This PDF file contains the front matter associated with SPIE Proceedings Volume 12273, including the Title Page, Copyright information, Table of Contents, and Conference Committee Page.

Key issues in the design of any passively modelocked laser system are determining the parameter ranges within which it can operate stably, determining its noise performance, and then optimizing the design to achieve the best possible output pulse parameters. Here, we review work within our research group to use computational methods based on dynamical systems theory to accurately and efficiently address these issues. These methods are typically many orders of magnitude faster than widely used evolutionary methods. We then review our application of these methods to the analysis and design of passively modelocked fiber lasers that use a semiconductor saturable absorbing mirror (SESAM). These lasers are subject to a wake instability in which modes can grow in the wake of the modelocked pulse and destroy it. Even when stable, the wake modes can lead to undesirable radio-frequency sidebands. We demonstrate that the dynamical methods have an advantage of more than three orders of magnitude over standard evolutionary methods for this laser system. After identifying the stable operating range, we take advantage of the computational speed of these methods to optimize the laser performance over a three-dimensional parameter space.

We present a model that calculates the reflected intensity of a high-energy laser irradiating a metallic target. It will enable us to build a laser safety model that can be used to determine nominal ocular hazard distances for high-energy laser engagements. The reflection was first measured in an experiment at 2 m distance from the target. After some irradiation time, the target begins to melt and the reflected intensity presents intensity patterns composed of caustics, which vary rapidly and are difficult to predict. A specific model is developed that produces similar caustic patterns at 2 m distance and can be used to calculate the reflected intensity at arbitrary distances. This model uses a power spectral density (PSD) to describe the melting metal surface. From this PSD, a phase screen is generated and applied onto the electric field of the laser beam, which is then propagated to a distance of 2 m. The simulated intensity distributions are compared to the measured intensity distributions. To quantify the similarity between simulation and experiment, different metrics are investigated. These metrics were chosen by evaluating their correlation with the input parameters of the model. An artificial neural network is then trained, validated and tested on simulated data using the aforementioned metrics to find the input parameters of the PSD that lead to the most similar caustics. Additionally, we tested another approach based on an autoencoder, which was tested on the MNIST dataset, to eventually generate a phase screen directly by using the caustics images.

Whereas laser weapon systems are foreseen as a new possibility to counter unmanned aerial vehicles (UAVs), a better understanding of the complex phenomena occurring during the interaction of a high-energy laser beam with a flying structure is required to use those new innovative defense devices in an efficient but also a secure way. This paper first presents multiple material characterizations performed on glass fibers-reinforced plastics (GFRP), from which near-infrared spectroscopic data and high-temperature thermodynamic results are later implemented into multiphysics simulations. Numerical outcomes from the models are then compared to experimental recordings arising from laser trials carried out with varying power densities (75, 150 and 300 W/cm²) and with illumination times of several seconds. A very good agreement is shown between temperature data collected during laser experiments and temperature values from computations.

The field of Space Situational Awareness (SSA) has seen increasing interest in recent years. As SSA is covering a wide range of applications, the activities are categorized in three main areas: space weather, near-earth objects and Space Surveillance and Tracking (SST). The latter one is covering active and inactive satellites and other man-made objects orbiting Earth. Consequently there are activities focusing on debris and inactive satellites on one hand, which can be seen as maintenance activities allowing humankind long-term access to space. On the other hand, SST covering active satellites is in principle a safety measure for space-based missions. Key words that arise in this context are detection, tracking and identification. These are the key ingredients for in-orbit threat detection. When these ingredients are mastered and dedicated solutions are developed, there is only a small step to go using the gained knowledge and same abilities to perform optical countermeasures.

At Jena-Optronik GmbH, we already have a variety of sensors for specific applications in the field of SST available. Our product range covers visible cameras, LiDARs and specific laser detection sensors. Consequently, we recently started increasing our activity in SST to develop dedicated solutions tailored to the specific needs of this application field. Currently, a thermal infrared camera is in development and furthermore we plan to combine all necessary sensors for SST of active satellites in a dedicated SST sensor suite. This suite contains a control electronics unit, which combines all data from the connected sensors to form one dataset that covers the satellite surrounding volume and identifies incoming threats. In a next step active sensors can be used to follow incoming threats and hinder them from observing the satellite with the SST sensor suite and in particular its other payloads.

This paper will provide an introduction into the topic. It will cover an overview of the concepts and needs for different sensors forming a SST sensor suite. Furthermore, the latest advances in our developments are presented and future steps and activities as well as sensors are motivated.

