Richard D. Rallison

Holographic polarization-separation elements

Volume holographic optical elements exhibit a property that we call bidiffringence, in which the diffraction efficiency of the element is strongly dependent on the polarization of the incident beam. Dualelement, volume holographic polarization separators utilizing bidiffringence can provide high extinction ratios, and they can provide greater angular separation of the polarized beams and greater flexibility in the beam-separation parameters than conventional birefringent elements. We designed and constructed holographic polarization separators using dichromated gelatin as the holographic medium. Experimental results are compared with the results predicted by the Kogelnik coupled wave theory.

Polarization properties of gelatin holograms

Dichromated gelatin exhibits variable changes in effective refractive index (n) from 1.54 before exposure to less than 1.25 as it expands during processing. This aerogel like effects causes aberrations in diffractive optics and Kogelniks theory predicts strong polarization separation in gratings at many different angles other than 90 degrees. The diffraction efficiency of both S and P polarizations at any angle is dependent on the product of thickness and index modulation while the angle inside the medium is dependent on n. We investigated predicted conditions where only one polarization would be diffracted and subsequently proved n varies from about 1.4 to 1.2 after processing and depends on the film thickness and processing procedures. Transmission gratings made at angles from 36 to 66 degrees were fit to mathematical models as proof of the phenomena, some performed with extinction ratios greater than 100:1. We were also able to demonstrate a similar range in conformal reflection structures and to design a novel polarizer. The calculation of exposure geometries for display holograms becomes more accurate when index change is included in the formulas but some results remain hard to explain.

Wavelength compensation by time reverse ray tracing

We specified 406 mm diameter off axis powered HOEs to operate at 1.06 microns and for trials at 670 nm, then looked for ways to construct them at 488 nm. We were able to write holographic surfaces directly in Zemax, then play back other wavelengths through them to display the aberrations that needed to be canceled. Zemax had easy entry and optimization routines that allowed us to do cut and try construction variations on screen rather than on the table. We designed construction set ups using only off the shelf lenses and mirrors and were able then to fabricate the HOEs with only small incremental improvements being made on the table. The computed spot sizes were less than .5 mm diameter and constructed holographic optics were on Bragg with outputs as small as .5 mm and high efficiencies.

Dichromated gelatin--some heretical comments

This paper looks at some ideas old and new that relate to the formation of images in dichromated gelatin. The traditional view that the dichromated system hardens gelatin thus preventing solubilization of the material must be balanced against other observations such as that of the reduction of the bulk index of the gelatin layer and the appearance of gelatin in the processing solutions. We revisit the problem and take a look at some new chemical ideas that relate to the behavior of gelatin during bleaching of silver halides. A new method for the processing of silver halide-sensitized gelatin (S.H.S.G.) is proposed which endows the silver halide emulsion with more than three orders of magnitude of speed when compared with D.C.G. per se.

Combat vehicle stereo HMD

Combat vehicles of the future may be devoid of direct vision ports but will contain multiple displays creating a virtual environment. The transition from real to virtual can be facilitated by the use of a helmet-mounted display (HMD) that projects a portion of the virtual environment over the real world. The authors propose a simple, light-weight color stereo projection system that has the potential for meeting most of the desired characteristics at a reasonable cost. Imaging is accomplished using CR39 ophthalmic substrates off- axis 15 deg and distorted to correct for astigmatism. The images from two sources are transferred to the focuses by coherent fiber image conduit shaped at one end to minimize field curvature. The demonstrated field of view (FOV) is 15v X 40h deg using readily available image conduit and colored transparencies; maximum FOV is 60v X 90h deg.

Holographic Optical Elements (HOES) In Dichromated Gelatin (DCG):Progress

In the early seventies a number of researchers noted that DCG was in some cases an ideal material for HOE fabrication. B.J. Chang and Andres Graube both did extensive characterizations of the material and many others have contributed since then. A large number of HOES have been made since then and a few are in industrial or commercial machines already. It would take a very long time to describe all the variations and applications that have been reported. In this paper we discuss only a few applications where non-standard practices may have been employed in meeting the design objectives. The emphasis is on the fabrication solutions and on any unusual or previously thought unlikely results.

