Photonics

Photonic-Crystal-Based Devices for Commercial Applications

Several analog-to-digital converter (ADC) devices and part of a multispectral receiver have been designed, fabricated, and tested to demonstrate their feasibility as part of an effort to establish the organizational and technological foundation for development of photonic-crystal-based devices for commercial and military applications. Also known as photonic-band-gap devices, photonic crystals contain periodic structures having feature sizes in the submicron range — less than the wavelengths of light that the devices are intended to handle. Photonic crystals can be fabricated by techniques used in the integrated-circuit and microelectromechanical systems (MEMS) industries.

Posted in: Briefs, Photonics, Optics
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Infrared Data Link Using an MQW Modulator on a Retroreflector

A n infrared data link between a ground station and a small uninhabited helicopter in flight has been demonstrated in an effort to develop a type of system for free-space optical communication between (1) a larger and relatively stationary platform, and (2) a smaller and relatively mobile platform. In a system of this type, rather than using laser transmitters with their associated gimbaled telescopes and pointing/tracking subsystems on both platforms, one uses only a single such laser transmitter on the larger platform (in this case, the ground station). The single laser transmitter is capable of tracking the smaller platform (in this case, the helicopter) and transmitting data to the smaller platform in the conventional way via modulation of the outgoing laser beam. The field of view of the receiver on the smaller platform is wide enough to capture the laser beam, without need for a large receiving telescope and its aiming subsystem. For transmitting data from the smaller to the larger platform, a large fraction of the laser power incident on the smaller platform is modulated and retroreflected to the larger platform, by means of an InGaAs-based multiple-quantum-well (MQW) light modulator on a cornercube retroreflector (see Figure 1).

Posted in: Briefs, Photonics, Data exchange, Optics, Rotary-wing aircraft
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Continuous-Wave Atom Laser

Progress has been made toward realization of a continuous-wave, phase-andamplitude- stable atom laser based on magnetic guiding, magnetic compression, and continuous distributed evaporative cooling of a sparse cloud of 87Rb atoms. This apparatus is intended to serve as a prototype of sources of coherent matter waves for future atom-interferometric field and motion sensors.

Posted in: Briefs, Photonics, Lasers, Sensors and actuators
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Adaptive Optoelectronic Eyes

Aprogram of research has built a technological foundation for further development of systems that could be characterized, variously, as "smart" cameras or adaptive optoelectronic eyes. A system according to this concept would function in an optimal manner to capture images under a variety of lighting conditions and would be capable of semi-autonomous recognition of objects.

Posted in: Briefs, Photonics, Imaging, Optics
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Heterodyne RF/Optical Links Utilizing Integrated Photonics

A program of research and development has been directed toward the goals of demonstrating (1) ultra-low-noise communication links in which information is conveyed by phase-modulated radio-frequency (RF) carrier signals that are, in turn, used to modulate laser-generated optical carrier signals and (2) implementation of transmitters and receivers in such links by means of several key integrated optoelectronic and photonic components. Notably, the scheme for integrating these components is based on the use of asymmetric twin optical waveguides (see figure) that afford design versatility in that they enable the use of a broad range of components useful in RF/photonic applications.

Posted in: Briefs, Photonics, Optics, Waveguides, Research and development
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Fast Liquid-Crystal-on-Silicon Spatial Light Modulators

Stressed-liquid-crystal (SLC) light-modulating devices suitable for use as liquidcrystal- on-silicon (LCOS) spatial light modulators (SLMs) that could operate in nearand mid-infrared wavelength ranges have been demonstrated. These SLC devices were conceived to exploit the SLC electrooptical effect, which makes it possible to obtain response times shorter than those of prior infrared LCOS SLMs.

Posted in: Briefs, Photonics, Optics, Semiconductors, Silicon alloys
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Detecting Change in 3D by Use of an Evidence Grid

Astudy has been undertaken to evaluate a method of detecting change in a three-dimensional (3D) terrain map generated from data acquired by an imaging ladar system carried aboard a moving unmanned ground vehicle (UGV) on patrol. The proposed method involves the use of an evidence grid (described below) in comparing data acquired on a second patrol with data acquired on the first patrol along the same route, in order to determine which, if any, volume elements (voxels) in a 3D map representing the terrain have changed from free space to occupied or vice versa. For the purpose of the method, it is assumed that the terrain is static during each patrol and the only changes of interest occur between patrols. It was recognized in the study that these assumptions are unlikely to hold in realistic scenarios. This study was intended to be a precursor to a study of a method for recognizing a moving obstacle C, particularly a moving pedestrian C during a patrol by an autonomous UGV.

