Showing papers on "Phase (waves) published in 1989"

Landau band conductivity in a two-dimensional electron system modulated by an artificial one-dimensional superlattice potential.

Abstract: We investigate the magnetoresistance of a quasi-two-dimensional electron system subject to a one-dimensional superlattice potential created by field effect in gated AlGaAs/GaAs heterojunctions. At low temperatures this potential gives rise to a new type of magneto-resistance oscillations with a period governed by the ratio of the classical cyclotron diameter 2${\mathrm{R}}_{\mathrm{c}}$ to the superlattice period a. The oscillations are quantitatively explained in period, phase, and magnitude by the formation of Landau bands in the two-dimensional electron system under the influence of the periodic potential.

Phase aberration correction in medical ultrasound using speckle brightness as a quality factor

Abstract: Medical ultrasonic images are degraded by tissues with inhomogeneous acoustic velocities. The resulting phase aberration raises the off-peak response of the imaging system's point spread function (PSF), decreasing dynamic range. In extreme cases, multiple images of a single target are displayed. Phase aberration may become a limiting factor to image quality as ultrasonic frequency and aperture size are increased in order to improve spatial resolution. A method is proposed to correct for unknown phase aberration, which uses speckle brightness as a quality factor. The phase delays of a phased array transducer are modified, element by element, to maximize mean speckle brightness in a region of interest. The technique proposed is analogous to the correction technique used by Muller and Buffington [J. Opt. Soc. Am. 64 (9), 1200-1209 (1974)] to adaptively focus incoherent optical telescopes. The method is demonstrated using a computer model with several different simulated aberration profiles. With this model, mean speckle brightness is calculated using the two-dimensional PSF. Experiments have also been conducted in which speckle brightness is shown to increase as the phase delays of an ultrasonic scanner are modified in order to compensate for a rippled aberrating layer made of silicone rubber. The characteristics of the proposed method, and the possibility of employing it clinically to correct for unknown inhomogeneities in acoustic velocity, are discussed.

Interdependence of EEG signals: linear vs. nonlinear associations and the significance of time delays and phase shifts.

Abstract: To investigate the degree of interdependence of EEG signals, we have to use signal analysis methods. Three of these are described and their performance is compared: the cross-correlation (coherence and phase), the average amount of mutual information (AAMI) or the normalized AAMI, also called transmission coefficient T, and the correlation ratio h2 that is a general measure of nonlinear fit between any two signals. The three methods were applied to simulated and real signals in order to put in evidence how nonlinear relationships may affect differently these three measures of association. The nature of the interdependence between EEG signals is not characterized only by the degree of association, but also by the corresponding phase relationship. A basic question is whether such a phase shift can be interpreted as a transmission delay. However, a fundamental problem is that a phase shift may be difficult to interpret in terms of a biophysical model. A procedure is described in order to solve this problem. This involves computing the phase spectrum between the pair of signals, estimating the gain of the corresponding linear transfer function and the associated minimum phase. By subtracting the minimum phase from the phase spectrum, a corrected phase function can be obtained. From the slope of this phase function, a transmission delay can be estimated. This procedure is illustrated by applications to simulated and real EEG signals. It is demonstrated that from phase shifts we may estimate transmission delays between at least certain classes of EEG signals. In this way we can asses, unambiguously, how the transmission of information between different brain sites develops.

Circuit for driving ultrasonic transducer

Abstract: Circuit for use in a surgical operation including a phase lock loop having a voltage controlled oscillator, and a phase comparator for comparing a phase of a voltage of a driving signal and a phase of a signal representing the vibration phase of the ultrasonic transducer to derive a phase difference therebetween which is applied to the voltage controlled oscillator as a frequency control voltage such that the driving signal is phase-locked with a resonance frequency of the ultrasonic transducer, the improvement being characterized in that during a start period, a reference signal having a frequency which is increased monotonously is applied to the phase comparator such that the frequency of the driving signal is increased until the driving signal is phase-locked with the vibration phase of the ultrasonic transducer vibrating at the resonance frequency. After the phase-lock condition has been attained, the signal representing the vibration phase of the ultrasonic transducer is applied to the phase comparator instead of the reference signal.

