High-power Laser Generation Research Articles

Mid-infrared seven-core chalcogenide fiber with ultra-large mode field area and high beam quality

A seven-core chalcogenide fiber with an ultra-large mode field for mid-infrared range of 2.5–11 μm is designed and fabricated. Through manipulation of the core radius and pitch in the seven-core configuration, we are engaged in a comprehensive exploration of crosstalk characteristics and the mode field area (MFA). In addition, the relationship between the parameters of seven-core fiber for infrared and the beam quality of the output laser is analyzed for the first time. A theoretical MFA of 8914 μm2 can be calculated with a core radius of 24 μm and the pitch of 50 μm. This impressive MFA is realized through the deployment of an improved drilling technique in the fabrication of a Ge–As–Se seven-core fiber. The fiber has a relatively low loss at the wavelength range of 2.5–11 μm, and the minimum loss is 1.4 dB m−1 at 8.5 μm. The measured MFA of the fiber at 10.6 μm is 7364 μm2, which is 6.2 times higher than that of traditional stepped single-core fiber, but slightly lower than the theoretical value. The power delivery capability of the fiber has been significantly improved about two times compared with that of single-core fiber. The output beam quality factor M 2 is calculated as 1.13. In all, the seven-core fiber exhibits substantial potential for high-power laser propagation with high quality and flexibility.

Thermal effect of high-power laser propagation in complex channels and its mitigation

We investigate the channel thermal effect resulting from the gas thermal blooming effect and mirror thermal deformation on the beam quality, and we propose a mitigation measure for blowing purge gases. Without gas blowing, the gas thermal blooming effect opposes the influence of mirror thermal deformation on the beam phase, resulting in significant phase distortion and offset. Noticeable distortion and divergence of the laser spot was also observed. After blowing purge gases, the beam phase was primarily determined by the mirror thermal deformation. Simultaneously, the mean-squared beam width decreased uniformly, improving the symmetry and energy concentration of the laser spot. However, the laser spot tended to stabilize when the gas velocity exceeded a certain threshold.

Neutron dose assessment in laser-generated ultra-short pulsed fields.

ELI Beamlines is one of the pillars of the Extreme Light Infrastructure European Research Infrastructure Consortium (ELI ERIC), the European project aiming at building the next generation of high power lasers for fundamental research and industrial applications. Several high-power lasers are hosted by the ELI Beamlines facility. Even at a power lower than the nominal one, when interacting with a target, the laser can generate mixed ionizing radiation fields of unique nature. One of the major laser systems, High-repetition-rate advanced petawatt laser system (HAPLS) was already used in commissioning experiments. Detecting the neutrons generated during these experiments has been a challenging task, since certain difficulties were faced. First, the experimental conditions were frequently altered during the commissioning phase (such as laser beam parameters, experimental geometry or target type). Next, the extremely short duration of the ionizing radiation pulse generated by the laser (~10-14s) complicated the correct interpretation of the data provided by the detectors designed and calibrated in standard fields. Here, one commissioning experiment is described, together with the means of addressing the problem of the detection of the ionizing radiation and the lessons learned in this endeavour.

Control systems and data management for high-power laser facilities

Abstract The next generation of high-power lasers enables repetition of experiments at orders of magnitude higher frequency than what was possible using the prior generation. Facilities requiring human intervention between laser repetitions need to adapt in order to keep pace with the new laser technology. A distributed networked control system can enable laboratory-wide automation and feedback control loops. These higher-repetition-rate experiments will create enormous quantities of data. A consistent approach to managing data can increase data accessibility, reduce repetitive data-software development and mitigate poorly organized metadata. An opportunity arises to share knowledge of improvements to control and data infrastructure currently being undertaken. We compare platforms and approaches to state-of-the-art control systems and data management at high-power laser facilities, and we illustrate these topics with case studies from our community.

A Comprehensive Review on Amplification of Laser Pulses via Stimulated Raman Scattering and Stimulated Brillouin Scattering in Plasmas

