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At least 127 records · Page 7

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NASA NTRS1983

Laser induced asymmetry and inhomogeneous broadening of the microwave lineshape of a gas cell atomic frequency standard

The possibility of replacing the RF discharge lamp in a rubidium gas cell clock with a single mode laser diode is addressed. Since the short term stability of the rubidium frequency standard is limited by the shot noise of the photodetector, an increased signal-to-noise ratio due to more efficient laser diode optical pumping might improve the short term performance. Because the emission wavelength of the laser diode can be tuned, improved long term performance could be gained through the control of the light shift effect. However, due to the nature of the gas cell frequency standard, various physical phenomena are strongly coupled in their effect on the frequency output, and thus careful consideration must be given to any change in one parameter because of its interrelation with other parameters. Some investigations concerning the coupled effect of the optical and microwave fields in the rubidium atomic clock are reported. It is shown that this type of coupling is an important consideration for any attempt to incorporate a laser diode into a gas cell clock.

Camparo, J. C.↗

NASA NTRS1984

Theoretical and experimental analysis of laser altimeters for barometric measurements over the ocean

The statistical characteristics and the waveforms of ocean-reflected laser pulses are studied. The received signal is found to be corrupted by shot noise and time-resolved speckle. The statistics of time-resolved speckle and its effects on the timing accuracy of the receiver are studied in the general context of laser altimetry. For estimating the differential propagation time, various receiver timing algorithms are proposed and their performances evaluated. The results indicate that, with the parameters of a realistic altimeter, a pressure measurement accuracy of a few millibars is feasible. The data obtained from the first airborne two-color laser altimeter experiment are processed and analyzed. The results are used to verify the pressure measurement concept.

Tsai, B. M.↗

NASA NTRS1983

X-ray observations of LMC X-3 with the monitor proportional counter aboard the HEAO 2 Einstein observatory - A comparison with Cygnus X-1

A comparison is presented of the black hole candidates LMC X-3 and Cygnus X-1 based on Einstein observations of LMC X-3 with the monitor proportional counter. A spectral analysis shows LMC X-3 to be more like the typical bright galactic X-ray source than Cygnus X-1. A search for periodic pulsations over a period range from 0.2 ms to over 1000 s set upper limits at the 90 percent confidence level of the order of 10 percent. An analysis of the aperiodic variability of LMC X-3 shows none of the shot noise behavior characteristic of Cygnus X-1. The absence of distinctive X-ray properties common to both sources suggests that the identification of black hole candidates on the basis of X-ray properties similar to Cygnus X-1 (or LMC X-3) is not reliable.

Weisskopf, M. C.↗

NASA NTRS1984

Short-term X-ray variability of the globular cluster source 4U 1820 - 30 (NGC 6624)

Analytical techniques for improved identification of the temporal and spectral variability properties of globular cluster and galactic bulge X-ray sources are described in terms of their application to a large set of observations of the source 4U 1820 - 30 in the globular cluster NGC 6624. The autocorrelation function, cross-correlations, time skewness function, erratic periodicities, and pulse trains are examined. The results are discussed in terms of current models with particular emphasis on recent accretion disk models. It is concluded that the analyzed observations provide the first evidence for shot-noise variability in a globular cluster X-ray source.

Stella, L.↗

NASA NTRS1985

Theoretical and experimental analyses of the performance of two-color laser ranging systems

The statistical properties of the signals reflected from the retroreflector equipped satellites were studied. It is found that coherence interference between pulse reflections from retroreflectors of different ranges on the array platform is the primary cause of signal fluctuations. The performance of a cross-correlation technique to estimate the differential propagation time is analyzed by considering both shot noise and speckle. For the retroreflector arrays, timing performance is dominated by interference induced speckle, and the differential propagation time cannot be resolved to better than the pulse widths of the received signals. The differential timing measurements obtained over a horizontal path are analyzed. The ocean-reflected pulse measurements obtained from the airborne two-color laser altimeter experiment are presented.

Im, K. E.↗

NASA NTRS1984

Photometry of CPD - 48 deg 1577

U, B, and V photometry is presented for CPD - 48 deg 1577, the newly discovered cataclysmic variable with the greatest known apparent brightness. The light curves are dominated by flickering but do suggest an orbital period of nearly 4 hr. Power spectra of 5 nights of U-band observations failed to reveal the presence of any periodic or quasi-periodic oscillations. Autocorrelation functions suggest that two time-scales exist in the flickering activity, one of 77 s and the other 500 s. The implications for a shot noise model of flickering are discussed.

