Showing papers on "Phase (waves) published in 1995"
TL;DR: In this paper, a large data set of fundamental-mode Rayleigh and Love waveforms was used to obtain global phase-velocity maps in the period range 40-150 s.
Abstract: SUMMARY
Although much is known of the 3-D structure of the Earth, existing models do not make use of much that is known about the large structural perturbations near the surface. It has long been known, for example, that continental and oceanic crustal structures are quite different, and that these differences are evident in the dispersion of Love and Rayleigh waves sampling continental and oceanic paths. Such differences are largest at periods of less than about 100 s. Existing global models do not adequately account for such data, and make allowances for crustal structure in a very approximate way, owing to the incompleteness of information on the global distribution of crustal parameters. As a result, variations in, for example, crustal thickness translate themselves into model artefacts extending to great depth. This can be seen as one aspect of the imperfect resolution of the existing global models. In order to construct higher resolution models of the Earth's outer shell (0-200 km depth), it is necessary to gain more precise knowledge of near-surface structure by incorporating data that have sensitivity to the details of the depth distribution of heterogeneity near the surface. As a first step we analyse a large data set of fundamental-mode Rayleigh and Love waveforms to obtain global phase-velocity maps in the period range 40–150 s. Minor and major arc phase velocities have been determined from about 24 000 digital GDSN and GEOSCOPE seismograms recorded between 1980 and 1990. In order to make such measurements in an automatic way, we have developed a method, using non-linear waveform inversion, in which velocity and amplitude, as a function of frequency, are expanded in B-splines. The waveform data are inverted for the B-spline coefficients, with the application of an explicit smoothness constraint that protects against unwanted effects, such as those due to notches in the amplitude spectra, and avoids some of the problems associated with the phase ambiguity. The cost function (which is minimized in a least-squares sense) presents many local minima, and a good initial model is needed; this is derived by integration of group velocities.
The measurements made using this new technique are then used in a global inversion for phase-velocity distributions of Love and Rayleigh waves, expressed in terms of a spherical harmonic expansion. We show resulting phase-velocity maps up to degree and order 40. These maps are corrected for possible artefacts due to the truncation of the spherical harmonic expansion. We present a detailed resolution analysis which shows that global lateral resolution for surface-wave tomography is of the order of 2000 km. Love-wave phase velocities show a high correlation with known upper mantle structure at long periods and with crustal structure at shorter periods. Similarly, Rayleigh-wave phase velocities correlate well with known tectonic features, but show no clear crustal signature owing to their different sampling of the structure with depth.
314 citations
TL;DR: In this article, an iterative range filtering technique was proposed to separate the differential propagation phase and differential backscatter phase under a wider variety of conditions than is possible with a simple range filter.
Abstract: Copolar differential phase is composed of two components, namely, differential propagation phase and differential backscatter phase. To estimate specific differential phase KDP, these two phase components must first be separated when significant differential backscatter phase is present. This paper presents an iterative range filtering technique that can separate these phase components under a wider variety of conditions than is possible with a simple range filter. This technique may also be used when estimating hail signals from range profiles of dual-frequency reflectivity ratios.
252 citations
TL;DR: In this article, Fourier analysis of phase shift algorithms is used to predict measurement errors as a function of the frequency, the phase, and the amplitude of the vibrations in phase shift interferometry.
Abstract: Unexpected mechanical vibrations can significantly degrade the otherwise high accuracy of phase-shifting interferometry. Fourier analysis of phase-shift algorithms is shown to provide the analytical means of predicting measurement errors as a function of the frequency, the phase, and the amplitude of vibrations. The results of this analysis are concisely represented by a phase-error transfer function, which may be multiplied by the noise spectrum to predict the response of an interferometer to various forms of vibration. Analytical forms for the phase error are derived for several well-known algorithms, and the results are supported by numerical simulations and experiments with an interference microscope.
