Showing papers on "Adaptive optics published in 1984"

Adaptive spherical lens.

Abstract: This paper reports the creation of a tandem liquid crystal cell arrangement capable of simulating the performance of a spherical lens. Electrical modulation of the index of refraction creates focusing behavior. The modulation is induced by a set of electrodes whose individual voltages are arranged to provide a cylindrical exit wave front for a uniform plane-wave input. Two such cells with orthogonal electrodes arranged in cascade create spherical lens performance. Experimental evidence using a lens with a relatively small number of electrodes is presented. Applications for such structures, once improved, would include real-time focus adjustment for optical disk readers and hand-held photographic equipment as well as aberration compensation for long path-length optical systems.

Intracavity adaptive optics. 4: Comparison of theory and experiment.

Abstract: A numerical analysis of the system performance of the intracavity adaptive optics experiment is presented. The analysis begins with a detailed comparison of the calculated and experimentally measured sensitivities of the unstable laser to both intracavity phase-tilt and astigmatism aberrations. The appropriate sensitivity measures considered here will be the outcoupled power and the far-field beam quality and power-in-the-bucket due to the field distribution outcoupled from the cavity. Of primary importance here is the correction obtained by the multidither zonal COAT system when a known intracavity phase aberration is introduced. The experimental performance with either beam quality or power in the bucket optimization is numerically assessed along with a comparison of intracavity and extracavity adaptive optic control.

Window aberration correction in laser velocimetry using multifaceted holographic optical elements

Abstract: Multifaceted holographic optical elements were used to correct aberrations introduced into the optical system of a laser Doppler velocimeter by a nonflat window. For this experiment, three green beams, each separated by 41.25 mm in the vertical plane, and two blue beams, separated by 82.50 mm in a horizontal plane, are brought into coincident focus inside a thick walled cylinder. The focal power to bring these beams to a common focus and the aberration correction required by the presence of the cylinder were built into the holographic elements for characteristic locations within the cylinder. The principle demonstrated using the elements described is applicable for an arbitrary window geometry.

Interferometric Method for Optical Testing and Wavefront Error Sensing

Abstract: A new interferometric method for optical testing has been developed. The test arrangement is similar to that of the Hartmann method using a screen with a rectangular grid of holes. The detection takes place close to the focus where the Hartmann images are partly overlapping and interfering. The positions of the interference maxima contain information on the wavefront errors of the optical system. As the size of the interference maxima is considerably smaller than the size of the Hartmann images obtained with the same screen, the accuracy of the position measurements is respectively better. Also much smaller hole spacing can be used. An application of the method could be wavefront error sensing in active optical systems. A CCD camera could ideally be used as detector for the measurement of the wavefront errors and for generating the error signals to the active optical system.© (1983) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Ultraviolet resolution of large mirrors via Hartmann tests and two-dimensional fast Fourier transform analysis

Abstract: Large mirrors appear to be limited in resolution by their medium scale defects (mirror surface "ripples" of small amplitude and of 4 to 12 cycles per diameter). The ultraviolet resolutions of the Pic du Midi (PDM) 2 m mirror, the Canada-France-Hawaii (CFH) 3.6 m mirror, and the Space Telescope (ST) 2.4 m mirror have been studied. Hartmann screen tests were used to determine the wavefront error of the PDM and CFH mirrors, while only semiquantitative reports were used in the case of the ST mirror. Point spread function and modulation transfer function were determined by two-dimensional fast Fourier transform analysis, a necessary technique in the ultraviolet, where the small phase defects of mirrors are no longer negligible (and also because the mirrors' irregularities do not possess a particular arrangement or a given symmetry). If the three mirrors appear to be nearly limited by diffraction in the visible (except the CFH), in the ultraviolet at Lyman a 121.6 nm the resolution is inferior by more than a factor of 10 to the diffraction-limited resolution. While the ultra-violet wavelength range and very high resolution (0.014 arcsec with a diffrac-tion-limited Space Telescope at Lyman a) are highly desirable, such a limitation imposed by mirror surface quality has to be mentioned. The effects of the correlation length, amplitude of defects, and Hartmann screen sampling are presented, as well as some comments on the inadequacy in the ultraviolet of analytical methods compared to two-dimensional numerical simulations.