Williams, G. A.↗

NASA NTRS1985

Time-resolved speckle effects on the estimation of laser-pulse arrival times

A maximum-likelihood (ML) estimator of the pulse arrival in laser ranging and altimetry is derived for the case of a pulse distorted by shot noise and time-resolved speckle. The performance of the estimator is evaluated for pulse reflections from flat diffuse targets and compared with the performance of a suboptimal centroid estimator and a suboptimal Bar-David ML estimator derived under the assumption of no speckle. In the large-signal limit the accuracy of the estimator was found to improve as the width of the receiver observational interval increases. The timing performance of the estimator is expected to be highly sensitive to background noise when the received pulse energy is high and the receiver observational interval is large. Finally, in the speckle-limited regime the ML estimator performs considerably better than the suboptimal estimators.

Tsai, B.-M.↗

NASA NTRS1986

Estimation of the differential pulse propagation times in two-color laser ranging systems

Two-color laser ranging systems can be used to determine the atmospheric delay by measuring the difference in propagation times between two optical pulses transmitted at two different wavelengths. In this paper, the performance of a cross-correlation technique for estimating the differential propagation time is analyzed by considering both speckle and shot noise. For the flat diffuse targets, the differential timing accuracy is highly dependent on the receiver bandwidth and the characteristics of the time-resolved speckle. At low signal levels, when the receiver bandwidth is chosen properly, the performance of the correlation estimator is comparable with that of the maximum-likelihood estimator. At high signal levels, however, speckle places a fundamental limit on the performance of the correlation estimator. For the cube-corner reflector arrays, timing performance is dominated by partially developed speckle. The differential propagation time cannot by resolved to better than the pulse widths of the received signals.

Im, K. E.↗

NASA NTRS1986

Evaluation and characterization of hybrid arrays

An Infrared Camera was built for astronomical observations and for testing arrays for satellite based work. The detectors are sensitive to mid infrared wavelengths. The camera was tested at the University of Arizona 61 and 90 inch telescopes and the NASA IRTF. These tests have demonstrated a sensitivity consistent with photon shot noise with an approximately 10% quantum efficiency for each pixel when the camera was used with a 1.6% spectral filter or when used with a Fabry-Perot having a resolving power of 2000. Initial measurements of read out noise, dark current, cross talk, and hysteresis were made. The detector chip in current use is a hybrid array manufactured by the Hughes Aircraft Company. Because part of the goal was to develop and advance technology, it was decided to work with a switched MOSFET array. This architecture is very promising, and the detector parameters will be quantified carefully.

Arens, J. F.↗

NASA NTRS1986

A synchronization technique for optical PPM signals

A technique for maintaining synchronization between optical PPM (pulse-position modulation) pulses and a receiver clock by means of a delay-tracking loop is described and analyzed. The tracking loop is driven by a doubly stochastic Poisson process that contains information about the location of the desired slot boundaries. The slot boundaries are subject to slowly varying random delays that are ultimately tracked by the loop. The concept of fractional rms delay error is introduced to quantify the effects of signal and background induced shot noise on the performance of the delay-tracking loop.

Vilnrotter, V. A.↗

NASA NTRS1986

Ultrasensitive detection of atmospheric trace gases using frequency modulation spectroscopy

Frequency modulation (FM) spectroscopy is a new technique that promises to significantly extend the state-of-the-art in point detection of atmospheric trace gases. FM spectroscopy is essentially a balanced bridge optical heterodyne approach in which a small optical absorption or dispersion from an atomic or molecular species of interest generates an easily detected radio frequency (RF) signal. This signal can be monitored using standard RF signal processing techniques and is, in principle, limited only by the shot noise generated in the photodetector by the laser source employed. The use of very high modulation frequencies which exceed the spectral width of the probed absorption line distinguishes this technique from the well-known derivative spectroscopy which makes use of low (kHz) modulation frequencies. FM spectroscopy was recently extended to the 10 micron infrared (IR) spectral region where numerous polyatomic molecules exhibit characteristic vibrational-rotational bands. In conjunction with tunable semiconductor diode lasers, the quantum-noise-limited sensitivity of the technique should allow for the detection of absorptions as small as .00000001 in the IR spectral region. This sensitivity would allow for the detection of H2O2 at concentrations as low as 1 pptv with an integration time of 10 seconds.

