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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

Fiber optics for the future - wavelength division multiplexing

Optical wavelength division multiplexing (WDM) systems, with signals transmitted on different wavelengths through a single fiber, can have increased information capacity and fault isolation properties over single wavelength optical systems. This paper describes a typical WDM system. The applicability of future standards to such a system are discussed. Also, a state-of-the-art survey of optical multimode components which could be used to implement the system are made. The components to be surveyed are sources, multiplexers, and detectors. Emphasis is given to the demultiplexer techniques which are the major developmental components in the WDM system.

Spencer, J. L.↗
NASA NTRS1984

Properties of the Upper Tropospheres of Uranus and Neptune Derived from Observations at Visible to Near-Infrared Wavelengths

Photons at wavelengths between 0.3 and 4.5 microns penetrate the atmospheres of Uranus and Neptune to pressures between about 0.01 bar and 10 bars. This pressure range brackets the radiative convective boundary in both atmospheres and is therefore designated upper troposphere. Physical processes which govern the transfer of radiation in Uranus's and Neptune's atmospheres at these wavelengths include Rayleigh/Raman scattering by hydrogen, scattering and broadband absorption by suspended aerosol particles and absorption in discrete bands and lines by methane and hydrogen. Consequently, tropospheric properties constrained by observations at these wavelengths include optical properties and distribution of aerosol particles, methane/hydrogen ratio, and ortho/para hydrogen ratio. Recent observations of Uranus and Neptune in this spectral range, are reviewed and compared with predictions based on models of the atmospheric structures. Significant results for Uranus include the presence of an opaque lower boundary to the visible atmosphere very near the level corresponding to 2 bars pressure, and consequently a methane/hydrogen ratio no less than 3 percent.

Bergstralh, J. T.↗
NASA NTRS1985

Extension of long wavelength response by modulation doping in extrinsic germanium infrared detectors

A new concept for infrared detectors based on multilayer epitaxy and modulation doping has been investigated. This permits a high doping concentration and lower excitation energy in the photodetecting layer as is necessary for longer wavelength response, without incurring the detrimental effects of increased dark current and noise as would be the case with conventional detector designs. Germanium photodetectors using conventional materials and designs have a long wavelength cutoff in the infrared at 138 microns, which can only be extended through the inconvenient application of mechanical stress or magnetic fields. As a result of this approach which was arrived at from theoretical considerations and subsequently demonstrated experimentally, the long wavelength cutoff for germanium extrinsic detectors was extended beyond 200 microns, as determined by direct infrared optical measurements.

Hadek, V.↗
NASA NTRS1985

Intermediate-wavelength magnetic anomaly field of the North Pacific and posible source distributions

A technique that eliminates external field sources and the effects of strike aliasing was used to extract from marine survey data the intermediate wavelength magnetic anomaly field for (B) in the North Pacific. A strong correlation exists between this field and the Magsat field although a directional sensitivity in the Magsat field can be detected. The intermediate wavelength field is correlated to tectonic features. Island arcs appear as positive anomalies of induced origin likely due to variations in crustal thickness. Seamount chains and oceanic plateaus also are manifested by strong anomalies. The primary contribution to many of these anomalies appears to be due to a remanent magnetization. The source parameters for the remainder of these features are presently unidentified ambiguous. Results indicate that the sea surface field is a valuable source of information for secular variation analysis and the resolution of intermediate wavelength source parameters.

Labrecque, J. L.↗
NASA NTRS1984

Observations of solar radio bursts from meter to kilometer wavelengths

New results obtained with the Clark Lake multifrequency radioheliograph at meter-decameter wavelengths and from satellite multifrequency directive observations at hectometer and kilometer wavelengths are reviewed. Evidence is presented that type III electrons propagate in dense coronal streamers and that frequently observed microbursts (presumably type III) at meter-decameter wavelengths are due to plasma radiation. Observations of hectometer and kilometer type III radio storms which reveal information about active region structures, the interplanetary magnetic field configuration, and solar wind acceleration are discussed. Kilometer type II bursts and interactions between type III electrons and interplanetary shocks are examined, and some new results on shock-associated events are presented.

Kundu, M. R.↗
NASA NTRS1985

What are the best radar wavelengths, incidence angles and polarizations for geologic applications? A statistical approach

Linear discriminant analysis of multifrequency and multipolarization radar scatterometer data of lava flows and sedimentary rocks indicates that the lava flows can be separated by age and the sedimentary rocks can be discriminated from one another. The optimum wavelengths, polarizations and incidence angles among those available for these problems was determined by the discriminant analysis program. For separation of the lava flows, shorter wavelengths, smaller incidence angles and horizontal polarization are best. A SIR-C radar configuration could provide nearly complete discrimination of these lava flows. Conversely, the longer wavelengths, larger incidence angles and vertical polarization was preferred for sedimentary rocks, perhaps due to the slight vegetation cover. Satisfactory classification of sedimentary rocks requires more radar data than for the lavas. These results are potentially useful both for radar system configuration and for geological applications. The method developed here may provide a rationale for user specification of imaging system parameters.

