Fabrication Of Holographic Optical Elements Research Articles

Fabrication of holographic optical elements on silver by nanosecond IR laser source

Holographic optical elements (HOE) are fabricated on silver by means of laser engraving using a cost effective ns fiber laser system developed for this scope, emitting at 1070 nm, for the first time to the authors' knowledge. Various HOEs are designed using commercially available software and fabricated on the sample surface. The developed methodology is presented and analyzed in terms of laser parameters. The fabricated HOEs are evaluated both structurally and optically.

Angle-compensated holographic wave front printing for the fabrication of holographic optical elements operating in the infrared

We introduce a technique for recording volume holographic optical elements (vHOEs) designed for operation at a wavelength different from the recording wavelength. This technique relies on our holographic wave front printer, which employs two spatial light modulators for recording wave front generation. We present the procedure for computing the required recording wave fronts accurately and noniteratively from the desired vHOE optical transformation function. We exploit this technique for the fabrication of a converging lens vHOE with an operation wavelength of 915 nm at a recording wavelength of 515 nm. Finally, we demonstrate the holographic lens functionality directly in the infrared spectral region.

Investigation of the sensitivity to humidity of an acrylamide-based photopolymer containing N-phenylglycine as a photoinitiator

Sensitivity of holographic recording materials to the relative humidity (RH) of the environment often restricts their use in fabrication of holographic optical elements and other applications. It is important to develop materials with little or no sensitivity to humidity. In this paper the humidity response of transmission gratings recorded in an acrylamide-based photopolymer containing N-phenylglycine (NPG) as a photoinitiator is studied at RH=20–90%. The hologram is found to be completely insensitive to humidity at RH below 70% and its diffraction efficiency remains constant. A decrease in diffraction efficiency is observed at RH=80–90% but this decrease is fully reversible, demonstrating quantitatively the NPG photopolymer’s excellent resistance to humidity.

Using acrylamide-based photopolymers for fabrication of holographic optical elements in solar energy applications

A holographic device is under development that aims to improve light collection in solar cells. The aim is to explore the potential of using photopolymer holographic optical elements (HOEs) to collect light from a moving source, such as the sun, and redirect it for concentration by a holographic lens. A working range of 45° is targeted for such a device to be useful in solar applications without tracking. A photopolymer HOE is capable of efficiently redirecting light, but the angular selectivity of a single grating is usually of the order of one degree at the thicknesses required for high efficiency. The challenge here is to increase the angular and wavelength range of the gratings so that a reasonable number may be multiplexed and/or combined to create a device that can concentrate light incident from a large range of angles. In this paper, low spatial frequency holographic recording is explored to increase the angular and wavelength range of an individual grating. Ultimately, a combination of gratings will be used so that a broad range of angles of incidence are accepted. A design is proposed for the combination of such elements into a holographic solar collector. The first step in achieving this is optimization of recording at low spatial frequency. This requires a photopolymer material with unique properties, such as a fast monomer diffusion rate. This paper reports results on the efficiency of holograms recorded in an acrylamide-based photopolymer at low spatial frequencies (100, 200, and 300 l/mm). The diffraction efficiency and angular selectivity of recorded holograms have been studied for various photopolymer layer thicknesses and different intensities of the recording beams. A diffraction efficiency of over 80% was achieved at a spatial frequency of 200 l/mm. The optimum intensity of recording at this spatial frequency was found to be 1 mW/cm2. Individual gratings and focusing elements with high efficiency and FWHM angles of 3° are experimentally demonstrated.

Shrinkage during holographic recording in photopolymer films determined by holographic interferometry

Shrinkage of photopolymer materials is an important factor for their use in holographic data storage and for fabrication of holographic optical elements. Dimensional change in the holographic element leads to a requirement for compensation in the reading angle and/or wavelength. Normally, shrinkage is studied at the end of the polymerization process and no information about the dynamics is obtained. The aim of this study was to use holographic interferometry to measure the shrinkage that occurs during holographic recording of transmission diffraction gratings in acrylamide photopolymer layers. Shrinkage in photopolymer layers can be measured over the whole recorded area by real-time capture of holographic interferograms at regular intervals during holographic recording using a complimentary metal-oxide-semiconductor camera. The optical path length change, and hence the shrinkage, are determined from the captured fringe patterns. Through analysis of the real-time shrinkage curves, it is possible to distinguish two processes that determine the value of shrinkage in the photopolymer layer. These processes are ascribed to monomer polymerization and crosslinking of polymer chains. The dependence of shrinkage of the layers on the conditions of recording such as recording intensity, single or double beam exposure, and the physical properties of the layers, such as thickness, were studied. Higher shrinkage was observed with recordings at lower intensities and in thinner layers. Increased shrinkage was also observed in the case of single beam polymerization in comparison to the case of double beam holographic exposure.

Absorption and scattering in photo-thermo-refractive glass induced by UV-exposure and thermal development

Photo-thermo-refractive (PTR) glass is a multicomponent photosensitive silicate glass that, after successive UV-exposure and thermal treatment, exhibits a refractive index change that results from the precipitation of nano-crystalline NaF. This glass is successfully used for the fabrication of holographic optical elements (volume Bragg gratings) that dramatically enhance properties of numerous laser systems and spectrometers. In this paper, induced absorption and scattering that determine efficiency of such elements were studied. It is found that the main contribution to induced absorption is produced by several types of silver containing particles having absorption bands with maxima in the blue-green region with exponential tails extending to the near IR spectral region. Evolution of all absorption bands was studied for different conditions of UV exposure and thermal development. Complex mechanisms of interconversion of silver containing particles is demonstrated as well as the fact that some of these particles can be associated with catalyzers of the nucleation process. It is also found that induced scattering obeys the classic Rayleigh law with an intensity depending on the conditions of UV exposure and thermal development. For short development times, scattering increases with dosage because of increased volume fraction of crystalline phase. For long development times, scattering decreases with dosage because of decreased size of individual crystals.