Research in the Optical Sciences.

Abstract: : Research during the fifth year of contract F49620-80-C-0022 is described Discussed are: optical bistability in thin evaporated films; long-range surface-plasmon polaritons; nonlinear guided wave interactions; theory of two-photon Doppler-free spectroscopy; x-ray image intensifiers with electronic readout; optical bistability; optical bistability experiments to improve solid-state devices and basic understanding; modulated emittance spectroscopy; high-resolution wavefront sensing through the atmosphere; aberrated Gaussian beams; ion beam processing of optical coatings on plastics; optical coatings for the x-ray to ultraviolet wavelength range The degrees awarded to students receiving JSOP support are listed Also included are the papers published under JSOP support from 1979 to 1983 Originator-supplied keywords include: Optics, and Optical sciences

Active Optical Focusing Of The Laser Scanner Ophthalmoscope

Abstract: Recently a new method of retinal imaging by laser scanning ophthalmoscopy was developed that overcomes the intensity problem of conventional fundus photography and promises further advantages. The laser scanning ophthal-moscope has been modified by incorporating an adaptive optical feedback system for laser beam control. For wave-front reconstruction maximum likelihood methods and the Gerchberg-Saxton-algorithm are applied. The system essentially provides an elimination of optical eye aberrations which diminish the fundus image quality. On the other hand by active focus control and/or wavefront sensing the aberrations of the human eye like astigmatism of the cornea and spherical aberration of the lens can be measured.

Optical methods of laser radar field processing

Abstract: The book describes modern methods of receiving laser signals, viz., by interferometry, by nonlinear optics, by laser amplification, and by heterodyne mixing. The theory of these methods is considered as applied to the problem of laser radars. Investigated in detail are features such as the resolving power of interferometry and nonlinear-optics method, the efficiency of parametric conversion of laser signals, the sensitivity of laser amplifiers, and problems of optimal heterodyne detection. Methods of speckle interferometry and adaptive optics are described. The book is aimed at a large circle of scientists and engineers active in the problem of recording of laser signals, as well as to students in advanced courses of the corresponding specialties.

Modal adaptive optics basis functions for obscured apertures.

Abstract: The effectiveness of using shifted Zernike polynomials as basis functions for a modal adaptive optics phase compensation system has been investigated for centrally obscured apertures. Several different types of phase aberration mechanisms have been considered including atmospheric turbulence and laser resonator gain media distortions. Comparisons of wave-front fitting errors indicate that improved fits result, relative to centrally obscured Zernikes polynomials, for cases in which the phase aberrations have high spatial frequencies.

Accurate Assessment Of The Optical Quality Of Infrared Systems

Abstract: The development and operation of a 300 mm aperture, high precision scanning interferometer with associated software for the semiautomatic testing of far-infrared lenses and system are described. The instrument is capable of analyzing and evaluating systems in terms of the optical transfer function (OTF), point spread function (PSF), wavefront aberration, ray intersection patterns, polynomial fitting, aberration contouring, and refractive index variation mapping and of providing a large range of representatives of data. The speed of the analysis is commensurate with established techniques with an improvement in the accuracy and flexibility of use of the instrument over alternative measuring systems. The introduction of this instrument has allowed the complete diagnosis of system defects, along with an accurate measure of any error and its effect on system performance in terms of wavefront error, modulation transfer function (MTF), etc. Results on a variety of systems which exhibit errors are presented, and their root cause and subsequent cure are discussed. The value of such an instrument for both prototype building and production runs is described.

