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At least 559 records · Page 31

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NASA NTRS1986

Lidar telescope overlap function and effects of misalignment for unstable resonator transmitter and coherent receiver

Effects of lidar telescope overlap function modeling on the accuracy of aerosol backscatter measurements are discussed. The treatment pertains primarily to a lidar arrangement which combines high sensitivity and a large degree of geometrical signal compression but places stringent requirements on the accuracy of the overlap function modeling - coherent IR lidar with noncoaxial (side-by-side) transmit and receive telescopes. A model is presented which includes an unstable resonator transmitter and accommodates both coaxial and noncoaxial arrangements, and comparison is made to experimental results including effects of misalignment.

Ancellet, G. M.↗

NASA NTRS1986

Picosecond coherent anti-Stokes Raman scattering (CARS) study of vibrational dephasing of carbon disulfide and benzene in solution

The vibrational dephasing of the 656/cm mode (nu1, a1g) of CS2 and the 991/cm mode (nu2, a1g) of benzene have been studied as a function of concentration in mixtures with a number of solvents using a ps time-resolved CARS technique. This technique employs two tunable synchronously-pumped mode-locked dye lasers in a stimulated Raman pump, coherent anti-Stokes Raman probe time-resolved experiment. Results are obtained for CS2 in carbon tetrachloride, benzene, nitrobenzene, and ethanol and for benzene nu2 in CS2. The dephasing rates of CS2 nu1 increase on dilution with the polar solvents and decrease or remain constant on dilution with the nonpolar solvents. The CS2/benzene solutions show a contrasting behavior, with the CS2 nu1 dephasing rate being nearly independent of concentration whereas the benzene nu2 dephasing rate decreases on dilution. These results are compared to theoretical models for vibrational dephasing of polyatomic molecules in solution.

Perry, Joseph W.↗

NASA NTRS1987

Resonance between coherent whistler mode waves and electrons in the topside ionosphere

Landau resonance and cyclotron resonance of coherent whistler mode waves and energetic electrons are explored for magnetoplasmas with appreciable gradients in the plasma density and magnetic field strength. It is shown that in the topside ionosphere of the earth near the ion transition height the gradients in plasma density and magnetic field strength along a magnetic field line may match in a way which enhances both Landau and cyclotron interactions between waves and electrons at the loss cone pitch angle. The pitch angle scattering induced by a signal from a ground-based VLF transmitter in the ionosphere above the transmitter has been estimated and compared to the pitch angle scattering induced by naturally occurring ELF hiss through cyclotron resonance. It is found that the expected scattering due to plasmapheric hiss is an order of magnitude larger than that due to Landau resonance in the topside ionosphere. Pitch angle scattering due to cyclotron resonance in the topside ionosphere, however, may be larger by a factor of 2. It is suggested that the 'fast Trimpi' effect may be caused by a cyclotron resonance interaction in the topside ionosphere.

Neubert, T.↗

NASA NTRS1987

Dual laser diode array emission coherently summed in an external cavity

The output power from two ten-element laser diodes arrays has been efficiently summed in an external cavity to obtain a coherent, continuous wave beam. In this technique, laser emission from each front surface antireflection coated diode array is stimulated by injection of emission from a second such array. This method was also applied to single-element diode lasers. The output beam characteristics such as spectral distribution, far-field and near-field beam patterns, and overall efficiency have been measured.

Hemmati, Hamid↗

NASA NTRS1987

Coherent laser radar at 1.06 micron using Nd:YAG lasers

A coherent laser radar system operating at the 1.06 micron Nd:YAG laser wavelength has been built and operated. A laser-diode-pumped monolithic ring laser served as the master oscillator. A single flash-lamp-pumped zigzag slab amplified the oscillator output to a power of 2.3 kW. Single-mode optical fiber was used to collect and mix the return signal with the local-oscillator output. Signals from clouds at a range of 2.7 km and from atmospheric aerosols at a range of 600 m were detected.

Kane, Thomas J.↗

NASA NTRS1986

Observations of coherent transverse ion acceleration

DE-1 retarding ion mass spectrometer (RIMS) observations of transverse O(+) acceleration in the topside ionosphere are reported and analyzed. The operation and capabilities of RIMS are reviewed, and the data are presented graphically and characterized in detail. Torus or ring O(+) distributions with radii 10 km/s are observed, consistent with coherent transverse acceleration to 10-eV energies in the bulk-plasma reference frame, and conical hot tails at energies above the 50-eV RIMS maximum are noted. Possible mechanisms for these phenomena are discussed.

Moore, T. E.↗

NASA NTRS1986

A parametric study of various synthetic aperture telescope configurations for coherent imaging applications

The comparative advantages of synthetic aperture telescopes (SATs) of segmented primary mirror and common secondary mirror type, on the one hand, and on the other those employing an array of independent telescopes, are discussed. The diffraction-limited optical performance of both redundant and nonredundant subaperture configurations are compared in terms of point spread function characteristics and encircled energy plots. Coherent imaging with afocal telescope SATs involves a pupil-mapping operation followed by a Fourier transform one. A quantitative analysis of the off-axis optical performance degradation due to pupil-mapping errors is presented, together with the field-dependent effects of residual design aberrations of independent telescopes.

Harvey, James E.↗

NASA NTRS1987

Coherent structures and dynamical systems

Any flow of a viscous fluid has a finite number of degrees of freedom, and can therefore be seen as a dynamical system. A coherent structure can be thought of as a lower dimensional manifold in whose neighborhood the dynamical system spends a substantial fraction of its time. If such a manifold exists, and if its dimensionality is substantially lower that that of the full flow, it is conceivable that the flow could be described in terms of the reduced set of degrees of freedom, and that such a description would be simpler than one in which the existence of structure was not recognized. Several examples are briefly summarized.

