Showing papers on "Phase (waves) published in 1992"

The theoretical basis for the determination of optical pathlengths in tissue: temporal and frequency analysis

Abstract: A concise theoretical treatment is developed for the calculation of mean time, differential pathlength, phase shift, modulation depth and integrated intensity of measurements of light intensity as a function of time on the surface of tissue, resulting from either the input of picosecond light pulses, or radio frequency-modulated light. The treatment uses the Green's function of the diffusion approximation to the radiative transfer equation, and develops this and its Fourier transform in a variety of geometries. Detailed comparisons are made of several of these parameters in several geometries, and their relation to experimentally measured clinical data. The limitations of the use of phase measurements is discussed.

Self-focusing and self-defocusing by cascaded second-order effects in KTP.

Abstract: We monitor the induced phase change produced by a cascaded χ(2):χ(2) process in KTP near the phase-matching angle on a picosecond 1.06-μm-wavelength beam using the Z-scan technique. This nonlinear refraction is observed to change sign as the crystal is rotated through the phase-match angle in accordance with theory. This theory predicts the maximum small-signal effective nonlinear refractive index of n2eff≅±2×10−14 cm2/W (±1 × 10−11 esu) for an angle detuning of ±5° from phase match for this 1-mm-thick crystal with a measured deff of 3.1 pm/V For a fixed phase mismatch, this n2eff scales linearly with length and as deff2; however, for the maximum n2eff the nonlinear phase distortion becomes sublinear with irradiance for phase shifts near π/4.

The geometric phase

Abstract: When a quantum system evolves so that it returns to its initial physical state, it acquires a 'memory' of this motion in the form of a geometric phase in the wavefunction. This phase has observable consequences in a wide range of physical systems, and its presence has now been convincingly demonstrated, for example, in optical and nuclear magnetic resonance experiments.

The Phase Rotor Filter

Abstract: We report creation by photolithography techniques of the phase rotor filter, an optical element whose complex transmittance depends in a linear fashion on the azimuth angle. Relationships are given that describe the scalar diffraction of coherent light by the rotor filter. The results of the numerical simulation and experiments are discussed.

Design and assessment of symmetrical phase-shifting algorithms

Abstract: Conventional phase-shifting algorithms based on a least-squares estimate use N samples over an incomplete period of the sampled waveform We introduce a class of phase-shifting algorithms having N + 1 samples symmetrically disposed over one full period of the sampled waveform Fourier analysis techniques are used to derive these algorithms and modify them to improve their performance in the presence of phase-shift errors The algorithms can be used in phase measurement systems having periodic, but not necessarily sinusoidal, waveforms

Investigation and compensation of the nonlinearity of heterodyne interferometers

Abstract: The non-linearity of heterodyne interferometers is mainly influenced by the non-orthogonality and ellipticity of the linearly polarized partial beams of the laser and by the limited extinction capability of the polarizing beam splitters. The phase errors at the interferometer output can be detected by measuring the phase difference between the two orthogonal directions in the output beam. This arrangement can be used to check an interferometer set-up for causes of non-linearity and to eliminate them by adjustment, replacement etc. When the phases of the two orthogonal output signals are measured separately and compared with the reference signal, and when the arithmetical mean value is formed, except for a small residual error the phase value obtained is free from non-linearities.

Double through‐transmission bulk wave method for ultrasonic phase velocity measurement and determination of elastic constants of composite materials

Abstract: This paper describes a modification of a nondestructive ultrasonic method for measurements of the phase velocity of bulk waves in arbitrary directions in generally anisotropic materials. In the conventional method the through‐transmission technique is used for velocity measurements at a specified angle of incidence. When this angle is changed by rotation of the sample, the transmitted beam changes position, and so the position of the receiving transducer must be changed. This leads to experimental difficulties and loss of precision. In the double‐transmission technique, the ultrasonic wave is reflected from reflector plates behind the sample and returns via the same path to the same position on the transmitter/receiver working in pulse‐echo mode, which eliminates the necessity of readjusting the receiver position. It is also shown that for arbitrary direction of measurement in anisotropic materials, time‐delay measurements give phase velocity regardless of the angle of deviation between phase and group ve...

Frequency-domain interferometer for femtosecond time-resolved phase spectroscopy

Abstract: A new femtosecond time-resolved interferometer was developed that utilizes interference fringes in the frequency domain to obtain simultaneously difference phase spectra (DPS) and difference transmission spectra with a multichannel spectrometer. For the first time to our knowledge, transient oscillations were observed in DPS and the spectral shift of a probe pulse was time resolved together with the rise in DPS, which is clear evidence for induced phase modulation in absorptive materials.