Cooper, David E.↗

NASA NTRS1986

Quasi-periodic oscillations as a probe of galactic-bulge X-ray sources

Quasi-periodic oscillations (QPOs) with frequencies in the range 5-50 Hz have recently been discovered in X-rays from two bright galactic-bulge sources and from Sco X-1. A shot-noise model that provides a mathematical framework for analyzing the constraints imposed on physical models by the observed power density spectra is described. A variety of physical models are then examined briefly. One of the most promising, the beat-frequency-modulated accretion model, is adopted as a working hypothesis to consider the clues it may provide concerning the galactic-bulge sources. Finally, some key observations are noted.

Lamb, F. K.↗

NASA NTRS1986

Molecular clouds and periodic events in the geologic past

The suggestion that a claimed 30 Myr period in the geologic past resulted from cometary impacts following encounters with molecular clouds as the solar system oscillates about the galactic plane poses a well-defined problem in the theory of shot noise. All recent CO surveys of the Galaxy clearly indicate that the concentration of molecular clouds in the galactic plane is not sufficient to allow a statistically significant period to be extracted from the small number of dated events. Of the order of 1000 events is probably required to obtain a credible period.

Thaddeus, P.↗

NASA NTRS1986

A synchronization technique for optical PPM signals

Vilnrotter, V. A.↗

NASA NTRS1989

Characteristic-eddy decomposition of turbulence in a channel

Lumley's proper orthogonal decomposition technique is applied to the turbulent flow in a channel. Coherent structures are extracted by decomposing the velocity field into characteristic eddies with random coefficients. A generalization of the shot-noise expansion is used to determine the characteristic eddies in homogeneous spatial directions. Three different techniques are used to determine the phases of the Fourier coefficients in the expansion: (1) one based on the bispectrum, (2) a spatial compactness requirement, and (3) a functional continuity argument. Similar results are found from each of these techniques.

Moin, Parviz↗

NASA NTRS1991

Cosmic backgrounds from primeval dust

A general formalism is developed for describing the spectrum and the anisotropy of radiation from primeval dust as a nearly isotropic cosmic background with fluctuations. The theory incorporates spatial and temporal variations in the density of both luminosity and dust and is sufficiently robust to treat emission by point sources, as well as smoother emission, in high and low optical depth situations. The fluctuations are calculated using linear perturbation theory for arbitrary dust and luminosity clumping, and are calculated nonperturbatively for compact IR sources using shot-noise models. For many realistic models, a large accompanying NIR background is predicted. It is shown that, while spectal information alone provides only coarse information about the dust abundance and type, the redshift, and the other parameters, it may provide a powerful probe of pregalactic history when used in combination with the anisotropy signal.

Bond, J. R.↗

NASA NTRS1991

Normal-incidence soft X-ray telescopes

Photos obtained during 5 min of observation time from the flight of a 25-cm-diameter normal-incidence soft-X-ray (63.5 A) telescope on September 11, 1989, are analyzed, and the data are compared to the results expected from tests of the mirror surfaces. These tests cover a range of spatial periods from 25 cm to 1 A. The photos demonstrate a resolution close to the photon shot-noise limit and a reduction in the scattering of the multilayer mirror compared to a single surface for scattering angles above 1 arcmin, corrresponding to surface irregularities with spatial periods below 10 microns. These results are used to predict the possible performance of future telescopes. Sounding rocket observations might be able to reach a resolution around 0.1 arcsec.

Spiller, Eberhard↗

NASA NTRS1992

Solid-state lasers for coherent communication and remote sensing

Semiconductor-diode laser-pumped solid-state lasers have properties that are superior to other lasers for the applications of coherent communication and remote sensing. These properties include efficiency, reliability, stability, and capability to be scaled to higher powers. We have demonstrated that an optical phase-locked loop can be used to lock the frequency of two diode-pumped 1.06 micron Nd:YAG lasers to levels required for coherent communication. Monolithic nonplanar ring oscillators constructed from solid pieces of the laser material provide better than 10 kHz frequency stability over 0.1 sec intervals. We have used active feedback stabilization of the cavity length of these lasers to demonstrate 0.3 Hz frequency stabilization relative to a reference cavity. We have performed experiments and analysis to show that optical parametric oscillators (OPO's) reproduce the frequency stability of the pump laser in outputs that can be tuned to arbitrary wavelengths. Another measurement performed in this program has demonstrated the sub-shot-noise character of correlations of the fluctuations in the twin output of OPO's. Measurements of nonlinear optical coefficients by phase-matched second harmonic generation are helping to resolve inconsistency in these important parameters.

Byer, Robert L.↗