The demand for high-intensity lasers has grown ever since the invention of lasers in 1960, owing to their applications in the fields of inertial confinement fusion, plasma-based relativistic particle accelerators, complex X-ray and gamma-ray sources, and laboratory astrophysics. To create such high-intensity lasers, free-running lasers were either Q-switched or mode-locked to increase the peak power to the gigawatt range. Later, chirped pulse amplification was developed, allowing the generation of peak power up to 1012 W. However, the next generation of high-intensity lasers might not be able to be driven by the solid-state technology alone as they are already operating close to their damage thresholds. In this scenario, concepts of amplification based on plasmas has the potential to revolutionize the laser industry, as plasma is already a broken-down medium, and hence does not pose any problems related to the damage thresholds. On the other hand, there are many other aspects that need to be addressed before developing technologies based on plasma-based amplification, and they are being investigated via theoretical and numerical methods and supported by several experiments. In this report, we review the prospects of employing plasma as the medium of amplification by utilising stimulated scattering techniques, such as the stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS) techniques, to modulate high-power laser pulses, which would possibly be the key to the next generation of high-power lasers. The 1980s saw the commencement of research in this field, and possibilities of obtaining high peak powers were verified theoretically with the help of numerical calculations and simulations. The extent of amplification by these stimulated scattering schemes are limited by a number of instabilities such as forward Raman scattering (FRS), filamentation, etc., and here, magnetised plasma played an important role in counteracting these parasitic effects. The current research combines all these factors to experimentally realise a large-scale plasma-based amplifier, which can impact the high-energy laser industry in the near future.

Kilowatt random Raman fiber laser with full-open cavity.

Random Raman fiber laser (RRFL) has been widely studied in high-power laser generation due to its special lasing characteristics. However, all previous high-power results are based on the half-open cavity. In this letter, we demonstrate an applicable high-power RRFL with the simplest structure, that is, a full-open cavity. The lasing dynamic and output characteristics are theoretically and experimentally studied. Laser source with multi-longitudinal modes can result in the random laser output from one side even in the full-open cavity. The ratio of the backward output power is mainly determined by the reflectivity of fiber ends. The experimental results show that such a simple structure can easily generate kilowatts of random laser power and is a promising setup to achieve higher output power, which is also an important platform to study the laser dynamic in high-power full-open cavity without any point-action or regular distributed reflectors.

Continuous wave high-power laser propagation in water is affected by strong thermal lensing and thermal blooming already at short distances

When laser beams propagate through media with non-vanishing absorption, the media is heated resulting in a change of the refractive index, which can lead to thermal lensing and thermal blooming. However, experimental details about both phenomena for propagations in water are lacking, especially for high-power lasers in the kilowatt range. We show that significant thermal lensing occurs only for high input powers before the onset of convective flow, while for low input powers, no strong thermal lens arises. After the onset of water flow, thermal blooming occurs at low input powers comparable to that known for propagations over kilometres in the air. However, for high input powers a thermal blooming on a qualitatively higher level is shown. By wavefront sensing, the change of refractive index distribution in water is investigated. This clearly shows the fast development of a strong thermal lens for high input powers and the onset of convection. Furthermore, a qualitatively good agreement of the accompanying simulations is observed. It is found that the absorption coefficient is linear with a value of mu ={13.7},{mathrm{m}^{-1}} at least up to 7.5 kW, i.e. 8 mathrm{kW/cm}^2. However, the directed transmission into an aperture is only constant before any thermal lensing of blooming occurs.

Preface

The Military Technical College, sponsored by the Egyptian Ministry of Defense, is honored to organize the ninth International Conference on Mathematics and Engineering Physics in the period of July 7-9, 2020. The conference provides a valuable opportunity to all the research people in Egypt and all over the world to exchange and update their knowledge in the designated fields of interest The papers accepted by the scientific committee were presented during the conference period in 3 sessions. The conference proceedings include only the full text papers, which were presented during the conference and approved by the respective session boards. Accepted papers are arranged in 3 sessions in the following scientific branches: Engineering Mathematics, this includes: • Numerical Analysis, • Mathematical Analysis. • Probability and Statistics. • Fuzzy Mathematics • Optimization Engineering Physics, this includes: • Applied Optics & Holographic Techniques. • Generation of High Power Laser & Simulation. • Lasers and its applications in medicine. • Laser – Material or Biological Tissue Interaction. • Nanoparticles Technology. List of Conference High Committee, Conference Steering Committee, Conference Organizing Committee, Scientific Committee, Contributors, Authors Index, 001_ ICMEP-10, 002_ ICMEP-10, 003_ ICMEP-10, 004_ ICMEP-10, 005_ ICMEP-10, 006_ ICMEP-10 are available in this pdf.

Nonlinear wakefields and electron injection in cluster plasma

Laser and beam driven wakefields promise orders of magnitude increases in electric field gradients for particle accelerators for future applications. Key areas to explore include the emittance properties of the generated beams and overcoming the dephasing limit in the plasma. In this paper, the first in-depth study of the self-injection mechanism into wakefield structures from non-homogeneous cluster plasmas is provided using high-resolution two dimensional particle-in-cell simulations. The clusters which are typical structures caused by ejection of gases from a high-pressure gas jet have a diameter much smaller than the laser wavelength. Conclusive evidence is provided for the underlying mechanism that leads to particle trapping, comparing uniform and cluster plasma cases. The accelerated electron beam properties are found to be tunable by changing the cluster parameters. The mechanism explains enhanced beam charge paired with large transverse momentum and energy which has implications for the betatron x-ray flux. Finally, the impact of clusters on the high-power laser propagation behavior is discussed.