209 citations
TL;DR: Experimental tests confirmed that the system could identify and locate both scattering and absorbing inhomogeneities in a tissue-simulating phantom and accuracy of the initial estimates for the optical properties was confirmed.
Abstract: Diffuse optical tomography is an imaging technique whereby spatial maps of absorption and scattering coefficients are derived from the characteristics of multiply scattered light transmitted through the object. The system described here used four intensity-modulated light sources and measurements of the intensity and phase (relative to each source) at 16 or 20 detectors on the surface of a 10 cm diameter cylinder. An iterative Newton-Raphson algorithm was used to estimate the absorption and scattering coefficients at each pixel in a 17 x 17 array minimizing the difference between measured and calculated values of the intensity and phase at the measurement sites. Forward calculations of the intensity and phase were based on a multigrid finite-difference solution of the frequency domain diffusion equation. Numerical simulations were used to examine the resolution, contrast, and accuracy of the reconstructions as well as the effects of measurement noise, systematic uncertainties in source-detector location, and accuracy of the initial estimates for the optical properties. Experimental tests also confirmed that the system could identify and locate both scattering and absorbing inhomogeneities in a tissue-simulating phantom.
199 citations
TL;DR: The authors have developed a technique based on a solution of the Poisson equation to unwrap the phase in magnetic resonance (MR) phase images that is robust in the presence of noise and applies to the 3-point Dixon technique for water and fat separation.
Abstract: The authors have developed a technique based on a solution of the Poisson equation to unwrap the phase in magnetic resonance (MR) phase images. The method is based on the assumption that the magnitude of the inter-pixel phase change is less than /spl pi/ per pixel. Therefore, the authors obtain an estimate of the phase gradient by "wrapping" the gradient of the original phase image. The problem is then to obtain the absolute phase given the estimate of the phase gradient. The least-squares (LS) solution to this problem is shown to be a solution of the Poisson equation allowing the use of fast Poisson solvers. The absolute phase is then obtained by mapping the LS phase to the nearest multiple of 2 K from the measured phase. The proposed technique is evaluated using MR phase images and is proven to be robust in the presence of noise. An application of the proposed method to the 3-point Dixon technique for water and fat separation is demonstrated. >
183 citations
TL;DR: In this paper, simple cells were measured as a function of contrast and the frequency of sine-wave grating patterns in order to explore the effect of contrast on the spatial and temporal phase transfer functions and on the spatiotemporal receptive field.
Abstract: The responses of simple cells (recorded from within the striate visual cortex) were measured as a function of the contrast and the frequency of sine-wave grating patterns in order to explore the effect of contrast on the spatial and temporal phase transfer functions and on the spatiotemporal receptive field. In general, as the contrast increased, the phase of the response advanced by approximately 45 ms (approximately one-quarter of a cycle for frequencies near 5 Hz), although the exact value varied from cell to cell. The dynamics of this phase-advance were similar to the dynamics of the amplitude: the amplitude and the phase increased in an accelerating fashion at lower contrasts and then saturated at higher contrasts. Further, the gain for both the amplitude and the phase appeared to be governed by the magnitude of the contrast rather than the magnitude of the response. For the spatial phase transfer function, variations in contrast had little or no systematic effect; all of the phase responses clustered around a single straight line, with a common slope and intercept. This implies that the phase-advance was not due to a change in the spatial properties of the neuron; it also implies that the phase-advance was not systematically related to the magnitude of the response amplitude. On the other hand, for the temporal phase transfer function, the phase responses fell on five straight lines, related to the five steps in contrast. As the contrast increased, the phase responses advanced such that both the slope and the intercept were affected. This implies that the phase-advance was a result of contrast-induced changes in both the response latency and the shape/symmetry of the temporal receptive field.
172 citations
TL;DR: It is shown how a minimum-cost-matching graph-theory method can be used to find the set of cuts that has the global minimum of total cut length, in time approximately proportional to the square of the number of sources.