Direct Interferometric Measurement of Static and Nonlinear Wavefront Distortions of a High Power Glass Laser Beam

Abstract: The wavefront distortion of a large aperture (200 mm) laser beam from a high power glass laser system was directly measured using an interferometric method. The measured static and nonlinear wavefront distortions were used to calculate the intensity distributions at the far field and were then compared with the experimental data. The relationship between the wavefront distortion and the far field pattern thus established is very useful for characterizing the performance of the laser system.

Solar Adaptive Optics.

Abstract: : The problem of imaging the solar surface is considered. A theoretical study is presented that indicates that traditional adaptive optic techniques are of limited usefulness for solar imaging. A new method for adaptive optic image sharpening for extended sources is described. The incoming wavefront has added to it controlled amounts of various aberrations. The image quality is sensed and an error signal derived that allows iterative corrections to the wavefront to be determined. The measure of the image quality that is used is an integrated power spectrum of the image plane distribution. It is shown that quantity is maximized for an unaberrated input wave. Results sharpening under conditions of moderately severe turbulence.

A Miniature Focus Sensor

Abstract: A miniature focus sensorAlan J. MacGovernItek Optical Systems, a division of Itek Corporation,and a wholly owned subsidiary of Litton Systems, IncorporatedAbstractThe ability to rapidly measure the focus of images acquired by optical systems is impor-tant in many imaging applications. Itek Optical Systems, building on its extensive wave -front sensing and adaptive optics capability, has developed an electro -optic focus sensordirectly mounted to a Nikon Camera. The sensor operates by measuring the relative positionof the image formed by two halves of the optical aperture. The measurement is performedusing a scanning technique and a processing algorithm implemented by digital electronics.Details of the sensor design with results of its operation are presented.IntroductionIn situations requiring rapid photography of scenes either through complex optical trains(such as submarine periscopes) or at imprecisely known distances, a common cause for loss inresolution is improper focus. Itek Corporation has developed a breadboard focus sensorattached to a Nikon F -3 camera that operates on the scene image directly and generates asignal directly proportional to the displacement of the image from proper focus. Thisapproach utilizes a focal plane chopping technique in a manner similar to that of Itek'sshearing interferometers, and a very sensitive digital algorithm developed for precisemeasurement of image motion. The sensor has been extensively tested using a 150mm focallength lens attached to a Nikon F -3 camera for a variety of scenes and illumination con-

Computer Model For Evaluating Synthetic Aperture Propagation

Abstract: A computer model is described that can generate the Point Spread Function (PSF) for a multiaperture optical system. The model basically does a Fourier transform of the multipupil wavefront to propagate the wave front to the far field. In our model the PSF is synthesized in the far field by squaring the net complex field resulting from the individual relative phase shifts. The wavefront for each subaperture can be expanded in terms of Zernike polynomials. Arbitrary amounts of aberration can be assigned to each individual array telescope. Using the program we did a Monte-Carlo wavefront error analysis that demonstrated general correspondence between RMS wavefront error and encircled energy. The effects of piston on beam steering and shaping are presented.© (1984) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Field test experiments using pseudo-conjugation

Abstract: One common adaptive optics arrangement consists of a reference point source which produces a wave front that passes through an aberrating media and is returned back through the aberrating media by an adaptive optical system,1 Ideally this system produces the phase conjugate of the aberrated wave front and returns the wave front perfectly focused back at the reference source, Most phase conjugate adaptive optics depend on flexible reflective surfaces or one of several techniques closely related to real-time holography such as four-wave mixing.2 It has been pointed out by as early as 1978 by Orlov et al.3 that it is possible, in certain circumstances, to correct nonspecific time-varying distortions in a return wave arrangement using a retroreflective array to obtain an approximate conjugate described in most literature as a pseudo-conjugate.4,5

Mirrors and Windows for High-Energy Lasers: The Wavefront Distortion Problem

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 wavefront distortions resulting from the absorption of some fraction of the incident laser-beam energy The first part of the paper concerns actively cooled HEL reflectors; a generalized mirror/heat-exchanger model is used for evaluating both irradiance-mapping and coolant-pressure induced distortions The theory of thermal lensing of HEL windows is reviewed in part 2; the purpose is to clarify the role of stress-induced birefringence effects and to apply the figure-of-merit concept to key laser-window material candidates

