Showing papers on "Phase (waves) published in 1994"

Complex wave-field reconstruction by using phase-space tomography

Abstract: A new class of phase-retrieval methods for 2-D fields is introduced. Phase-space tomography can be used for the experimental determination of the amplitude and phase structure of a quasi-monochromatic wave field in a plane normal to its propagation direction. The wave field ψ(r) may represent either a scalar electromagnetic (EM) field or the quantum-mechanical (QM) wave function of a matter wave. The complex wave field may be coherent or partially coherent, in which case the method reconstructs the two-point spatial correlation function, Γ(r, r′) = ⟨ψ(r)ψ*(r′)⟩. (In the QM case, the analogous quantity is the density matrix.) The experiment uses only intensity measurements and refractive optics (lenses), and the data-analysis algorithm is noniterative and requires no deconvolution.

Automatic alignment of optical interferometers

Abstract: We present a description of a system for automatic alignment of optical interferometers. The technique relies on using differential phase modulation to permit the detection of the phase difference between two fundamental-mode Gaussian beams at the output of an interferometer. Measurements of the spatially varying phase difference between the two beams by use of one or more multielement photodiodes permits information to be derived about the mismatch in overlap between the phase fronts at the output of the interferometer.

A unified theory of dielectrophoresis and travelling wave dielectrophoresis

Abstract: We show for the first time that a particle in a non-uniform AC electric field experiences a dielectrophoretic force arising from spatial non-uniformities of the magnitude and phase of the field interacting, respectively, with the in-phase and out-of-phase components of the induced dipole moment.

Measurement of large plane surface shapes by connecting small-aperture interferograms

Abstract: We propose a method to obtain the shape of a large plane surface by connecting phase distributions measured by a small-aperture interferometer. These separately measured phase distributions cannot be connected directly because the object will tilt or have vertical displacement during the measurements. To correct these errors, the measurements are made so that the adjacent interferograms have common areas, and these interferograms are connected to minimize the difference of the phase distributions in the common areas. A matrix equation is derived to obtain coefficients to correct tilt and vertical displacement, and the accuracy of connection increases in proportion to an exponent of 1.5 of the width of the common area.

Comparison of ultrashort-pulse frequency-resolved-optical-gating traces for three common beam geometries

Abstract: We recently introduced frequency-resolved optical gating (FROG), a technique for measuring the intensity and phase of an individual, arbitrary, ultrashort laser pulse. FROG can use almost any instantaneous optical nonlinearity, with the most common geometries being polarization gate, self-diffraction, and second-harmonic generation. The experimentally generated FROG trace is intuitive, visually appealing, and can yield quantitative information about the pulse parameters (such as temporal and spectral width and chirp). However, the qualitative and the quantitative features of the FROG trace depend strongly on the geometry used. We compare the FROG traces for several common ultrashort pulses for these three common geometries and, where possible, develop scaling rules that allow one to obtain quantitative information about the pulse directly from the experimental FROG trace. We illuminate the important features of the various FROG traces for transform-limited, linearly chirped, self-phase modulated, and nonlinearly chirped pulses, pulses with simultaneous linear chirp and self-phase modulation, and pulses with simultaneous linear chirp and cubic phase distortion, as well as double pulses, pulses with phase jumps, and pulses with complex intensity and phase substructure.

Comparing the fundamental frequencies of resolved and unresolved harmonics: Evidence for two pitch mechanisms?

Abstract: Four experiments measured sensitivity (d′) to differences in fundamental frequency (F0) between two simultaneously presented groups of frequency‐modulated harmonics. Each group was passed through a bandpass filter in either a LOW (125–625 Hz), MID (1375–1875 Hz), or HIGH (3900–5400 Hz) frequency region. In the first two experiments, a dynamic F0 difference (ΔF0) was created by introducing a 180° disparity between the frequency modulations imposed on the two groups. Experiment 1 measured sensitivity to such ΔF0’s between a MID group with a baseline F0 of 125 Hz and all components summed in sine phase, and a HIGH group, in four conditions. When the baseline F0 of the HIGH group was also 125 Hz, performance was good when its components were summed in sine phase and bad when they were in alternating phase. Conversely, when the HIGH F0 was 62.5 Hz, performance was better for alternating phase than for sine phase, consistent with alternating phase doubling the internal representation of HIGH group’s F0. Similar...