Abstract: An algorithm for unwrapping noisy phase maps by means of branch cuts has been proposed recently. These cuts join discontinuity sources that mark the beginning or end of a 2π phase discontinuity. After the placement of branch cuts, the unwrapped phase map is unique and independent of the unwrapping route. We show how a minimum-cost-matching graph-theory method can be used to find the set of cuts that has the global minimum of total cut length, in time approximately proportional to the square of the number of sources. The method enables one to unwrap unfiltered speckle-interferometry phase maps at higher source densities (0.1 sources pixel(-1)) than any previous branch-cut placement algorithm.
164 citations
Patent•
19 Jun 1995TL;DR: In this article, the phase assignment for a phase shift mask is determined by a technique which determines, without assignment conflict, the intersection of the gate pattern with the active gate pattern and divides the intersection into categories of stacks where a slightly different phase assignment rules is employed for the different stacks.
Abstract: A method of performing poly level lithography in manufacturing an integrated circuit using a phase shift mask in a step and repeat optical tool where the phase assignment for said phase shift mask is determined by a technique which determines, without assignment conflict, the Intersection of the gate pattern with the active gate pattern and which divides the Intersection into categories of stacks where a slightly different phase assignment rules is employed for the different stacks.
124 citations
TL;DR: A direct-search scheme is used to adjust gain and phase in one amplifier branch, guided by the measured output signal in adjacent channels, and the functionality of the method has been verified by means of simulation.
Abstract: The Letter describes a new method to correct for gain and phase imbalances in LINC transmitters. A direct-search scheme is used to adjust gain and phase in one amplifier branch, guided by the measured output signal in adjacent channels. The functionality of the method has been verified by means of simulation.
111 citations
TL;DR: This work measures the spectral phase of femtosecond optical pulses using a time-frequency analog of Young's doubleslit interference using a pair of narrow slits in an opaque sheet to measure the temporal phase of a family of beat frequencies obtained over a range of slit spacings.
Abstract: We measure the spectral phase of femtosecond optical pulses using a time-frequency analog of Young's doubleslit interference. A pair of narrow slits in an opaque sheet selects two spectral frequencies from the femtosecond pulse spectrum in a zero-dispersion pulse stretcher. Measurement of the temporal phase of a family of beat frequencies obtained over a range of slit spacings yields the desired spectral phase directly. We demonstrate this technique by accurately measuring the quadratic phase added to 80-fs optical pulses by a 6.5-cm block of BK-7 glass.
107 citations
TL;DR: In this paper, an extension to the peaklist optimization procedure is proposed, in which one overall phase refinement cycle consists of tangent expansion, E-map, peak search and elimination of peaks to achieve a maximum correlation coefficient between E o and E c.
Abstract: An extension to the peaklist optimization procedure is proposed, in which one overall phase refinement cycle consists of tangent expansion, E-map, peaksearch and elimination of peaks to achieve a maximum correlation coefficient between E o and E c This procedure appears to be able to solve large structures from random phases given data to atomic resolution The power of the method can be substantially increased by starting with slightly better than random phases, obtained for example from threefold Patterson vector superposition minimum functions or rotation searches using a fragment of known geometry These two sources of phase information require expansion of the data to the space group P1, which also appears to be a useful strategy when starting from random phases This real/reciprocal space recycling procedure was successful in solving two small known proteins and three unknown 200+-atom small-molecule structures An investigation of the influence of the resolution on the peaklist optimization algorithm shows that there is a marked deterioration in the effectiveness as the resolution of the data is truncated, the deterioration being particularly marked between 12 and 13 A
Patent•
10 Jul 1995
TL;DR: In this paper, a phase shifter was used to change the phase difference between the beams in the interferometer while data from both cameras (10,11) was taken by a frame grabber and saved in a computer.
