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Journal of Directed Energy
Volume 6, Number 2 Spring 2017

The papers listed below constitute Volume 6, Number 2 of the Journal of Directed Energy.
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Thermal Damage behind HEL-Irradiated Carbon Fiber-Reinforced Polymer Skin
Jorge D. Garcia, Peter Joyce, and Cody Brownell; United States Naval Academy

While lasers can defeat targets such as unmanned aerial vehicles (UAVs) through material degradation and penetration, it is also possible to defeat a target without penetration due to heat re-radiated to internal components. This possibility was investigated by using a thin carbon fiber-reinforced polymer (CFRP) to represent a UAV skin, and a steel disc to represent an internal component. A FLIR A325sc infrared camera was used to measure temperatures on the back of the CFRP and a Type K thermocouple to measure steel disc temperature. Results confirmed that even without penetration, high energy lasers (HEL) pose a serious threat to military vehicles due to high temperatures of internal components. In addition, results indicated that the presence of the internal component near the CFRP skin contributed to even greater heating of the back surface by reflecting and radiating heat back to the CFRP. The temperature measurements using the IR camera were further validated in separate tests using a CFRP with embedded fiber Bragg grating sensors. After testing, a basic numerical model was developed and compared to the experimentally determined results. The model did not match the experiments because it was a unidimensional model and it did not account for changing material properties of the CFRP.
KEYWORDS: Carbon fiber-reinforced polymer, Unmanned aerial vehicle, Fiber Bragg gratings
PAGES 119-136

Designing Better Sodium-Guidestar Lasers for Adaptive Optics with Experimental Results and Modeling
Shawn Hackett, Robert Johnson, and Jack Drummond; Air Force Research Laboratory

To date, modeling of sodium-guidestar mesospheric excitation for use in adaptive optics with large telescopes has been limited in application due to a lack of experimental results. Recent experimental evidence from the Starfire Optical Range (SOR) 3.5-m telescope with the new Toptica Photonics SodiumStar 20/2 is presented with an accompanying model, which accurately predicts the sodium-guidestar return flux observed at SOR to within 10%. The results from these experiments and this model are shown to be extendable for future guidestar research and development allowing for an optimization of guidestar return flux by modifying laser-design parameters such as laser bandwidth, laser power, repumping ratio, and launch-telescope design.
KEYWORDS: Guidestar modeling, Sodium guidestar, Starfire Optical Range, Fiber laser guidestar, VECSEL guidestar
PAGES 137-148

Modeling the Impact of Slow Varying Offsets between HEL Line of Sight and Target Aimpoint
G. Erten and B.H. Robinson, Raytheon Space and Airborne Systems

Many existing high energy laser (HEL) weapon modeling and simulation tools include line of sight (LOS) jitter among their inputs. Moreover, the literature on the subject provides a formula to compute a jitter Strehl ratio. Besides jitter, the HEL beam is also subject to slow varying, quasi-static LOS disturbances, namely, offsets between its actual HEL LOS and the desired "perfect-aim" LOS to the center of the aimpoint. This paper describes the geometric formulation of the problem associated with accounting for HEL LOS to aimpoint offsets using a 2D Gaussian approximation of the HEL beam. A closedform solution for power in the bucket (PIB) is then derived as a function of aimpoint size, HEL LOS to aimpoint offset, and the standard deviation of the 2D Gaussian HEL beam approximation function. The impact of jitter and aimpoint offset is thus simultaneously assessed since LOS jitter can be included by increasing the standard deviation of the Gaussian approximation for the HEL beam.
KEYWORDS: High energy laser (HEL), HEL modeling, Line of sight (LOS), Aimpoint maintenance
PAGES 149-158

Incorporating Refraction into Satellite Safety
John Dewsnap, Steven Gabriel, Heather Witts, and Udayan Bhapkar; Serco, Inc. and other affiliations

The U.S. Department of Defense is exploring the potential for employment of laser weapons through field demonstrations, exercises, and live-fire experiments. Satellite safety from unintended laser illumination is an on-going concern. A standardized operational solution for unintended laser illumination called the Joint Laser Deconfliction and Safety System (JLDSS) is being pursued by the Joint Directed Energy Transition Office, the Naval Surface Warfare Center, and the Laser Clearinghouse (LCH). Currently, in the precursor to JLDSS, atmospheric refraction of the laser beam is accounted for in satellite safety calculations by adding a fixed value to the static keep-out cone (KOC). This is a worst-case value, and is excessive for all pointing except near the horizon. The current development effort seeks to dynamically compute the refraction, and only the residual component of the refraction error is incorporated into the KOC as a function of pointing elevation, thereby improving deconfliction accuracy and increasing allowable firing times. In this paper we survey various refraction algorithms and explore ways to arrive at an accurate equation that is simple enough to be used in real time. The relative merits of several approaches are examined with a view toward accuracy, correct handling of units, and coding pitfalls. The paper also discusses refraction sensitivity to wavelength, humidity, and pressure. The bounds of predictability with respect to extreme weather conditions and atmospheric ducting are reviewed. Finally, recommendations are made for implementing an equation for refraction and adjustments to the KOC. The article contributes to understanding of current efforts to optimize JLDSS design.
KEYWORDS: Refraction, Deconfliction, Predictive avoidance, Laser Clearinghouse, Keep-out cone
PAGES 159-174

On the Clustering of Rare Earth-Dopants in Fiber Lasers
John Ballato and Peter Dragic; Clemson University and other affiliations

