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Journal of Directed Energy
Volume 5, Number 2 Winter 2014

The papers listed below constitute Volume 5, Number 2 of the Journal of Directed Energy. Print copies of issues of the Journal of Directed Energy are available through the online store..
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Incorporation of Probabilistic Effects in Reflected Laser Hazard Methodology
Edward Early, George Megaloudis, Justin Zohner, Paul Kennedy, and Robert Thomas; TASC, Inc. and other affiliations

The goal of this work is to provide an approach to accurately calculate the probability of exceeding a safety or injury threshold associated with laser engagements of targets. The reflected laser hazard methodology described here considers only specular reflections from targets, since these are generally the most hazardous. Analytical expressions, based on the geometry of the engagement scenario, use the cone of reflected rays in the specular direction to calculate the irradiance and exposure time at observer positions. These quantities are, in turn, used to calculate the extent of the hazard region. Monte Carlo techniques were incorporated into the methodology to account for stochastic variations in selected input parameters. A method was developed to accumulate results at a common set of observer positions based on the nominal, deterministic engagement scenario. The methodology considers discrete time steps, so the key to the method was to use a fixed set of reflected rays, which are invariant with respect to time and the stochastic input parameters. The rays sweep through space, so a series of intersection and interpolation calculations provides the irradiance and exposure time at the set of observer positions for each instance of the stochastic input parameters. The results from multiple stochastic instances over a plane of observer positions are displayed as contours of the probability for exceeding a specified threshold value. In the case of safety, this value is the maximum permissible exposure, while for injury it is a fixed probability. The probabilistic techniques were applied to an example scenario using probability densities for the input parameters of target trajectory, laser power, and target material reflectance magnitude and divergence angle.
KEYWORDS: Hazard region, High-energy laser, Laser safety, Monte Carlo methods, Probabilistic hazard analysis, Probability of injury, Reflected beam
PAGES 105-128

Scalable Pump Source for Diode-Pumped Alkali Laser
F. William Hersman, Jan H. Distelbrink and David W. Watt, University of New Hampshire and other affiliations

External-cavity diode laser systems are well suited for diode-pumped alkali laser (DPAL) systems due to their high power efficiency and excellent wavelength control under changing thermal loads. By conditioning the characteristics of feedback power, external cavities can narrow the spectral bandwidth and limit transverse modes of diode laser bars. Existing configurations use low-efficiency diffraction gratings at the Littrow angle to send back to the diodes a small fraction of the power, while directing the majority of the power forward in the output beam. We previously reported that a stepped mirror allows a single external cavity to condition the output of a stack of diode array bars. In this report, we describe a new approach to use a single external cavity to condition the output of up to 1000 or more diode array bars. A high-efficiency grating will be used to feed back essentially all the locked power, and power splitters will then distribute the power to multiple diode-array stacks. A single module is capable of 100-kW power output into a mode-limited, slowly diverging beam with a spectral width as low as 0.05 nm..
KEYWORDS: Rubidium vapor pump laser, External cavity, Spectral narrowing, Littrow configuration
PAGES 129-136

Numerical Modeling of High-Energy Laser Effects in Polymer and Composite Materials
Andrew C. Tresansky, Peter Joyce, Joshua Radice, and Joe Watkins; United States Naval Academy

A numerical heat transfer model to capture heat flow and material damage to polymers and carbon fiber–reinforced polymer composites irradiated by a laser beam is presented. The model uses the COMSOL Multiphysics® package to take into account the multiple interactions in effect during this type of irradiation. Experiments were conducted to validate the model by measuring the recession rate in polymer and polymer composite samples during irradiation.
KEYWORDS: Composite, Carbon fiber, COMSOL Multiphysics®, Laser effects
PAGES 137-158

Optical Directed-Energy Beam Directors: Enabling Capabilities for the War Fighter
Paul Konkola, Eyekon Systems LLC

Several beam director architectures have been implemented in Department of Defense programs that use inertial sensing and alignment laser beams through the optical train. We review the prior art in stabilization and tracking that is relevant to optical directed energy. Then we discuss an alternative architecture that has attractive system-level benefits when using technology developed by Eyekon Systems, especially for tactical applications of directed energy. This architecture simplifies the alignment loop metrology but requires beam director performance that is not available with status-quo beam directors. The system enables tactical applications of directed energy that would otherwise be impractical or suffer from limited performance. Tactical applications will use new classes of weapons that may have an optical aperture of only 30 cm. We also examine basic target service–rate capabilities that are relevant for directed energy scenarios. These issues must be thoroughly considered when developing laser weapon systems that the war fighter might adopt. Similarly, it is likely that system size, weight, and power and target service rates will be important metrics for future beam directors. Finally, we introduce the Eyekon Systems beam director technology that may help accelerate adoption of laser weapon systems since it eliminates major shortcomings of status-quo approaches for tactical applications.
KEYWORDS: Directed energy, Beam director, Optical beam steering
PAGES 159-174