Abstract: A method and system for providing interferometric measurements having reduced sensitivity to vibrations. An interference pattern from an interferometer (35) is amplitude split into first and second interferograms and imaged onto first and second detectors (10,11), respectively, such as CCD cameras (10,11). The two cameras (10,11) have different data acquisition rates, such as a fast:slow camera ratio of at least 2:1, and are so disposed such that the image fields substantially overlap and are synchronized with each other so that the frame integration periods are identical and overlap in time. During data acquisition a phase shifter (45) changes the phase difference between the beams in the interferometer (35) while data from both cameras (10,11) is taken by a frame grabber (15) and saved in a computer (25). During data analysis, the phase at each image point on each frame of the fast data set is calculated by the computer (25), with the phase difference between interferograms in the slow data set being determined from the phases derived from the fast data set and with the slow data set then being analyzed for phase at each image point using the measured phase increments.
TL;DR: In this article, the frequency dependence of phase velocity in all waveguide modes and the radial dependence of complex particle displacement amplitude in the two low-frequency modes (ET0 and ET1) were investigated.
Abstract: Axisymmetric propagation in a liquid‐filled elastic tube waveguide is considered, with emphasis on the two modes existing down to zero frequency. Previous work by Del Grosso is used as the basis of the theoretical description of modal phase velocities and particle displacement profiles in such waveguides. It is shown that certain combinations of material properties can produce a mode which, in the zero frequency limit, has plane‐wave motion in the liquid. Two examples of waveguides with very different wall compliance, aluminum/water and PVC/water, are studied numerically and experimentally. Numerical calculations are used to show the frequency dependence of phase velocity in all waveguide modes and the radial dependence of complex particle displacement amplitude in the two low‐frequency modes (ET0 and ET1). Contrasting behavior in the two waveguides is seen—approximate plane‐wave motion in the liquid occurs in the ET0 mode of the aluminum/water waveguide, but in the ET1 mode of the PVC/water waveguide. Experimental measurements of the frequency dependence of phase velocity in the ET0 and ET1 modes of these waveguides are also presented. Good agreement with numerical predictions is obtained in both cases, although experimental difficulties more severely limit the frequency range of measurement in the PVC/water waveguide.
TL;DR: In this paper, it was shown that the turbulence spectrum cannot be inferred from phase measurements when large amplitude fluctuations are observed at the receiver, but it can be recovered from imaging optics and wide angle phase sensitive receivers.
Abstract: Many anomalous features observed in reflectometer measurements of turbulent fluctuations in tokamak plasmas, such as loss of coherent reflection, large amplitude fluctuations, large angular divergence of the reflected waves, and correlation lengths of the order of the free‐space wavelength of the probe beam, can be explained by modeling the plasma fluctuations as a poloidally varying random phase grating located at the cutoff with a phase magnitude given by 1D geometric optics. A key result of this analysis is that the turbulence spectrum cannot be inferred from phase measurements when large amplitude fluctuations are observed at the receiver. However, the turbulence spectrum may still be recovered from phase measurements by use of imaging optics and wide angle phase sensitive receivers.
Patent•
3 Oct 1995TL;DR: In this article, a hybrid decision feedback equalizer (DFE) structure that approaches optimal DFE performance is described, which includes both an intersymbol interference DFE and a noise predictive DFE (NP-DFE).
Abstract: A hybrid type of decision feedback equalizer (DFE) structure that approaches optimal DFE performance is described. The hybrid DFE includes both an intersymbol interference DFE (ISI-DFE) and a noise predictive DFE (NP-DFE). The hybrid DFE structure is designed so that one form of tap values dominates over the other. This allows only the dominate tap values to be used by the transmitter during precoding and reduces the transmitter complexity. In particular, a hybrid DFE is designed so that the I(z) coefficient values dominate over the N(z) coefficient values, where the notation I(z) and N(z), as known in the art, represent the resulting coefficient values after adaptation of the ISI-DFE and the NP-DFE, respectively. A respective Tomlinson precoding scheme in the transmitter uses only the I(z) values. Since only the I(z) values are used in the precoder of the transmitter, the NP-DFE of the receiver is kept active during the communications phase. Conveniently, the continued use of the NP-DFE in the receiver allows the receiver to track small changes in the channel.