Modified phase contrast for recording of holographic optical elements

A modified phase contrast method is described for the fabrication of holographic optical elements with in-line diffraction patterns. In this method, a liquid crystal phase modulator is addressed by computer-generated holograms to achieve variable phase gratings. The phase gratings are imaged onto photosensitive material by a phase contrast method without a Fourier filter. We show the analytical treatment of the modified phase contrast method and demonstrate the application in holographic recording.

Holographic Characteristics of an Acrylamide/Bisacrylamide Photopolymer in 40–1000 µm Thick Layers

In this study we analyze the holographic behaviour of an acrylamide/bisacrylamide photopolymer in layers that range in thickness from 40 to 1000 µm. The photopolymer is composed of acrylamide as polymerizable monomer, N,N' methylene-bis-acrylamide as crosslinker, triethanolamine as radical generator, yellowish eosin as sensitizer and polyvinyl alcohol as binder. The composition and method of depositing the solution varies depending on the desired thickness of the final layer. For each thickness we analyze the holographic behaviour of the material during recording of unslanted diffraction gratings using a continuous argon laser (514 nm) at an intensity of 5 mW/cm2. The response of the material is monitored in real time with an He-Ne laser. The results obtained for the different parameters evaluated vary considerably depends on the layer thickness. Therefore, the different potential applications of the material (fabrication of holographic optical elements, use as recording material in holographic interferometry, or manufacture of holographic memories) depends on its thickness.

A Practical See-Through Head Mounted Display Using a Holographic Optical Element

This paper describes a practical method for design and fabrication of holographic optical elements (HOEs) as off-axis imaging optics. The authors have designed and fabricated a compact glass-like and see-through display using HOE and a total internal reflection prism. Observers can see through the display because wavelength selectivity of the HOE provides high transparency. Wave front of the HOE is designed by commercial software, though the actual fabrication method is not provided. The authors propose a practical method to fabricate HOEs, which can be applied to a wide range of imaging optics.

Highly sensitive photopolymerizable dry film for use in real time holography

A photopolymerizable holographic material has been obtained and characterized using agent N,N′-dihydroxiethylenbisacrylamide as crosslinking. A sensitivity of 5 mJ/cm2 and a diffraction efficiency around of 70% have been reached for holograms recorded at 633 nm with a spatial frequency of 1000 lines/mm. High sensitivities can be obtained with low intensities, and this is important in applications such as holographic interferometry and the fabrication of holographic optical elements.

Use of a pulsed neodymium laser in the fabrication of relief phase holographic optical elements

The surface of a recording medium was illuminated with a nonuniformity of less than 10% using second-harmonic radiation from a neodymium laser and the characteristic features of the illumination were considered. A laboratory technology was developed for fabricating 10-cm diameter holographic optical elements. This was based on optimizing the characteristics of a dichromated gelatin film, while obtaining a surface relief by recording with a 0.53-μm radiation pulse. The high image quality formed by a holographic optical element, with a low light scattering level, provides grounds for predicting extensive use of solid-state lasers in the fabrication of holographic optical elements.

Holographic solar concentrators

The approach selected is the fabrication of holographic optical elements which will focus to either a line or a point. A concentrating mirror is replicated in the hologram, which consists of dichromate gelatin exposed to a laser beam. The dichromate gelatin can be processed to produce a non-uniform microstructure, which gives the hologram a significant waveband width. Even so, it becomes necessary to stack at least three holograms, with each reflecting a different region of the solar spectrum, if we are to reflect most of the solar energy. To achieve high efficiency, it is necessary to obtain adjacent quasi-square waves for the efficiency—wavelength profile of each of the holograms in the stack. Profile information was obtained by the use of a monochromator coupled to a computer. An optical efficiency in excess of 50% was measured for a three-hologram stack. This represents approximately 70% of the efficiency achievable within the limited measuring range of the monochromator. A line-focus holographic concentrator model has been built for demonstration purposes. A cost analysis for mass producing holographic concentrators indicates that holographic concentrators become cost effective in relation to mass-produced conventional concentrators when the holographic optical efficiency exceeds 70%. To surpass this number, it becomes necessary to produce a five-hologram stack and to broaden the monochromator optical sensitivity range.

Dichromated Gelatin Holograms And Their Applications

Dichromated gelatin holograms, properly recorded and processed, have so little scattering and absorption that they are nearly indistinguishable from clear glass plates. Dichromated gelatin is the best material today for the fabrication of holographic optical elements and certain reflection type image holograms because of its low scattering, large refractive index modulation capacity, moderate exposure level, high resolution capacity, good environmental stability with a cover plate, and other unique properties such as reprocessibility and relatively easy hologram thickness control. We first review the mechanism of hologram formation in dichromated gelatin and holographic characteristics and then discuss several promising applications of dichromated gelatin holograms such as holographic optical elements (or holographic optics components) and information storage and display.

Dichromated gelatin for the fabrication of holographic optical elements

Hardened dichromated gelatin is the best material available today for the fabrication of holographic optical elements (HOEs) because of its low scattering, large refractive index modulation capacity, and other unique properties such as reprocessibility. On the basis of experimental data accumulated during the last several years, we describe holographic characteristics of the dichromated gelatin, including exposure characteristics with a bulk exposure model, hologram thickness change, and environmental stability. Basic chemistry, hologram formation in the dichromated gelatin, and generalized processing techniques are also discussed.