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At least 595 records · Page 33
All sourcesNASA NTRSDOE OSTI
NASA NTRS2002

Second order filter response with series coupled silica microresonators

We have demonstrated an approach for fabricating a photonic filter with second order response function. The filter consists of two germania-doped silica microtoroidal or microspherical resonators cascaded in series. We use UV irradiation to tune the mode of one microcavity to bring it close to the mode of the second microcavity. This approach produces a filter function with much sharper rolloff than can be obtained with the individual microresonators.

processing lithium niobate↗
NASA NTRS2002

Coupling Finite Element and Meshless Local Petrov-Galerkin Methods for Two-Dimensional Potential Problems

A coupled finite element (FE) method and meshless local Petrov-Galerkin (MLPG) method for analyzing two-dimensional potential problems is presented in this paper. The analysis domain is subdivided into two regions, a finite element (FE) region and a meshless (MM) region. A single weighted residual form is written for the entire domain. Independent trial and test functions are assumed in the FE and MM regions. A transition region is created between the two regions. The transition region blends the trial and test functions of the FE and MM regions. The trial function blending is achieved using a technique similar to the 'Coons patch' method that is widely used in computer-aided geometric design. The test function blending is achieved by using either FE or MM test functions on the nodes in the transition element. The technique was evaluated by applying the coupled method to two potential problems governed by the Poisson equation. The coupled method passed all the patch test problems and gave accurate solutions for the problems studied.

Chen, T.↗
NASA NTRS1983

Coupling of radiation into thin film modes by means of localized plasma resonances

The interaction between the surface plasmon mode that propagates at a metal dielectric interface and the localized plasma resonances (LPR) is investigated experimentally in Ag-island films. A stair-stepped sample geometry comprising a glass substrate, a continuous 50-nm Ag film, an LiF spacer film of thickness d = 5-60 nm, and an Ag-island film of mass thickness 3 nm is used in near-normal-reflectivity and plasmon-propagation-constant (k) determinations. The results are presented graphically and discussed. The overall shape of the reflectivity curves is found to be characteristic of Ag films, but with a dip at about 400 nm (corresponding to the absorption resonance of the island film) which is most pronounced with d = 25 nm. It is inferred that the island resonances are strongly coupled to a continuous-film dissipative mechanism at this d value. This inference is supported by the fact that the variation in k, correctd for LiF effects and plotted as a function of d, is greatest at around d = 25 nm. The implications of this finding for broad-band coupling into a thin-film mode, LPR enhancement of waveguide nonlinear effects, and new surface-enhanced-Raman-scattering geometries are indicated.

Holland, W. R.↗
NASA NTRS1985

Oscillator strengths and collision strengths for neutral sulfur

Configuration-interaction target wave functions are used in the present calculation of collision strengths, for electron impact excitation of neutral sulfur from the group 3p4 3P state to excited states 3p3 4s 3S0, 3p3(4S0)3d 3D0, and 3p3 4s 3P0, in a close coupling approximation for the energy range up to 1,000,000 K. Configuration-interaction target wave functions are used in the calculation of collision strengths, and oscillator strengths for various triplet transitions are reported together with transitions between 3p4 1D and 3p4 1S and other singlet-excited states.

Ho, Y. K.↗
NASA NTRS1969

Radiative decay of polyatomic molecules.

Radiative decay of polyatomic molecules, applying Green function form for transition probability to decay of manifold of closely spaced coupled levels

Freed, K. F.↗
NASA NTRS1979

Improvements in surface singularity analysis and design methods

The coupling of the combined source vortex distribution of Green's potential flow function with contemporary numerical techniques is shown to provide accurate, efficient, and stable solutions to subsonic inviscid analysis and design problems for multi-element airfoils. The analysis problem is solved by direct calculation of the surface singularity distribution required to satisfy the flow tangency boundary condition. The design or inverse problem is solved by an iteration process. In this process, the geometry and the associated pressure distribution are iterated until the pressure distribution most nearly corresponding to the prescribed design distribution is obtained. Typically, five iteration cycles are required for convergence. A description of the analysis and design method is presented, along with supporting examples.

Bristow, D. R.↗
NASA NTRS1980

Composition of the nighttime ionospheric F 1 region near the magnetic equator

The effects of vertical E x B transport on NO(+), O2(+) and O(+) densities in the nighttime equatorial ionospheric F 1 region are investigated. Ion densities are calculated as functions of altitude, latitude and local time by the numerical solution of coupled, time-dependent ion continuity equations, taking into account production, loss by charge exchange and dissociative recombination and transport by diffusion and E x B drift. The results of the calculations are compared with measurements of NO(+), O2(+) and O(+) ion densities obtained at low altitudes by a mass spectrometer on board the Atmospheric Explorer C satellite, and are found to be consistent with the observations, suggesting that in the equatorial region, vertical transport by E x B drift is primarily responsible for producing the observed NO(+), O2(+) and O(+) density profiles. In addition, the reaction of O2(+) with N(4S) is found to be an important sink for O2(+) and a source of NO(+) ions. Implications of the observed and calculated near constancy of electron and ion densities with altitude when NO(+) is the dominant ion on the growth of large-scale irregularities are also considered

