Showing papers on "Wavefront published in 1978"

Active optics: A new technology for the control of light

Abstract: Active optical systems are those in which real-time control over optical wavefronts is employed to optimize system perference in the presence of random distrurbances. Applications include 1) outgoingwave systems for maximizing the power density of a laser beam on a target and 2) received-wave systems for maximizing the angular resolving power of a telescope viewing a distant object through a turbulent atmosphere. In this tutorial-review paper, the basic concepts of active optics systems and their historical evolution are discussed, from early figurecontrol systems with a servo bandwidth of less than 1 Hz to the recently developed high-bandwidth systems for atmospheric compensation with bandwidths of several hundred hertz. A critical comparison of the various approaches to wavefront sensing is then made covering both coherent (laser) and incoherent (white-light)systems. Current techniques for wavefront correction including Bragg cells, segmented mirrors, thin-plate deformable mirrors, monolithic mirrms, and membrane mirrors are described. The performance analysis and optimization of closed loop systems is covered using two basic models. The paper concludes with a review of the design and performance of five current experimental active optical systems, with some comments on future applications.

Optical shop testing

Abstract: Fringe scanning techniques, now renamed heterodyning or phase shift interferometry, are covered in a completely rewritten chapter. New chapters have been added to cover wavefront fitting and evaluation as well as holographic and Moire methods. The chapter on parameter measurements has been completely rewritten and an appendix added suggesting appropriate tests for typical optical surfaces.

Velocity estimation and downward continuation by wavefront synthesis

Abstract: A “wave stack” is any stack over a common shot or geophone gather in which the moveout is independent of time. It synthesizes a particular wavefront by superposition of the many spherical wavefronts of raw data. Unlike the common midpoint stack, wave stacks retain the important property of being the sampling of a wave field and, as such, permit wave‐equation treatment of formerly difficult or impossible problems. Seismic sections of field data generated by wave stacks that synthesized slanted downgoing plane waves showed a similarity in appearance to the common midpoint stacks. In signal‐to‐noise ratio they lay between the single offset section and the midpoint stack. The angle selectivity of the slanted plane‐wave stacks permitted detection of a reflector that was not visible on either the midpoint stack or the raw gathers. Simple velocity estimation in slant frame coordinates differs only in detail from standard frame coordinates. Because of the wave field character of data that have been slant plane‐wa...

Holographic head-up displays

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 used as the combiner for presenting a direct view of the exterior on which are superimposed image signals generated by a source device such as a cathode ray tube and transmitted to the combiner through an optical system including a relay lens containing tilted and decentered optical elements to compensate for the aberration present in the holographic optical element. 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 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.

Disruption of wavefronts: statistics of dislocations in incoherent Gaussian random waves

Abstract: Wavefront dislocations-i.e. singularities of the phase of a wave psi in the form of moving lines in space where mod psi mod vanishes-are studied for initially plane waves that have passed through a random space and time-dependent phase-changing screen. For transmitted waves that are Gaussian random, incoherent, quasi-monochromatic and paraxial the following quantities are calculated in terms of the statistics of the phase screen: dislocation densities, i.e. the average number of dislocation lines piercing unit area of variously-oriented surfaces, and dislocation fluxes, i.e. the average number of dislocation lines crossing unit length of variously-directed lines in unit time.

Continuous-wave backward-wave generation by degenerate four-wave mixing in ruby

Abstract: This is the first reported demonstration of cw wavefront conjugation by degenerate four-wave mixing in a solid. Conversion efficiencies of 3% for conjugate-wave generation were obtained in a ruby crystal using a cw argon-ion laser.

Removal Of Test Optics Errors

Abstract: At times the fabrication errors in test optics approach the magnitude of the desired residual errors in a finished optic. We discuss the problem of removing the rotationally asymmetric test optics errors from a composite wavefront of test optics errors and residual finished optics errors.© (1978) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Optical interferometer system with CCTV camera for measuring a wide range of aperture sizes

Abstract: Apparatus is disclosed for the generation and display of interference patterns for the purpose of quantifying the distortion of either a surface or a transmitted wavefront. The invention comprises, in one embodiment, means to align the elements comprising the interferometer cavity, means to continuously magnify, i.e., zoom, the aperture diameter of the interference pattern, means to focus either the measured or limiting aperture onto the face of the vidicon, and means for transducing both the alignment spots and the interference pattern with a standard CCTV camera.

Designing and constructing thick holographic optical elements.

Abstract: Previously, holographic optical elements have been constructed with two spherical waves. Better image quality and efficiency can be achieved by constructing thick HOE's with spherically aberrated wavefronts. We include an exact lens equivalent for a HOE to allow computer optimization of such systems. The effect of changes in the index of refraction and thickness of the emulsion are considered along with the quality of the substrate. Optical systems to construct such HOE's are discussed and analytically described. Many examples are presented.

