Showing papers on "Phase (waves) published in 1991"

Fluorescence-detected wave packet interferometry: Time resolved molecular spectroscopy with sequences of femtosecond phase-locked pulses

Abstract: We introduce a novel spectroscopic technique which utilizes a two‐pulse sequence of femtosecond duration phase‐locked optical laser pulses to resonantly excite vibronic transitions of a molecule. In contrast with other ultrafast pump–probe methods, in this experiment a definite optical phase angle between the pulses is maintained while varying the interpulse delay with interferometric precision. For the cases of in‐phase, in‐quadrature, and out‐of‐phase pulse pairs, respectively, the optical delay is controlled to positions that are integer, integer plus one quarter, and integer plus one half multiples of the wavelength of a selected Fourier component. In analogy with a double slit optical interference experiment, the two the two pulse experiments reported herein involve the preparation and quantum interference of two nuclear wave packet amplitudes state of a molecule.These experiments are designed to be sensitive to the total phase evolution of the wave packet prepared by the initial pulse. The direct de...

Multipoint dixon technique for water and fat proton and susceptibility imaging

Abstract: Extensions to a previously described three-point Dixon magnetic resonance imaging technique are presented that use alternative water/fat phase-encoding strategies. The technique is generalized to phase encoding of (-theta, O, theta) or (O, theta, 2 theta) radians, and the signal-to-noise ratio (S/N) performance is evaluated. It was found that a theta of 2 pi/3 radians has optimal S/N but that a theta of pi radians is a good compromise and that phase encoding of (O, pi, 2 pi) radians offer an advantage over the previous method, which used (-pi, O, pi) increments, in that a T2' (intravoxel susceptibility dephasing) image may be obtained in addition to the usual water, fat, and Bo images. A new four-point method with phase encoding of (O, pi, 2 pi, 3 pi) radians that can also provide a measure of the spectral width of the fat resonance is suggested. The disadvantages of the method are the extra imaging time and low S/N efficiency.

Phase-based disparity measurement

Abstract: The measurement of image disparity is a fundamental precursor to binocular depth estimation. Recently, Jenkin and Jepson (in Computational Processes in Human Vision (V. Pylyshyn, Ed.), Ablex, New Jersey, 1988) and Sanger (Biol. Cybernet, 59, 1988 , 405–418) described promising methods based on the output phase behavior of bandpass Gabor filters. Here we discuss further justification for such techniques based on the stability of bandpass phase behavior as a function of typical distortions that exist between left and right views. In addition, despite this general stability, we show that phase signals are occasionally very sensitive to spatial position and to variations in scale, in which cases incorrect measurements occur. We find that the primary cause for this instability is the existence of singularities in phase signals. With the aid of the local frequency of the filter output (provided by the phase derivative) and the local amplitude information, the regions of phase instability near the singularities are detected so that potentially incorrect measurements can be identified. In addition, we show how the local frequency can be used away from the singularity neighbourhoods to improve the accuracy of the disparity estimates. Some experimental results are reported.

Optical heterodyne profilometry

Abstract: A noncontact optical technique for the measurement of surface profile is described, which has a height sensitivity of the order of 1 A. It is based on a common path heterodyne interferometer in which two orthogonally polarized beams of slightly different frequency are focused on the surface to be measured. One focal point acts as a reference as the other point circularly scans the surface. The phase of the beat frequency of the interfering return beams is directly proportional to the surface height. The results of a surface measurement include graphical displays of the surface profile, autocovariance function, spectral density function, stability, and repeatability. Comparison with other instruments is also discussed.

Source location in thin plates using cross-correlation

Abstract: In this paper an alternative method to first threshold crossing for acoustic emission (AE) source location is presented. For wave propagation in dispersive media, the accuracy of source location can be improved by locating corresponding phase points on the transducer outputs to determine the difference in arrival times. The phase point location was done by cross‐correlating the transducer outputs with a single frequency cosine wave modulated by a Gaussian pulse. Experiments were performed using a lead break as the AE source on the surface of an aluminum plate. Due to the plate geometry and source orientation, the wave produced was highly dispersive. Although this wave was unsuitable for first threshold crossing techniques, the time differences needed for triangulation could be determined using the cross‐correlation technique.

