Showing papers on "Adaptive optics published in 1983"

Two-dimensional fringe-pattern analysis.

Abstract: Two-dimensional sinusoid fitting and Fourier transform methods of analyzing fringes to determine the wave-front topography are described. The methods are easy to apply because they do not require finding fringe centers and fringe orders. Also, they are accurate. For an active optics experiment in which we have used these techniques, experimental noise exceeds the error resulting from analysis of noise-free theoretical fringe patterns.

Interferometer with a self-pumped phase-conjugating mirror

Abstract: An interferometer having a self-pumped phase-conjugating mirror is demonstrated. This device eliminates the effects of turbulence or optical distortion in the beam path yet responds to any uniform phase change.

Full Aperture Testing With Subaperture Test Optics

Abstract: This paper is an overview of recent work of the Lockheed Palo Alto Research Laboratory on the use of subaperture test optics to evaluate the performance of large optical systems. Supported by selected subscale experiments, a theory has been developed that addresses two test conditions, each based on the use of a known test flat in a double-pass configuration with a collimated optical system of unknown quality. The two test conditions, in order of increasing theoretical complexity, are: (1) a single test flat covering only a portion of the full-system aperture, and (2) multiple (not necessarily coherent) test flats. Analyses predict limited utility of a single test subaperture as a function of a subaperture size and location, and aberration content. Multiple subapertures viewing the full system are shown to give good results for higher order aberrations even when the individual test flats are unphased and contain large relative tilt errors. The test techniques described here are fully scalable to future optical systems of arbitrary size. This paper summarizes the theoretical basis for subaperture testing, gives quantitative performance predictions for some selected cases, and presents the results of supporting experimental work.

Transfer Functions for Partially Coherent SAR Systems

Abstract: An impulse response function and a transfer function of use for partially coherent systems are presented. Partially coherent systems are those that transform a complex amplitude input to an output intensity and are encountered in many guises including optical systems, radiometers, scatterometers, astronomical radars, and synthetic aperture radar. (In the fully coherent or noncoherent limits, linear analysis and the associated system descriptions are well known in terms of amplitude or modulation transfer functions, respectively.) Properties are derived herein for partially coherent systems that describe performance dependence on system phase and amplitude match to input signal characteristics and to system and signal partial coherence. Such systems are shown to be energy conservative in the event of coherence variations. Output spectral properties that describe the dependence of multiplicative Rayleigh noise (speckle) on input signal coherence, system coherence, and phase match are presented. Unlike linear systems, partially coherent systems require both (1) the impulse response and (2) a quantitative measure of scene/system coherence to adequately describe their performance. The general formulation of this paper is applied to synthetic aperture radar (SAR) imaging systems to derive closed-form results of importance for that discipline. It is shown that the mean output from a SAR for a random distributed scene input is not dependent on system focus, system coherence, or scene coherence, whereas imagery of scene detail is dependent on these same parameters. The speckle spectrum is a function of system coherence, but not a function of scene coherence or system focus.

