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At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Symmetric second-harmonic generation in sub-wavelength periodically poled thin film lithium niobate

Second-harmonic generation (SHG) extensively employs periodically poled nonlinear crystals through forward quasi-phase-matching to achieve efficient frequency conversion. As poling periods approach sub-micrometers, backward quasi-phase-matching has also been demonstrated, albeit by utilizing pulsed laser drives. The realization of symmetric second-harmonic generation, characterized by counterpropagating pumps, however, has remained elusive despite theoretical predictions. The main challenge lies in achieving strong nonlinear coupling with the poling period below half the wavelength of the second-harmonic light. The recent emergence of high-quality ferroelectric lithium niobate thin films provides an opportunity for achieving precise domain control at submicron dimensions. In this paper, we demonstrate reliable control of ferroelectric domains in a thin film lithium niobate waveguide with a poling period down to 370 nm, thereby realizing highly efficient continuous-wave pumped symmetric SHG. This demonstration not only validates the feasibility of achieving subwavelength periodic poling on waveguides but could also enable submicron ferrolectric domain structures to be leveraged in integrated photonics and nonlinear optics research.

Optics↗
NASA NTRS1974

Digital redesign of continuous systems by matching of states at multiple sampling periods

This paper presents a method of digital approximation of a continuous-data system by the matching of states at multiple sampling periods. Two alternate methods are presented; one method samples the states every sampling period, and alters the forward and feedback gains, so as to match the states of the two systems at the end of N sampling periods. The second method requires the sampling of the states every N sampling periods, with the change of gains at each sampling period. An illustrative example using the one-axis dynamics of the Skylab Satellite is presented.

Yackel, R. A.↗
NASA NTRS1974

Aeroelastic stability of periodic systems with application to rotor blade flutter

The dynamics of a helicopter blade in forward flight are described by a system of linear differential equations with periodic coefficients. The stability of this periodic aeroelastic system is determined, using multivariable Floquet-Liapunov theory. The transition matrix at the end of the period is evaluated by: (1) direct numerical integration, and (2) a new, approximate method, which consists in approximating a periodic function by a series of step functions. The numerical accuracy and efficiency of the methods is compared, and the second method is shown to be superior by far. Results illustrating the effect of the periodic coefficients and various blade parameters are presented.

Friedmann, P.↗
NASA NTRS1975

On relative periodic solutions of the planar general three-body problem

We describe two relatively simple reductions to order 6 for the planar general three-body problem. We also show that this reduction leads to the distinction between two types of periodic solutions: absolute or relative periodic solutions. An algorithm for obtaining relative periodic solutions using heliocentric coordinates is then described. It is concluded from the periodicity conditions that relative periodic solutions must form families with a single parameter. Finally, two such families have been obtained numerically and are described in some detail.

Broucke, R.↗
NASA NTRS1976

On the mathematical conditions for the existence of periodic fluctuations in non-uniform media

The assumption that periodic excitations result in periodic responses for the case of waves propagating through nonuniform media is examined by investigating the periodic solutions of linear hyperbolic differential equations whose coefficients vary with position and whose solution must satisfy periodic boundary or source data. It is shown that the nature of the coefficients of undifferentiated terms of the differential system is crucial in determining whether the solution is periodic. Particular attention is paid to the propagation of infinitesimal pressure waves through the nonuniform steady flow of a lossless fluid.

Gunzburger, M. D.↗
NASA NTRS1976

Waves in active and passive periodic structures - A review

The theory and recent applications of waves in periodic structures are reviewed. Both the Floquet and coupled waves approach are analyzed in some detail. The theoretical part of the paper includes wave propagation in unbounded and bounded active or passive periodic media, wave scattering from periodic boundaries, source radiation (dipole, Cerenkov, transition, and Smith-Purcell) in periodic media, and pulse transmission through a periodic slab. The applications part covers the recent development in a variety of fields: distributed feedback oscillators, filters, mode converters, couplers, second-harmonic generators, deflectors, modulators, and transducers in the fields of integrated optics and integrated surface acoustics. Work on insect compound eyes, mechanical structures, ocean waves, pulse compressions, temperature waves, and cholesteric liquid crystals, and particles interaction with crystals is briefly reviewed, especially in the case of zeolite crystals and superlattices. Recent advances in fabrication techniques for very fine gratings are also covered.