Anderson, D. N.↗
NASA NTRS1983

Aluminum/ammonia heat pipe gas generation and long term system impact for the Space Telescope's Wide Field Planetary Camera

In the Space Telescope's Wide Field Planetary Camera (WFPC) project, eight heat pipes (HPs) are used to remove heat from the camera's inner electronic sensors to the spacecraft's outer, cold radiator surface. For proper device functioning and maximization of the signal-to-noise ratios, the Charge Coupled Devices (CCD's) must be maintained at -95 C or lower. Thermoelectric coolers (TEC's) cool the CCD's, and heat pipes deliver each TEC's nominal six to eight watts of heat to the space radiator, which reaches an equilibrium temperature between -15 C to -70 C. An initial problem was related to the difficulty to produce gas-free aluminum/ammonia heat pipes. An investigation was, therefore, conducted to determine the cause of the gas generation and the impact of this gas on CCD cooling. In order to study the effect of gas slugs in the WFPC system, a separate HP was made. Attention is given to fabrication, testing, and heat pipe gas generation chemistry studies.

Jones, J. A.↗
NASA NTRS1985

Three-dimensional vibration analysis of a uniform beam with offset inertial masses at the ends

Analysis of a flexible beam with displaced end-located inertial masses is presented. The resulting three-dimensional mode shape is shown to consist of two one-plane bending modes and one torsional mode. These three components of the mode shapes are shown to be linear combinations of trigonometric and hyperbolic sine and cosine functions. Boundary conditions are derived to obtain nonlinear algebraic equations through kinematic coupling of the general solutions of the three governing partial differential equations. A method of solution which takes these boundary conditions into account is also presented. A computer program has been written to obtain unique solutions to the resulting nonlinear algebraic equations. This program, which calculates natural frequencies and three-dimensional mode shapes for any number of modes, is presented and discussed.

Robertson, D. K.↗
NASA NTRS1986

Theoretical studies of Resonance Enhance Stimulated Raman Scattering (RESRS) of frequency doubled Alexandrite laser wavelengths in cesium vapor

It is well known that the presence of a real atomic level which is nearly resonant with the pump field can greatly enhance the Raman emission cross section. In order to accurately calculate the Raman gain in systems where resonance enhancement plays a dominant role, expressions for the pump and signal susceptibilities must be derived. These expressions should be valid for arbitrary field strengths in order to allow for pump and signal saturation. In addition, the theory should allow for arbitrary longitudinal and transverse relaxation rates. This latter point is extremely vital for three level atomic systems such as the alkali earth metals since they do not have population reservoirs and can have widely varying spontaneous lifetimes on the three pertinent transitions. Moreover, the dephasing rates are strong functions of electron states and are therefore also different for the three coupled pairs of levels. These considerations are not as important when molecular systems are concerned since the large reservoir of rotational states serve to produce essentially equal longitudinal recovery rates for the population of the three levels. The three level system with three arbitrary longitudinal and transverse relaxation rates was solved. There is no need for setting either pair of rates equal and the expressions are valid for arbitrarily strong fields.

Lawandy, N. M.↗
NASA NTRS1987

The effects of stimulated star formation on the evolution of the galaxy. III - The chemical evolution of nonlinear systems

The evolution of models for star formation in galaxies with disk and halo components is discussed. Two phases for the halo (gas and stars) and three for the disk (including clouds) are used in these calculations. The star-formation history is followed using nonlinear phase-coupling models which completely determine the populations of the phases as a function of time. It is shown that for a wide range of parameters, including the effects of both spontaneous and stimulated star formation and mass exchange between the spatial components of the system, the observed chemical history of the galaxy can easily be obtained. The most sensitive parameter in the detailed metallicity and star-formation history for the system is the rate of return of gas to the diffuse phase upon stellar death.

Shore, Steven N.↗
NASA NTRS1989

Metal Film Increases CCD Quantum Efficiency

Thin layer of platinum or other high-work-function metal applied to back side of rear-illuminated charge-coupled device (CCD) achieves quantum efficiency (QE)-pinned state, an ideal condition allowing sensor to achieve 100-percent internal charge-collection efficiency within its photosensitive volume. Metal layer, called flash gate, easily applied by tungsten vacuum deposition during last step of sensor fabrication.

