Showing papers on "Wavefront published in 1979"

Influence of cardiac fiber orientation on wavefront voltage, conduction velocity, and tissue resistivity in the dog.

Abstract: When the canine epicardium is stimulated, the spread of epicardial excitation is 2.4 times faster parallel to the long axes of the cardiac fibers than perpendicular to them. Likewise, gross tissue resistivity is lower parallel to fibers by a factor of 3.2, and the voltage across the depolarization wave is approximately three times as great in the longitudinal direction. Equations are presented which relate these variables. Theoretical considerations confirm the experimental finding that the potentials around a wave of depolarization cannot be accounted for by the conventional hypothesis that the wavefront is a uniform double-layer current source.

Wavefront sensing by phase retrieval

Abstract: Adaptive optics with controllable delay in a matrix of regions focuses the image of an object upon a detector array in an image plane that provides a signal set representative of the radiant energy incident upon a matrix of locations in the image plane An image signal processor processes this signal set to provide a wavefront error signal representative of the estimated wavefront phase distortion of the radiant energy incident upon the detector array A control system responds to this wavefront error signal to effect alteration of the phase delay introduced by the adaptive optics to reduce the phase distortion of radiant energy incident upon the detector array

Phase-conjugate wavefront generation via real-time holography in Bi 12 SiO 20 crystals

Abstract: Continuous-wave phase-conjugate wavefront generation via real-time holography in Bi12 SiO20 crystals is presented. The efficiency of the device is analyzed in terms of wavefront reflectivity. Electro-optic configurations allowing gain in the reflected wave intensity are considered. Applications include real-time imaging through phase-disturbing media, optical processing, and interferometry.

A comparasion of three resequencing algorithms for the reduction of matrix profile and wavefront

Abstract: Three widely-used nodal resequencing algorithms were tested and compared for their ability to reduce matrix profile and root-mean-square (rms) wavefront, the latter being the most critical parameter in determining matrix decomposition time in the NASTRAN finite element computer program. The three algorithms are Cuthill–McKee (CM), Gibbs–Poole–Stockmeyer (GPS), and Levy. Results are presented for a diversified collection of 30 test problems ranging in size from 59 to 2680 nodes. It is concluded that GPS is exceptionally fast, and, for the conditions under which the test was made, the algorithm best able to reduce profile and rms wavefront consistently well. An extensive bibliography of resequencing algorithms is included.

Optical scanner using plane linear diffraction gratings on a rotating spinner

Abstract: An optical scanning system including a spinner containing at least one plane linear diffraction grating. As the spinner is rotated, a plane reconstruction wave of wavelength λ r is directed onto the grating creating a diffracted wavefront which can be focused onto an image plane. The grating is constructed so that the ratio of the reconstruction wavelength λ r to the grating period d is a value lying between the range of 1 and 1.618. The angles of incidence and diffraction of the reconstruction wavefront in a preferred embodiment, approximate 45°.

Influence of certain radiation parameters on wavefront reversal of a pump wave in a Brillouin mirror

Abstract: A study was made of wavefront reversal, accompanying stimulated Brillouin scattering in various media, and of the way it was affected by the geometrical, temporal, and polarization characteristics of the exciting radiation. In an oscillator-amplifier system, practically complete compensation was achieved for phase distortions of the oscillator signal introduced by the amplifier components. The output energy of the light beam was then 3 J, its diameter was 4 mm, and its divergence (diffraction limited) was 0.3 mrad. The results of the experiments indicated a practical possibility of applying the wavefront reversal phenomenon in high-power multichannel lasers.

Effect of Mid- and High-Spatial Frequencies on Optical Performance

Abstract: In many of today's telescopes the effects of surface errors on image quality and scattered light are very important. The influence of optical fabrication surface errors on the performance of an optical system is discussed. The methods developed by Hopkins (1957) for aberration tolerancing and Barakat (1972) for random wavefront errors are extended to the examination of mid- and high-spatial frequency surface errors. The discussion covers a review of the basic concepts of image quality, an examination of manufacturing errors as a function of image quality performance, a demonstration of mirror scattering effects in relation to surface errors, and some comments on the nature of the correlation functions. Illustrative examples are included.

