Michael T. Valley

Generation of infinitely long phase screens for modeling of optical wave propagation in atmospheric turbulence

Numerical modeling of optical wave propagation in atmospheric turbulence is traditionally performed by using the so-called ‘split’-operator method, where the influence of the propagation mediums refractive index inhomogeneities is accounted for only within a set of infinitely narrow phase distorting layers (phase screens). These phase screens are generated on a numerical grid of finite size, which corresponds to a rather narrow slice (spatial area) of atmospheric turbulence. In several important applications including laser target tracking, remote sensing, adaptive optics, and atmospheric imaging, optical system performance depends on atmospheric turbulence within an extended area that significantly exceeds the area associated with the numerical grid. In this paper we discuss methods that allow the generation of a family of long (including infinitely long) phase screens representing an extended (in one direction) area of atmospheric turbulence-induced phase distortions. This technique also allows the generation of long phase screens with spatially inhomogeneous statistical characteristics.

Image enhancement by local information fusion with pre-processing and composed metric

An imaging enhancement technique capable of mitigating the effect of atmospheric turbulence on wide field-of-view images is presented. The method is based on the local information fusion from a set of short-exposure anisoplanatic images. Since anisoplanatism affects short-exposure images locally, the synthetic imaging algorithm uses local image quality information. Pre-processing of the source data and use of a composed image quality metric are considered. The synthetic imaging technique is tested on experimental data. The synthesized images show an improvement in image quality compared to the original set of images.

Anisoplanatic imaging through atmospheric turbulence: brightness function approach

We present the development of a novel technique for numerical simulation and analysis of wide field-of-view (FOV) incoherent and anisoplanatic imaging of an object through volume turbulence. This technique is based on the recently developed brightness function method [J. Opt. Soc. Am. A, v. 22, p. 126 (2005)]. We present computer simulation results demonstrating the anisoplanatic turbulence effects on an object image quality.

Picosecond Raman compression laser at 1530 nm with aberration compensation

A passively Q-switched Nd:YAG laser with a master-oscillator power-amplifier configuration based on Brillouin and Raman pulse compression has been developed. The laser operates at 100 Hz repetition rate, producing 50 mJ pulses of approximately 30 ps duration at 1530 nm wavelength with near-diffraction-limited beam quality (M2≤1.2). The effect of spherical aberration in thermally loaded Nd:YAG rods was studied, and efficient aberration compensation was achieved by use of a specially designed aspheric element.

Removing orbital debris with pulsed lasers

Orbital debris in low Earth orbit (LEO) are now sufficiently dense that the use of LEO space is threatened by runaway collisional cascading. A problem predicted more than thirty years ago, the threat from debris larger than about 1cm demands serious attention. A promising proposed solution uses a high power pulsed laser system on the Earth to make plasma jets on the objects, slowing them slightly, and causing them to re-enter and burn up in the atmosphere. In this paper, we reassess this approach in light of recent advances in low-cost, light-weight segmented design for large mirrors, calculations of laser-induced orbit changes and in design of repetitive, multi-kilojoule lasers, that build on inertial fusion research. These advances now suggest that laser orbital debris removal (LODR) is the most costeffective way to mitigate the debris problem. No other solutions have been proposed that address the whole problem of large and small debris. A LODR system will have multiple uses beyond debris removal. Inte...

Application of stereo laser tracking methods for quantifying flight dynamics-II

Conventional tracking systems measure time-space-position data and collect imagery to quantify the flight dynamics of tracked targets. One of the major obstacles that severely impacts the accuracy and fidelity of the target characterization is atmospheric turbulence induced distortions of the tracking laser beam at the target surface and imagery degradations. Tracking occurs in a continuously changing atmosphere resulting in rapid variations in the tracking laser beam and distorted imagery. These atmospheric effects, in combination with other sources of degradation, such as measurement system motions (e.g. vibration/jitter), defocus blur, and spatially varying noise, severely limit the useful and accuracy of many tracking and analysis methods. This paper discusses the viability of employing stereo image correlation methods for high speed moving target characterization through atmospheric turbulence. Stereo imaging methods have proven effective in the laboratory for quantifying temporally and spatially resolved 3D motions across a target surface. This technique acquires stereo views (two or more) of a test article that has an applied random speckled (dot) pattern painted on the surface to provide trackable features on the entire target surface. The stereo views are reconciled via coordinate transformations and correlation of the transformed images. The principle limitations of this method have been the need for clean imagery and fixed camera positions and orientations. However, recent field tests have demonstrated that these limitations can be overcome to provide a new method for quantifying flight dynamics with stereo laser tracking and multi-video imagery in the presence of atmospheric turbulence.

Picosecond eye-safe Raman laser for advanced ranging and tracking

Commercial ranging systems (both beam modulation and pulsed time of flight systems) have an operating range and accuracy far below the needs for moving target system qualification testing and model validation. A new, eye-safe, long operating range, accurate (order of cm) ranging system must be developed. The most feasible approach to achieve this capability requires an ultrashort pulse laser system in conjunction with time-of-flight measurement methods. Accordingly, the Institute of Applied Physics and Sandia National Laboratories are developing an advanced ranging system for use in Sandias mobile laser tracker for quantification of the flight dynamics of high speed moving targets. Key to this development is a new laser. This paper presents a new picosecond Raman laser capable of range measurements to 50 km with 1 cm accuracy in the presence of atmospheric turbulence.

Modeling of atmospheric parameters for laser tracing scenarios (near horizontal low-latitude propagation paths)

Investigations of the dynamics of turbulent characteristics were carried out for different tracking paths based on theoretical equations. A multi-screen model and single-screen one for turbulent atmosphere have been constructed for numerical simulation of laser beam propagation along atmospheric paths within the framework of the paraxial approximation. These models are suitable for simulation of the propagation along both homogeneous and inhomogeneous paths. Within this model, the Fried radius, the scintillation index, the effective beam radius, and the coherence length of radiation were calculated. The values obtained in the numerical experiment were compared with those calculated analytically.

Turbulence compensation using micromirror arrays: the array design

An array of micromirrors can be used to correct the wavefront aberrations due to atmospheric turbulence. A simple method is presented for estimating the number of piston-only micromirrors needed to correct the seeing. We also compute how many piston-tip-tilt micromirrors are required. The three-actuator micromirrors are found to produce a more efficient solution, requiring 4X fewer actuators for the same improvement in the Strehl ratio.

Energy and statistical characteristics of optical radiation reflected from an infinite retroreflector array

The energy and statistical characteristics of laser radiation reflected from an infinite surface in the form of an array of single retroreflectors have been investigated. The study of the reflecting properties of such a surface involved the calculation of the coherence function of the radiation in the reflection plane. Rigorous and high-accuracy approximate expressions have been obtained for this characteristic. The intensity in the far zone and the coherence function of the reflected radiation at an arbitrary distance from the surface have been calculated. Approximate equations have been derived for these characteristics of the radiation. The results of numerical simulation by the Monte Carlo technique have been compared with the rigorous and approximate calculations. It has been shown that in the most significant cases the approximate equations proposed give a deviation within 5% from the rigorous ones and from the results of averaging over numerical realizations. The approximate equations obtained have been used to solve the problem of radiation propagation along sensing paths, including the forward propagation through the turbulent atmosphere, reflection, and backward propagation.