Since defense technologies greatly depend on Low Earth Orbit (LEO) military surveillance satellites, anti-satellite (ASAT) operation is the first reaction after a war breaks out. Surveillance satellites with the altitude of <400 km can be small and low cost, which realizes many deployments like drones in the sky. Therefore, ASAT operation targeting such a low-cost satellite by a multi-stage missile is costly. This gives an expectation for a high energy laser (HEL) as an ASAT weapon. However, the use of a ground-based laser (GBL) is limited to use only in clear sky conditions. A space-based laser (SBL) seems to realize an ideal ASAT weapon, but many SBLs are necessary to disable even a few satellites because the SBLs need to go around the Earth. Meanwhile a ballistic-missile defense system using a Chemical Oxygen-Iodine Laser (COIL) carried by a high-altitude airship (HAA) was proposed to intercept a ballistic missile. Since an HAA can always stay in a clear sky, it should also be used for ASAT operations. Only a single system can disable many satellites since it can stay at a fixed location. The beam profile is simulated to estimate the beam intensity at the target satellite. Since the laser beam propagates upward obliquely to the target, altitude-dependent atmospheric turbulence must be considered. The calculation results show that the beam intensity can be high enough to damage the solar panels of the satellite.

High-power laser plays an important role in many fields, such as directed energy weapon, optoelectronic contermeasures, inertial confinement fusion, industrial processing and scientific research. The uniform nearfield is the important part of the beam quality for high power lasers, which is conducive to maintaining the high spatial beam quality in propagation. We demonstrate experimentally that the spatial intensity distribution at the output is well compensated in the complex high-power solid-state laser system by using the small-aperture spatial light modulator (SLM) in the front stage. The experimental setup is a hundred-Joule-level Nd:glass laser system operating at three wavelengths at 1053 nm (1ω), 527 nm (2ω) and 351 nm (3ω) with 3 ns pulse duration with the final output beam aperture of 60 mm. While the clear arperture of the electrically addressable SLM is less than 20 mm. In the beam shaping system, the key point is that the front-stage SLM needs to precompensate the gain nonuniform of the laser system. Liquid crystal SLM is an effective active beam shaping device through adjusting each pixel transmittance to improve the spatial beam quality of the output laser, which can also be used as a binary optical element (BOE) with each pixel transmittance 0 or 1 to realize spatial beam shaping for high-power lasers. We present and demonstrate an efficient shaping method of the SLM used as BOE based on diffraction principle. The method can be used to control the output nearfield actively by compensating the spatial nonuniformity of transmission and amplification in the high power laser system. Results show the output nearfield beam quality improves significantly after shaping by using this method with the fluence contrast changing from 22% to 11.3% within only 2 shots in the single-shot operation laser.

The application of the solid state lasers has been greatly expanded in many domains. The laser system is required to be highly integrated. In this paper, an integrated design scheme of laser diode pumped solid state laser is proposed. The size of the device is reduced by integrating the laser power source with LD pump module. The MCU and low-temperature drift drive circuit is utilized as the hard core. The simplified effective pre-stage DC-DC power source and a quasiresonant full-bridge temperature control circuit are designed. The deep negative feedback for the discharge control loop is concluded in the design. The TEC is used and the control accuracy is ±0.5℃ by the using of predictive PID. The ripple and rise time of the current waveform is tested and the stability of the power supply operated at 20Hz is verified by the load test. The experiment results reveal that this system has good load capacity and stability. Compared with the previous power supply system, this system is small and compact. The power supply efficiency is improved by more than 10% and the size of device is greatly reduced.

With the rapid development of optoelectronic information technology, the infrared light source with the wavelength of 1~5μm has many potential application prospect in the field of spectroscopy, laser radar, remote sensing, laser communication and so on. The infrared laser can be obtained by many methods. Optical Parametric Oscillator (OPO) is one of the most important methods. It has the advantages of small size, light weight, stable performance, good reliability, high beam quality and the wide spectral range. In this paper, we design an OPO optical arrangement with the MgO:PPLN crystal which is pumped by a high repetition rate acousto-optic Q-switched Nd:YVO₄ laser. The crystal is fixed in a small case that the temperature could be precisely controlled. In the experiment, we heated the MgO: PPLN crystal to different temperatures. By adjusting the temperature, the relationship between the temperature and the output power is recorded. Theory analysis of the laser is given to illustrate the phenomenon. The effect on the output power and the transfer efficiency is detailed studied by varying the temperature of the MgO: PPLN crystal.