Holographic head-up display system

Abstract: There is disclosed an integrated optical design of head-up displays suitable for use in aircraft cockpits and the like. The display system is comprised of a holographic optical element (11) used as the combiner for presenting a direct view of the exterior on which are superimposed image signals generated by a source device (51) such as a cathode ray tube (12) and transmitted to the combiner through an optical system (10) including a relay lens containing tilted and decentered optical elements to compensate for the aberration present in the holographic optical element (11). There are further disclosed four relay lens design forms particularly suited for use in the design of such holographic head-up displays. The disclosure includes integrated designs in which (a) the holographic element (11) may or may not be constructed with aberrated wavefront and (b) a beam splitter is incorporated for insertion of a stand-by sight, possibly-also being tilted for the correction of aberrations.

Acoustic Emission From a Brief Crack Propagation Event

Abstract: Acoustic emissions produced by elementary processes of deformation and fracture at a crack edge are investigated on the basis of elastodynamic ray theory. To obtain a two-dimensional canonical solution we analyze wavefront motions generated by an arbitrary distribution of climbing edge dislocations emanating from the tip of a semi-infinite crack in an unbounded linearly elastic solid. These wavefront results are expressed in terms of emission coefficients which govern the variation with angle, and phase functions which govern the intensity of the wavefront signals. Explicit expressions for the emission coefficients are presented. The coefficients have been plotted versus the angle of observation, for various values of the crack propagation speed. The phase functions are in the form of integrals over the emanating dislocation distributions. Specific dislocation distributions correspond to brittle fracture and plastic yielding at the crack tip, respectively. Acoustic emission is most intense for brittle fracture, when the particle velocities experience wavefront jumps which are proportional to the stress-intensity factors prior to fracture. An appropriate adjustment of the canonical solution accounts for curvature of a crack edge. Such effects as focussing, finite duration of the propagation event, and finite dimensions of the crack are briefly discussed. As a specific example, the first signals generated by brittle Mode I propagation of an elliptical crack are calculated.

Small-signal wavefront reversal in nonthreshold reflection from a Brillouin mirror

Abstract: A method is proposed and implemented for small-signal wavefront reversal during stimulated Brillouin scattering in an optical waveguide. This method may find applications in the wavefront reversal of nanosecond pulses.

Aplanatic optical system containing two aspheric surfaces

Abstract: The design of an aplanatic optical system with two aspheric surfaces according to a method developed by Vaskas is treated in more detail. The shape of each aspheric surface is determined by a differential equation. It is shown that the ray trajectories through the optical surfaces, located between the two aspheres, are found by solving a system of N linear equations where N is the number of intermediate surfaces. An example of an aspherized aplanatic system is given.

Dispersion-free amplification and oscillation in phase-conjugate four-wave mixing in an atomic vapor doublet

Abstract: This paper presents theoretical and experimental descriptions of a novel application of the dispersion-free point associated with a pair of adjacent doublet transitions in atomic and molecular systems. At this point it is possible to observe enormous pulse amplification of a phase-conjugated wavefront and intense visible oscillation without mirror feedback in the degenerate four-wave parametric mixing in sodium vapor doublet at the characteristic wavelength of 5894 A. At the dispersion-free point one also observes an ac Stark-free condition which permits the generation of a new type of coherent optical radiation called "cooperative phased-array radiation" (COPAR) from a phased-array system of coherently excited atoms in the medium at the wavelength.

Properties of resonators with wavefront-reversing mirrors

Abstract: An analysis is made of the mode structure of a plane-parallel resonator with wavefront-reversing mirrors. An integral equation is derived for the oscillation modes of the resonator. It is shown that these modes are identical with those in a confocal resonator of the same length.

Interaction of a compressional impulse with a slot normal to the surface of an elastic half space

Abstract: Summary. An improved finite difference scheme is applied to simulate wave propagation in the vicinity of a slot normal to the surface of an elastic half space. It provides visualization of the scattered wave pattern at a sequence of time steps, and also the components of displacement as functions of time at a series of observation points. After being hit by a normally incident plane P pulse, the slot oscillates with two main cycles and two shear-compressional pairs of diffracted waves, and also Rayleigh pulses, are scattered from it. The resulting wavefronts are parallel to the vertical surfaces of the slot and curve in semicircular arcs around the bottom of the slot. Experimental tests of the theory were performed, using 0.5-6 MHz ultrasonic pulses on duralumin cylinders with surface-breaking slots ranging from 0.5-2 mm in width and from 2-6 mm in depth. The numerical results were confirmed by these experiments.

Spatial-polarization wavefront reversal in four-photon interaction

Abstract: A theoretical analysis is made of wavefront reversal of depolarized light in the four-photon interaction. Methods for implementing full spatial-polarization reversal are suggested.

