Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “Coherence”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 883 records · Page 49
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

Dynamics of coherent structures in a plane mixing layer

An incompressible, time developing 3-D mixing layer with idealized initial conditions was simulated numerically. Consistent with the suggestions from experimental measurements, the braid region between the dominant spanwise vortices or rolls develops longitudinal vortices or ribs, which are aligned upstream and downstream of a roll and produce spanwise distortion of the rolls. The process by which this distortion occurs is explained by studying a variety of quantities of dynamic importance (e.g., production of enstrophy, vortex stretching). Other quantities of interest (dissipation, helicity density) are also computed and discussed. The currently available simulation only allows the study of the early evolution (before pairing) of the mixing layer. New simulations in progress will relieve this restriction.

Hussain, Fazle↗
NASA NTRS1989

Monolithic narrow-linewidth InGaAsP semiconductor laser for coherent optical communications

A design for a monolithic narrow-linewidth InGaAsP diode laser has been developed using a multiple-quantum-well (MQW) extended-passive-cavity distributed-Bragg-reflector (DBR) laser design. Theoretical results indicate that this structure has the potential for a linewidth of 100 kHz or less. To realize this device, a number of the fabrication techniques required to integrate low-loss passive waveguides with active regions have been developed using a DBR laser structure. In addition, the MOCVD growth of InGaAs MQW laser structures has been developed, and threshold current densities as low as 1.6 kA/sq cm have been obtained from broad-stripe InGaAs/InGaAsP separate-confinement-heterostructure MQW lasers.

Palfrey, S. L.↗
NASA NTRS1988

Coherent HF radar backscatter from small-scale irregularities in the dusk sector of the subauroral ionosphere

This paper describes the characteristics of backscatter from decameter-scale ionospheric plasma density irregualarities, observed with an impressive regularity by the Goose Bay (Labrador) high-frequency (HF) radar in the dusk sector of the winter ionosphere, and discusses the relation of the scatter to the midlatitude trough. It is shown that this dusk scatter can be readily distinguished from other types of late afternoon/early evening scatter by the extreme equatorward position of its source region and by the low values of its associated radar Doppler velocities (not above 200 m/s) and spectral widths (not more than 200 m/s). A comparison of the radar observations with nearly simultaneous particle precipitation data obtained with the DMSP F6 satellite demonstrated that the source region of the backscatter lies within the subauroral ionosphere. It is shown that the characteristics of dusk scatter are compatible with the Spiro et al. (1978) model of the electrodynamics of the midlatitude trough.

Ruohoniemi, J. M.↗
NASA NTRS1988

Coherent mesoscale convection patterns during northward interplanetary magnetic field

Coincident DE 2 satellite plasma-drift and particle measurements and all-sky imaging photometer data were used to construct two-dimensional convection patterns in a study of polar ionospheric convection in the presence of subvisual-intensity soft-particle excited 6300-A sun-aligned polar cap arcs. Results show a highly anisotropic temporally stable convection with greater order than has previously been suspected.

Carlson, H. C.↗
NASA NTRS1989

Impact of random pointing and tracking errors on the design of coherent and incoherent optical intersatellite communication links

Given the rms transmitter pointing error and the desired probability of bit error (PBE), it can be shown that an optimal transmitter antenna gain exists which minimizes the required transmitter power. Given the rms local oscillator tracking error, an optimum receiver antenna gain can be found which optimizes the receiver performance. The impact of pointing and tracking errors on the design of direct-detection pulse-position modulation (PPM) and heterodyne noncoherent frequency-shift keying (NCFSK) systems are then analyzed in terms of constraints on the antenna size and the power penalty incurred. It is shown that in the limit of large spatial tracking errors, the advantage in receiver sensitivity for the heterodyne system is quickly offset by the smaller antenna gain and the higher power penalty due to tracking errors. In contrast, for systems with small spatial tracking errors, the heterodyne system is superior because of the higher receiver sensitivity.

Chen, Chien-Chung↗
NASA NTRS1989

Coherent tunable far infrared radiation

Tunable, CW, FIR radiation has been generated by nonlinear mixing of radiation from two CO2 lasers in a metal-insulator-metal (MIM) diode. The FIR difference-frequency power was radiated from the MIM diode antenna to a calibrated InSb bolometer. FIR power of 200 nW was generated by 250 mW from each of the CO2 lasers. Using the combination of lines from a waveguide CO2 laser, with its larger tuning range, with lines from CO2, N2O, and CO2-isotope lasers promises complete coverage of the entire FIR band with stepwise-tunable CW radiation.

Jennings, D. A.↗
NASA NTRS1989

Stochastic estimation of coherent structures in turbulent boundary layers

The stochastic estimation provides a means of estimating conditional averages from unconditional statistics, namely, the two-point correlation tensor. This technique is applied to three-dimensional data obtained in a fully turbulent boundary layer. The results of linear and nonlinear estimates are compared to the corresponding conditional averages and the agreement is found to be excellent, with little improvement for nonlinear estimates. Extensions of the technique to space-time estimates of one- and two-point conditional averages are also presented, the latter providing a means to examine structures associated with a given length scale.

Guezennec, Y. G.↗
NASA NTRS1989

Highly coherent long cavity GaAs/AlGaAs single-quantum-well lasers

The spectral properties of ridge-waveguide graded-index separate-confinement heterostructure single-quantum-well (SQW) GaAs/AlGaAs lasers are measured. Long-cavity lasers (800 microns) exhibit remarkably pure CW single-longitudinal-mode spectra in spite of the short cavity mode spacing. At output power 5 mW, the sidemode suppression exceeds 24 dB and the linewidth is 1.5 MHz. The linewidth-power product (LPP) is 6.4 MHz mW. Measurements of LPP as a function of cavity length L gives a 1/L-squared dependence, in agreement with theory. The results are used to deduce the linewidth enhancement factor alpha at the gain peak wavelength and its dependence on the excitation level. The sublinear gain-carrier density relation in the SQW results in an increase in alpha with increasing carrier density (decreasing cavity length). A decrease in alpha was observed for lasers operating at the second quantized state due to a recovery of the differential gain.

