4,887 research outputs found
Model of Thermal Wavefront Distortion in Interferometric Gravitational-Wave Detectors I: Thermal Focusing
We develop a steady-state analytical and numerical model of the optical
response of power-recycled Fabry-Perot Michelson laser gravitational-wave
detectors to thermal focusing in optical substrates. We assume that the thermal
distortions are small enough that we can represent the unperturbed intracavity
field anywhere in the detector as a linear combination of basis functions
related to the eigenmodes of one of the Fabry-Perot arm cavities, and we take
great care to preserve numerically the nearly ideal longitudinal phase
resonance conditions that would otherwise be provided by an external
servo-locking control system. We have included the effects of nonlinear thermal
focusing due to power absorption in both the substrates and coatings of the
mirrors and beamsplitter, the effects of a finite mismatch between the
curvatures of the laser wavefront and the mirror surface, and the diffraction
by the mirror aperture at each instance of reflection and transmission. We
demonstrate a detailed numerical example of this model using the MATLAB program
Melody for the initial LIGO detector in the Hermite-Gauss basis, and compare
the resulting computations of intracavity fields in two special cases with
those of a fast Fourier transform field propagation model. Additional
systematic perturbations (e.g., mirror tilt, thermoelastic surface
deformations, and other optical imperfections) can be included easily by
incorporating the appropriate operators into the transfer matrices describing
reflection and transmission for the mirrors and beamsplitter.Comment: 24 pages, 22 figures. Submitted to JOSA
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Magnetic characterization of perpendicular recording media
In this paper, we describe techniques for the magnetic characterization of perpendicular recording media. Such measurements made using traditional techniques, such as the vibrating sample magnetometry (VSM) and alternating gradient force magnetometer (AGFM), have to be corrected for the sample shape demagnetizing factor, which is often found not to be equal to -4p. For measurements other than the simple hysteresis loop, such as remanence curves, this correction must be carried out in real time and we describe the method by which this can be achieved and the process for achieving the correct demagnetization of perpendicular films prior to measurements of the isothermal remanent magnetization curve. A further complication is that real perpendicular media have a soft underlayer beneath the recording layer, which swamps and confuses signals from instruments such as VSM or AGFM. Hence, we describe the construction and use of a magnetooptical Kerr effect magnetometer, which does not penetrate significantly into the soft layer and enables the perpendicular layer to be measured independently. We describe the properties of a traditional alloy perpendicular medium and a Co-Pd multilayer system, which in the latter case exhibits multiple switching behavior. We also address the issue of the effect of the soft underlayer on the coupling in similar longitudinal films and find that the presence of the underlayer induces significant additional coupling effects that may well give rise to an increase in noise in recorded signal
Electrodynamics of Media
Contains research objectives and reports on one research project.Joint Services Electronics Programs (U. S. Army, U.S. Navy, and U.S. Air Force) under Contract DA 36-039-AMC-03200(E
Spatial and temporal filtering of a 10-W Nd:YAG laser with a Fabry-Perot ring-cavity premode cleaner
We report on the use of a fixed-spacer Fabry–Perot ring cavity to filter spatially and temporally a 10-W laser-diode-pumped Nd:YAG master-oscillator power amplifier. The spatial filtering leads to a 7.6-W TEMinfinity beam with 0.1% higher-order transverse mode content. The temporal filtering reduces the relative power fluctuations at 10 MHz to 2.8 x 10^-/sqrtHz, which is 1 dB above the shot-noise limit for 50 mA of detected photocurrent
Spectral characteristics for a spherically confined -1/r + br^2 potential
We consider the analytical properties of the eigenspectrum generated by a
class of central potentials given by V(r) = -a/r + br^2, b>0. In particular,
scaling, monotonicity, and energy bounds are discussed. The potential is
considered both in all space, and under the condition of spherical confinement
inside an impenetrable spherical boundary of radius R. With the aid of the
asymptotic iteration method, several exact analytic results are obtained which
exhibit the parametric dependence of energy on a, b, and R, under certain
constraints. More general spectral characteristics are identified by use of a
combination of analytical properties and accurate numerical calculations of the
energies, obtained by both the generalized pseudo-spectral method, and the
asymptotic iteration method. The experimental significance of the results for
both the free and confined potential V(r) cases are discussed.Comment: 16 pages, 4 figure
The mean magnetic field of the sun: Observations at Stanford