TL;DR: In this paper, the carrier frequency of the imaging pulse is higher in the center of the field of view than at the edges, which results in a pulse transmitted into the body whose frequency is highest in a center portion of the scan, and is reduced in a controlled fashion as the steering angle is increased to mitigate grating lobe artifacts.
Abstract: Method for scanning a field of view using a scan format, wherein the carrier frequency of the imaging pulse is higher in the center of the field of view than at the edges. The frequency variation can be accomplished on transmit by modulating appropriately delayed programmable initial waveform information samples with a programmable carrier frequency (T-102). This results in a pulse transmitted into the body whose frequency is highest in the center portion of the scan, and is reduced in a controlled fashion as the steering angle is increased in order to mitigate grating lobe artifacts. The technique preserves signal energy because modulation merely translates the signal in frequency substantially without modification of the pulse shape itself. The technique is also useful on receive, wherein demodulation to or near baseband followed by post-beamformation predetection remodulation (R-100) can correct for systematic scan-line-to-scan-line phase variations to ensure scan-line-to-scan-line phase coherency for subsequent coherent processing across scan lines or for coherent image formation (R-26) using the phase and amplitude information from multiple beams.
TL;DR: To present a comparison of the efficiency and the performance of the designed diffractive phase elements by use of the geometrical transformation technique, the Gerchberg-Saxton algorithm and the Yang-Gu algorithm for beam shaping, simulation computations are detailed for rotationally symmetric beam shaping.
Abstract: A design method based on the Yang-Gu algorithm [Appl. Opt. 33, 209 (1994)] is proposed for computing the phase distributions of an optical system composed of diffractive phase elements that achieve beam shaping with a high transfer efficiency in energy. Simulation computations are detailed for rotationally symmetric beam shaping in which a laser beam with a radially symmetric Gaussian intensity distribution is converted into a uniform beam with a circular region of support. To present a comparison of the efficiency and the performance of the designed diffractive phase elements by use of the geometrical transformation technique, the Gerchberg-Saxton algorithm and the Yang-Gu algorithm for beam shaping, we carry out in detail simulation calculations for a specific one-dimensional beam-shaping example.
Patent•
18 Jul 1995TL;DR: In this article, the phase of a clock whose phase is synchronized with the input digital signal is generated on the basis of the sampling output is used to control the clock's phase.
Abstract: A specific pattern in an inputted digital signal is detected (13) and the input digital signal is sampled and held (14) in accordance with such a detection output. A clock (15-17) whose phase is synchronized with the input digital signal is generated on the basis of the sampling output. With such a construction as mentioned above, the phase of the clock can be optimally controlled by a simple construction. The clock can be precisely extracted from the input digital signal, so that the operation of a circuit is also stable.
Patent•
8 Sep 1995TL;DR: Synchronous detection of fine position servo information within a partial response maximum likelihood (PRML) data channel Servo information (17) is recorded as a pair or series of fractional-track-width sinewave concurrent burst patterns producing an on-track phase generating a position error signal varying linearly about track centerline and at least one off track phase generating an off-track signal related to track boundary Head (26) generates on- track phase and offtrack phase analog signals that are gain normalized (40) and synchronously quantized (46) Mulfiplying by
Abstract: Synchronous detection of fine position servo information within a partial response maximum likelihood (PRML) data channel Servo information (17) is recorded as a pair or series of fractional-track-width sinewave concurrent burst patterns producing an on-track phase generating a position error signal varying linearly about track centerline and at least one off-track phase generating a position error signal related to track boundary Head (26) generates on-track phase and off-track phase analog signals that are gain normalized (40) and synchronously quantized (46) Mulfiplying by a normalization factor from a correlation signal generator (64) during servo sampling intervals provides normalized samples, integrating (66) on-track phase normalized samples provides on-track position error signal (82), and integrating (66) off-track phase normalized samples provides an off-track position error signal (88) A discrete matched filter is also used
26 May 1995
TL;DR: In this article, the authors derived from the rigorous solution of Maxwell's equations using the EMFlex and TEMPEST programs for an etched-quartz fabrications process, and the resulting diffracted orders were translated into an effective transmission and phase based on a thin mask approximation.
