In the last years many efforts have been invested in the development of augmented reality devices. Depending on the application different constraints need to be faced. Providing full color see-through augmented reality on eyeglasses with ophthalmic correction, and valid for a wide range of use cases, is one of the most challenging and ambitious applications. We are within a European Project aiming to this goal. One of the key components in this eyewear is the holographic lens mirror (HLM), acting as the beam combiner responsible for the see-through capability. In this work, we present the main goals of this European Project and, more specifically, the holographic approach being developed for the HLM within our research group.
In this work, reflection gratings has been stored in Bayfol X200 from Covestro. Up to two high-efficiency reflection holograms have been multiplexed in the same layer due to the physical thickness of the material films, 16μm. The less material thickness, the fewer number of high diffraction efficiency multiplexed holograms we can store in it. Once multiplexed the pairs of holograms at different recording angles we have stuck several layers together, forming a multilayer system. From this multilayer system, we have obtained a wide diffracted spectrum were the result is the superposition of all the multiplexed gratings recorded in each layer. Therefore, this system has a good chromatic selectivity and allows us to select the reflected wavelengths and the spectral wide for each application.
The storage of time-stable holographic gratings in photohydrogels when the material is immersed in liquid media represents a great challenge at present. A very important stage in the process of storing holograms in photohydrogels are the washing stages to eliminate the remains of the components that have not reacted in the photochemical reaction. The main goal of this work is focusing on the study of the optimization of the washing stages of the photohydrogels based on acrylamide and N,N’-methylenebis(acrylamide) once unslanted transmission holograms have been stored. For the purpose of determining the compositions of the wash solutions, High-Performance Liquid Chromatography and UV-visible measurements have been employed in our system. PBST and DMSO:H2O 6:4 (v/v) are used as solvents in the washing stages. The diffraction efficiencies are measured during the washing stages and after the storing of the holograms during several days in PBST. Maximum diffraction efficiencies of 38.0 and 27.6% are reached when PBST and DMSO:H2O 6:4 are employed, respectively.
The technology we are developing consists of the use of coordination compounds with metals to carry out this detection of Acetic acid. This compound normally reacts with acetic acid changing its colour, making it a suitable compound for use as a detector. The proposed method allows detecting acetic acid in any medium, whether in solution, in the gas phase, in the solid phase, or in any combination of these. Upon contact with the acid, a colour change occurs that can be detected visually or through optical means. After its use, the active medium can be regenerated by a simple procedure and be available again for new use. This allows the creation of simple and intuitive detection devices, usable by non-experts and that can be regenerated and reused. The main advantage of this sensor is to allow the specific detection of acetic acid and quantification of its concentration, using coordination compounds with metals that are present in the yellow dye.
Maximizing phase modulation in photopolymers remains a challenge in order to use these materials to fabricate photonics devices. Different material compositions and irradiation conditions have been studied in order to achieve it. One of the main conclusions has been that with continuous laser exposure better results are achieved. However, our results show that higher phase modulation can be achieved using pulsed laser. The study has been done with crosslinked acrylamide-based photopolymers (AA/PVA), Biophotopol and Holographic Polymer-Dispersed Liquid Crystals (HPDLC) exposed with a pulsed laser (532 nm). Thus, phase modulation increases of 8-15% have been achieved between pulsed laser and continuous laser exposure, with a maximum phase depth of 3π radians in AA/PVA, ~3π/2 in Biophotopol and ~π in H-PDLC. This opens the door to the use of this photopolymer in large-scale manufacturing, such as H-PDLC photopolymers to fabricate tunable lenses using the laser-induced direct transfer (LIFT) technique.
Polymer nanocomposites are designed and engineered on a nanometer scale with versatile applications including optics and photonics. During the last two decades, different photopolymerizable nano-compounds were introduced and developed to modify polymer properties. In this sense, inorganic and organic nanoparticles have been introduced to increase the refractive index modulation and/or to reduce the shrinkage. Liquid crystal polymer composites have been added to the category of active photopolymer materials with an electrically switchable option. Nowadays, in the design of smart glasses some problems remain on the table, like power consumption, the limitation of the resolution, the wide field of view, etc. The inclusion of holographic optical element has provided some possible solutions. In particular the photopolymers have been reported a good system to bring the photons produced in the image creation to the eye. Our group proposed an alternative scheme using transmission holographic elements. The fabrication architecture was tested with different photopolymers in order to optimize their chemical composition, and we proposed three schemes adapted to each material properties. In this paper we study the influence of the initiator concentration, for Holographic polymer dispersed liquid crystal photopolymer, on the refractive index modulation and on the tunable properties of these holographic optical elements.
In recent years, the development of low-toxicity photopolymers for holographic recording in reflection mode has reached great importance. One of the main advantages of reflection holograms is that they can be reconstructed using white light, which has enabled that many researches have focused on the development of sensors for different types of analytes. Optical data storage and three-dimensional multiplexing of reflection holograms to improve the data storage density have also been investigated through reflection holograms. However, the photopolymers used in these researches have certain undesirable features such as the toxicity of some of their components and non-environmentally compatibility. The common used hydrophilic photopolymers content poly(vinyl alcohol) (PVA) or gelatine binder and monomers related to acrylamide. This compound has a high potential to cause cancer. For this reason, we developed a photopolymer called “Biophotopol” as a recording holographic material for optical applications. “Biophotopol” have a low toxicity, good recycling properties and is environmental-friendly. The basic formulation of “Biophotopol” includes an initiator system which is a free radical generator composed by triethanolamine as co-initiator and plasticizer and sodium salt 5’- 72 riboflavin monophosphate as sensitizer dye, sodium acrylate as polymerizable monomer and PVA as inert binder polymer. Additionally, a cross-linking agent, as N,N’-(1,2-dihydroxyethylene)bisacrylamide (DHEBA) , can also be added. Volume transmission gratins and holographic lens have been fabricated in this photopolymer but not researches have been done to store reflection gratins. Is know that for higher spatial frequencies, the diffraction efficiency decreases considerably as the spatial frequency increases. In this sense, the general aim of this work has been fabricated reflection gratings in the symmetrical experimental setup in “Biophotopol” and to study the dependence of diffraction efficiency on physical thickness, recording intensity and exposure energy. First, films physical thickness was investigated. The maximum diffraction efficiency was obtained for thicknesses around 145 µm. The photopolymer layers uniformity was highly sensitive to drying and environmental conditions during the exposure stage. Therefore, both conditions were investigated and very controlled. An increase in diffraction efficiency was observed when the photopolymer films were cured with a LED lamp to improve the stability of the reflection holograms. The residual dye is eliminated during this process. The maximums diffraction efficiencies around 30 % were obtained for reflection gratings with a spatial frequency of 4738 lines/mm. The index modulation and optical thickness were obtained by fitting procedure through coupled wave theory. Experimental and theoretical results have been interpreted to modify the photopolymer formulation and exposure conditions in order to increase the diffraction efficiencies.