Interferometric testing with computer-generated holograms: aberration balancing method and error analysis.

Abstract: When testing aspheric surfaces with a computer-generated hologram, some problems should be considered. In this paper, first, we compare two types of hologram: Lohmann and interference. The phase error in the Lohmann type hologram is estimated, and a method of compensating the error is described. Second, we discuss the relation between the shape of the required wavefront and the number of resolution cells of the hologram. Since testing smaller f number optical elements increases the required number of resolution cells of the hologram, we propose the aberration balancing method to reduce the number of resolution cells. The optimum values of the defocus aberration are calculated. Especially, it is shown that the number of resolution cells in the hologram is capable of being reduced to 25%. Third, we discuss the error due to incorrect hologram size and due to misalignment of the optical system when the aberration balancing method is applied. Finally, an experimental example for testing an aspheric mirror 150 mm in diameter and 300 mm in focal length is given.

Wavefront Error Compensation Capabilities Of Multi-Actuator Deformable Mirrors

Abstract: The concept of a multi-actuator deformable mirror as a high-pass spatial frequency filter which operates upon the spatial frequency spectrum of the incident wavefront error allows us to apply the well-known techniques of linear systems theory to the wavefront error compensation process. The deformable mirror filter function, given by the ratio of the spatial frequency spectrum of the residual wavefront error after compensation to the spatial frequency spectrum of the uncorrected wavefront error, can be considered to be a transfer function characterizing the wavefront error compensation capabilities of the mirror. This concept has been implemented by constructing a realistic transfer function from measured actuator influence function data and incorporating the effects of several different types of wavefront sensors and control algorithms. For the special case of random wavefront errors with Gaussian statistics this transfer function can be used to predict the variance of the residual wavefront error as a function of the autocovariance length of the uncorrected wavefront error for any desired actuator density. The resulting design curves provide a simple method of determining the actuator density and stroke requirements of a deformable mirror capable of achieving a specified degree of wavefront error compensation. The actuator influence function is also explored as a variable in the design of deformable mirrors. The simple analytic model developed here is a valuable tool in the design and evaluation of wavefront error compensation systems.© (1978) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Deformable liquid mirror

Abstract: A liquid mirror for high energy lasers. A working liquid such as mercury, used as the reflecting surface for laser light. Dielectric enhanced reflecting techniques can be used to increase significantly the amount of reflection from the working liquid. The liquid is contained in a pressure cell behind a transparent optical flat. By varying the pressure cell of the liquid, compensation for the aberration caused by heat absorption can be made. These same pressure changes can be used to correct for laser beam wavefront distortions produced elsewhere in the laser, director or atmosphere.

The quasi-steady regime in critically initiated detonation waves

Abstract: Experimental work is described on the initiation of spherical detonation waves, in oxyacetylene with various diluents, using an exploding wire and a lead azide pellet as sources. The transition of the spherical blast wave to a detonation, when the source energy is either above or at the critical value, is studied by three methods. Changes in wave structure are inferred from the cellular patterns obtained on smoked foils orientated radially to the flow: the wavefront velocity is derived from the Doppler frequencies and the pilot pressure variation is measured. For supercritical source energies the results obtained provide a detailed pressure history of the transition process. At critical source energy it is shown that the transverse wave structure, which is present in the initially overdriven wave, disappears when the low-velocity regime is established. The duration of the quasi-steady regime is approximately 10 mu s (or distance approximately 1 cm) before the establishment of a normal CJ wave.

Atmospheric delay fluctuations with scale sizes greater than one kilometer, observed with a radio interferometer array

Abstract: The Westerbork radio telescope samples the wavefront from a radio source at 10 equidistant points over a 1300 m baseline. Observing frequencies are 0.6, 1.4, and 5.0 GHz. When observing a point source, the instrument senses the distortion of the wavefront resulting from atmospheric inhomogeneities. It is found that irregularities with sizes larger than the array predominate. In many cases they are seen to move over the array at speeds characteristic of tropospheric winds. Their effect can be described by a parabolic phase variation with position, the coefficients of which fluctuate with time. A number of examples illustrate the widely varying character the time dependences may exhibit. Analysis of a large number of observations shows diurnal and seasonal variations in the intensity of the fluctuations similar to those found by other workers and generally interpreted as indicating a tropospheric origin. The fact that the 1.4 GHz fluctuations are about twice as strong as those at 5.0 GHz is in disagreement with such an interpretation. Analysis of more observational material will be needed to resolve this matter.

