Special Coverage

Lightweight Internal Device to Measure Tension in Hollow- Braided Cordage
System, Apparatus, and Method for Pedal Control
Dust Tolerant Connectors
Foldable and Deployable Power Collection System
Iodine-Compatible Hall Effect Thruster
Development of a Novel Electrospinning System with Automated Positioning and Control Software
2016 Create The Future Design Contest Open For Entries
Home

Spaceborne Telescope for Communication, Ranging, and Imaging

A report describes a program to develop a multi-function telescope to be carried aboard spacecraft of the planned X2000 series of planetary missions. [A related multi-function telescope was described in "Telescope for Imaging and Laser Communication" (NPO-20388), NASA Tech Briefs, Vol. 24, No. 1 (January, 2000), page 27a.] An important element of this program is minimization of instrument mass through combination of several optoelectronic subsystems into one telescope system with one aperture for transmission and reception of light. The functions to be performed by the telescope system include scientific imaging, reception of laser-altimeter return signals, and two-way optical communication. The communication functions include transmission of scientific data, reception of commands, and reception and retransmission of laser ranging signals. For communication, the telescope would be aimed at the sunlit Earth, by use of aiming techniques like those summarized in the cited previous article. Rates for transmission of scientific data to Earth from as far away as Jupiter would range from 100 kb/s (day) to 400 kb/s (night). The data rate for reception of commands from Earth would be 2 kb/s. A flight-qualified engineering model of the system is scheduled for completion in the year 2001.

Posted in: Briefs, TSP, Physical Sciences

Read More >>

Dislocation-Induced Changes in InxGa1 -xAs Quantum Dots

A report describes an experimental investigation that revealed a previously unknown type of spatial alignment of quantum dots (QDs) in InxGa1 -xAs/GaAs multilayer structures. Multilayer arrays of QDs (in the form of nanometer-sized InxGa1-xAs islands) were formed by alternately depositing 10-nm-thick layers of GaAs and 5-molecule-thick layers of In0.6Ga0.4As/GaAs on substrates of semi-insulating [001] GaAs with a 2° miscut toward the [101] direction (resulting in steps along the [010] direction). Specimens were examined by electron microscopy and cathodoluminescence (CL) spectroscopy. The QDs were observed to undergo a transition between step edge alignment along the [010] direction to counter step alignment (along the [100] direction). This transition apparently occurred when (or soon after) the fifth quantum-dot layer was deposited, and is apparently associated with the onset of a network of misfit dislocations at the In0.6Ga0.4As/GaAs interface. A change to larger QDs in smaller concentrations was also observed after formation of the network of dislocations. Strong near-infrared CL emission from the QDs was observed, despite the presence of dislocations.

Posted in: Briefs, TSP, Physical Sciences

Read More >>

Solitons on WDM Beams in a Nonlinear Optical Fiber

This paper sets the ultimate limit on the maximum amount of optical data pulses that can be sent through a single fiber in a given period under the wavelength-division-multiplexed (WDM) format. The discovery in 1973 that optical soliton on a single wavelength beam can exist in fiber is one of the most significant events since the perfection of low-loss optical fiber communication. This means that, in principle, data pulses may be transmitted in a fiber without degradation forever. This soliton discovery sets the ultimate goal for optical fiber communication on a single-wavelength beam. Another most significant event is the development of WDM transmission in a single-mode fiber. This means that multiple beams of different wavelengths, each carrying its own data load, can propagate simultaneously in a single-mode fiber. This WDM technique provides dramatic increase in the bandwidth of a fiber. However, due to the presence of complex nonlinear co-propagating pulses on different wavelength beams, it is no longer certain that WDM soliton can exist. The existence of solitons is a blissful event in nature. It is a marvel that the delicate balance between the dispersion effect and the nonlinear effect can allow a specially shaped optical pulse to propagate in the fiber without degradation. They occur only on single-wavelength beams. When beams with different wavelengths co-propagate in a single-mode fiber, such as in the WDM case, interaction of pulses on different beams via the nonlinear cross-phase-modulation (CPM) effect (the Kerr effect) is usually instrumental in destroying the integrity of solitons on these wavelength multiplexed beams. This paper shows that temporal solitons can exist on WDM beams in a single fiber under appropriate conditions. The existence of these solitons critically depends on the presence of the nonlinear CPM effect of the WDM beams. Just as the earlier single-beam soliton case, this discovery sets the ultimate goal for optical fiber communication on WDM beams.

