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In the present work, the viability of a novel recording geometry to produce reflection holographic couplers is analyzed. Recalling the idea of previous works, photopolymers are used as the recording material because they have been proven to be well-suited for the intended see-through application: the capability to provide a virtual image without compromising the information about the surrounding environment. Moreover, holography fundamentals give us the proper background to examine the proposed design, where no prisms or microlenses arrays are used. Aiming to support the analysis, we provide experimental evidence that the produced reflection holographic gratings exhibit the correct properties to work as a coupler, where the sensitivity of the material and its properties are studied and examined.

Estimating the actual parameters of real holographic volume gratings from diffraction efficiency measurements is challenging. The natural formation of the grating provides different phenomena, such as shrinkage, bending of the fringes, or non-homogeneous modulation as a function of the thickness, amongst other issues. This work proposes a deep learning Convolutional Neural Networks (CNNs) and Feedforward Neural Networks (FNNs) hybrid architecture capable of predicting the grating parameters from theoretical and experimental diffraction efficiency patterns. For the training set of this regression problem, Kogelnik’s Coupled Wave Theory simulated data has been employed. Our best model has been trained with an 8000-sized dataset of 80 points of diffraction efficiency patterns simulated from a range of values for the normalized grating wavelengths, index modulations, and optical depths. It achieves test losses under one per cent (mean absolute error) for predicting the normalized grating wavelengths, index modulations and optical depths.

The see-through application of holographic optical elements (HOEs) has gained a lot of attention in the scientific literature in recent years. Regarding this kind of holograms recorded in photopolymers, the shrinkage plays an important role in the final properties of the display, as it may alter the Bragg angle. The present work is focused on the characterization of a HOE as a coupler in a waveguide combiner, recorded in holographic transmission geometry without prisms in the experimental set-up. From this point of view, it has been found that for specific geometries where the diffracted beam cannot be trapped a priori, a significant coupling effect exists: this variation in the diffracted energy and the propagation angle inside the glass can be used to measure the shrinkage of the photopolymer for this HOEs recording.

It is a fact that augmented reality or virtual reality has burst into society with force: allowing the creation of programmed video games to be played with virtual reality glasses or the manufacture of glasses that act as mobile devices connected to the Internet ("Google glasses"). Augmented reality devices generally consist of a headset and a display system to show the user virtual information in addition to the real one. The systems used are the transparent optical screen and the mixed video screen. In this way, virtual images can be used that are shown to the user, mixed with reality or projected directly on the screen. On the other hand, the "glasses" have some advantages in relation to a direct viewing mobile screen, for example, hands-free and high privacy character. Among the different models of glasses proposed, transparent glasses are especially important for mobile use because they are safer and easier for the public to accept. However, combining high-quality images with clear visibility throughout the system remains a challenge. In this work, holographic waveguides will be manufactured in photopolymers and the experimental results will be contrasted with the numerical simulations carried out using the RCW models. An important fact that must be taken into account in the recording of the holographic waveguide is the possible recording of noise reflection gratings. In general, noise gratings in other devices extract a small amount of energy from the diffracted order, without any other spurious effects. However, in this particular device it will be shown that noise reflection gratings could create an unwanted afterimage. Since the reconstruction is performed at a different wavelength than the recording, the reflection noise grating may or may not be "visible" during the reconstruction, so it is possible to control the appearance of the gratings, for example, by controlling the thickness of the final waveguide. The gratings to performed the study were recorded

Since photopolymers were first used as a holographic recording medium in the 1970s, one of the most important goals to be achieved was a quantitative understanding of their behaviour. In general, photopolymers are composed of one or more monomers, a binder and a dye that absorbs light in a certain region of the spectrum. The absorbed photons initiate polymerisation in the bright regions creating a concentration gradient compensated by the diffusion of chemical molecules described by Fick’s law. The first models to simulate the etching process in this type of materials were proposed in the early 1990s; they were very simple models where the authors assumed a harmonic in the monomer concentration, a constant rate of polymerisation and diffusion without direct influence of the refractive index of the monomer and the polymer. From this point on, improvements started to be made in the so-called “diffusion models”. One of the most important breakthroughs, proposed by John Sheridan’s research group in 2000, is the non-local behaviour of the photopolymer due to the finite size of the polymer chains. This phenomenon affects the limit of holographic recording resolution. Our research group at the University of Alicante (Spain) had the honour to collaborate with John Sheridan’s group developing new advances in the modelling of photopolymers as an optical recording medium, such as the threedimensional expansion of the models, and the explanation of surface variations or models to explain the recording of diffractive optical elements in this type of materials. In this papSer, we review this collaboration to improve diffusion models applied to photopolymers.