Linear Methods In Phase Retrieval

Abstract: On-orbit wavefront sensing and active alignment control are essential features of many spaceborne optical systems currently being developed. Phase retrieval is an especially appropriate wavefront sensing technique for this application, because it directly monitors system image quality and eliminates or reduces the need for auxiliary wavefront sensors. Although the general phase retrieval problem is highly complex and requires sophisticated nonlinear estimation techniques, properly selected linear methods provide satisfactory and efficient solutions to a number of important special cases. This paper discusses the per­ formance of several such linear phase retrieval algorithms. One method yields noise-optimal estimates of small wavefront errors, while a second approach can be used with arbitrarily large errors but is much more sensitive to noise. These two phase retrieval algorithms are actually special cases of a general linear algorithm that can be tuned as a function of wavefront error characteristics, measurement noise statistics, and focal plane detector geometry.Introduction the utility of linear phase retrievalOptical systems with large apertures and high resolution over a wide field of view will be required for a variety of future infrared- and visible-wavelength space sensors. The optical forms selected for these systems frequently consist of fast, off-axis mirrors that are highly sensitive to misalignments. Passive structural approaches to achieving the required opto-mechanical tolerances in such telescopes are often not feasible because sensor weight must be kept low; periodic or active on-orbit alignment sensing and control are then necessary when conventional passive approaches are inadequate. Phase retrieval from focal plane imagery is an attractive approach to alignment error sensing for this application, because it controls alignment directly on the basis of optical system performance and avoids the physical complexity of auxiliary alignment sensors, which are subject to drift or failure.Although the general phase retrieval problem is highly complex and requires sophisticated nonlinear estimation methods for its solution,1"5 three aspects of the phase retrieval application summarized above suggest that a simpler approach may prove satisfactory in this case. For near- to mid-term applications, the wavefront errors to be estimated will arise from rigid body mirror misalignments or from low-frequency mirror thermal deformations. The number of wavefront degrees freedom will therefore vary from three to approximately twenty.The phase retrieval algorithm need not obtain an exact value for wavefront error in a single application, but must instead produce satisfactory optical alignment after several cycles of alignment error estimation and correction. Point sources in the form of stars or beacons can be assumed for many potential systems. Granted these simplifying assumptions, it appears that properly devised linear phase retrieval algorithms possess sufficient accu­ racy and range to "fine tune" the alignment of optical systems with wavefront errors of up to 0.3 to 0.4 waves rms. These linear algorithms impose comparatively modest signal processing requirements, and therefore would be valuable when (as is usual) sensor signal processing requirements must be minimized.The remainder of this paper describes the theory and application of linear phase retrieval algorithms in greater detail. First, the advantages and limitations of two early attempts at linear phase retrieval are described, and then an optimal linear algorithm that a) is tunable as a function of operating conditions, b) permits performance analysis via closed-form equations of expected estimation accuracy, and c) can be specialized to the two previously described methods as limit cases, is presented. Then several additional applica­ tions of this linear phase retrieval algorithm are discussed.90

Intracavity adaptive optic compensation of phase aberrations. III: Passive and active cavity study for a large N eq resonator

Abstract: The results of a numerical study of both the passive and the active mode-structure properties and far-field behavior of a linear, positive-branch, confocal unstable resonator with a large equivalent Fresnel number in the ideal unaberrated, intracavity phase-aberrated, and intracavity phase-corrected states are presented. A simple, saturable, gain-medium model of a homogeneously broadened CO2–electron-beam discharge laser system is employed in the active cavity study. The active cavity results presented here show that, at least for the level of gain saturation considered, the passive cavity theory of phase-aberration sensitivity remains applicable in this more-physical situation. Furthermore, the ideal correction to phase-tilt and curvature aberrations, derived from passive cavity considerations, remains exact in the presence of saturable gain. Finally, the influence of a multidithered, zonal deformable mirror on the cavity-mode structure and far-field behavior is analyzed in order to account for recent experimental tests of the intracavity adaptive optics concept.

Exact modeling of lineshape and wavenumber variations for off-axis detectors in Fourier transform spectrometers (FTS) sensor systems

Abstract: The utilization of detector arrays in the focal planes of FTS sensor systems allows simultaneous spectral and spatial measurements. However, spectral lineshapes and wavenumber locations depend upon the size and location of the detector elements with respect to the Haidinger fringe pattern of the FTS sensor. These spectral distortions can be generalized as a shift and shape change of the FTS sensor lineshape. Depending on the distortions that can be tolerated, a degree of field-widening can be obtained for a given Haidinger fringe pattern. An exact model for predicting the FTS lineshape distortions is presented. The model is applied to several contemporary applications in order to quantify the magnitude of distortions to be expected.

Dual-wavelength adaptive optical systems.

Abstract: A method for decomposition of phase difference error between measurements of atmospheric turbulence-induced phase distortion at two different wavelengths is described. Calculations are made of the phase difference errors in the first five Zernike radial modes for both ground-to-ground and ground-to-space transmission of laser radiation. It is found that the phase difference error compared with the uncorrected wavefront phase error is relatively insignificant in the first (tilt) Zernike mode but increases in significance with the order of the Zernike mode. Relative phase difference error is also found to depend on transmitted and received wavelengths, aperture diameter, propagation path, and strength of turbulence.