Abstract: Strategies for modifying both mask fabrication processes and design data for alternating phase-shifting masks to account for mask scattering phenomena are explored. Results were derived from the rigorous solution of Maxwell's equations using the EMFlex and TEMPEST programs for an etched-quartz fabrications process. By importing the resulting diffracted orders into VCIMAGE, full vector calculation of the aerial image from mask to wafer was obtained. From the rigorous mask simulations, the 0th and 1st diffracted orders were translated into an effective transmission and phase based on a thin-mask approximation. With this analysis technique, a 0.25 micrometers line-space grating for the baseline etched-quartz process (4X magnification) showed a transmission error of 7.2% and a phase error of 1.6 degree(s). In order to compensate for these errors, etch-back fabrication techniques, in which the quartz was recessed beneath the chrome, were evaluated to determine the extent to which the transmission and phase errors could be reduced. For the dual etch-back process typically in use today, a residual transmission error of approximately 0.5% could not be completely removed, even for etch-back depths greater than 200 nm. Correction of the phase errors was achieved by reducing the reactive-ion etch depth by 2-3 nm. Design manipulation, in which the 180 degree(s) opening was increased in size, required feature-dependent phase errors as large as 1 degree(s) were present.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
Patent•
9 Jan 1995TL;DR: In this paper, a flexure beam DMD array was used to allow analog phase modulation of reflected light and the resulting amplitude modulated wave is flicker free and does not need to be synchronized to optical image sensors.
Abstract: A phase-contrast DMD based image system 36 for projecting an amplitude and phase modulated image. A flexure beam DMD array 34 is used to allow analog phase modulation of reflected light 38. The phase modulation is converted to amplitude modulation by the phase-contrast imaging optics including a phase plate 42. The resulting amplitude modulated wave is flicker-free and does not need to be synchronized to optical image sensors.
TL;DR: A simple wedged shear plate interferometer is proposed for in situ measurement of twisted nematic liquid-crystal television's phase modulation and operating curve and the coupled-mode modulation holograms yield good-quality reconstructed images with the zero-order spot reduced to a minimum.
Abstract: Commercial twisted nematic liquid-crystal television provides coupled phase and amplitude modulation. We propose a simple wedged shear plate interferometer for in situ measurement of its phase modulation and operating curve. For a given operating curve, the coupled-mode modulation holograms are designed with an iterative method. We adjust the operating curve by rotating the polarizer and analyzer to obtain the optimal operating curve. The phase-mostly holograms yield good-quality reconstructed images with the zero-order spot reduced to a minimum. Experimental results are shown.
Patent•
28 Nov 1995TL;DR: In this article, an apparatus for removing a low-frequency interference signal from the phase sample is presented. But the demodulator may produce degraded performance when the inventive apparatus is applied to a radio signal that does not have lowfrequency interference, and alternatively selecting, in response thereto, either the filtered or original phase sample to be supplied to further demodulation circuitry.
Abstract: In a digital FM receiver having a demodulator that receives a radio signal and generates therefrom a phase sample whose behavior determines an output of the demodulator, an apparatus for removing a low-frequency interference signal from the phase sample converts the phase sample into a frequency sample which is then high-pass filtered to produce a filtered frequency sample. The filtered frequency sample is then converted into a filtered phase sample, which may be further demodulated in accordance with known techniques. Converting the phase sample into a frequency sample may be performed by a first order difference circuit. Converting the filtered frequency sample into the filtered phase sample may be performed by an integrator. Because the demodulator may produce degraded performance when the inventive apparatus is applied to a radio signal that does not have low-frequency interference, in another aspect of the invention the demodulator is further provided with a circuit for detecting the presence of the low-frequency interference signal in the radio signal, and alternatively selecting, in response thereto, either the filtered or original phase sample to be supplied to further demodulation circuitry. Several methods of detecting the presence of the low-frequency interference signal are disclosed.