A Tolerancing Procedure Based On Modulation Transfer Function (MTF)

Abstract: A procedure is described which calculates the variation of MTF with respect to the constructional parameters of a lens system including the effects of diffraction, spectral band, obscuration, and vignetting. The calculation is much faster than the common procedure of changing a parameter and re-evaluating, and is applicable to systems in any state of correction. Compensating parameters such as defocus and image plane tilt are included in the procedure to account for adjustments made during assembly of the lens. The basic approach is an expansion of the OTF integral into a power series up to second order in the parameter. Each of the terms in the expansion is an integral over the convolved aperture. The integration is carried out by tracing a grid of rays through the system and using the H. H. Hopkins algorithm. The system is evaluated only once; differential ray tracing being used to obtain the wavefront derivatives, and hence the MTF variation, for each parameter. The relationship between MTF and any parameter is quadratic for small variations of the parameter, but not necessarily symmetric with respect to positive and negative incremental changes in the parameter. The changes in MTF for both positive and negative perturbations of all the constructional parameters are combined in a statistical manner to arrive at a probable degradation for the manufactured and assembled system. In addition to MTF, the resulting change in distortion for each field is calculated based on the parameter changes and adjustments.© (1978) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Large aperture ac interferometer for optical testing.

Abstract: A 20-cm clear aperture modified Twyman-Green interferometer is described. The system measures phase with an AC technique called phase-lock interferometry while scanning the aperture with a dual galvanometer scanning system. Position information and phase are stored in a minicomputer with disk storage. This information is manipulated with associated software, and the wavefront deformation due to a test component is graphically displayed in perspective and contour on a CRT terminal.

Wavefront reversal in stimulated light scattering in a focused spatially inhomogeneous pump beam

Abstract: A theoretical analysis is made of stimulated light scattering in a pump beam possessing both fine-structure inhomogeneity (and thus, "gray" divergence) and a continuously varying amplitude and phase envelope due to focusing of the beam with a lens into an unbounded medium. An equation is derived to describe the gain of the pump-reproducing field component, and a solution of this equation, excitable by spontaneous radiation noise, is found in the constant pump field approximation. In this case, the scattered wave is localized in a narrow (compared with the pump wave) near-axial zone and, unlike scattering in a waveguide, the reproduction of the pump wave is partial and dependent on the correlation between the "gray" and diffraction-limited divergence. The disagreement between this theoretical conclusion and the complete reproduction observed earlier experimentally indicates the importance of saturation effects in the scattering of focused beams.

Piezoelectric wavefront modulator

Abstract: A piezoelectric wavefront modulator employs a plurality of widely-separated posts, which are formed upon the upper portion of a monolithic piezoelectric medium and are affixed to the lower portion of a flexible mirror. The posts extend from the top surface of the piezoelectric medium a sufficient distance, so that the mirror floats. The posts are surrounded by moats formed within the medium, which are configured to produce a highly sensitive device, wherein the post mirror actuators are substantially electromechanically isolated from each other, but not from the piezoelectric material adjacent the moat.

Echographic device for the real-time display of internal discontinuities of a test object

Abstract: A real-time display device or fast imager for ultrasonic echography comprises a first electroacoustic transducer array 2 for periodically and simultaneously emitting a multiplicity of parallel ultrasonic wave trains toward a body O to be examined and receives from it reflected ultrasonic wave trains which it converts into electrical echo signals. These signals are transmitted through respective amplifiers 4 to a second electroacoustic transducer array 3 or 30 which retransmits them in the form of regenerated ultrasonic waves to an acousto-optical converter including a transparent tank 5 which contains a piezo-optical liquid 6, serving as an acousto-optical interaction medium, wherein the regenerated waves are focused by one or two focalizing elements, i.e. acoustic lenses and/or mirrors or simply by an arcuate shape of the second transducer array 30. The ultrasonic wavefronts form streaks or schlieren at image points spatially correlated with reflection-generating points of the test object O, these schlieren coming into existence in an image space of the tank at different instants depending upon the spacing of the respective object points from the first transducer array 2. The image space is transluminated by an oscillating light beam of narrow elongate cross-section whose velocity matches the apparent schlieren speed in the direction of wave propagation; the diffracted part of the beam, clearing a spatial filter or mask, is intercepted by a visualizer such as a vidicon tube.

Optical heterodyne system for imaging in a dynamic diffusive medium

Abstract: A system for providing an image of an object from a coherent light signal transmitted from the object through a dynamic diffusive medium is provided with a planar surface receiving the signal wavefront, the received wavefront comprising an array of discrete picture elements. A local oscillator signal is mixed with each picture element in a selected order to provide light responsive signals which represent the distribution of power over frequency of each picture element, all of the frequency components of the light responsive signals being included in a first frequency range of selected bandwidth which is centered around an intermediate frequency. A narrow-band electronic filter receives each of the light responsive signals and removes all frequency components therefrom which are not included in a second, narrow, bandwidth centered around the intermediate frequency. An image of the object is provided by electronic apparatus which receives the filtered signals.