Strehl’s ratio for a two-wavelength continuously deformable optical adaptive transmitter

Abstract: Polychromatic adaptive optical systems differ from monochromatic systems in that the required phase compensation is measured at a wavelength (or wavelengths) different from that at which compensation is required Unfortunately, the effect of turbulence is dispersive, and consequently, because of the wavelength difference, the compensation will not be exact In an effort to determine the magnitude of the effect, a formulation of Strehl’s ratio for the polychromatic case has been developed and numerical data generated for both horizontal and vertical paths

Theory of intracavity adaptive optic mode control

Abstract: A theoretical treatment of the influence linear intracavity phase aberrations have upon the transverse mode structure of an unstable cavity and the ideal compensation of such aberrations by an intracavity adaptive optic element is provided. Numerical calculations of the aberration sensitivity and the corrective capabilities afforded by such an ideally configured adaptive optic element demonstrate the accuracy of this analysis. A numerical analysis of the system performance obtained in a recent adaptive laser experiment shows that the multidither approach does not yield the optimum correction. As a consequence, it is suggested that the best approach to applying adaptive optics to intracavity mode control is to calculate the precise mirror surface profile from measurements of the outcoupled phase structure weighted by the appropriate theoretical aberration sensitivity coefficient.© (1983) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Performance Evaluation Of An Edge-Actuated, Modal, Deformable Mirror

Abstract: The results of the evaluation of a breadboard, edge-actuated, modal, deformable mirror are given. The mirror was designed to correct for focus and two axes of astigmatism in the Zernike formulation using three actuators. The dynamic response of the mirror, with single mode input, is down 3 dB at 150 Hz with a phase shift of 75°. For single mode input, the surface deformation with static input is linear to at least ±0.1 10.6 (±1.1 um) with cross coupling to the other modes less than 10%. Calibration curves derived from the single mode input data are included. With multimode static inputs, the surface deformation deviation from the calibration curves is generally less than 0.02 10.6.

Fused Silica Mirror Evaluation For The Shuttle Infrared Telescope Facility (SIRTF)

Abstract: The SIRTF optics are intended for operation at 20 K (or less); it will be extremely inconvenient, expensive, and time consuming if it becomes necessary to accomplish all of the optical element testing, assembly, and alignment at comparable temperatures. The thermal strain behavior, including potential anisotropies and inhomogeneities, of a mirror substrate between room temperature and 20 K thus becomes a major factor in the selection of the substrate material, structural configuration, and joining methods for lightweight structures. With support from Space Projects, NASA Ames Research Center, an optical figure evaluation of a 0.65-meter, lightweight, fused silica mirror at a low-temperature goal of 20 K is being conducted. The design details of a thermal shroud, provisions for extracting heat from the low-conductivity mirror, and wavefront error sources other than the mirror surface are discussed and preliminary test results presented.

Image compensation in the presence of thermal blooming.

Abstract: The effect of image compensation in viewing extended targets through thermal blooming is discussed. A wave-optics propagation code simulating multiple point sources and a low-bandwidth return-wave adaptive optics system is used to determine the steady-state thermally induced phase distortions and wave-front correction through various Zernike modes. Incoherent point spread functions for the isoplanatic regions are generated and convolved with the appropriate object field to reconstruct the extended target image. Image distortion, degradation in peak irradiance, and adaptive optics loop stability are discussed with respect to degree of correction and wavelength sensitivity.

Demonstration of real-time compensation of atmospheric turbulence by nonlinear phase conjugation

Abstract: The first demonstration of the correction of atmospheric turbulence by nonlinear phase-conjugation techniques is presented. It is shown that severely distorted and wandering beams can dynamically be corrected to near-diffraction limit and locked to a prescribed spatial position. These experiments were performed over a 100-M rooftop range. A beam is propagated over the range, becoming aberrated, enters a phase conjugator, is conjugated, and retraverses the atmospheric path canceling the aberrations. The conjugate signal is generated by the nonlinear optical process of four-wave mixing in sodium vapor at 5890 A. In this way the conjugate, or time reversed signal, is produced by a process involving no moving parts in contrast to conventional adaptive optics techniques involving deformable mirrors.

