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At least 1,657 records · Page 92

Carrier Dynamics of Polar, Semipolar, and Nonpolar InGaN/GaN LEDs Measured by Small-Signal Electroluminescence

The carrier dynamics in InGaN/GaN LEDs are directly tied to their efficiency and maximum modulation speed, which are important metrics for solid-state lighting, displays, and optical communication. We measure the carrier dynamics of nonpolar, semipolar, and commercial c-plane InGaN/GaN LEDs using small-signal electroluminescence methods. Rate equations and a small-signal circuit are used to model the carrier dynamics. The model is fit to the optical frequency response and input impedance of the LEDs to extract the carrier lifetimes and the recombination rates. The results offer insight into the underlying causes of efficiency droop and the green gap, and inform device design strategies.

42 ENGINEERING↗

A new real-time non-coherent to coherent light image converter - The hybrid field effect liquid crystal light valve

A new, high-performance device has been developed for application to real-time coherent optical data processing. The new device embodies a CdS photoconductor, a CdTe light-absorbing layer, a dielectric mirror, and a liquid crystal layer sandwiched between indium-tin-oxide transparent electrodes deposited on optical quality glass flats. The noncoherent image is directed onto the photoconductor; this reduces the impedance of the photoconductor, thereby switching the ac voltage that is impressed across the electrodes onto the liquid crystal to activate the device. The liquid crystal is operated in a hybrid field effect mode. It utilizes the twisted nematic effect to create a dark off-state and the optical birefringence effect to create the bright on-state. The liquid crystal modulates the polarization of the coherent read-out light so an analyzer must be used to create an intensity modulated output beam.

Grinberg, J.↗

Fiber-optic rotation sensor technology

A concept for an all-waveguide fiber-optic rotation sensor is discussed, and the results of preliminary tests of key elements are described. A single channel waveguide coupler design provides the functions of an optical switch, a 3-dB beam splitter, a phase retarder, and a signal modulator, all of which may be formed on the same chip and interconnected by channel single-mode waveguides. Preliminary test results for the waveguide coupler and for a rotation sensor without the coupler are presented. Signal processing, polarization control, and interconnection of the waveguide components are discussed.

Goss, W. C.↗

Optical residue addition and storage units using a Hughes liquid crystal light valve

Optical addition and storage units are described in this paper. These units are implemented using the Hughes Liquid Crystal Light Valve (LCLV) as a spatial light modulator using residue arithmetic for a numerical representation. The main hardware components of the design, besides the light valve, include an array of single-mode optical fibers that provide input information, a polarizing prism in combination with quarter-wave and half-wave retarders for residue arithmetic implementation in the adder, and a holographic array for spatial stability in the storage unit.

Habiby, S. F.↗

Nonlinear self-contraction of electron waves

Laboratory evidence is presented of modulationally unstable electron wave packets which can be described by a nonlinear geometrical optics theory. Growth times for self-contraction are found to be much faster than ion response times and the bursts do not appear to be related to Zakharov Langmuir-wave collapse.

Intrator, T.↗

Error control coding for satellite and space communications

The optical direct detection channel is discussed. It is shown how simple trellis coded modulation can be used to improve performance or increase throughput (in bits per second) without a bandwidth expansion and no performance loss. In fact, a modest performance gain can be achieved. The concentration is on signals derived from the pulse-position modulation format by allowing overlap.

Georghiades, Costas N.↗

Interferometric Fiber-Optic Gyroscope

Integrated three-waveguide directional coupler functions as polarizer and splitter. Designed with transverse electric (TE) polarization in bar state (two coupling lengths) and transverse magnetic (TM) polarization in cross state (one coupling length). Intended for eventual fabrication as in mass-producible integrated optical circuit that provides advantages including low drive voltage, large-bandwidth phase modulation, preservation of polarization in transmission between devices on same substrate, and low cost.

Depaula, Ramon P.↗

Rotational modulation and flares on RS CVn and BY Draconis stars. XIII - IUE spectroscopy and photometry of II Pegasi during September 1986

UV spectroscopy obtained for the RS CVn star II Pegasi in September, 1986, indicates the presence of a rotational modulation effect for the Mg II h and k and O I 1305 A lines; the largest flux is observed when the primary spot concentration is nearly on the opposite hemisphere, and the rotational modulation is noted to be of opposite sense to that observed in 1981. Optical photometry shows the largest concentration of spots on one hemisphere ever observed on II Peg, implying a spot group covering over 50 percent of the projected area of one hemisphere.

