Directed Energy Weapons (DEW) utilize a variety of techniques to concentrate destructive levels of energy over vast distances almost instantaneously. For this reason, directed energy systems are considered to be candidate weapons for intercepting intercontinental ballistic missiles (ICBM's) as they are being launched, and before they are able to release their multiple, independently targeted reentry vehicles (MIRV'S bearing nuclear warheads. Considerable leverage is gained by the defense since a single missile killed during this boost phase will eliminate not only 6 to 10 RV's, but also perhaps hundreds of decoys which would otherwise be deployed to confuse and saturate subsequent defense layers.
A self-referencing interferometer that was designed to be an integrated system for measuring wavefronts from laser and laser diode sources is described. Experience has demonstrated that this system is applicable for measurements in a wide variety of optical test situations, many of which will be described here.
Conventional phase-shifting interferonetry can he adopted for pulsed wavefront sensing by introducing the phase shifts in parallel channels. The paper describes a three-channel phase detector for polarization interferometers, with signal processing modules that are suitable for large-scale parallel processing.
A variant of Kirchhoff's diffraction theory is used to obtain expressions for the light field diffracted from distorted grating patches on curved mirrors. Explicit formulae are obtained for the fields in the vicinity of the wavefront sensor for certain special cases, and numerical calculations of the diffraction pattern are carried out as a function of grating parameters.
"Seeing" effects of the Earth's atmosphere cause motion and blur of stellar images produced by ground-based telescopes. In order to sharpen the images over long exposures it is necessary to add a camera which produces an image of the pupil, i.e. of the primary mirror, followed by the reimaged field of stars. A vital feature of such a camera is the provision for analyzing the imperfections of the image of a guide star at the center of the field. Any movement of the guide star in excess of a small threshold is sensed and corrected by feedback to an active mirror located at the pupil. Small, rapid motions of the active mirror sharpen both the guide star and the nearby images that lie within the "aplanatic patch" where seeing effects are correlated with those of the guide star. The aplanatic patch covers a field of about 2.5 arcmin at most. Usually, some magnification of the images is required to match the pixel size of the detector. Optical designs are given of cameras optimized for use at the secondary foci of two different telescopes: the 3.6 meter Canada-France-Hawaii Telescope (CFHT), which has classical optics (paraboloidal primary); and the 2.2 metre University of Hawaii telescope, which has Ritchey-Chretien optics (hyperboloidal primary). Absorption in the infrared is avoided by using only mirrors in the cameras.
A conventional radial-shear interferometer is converted to a multi-channel phase-shifted interferometric system. Simple diffraction gratings are used to obtain three 90 degree phase-shifted interferograms simultaneously in Mach-Zender type interferometer layout. Parallel acquisition and digital data processing of the three interferograms make it possible to measure wavefront phases from either pulsed or CW light sources. Basic principles of this new interferometer are discussed and also preliminary laboratory results are shown.
A 10-11 cm aperture stellar scintillometer can measure atmospheric isoplanatic angles reliably provided the normalized variance and the zenith angle of the data are sufficiently, restricted. Within these constraints, the normalized variance data has a zenith angle power law that is quite close to the theoretical prediction. This is a self-consistent indication that the instrument is actually measuring the isoplanatic angle.
The transverse coherence length (r0) was directly measured approximately every 5 minutes from 29 August to 28 September 1984 at MTh, I,=1). An MTF-device was used with stellar sources for the ro observations. The diurnal variation of ro is discussed and related to processes affecting optical turbulence in the planetary boundary layer. A complete heat exchange study involving radiative, sensible heat, latent heat and soil heat fluxes is presented and related to ro values as well as to surface measured Cn. Of particular interest to the problem is the observed surface thermal activity. Vertical profile measurements of Cn, obtained from thermosondes at various times of the day and night, are presented to show the contributions of optical turbulence at various levels in the atmosphere to ro. Cn2 development as related to solar activity is discussed. The measured Cn2 profiles are compared to appropriate models.
