Reflections of high-energy lasers from surfaces can present hazards to persons and instruments at significant distances. Heating from these lasers causes dynamic changes in the reflection characteristics of surfaces they illuminate. As such, reflections from these surfaces cannot be properly modeled with static bidirectional reflectance distribution functions (BRDFs), but require time-dynamic BRDFs. Moreover, the time-evolution of the surface reflections is not deterministic, but can vary even when the materials and illumination conditions are nearly identical, such that only probabilistic characterization is realistic. Due to the swiftly changing nature of the reflections, traditional BRDF measurements with goniometric instruments are impossible, so BRDFs must be deduced from images of the reflected light incident on a screen intercepting a portion of the reflection solid angle. A new BRDF model describes these complex probabilistic dynamic BRDFs with only four intuitive parameters for a given laser wavelength, irradiance, and duration, where these parameters have central values and statistical variances over discrete physical states corresponding to surface conditions. An automated procedure determines appropriate parameter values and variances from captured screen images, requiring only a single angle of laser incidence. Parameters from sample tests illustrate the model.
KEYWORDS: Bidirectional reflectance distribution function, BRDF, Dynamic BRDF, Probabilistic BRDF, High-energy laser reflections

High Energy Laser (HEL) systems are pushing the test and evaluation enterprise to field HEL sensors that provide measurement fidelity and repeatability. Underpinning that requirement is the establishment of measurement traceability to the International System of Units (SI) via primary measurement standards. The National Institute of Standards and Technology (NIST) developed a novel primary standard for measuring HEL radiative power, named the Radiation Pressure Power Meter (RPPM), which does not consume the laser beam. This new standard provides opportunities to calibrate any device under testing (DUT) with HELs, keeping the standard in-situ. SemQuest fielded an irradiance measurement instrument, BITS, for the US Army in 2012. Calibration testing was accomplished prior to the advent of RPPM, and relied on imaging sensors and ball calorimetry to derive the Global scaling factor, G, for the calibration at 2.38. An opportunity to explore and evaluate multiple DUTs with RPPM in November 2017 provided BITS with 39 RPPM-anchored measurements of HEL powers and the associated measurement uncertainties. Using G = 2.38, BITS was registering the incident HEL power approximately 13% ± 4% higher than that measured by the RPPM. Adjusting G to a value of 2.08, the power was within 2.23% uncertainty (2U) of the RPPM measurement.
KEYWORDS: High energy laser, HEL, Directed energy test and evaluation, DET&E, Irradiance, Calibration

As high-power directed energy (DE) systems progress toward deployment in mobile applications, having a low Size, Weight, and Power (SWaP) metric becomes critical to mission success. Typically, the size and weight of a DE system is derived from three major subsystems including the High Energy Laser (HEL) or High Power ElectroMagnetic (HPEM) device, the power delivery system, and the thermal management system (TMS). In this investigation, a high-level SWaP analysis was performed on the power delivery and TMS subsystems applied to a generic mobile DE system. Two-phase (TP) and single-phase liquid (SP) cooling systems were considered in the following arrangements: real-time continuous (current state-of-the-art), solid-liquid phase change material (PCM) transient thermal storage (TTS), liquid-vapor TTS, and once-thru cooling for a total of six TMS configurations. Directed energy events were studied over a range of durations (0 - 180 s) and powers (10 - 1000 kW). As a result of this investigation, it was found that the major components in each TMS could be feasibly kept to less than 1.5 kg/kW over the full range of events. In general, the TTS systems contain far more complexity and offer only minor SWaP improvement over real-time continuous systems for short event times, while becoming detrimental for extended events. In all cases, two-phase systems provided better SWaP performance than their single-phase counterparts. The once-thru cooling system greatly outperformed any other system for cumulative event times up to approximately 6 minutes. Finally, it was found that the power delivery system using batteries provided a substantial proportion of the overall system weight. The use of an alternative power source has the potential to significantly improve SWaP performance.
KEYWORDS: Mobile directed energy, Thermal management, Once-thru cooling, Transient thermal storage, Phase change material

Advances in directed energy weapons technology are leading to fielded systems for the U.S. military and may also become threats as peer competitor nations are also developing these technologies for their militaries. U.S. military forces need to be prepared to operate in future threat environments that include directed energy weapons. The Naval Postgraduate School is conducting counter directed energy weapons research to characterize directed energy threat environments and develop solution concepts for protecting naval assets against this developing problem space. The initial study focused specifically on high energy lasers as the adversarial threat and on naval unmanned aerial vehicles as a type of military asset that will be particularly vulnerable in this threat environment. The study identified solution concepts for defending unmanned aerial vehicles against adversarial high energy lasers and developed an analytical tool for determining lethality effects over a range of threat scenario parameters.
KEYWORDS: Directed energy weapons, High energy lasers, Counter directed energy weapons, Unmanned aerial vehicles