Solid-state lasers are widely used in many applications. The LD pumping module is the power source of the laser system. A well-designed pumping light field can achieve high efficiency and high quality laser output. In this paper, a mathematic model of the side-pumping module is constructed. The light field of a single LD bar is tested and simulated with different parameters with the ray tracing method. Then the pump power distribution in the cross section of the LD array is analyzed, which indicates the absorption and distribution in the laser rod. An improved method by increasing the pumping sides is proposed to achieve higher pumping homogeneity. Simulation results show that the pumping homogeneity is improved. A pumping module is produced according to the design. After a small adjustment of the mathematic model, the test result of the rod cross section photos fits the simulation well. Then an optical resonant cavity with a Nd: YAG laser rod is set up. The laser with the beam quality near the diffraction limit is obtained. The LD pumping module can be used in high power MOPA laser systems. The improved mathematic model can be used to simulate and design side-pumped lasers in future.

High power laser weapons have made impressive technical progress and have demonstrated their capabilities in shooting down numerous targets in live demos. National demonstrator systems on military platforms are now in preparation, for instance the integration of a 20 kW laser effector on a maritime platform. The core component of a laser weapon system is the militarized high energy laser (HEL). In this contribution the development of a 20 kW HEL for defence application will be presented. The laser concept is based on robust, compact and efficient high-power single-mode fiber laser modules. Power scaling to 20 kW is achieved with spectral beam combining. System architecture and laser data will be discussed.

The laboratory model of 10-kW class laser effector was designed and assembled. The special laboratory setup for characterization of its parameters and research on interaction with materials was developed. As a result of dynamic thermaloptic phenomena inside laser source and measurement setup the observed laser beam distributions in far field present features of 4D spatial-temporal, non-stationary stochastic process, thus averaging in given plane over long exposures times was not justified. Measurement of laser beam parameters directly in far field and Wavefront Sensing Measurement by Shack-Hartmann method were applied in experiment. To analyze the experimental data of distorted wavefront measurements Wigner Transform method was applied. Beam quality and brightness determined via Wigner approach was changed in the same way as the direct measurements of beam parameters in far field. The deterministic aberration as a result of dynamic thermal-optic effects depending on averaged laser power was found, which can explain non-Gaussian profiles in the vicinity of focal plane.

We present a design and analysis for the surface gain medium slab laser. This single slab laser is potential to output 12kW class laser. The experiment already achieved 5kW CW laser power output.

We report on recent experimental and theoretical studies of cesium laser, where we focused on the beam propagation factor M² in flowing-gas Cs DPALs with stable optical resonators and of its dependence on the resonator geometry. The measured results were modeled by multi-transverse-mode model [Auslender et al., Opt. Express 25, 19767 (2017)]. Conditions for substantial improvement of the output laser beam quality, reducing M² to close to unity, are found. In particular we show how changing the length of the resonator, and/or the radius of curvature of the high reflection mirror, leaving all other parameters of the laser unchanged, makes it possible to control the beam quality.

Optically pumped rare gas lasers have the potential to provide high-power output with excellent beam quality. They lase at wavelengths that are readily transmitted by the atmosphere, and their optical characteristics are closely similar to those of diode-pumped alkali vapor lasers (DPAL’s). As compared to DPAL’s they present two significant advantages. The first is that they use entirely inert reagents that are gases at ambient temperature (Helium plus a heavier rare gas). As there is no chemistry involved, these devices can be operated in a completely closed-cycle mode. A second advantage is derived from the energy level structures of metastable rare gas atoms. While the sp transitions of alkali metals will provide just one laser wavelength for a given element, each rare gas atom can offer a range of different output wavelengths from the first manifold of sp transitions. The primary technical challenge for the optically pumped rare gas laser is the requirement to generate the lasing medium (metastable Rg(3P2) where Rg=Ne, Ar, Kr or Xe) at a density of approximately 1e13 cm-3 in the presence of helium at total pressures in the range 0.5 – 1.0 atm. For this application we have developed a quasi-CW discharge driven by a high repetition rate power supply. The frequency and time duration of the pulses are tailored to exploit the non-steady high field characteristics of the pulsed breakdown, while sustaining a temporally steady argon metastable concentration in the gain medium. This system has been scaled to a 912.3 nm output of ~4 W when pumped by a 20 W diode laser. Recently, the pulsed discharge system has been improved by increasing the upper limit for the voltage available from the power supply. The present system operates at voltages up to 2200 V with a nominal pulse duration of 50 ns and pulse repetition frequency of 100 kHz. This has been used to sustain a discharge in 1 atm of a He/Ar mixture that produces an Ar* metastable density of 2e13 cm-3, a path length of 3 cm and a total volume of 1.2 cm3 (0.64x0.64 cm2 cross section). A diode pumped Ar* laser that employed this discharge has been operated for extended periods with no sign of performance degradation. As expected for a system that uses only inert gases, there was no indication of window damage or chemical activity. Computational models indicate that scaling to the 100 kW level is feasible with a discharge volume of 10 cm3 and discharge power of 200 W.