Jimenez, Javier↗

NASA NTRS1987

Coherent structures and modeling: Some background comments

Coherent structures are discussed as a sequence of events (identifiable motions) in the flow which convert significant amounts of mechanical energies of the mean flow stream, into turbulent fluctuations. The use of structure information in modeling is also discussed.

Kline, S. J.↗

NASA NTRS1988

Development of a coded 16-ary CPFSK coherent demodulator

Theory and hardware are described for a proof-of-concept 16-ary continuous phase frequency shift keying (16-CPFSK) digital modem. The 16 frequencies are spaced every 1/16th baud rate for 2 bits/sec/Hz operation. Overall rate 3/4 convolutional coding is incorporated. The demodulator differs significantly from typical quadrature phase detector approaches in that phase is coherently measured by processing the baseband output of a frequency discriminator. Baud rate phase samples from the baseband processor are decoded to yield the original data stream. The method of encoding onto the 16-ary phase nodes, together with convolutional coding gain, results in near quad PSK (QPSK) performance. The modulated signal is of constant envelope; thus the power amplifier can be saturated for peak performance. The spectrum is inherently bandlimited and requires no RF filter.

Clarke, Ken↗

NASA NTRS1987

Instabilities in a three-level coherently pumped laser

A theory for a coherently pumped, homogeneously broadened laser is developed which predicts instability at excitations 1.6 times threshold. The system exhibits a period-doubling sequence, chaos, and a period-three window.

Ryan, J. C.↗

NASA NTRS1987

Coherent lidar wind measurements from the Space Station base using 1.5 m all-reflective optics

This paper discusses the space-based measurement of atmospheric winds from the point of view of the requirements of the optical system of a coherent CO2 lidar. A brief description of the measurement technique is given and a discussion of previous study results provided. The telescope requirements for a Space Station based lidar are arrived at through discussions of the desired system sensitivity and the need for lag angle compensation.

Bilbro, J. W.↗

NASA NTRS1988

Diagnostics of silane and germane radio frequency plasmas by coherent anti-Stokes Raman spectroscopy

In situ plasma diagnostics using coherent anti-Stokes Raman spectroscopy have shown different dissociation characteristics for GeH4 and SiH4 in radio frequency (rf) plasma-enhanced chemical vapor deposition of amorphous silicon germanium alloy (a-SiGe:H) thin films. The GeH4 dissociation rate in rf plasmas is a factor of about 3 larger than that of SiH4. Plasma diagnostics have revealed that the hydrogen dilution of the SiH4 and GeH4 mixed plasma plays a critical role in suppressing the gas phase polymerization and enhancing the GeH4 dissociation.

Perry, Joseph W.↗

NASA NTRS1988

Unstable resonator antenna properties in coherent lidar applications - A comparative study

The antenna parameters of a coherent lidar in the far-field diffraction-limited regime, where the lidar transmitter consists of an unstable resonator with a Gaussian reflectivity profile output coupler, are investigated. A figure-of-merit combining lasr transmitter energy extraction efficiency with the far-field attributes of a given cavity configuration has been used to compare the lidar performance and relative merits of such systems as they relate to those of conventional unstable cavities with diffractive output coupling.

Tratt, David M.↗

NASA NTRS1988

Modelling the SIR-B image response to partially coherent seas

A mathematical model of transient seas modulated by coherent swells is developed as a Rayleigh-Poisson random process subject to deterministic correlations of scattering sites, as they appear in a time window defined by a SAR's range-Doppler measurements of the surface coordinates. This model of SAR response to sea surface dynamics is discussed as a Fourier domain predictor/corrector of a dynamic motion blur.

Tilley, D. G.↗

NASA NTRS1988

Two-way coherent Doppler error due to solar corona

Two-way coherent Doppler errors resulting from phase scintillations induced on the uplink by the solar corona are considered. It is shown that this error can be estimated by taking statistics on the differential Doppler measurements. Typical estimates for the error are given for four Sun-Earth-probe angles and for integration times ranging from 1 second to 1 minute. These results are based on data collected during the 1985 Voyager 2 conjunction.

Kinman, P. W.↗

NASA NTRS1988

Performance analysis and technical assessment of coherent lidar systems for airborne wind shear detection

Detailed computer simulations of the lidar wind-measuring process have been conducted to evaluate the use of pulsed coherent lidar for airborne windshear monitoring. NASA data fields for an actual microburst event were used in the simulation. Both CO2 and Ho:YAG laser lidar systems performed well in the microburst test case, and were able to measure wind shear in the severe weather of this wet microburst to ranges in excess of 1.4 km. The consequent warning time gained was about 15 sec.

Huffaker, R. Milton↗

NASA NTRS1989

Coherent Cerenkov radiation from the Spacelab 2 electron beam

The plasma environment of the Spacelab 2 mission was investigated through the deployment of the Plasma Diagnostics Package (PDP) by the Space Shuttle Orbiter and the Orbiter's ejection of a continuous 1-keV/50-mA electron beam along a field line. As the PDP flew by the beam, its plasma-wave instrument detected intense whistler-mode radiation originating from the beam. A detailed model has been developed of the coherent Cerenkov emission process, using a one-dimensional computer simulation of the beam to model the expected phase space structure of the electrons. The power calculated for the modeled 200-m beam segment can easily account for the measured whistler mode wave power.

Farrell, W. M.↗