TL;DR: In this article, the phase and amplitude of the reflectance of a thin film is recovered by adding to the unknown layered structure a known ferromagnetic layer, which is magnetized by an external magnetic field in a direction lying in the plane of the layer and subsequently perpendicular to it.
Abstract: Neutron reflectometry can determine unambiguously the chemical depth profile of a thin film if both phase and amplitude of the reflectance are known. The recovery of the phase information is achieved by adding to the unknown layered structure a known ferromagnetic layer. The ferromagnetic layer is magnetized by an external magnetic field in a direction lying in the plane of the layer and subsequently perpendicular to it. The neutrons are polarized either parallel or opposite to the magnetic field. In this way three measurements can be made, with different (and known) scattering-length densities of the ferromagnetic layer. The reflectivity obtained from each measurement can be represented by a circle in the (complex) reflectance plane. The intersections of these circles provide the reflectance.
Patent•
10 May 1995TL;DR: In this paper, a direct conversion receiver for a radio system has an antenna and blocking filter connected to an amplifier, and the input signal is split and mixed with an in-phase signal and a quadrature phase signal generated by an oscillator in a mixer circuit.
Abstract: A direct conversion receiver for a radio system has an antenna and blocking filter connected to an amplifier. The input signal is split and mixed with an in-phase signal and a quadrature phase signal generated by an oscillator in a mixer circuit. An output from each mixer circuit is applied to a low pass filter and to an input of a limiting circuit. The output from each low pass filter is applied to respective circuits, a first of which is arranged to sum the in-phase and quadrature phase signals, and the second of which is arranged to subtract the in-phase and quadrature phase signals to generate a respective output signal having an axis intermediate the in-phase and quadrature phase signal axes. These signals, together with the in-phase and quadrature phase signals are passed through a limiting circuit respectively, to a decoder circuit for recovering the data. The output of the limiting circuits represent signals quantized to eight possible phase states separated by 45°. In an alternative embodiment a ratiometric combiner may be used instead of the summation circuits, particularly for π/4-DQPSK modulation, where the phase excursion per symbol is ±45° or ±135° depending upon the bit pair combination. In the latter embodiment a minimum of eight axes are generated giving sixteen sectors and 22.5° phase resolution.
TL;DR: The integral equation-asymptotic phase (IE-AP) method as discussed by the authors uses an asymptotic solution to predict the relatively rapid phase dependence of the unknown current distribution, leaving a slowly varying residual function that can be represented by a coarse density of unknowns.
Abstract: A hybrid-procedure called the integral equation-asymptotic phase (IE-AP) method is investigated for scattering from perfectly conducting cylinders of arbitrary cross-section shape. The IE-AP approach employs an asymptotic solution to predict the relatively rapid phase dependence of the unknown current distribution, to leave a slowly varying residual function that can be represented by a coarse density of unknowns. In the present investigation, the current density appearing within the combined-field integral equation is replaced by the product of a rapidly varying phase function obtained from the physical optics current and a residual function. The resulting equation is discretized by the method of moments, using subsectional quadratic polynomial basis functions defined on curved cells to represent the residual function. Results show that the required density of unknowns can often be as few as one per wavelength on average without a significant loss of accuracy in the computed current density, even for scatterers with corners. >
Patent•
2 Mar 1995
TL;DR: In this article, a phase shifter is used to measure the damping of a resonator and the phase in the phase shifters is alternately set to two different values, and the difference between the frequencies corresponding to these two phase values is a measure for damping.
