Juris Upatnieks

Compact holographic sight

Size reductions of reflex type sights and head -up displays (HUDs) can be achieved with holographic components. For the reflex type sight, the image formed by a high -quality optical system is recorded in a holographic plate with an edge -illuminated reference beam. This hologram reconstructs the original image and duplicates its original quality. The system is compact, consisting of a hologram on a flat glass plate and a compact laser beam expansion optics. The illuminating beam enters the hologram plate through its edge. The laser is remotely located with light guided through a fiber optics cable to the sight. HUDs can also be made more compact by using a pair of gratings attached to a flat glass plate. The plate guides the display to a position in front of the pilot and occupies a small volume.

Edge-illuminated holograms

Design and construction considerations and performance characteristics of edge-illuminated holograms are described. The display system layouts presented are compact, monolithic, and achromatic, and the light usage efficiency is improved by recycling the undiffracted hologram-illuminating beam.

Noise suppression in coherent imaging.

Several noise suppression techniques in coherent imaging systems are described. For holographic imaging the diffuse wave, periodic phase modulation, and multiple wave techniques are compared and the implementation of the last is considered. For lens-type imaging systems the use of multiple incoherent waves results in excellent noise suppression.

Portable real-time coherent optical correlator

A real-time coherent optical correlator is mounted inside a 15- × 23- × 38-cm enclosure and features laser diode light sources and a liquid crystal incoherent-to-coherent image converter. Each of four reference filters is illuminated by a separate laser diode, real-time operation is achieved by the image converter, and a zoom lens forming the incoherent image allows a scale search. The correlator can be mounted on a tripod and performs correlations indoors or outdoors.

Compact Real-Time Coherent Optical Correlators

Efforts to fabricate real-time optical correlators in compact packages are under way. In particular, a brassboard unit mounted inside a 15 X23 X28 cm enclosure featuring laser diode light sources and a liquid crystal incoherent-to-coherent image converter has been built; operational experience with this correlator is discussed. Further miniaturization is possible, and some suggested geometries are examined in detail.

360-Degree Hologram Displays

360° holograms can be used for directly viewable image display both in information retrieval systems and in static displays. Images can be displayed with modest sized viewers that use a small section of the hologram for individual observers and with the capability for rapid interchange of holograms. Static displays that use the entire hologram are also realized in this concept. Holographic projector systems are also possible which can form two-dimensional images on a flat screen with parallax change along two dimensions. The design of viewers and projectors involves consideration of many parameters and tradeoffs that involve image quality, design complexity, appearance, and cost. Virtual image display units can use either laser or arc lamp light sources, while a laser beam is needed for projectors. Projectors require special design considerations related to the fact that the hologram con-tains the images in their original size and depth. Coherent speckle of projected images can be eliminated by use of fluorescent paints on the pro-jection screen. Several design examples of display units are described and shown.

Noise reduction in holographic images

Abstract Coherent noise, including the speckle pattern, is supressed in images that are formed by holograms and reimaged by lenses. The excess bandwidth of the holograms and a moving grating are used to form a semicoherent image with reduced noise level.

Holographic gunsights for small arms

Holographic gunsights were first demonstrated in the mid 1970s by researchers at the Environmental Research Institute of Michigan (ERIM) under contracts with the Air Force and the Army. The sights utilized He-Ne gas lasers and were designed for use with large weapons systems. With the advent of low cost visible laser diode, ERIM formed a new company, EOTech, to develop, manufacture and market a holographic gun sight for small arms. A hologram is used to reconstruct the image of a reticle pattern that appears at the target plane. Unlike red-dot sights, virtually any reticle pattern, 2D or 3D, can be formed. The design challenges include an opto-mechanical package that is compact, light weight and low cost which can withstand recoils up to 4,000 Gs and provide fine elevation/windage pointing adjustments, and optics that are aberration-free and stable over a wide temperature range. Manufacturing challenges include the mass production of high quality holographic optics at low cost and the precision alignment of the very low f/number optics.

Holographic gunsight: the next generation

Holographic gunsight was first conceived in the 1970s and prototypes were fabricated using a He-Ne laser as the illuminating source. The laser source was too costly and fragile and these prototype units were too bulky to be viable as a commercial product. With the advent of low cost laser diodes, EOTech introduced into the commercial market a compact holographic gunsight for small arms in 1996 which has since become one of the most popular gunsight in the U.S. and in Europe. In this paper, the design of the second generation holographic gunsight is described. The optical path travels predominantly in the vertical direction which reduces the length and the weight of the sight by a third. The optical design challenges include the generation of a stable holographic image with changes in the laser emission wavelength, circularization of the laser elliptical beam profile, and the production of high quality optics at low cost. The opto-mechanical design challenges include very fine angular adjustments, stability over large temperature range and the ability to withstand the recoil of powerful handguns.

Multiple Fourier transform generation for coherent optical correlators

An interferometrically generated off-axis holographic optical element images a laser diode light source to a 3 x 5 point array through 10 cm of glass. The element also reduces the elliptical beam cross section from 3:1 to 1.5:1.