Simultaneous phase-shift interferometer

Abstract: Interferometric testing of large optics over long path lengths has been hampered by vibration in the test set-up. The precision of phase measuring interferometry has not been able to provide measurements in vibration environments due to the time required to perform the required phase shift between multiple images of the interferogram. The simultaneous phase shift interferometer (SPSI) has eliminated effects of vibration from phase measurements by creating four separate phase shifted interferograms simultaneously, viewed with four CCD cameras. The CCD cameras provide electronic shutter exposure control which effectively 'freezes' the interference patterns producing high contrast interferograms even with severe vibration. Polarization optics are used to maintain the appropriate phase relationships between the four interferograms. Four separate synchronized video digitizers are used to digitize the interferograms to a maximum resolution of 380 by 240 pixels by 8 bits per pixel. The phase at each pixel in the interferogram is calculated by a PC/486 based microcomputer which also provides complete analysis and graphics of the measurement. Averaging of multiple measurements to reduce the effects of air turbulence is done automatically.

Phase step measurement and variable step algorithms in phase-shifting interferometry

Abstract: A novel method, utilizing Lissajous figures and ellipse fitting, of calculating the phase difference between interferograms obtained from a phase shifting interferometer is described. Intensity bias and intensity modulation of interferograms are also calculated using this technique. Two new phase extraction algorithms are presented which use intensity and phase step information to calculate a surface from interferograms acquired with uneven phase steps. One algorithm allows surface phase to be calculated from two interferograms. Preliminary results from the Lissajous figure technique and the presented algorithms are discussed.

Refractive indices of orthorhombic KNbO 3 . II. Phase-matching configurations for nonlinear-optical interactions

Abstract: Based on temperature-dependent refractive-index data of orthorhombic KNbO3 [ J. Opt. Soc. Am. B9, 380 ( 1992)], we analyze various nonlinear-optical second-order interactions. Phase-matching curves as a function of wavelength, propagation direction, and temperature are given. Both type I and type II phase matching are found to be possible in KNbO3 for second-harmonic generation and for sum-frequency generation and optical parametric oscillation. To demonstrate the precision of the calculations based on the new refractive-index data, we compare experimentally measured values with calculated values of acceptance angles, phase-matching angles, and phase-matching temperatures for some specific cases. A temperature-tuned optical parametric oscillator employing KNbO3 is demonstrated. Good agreement is found between calculated and experimental results.

Quantum limits on precision measurements of phase.

Abstract: We investigate the quantum mechanical bound to how precisely we can determine a phase shift given only a constraint on the mean total number of photons available. By considering how quickly one can gain information from data analysis, we derive the sensitivity achievable (in principle) for measurements involving even highly non-Gaussian noise. Using these results we calculate the sensitivity of several recent proposals for precision phase measurement, and show that no proposal to date beats the sensitivity believed achievable by squeezed state interferometry.

Phase-locked loop

Abstract: A phase-locked loop according to the present invention includes first and second frequency demultipliers, and a plurality of phase/frequency detectors. The first and second frequency demultipliers divide frequency of first and second signals by a predetermined number. Each of the plurality of phase/frequency detectors compares two signals supplied from the first and second frequency demultipliers. In accordance with a comparison result of the plurality of phase/frequency detectors, phase of the second signal is adjusted to be synchronized with the first signal.

Scattering of p-modes by a sunspot

Abstract: The acoustic scattering properties of a large sunspot are determined from a Fourier-Hankel decomposition of p-mode amplitudes as measured from a 68-hr subset of a larger set of observations made at the South Pole in 1988. It is shown that significant improvement in the measurement of p-mode scattering amplitudes results from the increased temporal frequency resolution provided by these data. Scattering phase shifts are unambiguously determined for the first time, and the dependence of the p-mode phase shift and absorption with wavenumber and frequency is presented.

Phase and gain error control system for use in an I/Q direct conversion receiver

Abstract: In a system for controlling for gain and phase errors due to mismatches between signal channels in direct conversion receiver having a pair of signal channels carrying I and Q baseband component signals which are in quadrature, new I′ and Q′ signals are generated (116,117) which may be viewed as analogs of the I and Q baseband components but which are related to twice the phase angle defined by the original I and Q baseboard components signal components. Phase angles ϑ A and ϑ B are determined (120,122) based on the original I and Q baseband components signals and the new I′ and Q′ signals. An error signal ϑ E is then formed (123) from these signals which can be analyzed (124) to determine the gain and phase errors g and p affecting the I and Q baseband components. These components can then be adjusted (101,102,103) to correct for gain and phase errors g and p in order to compensate for hardware mismatches between the signal channels in the receiver.