Elachi, C.↗
NASA NTRS1977

Higher-order Bragg coupling in periodic media with gain or loss

Wave propagation in longitudinally periodic media with gain or loss is considered. An extended coupled-waves approach and a Floquet approach are used to study index and gain (or loss) coupling in periodic media at the first few Bragg resonances by the use of Brillouin diagrams. Phase speeding or slowing that is dependent upon the type of coupling occurs in periodic media. Even and odd resonances display different dispersion characteristics. Several examples of coupling due to multiharmonic periodicities are also covered. Coupling or feedback strength can be continuously varied by changing the relative phase of multiharmonic periodicities.

Jaggard, D. L.↗
NASA NTRS1977

Further studies of the pulsation period and orbital elements of Centaurus X-3

The long- and short-term variability of the 4.8-s pulsation and the 2.1-day orbital periods of Centaurus X-3 are studied. The pulsation period decreases over 4 yr with a fractional change of -0.00028 per yr, but with rms fluctuations of 0.0002 s. In August-September 1972, a continuous transition from speedup to slowdown was observed. The orbital period also decreases over 4 yr with decrease of approximately 8 millionths per yr, and with significant fluctuations of the order of 0.00001 day over months. The orbital eccentricity is found to be about 0.0008. The pulsation-period variability is found to be consistent with a near balance between the Alfven and corotation radii in an accretion-disk model. The orbital-period variability is interpreted in terms of tidal circularization and possible mass transfer and loss.

Fabbiano, G.↗
NASA NTRS1977

Uranus' rotational period

A modified spectroscopic technique was applied to determine the rotational period of Uranus and the orientation of the projected spin axis of the planet. The method consists in obtaining a series of spectra for both a nonrotated planet disk and for a disk rotated 180 degrees. Ignoring the effects of seeing in analyzing the spectra for no rotation of the disk leads to a lower limit on the period, and for a disk rotated 180 degrees, ignoring the seeing in the modeling of the spectra leads to an upper limit on the period. When blurring from seeing and guiding errors are added to the modeling for both image orientations and the model spectra are compared to the observations, both the period and the degree of blurring are obtained simultaneously. The results gave a period of rotation of Uranus 23 + 5 or - 2 hr.

Trafton, L.↗
NASA NTRS1980

Physical loss of long-period comets

Loss of long-period comets from the solar system through a variety of physical processes is investigated. It is shown that planetary collision, defined as a comet passing within the Roche radius of any planet, has a probability of 1.35 x 10 to the -7th per perihelion passage for a long-period comet on a randomly oriented orbit. Sublimation of all volatiles for a one-kilometer radius hydrate ice nucleus would require on the order of 600 to 27,600 perihelion passages, depending on the surface albedo and assuming a perihelion distance of one astronomical unit. Analysis of observational records indicates that for dynamically 'new' comets in the Oort-Schmidt sense there is about a 10% probability of the comet randomly disrupting on its first perihelion passage. This probability drops to 4% per perihelion passage for older long-period comets, and less than 1% for short-period comets. The consequences of these loss mechanisms when incorporated into Monte-Carlo studies of long-period comet evolution are also discussed.

Weissman, P. R.↗
NASA NTRS1980

Periodic orbits of the asteroidal type in the circular restricted three-body problem

Periodic orbits of the asteroidal type in the circular restricted problem are studied by varying the period of the infinitesimal body (asteroid) and the mass ratio of the primaries (Sun-Jupiter mass ratio). The results indicate that asteroidal periodic orbits can exist for the actual Sun-Jupiter mass ratio for resonances of the Hecuba (2:1), Hilda (3:2) and Thule (4:3) groups, but not for resonances of higher consecutive integer ratios. It is also found that an asteroid can be placed in a periodic orbit at a position with mean motion between 2:1 and 3:2 even if Jupiter is about 9 times more massive than its actual value. However, an asteroid cannot be placed in a periodic orbit beyond the 4:3 resonance for the actual Sun-Jupiter mass ratio.

Kwok, J. H.↗
NASA NTRS1981

Observations of SMC X-1 with the monitor proportional counter on the Einstein /HEAO 2/ X-ray Observatory - The pulse period and its history

The results of the sixth determination of the pulse period of the X-ray source SMC X-1 are presented. The observations were made in April 1979. The pulse period at this epoch was found to be (0.713683 + or - 0.000032) s referred to phase zero of the binary motion. This result together with all available historical data is utilized to quantify the observed trend toward spin-up. It is found that simple secular trends are not applicable. This finding is interpreted as evidence for additional variations in the pulse period. The obtained results are discussed in the context of present theoretical models for period variations in the binary pulsating X-ray sources. The apparent absence of spin-down episodes in the period history of SMC X-1 is also considered.