Janesick, James R.↗
NASA NTRS1992

Ignition and combustion characteristics of metallized propellants

During this reporting period, theoretical work on the secondary atomization process was continued and the experimental apparatus was improved. A one-dimensional model of a rocket combustor, incorporating multiple droplet size classes, slurry combustion, secondary atomization, radiation heat transfer, and two-phase slip between slurry droplets and the gas flow was derived and a computer code was written to implement this model. The STANJAN chemical equilibrium solver was coupled with this code to yield gas temperature, density, and composition as functions of axial location. Preliminary results indicate that the model is performing correctly, given current model assumptions. Radiation heat transfer in the combustion chamber is treated as an optically-thick participating media problem requiring a solution of the radiative transfer equation. A cylindrical P sub 1 approximation was employed to yield an analytical expression for chamber-wall heat flux at each axial location. The code exercised to determine the effects of secondary atomization intensity, defined as the number of secondary drops produced per initial drop, on chamber burnout distance and final Al2O3 agglomerate diameter. These results indicate that only weak secondary atomization is required to significantly reduce these two parameters. Stronger atomization intensities were found to yield decreasing marginal benefits. The experimental apparatus was improved to reduce building vibration effects on the optical system alignment. This was accomplished by mounting the burner and the transmitting/receiving optics on a single frame supported by vibration-isolation legs. Calibration and shakedown tests indicate that vibration problems were eliminated and that the system is performing correctly.

Mueller, D. C.↗
NASA NTRS1995

Cooling Panel Optimization for the Active Cooling System of a Hypersonic Aircraft

Optimization of cooling panels for an active cooling system of a hypersonic aircraft is explored. The flow passages are of rectangular cross section with one wall heated. An analytical fin-type model for incompressible flow in smooth-wall rectangular ducts with coupled wall conduction is proposed. Based on this model, the a flow rate of coolant to each design minimum mass flow rate or coolant for a single cooling panel is obtained by satisfying hydrodynamic, thermal, and Mach number constraints. Also, the sensitivity of the optimal mass flow rate of coolant to each design variable is investigated. In addition, numerical solutions for constant property flow in rectangular ducts, with one side rib-roughened and coupled wall conduction, are obtained using a k-epsilon and wall function turbulence model, these results are compared with predictions of the analytical model.

Youn, B.↗
NASA NTRS1999

Coherent Pump-Probe Interactions and Terahertz Intersubband Gain in Semiconductor Quantum Wells

In recent years there has been considerable interest in intersubband-transition-based infrared semiconductor quantum well (QW) lasers because of their potential applications. In the mid-infrared range, both electrically-injected quantum cascade lasers [1] and optically-pumped multiple QW lasers [2] have been experimentally realized. In these studies, optical gain is due to population inversion between the lasing subbands. It was also proposed that stimulated Raman scattering in QW systems can produce net infrared optical gain [3j. In such a nonlinear optical scheme, the appearance of optical gain that may lead to intersubband Raman lasers does not rely on the population inversion. Since, in tile resonant Raman process (Raman gain is the largest in this case), the pump field induces population redistribution among subbands in the QW s ystem, it seems that a realistic estimate of the optical gain has to include this effect. Perturbative calculations used in the previous work [3] may overestimate the Raman gain. In this paper we present a nonperturbative calculation of terahertz gain of optically-pumped semiconductor step quantum wells. Limiting optical transitions within the conduction band of QW, we solve the pump-field-induced nonequilibrium distribution function for each subband of the QW system from a set of coupled rate equations. Both intrasubband and intersubband relaxation processes in the quantum well system are included. Taking into account the coherent interactions between pump and THz (signal) waves, we we derive the susceptibility of the QW system for the THz field. For a GaAs/AlGaAs step QW, we calculate the Thz gain spectrum for different pump frequencies and intensities. Under moderately strong pumping (approximately 0.3 MW/sq cm), a significant THz gain (approximately 300/m) is predicted. It is also shown that the coherent wave interactions (resonant stimulated Raman processes) contribute significantly to the THz gain.

Liu, Ansheng↗
NASA NTRS2002

Core Research Program, Year 5

Dramatic losses of bone mineral density (BMD) and muscle strength are two of the best documented changes observed in humans after prolonged exposure to microgravity. Recovery of muscle upon return to a 1-G environment is well studied, however, far less is known about the rate and completeness of BMD recovery to pre-flight values. Using the mature tail-suspended adult rat model, this proposal will focus on the temporal course of recovery in tibial bone following a 28-d period of skeletal unloading. Through the study of bone density and muscle strength in the same animal, time-points during recovery from simulated microgravity will be identified when bone is at an elevated risk for fracture. These will occur due to the rapid recovery of muscle strength coupled with a slower recovery of bone, producing a significant mismatch in functional strength of these two tissues. Once the time-point of maximal mismatch is defined, various mechanical and pharmacological interventions will be tested at and around this time-point in attempt to minimize the functional difference of bone and muscle. The outcomes of this research will have high relevance for optimizing the rehabilitation of astronauts upon return to Earth, as well as upon landing on the Martian surface before assuming arduous physical tasks. Further. it will impact significantly on rehabilitation issues common to patients experiencing long periods of limb immobilization or bed rest.

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