This PDF file contains the front matter associated with SPIE Proceedings Volume 11867, including the Title Page, Copyright information, and Table of Contents

Welcome and Introduction to Optical Countermeasures Conference

Welcome and Introduction to High Power Lasers Conference

This paper discusses the use of smoke obscurants as countermeasures against high energy lasers (HEL). Potential success of the smoke does not depend only the performance of the smoke. The transmission loss in the smoke is part of a chain of system components, including warning sensors, smoke launchers, etc.. The core of the paper deals with experimental work on the following research questions: - Does smoke attenuate an incoming beam of a HEL? - Does the HEL affect the smoke itself? The experimental set-up with the TNO 30kW HEL and the scale model for the smoke transmission path will be shown. Selected experimental results will be shown and discussed. Finally we will compare the results to theoretical calculations and we will analyze the properties of an ideal HEL attenuation smoke.

Multi-watt continuous wave operation has been demonstrated for broad-area, Fabry-Perot Quantum Cascade Lasers (QCLs). In addition to high optical power, increase in operational range for infrared countermeasures requires low atmospheric propagation losses for emitted radiation. Single-line operation tailored to low atmospheric losses can be achieved for QCLs utilizing the distributed feedback grating etched into the laser waveguide along full cavity length. An alternative solution explored here is to utilize the grating as an outcoupler, so-called distributed Bragg reflector (DBR) configuration. Since output facet reflectivity of only several percent is needed for high-performance QCLs, the DBR section can be made very short, on the order of several hundred microns, leaving the rest of the (optimized) laser waveguide unchanged. Top-metal DBR configuration with grating etched into the top cladding layers of the QCL structure offers the advantage of a low fabrication cost. Therefore, broad-area DBR QCLs with a top-metal grating promise a significant improvement in spectral brightness and at the same time a low fabrication cost. The main design principles for these devices will be discussed in this talk along with preliminary experimental data.

Unmanned aerial vehicles (UAV:s) have become an increasing threat in both civilian and military arenas. While military UAV:s often are relatively large and complex, the supply in the civilian hobby market is characterized by small, cheap and simple systems with the capacity to stream high-definition video, carry a variety of other sensors and transport critical goods (eg food or medicine) to hard-to-reach places. The criminal world has quickly realized how UAV:s can be used to smuggle weapons or drugs, for example. Militarily, UAV:s are established for reconnaissance, fire control and electronic warfare operations etc. Laser-guided weapons from a UAV, is an example of a widely used system for precision operations during later conflicts. This paper examines and summarizes various laser functions and their role for detecting, recognizing, tracking and combating a UAV. The laser can be used as a support sensor to others like radar or IR to detect end recognise and track the UAV and it can dazzle and destroy its optical sensors. A laser may also be used to sense the atmospheric attenuation and turbulence in slant paths, which are critical to the performance of a high power laser weapon aimed to destroy the UAV. This paper is part 2 dealing with the laser as a countermeasure towards the UAV optical sensors or used as a weapon.

The resistibility to corrosion of biocompatible metals is significantly reduced when plastic deformation of the surface layer is more than 0.5%. To increase the successful reliability of consolidation of bone tissues is proposed to use implants made of chemically pure titanium with a nanostructured biocompatible Ti-TiN coating to improve functional properties. An increase in functional properties is provided by improving coating deposition technology in an arc discharge plasma, which does not allow the formation of any impurities. The use of a high-current diffusion discharge in a vacuum chamber forms a controlled microstructure and the morphology of the implant surface, which increases adhesion during healing. The effectiveness of the proposed coating of implants is proved by the results of comparative tests on the study of the functional properties of samples without coating, with a coating applied by traditional technology and improved technology with a high-current diffusion discharge.

The continuously growing demand for high-precision complex-shaped parts combined with limited capabilities of conventional machining methods in their production have opened up high prospects for the use of Wire Electrical Discharge Machining (WEDM). The machining of ceramic composites with WEDM is an important area of highprecision machining. However, the high electrical resistance of such composites is a big problem, due to which WEDM of poorly conducting and semi-conducting materials cannot be monitored and controlled in the same way as that of electrically conductive materials. In particular, the adaptive control of the feed rate using monitoring of electrical parameters is no longer possible after the electrical conductivity of the workpiece material decreases to the level of semiconductors. This means that the development of monitoring and control principles and instruments for WEDM of semiconductor materials is critically important. In this work, after experimentally studying the physico-mechanical nature of EDM of semiconducting materials such as ceramic composites, recommendations for selecting the control criteria based on the vibroacoustic (VA) signal were developed. VA signals of the WEDM of semiconducting composite ceramics TiC+Al₂O₃ were measured to develop a more reliable method of preventing short circuits and wire electrode breakage than the conventional monitoring of voltage and amperage. We show that that the developed method is more efficient in detecting short circuits than conventional monitoring scheme and can thus be used as a foundation for the development of new adaptive control schemes of WEDM of semiconductor materials.

