Christopher W. Slinger

Computational challenges of emerging novel true 3D holographic displays

A hologram can produce all the 3D depth cues that the human visual system uses to interpret and perceive real 3D objects. As such it is arguably the ultimate display technology. Computer generated holography, in which a computer calculates a hologram that is then displayed using a highly complex modulator, combines the ultimate qualities of a traditional hologram with the dynamic capabilities of a computer display producing a true 3D real image floating in space. This technology is set to emerge over the next decade, potentially revolutionizing application areas such as virtual prototyping (CAD-CAM, CAID etc.), tactical information displays, data visualization and simulation. In this paper we focus on the computational challenges of this technology. We consider different classes of computational algorithms from true computer-generated holograms (CGH) to holographic stereograms. Each has different characteristics in terms of image qualities, computational resources required, total CGH information content, and system performance. Possible trade- offs will be discussed including reducing the parallax. The software and hardware architectures used to implement the CGH algorithms have many possible forms. Different schemes, from high performance computing architectures to graphics based cluster architectures will be discussed and compared. Assessment will be made of current and future trends looking forward to a practical dynamic CGH based 3D display.

Photodoped chalcogenides as potential infrared holographic media

The extension of holographic techniques from the visible to the infrared is important. Potentially, holographic diffractive elements have a large range of uses in this wave band. Examples include mirrors, lenses, filters, and beam combiners. All these elements would have similar advantages to those enjoyed by their visible band diffractive analogs. The metal photodissolution effect in chalcogenides shows promise as one of the few techniques for producing low-loss holographic materials for use at any given wavelength from 0.6 to beyond 16 microm. To date, the work has concentrated on the photodissolution of silver into arsenic sulfide glasses. Both bulk and surface relief gratings can be fabricated simply by holographic or mask exposure. In principle, kinoforms (e.g., blazed zone plates) and Fresnel lenses can also be made. The results of material studies show that phase gratings with high modulation and low absorption can be produced. A coupled-wave analysis is used to calculate the likely grating performance, and some initial grating characterization results are presented. The limitations of the medium are discussed and possible solutions are considered.

Progress and prospects for practical electroholographic display systems

Continuing advances in both computing and modulator techniques and technologies increase the likelihood of electro-holography displays becoming practical in the next five years or so. These displays aim to allow high quality, interactive, 3D images to be generated from compte held dat. Until now, large pixel counts have precluded any systems of practical utility. This paper will describe recent progress towards meeting the challenges of implementing such displays. Despite more than exponential increases in computer performance, interactive hologram calculation remains an issue. A significant part of the cost of any electro-holography product will be associated with the computational requirements. These are strongly influenced by the choice of computer generated hologram (CGH) type, the algorithm used to calculate the CGH and the computer architecture chosen for implementation. The leading optics will be discussed and some experimental results presented indicating performance, cost and image quality tradeoffs. Eventual choice will depend on the specifications of the required system. Another traditional bottleneck has been the optical modulator employed. As one of the leading candidates for practical implementation, the current and projected performance of the DERA Active Tiling system will be explored, and the latest experimental results presented. These will include the first published, full parallax, true CGH, 3D image replays from an Active Tiling channel.

Optical constants of Ag-photodoped As-S amorphous films

Abstract The optical constants of thin amorphous films yield important information on their structure and are also necessary for many applications, e.g. the modelling of holographic grating characteristics. This paper presents measurements of the refractive index and absorption coefficient from the visible (0.5 μm) to the far-IR (10μm) for both as-deposited and annealed As40S60 and As30S70 evaporated films photodoped with Ag. The refractive index difference between the undoped and the most heavily doped as-deposited films is > 0.5 over this wavelength region and both doped and undoped films are weakly absorbing in the IR, which indicates that these materials are suitable for the fabrication of diffractive IR optical elements. The change in the optical gap of these AsS films as a function of Ag content has been monitored and compared with that for bulk AsSAg samples: the gap decreases from 2.5 to 1.7 eV as the Ag content is increased to 30 at. %., which can be attributed to the smaller binding energy of the AgS bonds that are formed compared with that for AsS bonds.

The effect of heat on the metal photodissolution process in amorphous As40S60 films

Abstract The effect of heat on the Ag photodissolution rate in as-deposited As 40 S 60 chalcogenide films is reported. Raising the temperature of an As 40 S 60 film by ∼ 100°C during exposure increases the rate of Ag photodissolution by at least a factor of 10. The photodissolution process has an activation energy of ∼ 0.24 eV and this was found to be independent of whether laser or white light was used to stimulate the effect. Lateral Ag migration was found to be negligible at about 110°C and a grating was successfully recorded by mask exposure at this temperature.

