Showing papers on "Adaptive optics 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.

Correction of phase aberrations via stimulated Brillouin scattering

Abstract: We have obtained quantitative measurements on the correction of severely aberrated laser beams using stimulated Brillouin scattering (SBS) at 0.69 μm. We have shown that under certain conditions SBS can be used to restore an aberrated optical beam to its original unaberrated condition. When an optical beam double passes an aberrating region after reflecting from an “ordinary” mirror (i.e., a plane mirror) the aberration is twice that obtained from a single pass. However, when the aberrated beam enters a medium that allows SBS to occur, it emerges from its second pass through the aberrating medium in the same condition as that in which it originally entered. Quantitative experiments are described in which a single-mode ruby laser beam is intentionally aberrated by passing it through an etched plate. When the beam is allowed to double-pass the plate using an ordinary reflector (i.e., plane mirror), the beam divergence is more than 10 times the diffraction-limited divergence. However, when we replace the ordinary reflector with a cell in which SBS can take place, the SBS reflected beam is restored to diffraction-limited divergence when it is allowed to pass back through the aberrating medium. Applications of this time-reversal or phase-reversal technique for correcting aberrations in optical trains and atmospheric turbulence are discussed.

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.

Adaptive laser resonator.

Abstract: The feasibility of compensating for intracavity phasefront distortions using a multidither zonal COAT system has been demonstrated insided an unstable resonator cavity using a CO2 electric discharge laser. Experimental data are presented that verify that intracavity-induced degradation of the near-field and far-field intensity distributions can be corrected and the extracted output power improved by using adaptive resonator techniques.

Adaptive optics and short wavelength sources.

Abstract: The volume deals with adaptive optics, meaning optical systems that measure and correct wavefront aberrations in real time. In the jargon of the specialist, COAT (coherent optical adaptive techniques) reshapes rubber mirrors to compensate for such degrading effects as atmospheric turbulence, inhomogeneous laser mediums, and time varying figure errors in optical surfaces. Chapters 1 and 2 discuss the various approaches currently being pursued, with particular emphasis on high energy laser systems. Chapter 3 presents the fundamentals of (linear) adaptive optics without restriction to high energy systems. Chapter 4 details some actual improvements demonstrated through adaptive optics in an astronomical application. Chapter 5 discusses the exciting new approach of nonlinear, optical adaptive optics which offer virtually instantaneous response. Chapter 6 discusses the dramatic results being achieved in micromachined metal optics (again, special emphasis is on mirrors for high energy lasers). Chapters 7 and 8 describe recent progress toward achieving coherent vuv and x-radiation.

Laser Wavefront Analyzer For Diagnosing High-Energy Lasers

Abstract: The Laser Wavefront. Analyzer (LWA) determines the intensity and phase of a CO2 laser operating over a spectral range from 9.1 µm to 10.7 µm. The LWA is a sliding reference interferometer and measures differential optical phase over a 32 x 32 element format of the input beam. A resolution element is scanned every 10 µsec. The theory and general design aspects of the LWA are discussed in this paper. Phase and intensity data outputs obtained with the instrument during calibration tests performed by the Air Force Weapons Laboratory at Kirtland Air Force Base are presented. This calibration data was obtained by utilizing low power CO2 gas lasers with appropriate optical components to magnify and alter the phase front of the beam entering the analyzer sensor aperture. Phase accuracy and precision data from these calibration tests are also presented.

Experimental Studies Of Adaptive Laser Resonator Techniques (Alert)

Abstract: A standard multidither COAT servo system is used to control the surface figure of an 18-element deformable mirror located inside a CO2 unstable resonator. This adaptive laser resonator is capable of correcting for intra-cavity phase errors arising from disturbances in the laser medium or from distortions in the optical elements that form the laser cavity. This paper presents a brief discussion of the adaptive laser resonator as well as preliminary data on the correction of static and time-dependent resonator mirror tilt.

New Developments In Deformable Surface Devices

Abstract: For the past several years Itek has been involved with the development of deformable mirrors for active optical systems. Several innovative techniques have been developed to deal with the problems of device-design and resonance damping. Designing a device for a required amount of wavefront correction has been approached by utilizing a computer program, which calculates actuator voltages and error fit values based upon actuator response functions, arrays, etc. Resonant modes present in high bandwidth deformable surface devices have been dampened by energy absorption techniques. Resonance amplitudes have been attenuated by as much as 1/60.© (1978) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Phase Determination Of An Amplitude Modulated Complex Wavefront

Abstract: An electronic–optical method is described for obtaining the phase of an arbitrary complex 2-D wavefront. The method consists of intensity detection, limiting, pulse width normalization, differentiation, dc filtering, and integration. Theoretical analyses and experimental confirmation are included. Applications of this scheme that are discussed include space variant processing, adaptive optics, data transmission, kinoforms, holography, and optical data processing.

The Parallel Internal-Master-Oscillator Power-Amplifier for Phase Matching the Output Beams of Multi-Line Lasers.