Rationalizing energy consumption in the Electrical Discharge Machining (EDM) efficiently saves energy and improves machining quality. Since the conventional methods for estimating useful EDM energy are based on theoretical heat transfer studies or empirical assessments of processing conditions, the development of an industrially applicable method for assessing useful energy is an important problem. Here we show that the performance of the EDM process is directly related to acoustic emission (AE). The effectiveness of the proposed method has been evaluated in experiments. As part of the execution of the experiment, AlCuMg1 workpiece was machined using a copper electrode with different duty cycles with pulse widths varying from 10% to 80%. For comparative analysis, the root-mean-square vibroacoustic signal in the range of 1-10 kHz and the root-mean-square of the discharge current were used. It was found that the amplitude of the vibroacoustic (VA) signal monotonically increases with the increasing EDM performance. The properties of the VA signal allows using the VA monitoring to assess the performance of EDM, i.e., to determine the fraction of energy spent on removing the workpiece material. The advantage of the proposed method of monitoring is that the control of useful energy is carried out using accelerometers installed on the parts of the technological system on the workpiece side. The distance from accelerometers to the workpiece being processed can be quite large that is convenient for performing experiments. In particular, in the high frequencies range, the obtained results are protected from mechanical interference coming from drives, hydraulic units and wire rewinding mechanisms. Such VA signals are shown to be important indicators of EDM efficiency because they are observed only if the energy fluxes reach the workpiece surface. This provides a more reliable indication of raising concentrations of electroerosion products that prevents short circuits and breakage of wire electrodes.

We review recent work in which we developed a phase-matched model to study the transverse mode instability (TMI) in high-energy laser amplifiers. The standard models for TMI have contributions that vary rapidly compared to the beat period between the fundamental mode and the higher-order modes in the problem. In the phase-matched model, we neglect these rapidly varying contributions. We consider a realistic example with a Yb-doped fiber amplifier that is similar to the amplifier that was considered by Naderi et al. [Opt. Exp., 21(13), 16111 (2013)], but with a more realistic 10-m length. In this example, only one higher-order mode is present. We show that the computational speedup of the phase-matched model is on the order of 100 with no loss of accuracy even in the highly-saturated nonlinear regime.

Coherently Beam Combining System is sensitive to occurring in near-field disturbances: tilts, phase, amplitude, depolarization, defocusing and wavefront aberration. The analytical/numerical model for such purposes was developed. The tilts impact is determined mainly by angular diffraction size of emitter, whereas the size of lattice and number of emitters does not influence on result. The defocusing errors of endcaps in CBC combining optics resulting in the Guoy’s piston/phase errors in far-field must be mandatory compensated. Phase disturbance leading to Maréchal formula shows that the lattice architecture and specific beam profile plays second order role. The requirements on tilt and phase compensation do not increase with the complexity of CBC systems (number of emitters), paving the way for designing effective CBC systems with several hundreds of emitters.

Micro mirror arrays, such as the Digital Micromirror Device (DMD) from Texas Instruments, are amplitude modulating spatial light modulators. Binary holograms write spatial amplitude modulations to the device and with coherent sources can be used to control reflected beams through controlled diffraction. In this paper the inherent aberrations within the DMD device have been identified and corrected through the use of Zernike modes written to the array allowing a device profile to be used as a foundation characteristic upon which other applications can be improved. The approach used coopts a technique used for wavelength measurement using a DMD which focusses onto a camera. The tilting action of the mirrors means the device acts as a mixture of a lens and 2D blazed grating. In this case it also allows assessments of coherence length to be made based on a dual dispersion capability. A model for producing the Fresnel diffraction pattern from a specific DMD pattern is produced using Fresnel diffraction theory and shows good agreement with measured results.

This work presents a coherent beam combination (CBC) setup that consists of a four square tiled-aperture system with phase and tip-tilt control for each sub-beam. The system works at a wavelength of 1550 nm. Phase locking was performed with the stochastic parallel gradient descent (SPGD) algorithm, using the power reflected by a remote retro reflecting corner cube to a feedback detector located at the laser head. Experiments were performed for targets located at distances of 470 and 870 m during calm and partly cloudy conditions. The SPGD beam combining results are discussed and compared to optical wave propagation simulations. The optical wave propagation models that have previously been validated in a laboratory setting were extended to include atmospheric turbulence. In order to reproduce the experimental results it was important to include the propagation of the feedback signal from the retroreflector to the detector.