We have included a Parallel Aligned Liquid Crystal on Silicon (PA-LCoS) microdisplay in a Holographic Data Storage System (HDSS). This novel display, widely accepted as Spatial Light Modulator (SLM), presents some advantages and disadvantages. One of these disadvantages is the anamorphic and frequency dependent effect. In this work we want to test this effect and see its effects in the complete optical process involved in the HDSS. We will use stripe-based patterns with different orientation (vertical and horizontal). To check the limits, we will increase the data density by decreasing the minimum stripe width. For evaluating the degradation suffered by the data page, we use the Bit Error Rate (BER) as figure of merit. We make a BER calculation from the statistical analysis of the histogram. In addition to the anamorphic effects we evaluate the degradation effects introduced by the non-uniformity in the illumination. To this goal we divide the image in several regions that are processed in the same way that the entire image. The error analysis of the entire optical system is useful for its calibration and fine adjustment. Once we have characterized the experimental setup we introduce the holographic material. Thus, by making the same analysis, we can evaluate the errors introduced by the material. As holographic material we use Polyvinyl Alcohol Acrylamide (PVA/AA), that has been characterized and developed in previous works by our group.
The improving of the technology related to the Spatial Light Modulators (SLM), which can be used to modulate the wavefront of a light beam in many different applications in Optics and Photonics, has widespread their use in many new ways. In particular, the continue miniaturization of the pixel size let them be used as a master for Diffractive Optical Elements(DOE) recording applications. One of these displays isthe parallel-addressed liquid crystal on silicon (PA-LCoS) microdisplay, which offers easily the possibility of phase-only modulation without coupled amplitude modulation, but can be use also as an amplitude master just rotating the angles of two polarizers. Together with the DOEs, the optic recording material is also one of the crucial componentsin the system. Photoresist has been used classically for this purpose. Recently some works provide results of the incorporation of photopolymers, initially used for holographic recording, to fabricate DOEs. Among photopolymers, polyvinil alcohol/acrylamide (PVA/AA) materials have been studied firstly due to the accurate control of their optical properties and the ease of fabrication. Nevertheless, this kind of photopolymer presents a high level of toxicity due mainly to the monomer, acrylamide. In this sense, we made efforts to search alternative “green” photopolymers, one of these is called “Biophotopol”. This material presents good optical properties; although, it has two principal drawbacks: its refractive index modulation is lower than the PVA/AA one and the dye used presents very low absorption at 532 nm. In order to solve these problems for recording spherical diffractive lenses, in the present work we have explored different possibilities. On the first place, we have modified the fabrication technique of the solid layer to achieve thicker samples, on the second place, we have introduced a biocompatible crosslinker monomer. These two actions provide us a higher value of the phase modulation capability. On the third place, we have modified the dye to record DOE’s with the wavelength of 532 nm and obtain a direct comparison with the results obtained with PVA/AA materials.
There is an increasing demand for new holographic recording materials. One of them are photopolymers, which are becoming a classic media in this field. Their versatility is well known and new possibilities are being created by including new components, such as nanoparticles or dispersed liquid crystal molecules in classical formulations, making them interesting for additional applications in which the thin film preparation and the structural modification have a fundamental importance. Prior to obtaining a wide commercialization of displays based on photopolymers, one of the key aspects is to achieve a complete characterization of them. In this sense, one of the main parameters to estimate and control is the shrinkage of these materials. The volume variations change the angular response of the hologram in two aspects, the angular selectivity and the maximum diffraction efficiency. One criteria for the recording material to be used in a holographic data storage application is the shrinkage, maximum of 0.5%. Along this work, we compare two different methods to measure the holographic recording material shrinkage. The first one is measuring the angle of propagation for both diffracted orders ±1 when slanted gratings are recorded, so that an accurate value of the grating vector can be calculated. The second one is based on interference measurements at zero spatial frequency limit. We calculate the shrinkage for three different photopolymers: a polyvinyl alcohol acrylamide (PVA/AA) based photopolymer, one of the greenest photopolymers whose patent belongs to the Alicante University called Biophotopol and on the last place a holographic-dispersed liquid crystal photopolymer (H-PDLC).
Multiplexed diffraction gratings were recorded in 300 μm thick layers of Biophotopol photopolymer by using peristrophic multiplexing schema. Thirteen sinusoidal phase gratings were stored in a low toxicity recording medium. The diffraction efficiency conservation of the multiplexed diffraction efficiency obtained was studied along the time.
Holographic data storage systems (HDSS) have been a promising and very appealing technology since the first laser developments in the sixties. Impact of ongoing advances in the various components needs to be explored in its specific application to HDSS. In this sense, continuous progress is being produced in spatial light modulator (SLM) technology where parallel-addressed liquid crystal on silicon (PA-LCoS) microdisplays have replaced previous liquid-crystal displays (LCD) in most of optics and photonics applications. PA-LCoS microdisplays are well adapted to display phaseonly elements without coupled amplitude. In this paper, we analyse how PA-LCoS devices can also be used to display the widely applied binary intensity modulated (BIM) data pages. We also investigate hybrid-ternary modulated (HTM) data pages, which are very much demanding on the phase and amplitude modulation properties of an SLM. HTM data pages combine the ease of detection of BIM data pages, together with a large reduction of the DC term of the Fourier Transform of the data page. This reduction is necessary to avoid saturation of the recording material dynamic range. Simulated results show the magnitude of the expected DC term in the Fourier plane. We have verified the good performance of PA-LCoS to display BIM data pages. We have also obtained that pure HTM data pages cannot be produced with PA-LCoS devices, however, a rather close performance is obtained when implementing the pseudo-HTM data pages. In this work a more complete study of pseudo-HTM modulation is offered.
