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DIRECTED
ENERGY
PROFESSIONAL
SOCIETY
Journal of Directed Energy
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Volume 5, Number 4 |
Spring 2016 |
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The papers listed below constitute Volume 5, Number 4 of the Journal of Directed Energy.
Print copies of this, and other issues of the Journal of Directed Energy are available through the
DEPS online store.
DEPS members enjoy access to the complete technical paper(s) 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 |
The Effects of Jitter and Stationary Spot Size on Energy Transfer during a Laser Engagement
D. Blaine, J. Smith, K. Ludeman, J. Hartke, and L. Florence; U.S. Military Academy
Jitter on the target of a high energy laser system can have a significant effect on energy
transfer during an engagement with mortars or other artillery. Jitter in an actual target
engagement can be caused by turbulence of the projectile, the atmosphere, inconsistencies
within the tracking system of the laser, and jitter within the laser system itself. In this paper
we compare the effect of jitter on heat transfer with the effects of stretching the stationary
spot size in one direction with a cylindrical lens. Results show that stationary spot size of a
certain horizontal beam radius cannot be equated with sinusoidal jitter of equal amplitude.
KEYWORDS: Jitter, Stationary spot size
PAGES 309-318
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Fiber Optic Sensors for Nonintrusive Diagnostic Measurements of Rail Gun Electric and Magnetic Fields
Anthony Garzarella and Dong Ho Wu, Naval Research Laboratory
Measurements of the electric and magnetic fields within a rail gun can provide critical
electromagnetic details that can be used for diagnostic purposes as well as to support
modeling and simulation efforts. Fiber optic probes based on electro-optic (EO) and
magneto-optic (MO) materials are ideal for such measurements because of their small sizes
and nonmetallic structure. The output signals from the sensors are carried by fiber optic
cables and monitored on an oscilloscope, whose signal voltages follow a simple
proportional relationship to respective electric and magnetic fields. The main technical
challenge in this effort is to overcome the extreme mechanical, electromagnetic, and
acoustic environment within the rail gun bore, which can generate significant background
noise in the sensors. Overcoming this challenge is particularly critical in EO sensors, since
the magnetic field signature of the rail gun is large enough to yield acceptable signal-tonoise
ratios in our MO sensors. In this report, we describe our EO and MO sensors,
modifications necessary to improve their robustness and resilience in rail gun use, and
various electric and magnetic field data from the NRL rail gun.
KEYWORDS: Electro-optic, Magneto-optic, Rail gun, Sensor, Probe
PAGES 319-326
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High-Sensitivity, All-Dielectric Electric and Magnetic Field Sensors for RF and HPM Applications Up to 20 GHz
Dong Ho Wu and Anthony Garzarella, Naval Research Laboratory
All dielectric field probes based on electro-optic (EO) and magneto-optic (MO) crystals
are becoming attractive alternatives to conventional D-dot and B-dot sensors. Conventional
sensors are limited by their size, frequency response, metallic structures, and inability to
directly measure true waveforms in real time. A single EO sensor can measure electric fields
from dc to 20 GHz, and an MO sensor can measure magnetic fields from dc to 2 GHz. Both
sensors are small in size, minimally intrusive, and can be combined to form an integrated
sensor that measures both electric and magnetic fields. EO and MO sensors provide an
instantaneous signal output that precisely replicates the external field in real time, without
the need of intervening signal processing. In this report, we discuss recent advancements
in improving sensitivity and robustness of the sensors, as well as an evaluation of their use
at higher frequencies (up to 20 GHz).
KEYWORDS: Electro-optic, Magneto-optic, HPM, Sensor, Probe
PAGES 327-334
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Non-Intrusive High Voltage Measurement Inside a Coaxial Cable Using a Slab-Coupled Optical Sensor (SCOS)
Frederick Seng, Nikola Stan, Richard Selfridge, and Stephen Schultz; Brigham Young University
This paper introduces a method for measuring high-voltage discharges into an RG218/U
coaxial cable. Specifically, a slab-coupled optical sensor (SCOS) is inserted into an
RG218/U coaxial cable via a 3-mm diameter hole that is drilled and then filled with
epoxy. This paper explains the method for inserting the fiber optical sensor into the
coaxial cable as well as considerations that must be taken into account when inserting
the fiber optical sensor into the coaxial cable, such as the dielectric breakdown of the
cable, as well as reflections caused by the hole in the cable used to insert the sensor. This
paper shows and discusses SCOS measurements from a series of discharged pulses
ranging from 9.2 kV to 54 kV, simulating a high power-microwave weapon voltage discharge.
