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.
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.
One of the simplest active optical components is a device that allows rapid electronic control of the angular orientation of small mirrors. This paper describes a two-dimensional tip/tilt mirror positioner that has been shown to satisfy the exacting microradian requirements of beam stabilization and to be of use in solving some of the miscellaneous technically challenging HEL instrumentation problems.
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.
Conventional instruments for measuring or correcting refractive errors of the human eye contain large numbers of individual spherical and cylindrical lenses. Refracting instruments having continuously variable power (optometers) are mechanically simpler and allow smooth adjustment of power by the subject (subjective optometers) or smooth adjustment of power by automated photo-electronic sensing mechanisms (objective optometers). Optometers with spherical optical systems have had some clinical success, but those with spherocylindrical optical systems, described since the 1800's, have never been practically useful. Power ranges have been too small, scales have been non-linear, and adjustments have been awkward. Renewed interest in the automation of clinical refraction has led to the development of a spherocylindrical optical system overcoming the disadvantages of previous systems. Cylindrical lenses movable in a prescribed fashion along the optical axis provide continuously variable spherocylindrical power over a wide linear range. This system, in addition to forming the basis of a new refracting instrument, may provide a useful means for manipulation of spherocylindrical power in other optical applications.
The objective of this paper is to present in simple and condensed form,key physical aspects of adaptive optical systems so as to allow non-specialists to make first order estimates of performance and thus gain an appreciation of what such systems may offer in an application of interest. The historical review given at the outset of the preceding session indicated that two key components were required to implement such systems, namely a deformable or "rubber" mirror (or locally controlled refractor) and a wavefront sensor. In both elements spacial and temporal response capabilities adequate to the problem at hand were required. Once such wavefront correction and sensor devices were available, control theory suggested a number of the systems now covered under the generic name adaptive or active optics.
The operational limitation on many high-resolution optical systems and, in particular, on high-resolution microdensitometers, is determined by the system's ability to establish and hold critical focus under dynamic conditions . A precision focus control, utilizing pneumatic actuation and feedback, is described which provides the needed dynamic focusing to permit the acievement of submicron optical resolution. This focus control system, although initially designed as an integral subsystem of the Aerodyne Linear Microdensitometer, has been incorporated as an add-on to PDS 1050 Microdensitometers and to the David Mann 10-47 Microdensitometer. The Precision Focus Control, in its standard configuration, has provided focusing accuracy to better than ±0.5 µm at disturbance frequencies to 10 Hz. The set focus position can be varied from slightly above the target surface to 25 µm below the target surface, while focus can be maintained over a range of surface variations of more than 1.5 mm. Because of the pneumatic pressure reacting against the object surface, local film hold-down is automatically provided without any mechanical contact of the object surface. Object thicknesses of up to 6 mm can be accommodated.
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.
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.
Wavefront sensing is a critical function of any active optical system, and the sensor of choice depends quite critically on the application. The various sensors available are surveyed, and comparisons are developed in several operating scenarios.
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).
Classical interferometric measurement of mirrors is severely limited in accuracy as well as utility. When considered for use in characterizing dynamic performance of active mirrors, it is only of marginal utility. By a unique utilization of heterodyne interferometry, a method has been developed whose processes are ideally adaptable to the short frame times, high frame rates and high spatial frequencies required in active element measurements. Results showing accurate and high spatial resolution measurements will be presented. Concepts for high frame rates and noise immune systems will be also discussed.
By pulsing the illumination in an optical testing interferometer, the techniques of stroboscopy may be applied to interferometric testing. Stroboscopic interferometry can be used to visualize, in slow motion, the optical fringe motion in repetively excited test objects. Because the fringes are observed in slow motion, small dynamic fringe deformations (less than one fringe) become apparent even when they are not readily discerned as a reduction in fringe contrast in a time-averaged interferogram. An application of the technique is described in which the surface motions resulting from high order solid body resonances in an active optical mirror were made visible and recorded on a video tape.
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.
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.
Using laser radiation as the energy input to a rocket, it is possible to consider the transfer of large payloads economically between low initial orbits and higher energy orbits. In this paper we will discuss the results of an investigation to use a ground-based High Energy Laser (HEL) coupled to an adaptive antenna to transmit multi-megawatts of power to a satellite in low-earth orbit. Our investigation included diffraction effects, atmospheric transmission efficiency, adaptive compensation for atmospheric turbulence effects, including the servo bandwidth requirements for this correction, and the adaptive compensation for theral blooming. For these evaluations we developed vertical profile models of atmospheric absorption, strength of optical turbulence (CN2), wind, temperature, and other parameters necessary to calculate system performance. erformed for Our atmospheric investigations were for CO2, 12C1802 isotope, CO and DF wavelengths. For all of these considerations, output antenna locations of both sea level and mountain top (3.5 km above sea level) were used. Several adaptive system concepts were evaluated with a multiple source phased array concept being selected. This system uses an adaption technique of phase locking independent laser oscillators. When both system losses and atmospheric effects were assessed, the results predicted an overall power transfer efficiency of slightly greater than 50%.