The decomposition of turbulence-induced phase aberrations into Zernike polynomials is performed using simulation and numerical techniques, extending well-known analytic results when scintillation is weak. A spherical-wave geometry is assumed. Strong scintillation (Rytov variance > 0.3) has an impact on the distribution of aberration strength, and this impact depends on range and wavelength. A saturation effect is observed. Anisotropy affects the distribution of aberrations between nearby Zernike orders. Non-Kolmogorov exponents lower than 11/3 in magnitude tend to reduce the lower-order aberrations and slightly enhance the higher aberrations, as expected. The interplay of strong turbulence, anisotropy, and non-Kolmogorov exponents is also explored. Significant deviations from the existing weak-turbulence theory are found in some cases.
Fractal-based phase screens are compared to subharmonic-augmented FFT-based phase screens using both analytic and numerical statistical methods. Properties such as homogeneity and stationarity are investigated. It is shown analytically that augmented FFT-based screens are homogeneous and strict-sense stationary. Analytic means are also used to show that fractal-based phase screens are not stationary based on the definition of fractal Brownian surfaces. Corresponding numerical results show that the structure functions in both cases appear to be stationary or nearly so. It is shown that both types of phase screens must have “creasing” that has been observed in the power spectrum, due to edge effects. FFT-based screens without subharmonic augmentation, on the other hand, are shown to avoid such creasing. Sample results are also presented for imaging reconstructions with both types of screens.
Formation of images in strong turbulence (spherical-wave log-amplitude variance greater than 0.25) is a well-known challenge. Such imaging with coherent active illumination poses special difficulties due to the presence of laser speckle in the image, which is similar to atmospheric speckle and so it is difficult to separate the two forms of speckle. However, imaging with laser illumination has the potential benefit of enhanced signal and added information in the laser speckle. In this paper, several image processing approaches are investigated with simulation that show promise for good image quality in such conditions. These approaches include pupil-plane techniques that filter phase based on an assumed atmospheric spatial phase structure function, branch-cut filtering and intensity-weighted branch cut minimization. Also included in the analyses are focal-plane techniques. Significant improvements can be found for Rytov variances up to and beyond 0.4 and up to 10 atmospheric coherence lengths (r0) across the aperture, in uniform turbulence scenarios over a 30 km range with objects of limited extent. The approaches are optimized for fast execution and minimal number of image frames required to form an acceptable image. The results are compared using several image metrics and also compared with a corresponding idealized adaptive-optics approach using an incoherent beacon.
An important consideration for the laser beam propagation through a long atmospheric path is the geometry of the optical path and random variations in the refractive index due to atmospheric turbulence. Here, we consider a plane wave propagation through a 10km medium to investigate the deep turbulence effects on the beam propagation using phase screen approach. The turbulence effects are modeled by non-Kolmogorov descriptions of energy cascade theory, known as beta-model. The beta-model incorporates space-filling concepts for the turbulent eddies in the inertial ranges using fractal descriptions for the eddies. Metrics based on intensity and phase variances and number of zero intensity values are analyzed for various levels of turbulence intensity (measured by Cn2) by choosing the value of power law exponent from a range of eligible values. It is observed that metrics based on intensity variance saturate but metrics based on phase variance show potential for characterizing stronger turbulence effects.
Imaging of distant objects in a terrestrial environment involves propagation of light through significant turbulence. Conventional methods for imaging for these applications are radar or focal plane imaging. Both of these methods have limitations, such as object rotation rate and post-processing. A different class of imaging approaches involving coherent illumination has several advantages (a) reduced sensitivity to illumination conditions, (b) reduced sensitivity to object rotation, (c) the use of arrayed receivers that are lighter and lower in cost, and (d) snapshot reconstruction of aberrated images with just one or a few frames. These advantages must be balanced by the challenges of speckle noise in the image reconstructions, and more difficult algorithms. Coherent pupil-plane and focal-plane techniques are investigated for image formation. The associated algorithms include root-reconstruction techniques, phase smoothing methods, polynomial fit approaches, blind iterative deconvolution, and multi-frame blind deconvolution (MFBD). These techniques and algorithms are surveyed for speed and quality of image formation.
Video imagery is extremely useful in developing an intuitive sense for the effects of atmospheric turbulence in long, horizontal-path imaging. This is especially true when attempting to understand the effects of Non-Kolmogorov and anisotropic turbulence. We have created simulated video sequences featuring a static scene where parameters such as turbulence strength, power-law exponent, direction and degree of anisotropy are varied. Because these image sequences are simulated, it is possible to explore the effects of these parameters using non-subjective measures such as the Mean Squared Error. We find that changing the power-law exponent while leaving the Fried parameter fixed results in more blurring but less anisoplantic tip-tilt distortions at lower power laws. Images have subjectively less blurring and more global tip-tilt distortion when the power law is higher than the Kolmogorov value. We also observe that Mean Squared Error increases approximately linearly with the degree of anisotropy.