Active Mirror Segment Figure Control

Abstract: Experimental verification of the NEPSPP nonlinear finite-element mirror model and stable closed-loop correction of an optically aberrateel wavefront have been demonstrated. These results were obtained using a single 60-deg segment of a 45-cm diameter, 1-inch-thick Zero-dur spherical mirror (f/1.7) having a 25-percent central obscuration. Seventeen holographically produced Hartmann sampling gratings were used for the figure sensing in the closed loop experiments and 30 electrodynamic actuators for figure control. The entire system was under computer control. The mirror model predictions of the surface were verified by analysis of interferograms of the segment taken with a Point Diffraction Interferometer (PDI) and analyzed by the FRINGE program. Details of the experimental configuration and auantitative results are presented.© (1983) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Primary mirror figure control by laser autocollimation

Abstract: A method of monitoring the figure of the primary mirror of the University of Texas 7.6-m telescope is under study, in which slope errors are determined by reflection of laser beams from small flat mirrors cemented to the back of the thin meniscus primary mirror. The method requires calibration against a stellar image wavefront sensor but does not require continuous presence of the stellar image for operation. The method will be described, together with an error analysis. An optimum distribution of test points will be shown based on finite element and modal structural analyses of the mirror blank. A discussion of possible operational errors attributable to the laser system, based on similar experience at the University of Arizona - Smithsonian MMT Observatory, will be given.© (1983) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Real Time Metrology Using Heterodyne Interferometry

Abstract: The Air Force Weapons Laboratory (AFWL) located at Albuquerque, NM has developed a digital heterodyne interferometer capable of real-time, closed loop analysis and control of adaptive optics. The device uses independent phase modulation of two orthogonal polarizations of an argon ion laser to produce a temporally phase modulated interferogram of the test object in a Twyman-Green interferometer. Differential phase detection under the control of a Data General minicomputer helps reconstruct the phase front without noise effects from amplitude modulation in the optical train. The system consists of the interferometer optics, phase detection circuitry, and the minicomputer, allowing for complete software control of the process. The software has been unified into a powerful package that performs automatic data acquisition, OPD reconstruction, and Zernike analysis of the resulting wavefront. The minicomputer has the capability to control external devices so that closed loop analysis and control is possible. New software under development will provide a framework of data acquisition, display, and storage packages which can be integrated with analysis and control packages customized to the user's needs. Preliminary measurements with the system show that it is noise limited by laser beam phase quality and vibration of the optics. Active measures are necessary to reduce the impact of these noise sources.

Self-excitation of resonators with four-wave hypersonic reversing mirrors

Abstract: Conditions are found for the self-excitation of an adaptive resonator with four-wave hypersonic reversing mirrors. The threshold intensities of the pump waves of such mirrors are identified: when these values are exceeded, optical radiation is generated in a resonator of this kind. The threshold values of the pump wave intensities and the lasing frequencies are shown to depend periodically on the distance between reversing mirrors.

Recent Developments At The Multiple Mirror Telescope

Abstract: The Multiple Mirror Telescope (MMT) serves a dual function. It is a very large astronomical telescope with state-of-the-art instrumentation which is now being used for 85% of the time for astronomical research. It fills an equally important function as a testbed for new concepts in making large aperature telescopes at a modest cost. The MMT outperforms other telescopes in a number of areas, thus making it the benchmark against which future astronomical facilities will be measured. This paper summarizes the performance of the MMT.

Surface Error Analysis For The University Of Texas 7.6m Telescope Primary Mirror

Abstract: The techniques of modal control analysis derived by Creedon and Lindgren [1], an error analysis method proposed by Howell and Creedon [2], and various methods of finite element analysis are applied to a meniscus mirror of unusually large aspect ratio. The analyses predict the figure error for a set of characteristic error disturbances and correction strategies. Several arrangements of piston actuators are shown to he near optimum relative to the disturbance inputs considered, and in each case the related plots of modal coefficients, rms surface error, and the Howell and Creedon error analysis demonstrate the efficacy of the thin meniscus approach. In addition, the engineering approach to achieving a stable optical surface in this way is shown. Decomposing the distorted optical surface into modal terms gives it a natural order and allows the analyst to demonstrate that for some very harsh mechanical loads, relatively few modes are the chief contributors to the wavefront error. Therefore, by designing the active system for the control of the most significant modes, the mechanical engineering task becomes manageable.

