Indra Tengara

Compact HMD optics based on multiplexed aberration-compensated holographic optical elements

Head and helmet mounted displays (HMDs) can benefit greatly from new wide field-of-view, compact visor optics to project very high resolution (e.g., 5k X 4k) imagery. Physical Optics Corporation (POC) is developing novel, compact, lightweight wide field-of-view optics based on three-color multiplexed aberration-compensated holographic optical elements (MAC-HOEs). Taking advantage of the flexibility of holography, the HMD optics can be made compact using waveguide projection through the curved visor substrate, so that the see-through visor can have a wide field-of-view without large, bulky optical components. Using narrowband red-green-blue hologram multiplexing, MAC-HOEs can significantly reduce the chromatic and geometrical aberration introduced by conventional HOEs and refractive optics. In the initial phase of development, POC demonstrated the feasibility of the HMD optics through computer design and analysis, and by fabricating and demonstrating a MAC-HOE component.

Lippmann–Bragg broadband holographic mirrors

Lippmann–Bragg broadband volume holographic mirrors, with chirp normal to the surface, represent an entirely new class of grating. These gratings are presented and analyzed theoretically by using a combination of the multiple-beam interference method and Kogelnik’s local solution for uniform gratings. Particularly noteworthy is the new grating’s combination of a very high Bragg diffraction efficiency (>99.5%) with a large tunable bandwidth (from 5 to >300 nm).

Stratified volume diffractive elements: modeling and applications

We investigated a new type of optical element: volume diffractive optical element (VDOE). The uniqueness of the VDOE approach lies in the fact that it can exhibit a high diffraction efficiency (indigenous to a volume holographic optical element (HOE), while sporting a multi-diffractive layer structure amenable to standard lithographic manufacturing techniques. Computer design flexibility and the capability of effecting an arbitrary phase function in a VDOE provide a number of potential applications for these optical elements. Specifically, space telescope and image multiplication are discussed. Our theoretical modeling of the VDOE utilizes the rigorous coupled wave theory, which allows us to introduce an arbitrary VDOE/spacer layer thicknesses, grating slant angles, wavelengths, and incident angles. In addition, our model can simulate a multi-layer phase shifted VDOE structure (important in simulating HOE structures by a multi-layer VDOE design).

Second-order radiometric ray tracing

Conventional ray-tracing methods fail for non-Lambertian sources. To address this deficiency, we introduce a radiometric ray-tracing (R2T) method, applicable to quasi-homogeneous sources of arbitrary spatial coherence. Based on Fourier optics, applied to physical radiometry in the radiance transfer function second-order approximation, the R2T method retains the standard ray-tracing codes but modifies them to include phase-space weighting factors attached to conventional geometric rays.

Image tiling for a high-resolution helmet-mounted display

Head-mounted or helmet-mounted displays (HMDs) have long proven invaluable for many military applications. Integrated with head position, orientation, and/or eye-tracking sensors, HMDs can be powerful tools for training. For such training applications as flight simulation, HMDs need to be lightweight and compact with good center-of-gravity characteristics, and must display realistic full-color imagery with eye-limited resolution and large field-of-view (FOV) so that the pilot sees a truly realistic out-the-window scene. Under bright illumination, the resolution of the eye is ~300 μr (1 arc-min), setting the minimum HMD resolution. There are several methods of achieving this resolution, including increasing the number of individual pixels on a CRT or LCD display, thereby increasing the size, weight, and complexity of the HMD; dithering the image to provide an apparent resolution increase at the cost of reduced frame rate; and tiling normal resolution subimages into a single, larger high-resolution image. Physical Optics Corporation (POC) is developing a 5120 × 4096 pixel HMD covering 1500 × 1200 mr with resolution of 300 μr by tiling 20 subimages, each of which has a resolution of 1024 × 1024 pixels, in a 5 × 4 array. We present theory and results of our preliminary development of this HMD, resulting in a 4k × 1k image tiled from 16 subimages, each with resolution 512 × 512, in an 8 × 2 array.

Phase-Space Formalism and Ray-Tracing Modeling of Photometric Quantities

In this paper, radiometric ray tracing (R2T), based on phase-space formalism, is formulated.

Compensation of spacer-thickness variations in a holographic Fabry–Perot filter

The fabrication of a solid, holographically recorded Fabry-Perot interferometer that uses plate glass for the spacer has recently been reported. The component produced sharp, circular Fabry-Perot fringes in spite of its use of a plate-glass spacer. We develop a general theoretical characterization of such a component that accounts for its low sensitivity to spacer-thickness variations. We use the Kogelnik theory of volume holograms to calculate the phase change on reflection from the mirrors. This phase change results from the position of the fringes formed throughout the two holographic media during the recording process. An expression for the wavelength location of the transmission peak versus spacer-thickness variation is derived that agrees with the current experimental information available.

Liquid crystal modulated optical amplifier for night vision imaging

Image intensifier tubes, as part of night vision devices, have been the primary devices for the detection and amplification of near infrared light for night vision operations. In this paper, we demonstrate a novel all-optical night vision amplifier device with a potential to replace the image intensifier tube in night vision goggles. This image amplifier is based on a novel structure of semiconductor and spectrally tunable liquid crystal (LC) materials within a thin cell. The LC reacts to near-infrared (NIR) radiation but is unaffected by visible light, allowing see-through capability including visible-wavelength cockpit light. The technology is made very attractive by its high sensitivity, spatial resolution, and contrast without expensive, bulky, and heavy optics or high-voltage components.

Directionality of light shaping and displays

New approaches to light directionality and backlighting are described. They are discussed in the general context of the Liouville theorem and the second principle of thermodynamics; 3-D autostereoscopic applications are also discussed.

Diffraction efficiency side lobe suppression in stratified VDOEs

The potential utilization of volume diffractive elements (VDOEs) in a variety of sophisticated imaging applications has been established. VDOEs are capable of designed-in arbitrary phase distributions (indigenous to multi-layer diffractive structures), while maintaining high diffraction efficiencies (exhibited in volume holographic optical elements (HOEs)). Furthermore, multi-layer VDOE structure facilitates further improvement over HOEs. High diffraction efficiencies with simultaneous side lobe suppression can be achieved in VDOEs, while narrow bandwidths and desirable free spectral ranges are maintained. In this paper, we show that with proper utilization of VDOE and spacer thickness, we can achieve efficient side lobe suppression in the behavior of the diffraction efficiency, as a function of both incident wavelength and angle.