Different frequency conversion processes in the mid-IR were experimentally studied with slab RF discharge CO and CO₂ lasers and various nonlinear crystals ZnGeP₂, GaSe, BaGa₂GeSe₆, AgGaSe₂ and PbIn₆Te₁₀ : second and third harmonic generation of CO laser; sum and difference frequency generation of CO and CO₂ lasers. Mid-IR emission on hundreds of narrow spectral lines from 1.7 μm to 19.3 μm was obtained

Continued progress in the development of optical fiber-based lasers has led to the present state where further improvements in performance are limited by intrinsic optical nonlinearities. In order to manage such limitations, laser designers have largely adopted the approach of microstructuring the fiber to shift nonlinear thresholds to high optical powers. The nonlinearities are accepted as fixed and performance is enhanced through fiber geometric complexity. This talk treats a different option, which is to mitigate optical nonlinearities at their fundamental origin: the materials with which the light interacts.

Fibre lasers and amplifiers have proven their interest in high power laser systems thanks to their compactness and their stability against temperature and mechanical fluctuations. The increase of emitted optical power is currently limited by nonlinear effects due to the intrinsically high confinement of the optical field in fibres. Among all the existing nonlinear effects, the Stimulated Brillouin Scattering (SBS) is predominant in the case of single frequency or narrow spectrum optical fields. The SBS effect comes from the interaction between the incident optical field and the acoustic (density) waves in the optical fibre material, creating an effective Bragg grating for the incident wave. This interaction results in the creation of a back-reflected optical wave depleting the power of the incident one. Because of the nonlinear nature of SBS, the power transfer between the incident and the reflected optical waves depends on the incident power. It is usual to define a threshold power 𝑃𝑆𝐵𝑆 at which the reflected wave arises. In order to increase the power of single frequency fibre lasers, many developments have been done in the past decades to mitigate SBS. We can class the different SBS mitigation techniques in two categories depending on whether they act on the optical field properties or on the optical fibre itself. All the techniques belonging to the first category are based on an active control of the phase or the frequency of the incident optical wave, in order to spread the incident energy in the spectral domain: sinusoidal Phase Modulation (PM) [1], non-sinusoidal PM [2], Gaussian noise PM, Pseudo-Random Binary Sequence (PRBS) PM [3] or frequency chirp modulation [4]. In the second category, there are different ways to increase 𝑃𝑠𝑏𝑠. Among them, we can maximize the core radius of the fibre, minimize the fibre length [5], or apply temperature gradients [6], dopant concentrations gradients or strain gradients to broaden the SBS spectrum and thus lower its effective gain. We present here two patented techniques allowing longitudinal tensile or compression strain gradients on optical fibres. We performed successful realizations on a wide variety of optical fibres, ranging from single mode and Large Mode Area (LMA) passive fibres for power transport, to (single-mode and LMA) active fibres used in amplifiers architectures. Thanks to their passive nature and their insensitivity to wavelength, we believe these techniques to be highly suitable for high power narrow spectrum laser systems. Figure 1 : Experimental output power and pulse energy of two realizations of single frequency pulsed fibre amplifiers used in optical sensing systems. Left : Erbium doped amplifier at 1580 nm with tensile strain gradient on LMA active fibre. Right : ErYb co-doped single mode active fibre at 1545 nm with compresive strain gradient. Figure 1 presents the obtained output power and pulse energy of two single frequency amplifiers with high SBS threshold, thanks to our strain gradient techniques. Figure 1 (left) corresponds to a LMA Er doped fibre whose core diameter is 40 μm mounted on our tensile strain gradient device. It delivers 150 ns long pulses at 4 KHz repetition rate with up to 1800 W peak power, instead of the 600 W peak power limit in absence of SBS mitigation. Figure 1 (right) presents the achievements made with the compressive strain gradient technique. This single frequency single mode fibre amplifier emits up to 385 W peak power at 10 KHz repetition rate instead of 120 W peak power before applying the compressive strain gradient. The details of the two patented strain gradients techniques and some experimental implementations of these techniques in high SBS threshold fibre amplifiers will be presented. The advantages and limitations of such techniques for narrow spectrum high power laser sources will also be discussed. References [1] C. Zeringue, I. Dajani, S. Naderi, G.T. Moore and C. Robin (2012), A theoretical study of transient stimulated Brillouin scattering in optical fibers seeded with phase-modulated light. Opt. Express 20 (19), 1196-21213. [2] Y. Dong, Z. Lu, Q. Li and Y. Liu (2008) Broadband Brillouin slow light based on multifrequency phase modulation in optical fibers. J. Opt. Soc. Am. B 25 (12) [3] C. Robin, I. Dajani, C. Zernigue, A. Flores, B. Pulford, A. Lanari and S. Naderi (2013) Pseudo-random binary sequency phase modulation in high power Yb-doped fiber amplifiers Proc. SPIE 8601, 86010Z [4] J.O. White, D. Engin, M. Akbulut, G. Rakuljic, N. Satyan, A. Vasilyev and A. Yariv (2015) Chirped laser seeding for SBS suppression in a 100-W pulsed erbium fiber amplifier. IEEE J. Quant. Elec. 51 (6) [5] R. G. Smith (1972) Optical power handling Optical power handling capacity of low loss optical fibers as determined by stimulated Raman and Brillouin scattering. Appl. Opt., 11(11) [6] J. Hansryd, F.Dross, M. Westlund, P.A. Andrekson and S.N. Knudsen (2001) Increase of the SBS threshold in a short highly nonlinear fiber by applying a temperature distribution J. Light. Technol. 19 (11)

