Showing papers on "Phase (waves) published in 1993"

Characterization of arbitrary femtosecond pulses using frequency-resolved optical gating

Abstract: The frequency-resolved optical gating (FROG) technique for characterizing and displaying arbitrary femtosecond pulses is presented. The method is simple, general, broadband, and does not require a reference pulse. Using virtually any instantaneous nonlinear-optical effect, FROG involves measuring the spectrum of the signal pulse as a function of the delay between two input pulses. The resulting trace of intensity versus frequency and delay is related to the pulse's spectrogram a visually intuitive transform containing time and frequency information. It is proven using phase retrieval concepts that the FROG trace yields the full intensity I(t) and phase phi (t) of an arbitrary ultrashort pulse with no physically significant ambiguities. FROG appears to have temporal resolution limited only by the response of the nonlinear medium. The method is demonstrated by using self-diffraction through the electronic Kerr effect in BK-7 glass and 620-nm, linearly chirped, approximately 200-fs pulses of a few microjoules. >

Temporal phase-unwrapping algorithm for automated interferogram analysis.

Abstract: A new algorithm is proposed for unwrapping interferometric phase maps. Existing algorithms search the two-dimensional spatial domain for 2π discontinuities: only one phase map is required, but phase errors can propagate outward from regions of high noise, corrupting the rest of the image. An alternative approach based on one-dimensional unwrapping along the time axis is proposed. It is applicable to an important subclass of interferometry applications, in which a sequence of incremental phase maps can be obtained leading up to the final phase-difference map of interest. A particular example is quasi-static deformation analysis. The main advantages are (i) it is inherently simple, (ii) phase errors are constrained within the high-noise regions, and (iii) phase maps containing global discontinuities are unwrapped correctly, provided the positions of the discontinuities remain fixed with time. The possibility of real-time phase unwrapping is also discussed.

Using phase retrieval to measure the intensity and phase of ultrashort pulses: frequency-resolved optical gating

Abstract: We recently introduced a new technique, frequency-resolved optical gating (FROG), for directly determining the full intensity I(t) and phase φ(t) of a single femtosecond pulse. By using almost any instantaneous nonlinear-optical interaction of two replicas of the ultrashort pulse to be measured, FROG involves measuring the spectrum of the signal pulse as a function of the delay between the replicas. The resulting trace of intensity versus frequency and delay yields an intuitive display of the pulse that is similar to the pulse spectrogram, except that the gate is a function of the pulse to be measured. The problem of inverting the FROG trace to obtain the pulse intensity and phase can also be considered a complex two-dimensional phase-retrieval problem. As a result, the FROG trace yields, in principle, an essentially unique pulse intensity and phase. We show that this is also the case in practice. We present an iterative-Fourier-transform algorithm for inverting the FROG trace. The algorithm is unusual in its use of a novel constraint: the mathematical form of the signal field. Without the use of a support constraint, the algorithm performs quite well in practice, even for pulses with serious phase distortions and for experimental data with noise, although it occasionally stagnates when pulses with large intensity fluctuations are used.

Observation of self-trapping of an optical beam due to the photorefractive effect.

Abstract: Recently, a new type of spatial soliton has been predicted to be observable in a photorefractive crystal.1,2 For example, consider an input laser beam that is diverging because of diffraction as it passes through a photorefractive material. Interference between the beam’s spatial-frequency components can be considered to write photorefractive gratings on the crystal. These gratings, or spatial variations in the index of refraction, have one component that is in phase and another that is 90° out of phase with their parent intensity pattern.

A new phase-shifterless beam-scanning technique using arrays of coupled oscillators

Abstract: A method for electronic beam scanning in linear arrays of antenna-coupled oscillators is introduced which eliminates the need for phase shifters. It is shown that a constant phase progression can be established by slightly detuning the peripheral array elements, while maintaining mutual synchronization. This unusual nonlinear behavior is explained using coupled Van der Pol equations. A stability analysis provides theoretical limitations on the achievable interelement phase shift. When the phase of the coupling is zero, the theory predicts an interelement phase shift that can be varied continuously over the range -90 degrees > Delta theta >

Nonlinear analysis of phase relationships in quasi-optical oscillator arrays

Abstract: A dynamic theory of coupled oscillators is developed and applied to the class of loosely coupled quasi-optical oscillator arrays. This theory permits the calculation of stable, steady-state phase relationships between the oscillators. The distribution of free-running frequencies and the coupling parameters are most important in determining the behavior of the arrays. It is found that free-running frequencies of the peripheral elements have the strongest influence on the steady-state phase relationships. The influence of randomness in the frequency distribution is considered for the case of broadside beamforming, establishing a critical value for the coupling strength in order to maintain mutual synchronization with a specified maximum beam deviation. Techniques for simplifying the calculation of phase relationships for some common coupling parameters are also developed. >

1/f NOISE IN A TRAFFIC MODEL

Abstract: One-dimensional traffic flow is simulated by a cellular-automaton-type discrete model. As we increase the car density, the model shows a phase transition between a jam phase and a non-jam phase. By adding random perturbations we found a 1/f power spectrum in the jam phase, whereas a white noise is observed in the non-jam phase.

