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.
Photosensitive materials and optical information processing technologies based on holographic and photonic techniques are suffering a huge improvement. Furthermore Spatial Light Modulators (SLMs) based on LCoS microdisplays (PALCoS) open new possibilities to modulate the wavefront of a light beam. The improving of the models of the photopolymers as optical recording material together with the modelling of PALCoS, high resolution reflective devices, make possible the generation and recording of Diffractive Optical Elements (DOE) on the photosensitive materials. This DOEs have many important applications in photonics, communications of optical information processing. Working with a setup based on a LCoS display as a master, we can store complex DOEs. We used in this work PVA/AA based on acrylamide with coverplating and index matching system to avoid the influence of the thickness variation on the transmitted light in the material. With the 3-Dimensional diffusion model we can predict the DOE properties before recording and optimize the recording time and the exposure dose. Experimental data is compared with the simulation results to evaluate the accuracy of our model to reproduce the recording of any kind of complex DOE onto a photopolymer. This allows us to choose the appropriate characteristics for the material depending on the application and evaluate the influence of different parameters involved in the DOE generation. In this work we evaluate the simulation of the recording of optical vortexes, axicons, fork gratings and diffractive lenses comparing with the results using our experimental set-up.
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.
Photopolymers are classical holographic recording materials. Recently their chemical composition and the
fabrication techniques have been optimized for many new applications such as interconnectors, solar concentrations,
2-D photonic structures, or wave-guides. Their potential usefulness has been drastically increased by the
introduction of dispersed liquid crystal molecules; these components can be concentrated in the non-exposed zones
of the material by a photopolymerization induced phase separation process (PIPS). Therefore, by combining
polymer and dispersed liquid crystal (PDLC) has emerged as a new composite material for switchable diffractive
optical elements (DOEs). Parallel to the material advances some techniques have been proposed to record very low
spatial frequencies DOE’s. Different researchers have reported proposes to record DOE like fork gratings, photonics
structures, lenses, sinusoidal, blazed or fork gratings. In this work we have studied the behavior of a PDLC material
to record DOE’s with different spatial periods: from 1 μm, using holographic technique, to more than 200 μm,
Liquid Cristal on Silicon (LCoS) display working in mostly amplitude mode as a master. Due to the improvement in
the spatial light modulation technology and the pixel miniaturization, this technique permits us store gratings with
spatial frequencies until few microns. Additionally, this technology permits us an accurate and dynamic control of
the phase and the amplitude of the recording beam. In particular, for our case, to generate the blazed gratings, we
use an LCoS-Pluto provided by Holoeye with a resolution of 1920x1080 (HDTV) pixels and a pixel size of 7.7x7.7
m2.
We introduce a polyvinil alcohol/acrylamide (PVA/AA) photopolymer compound in a holographic memory testing
platform to provide experimental results for storage and retrieval of information. We also investigate different
codification schemes for the data pages addressed onto the parallel-addressed liquid crystal on silicon (PA-LCoS) device,
used as the data pager, such as binary intensity modulation (BIM), and hybrid-ternary modulation (HTM), and we will
see that an actual approximation for HTM can be obtained with a PA-LCoS device. We will also evaluate the effect of
the time fluctuations in the PA-LCoS microdisplays onto the BIM and HTM regimes. Good results in terms of signal-tonoise
ratio and bit-error ratio are provided with the experimental system and using the PVA/AA photopolymer produced
in our lab, thus showing its potential and interest for future research focused on this material with highly tunable
properties.
Photopolymers are optical recording materials appealing for many different applications such as holography, data storage, interconnectors, solar concentrations, or wave-guides fabrication. Recently the capacity of photopolymers to record diffractive optical elements (DOE’s) has been investigated. Different authors have reported proposes to record DOE like fork gratings, photonics structures, lenses, sinusoidal, blazed or fork gratings. In these experiments there are different experimental set-ups and different photopolymers. In this work due to the improvement in the spatial light modulation technology together with the photopolymer science we propose a recording experimental system of DOE using a Liquid Cristal based on Silicon (LCoS) display as a master to store complex DOE like cylindrical lenses. This technology permits us an accurate control of the phase and the amplitude of the recording beam, with a very small pixel size. The main advantage of this display is that permit us to modify the DOE automatically, we use the software of the LCoS to send the voltage to each pixel In this work we use a photopolymer composed by acrylamide (AA) as polymerizable monomer and polyvinyl alcohol (PVA). We use a coverplated and index matched photopolymer to avoid the influence of the thickness variation on the transmitted light. In order to reproduce the material behaviour during polymerization, we have designed our model to simulate cylindrical lenses and used Fresnel propagation to simulate the light propagation through the DOE and analyze the focal plane and the properties of the recorded lenses.
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.
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.
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.
Photopolymers are often used as a base of holographic memories displays. Recently the capacity of photopolymers to record diffractive optical elements (DOE’s) has been demonstrated. To fabricate diffractive optical elements we use a hybrid setup that is composed by three different parts: LCD, optical system and the recording material. The DOE pattern is introduced by a liquid crystal display (LCD) working in the amplitude only mode to work as a master to project optically the DOE onto the recording material. The main advantage of this display is that permit us modify the DOE automatically, we use the electronics of the video projector to send the voltage to the pixels of the LCD. The LCD is used in the amplitude-mostly modulation regime by proper orientation of the external polarizers (P); then the pattern is imaged onto the material with an increased spatial frequency (a demagnifying factor of 2) by the optical system. The use of the LCD allows us to change DOE recorded in the photopolymer without moving any mechanical part of the set-up. A diaphragm is placed in the focal plane of the relay lens so as to eliminate the diffraction orders produced by the pixelation of the LCD. It can be expected that the final pattern imaged onto the recording material will be low filtered due to the finite aperture of the imaging system and especially due to the filtering process produced by the diaphragm. In this work we analyze the effect of the visibility achieved with the LCD and the high frequency cut-off due to the diaphragm in the final DOE recorded into the photopolymer. To simulate the recording we have used the fitted values parameters obtained for PVA/AA based photopolymers and the 3 dimensional models presented in previous works.
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.
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