Larsson, Anders↗
NASA NTRS1989

Recoverable distributed shared virtual memory - Memory coherence and storage structures

This paper examines the problem of implementing rollback recovery in multicomputer distributed shared virtual memory environments, in which the shared memory is implemented in software and exists only virtually. A user-transparent checkpointing recovery scheme and new twin-page disk storage management are presented to implement a recoverable distributed shared virtual memory. The checkpointing scheme is integrated with the shared virtual memory management. The twin-page disk approach allows incremental checkpointing without an explicit undo at the time of recovery. A single consistent checkpoint state is maintained on stable disk storage. The recoverable distributed shared virtual memory allows the system to restart computation from a previous checkpoint due to a processor failure without a global restart.

Wu, Kun-Lung↗
NASA NTRS1990

Linear canonical transformations of coherent and squeezed states in the Wigner phase space. III - Two-mode states

It is shown that the basic symmetry of two-mode squeezed states is governed by the group SP(4) in the Wigner phase space which is locally isomorphic to the (3 + 2)-dimensional Lorentz group. This symmetry, in the Schroedinger picture, appears as Dirac's two-oscillator representation of O(3,2). It is shown that the SU(2) and SU(1,1) interferometers exhibit the symmetry of this higher-dimensional Lorentz group. The mathematics of two-mode squeezed states is shown to be applicable to other branches of physics including thermally excited states in statistical mechanics and relativistic extended hadrons in the quark model.

Han, D.↗
NASA NTRS1990

Perturbations to tropospheric oxidants, 1985-2035. I - Calculations of ozone and OH in chemically coherent regions

A one-dimensional model based on the sensitivity coefficients of Thompson (1990) is used to calculate regional changes in tropospheric O3 and OH due to specified global or regional changes in CH4 and CO emissions over the period 1985-2035. The methods applied are explained, and the results are presented in extensive tables and graphs and characterized in detail. A 10-15-percent increase in global tropospheric O3 and a 10-15-percent decrease in OH are predicted for both (1) global scenarios with continuing increases in CH4 and CO and (2) regional scenarios where emissions are limited in some regions but not in others; significant regional variation in O3 is predicted for (2). If depletion of O3 in the stratosphere and global warming are introduced into (1) or (2), however, they nearly cancel out the tropospheric O3 increases and OH decreases.

Thompson, Anne M.↗
NASA NTRS1989

A coherent digital demodulator for multiple signal formats and widely varying data rates

The Tracking and Data Relay Satellite System (TDRSS) uses four ground station demodulators for K-band signals with data rates from 1 kb/s to 300 Mb/s. The author discusses the feasibility of replacing these demodulators with a single digital demodulator that may be reconfigured by altering stored parameters to accommodate all signal formats and data rates. This implementation will reduce total ground station cost and facilitate automation of ground station operation. Analysis of system performance concentrates on the carrier tracking loop. Analytic and simulation results relate system performance to parameter values and signal format as data rate and power vary independently on the In-phase and quadrature channels. It is demonstrated that a single digital demodulator can support TDRSS-compatible signals at data rates conservatively extending from 1K symbols/s to 10M symbols/s, using off-the-shelf hardware with 6 or more bits of accuracy.

Mcguffin, Bruce F.↗
NASA NTRS1991

Solid-state lasers for coherent communication and remote sensing

Work in the stabilization of monolithic Nd:YAG lasers and the application of these lasers to nonlinear optical frequency conversion is discussed. The intrinsic stability of semiconductor diode laser pumped solid state lasers has facilitated a number of demonstration in external resonant cavity harmonic generation and stable optical parametric oscillation. Relative laser frequency stabilization of 0.3 Hz was achieved, and absolute stability of a few hundred hertz is anticipated. The challenge is now to reproduce this frequency stability in the output of tunable nonlinear optical devices. Theoretical and experimental work toward this goal are continuing.

Byer, Robert L.↗
NASA NTRS1990

Coherent structure induced pressure fluctuations in an elliptic jet

The fluctuating pressure near an M = 0.5 elliptic jet was examined. Layered structures of the pressure field were found. Just outside of the jet edge, the pressure is mainly produced by the passing vortices. A short distance away from this layer, the pressure fluctuations originating from the end of the potential core become dominant and are the source of the far field noise.

Schreck, S.↗
NASA NTRS1991

Far-field pattern of a coherently combined beam from large-scale laser diode arrays

The far-field pattern of a large-scale amplifier array (LSAA) consisting of a large number (2000) of diode laser amplifiers is numerically simulated, and the power collection efficiencies are determined. Random distributions of phase mismatches, misorientations, and element failures in the LSAA system are considered. Phase mismatches and misorientations of the element amplifiers are found to be the most critical parameters of those affecting the power-collection efficiency. Errors of 0.2 wavelength and 25 percent for phase and diffraction angle, respectively, cause a 10 percent reduction in power-collection efficiency. The results are used to evaluate the concept of space-laser power transmission. It is found that an overall transmission efficiency of 80 percent could be realized with a 5-m-diam. receiver at a distance of 10,000 km when an LSAA transmitter 6 m in diam. is aimed with state-of-the-art pointing accuracy.

Kwon, Jin H.↗