Doyle, J. G.↗

Optical observations of low-mass X-ray binaries and millisecond pulsars

We review recent results from optical observations of low-mass X-ray binaries (LMXBs) and millisecond pulsars. We discuss the optical and X-ray properties of those LMXBs which have been suspected of harboring black holes, reviewing in particular recent work on GX 339-4. We also present preliminary results from new optical observations of the ablating millisecond pulsar system PSR 1957 + 20. The orbital modulation is smooth and symmetrical, and is at least four magnitudes deep in R. However, the overall shape of the light curve can be reproduced by a simple model assuming blackbody reradiation from the secondary.

Callanan, Paul J.↗

System Would Generate Virtual Heads-Up Display

Proposed helmet-mounted electronic display system superimposes full-color alphanumerical and/or graphical information onto observer's visual field. Displayed information projected directly onto observer's retinas, giving observer illusion of full-size computer display in foreground or background. Display stereoscopic, holographic, or in form of virtual image. Used by pilots to view navigational information while looking outside or at instruments, by security officers to view information about critical facilities while looking at visitors, or possibly even stock-exchange facilities to view desktop monitors and overhead displays simultaneously. System includes acousto-optical tunable filter (AOTF), which acts as both spectral filter and spatial light modulator.

Lambert, James L.↗

Effects of Space Environments: Techniques and Lessons from Selected Flight Experiments

Data describing long-term materials performance under exposure to space environments has accumulated gradually over the past four decades. The authors present here selected results from several previous flight experiments and operational spacecraft, and a few examples from a current flight experiment. As examples of environmental effects on materials, analysis of Gortex samples from the Passive Optical Sample Assembly II (POSA II) experiment on the MIR-Shuttle docking module will be described. Results from a previous evaluation of radiation on silverized Teflon from over ten individual satellites will be summarized. Several examples of contamination effects on materials properties will be presented. These include outgassing of solar arrays onto nearby surfaces of the POSA I experiment and contamination of certain surfaces of the Long Duration Exposure Facility (LDEF). Results from experiments with silicone contamination on satellites in geosynchronous orbit will be compared with measurements on the Solar Maximum satellite and the LDEF. A number of techniques are being attempted to extend the range of exposure conditions present on selected experiments. Use of focusing concentrators on the Effect of Space Environments on Materials (ESEM), POSA, and Materials International Space Station Experiment (MISSE) will be described. A technique for obtaining time-resolved data from a passive materials experiment will be described. The status of an on-going materials flight experiment, MISSE, will be reported. Finally, the authors will draw some conclusions about the current state of knowledge relating to materials chosen for spacecraft applications. Understanding of degradation mechanisms, state of predictive models, and the need to strengthen model inputs will be discussed.

Golden, J. L.↗

Fault-tolerant processing system

A fault-tolerant, fiber optic interconnect, or backplane, which serves as a via for data transfer between modules. Fault tolerance algorithms are embedded in the backplane by dividing the backplane into a read bus and a write bus and placing a redundancy management unit (RMU) between the read bus and the write bus so that all data transmitted by the write bus is subjected to the fault tolerance algorithms before the data is passed for distribution to the read bus. The RMU provides both backplane control and fault tolerance.

Palumbo, Daniel L.↗

Optoelectronic tweezers for microparticle and cell manipulation

An optical image-driven light induced dielectrophoresis (DEP) apparatus and method are described which provide for the manipulation of particles or cells with a diameter on the order of 100 .mu.m or less. The apparatus is referred to as optoelectric tweezers (OET) and provides a number of advantages over conventional optical tweezers, in particular the ability to perform operations in parallel and over a large area without damage to living cells. The OET device generally comprises a planar liquid-filled structure having one or more portions which are photoconductive to convert incoming light to a change in the electric field pattern. The light patterns are dynamically generated to provide a number of manipulation structures that can manipulate single particles and cells or groups of particles/cells. The OET preferably includes a microscopic imaging means to provide feedback for the optical manipulation, such as detecting position and characteristics wherein the light patterns are modulated accordingly.