Power amplifier having nested amplitude modulation controller and phase modulation controller

Abstract: Transmitting signals containing amplitude modulated (AM) and phase modulation (PM) components requires a transmitter having AM and PM control loops (515, 517). The PM control loop provides phase modulation, frequency translation and phase predistortion for a transmitter. The phase predistortion/correction is accomplished by using an oscillator (505), thus, the amount of PA phase correction is essentially unlimited. Additionally, the PM control loop (517) is nested about a power amplifier (507) (PA), allowing the PM control loop (517) to correct for any distortion introduced by the PA (507).

Frequency-dependence of the action-perception cycle for postural control in a moving visual environment - relative phase dynamics

Abstract: When standing human subjects are exposed to a moving visual environment, the induced postural sway displays varying degrees of coherence with the visual information. In our experiment we varied the frequency of an oscillatory visual display and analysed the temporal relationship between visual motion and sway. We found that subjects maintain sizeable sway amplitudes even as temporal coherence with the display is lost. Postural sway tended to phase lead (for frequencies below 0.2 Hz) or phase lag (above 0.3 Hz). However, we also observed at a fixed frequency, highly variable phase relationships in which a preferred range of phase lags is prevalent, but phase jumps occur that return the system into the preferred range after phase has begun drifting out of the preferred regime. By comparing the results quantitatively with a dynamical model (the sine-circle map), we show that this effect can be understood as a form of relative coordination and arises through an instability of the dynamics of the action-perception cycle. Because such instabilities cannot arise in passively driven systems, we conclude that postural sway in this situation is actively generated as rhythmic movement which is coupled dynamically to the visual motion.

A minimal model of the single capacitor biphasic defibrillation waveform.

Abstract: UNLABELLED A quantitative model of the single capacitor biphasic defibrillation waveform is proposed. The primary hypothesis of this model is that the first phase leaves a residual charge on the membranes of the unsynchronized cells, which can then reinitiate fibrillation. The second phase diminishes this charge, reducing the potential for refibrillation. To suppress this potential refibrillation, a monophasic shock must be strong enough to synchronize a critical mass of nearly 100% of the myocytes. Since the biphasic waveform performs this protection function by removing the residual charge (with its second phase), its first phase may be of a lower strength than a monophasic shock of equivalent performance. A quantitative model was developed to calculate the residual membrane voltage, Vm, assuming a capacitive membrane being alternately charged and discharged by the first and second phases, respectively. It was further assumed that the amplitude of the first phase would be predicted by a minimum value plus a term proportional to Vm2. The model was evaluated on the pooled data of three relevant published studies comparing biphasic waveforms. The model explained 79% of the variance in the first phase amplitude and predicted optimal durations for various defibrillator capacitances and electrode resistances. Assuming a first phase of optimal duration, the optimal second phase duration appears to be about 2.5 msec for all capacitances and resistances now seen clinically. CONCLUSION The effectiveness of the single capacitor biphasic waveform may be explained by the second phase "burping" of the deleterious residual charge of the first phase that, in turn, reduces the synchronization requirement and the amplitude requirements of the first phase.

UV written reflection grating structures in photosensitive optical fibres using phase-shifted phase masks

Abstract: Reflection grating structures have been replicated in photosensitive optical fibres using phase shifted phase masks for the first time. The sensitivity of the narrow bandpass peak to alignment and position of the phase shift are reported.