Posted in: Briefs, TSP, Physical Sciences

Read More >>

Fast Observers for Spacecraft Pointing Control

A report discusses the design of fast stochastic observers for spacecraft pointing control. In this special context, "observers" signifies mathematical algorithms, implemented on computers aboard spacecraft, through which one processes sensory data (principally, the outputs of star trackers and gyroscopes) to estimate the states (attitudes and angular velocities) of the spacecraft. The development in the report was motivated by the presence of an attitude-dependent bias error in the star-tracker measurement associated with NASA's upcoming SIRTF (Space Infra-Red Telescope Facility) space telescope. This attitude-dependent bias term lies outside of basic linear estimation assumption, and the well-established Kalman theory is no longer optimal. The attitude-dependent bias term forces a step response through the dynamics of the onboard estimator each time the spacecraft is repositioned. If an optimal Kalman filter were used, its sluggish dynamics would create a long undesirable lingering output drift in the pointing response. While this drift error is small (e.g., at the arcsecond level) it cannot be ignored for space telescope applications, and is the main reason that Kalman filters are routinely replaced by simple observers on important missions with stringent pointing requirements such as the Hubble Space Telescope and SIRTF.

Posted in: Briefs, TSP, Information Sciences, Physical Sciences

Read More >>

Microfabricated High-Q Optical Resonators for Microphotonics

Ultra-high-Q microcavities would be mass-produced and integrated by techniques adapted from microelectronics. Submillimeter-sized, transparent, solid, truncated spheres and ellipsoids for use as optical resonators in integrated microphotonic devices would be made by microfabrication techniques like those used in the electronic industry to make integrated circuits, according to a proposal. Such resonators, heretofore denoted generally as "microspheres," have been described in several recent articles in NASA Tech Briefs. In a microsphere, resonance is achieved through glancing-incidence total internal reflection in one or more "whispering-gallery" modes, in which the light propagates in equatorial planes near the surface, with an integer number of wavelengths along a nominal closed circumferential trajectory. If the surface of the resonator is sufficiently smooth and a sufficiently close approximation of a sphere or ellipsoid, then in principle, the resonance quality factor (Q) is limited only by attenuation in the resonator material; for a microsphere made of fused silica, this translates to a potential to obtain Q "e1010.

Posted in: Briefs, TSP, Physical Sciences

Read More >>

Software To Detect Malfunctions in Anesthesia Equipment

Response times in critical situations can be reduced. To reduce response times and save lives in operating rooms, community trauma centers, and remote combat care facilities, a team of scientists working on behalf of Lyndon B. Johnson Space Center (JSC) has developed an artificial-intelligence alarm-management software system that detects malfunctions in esophageal intubation and anesthesia machines. This system uses CLIPS [the C-Language Integrated Production System] knowledge-based rules derived from real-time data supplied by a sheep model. Although this system is one of several current alarm-management software systems, it is vastly superior to the commercial software systems designed to perform the same or similar functions.

Posted in: Briefs, Physical Sciences

Read More >>

Sulfur Lamp With CaBr2 Additive for Enhanced Plant Growth

Addition of CaBr2 intensifies red light, which is favored by plants. Calcium bromide can be added to the sulfur filling in a sulfur lamp to increase the emission of red light for enhanced growth of plants. Red light is more efficacious for plant growth than is visible light at shorter wavelengths. The addition of CaBr2 increases the emission at wavelengths in the vicinity of 625 nm, where the quantum efficiency for photosynthesis is close to 1.

Posted in: Briefs, Physical Sciences

Read More >>

The U.S. Government does not endorse any commercial product, process, or activity identified on this web site.