Physical Sciences

Surface Gratings for Optical Coupling With Microspheres

Far-field coupling offers advantages over near-field coupling. A diffraction grating consisting of a periodic gradient in the index of refraction of a thin surface layer has been shown to be effective as a means of far-field coupling of monochromatic light into or out of the "whispering-gallery" electromagnetic modes of a transparent microsphere. This far-field coupling can be an alternative to the near-field (evanescent-wave) coupling afforded by prism- and fiber-optic couplers described in the immediately preceding article. Far-field coupling is preferable to near-field coupling in applications in which there are requirements for undisturbed access to the entire surfaces of microspheres. Examples of such applications include (1) a proposed atomic cavity in which cold atoms would orbit in a toroidal trap around a microsphere and (2) a photonic quantum logic gate based on coupling between a high-Q (where Q is the resonance quality factor) microsphere and trapped individual resonant ions.

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High-Speed Image Compression via Optical Transformation

A lens would be utilized as an optical Fourier transformer. A proposed method of compressing image data would exploit the well-known capability of a converging lens to generate the Fourier transform of an image by purely optical means, in much less time than is needed to compute the discrete Fourier transform of a sampled image by use of digital electronic circuits. Inasmuch as a transform (whether of the Fourier, discrete-cosine, or other type) is the most computation-intensive part of almost any electronic image-compression scheme, the speedup afforded by the proposed method could make the difference between success or failure in applications in which there are requirements to compress image data at high throughput rates. In addition, because high-speed digital image-processing circuits are typically power-hungry, the use of optical Fourier transformation can reduce power consumption.

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High-Speed Optical Image Compression at Lower Power

White-light holography would enable elimination of a power-hungry spatial light modulator. In an alternative to the optical image-compression method of the preceding article, the Fourier transform of the input image would be formed on the output plane by white-light holography, instead of by laser holography. The principal advantage of the alternative method would be decreased power consumption: A state-of-the-art liquid-crystal spatial light modulator needed to implement the method of the preceding article consumes about 10 W of operating power, and the liquid crystals must be maintained at a temperature near 25 °C. On the other hand, an image detector of the active-pixel-sensor (APS) type, needed to acquire the Fourier-transform image in both the method of the preceding article and in the alternative method, consumes only about 50 mW. Because the spatial light modulator would not be needed in the alternative method, the power consumption of the image-compression system could be greatly reduced.

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Biomorphic Gliders

Miniature robotic microflyers would gather scientific data to enable reconnaissance missions and deploy payloads on landing. Biomorphic gliders are small robotic microflyers proposed for use in scientific exploration of planetary atmospheres and terrains that capture some key features of insect and bird flight. Biomorphic gliders as biomorphic flight systems are a subset of biomorphic explorers. The multidisciplinary system concept of "Biomorphic Explorers" represents small, dedicated, low-cost explorers that possess some of the key features of biological systems, not easily captured by conventional robotic systems. Such features particularly include versatile mobility, adaptive controls, bioinspired sensor mechanisms, biomorphic sensor fusion, biomorphic communications, biomorphic cooperative behavior, distributed operations, and biomorphic energy generation/conversion. Significant scientific and technological payoff at a low cost is realizable by using the potential offered by a large number of such cooperatively operating biomorphic explorer units in concert with the traditional exploration platforms such as the lander/rover, orbiter, etc., for example.

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High-Heat-Flux Thermogravimetric Analysis With Radiography

A process and a laboratory setup to implement the process (see figure) have been devised to enable the acquisition of time-resolved data on the thermal decomposition of a specimen of a solid material exposed to a heat flux comparable to the heat flux in a typical rocket engine. The process is called "RTR-TGA" because it includes a combination of real-time radiography (RTR) and thermogravimetric analysis (TGA). In the process, one specimen surface (e.g., representing a surface exposed to flames in a rocket engine) is heated by a continuous-wave CO2-laser beam while the interior temperature of the specimen is measured and the specimen is observed by an x-ray apparatus that produces video images that can be recorded. The major advantage of this process over older processes for observing thermal decomposition of material specimens is that the environment to which the specimen is exposed approximates more closely the heating environment in a full-scale rocket engine.

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Thermogravimetric Analysis With Laser Heating

A thin specimen is radiantly heated from both sides.Laser thermogravimetric analysis (laser TGA) is a technique that yields time-resolved data on the thermal decomposition of a specimen of a solid material exposed to a heat flux comparable to the heat flux in a typical rocket engine. Like the technique described in the preceding article, laser TGA involves heating the specimen with a continuous-wave laser beam to obtain the required high heat flux. The utility of laser TGA is not restricted to rocket-engine materials; laser TGA could be used to study high-heating-rate thermal decomposition of almost any high-temperature insulating material.

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Ultralight Balloon Systems for Exploring Uranus and Neptune

A report proposes ultralight balloon systems to carry a 10-kg payload, including scientific instruments for exploring the atmospheres of Uranus and Neptune. The system masses to be transported to those planets would be kept low by not transporting balloon-inflating gases. Each system would include an upper balloon about 4 m in diameter (0.5 kg) connected via a small port (about 0.25 m in diameter) to a lower balloon about 15 m in diameter (6.4 kg). Through an opening in the lower balloon, the balloons would become filled with low-molecular-weight atmospheric gas (which has little methane content) during initial descent through the upper atmosphere. At some point in the descent, the opening would be closed. Thereafter, the collected gas would provide buoyancy in the higher-molecular-weight atmosphere (methane content ≈2 percent) in the exploration altitude range below the methane-cloud tops, and the lower balloon (used for collection only) would be dropped. The altitude could be held constant or could be regulated by alternately venting gas and dropping ballast, as is done on balloons in the terrestrial atmosphere.

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