Fiber amplifiers and lasers used in directed energy (DE) applications are enabled by light-emissive rare earth (RE) ions that are incorporated into silica glass. It is well established that the inclusion of alumina (Al2O3 ) into high silica-content glasses is necessary to mitigate clustering of RE dopants. Such RE clustering leads to concentration quenching and a reduction in spectroscopic performance. Within the high energy laser (HEL) community, the commonly stated role of the alumina is to "open up the glass network," thus permitting higher doping levels before the onset of concentration quenching. However, within the glass community, this interpretation is well known to be incorrect. Accordingly, this work provides a didactic description of the role of alumina in rare earth-doped glass optical fibers, and describes several avenues for future study and development to further advance HEL fiber lasers.
KEYWORDS: Fiber lasers, Optical fiber, Rare earth-doped glass
PAGES 175-181

Measurement of Potassium Electronic-Level Relaxation Cross-Sections Induced by Methane
M. D. Rotondaro, B. V. Zhdanov, M. K. Shaffer, and R. J. Knize; United States Air Force Academy

We have measured several of the potassium atom upper-electronic-level relaxation crosssections induced by collisions with methane. Highly excited states and ionization are important for understanding the operation of diode-pumped alkali lasers. The crosssections measured in this work range from 25 to 83 2. This work has demonstrated that adding a small amount of methane up to 20 Torr to the helium gas will significantly reduce the pathways for ionization..
KEYWORDS: Potassium, DPAL, Ionization, Cross-section
PAGES 182-186

Evidence and Implications of Differences in Atmospheric Optical Turbulence Behavior on Opposite Coastal Environments
William Bourque, Charles Nelson, and Douglas Nelson, U.S. Naval Academy and other affiliations

Over the course of 6 months, from January 2015 through June 2015, atmospheric optical turbulence measurements were made in a 3.9-km maritime optical link located on the coast of San Nicolas Island, California. Throughout the same period of time, weather parameters were measured on and around the island. The resulting data were compared to atmospheric measurements taken over the Chesapeake Bay by Navy Research Labs and Research Support Instruments Inc. from January 2007 through June 2007 in order to investigate similarities and differences between atmospheric optical turbulence behaviors in a range of maritime environments. Monthly mean values of Cn2 differ significantly in magnitude, however, following similar trends over time. The lack of obvious diurnal variations on San Nicolas Island confirmed theories about trends observed over the Chesapeake Bay. Additionally, Cn2 was found to have less dependence on air-minus-water temperature on San Nicolas Island than over the Chesapeake Bay. Results from the comparison of both environments highlight a need for continued investigation into the modeling of atmospheric propagation in a range of maritime environments, as well as near-maritime and littoral environments.
KEYWORDS: Optical turbulence, Atmospheric propagation, Cn2, Chesapeake Bay, San Nicolas Island
PAGES 187-197

Effects of Rotation and Inert Thermal Sinks on Laser Heating of Cold, Rolled-Steel Cylinders: Preliminary Experimental Results
D. Mauldin, L. O'Neill, I. De Mallie, F. Arnold, L. A. Florence, J. Hartke, D. O. Kashinski, J. E. Johnson, J. Lamb, R. Huffman, D. E. Riegner, N.F. Fell, T. Kreidler, G. Tamm and N. F. Fell; U.S. Military Academy and other affiliations

Rotation of and heat sinks inside a target increase high energy laser (HEL) engagement time required to achieve specified internal temperatures. Collaborative investigation into developing quantitative and qualitative models of non-static targets is underway in the Photonics Research Center at the United States Military Academy at West Point. Currently this collaborative investigation aims to ground theoretical thermal models with experimental data gathered from rotating steel cylinders engaged with a HEL. This article outlines the current experimental setup, cold-rolled steel targets, data acquisition system, and inert compounds used as heat sinks. Experimental results for a hollow cylinder as well as very preliminary qualitative results for a cylinder packed with a heat sink are reported. A qualitative comparison is also made with results from a preliminary theoretical simulation.
KEYWORDS: High energy laser, Thermal properties, Dynamic target
PAGES 198-208

Pulsed Discharge-Diode Pumped Ar* Laser
J. Han, M. C. Heaven, P.J. Moran, G.A. Pitz, E.M. Guild, C.R. Sanderson, and B. Hokr; Emory University and other affiliations

Optically pumped rare gas lasers are being investigated as potential high-energy, high beam quality systems. The lasing medium consists of rare gas atoms (Rg = Ne, Ar, Kr, or Xe) that have been electric discharge excited to the metastable np5 (n + 1)s, 3P2 state. Lasing is obtained by optically pumping the (n + 1)p - (n + 1)s transitions. Helium is used as the collisional relaxation agent. A technical challenge for the Rg* laser is the production of high Rg*-number densities (>1012 cm-3) in the presence of 0.5-1 atm of He. Previously, we demonstrated an Ar* laser that employed a pulsed discharge for metastable production. Optical pumping was provided by a CW diode laser. This system yielded a train of laser pulses of approximately 150-ns duration. In the present study we developed a pulsed discharge where the Ar* lifetime exceeds the time between discharge pulses. The first version produced a filament discharge with a 1.5-mm width. Diode laser pumping of this filament yielded CW lasing at an output power of 52 mW. Number density measurements indicated time-averaged Ar* densities in the range of (2-4)x1013 cm-3. A modification of the discharge system yielded CW Ar* production (1.4x1013 cm-3) over a path length of 2 cm, and a lasing output power of 3.1W.
KEYWORDS: Gas laser, Discharge kinetics, Metastable states
PAGES 209-219

Volume 6, Number 1, Journal of Directed Energy

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Last updated: 18 May 2017