Experimental Study of Laser Beam Welding Process Parameters on AISI 4130-309 Joint Strength

Abstract Laser Beam Welding (LBW) is extensively being utilized in manufacturing processes to join dissimilar metals and alloy steels because its special advantages of controlled heating, Low Heat Affected Zone (HAZ) and smaller weld bead formation. However, it is viewed as a strange strategy with applications normally and constrained to welding of thick plates of metals. With a new generation of high power lasers, there has been a renewed interest in thick section LBW (also known as Keyhole Laser Welding (KLW)). KLW makes the process a possibility for industrial applications dealing with thick metals for welding in power plants, pipelines, offshore structures, shipbuilding. The advantages provided by such LBW, is suitable for joining at high process speed, low heat input, and to achieve high productivity, leading to significantly reduction in process costs. LBW of dissimilar metals such as alloy steel and stainless steel is still a challenging task, particularly due to the formation of martensitic formation in HAZ, brittle phases in the weld and solidification cracking in the fusion zone. Such issues can significantly deteriorate strength of the welded joint. Therefore, the aim of the present work is to examine the basic phenomena that happen inside the different weld zones and their impact on weld quality in the wake of LBW joints. Taguchi L25 is chosen and experiments are conducted with LBW considering 2 mm thick dissimilar metals, i.e. AISI 4130 and AISI 309 stainless steel by varying Laser Power, Welding Speed, Beam Angle, Focal Point Position and Focal Length. The experimental output results that are measured for the mechanical properties like Ultimate Tensile Strength and Impact Strength. The Analysis of Variance (ANOVA) is carried out to obtain the influence of the process parameters and statistical evolution of the results. Firefly optimization algorithm is used to determine the optimal combination of the process parameters of the LBW

Extruded seven-core tellurium chalcogenide fiber for mid-infrared

A novel tellurium chalcogenide (ChG) multicore fiber has been fabricated via extrusion for the first time. The fiber has seven cores in a hexagonal structure, with one center core surrounded by six other ones. The loss of the fiber is less than 3.5 dB/m in a range of 5-11 µm, and the minimum optical loss is 0.38 dB/m at 6 µm. When light is coupled to each core, the maximum transmission power is up to 1.1 W at 10.6 µm. Supercontinuum spectrum covering 2-12 µm at a bandwidth of -30 dB has been demonstrated. This tellurium ChG multicore fiber shows great potential in the field of high-power laser propagation and wide supercontinuum generation in the mid-infrared.

Self-phase modulation and spectral broadening in millimeter long self-written polymer waveguide integrated with single mode fibers

Polymer waveguide bridges up to 1.2 mm in length were optically written between two single-mode optical fibres. Optical Nonlinearity of polymer waveguide was measured by coupling a high power ultra-short pulsed laser. Before waveguide fabrication fiber ends were treated by adhesion promoter which covalently bond to both the silica fibre and the photopolymer, also photopolymerization was conducted from both fibres. The new techniques improve both the mechanical properties and optical transmission of polymer bridges. The optical insertion loss was 1.1 dB for the longest waveguides. A considerably long interaction length of polymer waveguide allows spectral broadening and self-phase modulation features to occur in response to the high power laser propagation through the polymer bridges. The spectral broadening in polymer waveguide was much broader than that of 1.5 m plain fibre. The ultrafast nonlinearity of the polymer waveguides was determined to be of the order of 103 times the nonlinearity of silica.

Continuous-Wave Mid-Infrared Gas Fiber Lasers

Efficient continuous-wave laser emission of acetylene molecules at 3.1 μ m wavelength has been demonstrated in low-loss hollow-core fiber by optically pumping using a fiber-amplified diode laser around 1.53 μ m. In this paper, we report and compare the continuous-wave laser performance of acetylene molecules in the cavity and single-pass configurations. The use of low-loss antiresonant hollow-core fiber makes the single-pass configuration ideal for high-power laser generation and enables over 1.1 W continuous-wave mid-infrared laser output with over 33% slope efficiency relative to the absorbed pump power. The systematic characterization of power-scaling and pump power absorption properties demonstrates that the molecular kinetics inside hollow-core fiber determines the laser performance, and that low fiber attenuation is key to high-efficiency continuous-wave output.

Quadrature Michelson Interferometer for Temperature Measurement in the Core of Active Fiber of a High-Power Fiber Laser

We propose a new way for temperature measurement in the core of active optical fiber under high-power laser generation. The technique is based on quadrature fiber Michelson interferometer assisted with lock-in amplification for separated amplitude and phase modulations of the probe light, which allows to perform measurements in arbitrary heat transfer conditions. The technique is demonstrated on the Yb-doped fiber laser. The pump power up to 25 W under natural convection was applied. The value of convective heat transfer coefficient and its dependence on temperature difference between fiber and environment are determined from these measurements.