In order to model transport and effects of a particle beam, a source term must be defined. Particles from this source have an energy distribution, a distribution in their position, and a distribution in their direction. The Idaho Accelerator Center (IAC) owns an electron linear accelerator (LINAC) capable of producing pulses of electrons with kinetic energy up to 25 MeV. The previously uncharacterized 45° beamline energy distribution was measured for 14 nominal beam energies by use of a conducting loop and charge integrating circuit. The same line was used at a nominal energy of 21 MeV to characterize particle distribution in space and direction through exposures of EBT3 Gafchromic® film. Films were digitized using a Konica Minolta® Bizhub 454e scanner, and regions of interest were selected. From each color channel of these images, beam parameters were calculated under the assumption of a paraxial, Gaussian beam profile. It was found that the red channel analysis supported the paraxial, gaussian assumption. Analysis of green and blue channels showed distributions not consistent with a purely Gaussian profile. The blue channel response raises the possibility of a previously unknown EBT3 film response to non-ionizing radiofrequency radiation.
KEYWORDS: LINAC, Particle, Model, Energy, Phase, Film, EBT3

We use guided mode expansion (GME) to analyze surface etch photonic crystal (PhC) structures in order to evaluate photonic crystal surface emitting laser (PCSEL) designs and their optical modes. The three dimensional optical modeling reveals that the modal quality factor and modal coupling to the substrate vary periodically with increased PhC etch depth. We propose a method based on GME modeling to analyze the in-plane modes of finite-sized PCSEL lattices.
KEYWORDS: Photonic crystal, Diode laser, Photonic crystal surface emitting laser, Guided mode expansion

When a skin area is exposed to an electromagnetic beam, the beam energy penetrates into the skin tissue according to the absorption coefficient of the beam frequency. The evolution of the skin temperature is governed by the electromagnetic heating and the thermal conduction, which depend on the skin's material properties. In exposure tests, the skin surface temperature can be recorded with an IR camera at discrete time frames. The skin internal temperature, however, is not directly observable. In this study, we develop a method for inferring the internal temperature distribution solely from a time series of measured surface temperatures, in the absence of knowing the skin's material properties.
KEYWORDS: Electromagnetic heating, Heat equation, Analytical solution, Inferring internal distributions from surface measurements, Sensitivity to measurement noise

Anti-resonant hollow core fibers (AR-HCFs) have been investigated for several use cases relating to both single mode and multimode operation. Single mode, low-loss operation is desirable in telecommunications and in high-power delivery applications. These fibers can be designed such that the higher order modes couple out of the core into the surrounding capillaries and subsequently are highly attenuated compared to the fundamental mode. This property makes for an excellent candidate for high-power beam delivery as the beam quality increases by propagating down a properly designed fiber and therefore limits the output to only the fundamental mode. Recently, 1.2kW have been delivered through a Kagome type negative-curvature hollow core fiber (HCF), whereas only 300W CW have been delivered through an AR-HCF. This paper presents single mode transport of 600W (2.6 GW/cm2) 1070 nm CW laser, with 88% transmission in a 3.25m length of Anti-Resonant Hollow-Core fiber with 92% coupling efficiency.
KEYWORDS: Hollow core, Delivery fiber, Anti-resonant, Directed energy, Specialty Optical fiber

In order to protect an electronic system from high power microwaves (HPM), multiple hardening techniques can be used at various system levels. However, in many cases, there is limited understanding about the mechanisms of backdoor coupling and the key parameters at play. Such information could enable more effective mitigation techniques that reduce coupling and raise the threshold for disruption or failure. This study focuses on printed circuit board (PCB) backdoor coupling and provides insight into the relationships between certain design parameters and the degree of backdoor coupling. Existing work related to PCB backdoor coupling is limited, but quantitative studies are needed to better understand coupling mechanisms at the board level. In this work, we use Ansys High Frequency Structure Simulator (HFSS) to study the effects of multiple design techniques on PCB backdoor coupling. Each of these techniques is studied individually and in conjunction with other techniques to quantify the reduction in induced voltage at ports of interest. Both power plane and trace coupling are investigated under multiple conditions. We find that some techniques can reduce coupling by 20 dB to 30 dB in certain scenarios. Some of the simulated results presented in this study are also validated with a controlled experiment using an anechoic chamber and accompanying high frequency equipment. The data presented here suggest that there are PCB design guidelines that can be incorporated to harden systems and mitigate backdoor coupling in HPM environments.
KEYWORDS: Backdoor coupling, High intensity radiated fields, Printed circuit board coupling, Radiated susceptibility, Via fencing, Via stitching