Photonics

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.

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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.

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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.

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Surface-Emitting Laser Arrays Bring Light to the Top

Laser diodes are an integral part of everyday life, incorporated into commonplace items as diverse in function as laser pointers, fiber-optic communications systems, and DVD players. Manufacturers make most laser diodes by layering specially doped semiconductor materials on a wafer. By slicing tiny chips from these wafers to attain two perfectly smooth, parallel edges, they create very thin (tens of microns) waveguides. These waveguides define a resonating cavity that causes stimulated light to combine in a way that embodies a "laser" and propagates its lasing action. Although this process represents a highly successful and wellengineered means for producing semiconductor lasers, the lasers do not produce an optimum beam. Beam emission occurs from the small rectangular opening at the end of the chip, a configuration that results in an elliptically distorted beam as well as the loss of output efficiency. In addition, the output aperture's relatively small size can lead to destruction of the cleaved and polished end facet during the laser's high-power operation. Laser diodes produced using this process are also susceptible to substantial fluctuations in output wavelength and beam quality as a function of temperature. Furthermore, since the chip emits beam output from an edge instead of its top or bottom surface, manufacturers experience difficulty both in packaging various diode configurations and in combining the output beams of multiple laser diodes.

Posted in: Briefs, Photonics, Fiber optics, Lasers
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