Blom, R.↗
NASA NTRS1986

The wavelength dependence of phase coefficients

It is pointed out that spectral reflectance curves depend largely upon the optical constants of the surface materials. In the present study, the results of laboratory measurements are employed to investigate the wavelength dependence of disk-integrated phase coefficients of some materials, taking into account a basalt, the ordinary chondrite Bruderheim, the carbonaceous chondrite Allende, and sulfur. These materials have been selected as appropriate analogs to the surfaces of the moon, S- and C-class asteroids, and the Jovian satellite Io, respectively. It is shown that the observed wavelength dependence of phase coefficients can be predicted (at least qualitatively) once the spectrum of an object is known. Attention is given to phase variation of disk-integrated spectral reflectance curves, the wavelength dependence of phase coefficients, a comparison with observations, and the effects of surface roughness on phase coefficients.

Gradie, J.↗
NASA NTRS1986

Monitoring observations of selected SiO maser sources at 7 mm wavelength - 1977-1979

High-spectral-resolution SiO maser observations at 7 mm wavelength were made for 13 circumstellar objects and Ori A over a 34 month interval from January 1977 to November 1979. These data precede and adjoin the SiO monitoring data taken at 3 and 7 mm wavelengths by Lane (1982) and at 3 mm wavelengths by Nyman and Olofsson (1985) and thus provide an important link in the record of the temporal behavior of SiO masers over a period of several years. The time-averaged center of the weak SiO maser emission pedestal gives the stellar radial velocity. This reinforces the idea that the SiO maser emission pedestal is representative of the true structure of a stellar SiO maser, while the more visible and stronger, narrow SiO emission spikes represent chance coherences along the velocity range of the pedestal. Similarities between the SiO maser velocity patterns of VY CMa and Ori A may suggest similarities in their evolutionary stages.

Snyder, L. E.↗
NASA NTRS1986

VLA observations of Jupiter at 1.3 - 20 cm wavelengths

In order to study the vertical distribution of ammonia as a function of Jovian latitude, high resolution images were obtained with the VLA at 1.3, 2, 6 and 20 cm wavelengths. Although the interpretation of the data is quite complicated due to Jupiter's synchrotron radiation, which in fact is the dominant source of radiation at 29 cm, the belt-zone structure is clearly present at 2 and 6 cm wavelengths. At 1.3 cm near the center of the ammonia band, the structure is less pronounced, and at 20 cm it is absent. The data is currently being fitted with model atmosphere calculations. Since one probes in and through the visible cloud layers at these wavelengths (temperatures of 135 to 400 K), and the opacity is likely all provided by ammonia gas, a detailed vertical distribution of this gas can be obtained as a function of Jovian latitude. This ought to give insight in the formation processes of the white cloud layers in the zones and their absence above the belts.

Depater, Imke↗
NASA NTRS1986

Jupiter's zone-belt structure at radio wavelengths. I - Observations

VLA maps of Jupiter's thermal emission at 1.3, 2, 6, and 20 cm wavelengths are presented. The resolution at the four wavelengths is 0.218, 0.075, 0.108, and 0.144 Jovian radii, respectively. Bright bands across the disk are clearly visible at 2 and 6 cm. These bands agree with the dark belts seen at optical and the bright bands seen at infrared wavelengths.

De Pater, I.↗
NASA NTRS1987

Mercury cadmium telluride short- and medium-wavelength infrared staring focal plane arrays

Short and medium IR wavelength 64 x 64 hybrid focal plane arrays (FPAs) have been developed using sapphire-grown HgCdTe. The short wavelength arrays were developed for a prototype airborne imaging spectrometer, while those of medium wavelength are suitable for tactical missile seekers and strategic surveillance systems. Attention is presently given to results obtained for these FPAs' current-voltage characteristics, as well as for their characterization at different temperatures. The detector arrays were also mated to a multiplexer and characterized under different operating conditions. The unit cell size used is 52 x 52 microns.

Vural, Kadri↗
NASA NTRS1987

The generation of Tollmien-Schlichting waves by long wavelength free stream disturbances

The paper is primarily concerned with explaining how very long wavelength free stream disturbances are able to generate very short wavelength Tollmien-Schlichting waves in laminar boundary layers. Consideration is given to the case where the disturbances are of small amplitude and have harmonic time dependence and where the Mach number is effectively zero. It is shown that the free stream wavelength reduction occurs as a result of nonparallel flow effects which can arise from: (1) the slow viscous growth of the boundary layer, and (2) small but abrupt changes in surface geometry that produce only very weak static pressure variations. Analyses of these two mechanisms are carried out by linearizing the unsteady motion about an appropriate steady flow and asymptotically expanding the result in inverse powers of an appropriate Reynolds number. The analyses are compared with each other and with available experimental data, and they are used to explain the physics of the two mechanisms.

