polymeric substrate (photonic lab-on-the-chip).

Analysis of an ultrashort pulsed finite beam diffracted by volume gratings

We obtain analytical solutions of the coupled wave equations that describe the Bragg diffraction of ultrashort pulsed finite beams by a thick planar grating, using two-dimensional coupled wave theory. The diffraction properties for the case of an ultrashort pulsed finite beam with Gaussian profiles in both the time and spatial domains are investigated. The spectral bandwidth of the diffracted beam, the Bragg selectivity bandwidth and the diffraction efficiency of the volume grating are influenced by the geometry parameter and the input bandwidth. Therefore extra attention should be paid to designing optical elements based on volume gratings for use with ultrashort pulsed waves in applications of pulse shaping and processing.

Digital volume gratings

An optical element constructed by stacking a set of binary-phase grating sheets can simulate the functions of optically recorded volume gratings. Our electromagnetic numerical study also shows that if one of the grating sheets is replaced by another one with different grating period, power spectrum of the diffracted wave changes completely with extra diffraction orders. This property will claim strong advantage in security document applications. Analysis of alignment error reveals interesting phenomena concerning to how misalignment affects diffraction efficiency.

New Generation of Holographic Gratings Based on Polymer-LC Composites: POLICRYPS and POLIPHEM

Soft materials exhibit very attractive mechanical, optical, and electro-optical properties and easy processing techniques. Combined with holographic techniques, they can be patterned quite easily creating many unique structures. We present here few examples, of electrically-switchable passive opto-electronic components based on composite materials obtained from initial mixtures of photocurable monomers and liquid crystalline materials. Holographic exposure results in volume gratings consisting in alternate areas of photo-crosslinked polymer and phase separated LCs, respectively. We discuss the main features of this new generation of gratings and show how most of their performances can be enhanced with respect to other kinds of composite gratings, like H-PDLCs.

Comparison of Smith–Purcell radiation characteristics from gratings with different profiles

Gratings with different profiles such as lamellar, triangular, sinusoidal (so-called “volume” gratings) and “flat” gratings consisting of thin conductive strips separated by vacuum or dielectric gaps, have been used for investigation of Smith–Purcell radiation (SPR) in a number of experimental and theoretical reports. Some theoretical models predict that the flat grating is more preferable in respect to the radiation intensity. The azimutal dependence of SPR yield from volume and flat gratings is drastically different. However, till now the direct comparison of SPR characteristics for these profiles for the same experimental conditions is absent. We present some experimental results for such comparison obtained with the 6-MeV microtron in millimeter wavelength region under coherent conditions. Radiation angular distribution from gratings of different profiles has been studied in different spectral regions. The flat thin periodical target was demonstrated to have a superior radiation capacity. An additional result of this experiments is the observation of anomalous large long wave radiation intensity at small observation angles, which is not predicted by existing SPR theory. Calculations of SPR characteristics from volume and flat gratings using traditional models, presented in this article are in agreement with experiment only for a large polar angle.

Beam-width-dependent filtering properties of strong volume holographic gratings

The finite dimension of the incident beam used to read out volume holographic gratings has interesting effects on their filtering properties. As the readout beam gets narrower, there is more deviation from the ideal response predicted for monochromatic plane waves. In this paper we experimentally explore beam-width-dependent phenomena such as wavelength selectivities, angular selectivities, and diffracted beam profiles. Volume gratings in both reflection and transmission geometries are investigated near 1550 nm. Numerical simulations utilizing the technique of Fourier decomposition provide a satisfactory explanation and confirm that the spread of spatial harmonics is the main contributing factor.

Band-edge-induced Bragg diffraction in two-dimensional photonic crystals

Two-dimensional photonic crystals composed of two orthogonal volume diffraction gratings have been photogenerated in photopolymers. When the read beam is set at the Bragg angle, the diffraction efficiency of the transmission grating is strongly enhanced at the band edge of the reflection grating recorded in the material. Such a device provides Bragg operation and enhancement of the diffraction efficiency of the thin diffraction grating together with good wavelength selectivity. Such advantages could be interesting for optical signal processing.

Adiabatic three-wave volume hologram: large efficiency independent of grating strength and polarization

A transmission hologram with two volume gratings is considered in the regime of wave A diffracted into wave B via an intermediate weakly excited wave C. In analogy to the phenomenon of stimulated Raman adiabatic passage in nonlinear optics, this new scheme demonstrates diffraction efficiency with low sensitivity to the hologram's strength. A theory of three-wave adiabatic coupling has been developed and explored analytically. Numerical results show an example of a coupling profile that preserves high diffraction efficiency with almost no dependence on the hologram's strength, including the suppressed influence of polarization.