Grazing incidence interferometry: the use of the Linnik interferometer for testing image-forming reflection systems

Abstract: A simple interferometric method capable of displaying quantitatively the wave front aberration of any image-forming optical system is described. Its application for testing and aligning grazing incidence reflection optics at the same conjugates as those of short-wavelength use is demonstrated. The image-forming wave front from the system being tested is compared with a true spherical wave front generated within the interferometer from a point at the intended focus. The differences are displayed as a fringe pattern superimposed on an image of the exit pupil. Each fringe corresponds to one wavelength of separation between the actual image-forming wave front and the Gaussian reference sphere. The principle originates from a paper by W. P. Linnik published in Russian and German in 1933. A translation is included as an appendix. Four variations on Linnik’s design are discussed, one of which avoids the use of transmitting optics and normal incidence reflections altogether and could therefore be used at ultraviolet or soft x-ray wavelengths.

Slope and curvature measurement by a double-frequency-grating shearing interferometer: A shearing interferometer is presented which is capable of the direct independent measurement of all components of the first and second derivatives of deflection of reflecting plate models. The wavefront shearing is provided by a double-frequency grating

Abstract: This paper describes a shearing interferometer capable of the direct measurement of the slopes and curvatures of reflecting plates. The wavefront shear is produced by a double-frequency grating which is simply a grating containing two closely spaced frequencies. The first-order waves associated with each frequency form the interferogram, the contours of which are related to the model's slope or curvature depending upon system arrangement and recording procedure.

Simple closed-loop system for real-time optical phase measurement.

Abstract: A simple closed-loop system for estimating optical phase, called an interference phase loop, is presented. In this system the output intensity from an elementary interferometric phase sensor (e.g., Zernike phase contrast, homodyne, heterodyne, polarization, or shearing interferometer) is detected and used to drive a phase modulator in the path of the wavefront being measured. It is shown theoretically and experimentally that with large gain this configuration ignores amplitude fluctuations and unambiguously estimates phase at high speed over a dynamic range of multiple-pi radians. When self-interference (e.g., Zernike phase contrast) is employed, monochromatic light is not required.

Visualization and measurement of ultrasonic wavefronts

Abstract: The transmission of ultrasonic wavefronts through a medium and the reflection of these wavefronts from an object or interface depend upon the bulk mechanical properties of the medium or object. Measuring or visualizing the wavefront yields important information about various transmission media and objects under study and has found application in nondestructive evaluation, medical diagnosis, underwater imaging, and acoustic microscopy. This paper reviews the various methods by which ultrasonic wavefronts are detected, measured, and visualized. Special emphasis is placed on optical diffraction by ultrasound and optical interferometric detection of acoustic particle displacement. Shorter summaries are given of chemical, thermal, liquid crystal, electromechanical, and mechanical methods. The distinction between these methods are not always clear. For example, the displacement of an air/water interface by radiation force is a mechanical method even though the displacement may be recorded optically. Therefore, an attempt is made to distinguish between the physical effect of the wavefronts on the medium or measurement device as opposed to the process used to record that effect.

Reflective holographic scanning system insensitive to spinner wobble effects

Abstract: A holographic scanning system includes a reflective type holographic spinner having a plurality of diffraction gratings on its surface. A reconstruction wavefront incident on a non-grating surface of the spinner is reflected into a corrective optical element which displaces and redirects the wavefront so that it is incident at the grating at an angle corrected for the effects of spinner wobble.

Mutual coherence function of a wave front corrected by zonal adaptive optics

Abstract: The mutual coherence function (MCF) of a wave propagated through atmospheric turbulence and corrected by zonal adaptive optics is derived for a segmented corrector with either piston only or piston with a local tilt fit. We begin with an intuitive approach which is based on replacing the outer scale of turbulence in the uncorrected MCF with a quantity proportional to the segment size. This approach is especially useful in deriving asymptotes. A rigorous approach provides the full curves, and reduces to a single curve for piston only and a single curve for piston with tilt, after proper parametrization. The effect of amplitude fluctuations is also considered, since scintillation limits the ultimate degree of correction as the segment size goes to zero. Although the analysis is for a segmented corrector, the results apply fairly well to continuous corrector with actuator spacing equal to d/√3, where d is the segment diameter.

Real-Time Interferometer

Abstract: The fundamental measurement required for proof of quality of an optical system is the shape of its transmitted wavefront. A knowledge of wavefront error and the transmission characteristics of an optical system allow calculation (at the measurement wavelength) of all commonly used quality factors such as optical transfer function, point spread function, Strehi ratio, etc. This paper describes a recently developed shearing interferometer that is designed for general purpose laboratory use. Included is a description of the instrument characteristics and principle of operation as well as a comparison of measured and theoretical accuracy limits.