Posted in: Briefs, Photonics, Cartography, Lidar, Surveillance, Terrain, Autonomous vehicles, Military vehicles and equipment
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Patterned Gallium Arsenide Devices for Infrared Countermeasures

The US Air Force has a need for improved tunable laser sources—both in the midinfrared region, for developing infrared countermeasure (IRCM) applications, and in the longinfrared region, for addressing an increasing variety of threat sensors. Since few direct lasers exist in these spectral regions, scientists generally use nonlinear frequency conversion techniques to convert the output of available lasers into the desired longer

wavelengths.

Posted in: Briefs, Photonics
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Cyberspace Security via Quantum Encryption

Perfect information- theoretical security requires that the meaning of an encrypted message transmitted from point A to point B be statistically independent of the ciphertext in which that message is embedded. In other words, possession and analysis of the ciphertext must yield no information about the message sent. This article briefly describes cryptographic protocols exhibiting perfect, or nearperfect, security before addressing a new quantum data encryption protocol that employs quantum noise of light at the physical layer to buttress security based on mathematical complexity. This new protocol is called Keyed Communication in Quantum Noise, or KCQ. KCQ does not presently guarantee flawless informationtheoretical security; however, because of KCQ's physical-layer encryption in the quantum noise of light, some scientists believe that it enables better security than current secure communications systems based solely on mathematical complexity.

Posted in: Briefs, Photonics
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Dynamic Cavity Formation Imaging

When an Air Force bomber drops a penetrating munition, what happens as the warhead travels underground to the target? AFRL researchers at the Advanced Warhead Experimentation Facility (AWEF) recently captured X-ray images of laboratory-scale warheads as they penetrated sand targets at high speeds. These radiographs enhance the weapon design community's understanding of warhead/target interactions and aid in validating computer simulations as well.

To improve future warhead performance, AWEF researchers are striving to understand the physics behind a high-speed weapon's penetration of dry particulate media. The media under investigation undergoes very complex reactions during penetration because without the gluelike presence of moisture, the dry particles move freely with respect to each other. Consequently, the cavity formed around a high-speed projectile throughout its penetration collapses almost immediately after the projectile continues its progress. The diagnostic X-ray techniques that AFRL researchers are refining allow a glimpse of the temporary cavity, which provides a key indication of the penetration loading environment. Without knowledge of the physical interactions that occur during a weapon's penetration, weapon designers are limited to judging a new warhead's performance (e.g., penetration depth) through empirical trial-and-error methods. The incorporation of proven theoretical penetration models into advanced computer simulations will thus shorten the process of developing optimal warhead designs.

The current research effort is an expansion of a related study conducted in the 1970s. The previous study examined cylindrical rods traveling at conventional velocities through an unconfined sand trough. The recent experimental series, conducted in 2005, examined a variety of conventional warhead nose shapes in their highspeed penetration of confined sand targets. The investigated nose shapes included a sharply pointed ogive nose (see Figure 1), a blunted ogive nose (see

Figure 2), and a spherical nose.

Despite the past 30 years' progress in X-ray technology and the advent of digital processing, which enhances image contrast, obtaining useful images remains a challenge. For example, sand both absorbs and scatters X-rays. To maintain image quality, researchers therefore limited the target's diameter to 6 in., and to maximize the energy reaching the X-ray film, they kept the distance between the X-ray-generating power head and the film as small as possible. Due to the proximity of the equipment to the experiment's target location, however, flying fragments occasionally damaged both the film and the X-ray power head. As a result of this same proximity, the simultaneous triggering of the multiple X-ray heads produced shadows on the image. Throughout the experiments, researchers positioned the X-ray heads above and beside the projectile's expected flight path to show orthogonal views. This technique enabled the team to determine a projectile's pitch and yaw prior to its impact with the target and also provided a three-dimensional view of cavity formation during the penetration event. The researchers also found that by triggering a single X-ray pulse, they were able to mitigate the poor contrast and clutter caused by multiple shadow images. Despite the difficulties encountered, the series of experiments produced a dozen quality images.

The preliminary results surprised the research team. For example, the team discovered that the penetration cavity is much smaller than expected. Additionally, the area of the nose in physical contact with the target media is relatively small. In Figures 1 and 2, the light-colored objects are the projectiles and the dark region immediately surrounding each projectile is the cavity. In normal circumstances, the explosive payload would ride in the center of the projectile shaft. The thin white lines are the result of lead crosshairs placed on the film's exterior surface to align the X-ray head. Researchers are planning an additional series of experiments designed to increase the image database and refine the X-ray's diagnostic potential.

Understanding weapons penetration phenomena is essential to the efficient design of future weapons concepts. As a result of experimental efforts such as these, the intelligent warheads and other munitions of tomorrow may one day be capable of morphing their exterior shape to achieve the most efficient penetration of the media encountered.

Lt Christine E. Watkins, of the Air Force Research Laboratory's Munitions Directorate, wrote this article. For more information, contact TECH CONNECT at (800) 203-6451 or place a request at http://www.afrl.af.mil/techconn_index.asp. Reference document MN-H-05-16.

Posted in: Briefs, Photonics
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