The spot of Arago and its role in wavefront analysis

Abstract: The "spot of Arago" has been a controversial topic since its inception in 1818 when Poisson predicted its existence, which violated common sense, in an attempt to discredit Fresnel's wave theory of light. Arago performed the experiment and found the surprising prediction was true, thus putting Fresnel's theory on a firm technical foundation. In recent years, the spot of Arago, which exists as a bright spot at the center of the geometrical shadow of a circular obstruction, has caused substantial grief in various high energy laser applications and has come to be considered more of a nuisance than a curiosity. This paper suggests that the size and shape of the spot of Arago is characteristic of the wavefront aberrations of the incident beam and can therefore be used to advantage as a beam sample for wavefront analysis of annular beams. The implementation of this wavefront sampling scheme would eliminate the requirement for a special beam sampling optical component and thus reduce to a minimum the deleterious effects upon the beam frequently accompanying the use of such components. Both experimental and numerical results will be presented along with a discussion of the capabilities and limitations of this particular beam sample for performing various wavefront sensing functions.© (1983) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Actuator configurations for segmented mirror figure control

Abstract: Segmented mirrors with four and six segments over the full aperture are discussed with regard to the figure correction capability for both moment-and piston-type actuators. The number of actuators, ranging from 48 to 480 DOF over the full aperture, is analyzed for residual error for specific Zernike polynomial input errors. Tradeoff study results show that a very thin mirror will reduce required actuator forces, but if the mirror becomes a membrane and loses its bending stiffness, the figure correction capability degrades rapidly. In general, moment and piston actuators each have regions where one is superior to the other, but no generalization can be made for the cases presented in this paper. The results for a 65-cm-diameter mirror show that 50 to 100 actuators will provide very good wavefront correction capability for a rather broad spectrum of Zernike polynomial input errors.© (1983) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Wavelength dependence of adaptive optics compensation

Abstract: High energy laser systems usually attempt to focus a small spot of laser radiation on a distant receiver plane Aberrations within the optical system, as well as in the intervening atmosphere (turbulence), tend to spread the size of the beam in the focal plane and reduce the peak irradiance that could otherwise be obtained Adaptive optics technology in wavefront sensors and deformable mirrors can significantly reduce the phase errors on the laser beam and increase the laser fluence on the receiver plane The key to making this work is being able to measure the optical distortions accumulated along the actual laser beam path To obtain a wavefront on which to make this measurement, a wavefront sensor must optically share the full aperture of the optical path with the outgoing laser beam© (1983) COPYRIGHT SPIE--The International Society for Optical Engineering Downloading of the abstract is permitted for personal use only

Compensation of thermal defocusing by a three-element adaptive corrector

Abstract: A numerical simulation of a slow adaptive optical system is reported. It is suggested that the wavefront of a beam can be controlled by a phase corrector with three degrees of freedom: the angle of inclination of the beam and two focusing radii along mutually perpendicular directions. Various algorithms for the control of the corrector coordinates are considered and a study is made of the effectiveness of suppression of thermal defocusing in a moving nonlinear medium.

Adaptive Half-Symmetric Unstable Resonator With Internal Axicon (HSURIA) Studies I: Deformable Mirror Located In The Compacted Beam Region

Abstract: A deformable mirror in the compacted beam region of a HSURIA (half symmetric unstable resonator with internal axicon) is used to conduct intracavity adaptive optics experiments. The gain cell is a convection cooled CO2 electric discharge located in the compacted beam region. The closed loop performance of the resonator is optimized using multidither hillclimbing servo techniques to control the figure of the discrete actuator continuous surface deformable mirror which is driven zonally. Residual aberrations in the resonator conical optics (the w-axicon) are shown to be the dominant factor governing the mode for the open loop configuration. The extent of optimization of the resonator upon closure of the adap-tive optics control loop is determined by both the spatial frequency of the aberration with respect to the correction capability of the deformable mirror and the ability of the control to converge to the global rather than a local performance maximum. In addition to simple phase correction, mode modification by the intracavity deformable mirror must be considered as a probable response of the system especially in the presence of intracavity aberrations which are beyond the spatial frequency correction capability of the deformable mirror.© (1983) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

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.

Analytical modeling of DF laser propagation

Abstract: A system performance model is developed to predict the performance of a high-energy laser system taking into account the propagation through the beam control system and the atmosphere. The performance model essentially shows how the various subsystems, such as the laser device, intermediate transfer system, beam expander, target, etc., will interact with each other and how a closed-loop model for the atmospheric correction can be achieved. Such a model aids in the system design by identifying and optimizing the adaptive optic requirements for the laser beam cleanup and for the atmospheric compensation correction. The heart of such a model is the Perkin-Elmer Physical Optics Propagation (POP) code. The POP code has been successfully used in many high-energy laser programs. The general capabilities of the POP code and the performance model will be highlighted. Results from a test case using a chemical laser wavefront will also be presented.© (1983) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.