Novel Approaches to Phase Conjugation Utilizing Ultrafast Artificial Kerr Media

Abstract: Non-linear adaptive optics presents one approach for overcoming phase distortions encountered during propagation of laser radiation. If degenerate four-wave mixing is accomplished via quantum transitions, the temporal response of the medium to phase distortions is limited only by the speed of light. This, combined with the fidelity of the signal as well as the inherent mechanical simplicity of this technique, are encouraging features which suggest that phase conjugation can be utilized in actual laser systems. Unfortunately, most non-linear media which are utilized for phase conjugation are characterized by efficiencies which are too low for practical applications. This difficulty is particularly troublesome when only weak pump fields of only a few watts per square centimeter are available. Furthermore, one would prefer to utilize non-linear media which do not have resonance conditions.

Modal adaptive optics basis functions for obscured apertures: comment.

Abstract: In a recent paper, Schoen has compared the effectiveness of Barakat's C-functions (in general, they are not polynomials) and Zernike circle polynomials to fit an aberration function defined over an annular region. In the central obscured region, the C-functions and the Zernike circle polynomials are given their corresponding values at the inner edge of the annulus instead of zero. He concludes that the Cfunctions give a better fit for high spatial frequency aberrations but a worse fit for low spatial frequency aberrations. He finds, for example, that to fit a simple aberration such as a wavefront tilt, a large number of high spatial frequency Cfunctions are required. Such a conclusion is not surprising. We have shown earlier that, except for the rotationally symmetric aberrations, the C-functions represent no similarity to the classical aberrations used in optics, of which wave front tilt is an obvious simple example. Zernike annular polynomials derived in Ref. 3, on the other hand, do represent balanced classical aberrations for annular apertures, as do the Zernike circle polynomials for circular apertures. In a leastsquares sense, the Zernike annular polynomials would be the most optimum to fit an arbitrary annular aberration function. It may be worthwhile for Schoen to reinvestigate his problem using these Zernike annular polynomials. The Zernike annular polynomials have been discussed to a varying degree by several authors. Moreover, in Ref. 3, annular polynomials for nonuniform but radially symmetric illuminations of annular pupils are also derived. In particular, explicit closed-form expressions are given for the annular polynomials corresponding to classical primary (Seidel) aberrations for annular pupils with Gaussian illumination.

Phase Conjugation and Applications to Real Time Optical Treatment

Abstract: Over the past several years a new field of interest has emerged in coherent optics: non linear optical phase conjugation. That field include the possibility of performing operations in real time on the phase and amplitude of optical wavefronts. The use of such non linear interactions has resulted in numerous new applications such as adaptive optics, image transmission and amplification and non linear spectroscopy. In this paper we present at first the principle of phase conjugate mirrors and the rest of the lecture is devoted to applications of wave mixing experiments in photorefractive crystals. We will present the physical processes responsible of the index change and the application of phase conjugate wavefront generation to real time optical processing, dynamic holography and interferometry.

Adaptive focusing of a light beam under wind refraction conditions

Abstract: The aberration-free approximation is used to study adaptive control of the tilt and curvature of the wavefront of a light beam for the purpose of compensation of the thermal self-interaction. The efficiency of control in respect of the lowest aberrations is considered in the case of pulsations of the cross-wind velocity.

Emission of aberration-free radiation from lasers with active elements made of neodymium glass plates

Abstract: An experimental investigation was made of the optical quality of active elements made of GLS-22 neodymium glass plates with reflection from polished side surfaces, as used in an UMI-35 laser system. It was found that the active element was optically equivalent to a cylindrical lens. A continuously acting phase corrector compensated wavefront distortions in the active element and ensured that the divergence of the output radiation at the exit aperture was close to the diffraction limit.