Planar optical light guides that are suitable for compact, relatively large virtual image projection displays, of either see-through or non-see-through capabilities, are presented. Such light guides are comprised of three diffractive elements that are recorded on a single substrate. The basic principles, design methods, experimental procedures, calculated as well as experimental results are presented. The results reveal that a relatively large field of view and uniform luminance over the entire output image can be obtained, even when the distance from the light guide to the viewer is relatively large.
The development of replication processes of arrays of diffractive optical elements (DOEs) on planar substrates serving as optical light-guides (OLGs) by soft nano-lithography are described.The master DOE arrays were recorded by holographic interferometry in photoresist on the OLG substrate. These master OLGs carrying nanometer surface-relief
grating structures were then transferred into a thermally curable Si-elastomer serving as the molding tool for the production of the replica. The replica were formed by casting UV-curable photopolymers onto a planar substrate inserted into the mold. The replication process variables were then designed and evaluated by the statistical Taguchi technique. The replica OLGs were compared to the master OLGs both in terms of surface flatness and grating fidelity. The results show that optimal processes yielded both good OLG flatness (<λ/2)and high grating fidelity (~1).
Flexible PDLC-based devices having 'all-plastic'-made transparent electrodes were developed. Unlike the relatively non-flexible ITO-made electrodes, their polymer counterparts are mechanically flexible, allowing the fabrication of flexible devices. Other advantages of devices containing a conducting polymer are the reduction in operating energy consumption and the anticipated reduction in manufacturing costs. The electrooptical switching and imaging properties of devices having 'all-plastic' transparent electrodes were found to match those of their ITO-based counterparts. The optimal operation voltage in terms of switching rise times and optical transmittance shoed significant dependence on dimensional cell parameters, while the switching fall time was practically independent of these parameters.
Hydrophilic photoactive plastic matrices are investigated by holographic and wave-guiding techniques. The photoactive matrices are based on acrylate type monomers, which are dissolved together with xanthine dyes and other additives in poly(vinylalcohol). Dry plastic coatings are obtained by casting the aqueous polymer solutions on glass substrates. Photo-recording occurs in real-time and in-situ, without any post-exposure processing. This paper first describes the recording and diffraction properties of thick asymmetric holographic volume gratings. Such holographic gratings exhibit very high diffraction efficiencies (DEs) approaching 80 percent into free space. Here, they are considered for coupling into and coupling out of wave-guide layers at 850 nm. Our investigations indicated good wave-guiding properties in both, unexposed and exposed photopolymer layers. Grating couplers exhibited angular and spectral bandwidths of about 0.15 degrees and of about 1 to 2 nm, respectively. Coupling DEs of over 50 and de-coupling DEs reaching 6 percent were determined. The large difference between coupling and de-coupling DEs are explained by the losses due to an increase in angular divergence as a result of diffraction and modal dispersion.
Plastic optical elements are used extensively for high- volume, low-quality, commercial applications. The advantages of plastic over glass elements include low weight, ease of modeling, high shock resistance, and low price in mass production. The use of plastic optical elements in military systems is limited by stringent environmental demands, and by the high image quality which is usually required. The disadvantages of plastic from this point of view are softness, low chemical resistance, thermal defaces, and inhomogeneity. Display systems are located in the same environment as the user. In an enclosed space, such as a vehicle, the requirements for environmental conditions can be greatly eased. In addition, visual optical systems may incorporate a focusing adjustment which can be controlled by the viewer. In combination with a limited temperature range, this solves the problem of thermal defaces. The resolution required in display systems is generally far from the diffraction limit, which reduces the significance of inhomogeneity. In addition, the eye pupil is usually smaller than the lens diameters, so that inhomogeneity only produces a local effect in the field of view. It would thus appear that plastic optical elements are a practical option for display systems, despite relatively severe environmental conditions.In this paper, the application of these considerations will be illustrated by a practical optical design for an airborne military display system incorporating plastic optical elements. The points of image quality and environmental requirements will be addressed.
Grating formation in photoactive polymers are monitored by holographic recording. The photopolymers are based on acrylamide monomers, which are dissolved together with xanthine dyes in polyvinyl alcohol. Thin plastic coatings are obtained by casing on glass substrates. Photorecording occurs quasi-real-time and in-situ, meaning that no wet- chemical or post-thermal/photochemical processing is required. Formulations have been found, which produce large enough refractive index modulations, so that very high diffraction efficiencies can be obtained, when the recording beam angles are symmetric. Unfortunately, DEs significantly drop, when recording angles are highly asymmetric. The origin of this effect is shown to stem from grating anomalies, in that the slanted fringes bend due to nonlinear shrinkage effects during recording. The introduction of cross-linking and gelling agents stabilize the formed grating structures against dimensional distortions. These photopolymer layers have potential in photonics applications, such as holographic optical elements and waveguide structures.
Photoactive polymer matrices are investigated by and for holographic recording at 514 nm. The photopolymers are based on acrylamide monomers, which are dissolved together with xanthine dyes and other additives in polyvinylalcohol. Dry plastic coatings are obtained by casting the aqueous polymer solutions on glass substrates. Photorecording occurs in real-time and in-situ, without any post-exposure processing. This paper describes the influences of chemical additives on the photorecording process. Specifically, the addition of diphenyl iodonium chloride (DPI-Cl), in conjunction with triethanolamine (TEA), significantly increases the exposure sensitivities, by a factor of over three (to about 15 mJ/cm2). This sensitizing effect is shown to originate from a superadditive effect between TEA and DPI-Cl. The mechanism of the superadditive effect is discussed by a proposed reaction model. The exposure sensitivities are also significantly influenced by the PVA binder parameters, such as average molecular weights and degree of hydrolization. The present formulations produce large enough refractive index modulations, so that very high diffraction efficiencies (DEs > 90%) are obtained. The dynamic range of refractive index modulations was increased from 0.014 to 0.018 by the addition of glutaraldehyde crosslinking, which also improves the dimensional stability of the holograms.
Real-time recording of mid-infrared laser radiation in photochromic spiropyran-MMA copolymer materials is investigated. The laser radiation, derived from a CO2 laser, bleached pre-colored merocyanine, thereby converting it back to spiropyran. The exposure sensitivity at 50 percent of the initial merocyanine optical density was about 0.1 J/mm2 at 10.6 micrometers laser wavelength and 0.2 J/mm2 at 10.5 micrometers . The difference of sensitivity at the two laser wavelengths points to differences in thermal energy dissipation mechanisms. The characteristics and examples of imagery, recorded in these photochromics, are presented. A resolution equal or better than 40 lines/mm was achieved.
Surface photodeposition is a photon assisted process, by which thin films can be formed on substrates immersed in colloidal solutions. Holographic gratings of various spatial frequencies have been recorded by photodeposition of amorphous selenium colloids. The holographic surface relief grating formation is described in relation to the modulation transfer function of colloidal photodeposition. Spatial frequencies of about 1500 lines/mm can be recorded with amorphous selenium, whose colloidal particle sizes range from 30 to 80 nm.
Photodeposition is an emerging new thin-film material deposition and optical image patterning technique with photographic recording characteristics. In the past, lasers and incoherent light sources were exploited for photodeposition of various materials in micropatterns. Here we report that photodeposition can also be used for recording holographic relief gratings of 2000 lines/mm and periodically modulated depths of 1 to 20 nm.