Abstract: A resonator (1) is vibrating close to its resonance frequency. The vibration is excited by one or a first transducer (2) connected to an oscillator (11). The vibration is measured by the transducer (2) or a second transducer (3) and stabilised by a phase-locked feedback loop comprising a phase sensitive detector (33), a feedback controller (36), a phase shifter (17) and means (15) to evaluate the measured frequencies. In order to measure the damping of the resonator (1), the phase in the phase shifter (17) is alternately set to two different values. The difference between the frequencies corresponding to these two phase values is a measure for the damping of the system. One or more switches (19, 28, 27) and a gate generator (16) make sure that excitation and measurement do not occur at the same time. Thereby, any cross-talk between driving and sensing transducers is completely eliminated. Also, a single transducer can be used to both excite and measure the vibration. Thereby, a very compact transducer set-up can be implemented and the precision and range of the measurement is improved.
Patent•
19 Apr 1995TL;DR: In this article, a phase shifter for shifting the phase of a radio frequency F (F) signal was proposed. But the phase shift was not maintained substantially. And the phase shifters were not suitable for high frequency applications.
Abstract: A ferroelectric phase shifter for shifting the phase of a radio frequency F) signal. The phase shifter includes a conductor line, a ground plane and a ferroelectric element between the conductor line and the ground plane to form a microstrip circuit through which the RF signal propagates. The ferroelectric element has a dielectric constant that can be varied as a function of a DC voltage applied to the ferroelectric element wherein the speed of the RF signal propagating through the ferroelectric element is a function of the dielectric constant. The phase shifter further includes a DC voltage source connected across the conductor line and the ground plane. The DC voltage source applies a variable DC voltage to the ferroelectric element in response to a control signal thereby to vary the dielectric constant of the ferroelectric element. The phase shifter further includes a controller circuit operating at a test frequency having a synchronous detector for detecting changes in the dielectric constant of the ferroelectric element. The controller circuit provides the control signal to the DC voltage source to vary the applied DC voltage as a function of the detected changes. In this manner, changes in the dielectric constant over time are compensated for and the phase shift is maintained substantially constant.
TL;DR: It is shown that quantitative measurements are possible for low-intensity object beams (8 µW/cm(2)) and a large observed area and the temporal interferogram erasure is studied and is found weakly perturbative for the measured phase.
Abstract: A bismuth silicon oxide crystal is used in the diffusion regime as a dynamic recording medium in a real-time holographic interferometer based on anisotropic self-diffraction. This device is connected with an interferogram-analysis method that uses the phase-shifting technique for quantitative measurement of diffusive-reflecting object deformations. In addition to the usual error sources in phase shifting, the temporal interferogram erasure is studied and is found weakly perturbative for the measured phase. It is shown that quantitative measurements are possible for low-intensity object beams (8 μW/cm2) and a large observed area. A practical situation of defect monitoring in a composite structure is presented.
TL;DR: Fast Fourier beam-propagation methods for simulating the roles of internal refractive effects and external propagation from nonlinear media are introduced in this article, which is applied to model picosecond Z-scan measurements for the induced absorber, the dye Chloro-Aluminum Phthalocyanine, at 532 nm.
Abstract: Fast Fourier beam–propagation methods (BPM’s) for simulating the roles of internal refractive effects and external propagation from nonlinear media are introduced. These techniques are applied to model picosecond Z-scan measurements for the induced absorber, the dye Chloro-Aluminum Phthalocyanine, at 532 nm. Within the thin-sample approximation an incident Gaussian beam is taken to experience a change in phase profile on propagation through the medium but remains of Gaussian amplitude profile. Outside this approximation one must determine both the phase and the amplitude profiles at the sample exit face that are due to the influence of nonlinear refraction (and nonlinear absorption) on the beam propagating through the medium. The BPM technique allows this to be achieved efficiently, and the external propagation technique enables a single discrete fast Fourier transform to be used to describe the subsequent external propagation of the non-Gaussian-shaped beams. The analysis is especially useful for such self-enhancing nonlinearities as one would wish to exploit in optical limiting.