Measurement of small phase shifts using a single-mode optical-fiber interferometer

Abstract: Periodic phase changes in the 10(-6) -rad region have been induced and detected in a single-mode all-fiber Mach- Zehnder interferometer by stretching the fiber with a piezoelectric cylinder driven at frequencies between 40 and 10(4) Hz.

Multifrequency behavioral patterns and the phase attractive circle map

Abstract: With relative phase as a collective variable or order parameter, phase attractive dynamics can capture the temporally coherent behavior of a large number of different experimental systems. We present results from multifrequency coordination experiments in humans showing: a) that phase attraction persists especially for low order frequency ratios; b) that short-term jumps from one phase relation to another occur within a frequency ratio; c) that the most stable frequency-ratios are low order; and d) that transitions frequently occur from higher order (e.g. 5?2, 4?3) to lower order (2?1, 1?1) frequency ratios. We study a modified sine circle map with built-in phase attractive dynamics that qualitatively accounts for these results. In this phase-attractive map, patterns arise from competition between external driving and intrinsic phase attractive dynamics. The relative strength of extrinsic and intrinsic parameters determines the width of Arnol'd tongues, thereby influencing the delay or acceleration of irregular behavior. Behavioral complexity is inversely proportional to tongue width, thus accounting for the relative difficulty of performing different multifrequency behaviors and why "errors" in such behavior are often seen to occur.

Observation of a nonclassical Berry's phase for the photon.

Abstract: Coincidence detection of photon pairs produced in parametric fluorescence, in conjunction with a Michelson interferometer in which one member of each pair acquired a geometrical phase due to a cycle in polarization states, has allowed the first observation of Berry's phase at the single-photon level. We have verified that each interfering photon was essentially in an n=1 Fock state, by means of a beam splitter following the interferometer combined with a triple-coincidence technique. The results can be interpreted in terms of a nonlocal collapse of the wave function.

Linear approximation for measurement errors in phase shifting interferometry

Abstract: This paper shows how measurement errors in phase shifting interferometry (PSI) can be described to a high degree of accuracy in a linear approximation. System error sources considered here are light source instability, imperfect reference phase shifting, mechanical vibrations, nonlinearity of the detector, and quantization of the detector signal. The measurement inaccuracies resulting from these errors are calculated in linear approximation for several formulas commonly used for PSI. The results are presented in tables for easy calculation of the measurement error magnitudes for known system errors. In addition, this paper discusses the measurement error reduction which can be achieved by choosing an appropriate phase calculation formula.

Phase modulation spectroscopic system

Abstract: A phase modulated spectroscopy (PMS) system for detecting a pathophysiological condition in a subject includes two laser diodes emitting radiation at wavelengths of about 754-760 nm and 816-840 nm. These two laser diodes are modulated sinusoidally at about 220 MHz. The signals from each laser diode are brought to the subject by a bifurcated plastic optical fiber of 1 mm diameter. After transmission through the subject, the signals are detected by a Hamamatsu R928 photomultiplier (PMT), which generates two experimental signals. The experimental signals are compared to a reference signal in a phase detector. The respective phase shifts experienced by the signals are combined to form sum and difference signals, which are correlated to a pathophysiological condition.

Two-phase optical inspection method and apparatus for defect detection

Abstract: A method and apparatus for inspecting the surface of articles, such as chips and wafers, for defects, includes a first phase of optically examining the complete surface of the article inspected by scanning its complete surface at a relatively high speed and with an optical beam of relatively small diameter, particularly a laser beam, and a second phase of optically examining with a relatively high spatial resolution only the suspected locations for the presence or absence of a defect therein.

Electron Image Plane Off-axis Holography of Atomic Structures

Abstract: Fundamentally, Transmission Electron Microscopy is wave optics, i.e. the object structure is encoded in the amplitude and phase of the emergent electron wave, which is then transferred into the image wave by the subsequent lenses. In conventional microscopy, only the intensity, i.e. amplitude squared, is recorded, whereas the phase is lost. This gives rise to severe loss of information about the object structure. Therefore, electron holography, which furnishes amplitude and phase separately, offers the only path to a complete understanding of the object structure including atomic positions and species, as well as intrinsic electric and magnetic fields. By means of an electron biprism the image wave is superimposed on a coherent plane reference wave giving rise to an interference pattern called a “hologram”. The position of the electron biprism in the optical path has to be optimized with respect to both intended lateral resolution and field of view. By means of light optical or numerical image processing, the amplitude and phase of the image wave are reconstructed from the hologram. For interpretation of the image wave in terms of the object, one has to understand in detail not only the interaction with the object but also the transfer of the object wave into the image wave. The conversion of amplitude and phase of the object wave into amplitude and phase of the image wave by means of lenses is described; the aberrations of these lenses affect this process and holography allows us to correct these aberrations during the reconstruction of the wave from the recorded hologram. Performance, as measured by achievable resolution and field of view, is finally limited by the degree of lateral coherence, i.e., by the brightness of the electron source.