Due to the growing number of small and agile unmanned aerial vehicles (UAVs), including consumer micro-drones, appropriate countermeasures technologies are necessary to protect public and military forces, to increase critical infrastructure resilience, and to secure exchange of data. Most of the countermeasure requires reliable up-to-date position information of the approaching threats. Beside precise determination of the angle-of-arrival laser pulse time-of-flight information is one of the promising technologies to measure the distance to a target. Laser Range Finders (LRF) are typically used for long ranges to large objects or slowly moving targets.

Within the scope of this paper we are going to show a method to enhance laser ranging capabilities to small and fastmoving UAV targets. Dealing with small and agile targets the primary limitation of many laser ranging systems is the reduced hit rate. The restricted torque of the pan-tilt-unit drives is not able to align the LRF in the direction of agile UAV targets in the sky. In this paper, we will present a method using an additionally piezo steering device to reduce this residual tracking error. To estimate the improvement, we are going to compare results under same conditions with and without the fast steering device. Experimental evaluations show an improvement of the LRF hit rate during high accelerations of micro UAVs. We present theoretical analysis and experimental results of UAV laser range measurements under realistic environmental conditions.

Small unmanned aerial vehicles (UAVs) are increasingly becoming a challenge for both civilian and military security. Even simple modifications of these inexpensive and widely available systems can create a serious threat which potentially causes major damage. Using signals from global navigation satellite system (GNSS), UAVs are able to operate over long distances and to find their target zone without remote control of a pilot. In this flight mode, radio direction finder cannot detect any transmitted signal. Typical countermeasures like RF jamming or Wi-Fi hacking become ineffective to stop the threat.

The DLR smart GPS spoofing approach is a promising technology for a cost-effective countermeasure against such autonomously flying Micro UAVs. Several dedicated devices must reliably interact to fulfil this task. The optical identification and position measurement of the UAVs was developed at the DLR Institute of Technical Physics. By optimizing the tracking processes and the laser ranging systems, even small and agile targets can be tracked. The measured position and flight data are forwarded to a GPS spoofing system developed by the DLR Institute of Communications and Navigation. The emitted GPS signal is modified in such a way that the UAV leaves its original flight trajectory and is redirected to a save one. Outside the protected area the UAV can be forced to ground without collateral damages. The feedback loop via the independent optical position measurement ensures that the desired flight trajectory is maintained. The basic functionalities of the smart GPS spoofing countermeasure were successfully demonstrated in realistic field tests. The optical setup and first results will be discussed.

We propose to present our Laser Directed Energy Weapon system HELMA-P and the results of a several weeks C-UAV tests campaign in France. During this campaign we successfully demonstrated capability of detection and neutralization in a few seconds of different kind of drones (different types of rotary wings and fixed wings), at a distance of up to 1Km (Test Facility limited) with different weather conditions. Our small and smart approach allows CILAS to supply a cost effective and flexible C-UAV solution, easy to integrate on different kinds of platforms and minimizing potential risk of collateral damages by using the full potential of a well integrable single fiber Laser source.

This article reports a contribution to laser vulnerability involving a series of experiments of High-Energy Laser (HEL) effects, at the Vulnerability Test Facility (VTF), in Talence, France. The activity is realized in the framework of the Tactical Advanced Laser Optical System (TALOS) European project led by CILAS, as part of the “Preparatory Action on Defense Research” (PADR), with the objective of creating European capabilities in Laser Directed Energy Weapon (LDEW) technologies. The trial facility implements a 1.07µm continuous wave multi-kW high-energy modular laser source and is highly capable in emulating representative laser engagements. The targets of interest include miscellaneous target envelop materials, UAV components and mini-drones. The trials generate plenty of data and observations useful for a target vulnerability assessment. This constitutes elements to progress in HEL vulnerability and is a first step in clearing the path for a technical roadmap of a future LDEW capability.

This project has received funding from the European Union’s Preparatory Action on Defense Research under grant agreement No. 831726. The statements made herein represent the opinion and findings of the authors. The European Union and the European Defense Agency are not responsible for any use that may be made of the information they contain.

