This PDF file contains the front matter associated with SPIE Proceedings Volume 12739, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

We present design, simulation and benchtop demonstration of a beam combining system for use in coherently combined fiber arrays with >1kW per channel. A beam combiner assembly using laser-smoothed, monolithic freeform beam shaping and phase correction optics is designed and manufactured to meet the low-SWaP and high efficiency targets set for deployable LDEW systems. We report on achievable power-in-the bucket in coherently combined system, perchannel power handling capability and scalability to a larger number of channels.

We present our latest results in power scaling of thulium-doped fiber lasers in the 2 μm region based on coherent beam combination with tiled aperture technique. The investigation of a high-power laser system based on coherent beam combination was divided into three individual experiments. First a MOPA architecture was studied with focus on power scaling to kW level with a broad linewidth. Second another MOPA setup was developed to match the requirements for coherent beam combination. Lastly, the combination of milli-watt level channels was investigated using a SPGD algorithm. The performance of these systems will be presented.

We present experimental results of the disruption of a free-space continuous wave (CW) laser beam through its interaction with a separate, ultrafast laser induced filament bundle. Break-up of the CW beam and a significant reduction of its far-field peak irradiance has been predicted by numerical modelling and confirmed by in experiments. The degree of disruption is measured in laboratory-scale tests and found to depend on pulse energy, crossing angle and filament repetition rate. Disruption is also observed to exceed that predicted by our model. These effects are quantified experimentally and compared with numerical predictions and possible explanations for discrepancies are presented and future development steps discussed.

The propagation of a high-energy laser (HEL) beam through the atmosphere causes stray light emission of much higher average intensity than from conventional cw or pulsed laser sources. Particularly interesting are the events of laser emission interaction with aerosol and dust particles, leading to powerful scattered radiation pulses. Their peak power may significantly exceed the average level of the scattered emission and may seriously impact evaluation of nominal ocular hazard distance (NOHD) and thus laser safety aspects of HEL outdoor operations.

Since high-power fiber lasers are emerging in the field of directed energy, the fundamental interaction of these lasers with target materials has to be investigated. For this purpose, experiments with short propagation distances can be performed. The aspects of high-power propagation through the atmosphere over large distances are excluded and go beyond the scope of this work. We show experimental results of laser-matter interaction with up to 120kW continuous wave laser power. The targets are 10mm thick plates of aluminum and steel as representative materials commonly used for construction purposes. A decreasing perforation time is observed with increasing laser power. For low powers, the aluminum samples show a much higher perforation time compared to the steel samples. This changes at roughly 80 kW. Above this value the steel samples withstand the laser irradiation longer. This behavior can be attributed to the much higher thermal conductivity of aluminum compared to steel. The change of dominant effects from low to high laser powers is discussed. One further important aspect is the effective energy coupling into the sample. This is investigated by finite element simulations with an adjustable absorptivity. For the steel samples a high absorption rate of around 75% is observed at low laser powers, which drops down to roughly 15% at the highest laser powers. For the aluminum samples, on the other hand, an almost constant value of around 15% is observed.

While the effectiveness of innovative high-energy laser demonstrators against lightweight drones is daily being shown in several countries around the world, the understanding of the phenomena occurring during the intensive interaction of the laser beam with the target, which will also later lead to a more efficient and safer use, is far from being exhaustive. This paper focuses on the vulnerability of different kinds of rotating drone propellers submitted to high-energy laser irradiations up to 10 kW. The near-infrared spectral response and the high-temperature thermal behavior are first presented, emphasizing that both type of propellers mainly differ in color only (glossy white and matt black). The high-energy laser trial setup is further detailed with a special focus on the propeller test bench and on a dual-wavelength radiometric device specifically developed to record the temperature of a high-speed rotating target (5000 rpm). Finally, the experimental outcomes are discussed, illustrating the major role of the color of the propeller of course, but also of the laser pointing area and its distance from the rotation center of the propeller.

The aim of this study is to investigate the degradation of image quality due to the increased absorption of (IR) window materials such as germanium, silicon at elevated temperatures. In a prior study, it was observed that high-energy laser irradiation on the front window of a thermal camera caused the resulting image to become foggy and eventually disappear. Although the laser’s wavelength is not transmitted by the IR window, its irradiance is partially absorbed, causing the germanium lens to heat up and become opaque, emitting its own thermal IR light which is detected by the sensor. This fogging effect can persist for several seconds after the laser is turned off. This provides a way to temporarily dazzle optical sensors with high-energy lasers without causing permanent damage, this is called pseudo-inband dazzling The primary objective of this study is to evaluate the degree of image quality degradation caused by the loss of contrast resulting from pseudo-inband dazzling. The transmission and emissivity of the windows are evaluated at different temperatures by observing them through portholes using a thermal camera. Additionally, a target with known temperature contrast is observed to directly assess the loss of contrast. The study evaluates not only germanium but also silicon windows using a medium and long wave infrared camera.

We present a study of perforation time for a polymer material. Two different colors of the same polymer were investigated: natural and black. The study compares two different fiber laser wavelengths: 1 μm and 2 μm. The beam diameter on the polymer material was kept the same to provide a fair comparison between wavelengths. The irradiance was varied between 0.1 and 0.5 kW/cm². Over the studied cases the perforation time was found to be shorter for the 2 μm fiber laser.

This article reports an experimental contribution to High Energy Laser (HEL) engagement in urban combat by considering two key aspects: safety risks for the population due to Ytterbium beam reflectivity on target and vulnerability performance on drone cell structures by comparing Ytterbium and Thulium laser source technologies. A first phase is dedicated to the development of a proprietary specific high-reflective diffusive Lambertian screen, capable of withstanding high-temperature loadings. Then, dynamic Bidirectional Reflectance Distribution Function (BRDF) trials are performed at the Vulnerability Test Facility (VTF), in Talence, France. A specific focus is brought to the effect of target degradation on reflectivity pattern and level. The effect of incident laser beam shape is investigated. Finally, the efficiency of two laser source wavelengths on the drilling of thermoplastics is presented. The targets of interest include miscellaneous UAV (mini-drones) envelop materials of various colors. Maturity of laser source technologies has reached a point where the emergence of Laser Directed Energy Weapons (LDEW) on various theatres of operations becomes a short-term reality. Various target types will be engaged and each of these targets may be defeated in a scalable way (hard-kill, soft-kill). In that respect, understanding the effects and risks of HEL on targets, here in urban environment, is crucial.