Achromatic phase matching for second harmonic generation of femtosecond pulses

Abstract: A scheme is proposed by which the phase matching bandwidth can be increased by adjusting higher-order terms in frequency shift in the phase-matching condition. This is achieved by introducing a spectral angular dispersion so that the different spectral components propagate at their phase-matching angles. This is equivalent to canceling the group velocity mismatch. The equations describing the requirements on the angular dispersion are discussed and applied to the particular case of type I phase matching. A possible experimental setup is shown that would meet all the conditions required. >

Method and apparatus for simultaneously measuring a plurality of spectral wavelengths present in electromagnetic radiation

Abstract: An apparatus and method simultaneously measures a plurality of spectral wavelengths present in electromagnetic radiation. A modulatable birefringent optical element is employed to divide a polarized light beam into two components, thereby producing a phase difference in two resulting light beams such that the two beams can be made to interfere with one another when recombined, the interference pattern providing the wavelength information required for the analysis of the incident light. The interferometer thus created performs in a similar manner to a Michelson interferometer, but with no moving parts, and with a resolution dependent on the degree of phase shift introduced by the modulator.

Phase-coherent control of photocurrent directionality in semiconductors.

Abstract: We demonstrate that one can generate and control photocurrents in semiconductors, without bias voltage, through multiple-frequency phase-coherent laser excitation of donors.

Phase-gradient algorithm as an optimal estimator of the phase derivative.

Abstract: The phase-gradient algorithm represents a powerful new signal-processing technique with applications to aperture-synthesis imaging. These include, for example, synthetic-aperture-radar phase correction and stellar-image reconstruction. The algorithm combines redundant information present in the data to arrive at an estimate of the phase derivative. We show that the estimator is in fact a linear, minimum-variance estimator of the phase derivative.

Measurement of magnitude and phase of harmonics generated in nonlinear microwave two-ports

Abstract: A method for simultaneously measuring the magnitude and phase of the harmonics generated by a microwave two-port is reported. The two-port under test is driven with a sinusoidal microwave signal strong enough to force it into nonlinear operation. Its output harmonics are measured in the frequency domain with a setup that includes a vector network analyzer. For phase calibration at the harmonic frequencies, a millimeter-wave Schottky diode is used as a reference device. The system allows the measurement of harmonics with a phase accuracy of about +or-10 degrees at 20 GHz (referred to f/sub 1/=5 GHz). It can be built for any frequency ( >

Method and arrangement for flow rate measurement by means of ultrasonic waves

Abstract: For flow rate measurement by means of ultrasonic waves, on a measuring tube two ultrasonic transducers are arranged spaced apart from each other, one operating as transmitting transducer and the other as receiving transducer. For determining the flow rate the phase shift between the transmission signal and the reception signal is measured digitally in that the measuring time intervals corresponding to the phase shift the periods of a signal are counted with a counting frequency which is substantially greater than the frequency of the transmission signal. The start phase which the counting frequency signal has at the beginning of each counting operation with respect to the measuring time interval is varied from measuring time interval to measuring time interval in a measuring cycle including a plurality of successive measuring time intervals and the counts obtained in the course of the measuring cycle are evaluated to form a measured value with increased measurement resolution.

Bifurcation in the phase probability distribution of a highly squeezed state.

Abstract: We calculate the phase distribution of a highly squeezed state using both a definition of a phase eigenstate and the area-of-overlap principle. This probability curve undergoes a transition from a single- to a double-peaked distribution when we decrease the product of squeeze and displacement parameters.