Defense applications require intrinsically stable and resilient laser systems. Using single- or few-mode output fibers, fiber-based high-power lasers can address these challenges and also feature excellent beam quality, allowing to achieve high power density at long focusing distance. At high power levels, these diffraction limited output beams can be spoiled by thermally induced transverse mode instabilities (TMI), which cause beam profile fluctuations and thus increase the M²-factor. As TMI are an interference-based effect, it is to be expected that there is a dependence on polarization. We have thus set up an analysis setup that allows to characterize the individual mode content of the fluctuating beam along with the full polarization (in terms of Stokes vector) of each individual contributing mode at kHz speed. We will present the setup and first results for high power systems. In order to manipulate the TMI threshold, it has been shown to be beneficial to distribute the heat load evenly along the fiber. On the other hand, monolithic fiber component availability favors a co-propagating pump approach. We will present a dual-tone seeding setup that allows for variable modification of the heat load position and discuss the impact on the TMI threshold.

TNO has expanded its 30 kW HEL research facility with the capability to monitor specular and diffuse reflections of the laser beam. A capture screen and high-speed camera focus on dynamic specular reflections, while 15 individually placeable probes monitor the diffuse component under different angles. This paper introduces the reflection measurement capability and discusses the behaviour of steel and aluminium coupons under high-energy laser irradiation. Laser-material interaction was found to be rather predictable in thermal behaviour up to the perforation event. Reflections, however, showed a highly dynamic pattern, varying in magnitude and direction and depending on bulk material, material surface condition, phase state of the material (solid or liquid) and geometry. The difficulty of assessing proper stand-off distances for laser safety is illustrated.

Coherent beam combining techniques aim to obtain high-power laser beams with good quality and thus enabling several specific applications such as long-range communications, remote power delivering or L-DEW applications. They have received a renewed interest given the very good performances obtainable with high power fiber lasers adopting MOPA architectures and phase locking feedback loops. In the case of fiber lasers, the specific power of a single amplifier is limited by the deleterious effect of SBS. One can enhance the threshold of SBS widening the bandwidth of the field in the amplifier but this contrasts with the need of a spectrally pure field for the adoption of CBC architectures based on phase detection. Arrays with a large number of elements have been demonstrated for narrow-band systems, while wideband phase locking has been reported only in the case of a limited number of elements. The adoption of multi-KW fiber laser amplifiers requires bandwidths of the order 30 GHz. In this paper, we report on the experimental demonstration of a 7-element array coherently combined up to a spectral bandwidth of 47 GHz. The architecture is based on a narrow line single mode master oscillator whose emission is phase modulated to widen the bandwidth. Amplified fields are summed in a tiled aperture geometry and far field PIB is adopted as a metrics for a perturbation hill-climbing algorithm. Phase-locking results and convergence dynamics are analyzed in relation to the bandwidth properties of the oscillator modulation.

Brightness is often listed among the most important laser characteristic for practical applications. It is a function of both output optical power and mode quality. Multi-watt continuous wave (CW) operation has been demonstrated for broad-area Quantum Cascade Lasers (QCLs) emitting at ~4.6µm. Transition of the broad-area configuration to shorter wavelengths is however non-trivial as laser thermal behavior rapidly deteriorates with reduction in emission wavelength below 4.6µm. In this work we discuss the main design principles of high brightness, broad area QCLs emitting at ~4.0µm. Building off a power scaling approach to increasing broad area QCL CW power, a figure of merit is utilized to predict dominant lasing transverse modes for QCLs. A discussion follows on the role of laser core dimensions on mode selection within a waveguide, including design guidelines for maintaining single transverse mode behavior while altering broad area QCL design for increased power.

We report on the realization of a multi-emitter quantum cascade laser system with optimized package volume. A multidimensional bonding of chip-on-substrate units allows for close packaging of several chips. The emission of several chip-on-substrate units with an emission wavelength around 3.9 µm are geometrically combined to achieve a multi-Watt emission power level while obtaining a symmetric beam profile of the emission. A dedicated integrated electronic circuit provides individual pulse control, which enables individual timing for each emitter.

We present our latest results on the development of Quantum Cascade Lasers with Watt-level output power in the mid-wave infrared (MWIR) and long-wave infrared (LWIR) spectral regions. Wall-plug efficiencies in excess of 10% and 5% at 1 W output power level at room temperature were demonstrated at wavelengths of 4.0 and 9.0 µm, respectively. A ruggedized packaging which is capable of withstanding environmental conditions including mechanical shocks, vibrations, extreme storage temperature excursions, extreme ambient temperature excursions during operation, and thermal shocks was developed and qualified.

We developed a compact, diode-end-pumped, eye-safe laser rangefinder transmitter, which is based on the passively Q-switched Er–Yb:glass laser with a Co:Spinel plate as a saturable absorber (SA). The linear cavity laser considers a concave and a plane mirror with the cavity length is only 20 mm. The repetition rate can be tuned from 1 Hz to 8 Hz at the wavelength of 1535 nm. Our laser system operates stably at peak power > 250 kW and pulse width of 4.5 ns.