Circuit for reducing distortion produced by an r.f. power amplifier

Abstract: A high frequency SSB radio transmitter has an envelope amplitude modulator for varying the envelope of an rf signal source based on an error signal from envelope detectors detecting the envelope of the input and output waveform It also has a first phase modulator in a main feedback loop for varying the phase of the input waveform based on differences detected in a phase detector between the instantaneous phase of the input and output rf signal To overcome the problem of spurious outputs from the phase detector resulting from the cross-over points of the SSB waveform when there are carrier breaks and other problems, a subsidiary, phase lock loop feeds a signal derived from the error signal to a second phase modulator to tend to hold the inputs to the phase detector in such a phase relationship that the output is zero To cope with large phase shift errors between the input waveform and the output which result when the power amplifier changes frequency or temperature variations eg at the antenna, a broadband phase shifting network is brought into operation when a dual voltage comparator senses that the signal fed to the second modulator passes a value corresponding to its extremes of adjustment

Stabilization of oscillator phase using a fiber-optic delay-line

Abstract: The authors describe an electronic oscillator phase stabilizer based on a fiber-optic delay-line. Excellent agreement between calculated and first experimental results was obtained for this fiber-optic stabilized electronic oscillator (FOSO) at 100 MHz and 7.8 GHz. Expressions are derived for the FOSO phase noise performance and optimum fiber length, based on the noise contributions of the various components. The predicted performance of the FOSO expected with improved system components is also calculated. The FOSO is tunable over an extremely wide frequency range, with phase noise performance rivaling that of good resonant-cavity stabilized oscillators, which are constrained to narrowband operation. >

Spatial-carrier phase-shifting technique of fringe pattern analysis

Abstract: The spatial-carrier phase shifting (SCPS) technique that is related to both Fourier transform and phase-shifting methods of calculating the phase is analyzed. In this technique, a large amount of tilt is introduced into an interferogram, so that the phase difference between successive pixels equals (pi) /2. Three successive pixels are used to recover the phase using the adequate standard phase shifting formula. The detailed error analysis is presented. Examples of application of this method in conventional and moire interferometry, as well as in optical triangulation, are shown. As the SCPS technique requires a single frame only for phase calculation, it is capable of making high-speed measurements or performing testing in adverse environments.

Apparatus and method for simultaneous measurement of film thickness and surface height variation for film-substrate sample

Abstract: The relative height variation and the thickness of a film of an object are simultaneously measured. A first interference pattern is produced for a calibration surface at a first wavelength and detected. Intensities of the first interference pattern are measured and used to compute a first group of phase values for each pixel. Intensity values of a point of the calibration samples are measured and used to compute a corresponding phase. A second interference pattern for the calibration surface is produced at a second wavelength and detected. Intensities of the second interference pattern are measured and used to compute a second group of phase values for each pixel. Intensity values of the point of the calibration surface are measured and used to compute a corresponding phase. A value for the surface height change Δh is computed by obtaining a linear combination of the corresponding phase values of the data groups. A drift value is computer by obtaining the difference between the first and second phase values at the point. A corrected surface height change value is computed for the calibration sample by adding the two. This procedure is repeated for a test surface to obtain its corrected surface height change. A corrected relative surface height change value ∴h' is computed by subtracting the corrected surface height change value of the calibration surface from the corrected surface height change value for the test surface for each pixel. Computed Δh' values are compared to corresponding values from a mathematical model to obtain a best fit value of film thickness t for each pixel and the height error due to phase change is subtracted.