Phase correction system for automatic focusing of synthetic aperture radar

Abstract: A phase gradient autofocus system for use in synthetic aperture imaging accurately compensates for arbitrary phase errors in each imaged frame by locating highlighted areas and determining the phase disturbance or image spread associated with each of these highlight areas. An estimate of the image spread for each highlighted area in a line in the case of one dimensional processing or in a sector, in the case of two-dimensional processing, is determined. The phase error is determined using phase gradient processing. The phase error is then removed from the uncorrected image and the process is iteratively performed to substantially eliminate phase errors which can degrade the image.

New phase shifting mask with self-aligned phase shifters for a quarter micron photolithography

Abstract: In order to markedly improve the resolution of photolithography without improving the resolution of exposure systems, the authors propose a simple and effective phase shifting mask technology. The mask has self-aligned phase shifters which do not require assistant patterns and/or complicated design of the phase shifter patterns, which are essential to the conventional phase shifting mask. The mask with a phase shifter size of 0.5 mu m reduces the width of photointensity to 60% of that without phase shifters, while keeping high contrasts. The authors have fabricated the phase shifting mask and obtained 0.2- mu m line resist patterns with a high-contrast resist profile by a KrF excimer laser stepper with resolution capability of 0.4 mu m. The proposed phase shifting mask method is extremely attractive for a future ULSI lithography tool in 256-Mb DRAM (dynamic RAM) and beyond. >

Linear readout of dynamic phase change in a fiber-optic homodyne interferometer.

Abstract: A new technique that provides linear measurement of dynamic phase change in a no-feedback, no-phase-bias fiberoptic interferometer is described. The phase measurement is unaffected by random changes in phase, source intensity, and fringe visibility. A minimum detectable phase shift of 0.1 rad has been measured for the configuration reported.

Comparison of experimental measurements and model predictions for radio‐frequency Ar and SF6 discharges

Abstract: Using a continuum model based on dc field data, the discharge physics of Ar and SF6 has been simulated for 13.5‐MHz excitation. In Ar, the ionization primarily occurs at the bulk–sheath interfaces and is temporally modulated with its maximum corresponding to the cathodic phase of the rf cycle. In SF6, the ionization is distributed throughout the bulk with a maximum occurring at the bulk–sheath interface and a temporal maximum occurring during the anodic phase. The plasma induced emission (PIE), which correlates with the predicted ionization rate, has been experimentally measured as a function of position and time and agrees well with the model predictions. These results explain the phase shift of the PIE between electronegative and electropositive discharges.

Phase sensitivity in atom-field interaction via coherent superposition

Abstract: Considering a system consisting of a two-level atom, initially prepared in a coherent superposition of upper and lower levels, interacting with a coherent state of the field, we show that the dynamics of the atom as well as the spectrum of the field are sensitive to the relative phase between the atomic dipole and the cavity field. It is shown that, for a certain choice of this phase, ``coherent trapping'' occurs in two-level atoms. In the case of spectra, for the same choice of the phase, instead of a three-peaked symmetric spectrum, we have an asymmetric two-peaked spectrum.

Correlation with a spatial light modulator having phase and amplitude cross coupling.

Abstract: In correlation filtering a spatial light modulator is traditionally modeled as affecting only the phase or only the amplitude of light. Usually, however, a single operating parameter affects both phase and amplitude. An integral constraint is developed that is a necessary condition for optimizing a correlation filter having single parameter coupling between phase and amplitude. The phase-only filter is shown to be a special case.

The Prediction of Two-Phase Turbulence and Phase Distribution Phenomena Using a K-κ Model

Abstract: A closed-loop analysis of phase distribution is presented. This analysis is based on the use of a two-fluid model. It is shown that the treatment of the turbulence distribution in the continuous phase is crucial if accurate predictions are to be made. A K-κ model is used to model turbulence in this study. Finally, recommendations are made as to how the state-of-the-art can be advanced.