Two-dimensional phase unwrapping using a minimum spanning tree algorithm

Abstract: Phase unwrapping refers to the determination of phase from modulo 2 pi data, some of which may not be reliable. In 2D, this is equivalent to confining the support of the phase function to one or more arbitrarily shaped regions. A phase unwrapping algorithm is presented which works for 2D data known only within a set of nonconnected regions with possibly nonconvex boundaries. The algorithm includes the following steps: segmentation to identify connectivity, phase unwrapping within each segment using a Taylor series expansion, phase unwrapping between disconnected segments along an optimum path, and filling of phase information voids. The optimum path for intersegment unwrapping is determined by a minimum spanning tree algorithm. Although the algorithm is applicable to any 2D data, the main application addressed is magnetic resonance imaging (MRI) where phase maps are useful. >

Photoplethysmographics using phase-division multiplexing

Abstract: First and second carrier signals, distinguishable by phase, are respectively applied to infrared and red energy emitters. A detector receives the sum of the energy after modulation at the infrared and red wavelengths. The signal received by the detector is then demultiplexed into its original first and second components, thereby allowing determining of both the infrared and red modulation components. The first and second carrier signals may comprise time-varying periodic signals with identical frequency and frequency spectra, such as a pair of sine waves which are indistinguishable except by phase and amplitude. A 90° phase difference is preferred, but any phase other than 0 or an integer multiple of 180° is workable. A carrier frequency which avoids excessive interference from ambient light is preferred.

One-dimensional modeling of the wavelength sensitivity, localization, and correlation in reflectometry measurements of plasma fluctuations

Abstract: The reflection of electromagnetic waves from a plasma cutoff layer has been used to examine properties of density fluctuations in fusion plasmas. In this paper an exact one‐dimensional model is used to show the relation between changes in the phase of the reflected wave and the location, magnitude, and correlation properties of density fluctuations. For long‐wavelength density perturbations the reflected phase can be simply related to the amplitude of fluctuating density and the density scale length, Ln, near the cutoff layer. However, the phase response falls substantially as the fluctuation wavelength approaches the free space wavelength of the reflected wave, λ0, and the location of the maximum response moves out in front of the cutoff layer following the wave matching condition kΛ= 2k ≊ 2η(x)k0. Thus, a measurement of the reflected phase is strongly weighted to and localized for phenomena whose wavelength is longer than the characteristic scale (λ20Ln)1/3. Because of this weighting and because the region of maximum response moves away from the cutoff layer for short‐wavelength fluctuations, there is also a limitation in any estimate of the density correlation length from the reflected phase. The correlation of phases between several different probe frequencies can be used to estimate a density correlation length no less than about four times the free space probe wavelength.

Apparatus and method for phase resolved fluorescence lifetimes of independent and varying amplitude pulses

Abstract: A flow cytometer measures phase fluorescence lifetimes by the phase shift of a reference signal and an emission from a particle or cell in a flow chamber (17). An acoustic optic modulator (13) modulates laser light with a sinusoidal wave of a predetermined frequency to excite particles or cells. Detectors respond to emissions of individual particles or cells in the form of an output signal pulse at the predetermined frequency. The output signal pulse is divided into equal pulses with each at the modulation frequency, the same amplitude and fidelity and amplitude. One part of the divided pulse is stripped of its envelope to pass the width thereof and out of band components are rejected. A variable amplifier passes a portion of the pulse above a preset level. A delay line sets a central part of the signal at a predetermined point in time. A circuit limits the attenuated one part. A double balance mixer multiplies and the relates the limited signal with a reference signal to determine the phase shift. The next step attenuates the signal to a preset level so it can be limited, then filtered to remove harmonics above the modulation frequency and an envelope below. A double balance mixer multiplies the limited signal with a reference signal and produces a multiple signal and determining the relative phase shift of the emissions.

InP/GaInAsP guided-wave phase modulators based on carrier-induced effects: theory and experiment

Abstract: An overview of the different contributions due to carrier-induced effects that appear in InP/GaInAsP guided-wave phase modulators is given. The authors review and calculate the band-filling effect in such devices and point out optimized structures. The fabrication of phase modulators and directional coupler switches based on a GaInAsP/InP heterostructure is described. These devices exhibit modulation characteristics in good agreement with the calculations having a phase modulation efficiency as high as 11 degrees /V mm and low optical losses. >

Physically reasonable analytic expression for the single-scattering phase function: errata.