One of the most promising phase optical recording mediums are photopolymers. In these materials, the use of an index matching component permits a better conservation of the stored information and, additionally, the study of the molecules migration and shrinkage/swelling phenomena separately. In general, the transmitted beam has the information of the thickness and refractive index modulation mixed. Therefore, we propose the introduction of a coverplate besides with an index matching liquid in order to improve the characterization and the conservation. The index matching techniques have been classically used for holographic recording materials. In principle, to obtain an accurate index matching we have to choose a liquid with refractive index very close to the mean of the polymer one. Then, when shrinkage takes place during recording, mainly due to the polymerization, the liquid will fill up the generated grooves minimizing the diffractive effects produced by the relief structure. In fact, in this work we study different index matching components for different photopolymers. The photopolymers analyzed in this work have a polyvinyl alcohol (PVA) as a binder and two different main monomers: one has acrylamide and the other one sodium acrylate. We have recorded very low diffractive gratings and studied their conservation for different index matching components.
Phase-only modulation is necessary in a great number of modern spatial light modulation applications, and the spatial light modulator (SLM) technology of choice is usually the parallel-aligned liquid crystal on silicon (PA-LCoS) microdisplay. Various degradation effects have been analyzed in the literature which may be introduced by SLMs and whose quantitative knowledge enables to select the best working conditions and/or to design specific compensation strategies to diminish negative effects. In this paper we concentrate on the phase flicker typically produced by PA-LCoS devices. The availability of a recent polarimetric-based method, the average Stokes polarimetric technique, to measure the linear retardance and its flicker amplitude eases the capability to simulate the performance of spatially varying phase multilevel elements typically addressed onto PA-LCoS devices. A representative element is the blazed grating. Recently we demonstrated the capability of the calibration provided by the average Stokes polarimetric technique to predict the performance of blazed gratings, both their average diffraction efficiency, static analysis, and its associated time fluctuation, dynamic analysis. In the present work we take advantage of the demonstrated predictive capability of our approach to analyse to find the wide range of applicability of PA-LCoS devices in applications in spite of flicker.
A wide range of chemical compositions are possible to design photopolymers. These materials are also appealing for diffractive and holographic applications due to their capability to modulate the refractive index and/or the thickness when illuminated. Some of the most interesting applications for photopolymers are the optical data storage, security systems, surface relief photo-embossing, diffractive and refractive optical elements, holographic elements, solar concentrators, optical detectors and hybrid optoelectronic 3-D circuitry. Looking for an optimized chemical composition for each application many different photopolymers compositions may be needed enabling a variety of materials properties: materials with low or high rates of monomer diffusion, low or high values of shrinkage, long or short length of polymer chains and low or high light absorption. In parallel many models are presented in order to predict the photopolymers recording and the post exposure evolution. In this work we use one of these experimentally checked models to study the influence of the material characteristics in the final diffractive optical element recorded in the material. We study the changes in the surface relief and in the refractive index in order to understand the importance of each material property in the final diffractive optical element recorded.
Parallel-aligned liquid crystal on silicon (PA-LCoS) displays have become the most attractive spatial light modulator device for a wide range of applications, due to their superior resolution and light efficiency, added to their phase-only capability. Proper characterization of their linear retardance and phase flicker instabilities is a must to obtain an enhanced application of PA-LCoS. We present a novel polarimetric method, based on Stokes polarimetry, we have recently proposed for the measurement of the linear retardance in the presence of phase fluctuations. This can be applied to electrooptic devices behaving as variable linear retarders, and specifically to PA-LCoS. The method is based on an extended Mueller matrix model for the linear retarder containing the time-averaged effects of the instabilities. We show experimental results which validate the predictive capability of the method. The calibrated retardance and phase fluctuation values can then be used to estimate the performance of the PA-LCoS device in spatial light modulation applications. Some results will be given.
Holographic polymer dispersed liquid crystals (H-PDLC) are made by holographic recording in a photo-polymerization induced phase separation process in which the liquid crystal molecules diffuse to dark zones in the diffraction grating. The devices with H-PDLC materials develop a dynamic behavior that may be modified by means of an electric field. We study a photopolymer formulation with high diffraction efficiency but with the problem of high electric conductivity. We use a bleaching post-exposure treatment to obtain devices with a better electro-optical performance.
Parallel aligned liquid crystal on silicon (PA-LCoS) displays have found wide acceptance in applications requiring phase-only modulation. Among LCoS devices, and PA-LCoS as a specific case, digital addressing has become a very common technology. In principle, modern digital technology provides some benefits with respect to analog addressing such as reduced interpixel cross-talk, lower power consumption and supply voltage, gray level scale repeatability, high programmability, and noise robustness. However, there are also some degradating issues, such as flicker, which may be enhanced. We analyze the characteristics of the digital pulse width modulated voltage signals in relation to their effect on the optical modulation capabilities of LCoS displays. We apply calibration techniques developed in our laboratory, basically the classical linear polarimeter extended to take into account the existence of flicker. Various digital sequence formats are discussed, focusing the analysis on the variations in the magnitude of the applied voltages across the LC layer. From this analysis, we obtain how to amplify the retardance dynamic range and how to enhance linearity in the device without enhancing flicker and without diminishing the number of available quantization levels. Electrical configurations intended for phase-only and intensity modulation regimes, useful in diffractive optics, are given.
High spatial frequencies in holographic gratings are difficult to obtain by limitations of the recording material. In this work, the results obtained after storing holographic transmission gratings with a spatial frequency of 2656 lines/mm in a material based on PVA/AA are presented. A chain transfer agent, the 4,4 '-azobis (4-cyanopentanoic acid) (ACPA) has been incorporated in the material composition to improve the spatial resolution. The concentration of the ACPA in the different compositions of the material has been modified in order to find the optimal concentration which gets obtain the maximum diffraction efficiency for high spatial frequencies.