Fabrication and properties of chalcogenide IR diffractive elements

The authors report on techniques used to manufacture IR diffractive elements in chalcogenide glasses and on measurements of the material properties relevant to the performance of these elements. The characteristics of the elements produced are also presented and compared with theoretical predictions. The fabrication process used is based on the photodissolution of Ag into amorphous As-S films. Both surface relief and volume phase modulated transmission elements have been made. The transmission of Ag photodoped and undoped As-S films was found to be >80% over the range 2-12 micrometers for films up to 2 micrometers thick, the main loss mechanism being reflection. The difference in refractive index between Ag photodoped and undoped As-S over the range 0.5-12 micrometers was 0.5 for the most heavily doped material, so that high modulations are achievable for phase gratings. Theory suggests that for these As-S materials, green illumination (e.g., 514.5 nm) is the most efficient for producing the deep structures required for many of these IR elements. Surface relief structures can be produced by removing undoped material with an alkali etchant (e.g., NaOH). For transmission gratings, any remaining metallic Ag must be removed, to avoid high losses: the most successful Ag etchant was found to be Fe(NO3)3 in water. For the bulk holographic transmission gratings produced, efficiencies of >33% were observed for first diffraction orders measured in air at 632.8 nm, the main loss mechanisms being absorption and reflection, with some scatter. Measurements at 1.5 micrometers have given efficiencies of >30%, stability requirements during holographic recording currently being the main limitation to higher efficiencies at these and longer wavelengths. The results of a theoretical analysis based on numerical solution of the appropriate coupled-wave equations and taking into account bulk losses with phase and absorption modulation are in good agreement with the observed diffraction efficiency data. Given the low material absorption in the IR, theoretical studies show that, with suitable coatings, >95% efficiency should be possible for properly optimized bulk gratings and blazed zone plates.

Application of chalcogenide glasses in integrated and diffractive optics

We report on the use of the metal photodissolution effect in chalcogenide glasses to fabricate structures for integrated and diffractive optics. In the case of As-S films photodoped with Ag, the photodoped material was found to be transparent from the near IR to beyond 12 μm. Electron microprobe analysis of the composition of the photodoped films revealed that Ag-photodoped As 40 S 60 consists mainly of two As-S-Ag phases whereas Ag-photodoped As 30 S 70 has a single composition corresponding to that of the crystalline compound smithite (AgAsS 2 ). In addition, a Secondary Ion Mass Spectroscopy (SIMS) analysis showed that for Ag-photodoped As 30 S 70 there is a gradual decrease in Ag concentration with depth, indicating that the optical constants may vary slightly through the film. Photodarkening was found to decrease with increasing sulphur content, becoming negligible near As 30 S 70 , which may therefore be the optimum composition for these applications. Both surface relief and phase gratings have been fabricated using the photodissolution of Ag in As-S films and diffraction efficiencies of up to 35% at 632.8 nm and 1.5 μm have been measured.

Quantitative evaluation of 3D images produced from computer-generated holograms

Advances in computing and optical modulation techniques now make it possible to anticipate the generation of near real- time, reconfigurable, high quality, three-dimensional images using holographic methods. Computer generated holography (CGH) is the only technique which holds promise of producing synthetic images having the full range of visual depth cues. These realistic images will be viewable by several users simultaneously, without the need for headtracking or special glasses. Such a data visualization tool will be key to speeding up the manufacture of new commercial and military equipment by negating the need for the production of physical 3D models in the design phase. DERA Malvern has been involved in designing and testing fixed CGH in order to understand the connection between the complexity of the CGH, the algorithms used to design them, the processes employed in their implementation and the quality of the images produced. This poster describes results from CGH containing up to 108 pixels. The methods used to evaluate the reconstructed images are discussed and quantitative measures of image fidelity made. An understanding of the effect of the various system parameters upon final image quality enables a study of the possible system trade-offs to be carried out. Such an understanding of CGH production and resulting image quality is key to effective implementation of a reconfigurable CGH system currently under development at DERA.

An adaptive coded aperture imager: building, testing and trialing a super-resolving terrestrial demonstrator

There is an increasingly important requirement for day and night, wide field of view imaging and tracking for both imaging and sensing applications. Applications include military, security and remote sensing. We describe the development of a proof of concept demonstrator of an adaptive coded-aperture imager operating in the mid-wave infrared to address these requirements. This consists of a coded-aperture mask, a set of optics and a 4k x 4k focal plane array (FPA). This system can produce images with a resolution better than that achieved by the detector pixel itself (i.e. superresolution) by combining multiple frames of data recorded with different coded-aperture mask patterns. This superresolution capability has been demonstrated both in the laboratory and in imaging of real-world scenes, the highest resolution achieved being ½ the FPA pixel pitch. The resolution for this configuration is currently limited by vibration and theoretically ¼ pixel pitch should be possible. Comparisons have been made between conventional and ACAI solutions to these requirements and show significant advantages in size, weight and cost for the ACAI approach.

Creation of large-area absorption grey-level CGHs of 3D objects: writing and printing regimes for high dynamic range with high printing speed

This paper discusses certain developments in our understanding of the writing of large-area CGH arrays of the absorption type. The calibration of grey-scale structures is considered, using different forms of the linearization process. It is also pointed out that the feature shape and packing fraction play a dominant role in the achievement of high dynamic range of the holographic recording.