Photopolymers are designed and engineered with versatile applications including optics and photonics. Holography is one of the classical porpoises that use photopolymers as holographic recording materials. The success of these materials can be seen in the market with the photopolymer fabricated by Covestro. Some of these holographic applications require a long-time life of the holograms recorded in photopolymers. Nevertheless, initial tests of Covestro holograms show significant degradation after less than one year of exposure even after sealing and degradation occurs under solar light exposition. In this sense, it is important to perform deeper studies of the different possibilities for hologram conservation. Usually, the first step after recording is the material cure, with UV or visible light, to eliminate the residual dye and monomer. With this process high efficiency holograms can also be obtained. Afterwards, an index matching technique can be used to cover the material with a glass or it is possible the application of aerosol sealant. In this paper we analyze the introduction of holograms between two glasses linked by pressure, using Bayfol HX 200 from Covestro as the recording material. In order to characterize the process, four different spatial frequencies were tested, which were stored either by transmission or reflection schemes. The data of the reconstruction step has been measured before and after the encapsulation. In addition, multiple holograms have been superposed in the same glass, where we have found that shrinkage is more significant.

Nowadays augmented reality, 3D Image, mixed reality and see-through applications are very attractive technologies due to their great potential. Holographic optical elements can provide interesting solutions for injection and extraction of the image in the waveguides that are part of the see-through devices. We have developed a coupled waveguide system based on slanted transmission gratings recorded in manufactured photopolymers. In this work we optimize our schedule to a commercial photopolymer for this high demanded application. We demonstrate that high diffraction efficiencies can be obtained if we optimize the recording geometry, recording intensity and recording time for this material. In addition, we study the effects of shrinkage in our holographic system. In general shrinkage is an important drawback for holographic applications, nevertheless we demonstrate how shrinkage can help these systems open new possibilities. Lastly, we show how to significantly improve the quality of the guided image.

The study of liquid crystal (LC) director distribution is an important area of research in materials science and technology. Parallel-aligned liquid crystal (PA-LC) devices have been extensively studied due to their applications in liquid crystal displays, optical devices, and sensors. Estimating the LC director distribution is a critical step in designing and optimising PA-LC devices. This work shows the results derived from apply- ing novel optimisation techniques to estimate the liquid crystal (LC) director distribution in parallel-aligned liquid crystal (PA-LC) devices. Moreover, the genetic algorithm (GA) has been applied and compared with the minimisation of the Frank-Oseen free energy through the Euler-Lagrange equations. The GA is a stochastic optimisation technique that can effectively explore the search space and find the global optimum. Overall, this study’s results demonstrate the GA’s effectiveness in estimating the LC director distribution in PA-LC devices. This approach can improve the performance and design of liquid crystal displays, optical devices, and sensors. Furthermore, it can be extended to other fields where the optimisation of complex systems is required. Further research is needed to optimise the GA parameters and to explore its potential in other applications.

This doctoral thesis presents the research carried out in developing and applying numerical methodologies to forming images by volume elements. The validity of the theoretical models used has been analysed by comparing the results obtained with the models generated with the results obtained using theoretical models that are well established in the scientific community. Furthermore, this knowledge has been applied to fabricating diffractive optical elements (DOEs) in PVA/AA photopolymers and the corresponding experimental validation with the developed theoretical models., En esta Tesis Doctoral se estudió la formación de imágenes en holografía de volumen. En primer lugar se desarrollaron diferentes modelos teóricos para simular el comportamiento de medios dieléctricos de volumen. En particular, se implementó un método no destructivo basado en la integral de Fresnel-Kirchhoff. Además del desarrollo de modelos teóricos que permitieron predecir el comportamiento de la radiación electromagnética en medios dieléctricos en esta Tesis se obtuvo también una contrapartida experimental. En ese sentido se fabricaron elementos ópticos difractivos mediante técnicas holográficas. En particular se fabricaron guías de onda holográficas con aplicaciones en la Realidad Aumentada.

Liquid crystal on silicon (LCoS) microdisplays are an element commonly found in advanced photonic applications. Phase-only modulation and spatial light modulators (SLMs) use parallel-aligned LCoS (PA-LCoS) microdisplays due to their large resolution and small pixel size. However, the crosstalk between neighbouring pixels induces several phenomena, i.e. the smoothing of the electric field in the interpixel region known as fringing fields, out-of-plane reorientation of the liquid crystal director, and diffraction effects due to the finite pixel grid pattern. These effects degrade the performance in SLM applications. Therefore, a rigorous numerical analysis is carried out based on a full 3D model that estimates the LC director distribution of the PA-LCoS microdisplay through uniform applied voltages. Once the LC director distribution is known, a numerical finite-difference scheme simulates light propagation across the structure. Here, the impact of crosstalk is analysed from the polarimetric point of view for different pixel and interpixel sizes. It has been found that the finite-patterned pixel scheme induces the phenomena previously addressed, degrading the responses of the Stokes vector components S0, and S1 and also the degree of polarization (DOP). Furthermore, the impact over these parameters is generally inversely proportional to the fill factor. To isolate the influence of diffraction, out-of-plane, and fringing fields effects is challenging since all phenomena are mixed in both time and space, however we have found some useful trends.