Abstract: An analytic phase function that reduces to the Rayleigh phase function for the scattering of unpolarized light is presented and compared with the traditional Henyey–Greenstein phase function. Comparisons between the proposed phase function and the phase function for three of Deirmendjian’s polydispersions are shown and applications to radiative transfer are demonstrated.

Detection and Printability of Shifter Defects in Phase-Shifting Masks II. Defocus Characteristics

Abstract: One drawback of phase-shifting lithography lies in the high printability of shifter defects. A transparent object on masks (shifter defects) produces lead or lag of the optical phase, which causes an unique printability with respect to focus level. We investigated the printability of shifter defects on phase-shifting masks in out-of-focus conditions. Two types of shifter defects related to the phase angle were investigated: (a) a phase error which is a shifter layer having an incorrect thickness and (b) a phase angle defect which is a defect having a phase angle other than 180°. Phase error causes asymmetrical pattern profiles. It was found that the illumination coherence factor (σ) has a strong influence on the allowance of the shifter thickness (phase error). The effect of phase angle defects located in a large clear area and in small features was investigated. The phase angle defects have an optimum focal plane other than the best focal plane. A defect with a low phase shift located in fine patterns has high printability. In order to establish a mask fabrication process for production-worthy phase-shifting masks, further investigation in the detection of low phase angle shifter defects is necessary.

Custom laser resonators using graded-phase mirrors

Abstract: The authors derive a simple algorithm for designing a stable graded-phase-mirror resonator. First, the desired output beam profile of the fundamental mode is propagated into the laser medium. The wavefront is then extracted and serves to determine the appropriate phase profile of the mirror. The diffractional analysis of the resonator using this graded-phase mirror indicates a very low loss for the fundamental mode with a very large discrimination of higher modes. Practical design parameters such as the geometric factor, the Fresnel numbers, and phase profile perturbations are discussed. The authors conclude that this type of resonator can increase significantly the mode volume and favor the single-mode operation of laser systems relying on a stable resonator geometry. >

Direct‐space methods in phase extension and phase determination. I. Low‐density elimination

Abstract: A density-modification procedure for phase extension and refinement is described which replaces all density less than one-fifth of the height of a light-atom peak by zero. Its effectiveness is demonstrated by applications to a small and a medium-size protein structure. With high-resolution data, for the small protein, it is possible to extend and refine from 3 to 1 A with a mean phase error less than 30°. Successful phase extension from 4 A is also possible. In general it is shown that phase extension to high resolution gives less error than extension to lower resolution. It has also been shown that for a small protein it is possible to obtain an ab initio solution of the structure by refining from a complete set of random phases for all reflexions.

Phase Fluorometry Using a Continuously Modulated Laser Diode.

Abstract: The authors report phase fluorometrlc measurements using directly amplitude-modulated 670- and 791-nm laser diodes as the excitation source. Fluorescence lifetimes ranging from 220 ps to 1.7 ns were measured using a commercial phase fluorometer. It was found that higher amounts of radio frequency power were required to achieve comparable degrees of modulation when the dc current on the diode was Increased and that rf power above a certain level created distortion In the light output. Results are shown for common cyanlne laser dyes emitting at wavelengths to 840 nm.

Demodulator for digitally modulated wave

Abstract: A QPSK modulated wave is inputted to an in-phase detector and an orthogonal detector. The detected components are converted to substantially a base band, and each component is digital-converted by A/D converters. Each digital component is spectrum-shaped by digital LPFs. The outputs of digital LPFs are inputted to a complex multiplier and calculated by use of first and second reproduction carriers and expressed as first and second calculation outputs, and inputted to a phase detector. The phase detector obtains phase difference data between the phase expressed by the first and second calculation outputs and a predetermined phase and quadrant data of the phase. The phase difference data is used for a PLL. The phase difference data is is inputted to a frequency error detection circuit detecting a frequency error. The frequency error output is smoothed by a filter of an AFC loop, and used as a control signal controlling the oscillation frequency of the local oscillation unit. If the frequency error is large, the frequency error detection circuit controls the AFC loop to be in an operation state and the PLL loop and the PLL loop to be in a fixed state. If the frequency error is small, the frequency error detection circuit contains the control state of the AFC loop and switches the PLL loop to be the operation state.