Among the existent technologies of spatial light modulator devices, parallel aligned liquid crystal on silicon displays
(PA-LCoS) have found wide acceptance. They are especially interesting since they provide phase-only modulation with
no coupling of amplitude modulation. Optimal use of these devices requires proper calibration of their modulation
capabilities in order to minimize some degradation effects found in the literature, such as flicker or anamorphic and
frequency dependent modulation. In this work we apply calibration techniques developed in our lab, basically the
classical linear polarimeter adapted to be able to take into account the existence of flicker. This method enables to obtain
both the average retardance and a good estimation of the magnitude of the phase fluctuation when flicker is present.
Various addressing formats are discussed and variations in the magnitude of the applied voltages are considered in order
to amplify the retardance dynamic range and to enhance linearity in the device. Finally, two electrical configurations
intended for phase-only and amplitude-mostly modulation regimes, useful e.g. in diffractive optics, are given.
Recently the possibility to record phase diffractive optical elements (DOEs) onto photopolymers has been explored. Two
of their properties when they are illuminated are useful to this goal: the relief surface changes and the refractive index
modifications. The recording intensity distribution with a sinusoidal profile is the easiest profile to record in a
holographic recording material, i.e. it can be obtained by the simple interference of two plane wave beams or
alternatively using a spatial light modulator. This second method is more flexible and opens the possibility to record a
wide range of diffractive elements such as binary, blazed gratings, diffractive lenses, etc. Sharp profiles may as well be
recorded. In general they present a clear smoothing of the edges due to various reasons: the cut-off frequency (a low pass
filtering) of the optical system, quality of the spatial light modulation, inhibition period, finite size of polymer chains,
monomer diffusion, and non-linearities in the recording process. In this work we have analyzed the importance of some
of these aspects of the photopolymer and the experimental set-up in order to record high quality DOEs. The
photopolymer analyzed is based on polyvinylalcohol/acrylamide. To achieve this goal we have used a diffusion model to
simulate the DOE’s recording with different recording intensities distributions.
Novel liquid crystal on silicon displays (LCoS) technology has found widespread use in a number of applications dealing
with the spatial modulation of the properties of a light wavefront. Parallel aligned LCoS (PA-LCoS) are especially
interesting since they provide phase-only modulation with no coupling of amplitude modulation. However, typically
LCoS are digitally addressed and this has been proven as a drawback since it causes fluctuations in the phase
modulation. In this work we concentrate on PA-LCoS. We analyse the application of a classical polarimetric method for
retardance measurement, which makes use of only linearly polarized light, typically used in wave plates. We analyse the
effect of phase fluctuations on the measurements and provide a refinement of the method which allows estimating both
the average retardance and the magnitude of the phase fluctuation. We demonstrate both theoretically and experimental
that this extension of a classical method is both simple and very well suited for the measurement of the modulation
properties of novel PA-LCoS.
We build a dynamic beam splitter with a holographic optical element (HOE). The laser light goes through the HOE and a fraction of intensity diffracted and transmitted could be tuned by an electric signal. We use holographic polymer
dispersed liquid crystals materials. It is made by holographic recording in which the liquid crystal molecules diffuse to
dark zones in the diffraction grating and they can be oriented by means of an electric field. The orientation of the liquid crystal produces a refraction index variation which changes the diffraction efficiency and therefore the grating has a dynamic behavior.
Holographic reflection gratings were stored in a PVA/AA based photopolymer material using symmetrical geometry.
Diffraction efficiency of the gratings was measured and a curing process was applied in the gratings to fix them. The
aim of this paper is to analyze the stability of stored gratings with over time, after to apply the curing process,
comparing the results obtained with and without curing.
Typically 4-f systems are considered as the basis for holographic memory setups. However, other geometries, such as the
convergent correlator, may also be considered. This is a setup widely used in optical processing architectures but not so
much explored in holographic data storage systems. It provides some benefits when used in optical processing such as
flexibility in the adjustment between Fourier filter dimensions and the Fourier transform of the scene. It also allows a
wider freedom in the choice of the optical systems (lenses) used in the setup since it is no longer necessary that their
focal lengths match, and the total length of the setup may be shortened. In this paper we make use of Fourier optics
techniques to analyze the validity and possible benefits of this setup in its application to holographic memories. We
consider the recording and the reconstruction steps. Both analytical expressions and simulated results are given.
The PEA photopolymer is composed of dipentaerythritol penta/hexa-acrylate as monomer and binder, N-vinyl
pirrolidone as crosslinker, ethyl eosin as dye and N-methyl diethanolamine as radical generator. This photopolymer is
suitable to work with dispersed liquid crystal molecules in dynamic holographic and diffractive applications. In order to
characterize these materials we have analyzed the behaviour of different compositions at zero spatial frequency limit.
This method is based on an interferometer that has been successfully applied in the phase-shift versus applied voltage
characterization of liquid-crystal displays, in addition to that it has been applied to characterize PVA/AA and
PVA/NaAO photopolymers. In PEA case there is no shrinkage since the photopolymer is coverplated. Samples have a
glass substrate as the cover plate. In our analysis we have studied the importance of the monomer, crosslinker and crystal
liquid molecules concentrations, in the phase shift produced in the layer during photopolymerization process.
We use 4,4' azo-bis-(4-cyanopentanoic acid) as chain transfer agent in a photopolymer with
triethanolamine/yellowish eosin as initiator system. It is possible to work in a particular conditions to
get a chain transfer effect minimizing the decomposition of 4,4' azo-bis-(4-cyanopentanoic acid) by
the sensitized dye. The improved photopolymer has a low scattering due to the low molecular weight
of the generated polymer chains. This is related to the chain transfer effect in the size of the polymer
chains. It is important to establish the optimum concentration of chain transfer agent to avoid
decreasing the maximum diffraction efficiency due to a low molecular weight of the polymer chains.
In this paper four different photopolymers are compared on the basis of their suitability for
holographic data storage. The optical recording parameters of these photopolymers were directly
determined using the zero spatial frequency limit. The behavior of cover plated and uncover plated
material layers was analyzed. Once the main parameters were determined, we proposed a novel model
to simulate the recording of relief diffractive elements onto the photopolymers without cover plating.