Large nonlinear phase shifts in second-order nonlinear-optical processes.

Abstract: We show that processes such as second-harmonic generation and subsequent downconversion, and parametric mixing in general, can lead to large field-dependent phase shifts for the input beams under a variety of conditions.

Properties of photon density waves in multiple-scattering media

Abstract: Amplitude-modulated light launched into multiple-scattering media, e.g., tissue, results in the propagation of density waves of diffuse photons. Photon density wave characteristics in turn depend on modulation frequency (ω) and media optical properties. The damped spherical wave solutions to the homogeneous form of the diffusion equation suggest two distinct regimes of behavior: (1) a high-frequency dispersion regime where density wave phase velocity Vp has a ω dependence and (2) a low-frequency domain where Vp is frequency independent. Optical properties are determined for various tissue phantoms by fitting the recorded phase (ϕ) and modulation (m) response to simple relations for the appropriate regime. Our results indicate that reliable estimates of tissuelike optical properties can be obtained, particularly when multiple modulation frequencies are employed.

Manipulation of free shear flows using piezoelectric actuators

Abstract: An air jet emanating from a square conduit having an equivalent diameter of 4.34 cm and a centreline velocity of 4 m/s is forced using four resonantly driven piezoelectric actuators placed along the sides of the square exit. Excitation is effected via amplitude modulation of the resonant carrier waveform. The flow is normally receptive to time–harmonic excitation at the modulating frequency but not at the resonant frequency of the actuators. When the excitation amplitude is high enough, the excitation waveform is demodulated by a nonlinear process that is connected with the formation and coalescence of nominally spanwise vortices in the forced segments of the jet shear layer. As a result, the modulating and carrier wave trains undergo spatial amplification and attenuation, respectively, downstream of the exit plane. Strong instabilities of the jet column are excited when the jet is forced at phase relationships between actuators that correspond (to lowest order) to the azimuthal modes m = 0, ±1, ±2, and −1 of an axisymmetric flow. The streamwise velocity component is measured phase locked to the modulating signal in planes normal to the mean flow. Resonantly driving the actuators with different carrier amplitudes results in a distorted mean flow having a featureless spectrum that can be tailored to provide favourable conditions for the introduction and propagation of desirable low-frequency disturbances.

Method and apparatus for defibrillation using a multiphasic truncated exponential waveform

Abstract: A method and apparatus for converting an arrhythmia of a heart using a biphasic truncated exponential waveform wherein the first phase is of shorter duration than the second phase.

Dual capacitor biphasic defibrillator waveform generator employing selective connection of capacitors for each phase

Abstract: An implantable defibrillator/cardioverter which generates a biphasic defibrillation/cardioversion waveform including a pulse generator comprising two capacitors and a pair of switches for connecting the capacitors in parallel during a first phase and in series during a second phase. The first phase has a small "tilt" between the leading edge voltage and the trailing edge voltage. The second phase has a leading edge voltage which is approximately twice the trailing edge voltage of the first phase.

Gravitational wave tails and binary star systems

Abstract: Gravitational wave tails are produced by back-scattering of the outgoing gravitational radiation (emitted by an isolated system) off the curved spacetime associated with the total mass of the system. This paper investigates the spectral (or Fourier) decomposition of gravitational wave tails at large distances from the system, at the 1.5 post-Newtonian order in the wave field. It is shown that the effects of wave tails are (i) to increase the amplitude of the Fourier components of the (linear) waves by a factor linearly depending on the frequency, and (ii) to add to the phase of the waves a supplementary phase depending on the frequency as omega ln omega . The latter frequency-dependent phase introduces a new effect which should be observable in any radiation containing more than one frequency, for instance in the radiation emitted by a binary star system orbiting a Keplerian ellipse with non-zero eccentricity, or in the radiation emitted by an inspiralling (compact) binary star system. We propose in this paper to include the tail-induced effects (i) and (ii) in the matched filters of the future data analysis of inspiralling compact binary signals in laser interferometer gravity-wave detectors (at least in future, very sensitive, such detectors). In, this way, the filters will be highly correlated with the actual signal, and in particular will remain, as the frequency of the signal increases, in accurate phase with it. The contribution of the wave tail in the total gravitational energy emitted by a binary system is also calculated, and a numerical application to the binary pulsar PSR 1913+16 is presented. We find that the tail-induced relative correction in the orbital PTh of the pulsar is equal to +1.65*10-7 (too small to be observed).