Novel writing technique of long and highly reflective in-fibre gratings

Abstract: A novel method for writing fibre Bragg gratings is introduced. The method consists of translating a UV beam along the fibre through a phase mask. Because the phase mask and the fibre are held together, the phase of the interference pattern generated by the mask is not altered when the beam is translated along the mask. Gratings up to 1.5 cm long, limited by the mask dimensions, and with 98.5% reflection have been written in that way.

Radiation boundary conditions for dispersive waves

Abstract: The author develops radiation boundary conditions for the numerical modeling of dispersive waves. During the construction of such boundary conditions, the goal is to simulate the outward radiation of waves near an artificial computational boundary. The velocities of the outgoing waves are typically involved in processes of this nature. A central problem in the dispersive case is that two different types of velocities are present, phase velocity and group velocity, and each can vary with wavenumber and frequency. With the boundary conditions developed in this paper, the user needs to specify some parameters; in the cases that are emphasized here, the parameters can be interpreted in terms of phase velocities of waves that are absorbed exactly at the computational boundary. The amount of reflected error is a continuous function of the parameters, and the performance of the boundary conditions is not sensitive to the choice of parameters. Good performance is obtained in numerical tests involving data that ha...

A comparative evaluation of several algorithms for phase aberration correction

Abstract: A common framework is presented to classify several phase correction techniques. A subset of these techniques are evaluated through simulations which utilize 2-D phase aberration profiles measured in the breast. The techniques are compared based on their ability to reduce phase errors, stability, and sensitivity to noise and missing elements in the transducer array. Significant differences are observed in these measures of performance when the size and location of the aperture area used to generate a phase reference signal are varied. Techniques that utilize a small correction reference region are more susceptible to noise and missing elements than techniques which use larger reference regions. The algorithms encounter problems in 2-D phase correction when making the transition from one row to the next, due to the low interelement correlation at the transition points. It is shown that the magnitude of the interelement correlation is the key parameter governing phase correction performance. >

System for optically measuring the surface contour of a part using more fringe techniques

Abstract: An optical measuring system comprises an illumination arrangement including a light source, grating, and lens, and an image acquisition arrangement, including a lens, grating, and camera. A mechanical translation device moves the grating in a plane parallel to a reference surface to effect a phase shift of a projected image of the grating on the contoured surface to be measured. A second mechanical translation device moves the lens to effect a change in the contour interval. A first phase of the points on the contoured surface is taken, via a four-bucket algorithm, at a first contour interval. A second phase of the points is taken at a second contour interval. A controller, including a computer, determines a coarse measurement using the difference between the first and second phases. The controller further determines a fine measurement using either the first or second phase. The displacement or distance, relative to the reference plane, of each point is determined, via the controller, using the fine and coarse measurements.

Noise in an ac biased junction: Nonstationary Aharonov-Bohm effect.

Abstract: We study excess noise in a quantum conductor in the presence of constant voltage and alternating external field. Becasue of a two-particle interference effect caused by Fermi correlations the noise is sensitive to the phase of the time-dependent transmission amplitude. We compute spectral density and show that at T=0 the noise has singular dependence on the dc voltage V and the ac frequency \ensuremath{\Omega} with cusplike singularities at integer eV/\ensuremath{\Elzxh}\ensuremath{\Omega}. For a metallic loop with an alternating flux the phase sensitivity leads to an oscillating dependence of the strength of the cusps on the flux amplitude.

Phase errors in multi-shot echo planar imaging.

Abstract: Field inhomogeneity related phase errors in multi-shot echo planar imaging (EPI) are directly visualized and analyzed in the spatial frequency domain data or 'k-space'. The echo time shift (ETS) technique incrementally moves the position of the echo train and improves the phase error function by redistributing phase discontinuities away from the center of k-space.