We have obtained optical turbulence profiles consisting of values for the statistical parameter Cn-squared derived from stellar scintillations. Each profile consists of the values for Cm-squared determined for a collection of seven vertical positions (2-20km). These determinations are based upon measurements comprising the spatial spectrum of intensity scintillations observed across wavefronts arriving from a given star during a two-minute period of time. The profiles in this data base were obtained from two different locations over a period of several years; 1) LAOTFJ Advanced Optical Test Facility at Verona, New York; 2) LAMOSJ ARPA Maui Optical Station at Maui, Hawaii. We have time-analyzed the statistical fluctuations in the Cn-squared values obtained. This analysis has revealed that the turbulence fluctuations are statistically homogeneous for only relatively short periods of time. We shall report on several systematic changes in the statistical properties which we have founa to occur over longer periods of time.
Atmospheric turbulence below 300 meters has been probed with an Echosondel acoustic sounder at the Mauna Kea, Hawaii observatory. Continuous hourly averages for several months yield "characteristic" site profiles of and Cn2 with altitude as well as the diurnal signatures. Near surface acoustic turbulence soundings are compared to the results of observations using microthermal probes at the NOAA Boulder Atmospheric Observatory in Colorado and at Mt. Livermore near. McDonald Observatory in Texas.
A general algorithm is presented for reconstructing a two-dimensional wavefront optical path difference (OPD) map from noisy slope or difference measurements by means of a least squares fit using complex exponentials. This form of modal estimation can be described as a filtering operation in the spatial frequency domain. Thus fast Fourier transform (FFT) algorithms can be used for rapid reconstruction. The reconstruction is unbiased also in the case of finite data arrays. The error propagation from the noisy measurement data to the integrated wavefront is minimal in a least squares sense. It is believed that this reconstruction algorithm can be implemented in an adaptive optical system by using commercially available array processor hardware, thus reducing the total system cost and the need for specialized hardware.
Acoustooptic methods for adaptive filtering of temporal signals are discussed. Two specific architectures are presented: one utilizing space-integration alone and the other combining both time and space integrating techniques. Performance issues regarding the space-integrating system are discussed in detail, and a description and experimental results for the space-time integrating filter are presented.
Adaptive optical processors may be specified in terms of the algorithms they execute, the architectures they employ, and the devices they use. This paper briefly considers, as examples, adaptive linear discriminant algorithms, register-level and threshold logic architectures, and multipliers and threshold logic elements implemented using integrated optics.
7/65/35 composition of PLZT (lead, lanthanum, zirconate, titanate) is a photosensitive ferroelectric ceramic. The surface of this composition of PLZT deforms. The deformations are proportional to the integral number of photons incident on the ceramic during the exposing cycle. This characteristic was used to store a computer generated hologram and was the subject of the diffraction efficiency measurements.
A relatively simple 'all-optical' system, called a self-referenced interference phase loop (IPL), is presented for measuring spatial and temporal phase fluctuations over an optical wavefront, and also adaptively correcting that wavefront and/or generating a conjugate wave. It is shown that this system can unambiguously (with no phase quadrant ambiguity) estimate and compensate phase in real time over multiple Tr radians of dynamic range/ and is essentially unaffected by simultaneous wave amplitude variations. Furthermore, this system requires no external reference wavefront and can operate on partially coherent and multispectral (i. e, "white light") wavefronts, in which case it estimates and compensates optical-path-distance errors. In addition, the IPL readily lends itself to high resolution "all-optical" implementations containing thousands to millions of spatial resolution elements. The IPL can also be operated as a bistable array in optical information processing and computing applications.
This paper is a summary of work described previously on the integrated optics wavefront measurement sensor.1-3 In addition, the previously reported concept to use integrated guided-wave optics to process an optical wavefront with the conjugate of its phase distortion3 is described. In this concept, the distortion of an optical wavefront is removed by producing, using linear techniques, the phase conjugate of the distortion over a large array of waveguides. We believe that the program, the first phase of which is summarized in this paper, is the first application of integrated guided-wave optics to the analog processing of optical wavefronts.
