Currently there are extensive modeling and measurement capabilities for the region extending from 100 ft above sea
surface to space, but few such capabilities exist for the region extending up to 10 ft above the sea surface. By measuring
and characterizing conditions in the marine boundary layer existing below 30 ft above the sea surface such as turbulence
and extinction, the optical communication capabilities of maritime vessels when operating at or near the surface may be
extended and enhanced. Key physical parameters such as absorption, scattering, and turbulence strength (C<sub>n</sub>
<sub>2</sub>) along the
propagation path have a degree of variability on meteorological conditions as well optical wavelength. Modeling of the
atmospheric environment is thus critical in order to generate a good understanding of optical propagation through the
atmosphere. NUWC is utilizing software provided by MZA to model Cn
2 and resultant beam propagation characteristics
through the near-marine boundary layer. We are developing the capability of near-marine boundary layer atmospheric
and turbulence measurements and modeling as well as optical laser link testing at outdoor test sites. Measurements are
performed with optical laser links (e.g., bit rate error), scintillometer, and particle image velocimetry (PIV) cameras,
while turbulence and propagation modeling is achieved using MODTRAN5, ATMTools, NSLOT, LEEDR, and
WaveTrain modeling and simulation code. By better understanding the effects of turbulence on optical transmission in
the near-marine boundary layer through modeling and experimental measurements, measures can be implemented to
reduce the bit error rate and increase data throughput, enabling more efficient and accurate communication link
We present and analyze experimental results of lab-based open-loop turbulence simulation utilizing the Adaptive Aberrating Phase Screen Interface developed by ATK Mission Research, which incorporates a 2-D spatial light modulator manufactured by Boulder Nonlinear Systems. These simulations demonstrate the effectiveness of a SLM to simulate various atmospheric turbulence scenarios in a laboratory setting without altering the optical setup. This effectiveness is shown using several figures of merit including: long-term Strehl ratio, time-dependant mean-tilt analysis, and beam break-up geometry. The scenarios examined here range from relatively weak (<i>D/r<sub>o</sub></i> = 0.167) to quite strong (<i>D/r<sub>o</sub></i> = 10) turbulence effects modeled using a single phase-screen placed at the pupil of a Fourier Transforming lens. While very strong turbulence scenarios result long-term Strehl ratios higher than expected, the SLM provided an accurate simulation of atmospheric effects for conventional phase-screen strengths.
The use of high-energy laser (HEL) weapon systems in tactical air-to-ground target engagements offers great promise for revolutionizing the USAF's war-fighting capabilities. Laser directed-energy systems will enable ultra-precision strike with minimal collateral damage and significant stand-off range for the aerial platform. The tactical directed energy application differs in many crucial ways from the conventional approach used in missile defense. Tactical missions occur at much lower altitudes and involve look-down to low-contrast ground targets instead of a high-contrast boosting missile. At these lower altitudes, the strength of atmospheric turbulence is greatly enhanced. Although the target slant ranges are much shorter, tactical missions may still involve moderate values of the Rytov number (0.1-0.5), and small isoplanatic angles compared to the diffraction angle. With increased density of air in the propagation path, and the potential for slow-moving or stationary ground targets, HEL-induced thermal blooming will certainly be a concern. In order to minimize the errors induced by tracking through thermal blooming, offset aimpoint tracking can be used. However, this will result in significant tilt anisoplanatism, thus degrading beam stabilization on target. In this paper we investigate the effects of extended turbulence on tracking (or tilt) anisoplanatism using theory and wave optics simulations. The simulations show good agreement with geometric optics predictions at angles larger than about 5 micro-radians (asymptotic regime) while at smaller angles the agreement is poor. We present a theoretical basis for this observation.
There is a current push to place large telescopes in geosynchronous earth orbit to improve the imaging resolution of earth observing systems. The concept is to use inflatable or deployable optical systems to get diffraction-limited imaging from a thirty-meter class telescope in space. While the resolution of conventional ground based telescopes is limited by near field atmospheric distortions, space based systems have no such limitation. However, the issue of signal strength must be considered. In this paper, the effects of noise on imaging performance are investigated in terms of the frequency domain signal-to-noise ratio. The resolution in the presence of noise is determined by the spatial frequency at which the SNR drops below unity. It is shown that for a power law (f<sup>-n</sup>) object spectrum the resolving power is not proportional to the square of the aperture diameter, D, but rather D<sup>m</sup> where m is less than two and dependent upon object spectrum power law relationship, n.