Para la simulación de la propagación electromagnética en medios dieléctricos existe en la literatura un gran número de métodos numéricos [1] como el método de elementos finitos, el método de diferencias finitas, el método de líneas, el método de momentos, etc; que han demostrado una gran eficiencia a la hora de predecir el comportamiento de la radiación electromagnética. No obstante, el coste computacional suele ser elevado en muchos de ellos. En este trabajo se propone resolver la ecuación de Helmholtz escalar mediante un método integral de volumen basado en el uso del tensor de Green [2]. Si bien se derivará una versión más sencilla, en la que el tensor de Green degenera en una función y que se pretende validar comparándola con otros métodos más rigurosos. Este método se aplicará al estudio de objetos dieléctricos de diferentes formas y estructuras.

A model describing the parameter and wavelength dependencies not only of retardance, but also its flicker in liquid-crystal-on-silicon spatial light modulators (LCoS SLMs), is presented. We provide results for different digital addressing sequences producing different levels of flicker.

We have developed a coupled waveguide system based on slanted transmission gratings recorded in photopolymers. In this work we compare the behavior of four different photopolymers for this high demanded application.

In this work we present an application of volume transmission gratings for the coupling of light inside and outside a holographic waveguide. The volume gratings were recorded on photopolymer, since they have demonstrated great advantages over other materials. The gratings were fabricated with a spatial frequency of 1700 lines/mm so as to couple light inside the glass substrate under total reflection condition. The results demonstrate the efficiency of this device to be used in augmented reality applications, for example as see-through glasses.

A numerical formulation based on the precise-integration time-domain (PITD) method for simulating periodic media is extended for overcoming the Courant-Friedrich-Levy (CFL) limit on the time-step size in a finite-difference time-domain (FDTD) simulation. In this new method, the periodic boundary conditions are implemented, permitting the simulation of a wide range of periodic optical media, i.e., gratings, or thin-film filters. Furthermore, the complete tensorial derivation for the permittivity also allows simulating anisotropic periodic media. Numerical results demonstrate that PITD is reliable and even considering anisotropic media can be competitive compared to traditional FDTD solutions. Furthermore, the maximum allowable time-step size has been demonstrated to be much larger than that of the CFL limit of the FDTD method, being a valuable tool in cases in which the steady-state requires a large number of time-steps.

In this work, we test a nondestructive optical method based on the Fresnel–Kirchhoff integral, which could be applied to different fields of engineering, such as detection of small cracks in structures, determination of dimensions for small components, analysis of composition of materials, etc. The basic idea is to apply the Fresnel–Kirchhoff integral method to the study of the properties of small-volume dielectric objects. In this work, we study the validity of this method. To do this, the results obtained by using this technique were compared to those obtained by rigorously solving the Helmholtz equation for a dielectric cylinder of circular cross-section. As an example of the precision of the method, the Fresnel–Kirchhoff integral method was applied to obtain the refractive index of a hair by fitting the theoretical curve to the experimental results of the diffraction pattern of the hair measured with a CCD camera. In a same manner, the method also was applied to obtain the dimensions of a crack artificially created in a piece of plastic.

Photopolymers have become an important recording material for many applications, mainly related to holography. Their flexibility to change the chemical composition together with the optical properties made them a versatile holographic recording material. The introduction of liquid crystal molecules in a photopolymer based on multifunctional monomer provides us the possibility to generate tunable holograms. The switchable holographic elements are a key point for see-through applications. In this work, we optimize the holographic polymer-dispersed liquid crystals composition to improve the performance of tunable waveguide couplers based on transmission gratings and specifically their response under an applied electric field. A variation around 60% in the transmission efficiency was achieved.

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 this work we present simulations of dielectric structures based on the volume integral method. The formalism is based on the solution of Helmholtz equation using the Green’s tensor technique. By using this technique, the second order differential equation (Helmholtz equation) is transformed into an implicit integral equation. In this work the two dimensional Green tensor will be derived in order to study dielectric structures with translational symmetry. The method is firstly compared with other methods existing in the literature for homogeneous circular dielectric cylinders in order to test the validity of the method proposed. Then numerical simulations of other structures such as dielectric cylinders with elliptical base are performed.

Through analytical expressions, three experimental-based normalizations are evaluated for the diffraction efficiency of blazed gratings on LCoS devices. We find that non-inclusion of the fill factor, usually the case, leads to wrong conclusions.