The design of the first Real-Time-Tomographic-Holography (RTTH) optical system for augmented-reality applications
is presented. RTTH places a viewpoint-independent real-time (RT) virtual image (VI) of an object
into its actual location, enabling natural hand-eye coordination to guide invasive procedures, without requiring
tracking or a head-mounted device. The VI is viewed through a narrow-band Holographic Optical Element
(HOE) with built-in power that generates the largest possible near-field, in-situ VI from a small display chip
without noticeable parallax error or obscuring direct view of the physical world. Rigidly fixed upon a medical-ultrasound
probe, RTTH could show the scan in its actual location inside the patient, because the VI would
move with the probe. We designed the image source along with the system-optics, allowing us to ignore both
planer geometric distortions and field curvature, respectively compensated by using RT pre-processing software
and attaching a custom-surfaced fiber-optic-faceplate (FOFP) to our image source. Focus in our fast, non-axial
system was achieved by placing correcting lenses near the FOFP and custom-optically-fabricating our volume-phase
HOE using a recording beam that was specially shaped by extra lenses. By simultaneously simulating and
optimizing the system's playback performance across variations in both the total playback and HOE-recording
optical systems, we derived and built a design that projects a 104x112 mm planar VI 1 m from the HOE using
a laser-illuminated 19x16 mm LCD+FOFP image-source. The VI appeared fixed in space and well focused.
Viewpoint-induced location errors were <3 mm, and unexpected first-order astigmatism produced 3 cm (3% of
1 m) ambiguity in depth, typically unnoticed by human observers.
Characterization of Volume Phase Holographic gratings at cryogenic temperatures have been conducted using a new test facility at Caltech. The new test bench includes a cryostat that allows large angles for incident and diffracted light. Gratings under tests are shielded from thermal background, and precisely and uniformly temperature controlled. Preliminary results are presented and show little temperature dependence of the efficiency function.
Large area volume phase holographic (VPH) gratings have been made for use in spectrographs attached to large telescopes and for scanning LIDAR systems. Examples of the transmitted wavefronts, the spectral efficiency measurements and other parameters such as uniformity, scatter, absorption and Q have been gathered and presented. Two exposure layouts have been used and are described along with some discussion of modulation and bulk index of processed DCG. A discussion of thickness regimes is given. A special case (Dickson) design is presented with examples of performance and some intrinsic properties.
Optical Doppler shifts may be detected directly by filtering and dispersing the return light. The direct detection method is far more relaxed optically, requiring only "photon bucket" collection optics. A narrowband interference filter and two etalons are enough to distribute the frequency shifted light into the radial pattern ofonly one free spectral range ofthe last etalon. This paper describes a novel system using three hoes to perform transmission, collection, filtering and redistribution of the imaged zonal signal light onto a linear string of detectors. The light collection optic is a large area scanning and focusing hoe made in DCG. This optic may also serves as the laser transmitting optic. Holographically recorded etalons of medium finesse are made to do final filtering of the return signal down to picometer bandwidths. Finally a 24-channel circle to point converter has been fabricated in DCG and also transferred to resist and subsequently etched into a fused silica substrate. Reactive ion etching (RIE) was attempted using a Faraday cage to form tilted fringes in the silica. An Ion mill was rebuilt to perform a similar task. All components worked well when made in DCG but problems with dry etching were not all resolved. Circle-to-point conversion has been used for collecting the output of an etalon where a suitable detector of equal area circular zones was not readily available. The output is converted to a line of points so that it may be read with a simple and readily available linear photon counting detector.