This study extends branch point tolerant phase reconstructor research to examine the effect of finite time delays and measurement error on system performance. Branch point tolerant phase reconstruction is particularly applicable to atmospheric laser weapon and communication systems, which operate in extended turbulence. We examine the relative performance of a least squares reconstructor, least squares plus hidden phase reconstructor, and a Goldstein branch point reconstructor for various correction time-delays and measurement noise scenarios. Performance is evaluated using a wave-optics simulation that models a 100km atmospheric propagation of a point source beacon to a transmit/receive aperture. Phase-only corrections are then calculated using the various reconstructor algorithms and applied to an outgoing uniform field. Point Strehl is used as the performance metric. Results indicate that while time delays and measurement noise reduce the performance of branch point tolerant reconstructors, these reconstructors can still outperform least squares implementations in many cases. We also show that branch point detection becomes the limiting factor in measurement noise corrupted scenarios.
Desirable features of any digital image resolution- enhancement algorithm include exact interpolation (for 'distortionless' or 'lossless' processing) adjustable resolution, adjustable smoothness, and ease of computation. A given low-order polynomial surface (linear, quadratic, cubic, etc.) optimally fit by least squares to a given local neighborhood of a pixel to be interpolated can enable all of these features. For example, if the surface is cubic, if a pixel and the 5-by-5 pixel array surrounding it are selected, and if interpolation of this pixel must yield a 4- by-4 array of sub-pixels, then the 10 coefficients that define the surface may be determined by the constrained least squares solution of 25 linear equations in 10 unknowns, where each equation sets the surface value at a pixel center equal to the pixel gray value and where the constraint is that the mean of the surface values at the sub-pixel centers equals the gray value of the interpolated pixel. Note that resolution is adjustable because the interpolating surface for each pixel may be subdivided arbitrarily, that smoothness is adjustable (within each pixel) because the polynomial order and number neighboring pixels may be selected, and that the most computationally demanding operation is solving a relatively small number of simultaneous linear equations for each pixel.
Proc. SPIE. 4493, High-Resolution Wavefront Control: Methods, Devices, and Applications III
KEYWORDS: Microelectromechanical systems, Bistability, Mirrors, Modulation, Control systems, Micromirrors, Digital micromirror devices, Chemical elements, Control systems design, Electromechanical design
This research involves the design and implementation of a complete line-addressable control system for a 32 X 32 electrostatic piston-actuated micromirror array device. Line addressing reduces the number of control lines from N<SUP>2</SUP> to 2N making it possible to design larger arrays and arrays with smaller element sizes. The system utilizes the electromechanical bistability of individual elements to hold arbitrary bistable phase patterns, a technique previously used on tilt arrays. The control system applies pulse width modulated (PWM) signals to the rows and columns of the device to generate a static phase pattern across the array. Three modes of operation are considered and built into the system. The first is the traditional signal scheme which requires the array to be reset before a new pattern can be applied. The second is an original scheme that allows dynamic switching between bistable patterns. The third and final mode considered is an effective voltage ramp across the device by operating above mechanical cutoff. Device characterization and control system testing are conducted on samples from two different foundry processes. The test results showed that the control system was successfully integrated, however individual bistable control was not successfully demonstrated on the micromirror arrays tested. The inability to demonstrate bistable control is attributed to flaws in the device and variations in snap-down voltage with the application of PWM signals below mechanical cutoff. Methods to correct these flaws for a future redesigned line- addressable device are proposed.
Wave optics propagation codes are widely used to simulate the propagation of electromagnetic radiation through a turbulent medium. The basis of these codes is typically the two dimensional Fast Fourier Transform (FFT). Conventional FFTs (i.e. the standard Matlab FFT) do not use parallel processing and for large arrays, the processing time can be cumbersome. This research investigates the use of network- based parallel computing using personal computers. In particular, this study uses the Air Force Institute of Technology (AFIT) Bimodal Cluster a heterogeneous cluster of PCs connected by fast Ethernet for parallel digital signal processing using an FFT algorithm developed for use on this system. The parallel algorithms developed for the Parallel Distributed Computing Laboratory could greatly increase the computational power of current wave optics codes. The objective of this research is to implement current parallel FFT algorithms for use with wave optics propagation codes and quantify performance enhancement. With the parallel version of the FFT implemented into existing wave optics simulation code, high resolution simulations can be run in a fraction of the time currently required using conventional FFT algorithms. We present the results of implementing this parallel FFT algorithm and the enhanced performance achieved over the Matlab FFT2 function.