KEYWORDS: Sensing, Test and evaluation, High power microwave, High voltage
PAGES 335-345
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Stochastic Parallel Gradient Descent Algorithm with Adaptive Gain for Atmospheric Turbulence Compensation
Daniel Whitley and Jessie Hazen, University of Alabama; Greg Finney and Christopher Persons, IERUS Technologies; and John Rakoczycr, NASA Marshall Space Flight Center
Stochastic parallel gradient descent (SPGD) is a metric-based approach for adaptive optics.
This approach optimizes a scalar metric by taking random perturbative steps for many
actuators simultaneously. Conventional adaptive optics uses a wavefront sensor to compute
the conjugate phase for the deformable mirror, requiring high levels of computational
resources. In contrast, the SPGD metric may be determined using a small region of a single
charge-coupled device camera by evaluating the power-in-the-bucket, or one of several
other metrics available. The focus of this research was to evaluate the performance of metric
adaptive optics for compensating atmospheric effects on laser beam propagation. The
SPGD algorithm was modified to use an adaptive gain to increase the convergence rate of
the algorithm while maintaining a low steady-state error. Perturbations that follow
Kolmogorov statistics were determined in Zernike space and shown to provide faster
convergence than simply perturbing actuator voltages. Results of laboratory experiments
to evaluate the performance of the SPGD algorithm using phase plates to simulate
atmospheric turbulence are presented.
KEYWORDS: Adaptive optics, Stochastic parallel gradient descent, Stochastic parallel gradient descent, Scalar metric
PAGES 346-354
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Impact of Partial Spatial and Temporal Coherence on Active Track and Active Imaging
Richard Holmes, Michael Werth and Jacob Lucas, Boeing LTS; V.S. Rao Gudimetla and Jim F. Riker, AFRL
It is well known that coherently illuminated objects will result in degraded images and
degraded track centroids, compared to an incoherently illuminated object. These
degradations from temporal coherence are manifested by increased intensity modulation
in an image caused by coherent constructive and destructive interference of reflections
from different parts of the object. To reduce such coherent interference, techniques for
reduction of temporal coherence are of interest to reduce interference from different depth
cells of the object. It is also well known that propagation of a spatially coherent laser
through the atmosphere and focused on a target can cause scintillation. This scintillation
can result in non-uniform illumination of a target that can result in a spatially modulated
active image and/or an erroneous active-track centroid. Hence one might also consider
techniques for spatial coherence reduction of a transmitted beam as a means for
mitigating such scintillation. In this effort, the impact of both spatial and temporal
coherence reduction are explored in parallel, addressing the possibility of reduction of
coherent artifacts described above, within the practical limits of existing spatial and
temporal modulation devices. In this preliminary study of the use of partially coherent
sources for directed-energy applications, simulation results show a factor of 2 or more
reduction in atmospheric scintillation on target using reasonable levels of spatial
modulation to reduce spatial coherence. On the other hand, little reduction in laser
speckle was found in simulated received imagery, using reasonable reductions of
temporal coherence of the transmitted beam.
KEYWORDS: Active tracking, Active imaging, Laser partial coherence
PAGES 355-380
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Probabilistic Risk Assessment Process for High-Power Laser Operations in Outdoor Environments
Brian K. Flemming, Selex ES Ltd; Paul K. Kennedy and Daniel F. Huantes, JBSA; and Matthew D. Flower, UK Ministry of Defence
Risk assessment for exposure to the output from a high-power laser system involves
consideration of the range of directions over which laser energy can be emitted, and the
damage that the incident laser energy could cause to an unprotected observer or sensor. A
deterministic laser hazard zone (LHZ) based on nominal hazard distances derived from
applicable maximum permissible exposure (MPE) limits, and incorporating all possible
directions in which laser energy could be fired, can be severely restrictive for testing and
training operation purposes with high-power laser systems on outdoor ranges. An
alternative approach based on the use of probabilistic risk assessment (PRA) techniques
can result in more practical, less onerous, clearance restrictions due to quantification of
the extant risk and introduction of the acceptable risk level concept. The probabilistic laser
hazard evaluation technique is based on an application of the formal risk assessment
process developed since the early 1980s, and which is also shown to underpin the
deterministic hazard assessment technique. This discussion is an extended version of a
paper presented at the 2015 International Laser Safety Conference (ILSC 2015).
KEYWORDS: Laser hazard analysis, Probabilistic risk assessment, High-energy laser
PAGES 381-407
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Volume 5, Number 4, Journal of Directed Energy
Last updated: 28 April 2016
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