Spatial Light Modulators (SLMs) typically use PA-LCoS microdisplays, known for high resolution and small pixel size, for phase-only modulation. However, different phenomena related to the cross-talk, fringing fields between pixels, and out-of-plane reorientation of the liquid crystal director degrade the performance of pixelated SLM devices. The technology faces the challenge of overcoming these phenomena as novel applications require higher pixel size resolutions and smaller pixel sizes. This work applies a numerical workflow that permits the analysis of the different parameters that set up a PA-LCoS microdisplay, e.g., fill factor, pixel size, external voltage and the influence of the LC director and the interaction of light through the structure. Specifically, we focus our analysis on the impact of high-frequency binary phase gratings on the radiometric and polarimetric response with orthogonal grating vectors. The heterogenous LC distribution as a function of the grey level along the 3D structure and the inhomogeneous distribution of the Stokes parameters are analysed, setting up a framework to analyse the nonlinear behaviour of the system and its impact on diffraction and polarimetric performance in future microdisplays with higher resolutions and smaller pixel sizes.
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 introduction of a mathematical sequence into a diffractive lens phase, such as the Fibonacci or Silver Mean Sequence, allows us to achieve multifocal behavior in the Fresnel regime. This study explores the use of photopolymers based on Polyvinyl Alcohol Acrylamide (PVA/AA) as an optical recording medium for these complex lenses. PVA/AA photopolymers were selected because of their versatility and good response in recording DOEs. Moreover, the use of a 4F optical system configuration and a Spatial Light Modulator (SLM) provides the flexibility to adjust lens radius and orders, enabling the creation of lenses with adaptable focal lengths and specific sizes. In addition, we model the material’s behavior using a 3D diffusion model with a coverplate and index matching, allow us to adjust some parameters to obtain higher diffraction efficiency. These lenses find practical applications in microscopy and ophthalmology, particularly as progressive intraocular lenses. Their ability to adjust focal points and sizes makes them invaluable for addressing vision problems, like presbyopia and enhancing optical devices used in medical and research settings.
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.
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.
In the last few years, the interest in storing volume holograms in photopolymers has increased enormously due to their applications in industry, the medical field, security, or renewal energy among others. The production of environmentally compatible photopolymers is one of the main focuses of Holography research. In this work, we have studied how to increase the diffraction efficiency of reflection holograms stored in a low-toxicity PVA-based photopolymer called Biophotopol. The holographic material has been doped with different types of nanoparticles (NPs) to achieve an increase in the refractive index modulation during the recording stage. Metallic NPs, obtained by physical and electrochemical methods have been used. The results obtained with all of them have been compared as a function of the concentration used, the size of the NPs, and the stabilization method used for their synthesis. A considerable increase in diffraction efficiency has been achieved by using NPs in the low-toxicity material. By using high refractive index NPs, the average refractive index of the holographic material increases and consequently the diffraction efficiency.
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.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.