The Air Force Phillips Laboratory is in the process of demonstrating an advanced space surveillance capability with a heterodyne laser radar (ladar) system. Notable features of this ladar system include its narrow (< 1.5 ns) micropulses, contained in a pulse-burst waveform that allows high-resolution range data to be obtained, and its high power (30 J in a pulse burst), which permits reasonable signal returns from satellites. The usefulness of these range data for use in reflective tomographic reconstructions of satellite images is discussed. A brief review of tomography is given. Then it is shown that the ladar system is capable of providing adequate range-resolved data for reflective tomographic reconstructions in terms of range resolution and sampling constraints. Mathematical expressions are derived which can be used to convert the ladar returns into reflective projections. Image reconstructions from computer-simulated data which include the effects of laser speckle and photon noise are presented and discussed. These reconstructions contain artifacts even in the absence of noise, due to the inadequacies of the standard tomographic problem formulation to accurately model the reflective projections obtained from the ladar system. However, object features can still be determined from the reconstructions when typical noise levels are included in the simulation.
The resolution achieved by an optical imaging system in the presence of the random effects of the atmosphere is severely degraded from the theoretical diffraction limit. Techniques exist for recovering near diffraction-limited performance of an imaging system in the presence of atmospheric turbulence. These image enhancement techniques include speckle imaging, deconvolution, and adaptive optics. A turbulence chamber has been designed and built for laboratory testing of current and future adaptive optics and image enhancement techniques. The turbulence is produced within a chamber consisting of two small fans and a heating element. The effects of the generated turbulence on optical propagation are directly measured by sensing the perturbed wavefront phase. The wavefront phases are measured using a shearing interferometer. The statistical properties of the turbulence are then characterized by means of estimating the phase structure function from the wavefront phase measurements. We found that the estimate of the phase structure function depends only on the magnitude of the separation between two points on the optical wavefront and follows the Kolmogorov 5/3 power law.
The Air Force Phillips Laboratory is upgrading the surveillance capabilities of its AMOS facility with a coherent laser radar system. A notable feature of this laser radar system is its short (approximately equals 1 ns) pulselength which allows high resolution range data to be obtained. The usefulness of this range data for use in reflective tomographic reconstructions of images of space objects is discussed in this paper. A brief review of tomography is given. Then the capability of the laser radar system to provide adequate range-resolved data is analyzed, both in terms of system parameters and signal-to-noise issues. Sample image reconstructions are presented and discussed.
The Air Force Phillips Laboratory is developing a coherent laser radar system to upgrade its space surveillance capabilities. Because of the short pulse length of this laser system, range resolved information can be obtained. This range information can be used to reconstruct images by reflective tomography. This paper presents results of simulations using four different transmission tomography algorithms to reconstruct images from reflective tomography data. The transmission tomography problem formulation is stated, a description of reflective tomography is given, and results of the simulations are presented.
This paper describes an experiment characterizing the statistical properties of laboratory generated turbulence using a shearing interferometer based wavefront sensor. The statistics of the turbulence have been characterized by taking many measurements of optical wavefronts that have propagated through the turbulence. The wavefront sensor is capable of both high time and spatial resolution. The wavefront sensor measures the wavefront phase over a circular area corresponding to the size of the propagated laser beam. Nearly 256 samples of the wavefront phase in both the x- and y-directions are sampled per measurement. From these optical phase measurements, a phase structure function was calculated. Laboratory generated turbulence that produces locally homogeneous and isotropic disturbances has been developed for the purpose of studying current and future adaptive optics and image enhancement techniques.
The ability to accurately measure the phase of the wavefront in an amplitude interferometer is fundamentally limited by the light level. Under high light conditions, the variance of the phase measurement is inversely proportional to the number of photons detected. In this paper, we review the basic theory of phase measurement for an optical heterodyne array imaging system for high light conditions. The theory is then extended to a sheared coherent interferometric photography (SCIP) system. Simulation and laboratory results verifying the theory and extending it to low light levels are then presented.