Expansion of channel number in optical demultiplexer using cascaded photopolymer volume gratings

A method to expand the number of channels in the optical demultiplexer (DMUX) using two cascaded photopolymer volume gratings is proposed and presented. A 0.4-nm-spaced 130-channel DMUX with the channel uniformity of 3.5 dB, the 3-dB bandwidth of 0.12 nm, and the channel crosstalk of -20 dB is experimentally demonstrated.

Translation-invariant object recognition system using an optical correlator and a super-parallel holographic random access memory

We demonstrate a translation-invariant VanderLugt correlator (VLC) for a set of images stored via holographic angle multiplexing of volume gratings in a polymeric substrate. The images read out from the volume gratings are optically correlated in a translation-invariant manner with a dc-suppressed holographic filter. The quality and efficiency of this correlator are observed to be nearly as good as that of another VLC with images directly from a spatial light modulator used as inputs. This experiment is the first step toward realizing a novel optical image recognition system capable of identifying a query image through an exhaustive search in a large database of filter images stored in an ultrahigh capacity superparallel holographic random access memory (SPHRAM). In this system, the identification of the query image is to be performed with a translation invariant, real-time VLC or a joint transform correlator (JTC).

Modeling of phase volume diffractive gratings, part 1: transmitting sinusoidal uniform gratings

A detailed model of the diffraction of plane and Gaussian beams on plane uniform phase Bragg gratings based on Kogelnik's theory of coupled waves is presented. The model describes transmitting gratings with arbitrary orientation in a plane-parallel plate taking into ac- count spectral width and angular divergence of laser beams along with material dispersion of a photosensitive medium. The model results are compared with experimental data for high-efficiency Bragg gratings in a photothermorefractive PTR glass. © 2006 Society of Photo-Optical Instrumenta- why our modeling is illustrated for those variations of grat- ing parameters that are typical for PTR VBGs: the refrac- tive index is in the range of 1.5 at wavelengths ranging from 0.4 to 2.7 m, refractive index modulation is up to 1000 ppm 10 3 , and grating thickness is from 0.2 to 20 mm. The goal of this work is to reduce Kogelnik's theory to practical formulas that enable practical modeling and de- sign of diffractive optical elements based on VBGs. This part of modeling considers diffraction of plane monochro- matic, divergent, and polychromatic laser beams on uni- form sinusoidal lossless transmitting volume gratings and compares the model with experimental results in PTR Bragg gratings. Further parts will describe modeling of re- flecting volume gratings holographic mirrors as well as the application of both transmitting and reflecting grating for spectral beam combining for different types of lasers.

Optical third harmonic generation during femtosecond pulse diffraction in a Bragg grating

We report on a novel phenomenon occurring when ultrashort laser pulses are diffracted in a Bragg grating recorded in a photo-thermo-refractive glass: two intense beams at the third harmonic of the incident laser frequency appear between the direct and the diffracted laser beams. A high conversion efficiency of about 0.04% at an incident intensity of 5 × 1011 W cm−2 in a 1 mm thick grating is attributed to self-phase matching for the third harmonic generation via Cherenkov radiation from the interfering third order polarization of the incident and diffracted beams. It is assumed that this is a general phenomenon for diffraction in a volume grating.

Holographic volume gratings in a glass-like polymer material

We demonstrate a possibility to write efficient and thermally stable volume holographic gratings in a glassy polymer material based on PMMA and phenanthrenequinone with layers prepared, by casting the liquid solution of ingredients on a substrate and drying to a solid state. A high concentration of phenanthrenequinone (up to 4 mol. %) makes it possible to use photosensitive layers of lower thicknesses (50–180 μm) for the recording of efficient holographic gratings. The exposing is followed by a thermal amplification of the grating due to diffusion of residual phenanthrenequinone molecules and fixation by an incoherent optical illumination. We present experimental temporal curves of the refractive index modulation and diffraction efficiency both under the exposure and the heating process. The behavior of the gratings under temperatures up to 140 °C has been studied.

High transmission efficiency for surface plasmon resonance by use of a dielectric grating

The efficiency of the transmission of surface plasmon waves by use of a dielectric diffraction grating is discussed. The Kretschmann device allows us to obtain a surface plasmon resonance that consists of an absorption peak in the reflection spectrum. When surface plasmon resonance occurs, the TM-polarization mode of the incident electromagnetic wave is neither transmitted nor reflected. The procedure to transform an 4bsorption peak into a transmission peak is described. Transmittivity of 68% is obtained for a simple structure that consists of a thin-film layer of Ag coated on a volume diffraction grating and embedded between two dielectric media. The results presented herein were obtained by numerical simulations that were carried out by use of an algorithm based on the rigorous coupled-wave theory.