We have developed the first commercial electro-optic contrast system for light microscopy and through our research we have provided new techniques for optical preprocessing to be used with vision systems.
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. Results from numerical simulations show that this technique shows promise for image sharpening under conditions of moderately severe turbulence.
rAtmospneric emission fluctuations due to spatial and temporal variations of the relative humidity along line-of-sight can he a significant source of noise for infrared sensor systems. A theoretical model of the infrared sky noise is described which can be used for arbitrary geometries. The model is in good agreement with clear daytime mountaintop observations.
Wavefront sensors that can operate at low light levels, be built from present technology components, and provide accurate wavefront phase estimates in real-time are required for use with adaptive optics systems. The use of estimation theory makes possible the evaluation of wavefront sensors without specification of the wavefront phase estimation algorithms. The Cramer-Rao method was used to find a lower bound on integrated rms wavefront sensor estimation error. In addition to an analysis of the general case, the error lower bound was numerically evaluated for the shearing interferometer wavefront sensor. Computer simulations of the atmosphere and wavefront sensor measurements including noise were per-formeo. Using an appropriate algorithm, the phase was estimated and the resulting phase error was compared with the lower bound. The results support the validity of using the Cramer-Rao lower bound to evaluate wavefront sensor performance.
Two Atmospheric Boundary Layer (ABL) models, the Kukaharets/Tsvang and the Kaimal models are used to obtain Cn2 profiles during daytime conditions for comparison with balloon-borne thermosonde measurements. Path averaged Cn2 measurements obtained on a 14 meter tower are used as inputs to the models. Values of ro2(coherence length) and 00 (isoplanatic angle) are calculated by integration of model expressions for Cn as a function of altitude. The VanZandt model is used to obtain ro and 00 conibutions from above the ABL. These are compared to values obtained by integration over the thermosonde Cn height profiles. Comparisons are also made to isoplanometer and MTF device measurements made near the ground.
A measurement program, jointly conducted by the Army's Atmospheric Sciences Laboratory (ASL) and the Air Force Geophysics Laboratory (AFGL) was undertaken at the High Energy Laser Instrumentation Development Laboratory (HIDL), White Sands Missile Range (WSMR) site, between 29 August and 28 September 1984. During this 31 day test period, meteorological and turbulence data were acquired 24 hours per day, seven days per week. A large data base of surface and tower measurements of wind, temperature, turbidity, heat fluxes, etc. and measurements of upper atmosphere turbulence, temperature, density, and humidity now exists. The purpose of this program was to characterize the optical turbulence and obtain cloud cover and distribution statistics in the general area of the HIDL test site. An overview of the test, representative data, and the data base is described.
Phase sensors that are most commonly used in the adaptive-optics area typically measure the gradient of the phase. A phase reconstructor is necessary to obtain the phase at the actuator positions of the deformable mirror. In the past reconstructors to obtain the optical phase from gradient measurements have been built using resistive nets. These nets simulate a least-squares reconstruction algorithm. There are other algorithms which can be used to mate wavefront sensors and deformable mirrors with different geometries or which can improve the noise performance by using the spatial correlation of the phase. These types of algorithms are difficult to implement and change using analog techniques. In addition, since the movement of an actuator can influence the position of adjacent actuators it is desirable to include this effect in the reconstructor. One may also want to remove the piston and the tip and tilt from the signal applied to the deformable mirror, and determine the values of the focus and tip and tilt terms in order to provide signals to auxiliary mirrors. A digital reconstructor can provide this capability. An approach to a digital reconstructor which can calculate an optical phase which is any linear function of the gradient measurements is described. This reconstructor is based on using a multiplier-accumulator circuit in each channel. A single phase value is calculated in each channel by summing the result of multiplying each gradient measurement by a stored matrix coefficient. Several sets of matrix coefficients are stored in memory to allow one to change the reconstruction algorithm quickly. The circuitry used and the time taken to perform the reconstruction will be described.