Holographic data storage systems

Abstract: A holographic data storage system for the storage and retrieval of data which includes in combination input means adapted to produce a number of coherent beams; wavefront matching means comprising at least one cylindrical lens for facilitating the discrimination of date; a storage medium adapted to store a number of data tracks, and capable of movement in a plane perpendicular to the plane of incidence; stepping means which is adapted to direct the number of beams produced by the input means on to the storage medium in a manner whereby a number of data tracks are recorded or retrieved, and read out means capable of controllable selective retrieval of date

Limits of existence of wavefront reversal in stimulated light scattering

Abstract: An analysis is made of the wavefront reversal (or reproduction) in pump beams with a highly developed transverse interference structure. When the difference between the pump and scattered field wavelengths is small, diffraction improves the reproduction process. On the other hand, pulling of the scattered field into the cores of diffraction filaments with high pump intensities (as in serpentine active microwaveguides) results in deterioration of the reproduction process but it increases the scattered-field increment. A calculation is made of the serpentine effect using a power-series expansion in terms of the intensity of a monochromatic pump beam with a randomly inhomogeneous transverse structure.

Lens design merit functions: rms image spot size and rms optical path difference.

Abstract: The chief lens design problem is to get all the optical paths from object point to image point equal within a small fraction of a wavelength to ensure that spherical wavefronts will be converging on all image points in the field of view. The LASL lens design program minimizes the lateral deviations of the rays from their ideal image points. Results given here show that this procedure also minimizes the optical path difference and that there is a linear relationship between the rms image spot size and the rms optical path difference.

Piezoelectric surface acoustic wave device with suppression of reflected signals

Abstract: A surface acoustic wave device has an electrode finger structure in which the width of finger portions external of envelopes indicating a weighted overlap length and the length of the spaces existing between adjacent finger portions measured widthwise thereof are substantially equal to one half of the wavelength of the acoustic surface wave, the envelopes are symmetric about the center line of a propagation path for the surface acoustic wave, and the finger portions and the spaces between the finger portions are in opposite positional relation to each other about the center line of the propagation path. With this electrode finger structure, occurrence of disturbing reflected signal at the weighted interdigital electrode, which is used as an input/output electrode of the surface acoustic wave device and occurrence of wavefront distortion are prevented and production yield of the device is improved.

Self-adjusting laser-target system for laser fusion

Abstract: An analysis is made of methods of increasing the radiation density in laser fusion facilities. The utilization of wavefront reversal by a stimulated-Brillouin-scattering (STBS) mirror substantially reduces the requirements for the manufacturing precision of the optical components, structural rigidity, and target feed precision. A detailed analysis is made of the optical and energy characteristics of such a system. Estimates show that a target chamber of ~1.5 m radius can be used with a focusing zone of ~10−3 cm diameter.

The Comparison of Complex Object Geometries Using a Combination of Electronic Speckle Pattern Interferometric Difference Contouring and Holographic Illumination Elements

Abstract: This paper demonstrates that two-wavelength speckle-pattern interferometry may be used to compare complex objects of nominally identical geometries. The principle of the technique is that the inspection object (usually of non-specular finish) is illuminated by a smooth 'master wavefront' at wavelength u 1. The u 1 interferogram formed between the speckle image of this object and an in-line reference beam is then subtracted from the corresponding pattern observed at a second wavelength u2. (u2 m u1 is typically 10 nm.) Speckle pattern correlation fringes observed in the subtracted pattern define differences between the inspection object and master wavefront of magnitude Gu1u2/(u2 m u1), where G is a function of the object geometry. Master wavefronts of complex geometry are reconstructed by holographic elements which initially record the light field reflected from a specular master component. This field is reconstructed to form the required illumination wavefront by re-illuminating the hologram with a wavef...

Nonlinear Optical Phase Conjugation for Laser Systems

Abstract: This paper deals with certain nonlinear effects that can directly generate a wavefront which is the phase conjugate of the incident field. These phenomena can be exploited in pulsed laser systems to remove aberrations in the optical train as well as ab-errations arising from turbulence in the atmosphere. We discuss our measurements of the effectiveness of this conjugation process using a ruby laser and stimulated Brillouin scattering in a CS2 waveguide device. Measurements are made of the divergence angle of the beam after correction. The application of conjugate processes realizable in SBS, SRS, parametric downconversion, and four-wave mixing is considered for typical CO2 laser systems. System gain, backscatter limitations and parasitic oscillation are discussed for typical pulsed amplifier systems. Amplified spontaneous emission and unwanted glint returns from optical defects appear to be the most serious limitations.