Dazzling of the human eye by lasers or bright light sources is a commonly used non-lethal measure to warn individuals or to reduce the human performance of an opponent in security or military scenarios. To study the deterioration of the human performance, e.g. in terms of the ability to drive a vehicle or with respect to task performance, like shooting precision, dazzle experiments on subjects are essential. Experiments with laser dazzlers require complex and careful preparation to ensure laser safety. Moreover, in many cases an approval from an ethics committee is required. In this publication, we propose an approach for the assessment of human performance degradation with regard to laser dazzle based on augmented and virtual reality (AR/VR). Instead of using a laser dazzler, the visual incapacitation is implemented by a virtual dazzle spot simulated on an AR/VR device. The achievable level of obscuration and veiling caused by the virtual dazzle spot is investigated experimentally and, in a further step, compared to experiments performed with real lasers as described in the literature.

High Energy Laser (HEL) is enjoying great interest around the globe, prompting explicit knowledge-building. TNO, the leading defence research institute of the Netherlands, has established a physics-based HEL system performance assessment tool, encompassing all aspects relevant to the effective use of HEL in the military operational environment. TNO also created two experimental facilities that provide validation data for the assessment tool. The tool and facilities are presented here, and a few examples of the validation efforts are given.

An experimental demonstration of laser beam coherent combining with active phase control has been performed using for the first time a Multi-Plane Light Converter device (MPLC). The MPLC as a beam combiner is designed as a spatial multiplexer which output modes form a Gaussian beam when superimposed constructively, reaching theoretically 100% efficiency. Moreover, reflective free-space design allows for handling high power. The experiment combines seven 1.5 μm continuous wave fiber lasers operated at a low power level in the tens of milliwatt range using the frequencytagging LOCSET technique (Locking of Optical Coherence by Single-detector Electronic-frequency Tagging) for the phase locking. 72-% power efficiency MPLC CBC is achieved with an output combined beam close to a Gaussian beam profile. M² is lower than 1.8 depending on the transverse direction, revealing an excellent quality for the combined beam. The output beam is more than 94 % linearly polarized. Simulation of the impact of atmospheric turbulence on the propagation of the seven laser beams up to 1 km is performed. We demonstrate that it is possible to compensate for most of the atmospheric propagation detrimental effects and to perform efficient MPLC CBC through strong turbulence.

Fiber-based high-power laser systems are advantageous for defense applications due to their intrinsic stability and resilience against challenging environmental conditions. Using single- or few-mode output fibers, an excellent beam quality can be achieved, allowing for tight focusing over long distance. However, thermally induced transverse mode instabilities (TMI) limit the obtainable diffraction-limited output power. We will show how thermally induced mode instabilities can be conveniently detected in-line in an all-fiber system. Towards this aim, we compare different detection locations and methods and show that characteristic signals can be detected in backwards direction. Expanding the system and applying these detection methods, we will investigate Raman-based high-power amplification for spectral shift in a passive fiber. We present the observation and identification of TMI due to Raman amplification and the tracing of their origin, locating it in the passive fiber section. By comparing different fiber and seed laser parameters, we deduce respective limiting parameters.

The goal of deploying a high-power laser directed energy effector on a mobile platform creates several challenges beyond the primary requirement of high laser power. The pump sources and beam conditioning optics that are used in industrial lasers do not provide the low volume and mass required for deployment on a mobile platform. Similarly, use of conventional discrete spherical and aspherical optical elements does not provide the level of beam control and efficiency required to achieve the necessary on-target power and spot size. We describe how freeform optics are used to realize pump sources and beam combining systems with the high levels of optical performance and efficiency, coupled with low mass and volume, required to meet the low-SWaP targets set for deployable LDEW systems.

We are demonstrating a BiBO crystal based three-stage OPCPA operating in the SWIR wavelength range from ~1800 nm to ~2000 nm for signal and from ~2100 nm to ~2400 nm for idler. This OPCPA was seeded with supercontinuum pulses from a 130 mm YAG crystal. Both the OPCPA and supercontinuum were pumped by transform-limited ~1.2 ps pulses from a Yb:YAG chirped pulse amplifier. In the second stage, a signal energy of ~220 μJ was achieved with a conversion efficiency of 10%, and the signal energy in the third stage exceeded 1 mJ, which corresponds to a conversion efficiency of 19%. Subsequently, the signal-to-idler energy transfer with efficiency of 20% was achieved by Transient Stimulated Raman Chirped-Pulse Amplification in a KGW crystal 30 mm long. Preliminary results were obtained on the compression of the amplified pulse. The laser will be used for remote detection of hazardous gases.