Generation and detection of nonclassical field states by conditional measurements following two-photon resonant interactions

Abstract: A simple scheme is presented that allows the generation and detection of nonclassical states of the electromagnetic (em) field with controllable (predetermined) photon-number and phase distributions. It is based on the two-photon resonant interaction of a single em field mode in a high-Q cavity with initially excited atoms crossing the cavity sequentially (one at a time). The sequence duration should be much shorter than the cavity-mode lifetime. Nonclassical states of the field are generated conditionally, by selecting only those sequences wherein each atom is measured to be in the excited state after the interaction. The field distribution resulting from a sequence of N such measurements is peaked about 2N positions in the phase plane, which evolve sinusoidally as a function of the atomic transit times and are therefore simply controlled. When these peaks are chosen not to overlap, the field state constitutes a generalized Schr\"odinger cat. By choosing them to overlap, we can make parts of the field distribution strongly interfere, giving rise to decimation of the photon-number distribution. In particular, this process can prepare Fock states with controlled photon numbers. The generated phase distribution can be detected by monitoring the pattern of revivals in the excitation of a ``probe'' atom.

Subaperture autofocus for synthetic aperture radar

Abstract: A subaperture autofocus algorithm for synthetic aperture radar (SAR) partitions range-compressed phase-history data collected over a full aperture into equal-width subapertures. Application of a one-dimensional Fourier transform to each range bin converts each subaperture data set into a full-scene image (map). Any linear phase difference, or phase ramp, between a pair of subapertures expresses itself as cross-range drift in their maps. A traditional autofocus algorithm fits a polynomial to inferred equal-width phase ramps. If the true phase error function contains significant high-order components, then polynomial regression generates a poor estimate of the phase error function. Instead of filling a polynomial, we fit a sinusoidal function through the inferred phase ramps. An example with a degraded SAR image shows how a sinusoidal correction improves image quality. We compare lower bounds on mean squared error (MSE) for polynomial and sinusoidal parameterizations. Sinusoidal parameterization reduces MSE significantly for model orders greater than five. >

Time-dependent equations for phase differences and a collective mode in Josephson-coupled layered superconductors

Abstract: A time-dependent equation for the interlayer phase differences is derived for Josephson-coupled layered superconductors. It generalizes the sine-Gordon equation for the phase in a standard Josephson junction to the case of multilayer systems. With the help of this equation, the dispersion of a collective mode is found at low temperatures. In highly anisotropic systems the gap in the spectrum of this mode lies below the superconducting gap and is suppressed strongly by a magnetic field parallel to the layers. The effect of this mode on the dielectric function and specific heat is calculated.

Harmonic heat flow in isotropic layered systems and its use for thin film thermal conductivity measurements

Abstract: A theoretical model is presented describing harmonic heat flow in a two layer system heated by a modulated Gaussian laser beam Amplitude and phase of the modulated temperature rise in the layers, as well as in the backing substrate and adjacent atmosphere, are calculated by solving the three‐dimensional heat conduction equation with a source term including exponential absorption of the laser light in one or two layers Heat conduction is assumed to be isotropic throughout the system, however, a thermal contact resistance between the two layers can be taken into account Results are presented for single and double layer systems of gold and various dielectric thin film materials on glass substrates Amplitude and phase of the harmonic temperature variation are calculated either as a function of position in the sample system or at the surface as a function of the laser beam modulation frequency It is found that both amplitude and phase of the calculated temperature rise exhibit typical thin film features i

Ultraviolet generation at 266 nm in a novel organic nonlinear optical crystal: l‐pyrrolidone‐2‐carboxylic acid

Abstract: A novel organic nonlinear optical crystal, l‐pyrrolidone‐2‐carboxylic acid (l‐PCA), has been found to be phase matchable for second‐harmonic generation (SHG) and sum‐frequency mixing down to the UV range at room temperature Tunable radiation down to 266 nm was obtained The nonlinear coefficient d14 of l‐PCA for the fundamental wavelength of 532 nm was determined to be 032 pm/V The angular acceptance (the full width at half‐maximum) of the type II phase‐matched SHG has also been measured to be 303 mrad cm which agreed with the calculated value This value is much larger than that of the type I phase‐matched SHG in β‐BBO

Multilevel phase gratings for array illuminators

Abstract: We describe a variety of multilevel phase structures that can be used to generate Lohmann's array illuminators. We report several experimental verifications of the synthesis of such multilevel phase structures by using simple binary curves in a conventional optical processor.