Relief surface changes provide interesting possibilities for storing diffractive optical elements on
photopolymers and are an important source of information for characterizing and understanding the
material behaviour. In this paper we also present a 3-dimensional model, based on direct parameter
measurements, for predicting the relief structures generated on the material. This model was
successfully applied to different photopolymers with different values of monomer diffusion.
In this work we present an analysis of non-slanted reflection gratings by using a corrected Coupled Wave Theory which
takes into account boundary conditions. It is well known that Kogelnik's Coupled Wave Theory predicts with great
accuracy the response of the efficiency of the zero and first order for volume phase gratings, for both reflection and
transmission gratings. Nonetheless, since this theory disregard the second derivatives in the coupled wave equations
derived from Maxwell equations, it doesn't account for boundary conditions. Moreover only two orders are supposed, so
when either the thickness is low or when high refractive index high are recorded in the element Kogelnik's Theory
deviates from the expected results. In Addition, for non-slanted reflection gratings, the natural reflected wave
superimpose the reflection order predicted by Coupled Wave theories, so the reflectance cannot be obtained by the
classical expression of Kogelnik's Theory for reflection gratings. In this work we correct Kogelnik's Coupled Wave
Theory to take into account these issues, the results are compared to those obtained by a Matrix Method, showing good
agreement between both theories.
We propose a rigorous electromagnetic analysis for a Photonic Crystal Fiber (PCF) geometry consisting of
multiple hollow slits that go across the fiber core along the propagation axis z. The slits are regarded as
invariant along the transverse dimension x but exhibit multiple sinusoidal bends in the y-z plane, which prevents
the transversal profile being constant along the z axis. To analyze and characterize the electromagnetic behavior
of the considered PCF geometry, we use a 2D Finite-Difference Time-Domain (FDTD) scheme assuming an
ultrashort incident pulse with a polarization angle of 45 degrees as the excitation source. Our analysis focuses
on three key aspects for the ultrashort pulse propagation through the slit array: pulse shaping and delay,
spatiotemporal dispersion and birefringence features. Numerical FDTD simulations illustrate the effect of the slit array parameters on the previous magnitudes. Our results demonstrate that the proposed structure provides with a wide and deep control over the pulse propagation and wavefront.
The photopolymers with a hydrophilic matrix as poly(vinyl alcohol), PVA, are versatile holographic recording materials
in hologram recording experiments. They use water as solvent and they can be made in layers with several thickness.
One of the photopolymers more used is composed of acrylamide as polymerizable monomer, PVA and water as binder.
The pair: triethanolamine, TEA, and the dye yellowish eosin, YE, is widely used as initiator system due to its high
sensitivity and efficiency. TEA is the radical initiator more used with dyes derived from fluorescein as YE because they
can generate a radical by redox reaction under dye excitation by light. The dye is bleached in this process because is
decomposed in the photoinitiation reaction. The ethylenediaminetetraacetic acid EDTA has a molecular structure very
similar to TEA and therefore could replace it in this kind of photopolymers.
The 4,4' azo-bis-(4-cyanopentanoic acid), ACPA, is a radical initiator that is soluble in water and usually used in
polymerizations in solution with thermal initiation. In this work, we use EDTA and ACPA in order to check their
properties as radical initiator in the photochemical reaction that takes place inside the photopolymer while a hologram is
being recorded. We will compare the results obtained with those derived from TEA and will evaluate the possibilities for
these substances.
Photopolymers are useful for different applications such as in the development of holographic memories or holographic
optical elements. Photopolymers have an undesirable feature, the toxicity of their components and their low
environmental compatibility, particularly if we analyse the life cycle of the devices made with these materials and their
interaction with the environment. In this sense the University of Alicante has patented new dry biocompatible
photopolymer: Biophotopol. Initially this new photopolymer was optimized to holographic memories application. The
main goal of the previous works was to achieve thick stable layers. On the other hand polyvinyl/acrylamide (PVA/AA)
photopolymers have been widely studied by many research teams. The main drawback of an AA-based photopolymer as
far as the environment is concerned is the acrylamide, a substance which has been known to be carcinogenic for many
years. Recent investigations have characterized PVA/AA based photopolymers at very low spatial frequencies. In
previous works we have proposed the application of interferometric techniques, both in transmission and in reflection, to
characterize in real-time the modulation performance of the photopolymers. We used this approach to characterize the
optical modulation properties of a PVA/AA photopolymer. With this scheme we mainly characterize the properties at
very low spatial frequencies, which can be useful to analyze the applicability of holographic recording materials in
another range of applications, such as recording of diffractive optical elements (DOEs). In this work we have compared
Biophotopol to PVA/AA photopolymers.
Liquid crystal displays (LCDs) are widely used as spatial light modulators (SLMs) in many applications (optical signal
processing, holographic data storage, diffractive optics...). In particular, as an alternative microoptics recording scheme
we have explored the possibility to use a LCD to display the diffractive optical element (DOE) to be recorded onto a
photosensitive phase material, so as to enhance the flexibility of the recording architecture. In this application the LCD
acts as an amplitude dynamic transparency. By means of an optical system we image the function addressed to the LCD
onto the recording material. The element to be recorded onto the phase material can be easily changed simply by
changing the function addressed to the LCD. Among the recording materials, photopolymers provide very attractive
capabilities. They present a great flexibility in their composition, the recording layer can be manufactured in a wide
range of possible thicknesses, and they are inexpensive. These properties make it an interesting material to generate the
phase DOEs. Both the composition and the thickness need to be optimized for the application to DOEs. In this work we
explore the results dealing with the calibration of the recording setup and the photopolymer material. We also analyse the
performance of phase-only diffractive lenses generated onto the photopolymer. Promising results have been obtained,
where the focalization of the diffractive lenses generated has been demonstrated.
In holographic recording photopolymers the diffraction efficiency develops according to Kogelnik
expression during hologram recording stage. In the ideal situation, the curve of diffraction efficiency
versus energetic exposure has a linear zone, although it is usual to find examples of deviations to this
behavior. In this investigation we study different cases in which there are deviations to ideal behavior,
proposing different hypothesis in order to explain the situations that can be experimentally obtained.