Incoherent Multifocus Hololens Design and Fabrication

Several 5 x 5 multifocus Hololenses have been produced with diffraction efficiencies between 20% and 75%. Low intermodulation noise was achieved by going off axis 16 degrees and using SHG and DCG materials to record the master and copies respectively. Astigmatism and Bragg tilt errors were minimized but images showed more coma than prior art. Both coherent and incoherent copies were made, the oherent copies were always low in efficiency because of very high intermodulation noise or because beam ratios were made high to avoid intermodulation noise. Incoherent copies proved to be only a little more difficult to fabricate and the master copy process in an index matching fluid proved to be more versatile as well as optically cleaner. Problems with uniformity from exposure to exposure were found and cured or probable causes were found. A limit to uniformity probably exists that relates to random coherent phasing of overlapping Bragg structures added to very small thermal and mechanical instabilities during exposure. All copies are made in a contact copy jig with each exposure running about 2 seconds. The process for obtaining an unaberrated master at 633 nm while making copies at 488 nm is described for both the coherent and incoherent methods. Test results for an incoherent 5x5 array working at 633 nm are given including an intensity profile of a spot, power distribution and output with crosstalk. Suggestions for further improvements are given.

Hologram Scanner Design And Fabrication In Dichromated Gelatin (DCG)

Two major applications of holographic scanners are considered, the first is the code reader scanner now in use in supermarkets and soon to be used in automated warehousing. The second is the multipurpose line scanner currently used in line printers and soon to be included in automated inspection systems. Code reader facets perform multiple functions, each one deflects and focuses laser light at a unique angle and scans a short arc, the return light from a bar code is collimated by the same facet and is subsequently focused through a small aperture. Ambient light is diffracted at other angles and focused at points all around the aperture giving a high signal to noise ratio and the large high efficiency facets gather sufficient return light so that photo diodes and low power lasers can be used in the system. Line scanners can be made in a large variety of sizes and configurations inexpensively and with perfect fidelity, each one being a holographic replica of a master hologram. Focused arcs as well as parallel straight lines and even arbitrary computer generated scans are possible. The limitations and considerations of such devices are discussed along with design criteria related to fabrication problems and actual production line results.

BRIGHTNESS INCREASE IN AN LCD STEREO DISPLAY

A practical head mounted display (HMD) has to be light enough and bright enough to wear and view without undue strain on the users head or eyes. A 10 pound CRT based helmet is not always out of the question but binocular or stereo HMDs using LCDs rather than CRTs need only weigh in at around one pound complete with electronics and are far more comfortable to wear. The space bandwidth product or pixel count of LCDs is now approaching that of CRT type displays but LCDs could use a big boost in brightness, especially for see thru designs. The see thru or head up style has many user advantages and this paper addresses ways to more efficiently transmit photons from the source to the eye in one such design. All of the components that are used to improve performance may be made holographically or in an alternate fashion. The most practical method of construction is probably a toss up for some components.

Using thick DCG, 30-100 μm

Thick holographic films are useful for making multiple recordings in the same volume and for reducing the amount of light diffracted into unwanted orders by a single recorded grating. Dichromated gelatin (DCG) is a material that may be used in thick layers and processed in a way that leads to behavior as a thick hologram. We investigated ways to coat and process layers up to 100 microns thick on glass. We found that the control of the modulation and integrity of the original exposed structure was a formidable task. The angular bandwidth was often smaller than the angular error and the angular error was sometimes a random variable over the surface and volume. Uniformly hardened films were made and exposed to uniform plane waves but the resulting recordings often lacked uniformity in every property but thickness. The lower range of thicknesses was far easier to work with and process than the higher range.