Circle-to-point conversion has been used for collecting the output of an etalon where a suitable detector of equal area circular zones was not readily available. The output is converted to a line of points so that it may be read with a simple and readily available linear CCD detector. The purpose of this work was to determine the feasibility and practicality of making circle-to-point optical converters with up to 24 equal area annuli in a 24 mm diameter form factor. Each annulus redirects light into a focus and all foci lie in a straight line with equal spacing between them. The principal working wavelengths were 532 and 355 nm. The first approach was a step and repeat fabrication process using high resolution stepping stages to register 24 masks and a master HOE. The second method consisted of writing software that would generate the desired optic in the form of a single binary mark. An 8X photo reducer and a 4f optical processor were also used to improve performance. Both methods worked well and many samples of each were delivered with the goals being met. Work on the UV portion could not be completed on time but a simple RIE machine was configured and exposures into photo-resist were successfully made using a DCG hybrid master. The resist copies were etched with CF4 and O2 to make copies in fused silica. Further work will require an ion source to reduce losses in the UV HOEs by etching at an angle to the surface.
In holography, DC-PVA it is an alternative real time recording materials useful for phase conjugation experiments and also a stable long term holographic storage medium needing no processing other than heat. To this we add the capability of greatly increasing the versatility of PVA by boosting the index modulation by almost 2 orders of magnitude. Simple two or three step liquid processing is all that is required to make it grow. This paper contains a simple recipe for mixing and using this versatile material. A wide variety of holographic recordings can be easily made in it with blue laser light.
PVA films have been used as mold releases, strippable coatings, binders for photopolymers and when sensitized with metals and/or dyes they have been used as photoresists, volume HOEs, multiplexed holographic optical memory and real time non destructive holographic testing. The list goes on and includes Slime and birth control. In holography, DC-PVA is a real time photoanisotropic recording material useful for phase conjugation experiments and also a stable long term storage medium needing no processing other than heat. Now we add the capability of greatly increasing the versatility of PVA by boosting the index modulation by almost two orders of magnitude. We can add broadband display and HOE applications that were not possible before. Simple two or three step liquid processing is all that is required to make the index modulation grow.
The wavelength scaling of an f# 2.5 off axis HOE from 488 nm to 1064 nm has been done. We canceled large induced astigmatism, and other higher order aberrations using a combination of 1 curved reflector, 1 cylindrical lens and one Null CGH bonded to the cylindrical lens. The task was made more difficult by a requirement to fill a 404 nm round aperture and make it focus to a 53 micron diameter spot at the 1/e clip level. The design procedure, the construction sequence and the measured results are presented as a work in progress.
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.
The dynamic recording range or the maximum index modulation obtainable in any holographic material limits the properties of the hologram. The product of the modulation times the thickness determines diffraction efficiency in any phase hologram. In a reflection hologram low uniform modulation yields narrow spectral and angular bandwidths and high modulation in a chirped structure yields high angular and spectral bandwidths. An ideal master hologram requires a just right efficiency and very low noise to produce the best possible copies. In a two beam copy scheme the efficiency is less important than the low noise because beams can usually be balanced to the just right range with a variable beam splitter. In a single beam copy scheme the balance and the low noise plus wide enough angular and spectral responses all have to be built into the master. DCG has all the right properties for a top quality master that will get the most from any copy film of marginal index modulation, single beam reflection copies in Dupont photopolymers are as bright as the material will allow in any copy configuration.
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.
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.
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.
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 Kogelnik's 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.
We review the properties and relative usefulness of 3 non silver Volume holographic recording materials that are available today. Dichromated Gelatin (DCG) will receive the most attention followed by Dupont Omnidex products and a light treatment of Polyvinyl Carbazole (PVK). Enhancement and control of color, bandwidth and diffraction efficiency of volume reflection holograms recorded in DCG and photopolymers is discussed. Methods of increasing the bandwidth while shifting the center frequency toward the red is given for photopolymers. Red pseudo color will be covered thoroughly so that the practitioner will have all the elementary tools to make full color and broadband DCG holograms from scratch. The entire DCG technology is disclosed as it relates to production of high quality display holograms that span the spectrum and may be narrowband and very deep or shallow and broadband.
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.
Enhancement of color, bandwidth and diffraction efficiency
of volume reflection holograms recorded in photopolymers is
discussed and demonstrated. A method of increasing.the bandwidth
while shifting the center frequency toward the red is given for
special cases in two well known photopolymers. Computer
modelling gives clues to what has happened to the structures in
some cases. Many more adjustment recipes than those given appear
to be possible and are likely to work as well or better.