The more important deviation experimentally observed is when the first maximum is lower than the
second one in the diffraction efficiency versus exposure curve during recording. A main hypothesis is
presented, which is related to the dye concentration into the layer and the molecular weight of the
polymer chains generated in the polymerization process.
Holographic data storage is a new optical technology which allows an important number of bits to be stored in a
recording material. In this work, holographic data pages were stored in a PVA/acrylamide photopolymer layer using a
peristrophic multiplexing method. This material is formed of acrylamide photopolymers, which are considered
interesting materials for recording holographic memories. A liquid crystal (LCD) device was used to modify the object
beam and store the data pages in the material. This LCD was used to modulate the object beam in order to reduce the
zero frequency of the Fourier Transform (FT) of the object. The scheme used to reduce this frequency was hybrid
ternary modulation (HTM).
Photopolymers are light sensitive materials with adequate characteristics for application as holographic recording media in new devices as optical elements and holographic memories. A specific set of parameters are required from the photopolymers: good energetic sensitivity to save energy during the recording process, an adequate spectral sensitivity, a higher resolution to let the necessary fidelity in the stored information reproduction, a better environmental compatibility including low toxicity, easy making management and good recycling properties. We present the main research lines in this area and the last results obtained with the new photopolymeric materials that we are developing.
Recently, we have proposed the application of interferometric techniques, both in transmission and in reflection, to
characterize in real-time the modulation performance of the photopolymers. In this work we use this approach to
characterize the optical modulation properties of a polyvinyl alcohol/acrylamide (PVA/AA) photopolymer. A double
beam interferometer is constructed, both in transmission and in reflection, in combination with the setup to expose the
recording material. Some benefits are provided by this approach: direct calculation of the properties of the material is
possible, index and surface modulation can be decoupled, and additional information can be obtained since the results are
not influenced by diffusion processes. With this scheme we mainly characterize the properties at very low spatial
frequencies, which can be useful to analyze the applicability of holographic recording materials in another range of
applications, such as recording of diffractive optical elements (DOEs). Comparison with the conventional holographic
characterization shows significant differences.
Holographic data pages were multiplexed in a PVA/acrylamide photopolymer layer. This material is formed of
acrylamide photopolymers which are considered interesting materials for recording holographic memories. A liquid
crystal device was used to modify the object beam and store the data pages in the material. Objects with different pixel
size have been used to simulate the data pages. A peristrophic multiplexing method is used to store a large number of
data pages in the same spot in the material. The Bit Error Rate (BER) was calculated fitting the histograms of the
images to determine the quality of the images.
Holographic data pages were stored in a PVA/acrylamide photopolymer. This material is formed of acrylamide photopolymers which are considered interesting materials for recording holographic memories. A liquid crystal device was used to modify the object beam and store the data pages in the material. During the storage process, some parameters like exposure time, beam ratio and intensity were controlled to obtain high quality images after the reconstruction process. The bit error rate was calculated to determine what parameters improve the quality of the image.
We present the characterization and the optimization of the phase and amplitude modulation of a liquid crystal display
(LCD) applied to holographic data storage. We discuss the main LCD modulation regimes demanded in the literature in
the application to holographic memories: binary amplitude, binary phase and hybrid ternary modulation (HTM). We
show how to obtain optimally these modulation regimes with a LCD. In our strategy it is essential the accuracy
demonstrated by the model we use to describe the modulation properties of the LCD. This accuracy allows for a reliable
computer search of the configurations enabling for an optimum LCD complex amplitude modulation. Results are given
for the calibration of a commercial LCD. Optimum configurations close to the ideal for each of the modulation regimes
are also obtained and the values of the various parameters characterizing the quality of these configurations are provided.
We have also analyzed the performance of these configurations to perform the Fourier transform (FT) of a binary data
page. We see that the optimum configurations obtained for the binary phase and the HTM regimes produce a
homogeneous distribution of the energy in the Fourier plane with no DC peak.
In this work we analyse the complex amplitude response of volume holograms. This analysis suggests a new scheme for encoding complex amplitude wavefront information onto an optical hologram which may prove useful for combining computer-generated holograms (CGH) onto volume holograms. Specifically, we analyse the amplitude and phase
modulation associated with a volume phase unslanted transmission grating. Using the expressions given by the Kogelnik's coupled wave theory (KCWT) we find that there is a coupling between the amplitude and the phase modulations. This coupling can be controlled to some extent by means of the reconstruction angle: we have found that the significant magnitude is the normalized Bragg detune angle. A large variation range of the grating strength is desirable, which is enhanced by the use of shorter wavelengths, and materials exhibiting overmodulation capability and/or with a high thickness.
The acrylamide photolymers are considered interesting materials for holographic media. They have high diffraction efficiency (ratio of the intensities of the diffracted and the incident beams), an intermediate energetic sensitivity among other materials and post-processing steps are not necessary, therefore the media is not altered. The layers of these materials, about 1 mm thick, are a suitable media for recording many diffraction gratings in the same volume of photopolymer using peristrophic multiplexing technique, with great practical importance in the field of holographic memories type WORM (write once read many). In this work we study the recording of diffraction gratings by peristrophic multiplexing with axis of rotation perpendicular to the recording media. The photopolymer is composed of acrylamide as the polymerizable monomer, triethanolamine as radical generator, yellowish eosin as sensitizer and a binder of polyvinyl alcohol. We analyze the holographic behaviour of the material during recording and reconstruction of diffraction gratings using a continuous Nd:YAG laser (532 nm) at an intensity of 5 mW/cm2 as recording laser. The response of the material is monitored after recording with an He-Ne laser. We study the recording process of unslanted diffraction gratings of 1125 lines/mm. The diffraction efficiency of each hologram is seen to decrease as the number of holograms recorded increases, due to consumption of the available dynamic range, in a constant exposure scheduling. It can be seen that the photopolymer works well with high energy levels, without excessive dispersion of light by noise gratings. In order to homogenize the diffraction efficiency of each hologram we use the method proposed by Pu. This method is designed to share all or part of the avaliable dynamic range of the recording material among the holograms to be multiplexed. Using exposure schedules derived from this method we have used 3 scheduling recordings from the
algorithm used. Additionaly, we use an exponential scheduling recording in order to correct the exposure times from the first iteration of the algorithm.
