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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.. DEPS members enjoy access the complete technical papers through links in the paper titles. Members should sign in to their online account and return to this page to access this additional content.  Join DEPS 
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, Highenergy laser, Laser safety, Monte Carlo methods, Probabilistic hazard analysis, Probability of injury, Reflected beam
PAGES 105128

Scalable Pump Source for DiodePumped Alkali Laser
F. William Hersman, Jan H. Distelbrink and David W. Watt, University of New Hampshire and other affiliations
Externalcavity diode laser systems are well suited for diodepumped 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 lowefficiency 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 highefficiency grating will be used to
feed back essentially all the locked power, and power splitters will then distribute the
power to multiple diodearray stacks. A single module is capable of 100kW power output
into a modelimited, 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 129136

Numerical Modeling of HighEnergy 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 137158

Optical DirectedEnergy 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 systemlevel
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 statusquo 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
statusquo approaches for tactical applications.
KEYWORDS: Directed energy, Beam director, Optical beam steering
PAGES 159174

Sensitivity Analysis and Characterization of VerticalCavity, SurfaceEmitting Lasers for Directed Energy Applications
E.A. Fennig, P.O. Leisher, G. Ragaunathan and K.D. Choquette; RoseHulman Institute of Technology and other affiliations
Verticalcavity, surfaceemitting 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 powerscalable 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 FabryPerot 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 oxideconfined 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 25micrometer pitch.
KEYWORDS: VCSEL, Sensitivity, Oxideconfined devices
PAGES 175180

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 highenergy laser fired at an incoming
projectile will inadvertently hit (not necessarily damage) a background satellite.
Typically called the counterrocket, artillery, and mortar (CRAM) 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 singleengagement 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 nearterm counterinsurgency
conflict gives a probability of hit of about 15.5 × 10–9 per engagement per satellite, or
15.5 nanohits, for satellites in sunsynchronous orbits. Another simulation was
constructed to represent a hypothetical major combat operation with longrange 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
3year counterinsurgency conflict and 20 sunsynchronous spacecraft of interest results
in a total of 0.0034 expected spacecraft hits during the conflict. Accumulating over a
notional 2week major combat operation and the same spacecraft results in 0.015
expected hits.
KEYWORDS: CRAM, Engagement modeling, Highenergy laser, Lasers for air defense, Monte Carlo sampling, Satellites, Tactical lasers
PAGES 181205

Volume 5, Number 2, Journal of Directed Energy
Last updated: 15 May 2016