Complete phase-strain model for structurally embedded interferometric optical fiber sensors

Abstract: The relation between a three-dimensional state of strain and the optical phase retardation in a single mode optical fiber is formalized by drawing together classical three-dimensional crystal optics and classical waveguide theory. Neumann's strain optic relations are described in a form usable in optical fiber sensor design. These relations are then combined with weakly guiding fiber theory to develop an integral which relates the optical phase shift in a structurally embedded interferometric optical fiber strain sensor to the induced three-dimensional strain field. This process leads to a previously undisclosed, additional waveguide dispersion term which contributes on the same order to the total strain induced phase retardation as does the term derived by Butter and Hocker (1978). Still, however, waveguide dispersion effects are found to be negligibly small, even in three- dimensional loading. Butter and Hocker's equation and the complete phase-strain model developed herein can give very similar results...

Analysis of a method of phase measurement of ultrashort pulses in the frequency domain

Abstract: A detailed analysis of a method of phase measurement of ultrashort light pulses is performed. It is shown that the output pulse of a zero dispersion compressor with a mask consisting of a narrow slit is a wide pulse delayed an amount equal to the phase derivative of the field spectrum with respect to frequency. State criteria to determine whether the approximations are valid are presented. The method is not affected by artifacts coming from diffraction effects in the slit or by the fact that only correlations are measured. The effects of the errors in positioning the elements are considered and minimized in the construction of the experimental apparatus. A novel technique for measurement of the derivative of the cross correlation is described, and it is used to locate the delayed output pulse. >

Nonlinear-interferometric generation of number-phase-correlated fermion states.

Abstract: We propose a nonlinear-interferometric scheme for generating two fermion states correlated in particle number or in wave phase. This scheme can be used for surpassing either the standard quantum limit of the number-partition noise or that of the interferometric phase sensitivity, up to a factor of the order of (total particle number${)}^{1/3}$. A possible experimental scheme in a mesoscopic system is discussed.

Interferometric Measurements of Phase Singularities in the Output of a Visible Laser

Abstract: We report the use of a simple interferometric technique which allows direct identification of phase singularities in laser fields. Phase singularities are observed in families of optical patterns formed via cooperative frequency mode locking in a continuous single longitudinal mode Na2 ring laser. The interferometric technique complements a previously reported astigmatic imaging method, and is superior in that it can be used to elucidate the structure of the higher order stationary patterns.

Upper bound of the diffraction efficiency of diffractive phase elements

Abstract: An upper bound of the diffraction efficiency of diffractive elements that only influence the phase of the illumination wave is derived. The derivation only utilizes the specification of the desired diffraction pattern. It is independent of the technique to design and fabricate the diffractive element. The theory is based on the transmittance approach to describe the effect of the element on the illumination wave.

Two-beam coupling measurements of grating phase in a photorefractive polymer

Abstract: We investigate in detail the grating properties of a recently discovered organic photorefractive polymer as a function of electric field by using two-beam coupling. Using an oblique-incidence geometry, we present measurements of index and absorption grating phase relative to the intensity interference pattern as well as measurements of the amplitude of both the absorption and index gratings. We find that in low electric fields a weak in-phase grating (possibly photochromic) and the low electro-optic coefficient prevent observation of a phase-shifted photorefractive grating. However, in moderate-to-high electric fields a much stronger photorefractive index grating with a phase shift approaching 90° dominates. The presence of an index grating with a 90° phase shift at high fields provides strong evidence that these polymers are indeed photorefractive.

Phase based vector modulator

Abstract: Vector modulation is effected by summing output signals from two phase modulators. The lack of amplitude modulation on the signals when summed permits non-linear processing of the signals, such as PLL frequency translation. In one embodiment, the phase modulators comprise fractional-N phase locked loops modulated with digital data words. In another, the phase modulators comprise direct digital synthesizers.

Direct digital FM waveform generator for radar systems

Abstract: A linear FM waveform generator employs direct digital synthesis and includes a digital linear FM signal generator, a digital-analog converter, and a sampler means. The linear FM waveform generator includes a frequency slope register coupled to a first stage integrator which in turn is coupled to a second stage integrator, all of which operate off a system clock. The output signal from the second stage integrator represents phase as a quadratic function of time. A stored sine look-up table is referenced to generate an output digital signal representing amplitude as a function of time. The output digital signal is converted to an analog linear FM waveform which in turn is coupled to a sampler that includes a step recovery diode and a fast switch. The sampler aperture is sufficiently small that the sampler output linear FM waveform includes base and higher harmonic spectra with amplitude attenuation small enough to enable a third or higher harmonic spectra to be used for generation of the output linear FM waveform. The sampler output is filtered to pass a higher harmonic spectra which is directly coupled to output multiplication and filter circuitry.