A solar telescope was built at Stanford University to study the organization and evolution of large-scale solar magnetic fields and velocities. The observations are made using a Babcock-type magnetograph which is connected to a 22.9 m vertical Littrow spectrograph. Sun-as-a-star integrated light measurements of the mean solar magnetic field were made daily since May 1975. The typical mean field magnitude is about 0.15 gauss with typical measurement error less than 0.05 gauss. The mean field polarity pattern is essentially identical to the interplanetary magnetic field sector structure (seen near the earth with a 4 day lag). The differences in the observed structures can be understood in terms of a warped current sheet model
Asymptotic stability, concentration, and oscillation in harmonic map heat-flow, Landau-Lifshitz, and Schroedinger maps on R^2
We consider the Landau-Lifshitz equations of ferromagnetism (including the
harmonic map heat-flow and Schroedinger flow as special cases) for degree m
equivariant maps from R^2 to S^2. If m \geq 3, we prove that near-minimal
energy solutions converge to a harmonic map as t goes to infinity (asymptotic
stability), extending previous work down to degree m = 3. Due to slow spatial
decay of the harmonic map components, a new approach is needed for m=3,
involving (among other tools) a "normal form" for the parameter dynamics, and
the 2D radial double-endpoint Strichartz estimate for Schroedinger operators
with sufficiently repulsive potentials (which may be of some independent
interest). When m=2 this asymptotic stability may fail: in the case of
heat-flow with a further symmetry restriction, we show that more exotic
asymptotics are possible, including infinite-time concentration (blow-up), and
even "eternal oscillation".Comment: 34 page
Evaluating the pronunciation component of text-to-speech systems for English: A performance comparison of different approaches
The automatic derivation of word pronunciations from input text is a central task for any text-to-speech system. For general English text at least, this is often thought to be a solved problem, with manually-derived linguistic rules assumed capable of handling `novel' words missing from the system dictionary. Data-driven methods, based on machine learning of the regularities implicit in a large pronouncing dictionary, have received considerable attention recently but are generally thought to perform less well. However, these tentative beliefs are at best uncertain without powerful methods for comparing text-to-phoneme subsystems. This paper contributes to the development of such methods by comparing the performance of four representative approaches to automatic phonemisation on the same test dictionary. As well as rule-based approaches, three data-driven techniques are evaluated: pronunciation by analogy (PbA), NETspeak and IB1-IG (a modified k-nearest neighbour method). Issues involved in comparative evaluation are detailed and elucidated. The data-driven techniques outperform rules in accuracy of letter-to-phoneme translation by a very significant margin but require aligned text-phoneme training data and are slower. Best translation results are obtained with PbA at approximately 72% words correct on a reasonably large pronouncing dictionary, compared to something like 26% words correct for the rules, indicating that automatic pronunciation of text is not a solved problem
Energies and wave functions for a soft-core Coulomb potential
For the family of model soft Coulomb potentials represented by V(r) =
-\frac{Z}{(r^q+\beta^q)^{\frac{1}{q}}}, with the parameters
Z>0, \beta>0, q \ge 1, it is shown analytically that the potentials and
eigenvalues, E_{\nu\ell}, are monotonic in each parameter. The potential
envelope method is applied to obtain approximate analytic estimates in terms of
the known exact spectra for pure power potentials. For the case q =1, the
Asymptotic Iteration Method is used to find exact analytic results for the
eigenvalues E_{\nu\ell} and corresponding wave functions, expressed in terms of
Z and \beta. A proof is presented establishing the general concavity of the
scaled electron density near the nucleus resulting from the truncated
potentials for all q. Based on an analysis of extensive numerical calculations,
it is conjectured that the crossing between the pair of states
[(\nu,\ell),(\nu',\ell')], is given by the condition \nu'\geq (\nu+1) and \ell'
\geq (\ell+3). The significance of these results for the interaction of an
intense laser field with an atom is pointed out. Differences in the observed
level-crossing effects between the soft potentials and the hydrogen atom
confined inside an impenetrable sphere are discussed.Comment: 13 pages, 5 figures, title change, minor revision
Quantum Zeno subspaces
The quantum Zeno effect is recast in terms of an adiabatic theorem when the
measurement is described as the dynamical coupling to another quantum system
that plays the role of apparatus. A few significant examples are proposed and
their practical relevance discussed. We also focus on decoherence-free
subspaces.Comment: 5 pages, 1 figure, to be published in Phys. Rev. Let
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