Phase-Locked Interferometry

Abstract: A technique is described which converts in real time the fringe pattern resulting from two interfering beams into a wavefront map. The interferometer incorporates, as part of a servo system, a piezoelectrically driven mirror that is capable of applying both a known optical phase offset and a periodic optical phase modulation to one beam. The ac signal detected by a single photodiode is processed to generate an error signal for the servo system and a signal proportional to the optical phase difference between the two beams. Three instruments built on this principle will be discussed. The instruments are capable of detecting phase differences with a precision approaching A/100 and may be used for measuring surface topography (roughness, irregularity) or transparent phase objects such as biological samples, microballoons and gradient index materials.

Optical heterodyne detection system and method

Abstract: A system to achieve heterodyne detection of optical (i.e., infrared, visible and ultraviolet) signal wavefront is disclosed. The system employs a holographic phase grating that, when illuminated by laser radiation, will generate a plurality of wavefronts each of which has a predetermined spatial distribution of amplitude and phase. The grating is part of an optical local oscillator that includes a laser that illuminates the holographic phase grating. The system further includes a mechanism to receive an optical-signal wavefronts and combine this wavefront with a plurality of local-oscillator wavefronts to provide combined optical wave fields that are applied as input to an array of discrete detector elements; each detector of the array optimally mixes the one local-oscillator wavefront directed to it with the optical-signal wavefront and generates therefrom an electrical output with optimum detection efficiency and with optimum amplitude and fixed phase relationships to the outputs of the other detectors of the array. The optical heterodyne array detection system produces detection of a plurality of optical-signal, spatial-resolution elements simultaneously.

Thermally Induced Optical Distortion In High-Energy Laser Systems

Abstract: High-energy laser (HEL) systems involve an optical train consisting of mirrors and windows, which may compromise the system's operation because of unavoidable irradiance-mapping aberrations resulting from the absorption of some fraction of the incident laser-beam energy. This paper describes an analytical investigation of relevant processes and discusses how laser-driven mirror/window distortions may affect the performance of HEL systems with regard to focal intensities and on-target fluences. An approximate expression for the brightness at the Gaussian focus is derived on the basis of a far-field degradation model and shown to be simple enough to allow HEL system designers to assess the capability of a contemplated optical train and to evaluate its behavior as a function of beam-power level and laser run-time. Two figures of merit are introduced for the purpose of characterizing the response of power-optics mirrors and windows in terms of thermally induced wavefront errors. For cooled mirrors, it is proposed to consider a figure of merit FoMm = K'eff/ (Am), where K'eff relates to the thermal conductivity, the heat-transfer coefficient, and the thickness of the faceplate, whereas AM measures the mirror absorptance, and E is the distortion coefficient. Similarly, the window figure of merit is FoMw = Cp/ (AW'x), where C'P represents the specific heat per unit volume, AW measures the window absorptance, and x is a distortion coefficient as defined in earlier investigations. On using these figures of merit, an important result emerges: if the window material has a positive distortion coefficient, window lensing suppresses the steady-state mirror-induced aberration at time tc = N(D/d)2FoMw/FoMm, where N is the number of relay mirrors and D/d is the telescope magnification. Under normal circumstances, the far-field irradiance exhibits strongly nonlinear features. Refocusing can eliminate the effect of mirror distortion but, ultimately, window distortion dominates and

Wavefront Dislocations and Their Analysis Using Catastrophe Theory

Abstract: We aim to shed some new light on the concept of wavefront dislocations by using ideas from catastrophe theory. Wavefront dislocations were originally defined as phase singularities in a complex wavefunction, which provides a very convenient way to analyse their properties. The reason why wavefront dislocations are common features of wavefields is explained by considering some typical global behaviour of wavefronts. Catastrophe theory tells us that in two dimensions we should consider the wavefronts around a cusp caustic. Consideration of the way dislocations are likely to be observed experimentally suggests that they may be regarded as wavefront catastrophes of a real wavefunction. We can theref.

Zoom null lens.

Abstract: A simple null lens is described that is moved back and forth relative to a point source to provide large amounts of variable wave front aberration. This device can be used to test a wide variety of optical components and systems.

Special Applications Of The Point-Diffraction Interferometer

Abstract: The basic principles of operation of the point-diffraction interferometer are briefly discussed. Properties of this interferometer such as formation of the reference wavefront at the image (allowing its use in a single-pass mode) and relative insensitivity to vibration make it especially convenient for testing large complex optical systems in situ. Examples of such kinds of tests are described.© (1979) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.