Geometric autogeneration of "hard" phase-shift designs for VLSI

Abstract: A method implemented in a computer aided design (CAD) system automatically generates phase shifted mask designs for very large scale integrated (VLSI) chips from existing circuit design data. The system uses a series of basic geometric operations to design areas requiring phase assignment, resolve conflicting phase assignments, and eliminate unwanted phase edges. This process allows automatic generation of phase shift mask data from any circuit design that allows for phase shifting. Since the dimensional input for all geometric operations is directly linked to the design ground rules given to the circuit designers, any designable circuit layout can also be phase shifted with this algorithm. The autogeneration of phase shift patterns around an existing circuit design is broken down into four major tasks: 1. Define areas that need a phase assignment; 2. Make a first pass phase assignment unique to each critical feature and define "runs" of interrelated critical features; 3. Propagation phase assignment through the "runs"; and 4. Design trim features.

Phase-Shifting Holographic Interferometry

Abstract: Quantitative data can be extracted from holographic interference fringes using Phase-Measurement Interferometry (PMI) techniques. These techniques are used to determine the phase of the secondary interference fringe pattern and can be divided broadly into spatial and temporal techniques. Temporal techniques introduce a known phase shift between the object and reference beams in an interferometer and take a series of data over time as the phase shift is varied. Spatial techniques rely on encoding the phase shift information spatially in a single interferogram by using a large number of fringes as a carrier for the phase information which are generated by tilting the reference wavefront relative to the test wave front. Temporal techniques which process data in electronics are known as heterodyne techniques and were discussed in the last chapter. The processing of spatial phase-measurement data will be discussed in the next chapter. Temporal techniques which process the data analytically on a point-by-point basis will be the concentration of this chapter.

Blazed phase liquid crystal beam steering

Abstract: In this paper we describe a novel liquid crystal device for rapidly steering laser beams for high-power and large-aperture applications. The device consists of an array of optical phase modulators that contain a thin, active, liquid crystal layer sandwiched between two substrates. The unique aspect of the device is that each phase modulator can produce a linear (blazed) phase gradient, rather than the constant phase profile typical of other liquid crystal beam- steering devices. It is designed for use over a wide range of wavelengths and is particularly well suited for the deflection of short-wavelength laser beam. In this paper, we will describe device design, theoretical performance (diffraction efficiency and time response), and present experimental results of a device built to deflect a 1.064-micrometers laser beam.

Phase-step insensitive algorithms for phase-shifting interferometry

Abstract: Two novel phase extraction algorithms, in which step sizes need not be known or equal, are described. One algorithm requires a minimum of five interferograms from which intensity offset and intensity modulation are calculated at each pixel and the relative phase is calculated for all pixels with respect to a reference pixel. The other algorithm requires two phase stepping mechanisms and a minimum of ten interferograms. The intensity characteristics and reference phase step are calculated at each pixel and a previously published algorithm is used for the final phase calculation.

Magnetic disk storage apparatus with phase sync circuit having controllable response characteristic

Abstract: A magnetic disk storage apparatus provides with a phase locked loop or a phase sync circuit including a phase comparator, a charge pump, a filter and a voltage-controlled oscillator. The phase sync circuit includes a register which is connected to an information processing system and adapted to store therein the response characteristics of the phase comparator, the charge pump, the filter and the voltage-controlled oscillator as instructed from the information processing system. In this way, in accordance with the information on the response characteristics from the information processing system, the phase sync circuit is controlled thereby to assure a stable operation even in the case of the data transfer speed varying between inner and outer track such as occurs in a magnetic disk.

Apparatus and method for finding a signal emission source

Abstract: An apparatus and method are described for direction finding the source of emf emissions including a monotone signal by detecting the phase changes of the monotone signal during movement relative to the source. A software embodiment measures the amount of phase change.