Phase Screen simulations for laser propagation through non-Kolmogorov turbulence are presented and the results for
scintillation index and correlation functions for the intensity are compared with the theory at low turbulence levels at
selected non-Kolmogorov exponents. Additional simulation results are presented the strong turbulence region. In
particular, effects of transitioning from Kolmogorov to non-Kolmogorov turbulence using their spectral equivalence
at the Fresnel scale (as suggested in the literature) on the scintillation index and correlation functions at the receiver
are examined for two example paths.
Remote Ultra-Low Light Imaging detectors are photon limited detectors developed at Los Alamos National
Laboratories. RULLI detectors provide a very high degree of temporal resolution for the arrival times of detected photoevents,
but saturate at a photo-detection rate of about 106 photo-events per second. Rather than recording a conventional
image, such as output by a charged coupled device (CCD) camera, the RULLI detector outputs a data stream consisting
of the two-dimensional location, and time of arrival of each detected photo-electron. Hence, there is no need to select a
specific exposure time to accumulate photo-events prior to the data collection with a RULLI detector - this quantity can
be optimized in post processing. RULLI detectors have lower peak quantum efficiency (from as low as 5% to perhaps as
much as 40% with modern photocathode technology) than back-illuminated CCD's (80% or higher). As a result of these
factors, and the associated analyses of signal and noise, we have found that RULLI detectors can play two key new roles
in SSA: passive imaging of exceedingly dim objects, and three-dimensional imaging of objects illuminated with an
appropriate pulsed laser. In this paper we describe the RULLI detection model, compare it to a conventional CCD
detection model, and present analytic and simulation results to show the limits of performance of RULLI detectors used
for SSA applications at AMOS field site.
As the demand for increased bandwidth continues to grow, the implementation of 10 Gbit/s systems is underway and telecommunications companies are now beginning to see a need for 40 Gbit/s per channel. One of the key enablers for building such 40 Gbit/s systems is the availability of high bandwidth receivers. At 40 Gbit/s the fundamental tradeoff between the transit time limited bandwidth and the internal quantum efficiency for conventional surface illuminated PIN detectors makes them unsuitable. It is therefore necessary to employ a waveguide photodetector design. Many 40 Gbit/s systems employ return-to-zero transmission formats and polarization interleaving. This added complexity places additional demands on both the bandwidth and the polarization dependence of these detectors. The high demands placed on the detector design make an optoelectronic integrated circuit approach for the receiver very difficult to realize. In this paper, we will discuss a 40 Gbit/s receiver with a responsivity greater than 0.7 A/W and polarization dependence of less than 0.1 dB over the wavelength range from 1500 nm to 1620 nm. This receiver has a 3dB bandwidth of 40 Ghz, a conversion gain as high as 120 V/W, < 20 pA/(root) Hz input referred noise, < -7 dBm sensitivity, low deviation from linear phase, and a linear output swing greater than 0.8 V p-p.
The effects of round-trip atmospheric turbulence on ladar for unresolved target detection are being investigated using a Monte Carlo code with many phase-screens to simulate atmospheric turbulence effects. These phase-screens are located along the outward path of the laser-mode and the inward path of the backscattered laser speckle pattern. The targets used are variable in size and smaller than the propagated laser-mode transverse dimension, and they are therefore termed 'unresolved'. In this paper previous round- trip turbulence analyses and data are reviewed, and the current Monte Carlo simulation code for unresolved targets is discussed. Simulation results to date are presented indicating that intensity fluctuations are best described by a new two-parameter K-distribution probability density function. This intensity distribution may then be used in deriving a ladar receiver-operating-characteristic for determining the target detection probability including round-trip turbulence.
Analytical expressions for the sensitivities of the S-parameters of a MESFET are presented. These expressions are useful for optimizing the gate and source metallization resistances for design of MESFETs. The final expressions are given in terms of general circuit impedances and therefore can be used in analyzing other solid state three terminal active devices.
The statistics of the intensity and
phase of a Gaussian laser speckle pattern
constitutes an important study in several
optical systems. Often such a study is limited
to the first and second order statistics
because calculations of the third and higher
order statistics, though striaght forward
involve lengthy algebraic manipulations. It
is demonstrated that the use of computeraided
symbolic manipulations simplifies the
calculation of the higher-order correlations,
in which research interests are growing.
Some examples are given.
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