We report the development of a model written in the COMSOL multi-physics environment to simulate the onset and shortrange propagation of laser filaments in standard atmospheres at sea-level and 10 km altitude. We show excellent agreement with analytical and open-source experimental data for the initial collapse distance of the laser, as well as the free carrier density of the resulting plasma. Limitations of the model are presented as well as the potential for further development, including the incorporation of additional physics to study the effect of filamentation on radio frequency and optical propagation.

Composite materials (GFRP) have shown a significant increase in their use for aerospace and military applications in recent years. Concurrent maturation of the technology in near Infrared High Energy Lasers has proven to be promising for use in directed energy weaponry. The binding matrix of many common aerospace composite materials are made of polymeric materials. The thermodynamics of GFRP as well as the thermo-mechanical behavior of such heterogeneous materials submitted to a typical laser weapon irradiation need therefore to be further explored to understand precisely the deterioration process induced by the illumination. Front and side views during the HEL (High Energy Laser) interaction process with the GFRP target material at different time delays clearly evidence the complicated nature of smoky plume and flame combustion creation during the irradiation process. During the energy deposition rise of the continuous laser wave, the GFRP target heats up due to absorption in the outer epoxy layer. Further increase of the energy reveals a smoky plume optically thin directly above the surface of the GFRP-target. Time resolved emission spectroscopy was used to investigate the chemical decomposition and surface temperature in the low and high temperature regime during the laser matter interaction process. The spectral emission in the visible range is dominated by continuum emission which provides the distribution temperature in time based on Planck`s law. In the low temperature regime (infrared wavelength bands from λ = 2μm up to λ = 11μm) a four channel infrared detector system was designed to retrieve the distribution temperature. The experimental, time resolved signal from four different infrared detectors at center wavelengths of λ = 3.348 μm, λ = 4.49 μm, λ = 7.41 μm and λ = 10.57 μm are used to reconstruct the Planck-function by a fitting routine with the temperature as parameter. The calibration of the system was made with a conventional Black-body source PY5. The obtained temperature distribution in time was then compared to measurements with a conventional pyrometer. With the concept of this four channel detector we overcome the difficulties of acquisition speed and single band nature of conventional pyrometers.