Research dealing with models to predict and understand the behaviour of photopolymers have generated many interesting studies considering a 2-dimensional geometry. These models suppose that the photopolymer layer is homogeneous in depth. Using this approximation good results can be obtained if the thickness of photopolymers is less than 200 μm. However, it is well known that Lambert-Beer's law predicts an exponential decay of the light inside the material. In recent years intensive efforts have been made to develop new holographic memories based on photopolymers. For this application the thickness of the layer is increased, usually to more than 500 μm, and Lambert-Beer's law plays a significant role in the recording step. The attenuation of the index profile inside these materials has been measured, showing that it is an important phenomenon. This attenuation limits the maximum effective optical thickness of the grating and shows that the 2-D models can not be applied in these cases. For this reason in this work a 3-dimensional model is presented to analyze the real behaviour of the photopolymers and study the variations in the
index profile in depth. In this work we examine the predictions of the model in the case of a general dependence of the polymerisation rate with respect to the intensity pattern, and the effects of varying the exposure intensity are also compared in 3-D cases. Finally, the limitation of the data storage capacity of the materials due to the Lambert-Beer law is evaluated.
The characterization of the behavior of photopolymers is an important fact in order to control the holographic memories
based on photopolymers. In recent years many 2-dimensional models have been proposed for the analysis of
photopolymers. These models suppose that the photopolymer layer is homogeneous in depth and good agreement
between theoretical simulations and experimental results has been obtained for layers thinner than 200 μm. The
attenuation of the light inside the material by Beer's law is an important factor when higher thickness are considered. In
this work we use a Finite-Difference method to solve the 3 dimensional problem. Now diffusion in depth direction and
the attenuation of the light inside the material by Beer's law are also considered, the influence of the diffusivity of
material in the attenuation of the refractive index profile in depth is analyzed.
A peristrophic multiplexing method is used to store various diffraction gratings at the same spot in the material. This material is formed of acrylamide photopolymers which are considered interesting materials for recording holographic memories. They have high diffraction efficiency (ratio between diffracted and incident beams), high energetic sensitivity and optical quality, and developing processes are not necessary. In this work, the photopolymer is composed of acrylamide (AA) as the polymerizable monomer, triethanolamine (TEA) as radical generator, N,N′methylene-bis-acrylamide (BMA) as crosslinker, yellowish eosin (YE) as sensitizer and a binder of polyvinyl alcohol (PVA). The
layers of material obtained are approximately 1 mm thick. Using holographic recording schedules, the exposure energy each hologram should receive in order to achieve uniform diffraction efficiency is optimized. The purpose of these recording schedules is to enable full advantage to be taken of the whole dynamic range of the material and to share it between the individual holograms. The Scheduled Exposure Method (SEM) and the Incremental Exposure Method (IEM) are the two multiplexing schedules used to determine the recording times. Having determined these times, the results obtained with both methods are compared to ascertain which method enables the greatest number of holograms to be recorded with the highest, most uniform diffraction efficiencies.
We present two laboratory experiments on optical image processing which show complementary modeling approaches. One experiment is based on an optical correlator, where a physical Fourier plane can be identified, and the other is based on the angular response exhibited by a volume grating (Bragg processing) where no Fourier plane is available. In the correlator the frequency content can be visualized as intensity variations in the Fourier plane, whereas in Bragg processing the angular plane waves decomposition of the object is the intuitive approach. Then we have two different approaches to synthesize the transfer function of the system. The combination of the two experiments in a student's lab helps for a deeper understanding of optical image processing, its linear systems mathematical background, and some aspects of volume holography.
In recent years the development of new holographic memories based in photopolymers have a great goal. A new commercial holographic memories has been optimized by Aprilis and Inphase. The advantages of this type of materials to store information are well know: their high capacity and their fast random access. In the behaviour of this type of material two important factors determinate the quality of the material, the capacity and the energetic sensitivity. The base of the high capacity of the holographic memories is the high Bragg’s angular selectivity. The narrow curve of the angular scan are obtained when the effective optical thickness of material is high and permits record many holograms turning the plate only 3°. Others important factors are the dye and monomer concentration, high values of the concentrations origins high values of the index modulation (higher stored capacity, better energetic sensitivity and large dynamic range) but in when the dye and monomer concentrations are high all the light is absorbed in the first 200 μm of the material and the effective optical thickness of the grating recorded decrease (decrease the stored capacity) and the values of the noise are high in this case. In this work this attenuation of the index modulation in depth is having into account using rigorous couple wave theory to obtain the limit of the effective optical thickness for each composition of photopolymer. The optimal composition to obtain high stored capacity, high index modulation, good energetic sensitivity, high dynamic range and low noise are found.
In this study we focus on the properties of acrylamide photopolymer layers about 1 mm thick with hydrofilic binder of polyvinyl alcohol and yellowish eosin as dye. We consider peristrophic multiplexing. Note the differences in the reconstruction step between the next two peristrophic multiplexing schemes: rotation axis parallel and rotation axis perpendicular to the hologram media. In the first case (parallel) a very thick media allows for a high storage density and a highly selective individual hologram reconstruction thanks to the Bragg angular selectivity. In the second case (perpendicular), an additional optical system is necessary to separate the various simultaneous holograms reconstructed. We show experimental evidence for the two schemes. We analyze the behaviour of the material when multiplexing diffraction gratings using peristrophic multiplexing. In the reconstruction step we observe differences, between the various multiplexed holograms, in their angular response shape. We consider that this is due to polymer diffusion in exposed areas or to the dye concentration variation with registered holograms number, because xantene dyes are bleached at the initiation step of radical polymerization reaction. The photopolymer composition is an important factor to be taken into account when single or multiple holograms are registered. The differences in both cases are considered in this work.