Defibrillator Waveform Generator for Generating Waveform of Long Duration

Abstract: An implantable defibrillator/cardioverter which generates a biphasic waveform having a duration of at least 15-80 msec and a small "tilt" between the leading edge and trailing edge voltages of each phase. In one embodiment, two capacitors are connected in parallel to create an effectively large capacitance to generate a slowly decaying waveform. In another embodiment, a high capacitance bipolar capacitor is used, which capacitance is at least 200 microFarads.

Representation of the Equatorial Stratospheric Quasi-Biennial Oscillation in EOF Phase Space

Abstract: A 35-year record of monthly mean zonal wind data for the equatorial stratosphere is represented in terms of a vector (radius and phase angle) in a two-dimensional phase space defined by the normalized expansion coefficients of the two leading empirical orthogonal functions (E0Fs) of the vertical structure. The tip of the vector completes one nearly circular loop during each cycle of the quasi-biennial oscillation (QBO). Hence, its position and rate of progress along the orbit of the point provide a measure of the instantaneous amplitude and rate of phase progression of the QBO. Although the phase of the QBO bears little if any relation to calendar month, the rate of phase progression is strongly modulated by the first and second harmonics of the annual cycle, with a primary maximum in April/May, in agreement with previous studies based on the descent rates of easterly and westerly regimes. A simple linear prediction model is developed for the rate of phase progression, based on the phase of the Q...

Highly sensitive object location in tissue models with linear in-phase and anti-phase multi-element optical arrays in one and two dimensions.

Abstract: Based upon previous observations of low-frequency photon diffusion waves within highly scattering tissue, this paper explores the "near-field" phenomena of such waves of approximately 10-cm wavelength with 200-MHz phase modulation equipment. Multiple-element source arrays consist of laser diode sources modulated at 180 degrees out of phase with respect to the other sources. The diffusing waves originating from the out-of-phase sources give, in the midplane, an amplitude null and a sharp phase transition. These may be observed in a highly scattering intralipid medium simulating the breast or brain (0.5% intralipid), 3-5 cm from the transmitting laser diodes. In the plane containing the array, there is a high sensitivity for a small volume of a hidden absorber (indocyanine green) deep within a highly scattering medium; 20 pmol in a volume of 70 microliters can be detected. Two-dimensional arrays consisting of four or more elements in two orthogonal planes give sensitivity on both axes similar to the one-dimensional array. Measurements show that in the presence of a light-absorbing object, the amplitude null and the interference plane becomes a curved surface which is deflected toward the heterogeneity. The degree of deflection is related to the volume and the absorption characteristics of the heterogeneity and provides detection of the heterogeneity, and thereby may provide localization information for the detection of small tumors within the human breast, or stroke volumes, aneurysms, and tumors in the human brain.

all optical switching devices based on large nonlinear phase shifts from second harmonic generation

Abstract: We show that the large nonlinear phase shifts obtained from phase‐mismatched second harmonic generation can be used to implement all‐optical switching devices such as a nonlinear Mach–Zehnder interferometer and a nonlinear directional coupler.

Coherent control of terahertz charge oscillations in a coupled quantum well using phase-locked optical pulses.

Abstract: We demonstrate that we can enhance, weaken, and also phase shift terahertz (THz) radiation emitted by optically excited quantum beats in a coupled quantum well. The changes in the evolution of the THz radiation are induced by exciting the sample with a second optical pulse, phase locked with the first. We observe phase shifts in the emitted THz radiation of 330 fs for a change in the optical-pulse separation of only 1.33 fs

Sagnac experiment with electrons: Observation of the rotational phase shift of electron waves in vacuum