Darbro, W.↗
NASA NTRS1981

Bifurcations from stationary to periodic solutions in a low-order model of forced, dissipative barotropic flow

The considered investigation is concerned with periodic solutions in the context of a forced, dissipative, barotropic spectral model truncated to three complex coefficients with constant forcing on only the intermediate scale. It is found that determining a periodic solution of this three-coefficient model also reduces to finding the algebraic roots of a real polynomial. In the derivation of this polynomial, a class of hydrodynamic spectral systems is described for which a periodic solution might be similarly specified. The existence of periodic solutions of the three-coefficient model is controlled by the roots of the stability polynomial of the basic stationary solution, which represents the simplest response to the constant forcing. When the forcing exceeds a critical value, the basic solution becomes unstable. Owing to the nature of the roots of the stability polynomial at critical forcing, bifurcation theory guarantees the existence of a periodic solution.

Mitchell, K. E.↗
NASA NTRS1982

The discovery of 50 minute periodic absorption events from 4U 1915-05

An analysis of X-ray observations shows that the steady flux from 4U 1915-05 undergoes periodic absorption dips with a period of 50 minutes. This period most probably represents the underlying orbital period of the system, and variations in the depth and duration of these events suggest that they are caused by a bulge on the edge of the accretion disk at the point where the gas stream impacts the disk. The mass-losing star in this system is probably a low-mass white dwarf, and the spectrum of the dips indicates that the metallicity of the absorbing material is at least a factor of 17 below solar values. The discovery of the 50-minute binary period supports earlier suggestions that X-ray burst sources are in binary systems.

White, N. E.↗
NASA NTRS1983

X Persei - Optical polarization variation on the 580 day binary-like period

Observations of periodicities in the optical polarization of the weak X-ray source X Persei are reported. Polarization was measured in the V band 400 times over the course of 356 nights during 1977-1982. The Q and U Stokes parameters derived from the measurements show a definite variability with a typical amplitude of 0.1%, which is strongest at periods greater than 100 days. In particular, a strong peak is observed with a period of 297 days, consistent with the second-harmonic component of the known 580-day radial velocity cycle. A component with 590-day periodicity is not ruled out by the observations, although it was not detected. Observations imply the 580-day period to be an effect of orbital or rotational processes, with the corotating scattering body either close to the light source or very large in extent, and a system inclination close to 80 deg.

Kemp, J. C.↗
NASA NTRS1983

Modal control of an unstable periodic orbit

Floquet theory is applied to the problem of designing a control system for a satellite in an unstable periodic orbit. Expansion about a periodic orbit produces a time-periodic linear system, which is augmented by a time-periodic control term. It is shown that this can be done such that (1) the application of control produces only inertial accelerations, (2) positive real Poincareexponents are shifted into the left half-plane, and (3) the shift of the exponent is linear with control gain. These developments are applied to an unstable orbit near the earth-moon L(3) point pertubed by the sun. Finally, it is shown that the control theory can be extended to include first order perturbations about the periodic orbit without increase in control cost.

Wiesel, W.↗
NASA NTRS1983

Wave scattering and guidance by dielectric waveguides with periodic surfaces

The extended-boundary-condition method is used to solve the wave scattering and guidance by a stratified medium with a periodic surface. First, the transition matrix for two sets of Flocquet waves incident from above and below a periodic surface separating the two media is derived. The results are applied to the diffraction of an incident plane wave by a multilayered medium having a periodic surface. The theoretical results can easily be applied to the guidance problem for which the complex guiding constant of the periodic structure is obtained. Numerical results for both the scattering and the guidance problems are obtained. The theory is compared with experiment for metallic gratings with surface-plasmon excitations. The calculated guiding constants of a thin film with a periodic sinusoidal surface are also compared with other numerical methods.

Chuang, S. L.↗
NASA NTRS1985

Radio emission from RS CVn binaries. I - VLA survey and period-radio luminosity relationship

A VLA survey of radio emission from 36 close binary stellar systems with RS CVn properties is reported. Eight new sources were detected. A summary of all published reports of radio emission from RS CVn systems is presented. There appears to be a correlation between maximum radio luminosity and rotational period, with a tentative functional form L(R) varies as P to the (-0.7) power. Rapid rotators (periods approximately 2 days) may be underluminous compared with the extrapolated trend from longer-period systems. The luminosity-period correlation probably results from a dynamo mechanism which produces strong magnetic fields and, in turn, enhances the nonthermal radio emission. The decrease in radio luminosity at short periods may be caused by a saturation of energy deposition in the chromosphere, possibly because the surface of the active star has become covered with spotted regions.

Mutel, R. L.↗