Aviation is exceptionally vulnerable to man-portable missile attacks (MANPADS), particularly during the critical stages of flight, e.g., take-off and landing. Consequently, aircraft require a further means of self-protection in addition to pyrotechnic flares. Laser Directed Infrared Countermeasures (DIRCM) target the infrared guidance system present in the majority of all MANPADS, resulting in sensor dazzle and possible damage-a soft kill approach. Unfortunately, current DIRCM systems, albeit highly effective against first and second-generation seekers, are less against imaging ones (third and fourth-generation). Our paper investigates a means to increase the effectiveness of dazzle by modulating the laser at a rate close to the frame rate of the imaging sensor, i.e., a strobing effect. A continuous-wave quantum cascade laser (QCL) at 4.6 microns illuminated a mid-infrared focal plane array imager, modulated by either an optical chopper or by periodically varying the current of the QCL. The laser beam and a representative target were combined optically using plano and off-axis parabolic mirrors, resulting in the imager viewing a dazzled scene at infinity. In summary, we demonstrate experimentally that the intermittency of the laser dazzle could improve the effectiveness of a DIRCM system.

This work presents a newly constructed setup for laser damage assessment at FOI, Sweden. The laboratory-based system is built around a 2 kW single mode fiber laser. The design, characterisation and performance of the system will be discussed. Initial studies on various fiber-reinforced plastics are presented. The time-to-penetration of different test-coupons is compared to analytical models and the range of applicability of these simple models will be discussed. Finally, laser damage studies on a hobby UAV will be shown.

High energy lasers (HEL) are increasingly seen as a versatile tool to counter observation systems by directly damaging or dazzling the electro-optical (EO) or infrared (IR) sensors used for detection, recognition, tracking, and targeting. The main mechanism through which high energy lasers affect their target is by heat. In the case of thermal sensors which use germanium optics, this heat is applied on the outer optical component as germanium isn’t transmissive at the typical wavelengths at which current HEL lasers operate. Germanium is a brittle material and therefore is prone to shattering due to mechanical stresses caused by local thermal expansion of the hotspot. Furthermore, germanium becomes opaque to thermal IR radiation at elevated temperatures and starts emitting radiation itself. This mechanism allows an out-of-band high-energy laser to indirectly dazzle thermal infrared sensors which is referred to as pseudo-in-band dazzling. Because this effect depends on the temperature of the germanium, the sensor can remain dazzled some time after the laser irradiation has stopped while the germanium lens is cooling down. To be able to assess the effectiveness of a laser beam to dazzle or destroy a germanium lens, one must know the evolution of the heat distribution throughout the lens. In this work a thermal simulation model is presented that takes in account several aspects that influence the propagation of heat in a realistic lens. The simulation results are compared to experimentally obtained results from an earlier measurement campaign. The potential impact of the incident radiation distribution on the heating and cooling times are discussed.

The technology of missiles and of their countermeasures is evolving continuously. High-power lasers are an option to encounter these threats. In order to understand their potential in such a scenario, it is vital to investigate the laser effects in the presence of a corresponding aerodynamic environment. Thus, experimental and numerical investigations were conducted cooperatively by Fraunhofer Ernst-Mach-Institut and the supersonic and hypersonic technologies department of DLR. An ytterbium fiber laser system was installed at the supersonic wind tunnel VMK. The laboratory was fit to meet necessary laser safety requirements. Combined subsonic and supersonic flow and high-power laser experiments with flow velocities up to a Mach number of three and a laser power up to 10 kW were realized. Two kind of tests were performed, focusing on laser beam distortion through aero-optical effects and on high-power laser effects, respectively. The interaction effects between aerodynamics, laser radiation and irradiated targets were studied on flat-plates as well as cylindrical and radome targets, simulating generic missile design. Irradiated objects consisted of steel, aluminum, carbon-fiber-reinforced polymer and the ceramic-based composite WHIPOX. While beam distortions were studied with a wavefront sensor, damaging processes were investigated by measuring the perforation time of the targets, as well as via high-speed imaging, thermography as well as Schlieren imaging. Numerical three-dimensional, steady, and uncoupled simulations were performed. The data indicated complex interactions between material, laser beam, and aerodynamics. This investigation can be used as an initial basis for further analysis of laser-material-aerodynamic interactions with respect to missile defense.