A new method of reconstructing and predicting an unknown probability density function (PDF) characterizing the
statistics of intensity fluctuations of optical beams propagating through atmospheric turbulence is presented in this
paper. The method is based on a series expansion of generalized Laguerre polynomials ; the expansion coefficients are
expressed in terms of the higher-order intensity moments of intensity statistics. This method generates the PDF from
the data moments without any prior knowledge of specific statistics and converges smoothly. The utility of
reconstructed PDF relevant to free-space laser communication in terms of calculating the average bit error rate and
probability of fading is pointed out. Simulated numerical results are compared with some known non-Gaussian test
PDFs: Log-Normal, Rice-Nakagami and Gamma-Gamma distributions and show excellent agreement obtained by the
method developed. The accuracy of the reconstructed PDF is also evaluated.
We present results from simulation of an Optical Synthetic Aperture radar system(OpSAR) using a wave optics propagation model. This model allows phase and amplitude errors to be introduced anywhere along the propagation path. We show the effects of uncompensated phase errors caused by atmospheric turbulence. We demonstrate azimuth phase error compensation using a sharpness metric based on entropy minization.
In this paper, we present the results of the phase diversity algorithm applied to simulated and laboratory data. We show that the exact amount of defocus distance does not need to be known exactly for phase diversity algorithm. We determine, through simulation, the optimum diversity distance. We compare the aberrations recovered with the phase diversity algorithm and those measured with a Fizeau interferometer using a HeNe laser. The two aberration sets agree with a Strehl of over 0.9. The contrast of the recovered object is found to be 10 times that of the raw image.
A methodology for analyzing an imaging sensor's ability to assess target properties is developed. By applying Cramer- Rao covariance analysis to a statistical model relating the sensor measurements to the target, a bound on the accuracy with which target properties can be estimated can be calculated. Such calculations are important in understanding how a sensor's design effects its performance for a given assessment task, and in performing feasibility studies or trade studied between sensor designs and sensing modalities. A novel numerical model relating a sensor's measurements to a target's three-dimensional geometry is developed in order to overcome difficulties in accurately performing the required numerical computations. An example use of the approach is presented in which the influence of viewing perspective on orientation accuracy limits is analyzed. The example is also used to examine the potential for improving the accuracy bound by fusing multi-perspective data.
The use of all classes of space systems, whether owned by defense, civil, commercial, scientific, allied or foreign organizations, is increasing rapidly. In turn, the surveillance of such systems and activities in space are of interest to all parties. Interests will only increase in time and with the new ways to exploit the space environment. However, the current space awareness infrastructure and capabilities are not maintaining pace with the demands and advanced technologies being brought online. The use of surveillance technologies, some of which will be discussed in the conference, will provide us the eventual capability to observe and assess the environment, satellite health and status, and the uses of assets on orbit. This provides us a space awareness that is critical to the military operator and to the commercial entrepreneur for their respective successes. Thus the term 'dual-use technologies' has become a reality. For this reason we will briefly examine the background, current, and future technology trends that can lead us to some insights for future products and services.
Sheared Coherent Interferometric Photography (SCIP) is an active imaging technique that allows near-diffraction limited imaging of objects through turbulent media. This paper presents computer simulation and laboratory results that illustrate the viability of the technique.
We investigate the way laser-speckle noise and limited signal power affect the quality of images reconstructed from diffraction field data obtained with a pupil-plane array of optical heterodyne detectors. Modeling the detected signal from each detector sub-element as a circular-complex Gaussian random variable and taking into account the random amplitudes of the detected signals arising from laser-speckle effects, we compute the SNR of the Fourier spectrum of the coherent intensity image formed from the array of heterodyne field measurements. The resulting SNR expression is compared to that obtained earlier, P.S. Idell (1988), for the same quantity estimated from photocount-limited, focal plane detector measurements. This comparison shows that the spatial frequency SNR performance of both systems is identical when the systems are operated at high signal-level operating conditions (number of signal photocounts per speckle much greater than one). When signal levels drop significantly below one signal photocount/speckle, we find that focal plane imaging performs somewhat better at estimating all but the very highest spatial frequencies.
A straight-forward computer simulation technique is presented for determining the spatial intensity distribution that can be expected when a rough object is illuminated with partially coherent light. The technique is useful for problems in which the illumination source is a laser whose spectral content can be described as a sum of delta function components. Laboratory and computer simulation results that illustrate illumination coherence effects on imagery obtained with the method known as imaging correlography are presented.
Second-, third-, and fourth-order intensity correlations measured in the field in the pupil plane are used to construct the amplitude and phase of the two-dimensional mutual coherence function. Information about the noncoherent object is derived by a two-dimensional spatial Fourier transform of the mutual coherence function. A computer simulation of the Fourier domain laser speckle patterns is used to provide data from which the expected second-, third-, and fourth-order intensity correlations are computed. These correlations are used in the program for the explicit reconstruction of the phase. In addition, the signal-to-noise ratio (SNR) is discussed with reference to the measured integrated intensity, ?0TI(t)dt, as compared to the theoretically assumed instantaneous intensity,I(t). The study of the SNR for the second-, third-, and fourth-order intensity correlations involves higher-order intensity correlations. With the assumed Gaussian statistics of the wave amplitude, the analytical expressions for the higher-order correlations are algebraically complex. The SNR for the third-order case is discussed. For further development, symbolic manipulation programs (e.g., DERIVE, MATHEMATICA, or MACSYMA) will be used. The discussion of the signal-to-noise ratio applies to intensity correlation interferometry (low light levels) for which the integration time, T, is large compared to the coherence time, ?c, that is, T >> ?c. We will consider the case for laser speckle interferometry for which ?c » T in our follow-up work.
It has been suggested that high resolution images of laser illuminated objects can be digitally synthesized from measurements of the wavefront slope (gradient) associated with the backscattered laser-speckle field. We describe the image synthesis procedure and present images reconstructed from computer simulated laser-speckle fields. Noise was added to simulated wavefront-difference measurements to illustrate the effect on the imagery. We also describe a Shack-Hartmann type of wavefront sensor that was designed and built at the Weapons Laboratory and initially used to investigate the distribution of ray directions in a speckle field. Imaging results obtained with the sensor in the laboratory are presented and we describe an adaptation of the basic imaging technique that can be used to image coherently illuminated objects through optical phase distortions.