A Ca optical frequency standard frequency stabilization by means of nonlinear Ramsey resonances

Abstract: An optical frequency standard based on frequency stabilization to Ramsey fringes observed in a Ca atomic beam is described. The important operation parameters influencing the uncertainty of such a standard are studied experimentally. Frequency shifts due to phase errors of separated traveling wave excitation can be strongly reduced by means of laser beam reversal. It is expected that the present fractional uncertainty of approximately 2.3×10−12 to realize the true line center can be reduced below the 10−14 level by applying atomic beam cooling and improved phase alignment.

Position detection for a brushless DC motor

Abstract: The present method and apparatus detects the position with an accuracy of π/m electrical radians (where m=the number of motor phases) within one electrical period, and provides enough information to be able to start in the correct direction with certainty. After starting the motor in an open loop mode of one or two steps, starting the rotor in the correct direction, the closed loop mode may be switched on, using a dynamic indirect position detection as is already well known in the technology. More specifically, the position at start is determined by the injection of short current pulses in different motor phases, each phase or pair of phases being energized first by a pulse of one polarity and of the opposite polarity. The sign of the difference between the induced voltages is detected. By performing a succession of these tests on different phases or pairs of phases of the standing motor, a table of results is established which clearly defines the position of the rotor relative to the motor phases. The same table then defines when polarity currents should be applied to each phase to reliably and certainly start the motor in the proper direction.

FFT-based aperture monitor for scanning phased arrays

Abstract: Method and means for monitoring the performance of a phase array antenna The antenna comprises an array of individual radiating element seach of which radiates a prescribed proportion of the energy to be transmitted thereby shaping such energy into a beam Individual phase shifters are associated with each radiating element, the phases of which control the direction of the beam pointing The method of the invention involves sampling the beam by means including a single receiver located along a fixed radial from the array The beam scans at a constant rate The samples are collected at non-uniform intervals of time during a beam scan The samples are, however, separated by equal increments of arcsine θ, where θ is the pointing angle of the beam The samples are analyzed by means including a Fourier transform to provide the value of the amplitude and phase of the signal radiated by each of the radiating elements Comparison of the amplitude and phase values for each radiating element as determined by analysis with design values for each element reveals any element or phase shifter which may be faulty The method can also be used to calibrate an antenna when it is first put into service

Phase-matched periodic electric-field-induced second-harmonic generation in optical fibers

Abstract: Phase-matched electric-field-induced second-harmonic generation is demonstrated in single-mode germania-doped silica fibers. A periodic second-order nonlinearity is induced by a simple interdigitated electrode structure, which can be rotated to permit phase matching between all propagating modes. The most efficient mode interaction between HE11ω and HE112ω is achieved at 1.064 μm by using a Q-switched Nd+3:YAG laser. In principle, phase matching at any propagating wavelength is possible. This technique could be applied to planar as well as cylindrical waveguides and can be used with many non-χ(2) materials. The asymmetry in the applied electric field enhances the optical-field overlaps between modes of dissimilar orders, and this is also demonstrated. A conversion efficiency of 4.0 × 10−4% has been obtained in unoptimized devices. Device optimization is also discussed.

Magnetic resonance imaging apparatus

Abstract: PURPOSE:To form an image stably and accurately, by removing noise, DC level shift and timing shift by providing a smoothing means, a DC correcting means and a phase correcting means. CONSTITUTION:The signal emitted from a spin excited by a series of pulse sequences is detected by a coil and amplified to perform phase sensitive detection to form a detection signal. A smoothing means 8 for performing the spike detection for detecting protruding noise, clip detection for detecting the clip state of a signal and the smoothing of the signal by moving averaging with respect to said detection signal, a DC correcting means 9 for detecting the DC level of the signal to correct the same and a phase correcting means 10 for calculating the unnecessary phase difference component of the signal and removing the phase difference calculated from the phase quantity of the whole are provided. Since the noise of a resonance signal derived from various factors of the system is removed and the signal is further smoothed and DC correction and phase correction are performed, the noise or artifact of a formed image is reduced and the stability of the system can be enhanced.