Sensitivity Analysis and Characterization of Vertical-Cavity, Surface-Emitting Lasers for Directed Energy Applications
E.A. Fennig, P.O. Leisher, G. Ragaunathan and K.D. Choquette; Rose-Hulman Institute of Technology and other affiliations

Vertical-cavity, surface-emitting lasers (VCSELs) have a wide variety of applications; typically they are used for sensing and data communications. There has recently been increasing interest in using power-scalable arrays for pumping and directed energy applications. The epitaxial growth of VCSELs requires extreme precision, and as such, special care must be taken with controlling the optical thickness of each layer. The performance of the structure is particularly sensitive to changes greater than ~10 nm in the thickness of the optical cavity, since its thickness is typically only ?/2. A sensitivity analysis was conducted on a VCSEL structure in order to better understand the limitations of growth from a theoretical perspective, and to minimize the costs associated with growth calibration. By examining the variance in both the Fabry-Perot resonant wavelength and the reflectance profile for the structure, it was possible to assess the tradeoff between performance and growth error and to provide data concerning the required accuracy of the growth process. The structure was grown using metalorganic chemical vapor deposition, and oxide-confined devices of varying aperture were fabricated. Characterization results are fully detailed; devices with an aperture of ~10 micrometer exhibited a maximum output power of 8 mW, enabling >1 kW/cm2 for arrays fabricated with 25-micrometer pitch.
KEYWORDS: VCSEL, Sensitivity, Oxide-confined devices
PAGES 175-180

On the Average Probability of Hitting a Satellite during a Laser Counterartillery Engagement
Roger Chapman Burk, United States Military Academy

This paper investigates the probability that a high-energy laser fired at an incoming projectile will inadvertently hit (not necessarily damage) a background satellite. Typically called the counter-rocket, -artillery, and -mortar (C-RAM) mission, such an engagement can be defined by parameters describing projectile trajectory, laser characteristics, laser firing, and spacecraft orbit parameters, so that a probability of hit can be accurately calculated from the geometry of the situation. The model discussed here takes into account laser location, laser pointing and angular sweep, laser beam angular divergence, and orbit height, inclination, and ascending node. The model does not take into account atmospheric refraction or absorption, assumes that the laser beam propagates beyond the intended target into space, and does not address the probability of damage given a hit. Based on the single-engagement probability calculation, Monte Carlo sampling is then used to find a general probability of hit. For each replication, the threat launcher is placed at a random distance and azimuth, the impact point randomly placed within 1 km of the laser, and the engagement placed randomly in the trajectory. The spacecraft parameters are selected randomly from a comprehensive set of 1417 orbital elements for actual operational or formerly operational spacecraft. A simulation constructed to represent defense against mortars in a near-term counterinsurgency conflict gives a probability of hit of about 15.5 × 10–9 per engagement per satellite, or 15.5 nanohits, for satellites in sun-synchronous orbits. Another simulation was constructed to represent a hypothetical major combat operation with long-range rocket and artillery threats in addition to mortars. This simulation yields 27.5 nanohits for the same set of satellites. An extensive sensitivity analysis explores how these results vary as the parameters describing spacecraft, projectile, laser, and engagement are changed, giving results from 0 to 549 nanohits. Hits increased as the laser site moved north, as the distance to the threat launch point increased, as the engagement moved away from the midpoint of the projectile trajectory, as laser beam quality deteriorated, as engagement duration increased, and as satellite altitude increased. Over all cases, the statistical 95% confidence interval was ±3% to 10%. Accumulating the scenario results over a notional 3-year counterinsurgency conflict and 20 sun-synchronous spacecraft of interest results in a total of 0.0034 expected spacecraft hits during the conflict. Accumulating over a notional 2-week major combat operation and the same spacecraft results in 0.015 expected hits.
KEYWORDS: C-RAM, Engagement modeling, High-energy laser, Lasers for air defense, Monte Carlo sampling, Satellites, Tactical lasers
PAGES 181-205

Volume 5, Number 2, Journal of Directed Energy

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