Goldstein, M. E.↗
NASA NTRS1987

Simultaneous 2 and 6 centimeter wavelength observations of a solar flare using the VLA

VLA observations of a solar active region and a flare are discussed. The event was observed at wavelengths of 2 and 6 cm simultaneously. Radio maps prior to the flare delineate the most important magnetic structures in the region. Interaction between these structures apparently led to preheating of plasma above the active region some 30 minutes prior to the flare. The 2 and 6 cm flare positions were coincident, and the time profiles of the burst at the two wavelengths were almost identical, implying that the same population of electrons was responsible for emission at the two wavelengths. Emission was probably nonthermal gyrosynchrotron radiation, and the physical conditions in the burst source are derived using this assumption.

Kundu, M. R.↗
NASA NTRS1987

On the simple models for the interpretation of centimeter-wavelength radio observations of asteroids

The predictions of the two-layer models used to interpret radio spectra of the asteroids have been examined. It is clear that one must treat the models with caution as the physics is highly simplifed. Although the predictions are in accord with physical expectations, careful attention to the circumstances of the observations is essential. The lack of short-centimeter wavelength measurements of dielectric properties is especially vexing. Some simple improvements to the models based on studies of the lunar radio emission by Keihm and using radar measurements of asteroid surface characteristics by Ostro and colleagues are suggested. Even so, the calculation of a realistic continuum-emission spectrum for asteroidal bodies from 20 cm wavelength to 20 microns wavelength remains a formidable task.

Webster, William J., Jr.↗
NASA NTRS1988

Analysis of algorithms for the retrieval of rain-rate profiles from a spaceborne dual-wavelength radar

The ability to retrieve rain-rate profiles from a dual-wavelength spaceborne radar system operating at 13.6 and 35 GHz is analyzed. The fundamental problem of extracting either the attenuation and/or the reflectivity from the backscatter echo, which contains both contributions, is addressed. Three algorithms, the backscatter, the attenuation coefficient, and the dual-wavelength methods, are examined. These algorithms are tested using four rain-rate profiles derived from radar measurements. In particular, measured (true) values are compared with calculated (retrieved) rain rates applying the algorithms with superimposed uncertainties assuming a suggested spaceborne dual-wavelength radar system. Error values of rain rates are determined where these values reflect failure of the assumptions utilized in the derivation of the algorithms, rain backscatter noise, and instrument noise. It is concluded that no single technique gives rise to a panacea in the making of accurate rain measurements and that difficulties exist with each method.

Goldhirsh, Julius↗
NASA NTRS1988

Fiber optic wavelength division multiplexing: Principles and applications in telecommunications and spectroscopy

Design and fabrication tradeoffs of wavelength division multiplexers are discussed and performance parameters are given. The same multiplexer construction based on prism gratings has been used in spectroscopic applications, in the wavelength region from 450 to 1600 nm. For shorter wavelengths down to 200 nm, a similar instrument based on longer fibers (500 to 1000 micrometer) has been constructed and tested with both a fiber array and a photodiode detector array at the output.

Erdmann, R. K.↗
NASA NTRS1988

On the wavelengths of (n + 1/2)f(ce) gyroharmonic emissions in the earth's magnetosphere

Using data from the University of Minnesota Plasma Wave Experiment aboard the IMP 6 (Explorer 43) satellite, the wavelengths of certain electrostatic waves in earth's magnetosphere are measured. The antenna response to electrostatic waves having several k vector distributions in space is calculated using the fact that the X and Y dipole antennas on IMP 6 are of unequal length. These results are used to determine the wavelengths of (n+1/2)f(ce) gyroharmonic emissions observed just beyond the plasmapause. These wavelengths are then used in conjunction with theoretical models to determine the energy of the electrons driving the waves, and a range of energies between several tens to several hundreds of electron volts is found. This technique is also applied to Langmuir waves observed upstream of earth's bow shock, and the results are found to be in good agreement with theory.

Filbert, Paul C.↗
NASA NTRS1989

On the wavelength dependence of apparent emissivity of asteroid microwave emissions - Ceres and Vesta

Observations of Ceres and Vesta demonstrate the need for modifying the physical parameters of the standard model procedure in treating single-wavelength observations. It is suggested that simultaneous IR and cm-wavelength data should be used in order to determine such properties as the surface dielectric characteristics, layer depth, and radio emissivity. If using the standard model, an emissivity of 0.8 should be adopted for wavelengths of longer than 1 cm.

Webster, William J., Jr.↗