A comprehensive three-dimensional modeling of static K diode pumped alkali laser (DPAL) and flowing-gas K and Rb DPALs is carried out. The cases of He/CH4 and pure He buffer gases are investigated and the output power and optical efficiency calculated for various pump powers, mole fractions of methane, buffer gas pressures and flow velocities. The model considers the processes of excitation of high levels of K and Rb, ionization, ion-electron recombination and heating of electrons which affect the diffusion coefficient of K and Rb ions. It explains the experimentally observed sharp increase in power in static K DPAL caused by the addition of a few percent of methane to He buffer gas and its decrease with further increase in the methane percentage [B.V. Zhdanov et al, Opt. Exp. 25, 30793 (2017)]. The predictions of the model for different He/CH4 mixtures are presented and verified by comparing them with experimental results for K flowing-gas DPAL [A. J. Wallerstein, Ph.D. dissertation (Air Force Institute of Technology, 2018)] and with the calculated results obtained using a simplified three-level model based on one-dimensional gas dynamics approach reported by A. Gavrielides et al [J. Opt. Soc. Am. B 35, 2202 (2018)].

The paper demonstrates results of our experimental study of THz emission from the plasma of a single-color laser filament for two laser wavelengths of 740 and 940 nm. Particular attention is paid to the study of a frequency-angular distribution of the THz emission within 0.1 – 3 THz spectral range. It is observed that different spectral components of THz radiation have different angular distribution, and an increase in the laser beam numerical aperture leads to a growth of the THz emission divergence, especially in its low-frequency range. The study revealed a significant effect of a laser pulse initial wavelength on the THz emission characteristics. Transformation of frequency-angular THz emission spectrum produced by a single-color (740 nm) laser filament plasma under an external electrostatic field of various strength is also experimentally studied. If there is no static electric field, THz emission is predominantly generated in the low-frequency spectral range around 0.1 THz and propagates within a hollow cone. When the electric field is applied, the transition from the hollow cone to a filled one is observed with the field strength rise, THz emission frequency being within of 0.3 - 0.5 THz. Higher frequency emission of ~1 THz fills the whole cone with the emission maximum along the laser filament axis. Furthermore, the angular distribution for the low-frequency THz emission depends significantly on the laser pulse energy in contrast to the case of no electric field. Namely, the laser pulse energy rise results in a decrease of the propagation angles for low-frequency THz emission and disappearance of the local minimum in the angular distribution on the propagation axis.

This paper will report the use of spectroscopy in a series of experiments in which a High Energy Laser (HEL) interacts with target materials. After a brief description of the 30 kW L3O laser facility at TNO, the experimental results will be discussed. The main research question is whether spectroscopy can contribute to analysing the behaviour of the material during the interaction while the HEL is illuminating the target material. Selected experimental results will be shown and discussed. Conclusions will be drawn on how spectroscopy can contribute to analysing the HEL-material interaction indoors and potentially outdoors.

A new approach to obtaining three-dimensional cutting edges on a tapered cutting surface is proposed based on an image obtained on a measuring machine in the control section. The method is developed based on forming a spline and evaluating the shape using a unique algorithm, which is in turn based on an analytical mathematical model. The developed method allows estimating the main geometric parameters of the cutting edge and analyzing the shape of the helical groove of the drill. In the experiments, the drill was installed on a rotary table capable of adjusting the position around its axis. The validity of the new approach was corroborated by physical tests and analysis of the set of images obtained using an optical measuring system.

Photoacoustic imaging (PAI) has gradually developed into a new and important imaging technology, which combines the high contrast of optical imaging and the high resolution of ultrasonic imaging to achieve a deeper imaging of biological tissue, and has been widely used in biological imaging. The basic principle of photoacoustic imaging is that a short pulse of laser is used to illuminate biological tissue. As photons pass through biological tissue, some of them are absorbed by biological molecules such as hemoglobin, DNA-RNA, fat, water, melanin and cytochrome. In PAI, the light absorption mechanisms generally include electron absorption, vibration absorption, stimulated Raman absorption and surface plasmon resonance absorption. The absorbed light energy is usually completely converted to heat energy by the non-radiative relaxation of the excited molecules, and the pressure wave caused by the heat energy is transmitted in the tissue to form ultrasonic wave, namely the photoacoustic signal. The photoacoustic signal reconstruction generated by the detection of ultrasonic transducer can be obtained from biological tissue. The absorption of blood in visible band is much higher than that of other tissues (except melanin) to achieve high resolution imaging of blood vessels without exogenous markers. Due to the characteristics of biological tissue, it has different degrees of absorption to different wavelengths of light, resulting in different heat and pressure waves generated, the final received ultrasonic wave is different, which will affect the final image results. Based on the finite element analysis method, this paper establishes four complete coupling modules in COMSOL software: coefficient form partial differential equation module, biological heat transfer module, solid mechanics module and transient pressure acoustic module. By setting different optical parameters to explore the degree of absorption of different light waves by biological tissues, so as to find the best wavelength of light for the experiment.