Abstract: A Sagnac experiment with electron waves in vacuum is reported. The phase shift caused by rotation of an electron biprism interferometer placed on a turntable has been measured. It was found to agree with prediction within error margins of about 30%. A compact ruggedized electron interferometer was used. It is based on a high-precision optical bench of 36-cm length. This interferometer is less sensitive by orders of magnitude to mechanical vibrations and electromagnetic stray fields than conventional electron interferometers. A beam of low-energy electrons (150\char21{}3000 eV) emitted by a field-emission electron source was used. For the most part, electrostatic electron optical components were employed. The magnified interference fringe pattern was intensified by a dual-stage multichannel-plate intensifier, recorded by a charge-coupled-device video camera, transmitted from the turntable to the laboratory system via a slip ring, and evaluated by an image-processing system. Both the rotation rate and the area enclosed between the two partial waves were varied (up to values of 0.5 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ and 3.9 ${\mathrm{mm}}^{2}$, respectively). Fringe shifts on the order of 5% of a fringe period were attained. Some historical aspects of the Sagnac effect as well as some aspects of its interpretation are mentioned. A brief informal discussion is included of the interpretation of the Sagnac phase shift as a geometric phase (``Berry phase'') caused by the global anholonomy of the local phase factor that is produced by the gauge field induced by rotation.

New compensating four-phase algorithm for phase-shift interferometry

Abstract: Phase-shift interferometry suffers from periodic systematic errors caused by erroneous reference phase adjustments and instabilities of the interferometer. A new method is described that uses only four interferograms and eliminates the errors caused by linear adjustment deviations of the reference phase or the mean phase in the interferometer. Test results confirm the theoretical predictions.

Phase statistics and decorrelation in SAR interferograms

Abstract: SAR interferometry requires a thorough phase control throughout the imaging process. The phase accuracy is affected by decorrelation effects between the individual surveys. The paper presents the probability density function of the interferometric phase and analyzes quantitatively the influence of decorrelation. In particular, phase aberrations typical for SAR processors are studied. The dependence of phase bias and variance on processor parameters is demonstrated. >

Synthetic aperture radar interferometry applied to ship-generated internal waves in the 1989 Loch Linnhe experiment

Abstract: Interferometer synthetic aperture radar images collected during the 1989 Loch Linnhe experiment showed mean Doppler variations across the phase of ship-generated internal waves that corresponded to “velocity” variations of the order of 50 to 100 cm/s. The in situ current data, however, showed surface currents associated with the internal wave features of the order of 5 to 10 cm/s and virtually ruled out the existence of surface currents as large as the interferometer-inferred values. In this paper we show how the pixel-to-pixel phase difference measured by the Jet Propulsion Laboratory interferometer is related to the mean Doppler frequency of the backscattered field. Model calculations are used to show how this frequency can sometimes change by a large amount, even when rather small surface currents are present. In particular, for winds blowing roughly across the internal wave features, as was the case for the interferometer runs in Loch Linnhe, computations based on our wave-current interaction and time dependent scattering models show that changes in the mean Doppler frequency corresponding to large velocities can, in fact, be produced from the much smaller measured surface currents. We show that the larger interferometer velocity estimates are essentially due to the different modulation strengths of the surface Bragg waves advancing toward and receding from the radar. Thus for these crosswind conditions, care must be taken in converting the phase differences measured by the interferometer to a surface current image. When the wind is aligned more nearly along the internal wave propagation direction, the mean Doppler shifts (and the phase differences) are dominated mostly by advection, and interferometer current estimates are more accurate. C band computations predict that if the antenna spacing is small enough so that the fields from the two antennas remain correlated, then the C band interferometer current estimates will be better than those at L band.

Atomic-position resolution by quadrature-field measurement

Abstract: An atom passing through a standing light field imparts a position-dependent phase shift to the field. By making a phase-sensitive measurement of the field, it is possible to resolve the atom's position to much less than the wavelength of the light. The field measurement results in the creation of virtual slits, and diffraction and interference phenomena may be observed. The phase measurements give welcher Weg information and enable ``quantum-eraser'' experiments to be realized.

Large nonlinear phase modulation in quasi-phase-matched KTP waveguides as a result of cascaded second-order processes

Abstract: Nonlinear spectral modulation and broadening have been observed in quasi-phase-matched KTP waveguides near 850 nm, indicative of self-phase modulation that is due to cascaded second-order processes. Numerical beam-propagation simulations indicate nonlinear peak phase shifts larger than π for 600 W of peak power in a 2.8-mm-long guide.

Measurement of the Aharonov-Casher phase in an atomic system.

Abstract: We describe a new experimental configuration suitable for observing the topological phase of Aharonov and Casher in atomic systems. Using this we have been able to show experimentally that the Aharonov-Casher phase is both independent of velocity and proportional to electric field and we have verified the predicted size of the effect with an accuracy of 4%.