Abstract: : A conceptual device optics system is discussed, which, in principle, allows combining, on a large mirror, the multiline outputs of an arbitrary number of separate laser devices. In principle, each lasing transition from each laser will be in phase with the corresponding transition from the other laser devices that illuminate the other parts of the mirror. This will be true even if the multiline lasing transitions fluctuate rapidly (go on and off). The concept involves the use of several equal size laser segments, possibly arranged in a polygon configuration for compactness. One segment is run as an oscillator; its output is split into a number of beams, which are used to drive the other laser segments as amplifiers. Because of the multiline nature of the laser mechanism, the basic requirement of the system is that the absolute lengths of the paths through the amplifiers from the oscillator output mirror to the large collecting mirror be equal. In this report, the concept is developed in its simplest form; however, it should be compatible with both adaptive optics beam control (multisection movable mirrors) and techniques to correct beam steering caused by anomalous dispersion or by other effects. (Author)

Characterization Techniques For Deformable Metal Mirrors

Abstract: Deformable mirrors must be subjected to a series of tests in order to accurately assess their structural integrity and predict their operational performance in a closed-loop system. These tests consist of monitoring the surface under various drive conditions. Among the parameters determined from the surface measurements are actuator surface influence functions, hysteresis effects and frequency response. They are used to characterize a deformable mirror by providing information about inter-actuator coupling, drive response linearity and frequency bandwidths. When piezoelectric actua-tors are used, large drive voltages are necessary, which when coupled with high drive frequencies and small surface motion, makes acquisition of accurate data a complex task. This paper describes and compares several laboratory techniques currently being used to measure these and other deformable metal mirror characteristics for two mirror types: dither (high frequency, small surface excursion), and control (low frequency, large surface excursions).

Characteristics Of Bismuth (BI) Photopot Position-Sensing Detectors And Their Use In Adaptive Optical Systems

Abstract: This paper describes measured characteristics of Bismuth thin film detector assemblies as they are used in different laser power and laser beam centroid position measuring applications. These uncooled detectors are insensitive to wavelength over a wide range of the spectrum (visible and infrared to at least 10.6 microns) and they have relatively wide bandwidth characteristics, that make them very useful in many applications.

Intracavity Tip And Tilt Adaptive Control Experiments

Abstract: With recent advances in miniature high performance tip and tilt mirror components (see details in another paper presented in this seminar), it is feasible to dynamically compensate for angular misalignment of intracavity optical components. The control system described in this paper seeks to maximize output power. The 2-axis active mirror mount is operated in a spiral scan mode searching for the optimum alignment in 2 dimensions. Data (frame) updating rates of 20 frames per second have been achieved by using a small 2" diameter beryllium mirror driven by a pair of "shaker motors." Extensions of this and similar approaches to different practical applications will be discussed together with preliminary experimental results.© (1978) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Analysis of techniques for imaging through the atmosphere

Abstract: : A detailed analysis of turbulence effects in speckle interferometry, i.e., the Labeyrie technique, and in speckle imagery, the Knox-Thompson algorithm, is presented. Particular concern is devoted to the problem of isoplanatism, as originally we had thought that the Knox-Thompson algorithm might have a very large isoplanatic field-of-view. Our analytic results show that the isoplanatic patch size is approximately the same for Knox-Thompson and Labeyrie methods, and nearly the same as for adaptive optics. It is noted that, unlike the adaptive optics systems for which the ability to record high spatial frequency details is lost due to anisoplanatism, for the speckle methods anisoplanatism does not suppress the high spatial frequency details of the image - it merely jumbles them so that no real information is developed. From these results, it appears that a direct application of speckle techniques will not provide a significant relief from the anisoplanatism problems of adaptive optics. As part of the analysis of the Knox-Thompson method, we develop the conditions for the allowable spatial frequency separation (something done previously only by simulation). We find that the allowable spatial frequency separation should be less than 0.427 r sub 0/lambda, and preferably should be less than 0.2 r sub 0/lambda.

Computer-aided deformable-mirror system using differential angle control.

Abstract: The angles between neighboring small mirror elements are controlled by the signals from a microcomputer through properly arranged bimorph actuators so that an effective deformable mirror system is realized. This system has the following special features: (a) The surface of the mirror is continuous, and only the angles at the connecting edges between neighboring mirror segments are changed. This results in a smooth and largely deformable mirror surface with a smaller number of elemental mirrors than the conventional COAT system. (b) The errors in the mirror system, introduced in the fabrication of each mirror element and in the construction of the system, can be compensated by properly controlling the voltages supplied to the bimorphs. The information about these errors is obtained by previous measurements and stored in the computer. An experimental system was constructed, and some experimental results are shown. The results show the effectiveness of the method.

Historical Review Of Adaptive Optics Technology

Abstract: The history of adaptive optics is outlined from the concepts of 25 years ago to the many forms in which it is being applied today. Several independent paths of development are traced, leading to the emergence of a new technology that combines the disciplines of optics and electronics. Speculations are made on possible directions for the future.© (1978) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.