In this work we analyse the very specific properties offered by volume holography when applied to image processing with no Fourier plane. Bragg diffraction, exhibited by holographic optical elements (HOEs), modifies the impulse response of an imaging system, facilitating spatial filtering operations with no need for a physical Fourier plane (Bragg processing). We show both experimental and simulated results with holographic phase gratings and with holographic lenses generated on a polyvinyl alcohol/acrylamide (PVA/AA) photopolymer. We determine which are the significant parameters to model the performance of the HOEs for Bragg filtering: orientation and bandwidth of the passband of the filter. We relate these spatial filtering parameters with their corresponding counterparts in volume holography. We also show how the local variation of these parameters is responsible for space-variance properties of the HOE when applied in Bragg processing. We have also analysed the impulse response characteristics of the Bragg filter together with the effects of the limited aperture of the imaging system.
The hard research about the models to predict and understand the behaviour of photopolymers have as a results many interesting works for 2-dimensional cases. These studies permits obtain the mains parameters that governs the process in photopolymers with maximum thickness about 200 μm. Historically this materials, dry layers of photopolymers, have been used in many attractive devices and now a new application of this type of material is being developed: the application of photopolymers as holographic memories. The main characteristics of these layers of photopolymers are their high thickness (higher than 600 μm). In order to optimize this layers the original photopolymer composition has been changed, then a new parameters estimation has be done. In this work this study is made using PVA/Acrylamide photopolymer with layers around 800 μm of thick. The values of monomer and polymer diffusion are obtained and the values of polymerization rate constant and residual monomer are calculated too using and first harmonic diffusion constant. The validity of this model to study the layers with high thickness is evaluated, because this type of materials only a few percent of the initial light arrive to the depth zones of photopolymer.
In order to achieve higher diffraction efficiencies of the volume
gratings stored in acrylamide based photopolymer, we introduce in the photopolymer a crosslinker (N,N'methylene-bis-acrylamide). The presence of this component increase the rate polymerization and the modulation of refraction index. The recording was performed using a holographic copying process. The original was a grating of 1000 lines/mm processed using silver halide sensitized gelatine, with diffraction efficiency around 50 % for a reconstruction wavelength of 532 nm. The main beam was split in two secondary beams by the
original grating, with an intensity ratio 1:1. The results obtained using the new composition of material are compared with the composition without crosslinker. In the other hand the no linearity of the material's response is also studied comparing the energetic sensitivity, diffraction efficiencies and index modulation of gratings recorded with pulsed and continuous laser. This study is realized fitting the angular scan of each grating using Kogelnik's theory. The gratings are recorded with wavelength of 532 nm when pulsed exposure is used and with wavelength of 514 nm when continues exposure is used. Using pulsed laser at 532 nm the photopolymer without crosslinker presents the diffraction efficiencies lightly smaller than 60%. In the other hand when the crosslinker has been
introduced in photopolymer composition, the diffraction efficiencies achieves are higher than 85 %.
In this work we explore the application of volume effects given by holographic optical elements (HOEs) to image processing operations. Bragg diffraction, exhibited by HOEs, modifies the impulse response of an imaging system, facilitating spatial filtering operations with no need for a physical Fourier plane. Both the holographic recording material we use and the specific HOE we design are important parameters. In the present communication we report our last results using a polyvinyl alcohol/acrylamide (PVA/AA) photopolymer. This compound combines good optical properties, ease of fabrication, and self-development capability. We are able to produce layers as thick as 1 millimeter (mm) with a diffraction efficiency higher than 80%. Using layers with a thickness of 0.1 mm and 1 mm we have recorded both holographic diffraction gratings and holographic lenses. When using the holographic gratings we can obtain image edge enhancement as demonstrated in previous works. As a novelty, in this work we show that when using the holographic lenses we obtain a space-variant image edge enhancement. We analyse the properties of this space-variant operation in terms of the reconstruction geometry and the local grating structure exhibited by the lens across its aperture.
Photopolymers are systems of organic molecules that rely on photoinitiated polymerization to record volume phase holograms. Characteristics such as good light sensitivity, large dynamic range, good optical properties and relatively low cost make photopolymers one of the most promising materials for write-one, read-many (WORM) holographic data storage applications. Thus, it is interesting to understand the mechanisms that control the way information is stored in photopolymer materials. Different authors have demonstrated that two processes play the main role in hologram formation: monomer polymerization and monomer diffusion. A number of models based on these two processes have been proposed and their prediction capability has been validated. In this work we extend the capabilities of the existent models by introducing another important characteristic: the attenuation of light through the depth of the material which happens in the recording process. In order to check the validity of the theoretical model that we propose, volume phase transmission gratings are recorded in a PVA/Acrylamide photopolymer with different spatial frequencies. Using the Rigorous Coupled Wave
Theory (RCWT) we show that we can obtain information about the higher harmonics in the recorded refractive index modulation. Comparison between simulated and experimental results validates the interpretation provided by the proposed model.
Photopolymers are considered one of the most interesting materials for holographic storage applications. The search for optimum chemical formulations to improve the quality of the final polymer materials has yielded a great deal of experimental work. different theoretical models have also been developed by different authors to clarify the mechanism of hologram formation inside photopolymer materials. These advances in both experimental and theoretical fields permit obtaining suitable compositions of photopolymer materials for data storage purposes. In this work we present the results of an optimization of a PVA/acrylamide recording material with a high thickness (over 1 mm) to obtain holographic memories. Diffraction gratings of 1125 lines/mm were recorded in this material by using the peristrophic multiplexing technique and using the light from an Argon laser at a wavelength of 514 nm.
The kinetics of photosensitive polymer holographic recording materials is examined. We discuss why a linear relationship between monomer concentration and polymerization rate does not satisfactorily explain current experimental results and propose a possible solution. Then, using the Rigorous Coupled Wave Model(RCWM) we examine the higher order grating components so as to more clearly understand the non-linear relationship between exposing intensity and polymerization.
In this study we analyze the behavior of thick dry layers of a PVA/acrylamide photopolymer when used as a holographic recording material. The thickness of the layers is greater than 150 μm and the holographic parameters studied are diffraction efficiency, sensitivity, stability and the possibility of overmodulation with high exposures.
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