Wu, Ming Chiang↗

Optoelectronic Tweezers for Microparticle and Cell Manipulation

An optical image-driven light induced dielectrophoresis (DEP) apparatus and method are described which provide for the manipulation of particles or cells with a diameter on the order of 100 micromillimeters or less. The apparatus is referred to as optoelectric tweezers (OET) and provides a number of advantages over conventional optical tweezers, in particular the ability to perform operations in parallel and over a large area without damage to living cells. The OET device generally comprises a planar liquid-filled structure having one or more portions which are photoconductive to convert incoming light to a change in the electric field pattern. The light patterns are dynamically generated to provide a number of manipulation structures that can manipulate single particles and cells or group of particles/cells. The OET preferably includes a microscopic imaging means to provide feedback for the optical manipulation, such as detecting position and characteristics wherein the light patterns are modulated accordingly.

Wu, Ming Chiang↗

Demonstration of Space Optical Transmitter Development for Multiple High Frequency Bands

As the demand for multiple radio frequency carrier bands continues to grow in space communication systems, the design of a cost-effective compact optical transmitter that is capable of transmitting selective multiple RF bands is of great interest, particularly for NASA Space Communications Network Programs. This paper presents experimental results that demonstrate the feasibility of a concept based on an optical wavelength division multiplexing (WDM) technique that enables multiple microwave bands with different modulation formats and bandwidths to be combined and transmitted all in one unit, resulting in many benefits to space communication systems including reduced size, weight and complexity with corresponding savings in cost. Experimental results will be presented including the individual received RF signal power spectra for the L, C, X, Ku, Ka, and Q frequency bands, and measurements of the phase noise associated with each RF frequency. Also to be presented is a swept RF frequency power spectrum showing simultaneous multiple RF frequency bands transmission. The RF frequency bands in this experiment are among those most commonly used in NASA space environment communications.

Nguyen, Hung↗

Modular Active Self-Assembling Space Telescope Swarms

The Phase I study included a detailed exploration of a specific mission concept based on this architecture - a 31-meter diameter version of the LUVOIR (Large Ultraviolet Optical Infrared Surveyor) mission concept with the same wavelength coverage and wave-front quality requirements. The study focused on establishing the feasibility of the proposed concept with respect to what were identified as the most challenging aspects of the mission: the ability to find orbital solutions using solar sail propulsion for both the initial transfers and subsequent rendezvous trajectories; and the ability to generate the required final primary mirror surface out of uniformly polished individual module mirrors using deformation via mechanical degrees of freedom. As a result of the Phase I work it was shown that both orbital and optical solutions are feasible, but will require technology advancement in multiple key areas. In particular, improvements to solar sail areal densities and loading factors will allow us to more easily find fully general solutions for the transfer trajectories, while lower mass, ultra-precise cryogenic linear actuators will likely be needed to form the final mirror shape. However, it was found that current state of the art in solar sails was sufficient for rendezvous trajectories once on-orbit, that the requisite wave-front quality could be achieved with actuation requirements not more stringent than those being carried by LUVOIR, and that there exists a well-defined design space in which the module spacecraft designs can be optimized. The technology development required to progress this work overlaps significantly with numerous NASA technology roadmaps, as well as the Exoplanet Exploration Program's technologies gap list, meaning that continued development of this concept would directly benefit a variety of NASA programs, as well as enabling the completely unique capabilities provided by a 31-meter optical space telescope...Our architecture requires only a single module type allowing us to directly leverage the cost and efficiency benefits of true mass production, in direct contrast to previously explored self-assembling swarm concepts, which typically include multiple module types.

Astronomy↗

Optical communications for future deep-space missions

The development of key technologies for an optical communications system for deep-space probes is discussed. Aspects of the technology include the pulse-position modulation technique for data transmission, a solid-state laser with a Nd:YAG crystal end, nondiffraction-limited receivers, and the application of avalanche photodiodes to improve photon detection. Mission benefits and flight opportunities and applications are discussed. Experiments to test optical communication systems are being planned for the Space Shuttle and the Mars Rover Sample Return misson. Future missions which will probably use the system include Cassini and the Mariner Mark II.

Rayman, Marc D.↗