The components used with high power lasers are frequently exposed to high thermal loads, which can cause a variety of detrimental effects including thermal deformation, which can lead to aberrations, and contamination-induced heating, which can lead to catastrophic component failure. In this presentation, we discuss several topics in thermal design for high power laser components. The first involves dirt or contamination. When a dirty optic is hit with high power laser light, the contamination absorbs the light very efficiently, sending the surface temperature to thousands of degrees. In order to prevent the surface from creating large numbers of free carriers due to the extreme temperature, the materials near the surface must have a very large bandgap. Indeed, we have shown that different materials can display more than an order of magnitude different CW (continuous wave) laser damage thresholds. Contamination-resistant optics must be designed solely of high bandgap materials. A second topic involves thermal mismatch in optical coatings. Thermal expansion mismatch between layers in a coating or between the coating and its substrate can deform an optical component, but such deformation can be eliminated at the coating design stage by optimizing the mechanical parameters of the coating in tandem with the optical properties. To first order, this is a matter of incorporating layer thickness, elastic modulus, thermal expansion, and intrinsic stress levels into a mechanical multilayer model that ensures flatness across a wide range of temperatures. Additional topics build on the first two. It has been found that there can be a significant reduction in film stress as the density of a thin film is reduced, and there seems to be a threshold for sharp transitions at a certain density level. These characteristics can be used to build low-stress contamination-resistant high reflectivity coatings from a single material with a high bandgap, such as SiO2. Finally, since optical elements are so sensitive to thermal deformation via changes in optical path length (shape change and refractive index), cavities formed by such elements can be used a probe of temperature variation or indeed other processes that change local refractive index, such as turbulence.

This work seeks to explore the interplay between material differences in the nonlinear index of refraction n₂ and zero group velocity dispersion wavelength λ_GVD in the formation of spectral broadening and supercontinuum generation. We present mid-wave infrared (MWIR) spectroscopic data of the transmitted 200 fs (FWHM) laser pulse through polycrystalline zinc selenide and polycrystalline zinc sulfide (Cleartran™) optical materials. Using MWIR laser wavelengths between 3.3 and 3.8 μm, we vary the laser energy between roughly 1 and 126 μJ and focus the laser pulses into the materials using lenses of four different focal lengths. This allows us to analyze not only how the nonlinear frequency broadening of the fundamental wavelength varies with increasing pulse energy, but also how it varies with increasing peak intensities when the pulse energy is kept fixed. These results are discussed along with possible implications on the importance of both laser energy and peak intensity in producing supercontinuum for a material with a given n₂ and λ_GVD. Understanding of these practical considerations is essential for assessing material effects to MWIR ultrashort pulsed lasers.

Laser experiments under combined mechanical tensile stress have been carried out to evaluate the operational vulnerability of fiberglass composite structures to a laser irradiation. It follows that the optical propagation of the laser beam inside the semi-transparent reinforced laminate is one of the key issues of this study as it rules every phenomenon occurring at a later stage. The Radiative Transfer Equation (RTE) has been used together with spectrometric measurements to assess the initial coupling of the monochromatic laser beam with the optically diffusing material. However, the partial absorption of the high-energy laser beam quickly leads to very-high temperatures on the irradiated area that can induce several phase transitions of the polymer. These changeovers and their influence on the optical and thermal properties of the system have been investigated with conventional methods (TGA and DSC) and reproduced with a time- and temperature-dependent kinetic scheme based on the Arrhenius equation. Finally, a 1D multiphysics model has been developed to reproduce the temperature evolution recorded during laser trials. Based on a time-explicit scheme, this computational approach shows a fairly good agreement and allows for a further understanding of the multiple phenomena occurring under a laser irradiation.

Numerical calculations based on the finite element method (FEM) were performed dealing with the thermal initiation of a confined explosive charge (here: TNT confined in a steel casing). These calculations show self-intensifying heating inside the explosive as soon as the temperature reaches a critical value. The point of initiation is located slightly inside the explosive charge. At the interface explosive to metallic wall, heat conduction into the wall prevents a fast heating whereas deeper inside the explosive, less internal energy is released due to the lower local temperature. As expected, the initiation times are getting shorter with increasing laser power. In addition, the parametric studies show that the critical temperature and the distance between the initiation point and the wall are decreasing with rising laser power density. As the FEM-calculations are very time consuming, simplified analytical considerations were applied to approximate the temperature and time necessary to achieve a thermal initiation. Starting from analytical solutions of the heat equation, that describe the thermal evolution inside the metallic shell for Gaussian shaped laser beams in the multiple kw-range, an iterative algorithm has been developed to determine the critical temperature and the initiation time. An extended analytical derivation also made it possible to determine the temperature evolution for a rotating tube.

This PDF file contains the front matter associated with SPIE Proceedings Volume 11162, including the title page, copyright information, table of contents, and author and conference committee lists.