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

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.

Angular sensitivities of volume gratings for substrate-mode optical interconnects

The angular sensitivities of slanted volume gratings (VGs) illuminated by three-dimensional (3-D) converging-diverging spherical Gaussian beams for substrate-mode optical interconnects in microelectronics are analyzed by application of 3-D finite-beam rigorous coupled-wave analysis. Angular misalignments about the z, y, and x axes that correspond to yaw, pitch, and roll misalignments resulting from manufacturing tolerances of chips are investigated. Two cases of linear polarization of the central beam of the Gaussian are considered: E perpendicular K and H perpendicular K, where K is the grating vector. From worst-case manufacturing tolerances, the ranges of yaw, pitch, and roll misalignment angles are alpha = +/-1.17 degrees, beta= +/-3.04 degrees, and gamma = +/-3.04 degrees, respectively. Based on these ranges of misalignment angles, the decreases of diffraction efficiencies for slanted VGs that are due to both the yaw and the roll misalignments are relatively small. However, the efficiency of substrate-mode optical interconnects achieved by slanted VGs could be reduced by 61.04% for E perpendicular K polarization and by 58.63% for H perpendicular K polarization because of the pitch misalignment. Thus the performance of a VG optical interconnect is most sensitive to pitch misalignment.

High-efficiency volume holograms recording on acrylamide and N,N′methylene-bis-acrylamide photopolymer with pulsed laser

In order to achieve a better understanding of the mechanisms of hologram formation and higher diffraction efficiencies in volume gratings stored in acrylamide based photopolymers, a crosslinker (N,N′methylene-bis-acrylamide) has been incorporated in the photopolymer to record holograms by pulsed laser exposure. The presence of this component increases the polymerization rate and refractive index modulation. The recording was performed using a holographic copying process. The original was a grating of 1000 lines/mm processed using silver halide sensitized gelatin. First, the effect of the pulse fluence was investigated. When the pulse fluence was optimized, the results obtained using the new composition of material were compared with those using the composition without a crosslinker. Using a pulsed laser at 532 nm the photopolymer without crosslinker presented diffraction efficiencies slightly less than 60%. On the other hand, when the crosslinker was introduced in the photopolymer composition, the diffraction efficiencies achieved were higher than 85%. The non-linearity of the material's response was also studied comparing the energetic sensitivity, diffraction efficiencies and index modulation of gratings recorded with pulsed and continuous laser exposure. This study was performed fitting the angular scan of each grating using Kogelnik's theory.

Three-dimensional converging–diverging Gaussian beam diffraction by a volume grating

The diffraction characteristics of a volume grating (VG) illuminated by a three-dimensional (3-D) converging-diverging Gaussian beam at conical incidence are investigated by applying 3-D finite-beam (FB) rigorous coupled-wave analysis (RCWA) based on the conventional 3-D RCWA in conjunction with two-dimensional plane-wave decomposition. The Gaussian beam is assumed to have an arbitrary incidence angle, an arbitrary azimuthal angle, and any linear polarization. The two cases with linear polarizations of the central beam of the Gaussian (E perpendicular K and H perpendicular K) are investigated. The diffraction efficiencies and the diffracted beam profiles for both unslanted VGs and slanted VGs (designed for substrate-mode optical interconnects) are presented. In general, the diffraction efficiencies of a converging-diverging spherical Gaussian beam diffracted by both unslanted VGs and slanted VGs increase and approach the central-beam results as the refractive-index modulation increases.

Polarization-dependent diffraction efficiency of a photorefractive volume grating and suppression of this efficiency

In terms of refractive-index ellipsoid of a uniaxial crystal, the relationship between the diffraction efficiency of a volume grating and the polarization state of a readout beam is theoretically analyzed. The direction of a refractive light beam and the corresponding refractive-index modulation will both be changed by a variation of the polarization state. In the polarization state of the readout beam, which may lead to a strong variation in the diffraction efficiency of the volume grating. This kind of polarization-dependent diffraction efficiency of a volume grating in an anisotropic crystal is extremely disadvantageous for some applications. A method to suppress the polarization-dependent diffraction efficiency by use of double volume gratings is presented, and experiments with LiNbO3:Fe crystal are also demonstrated. The experimental results indicate that this method can well suppress the polarization-dependent diffraction efficiency of a volume grating. Furthermore, the diffraction properties of the double volume gratings are almost independent of the polarization state of the readout beam. The relative values of the diffraction peaks are calculated on the basis of the relationship between index modulation and the state of polarization. The experimental values are in good agreement with the theoretical analyses.