The steady-state thermal blooming of the high-power laser propagation in the rain

We study the propagation of high-power laser in the rain. The effective permittivity of the rain is incorporated into the propagation model to study energy loss and further the steady-state thermal blooming of the high-power laser. With the help of simulations, we obtain the contour distribution of the laser beam at different propagation distance for different rain rates.

Atmospheric propagation and combining of high-power lasers.

In this paper, we analyze beam combining and atmospheric propagation of high-power lasers for directed-energy (DE) applications. The large linewidths inherent in high-power fiber and slab lasers cause random phase and intensity fluctuations that occur on subnanosecond time scales. Coherently combining these high-power lasers would involve instruments capable of precise phase control and operation at rates greater than ∼10 GHz. To the best of our knowledge, this technology does not currently exist. This presents a challenging problem when attempting to phase lock high-power lasers that is not encountered when phase locking low-power lasers, for example, at milliwatt power levels. Regardless, we demonstrate that even if instruments are developed that can precisely control the phase of high-power lasers, coherent combining is problematic for DE applications. The dephasing effects of atmospheric turbulence typically encountered in DE applications will degrade the coherent properties of the beam before it reaches the target. Through simulations, we find that coherent beam combining in moderate turbulence and over multikilometer propagation distances has little advantage over incoherent combining. Additionally, in cases of strong turbulence and multikilometer propagation ranges, we find nearly indistinguishable intensity profiles and virtually no difference in the energy on the target between coherently and incoherently combined laser beams. Consequently, we find that coherent beam combining at the transmitter plane is ineffective under typical atmospheric conditions.

Direct-drive inertial confinement fusion: A review

The direct-drive, laser-based approach to inertial confinement fusion (ICF) is reviewed from its inception following the demonstration of the first laser to its implementation on the present generation of high-power lasers. The review focuses on the evolution of scientific understanding gained from target-physics experiments in many areas, identifying problems that were demonstrated and the solutions implemented. The review starts with the basic understanding of laser–plasma interactions that was obtained before the declassification of laser-induced compression in the early 1970s and continues with the compression experiments using infrared lasers in the late 1970s that produced thermonuclear neutrons. The problem of suprathermal electrons and the target preheat that they caused, associated with the infrared laser wavelength, led to lasers being built after 1980 to operate at shorter wavelengths, especially 0.35 μm—the third harmonic of the Nd:glass laser—and 0.248 μm (the KrF gas laser). The main physics areas relevant to direct drive are reviewed. The primary absorption mechanism at short wavelengths is classical inverse bremsstrahlung. Nonuniformities imprinted on the target by laser irradiation have been addressed by the development of a number of beam-smoothing techniques and imprint-mitigation strategies. The effects of hydrodynamic instabilities are mitigated by a combination of imprint reduction and target designs that minimize the instability growth rates. Several coronal plasma physics processes are reviewed. The two-plasmon–decay instability, stimulated Brillouin scattering (together with cross-beam energy transfer), and (possibly) stimulated Raman scattering are identified as potential concerns, placing constraints on the laser intensities used in target designs, while other processes (self-focusing and filamentation, the parametric decay instability, and magnetic fields), once considered important, are now of lesser concern for mainline direct-drive target concepts. Filamentation is largely suppressed by beam smoothing. Thermal transport modeling, important to the interpretation of experiments and to target design, has been found to be nonlocal in nature. Advances in shock timing and equation-of-state measurements relevant to direct-drive ICF are reported. Room-temperature implosions have provided an increased understanding of the importance of stability and uniformity. The evolution of cryogenic implosion capabilities, leading to an extensive series carried out on the 60-beam OMEGA laser [Boehly et al., Opt. Commun. 133, 495 (1997)], is reviewed together with major advances in cryogenic target formation. A polar-drive concept has been developed that will enable direct-drive–ignition experiments to be performed on the National Ignition Facility [Haynam et al., Appl. Opt. 46(16), 3276 (2007)]. The advantages offered by the alternative approaches of fast ignition and shock ignition and the issues associated with these concepts are described. The lessons learned from target-physics and implosion experiments are taken into account in ignition and high-gain target designs for laser wavelengths of 1/3 μm and 1/4 μm. Substantial advances in direct-drive inertial fusion reactor concepts are reviewed. Overall, the progress in scientific understanding over the past five decades has been enormous, to the point that inertial fusion energy using direct drive shows significant promise as a future environmentally attractive energy source.

Fabrication of chalcogenide glass photonic crystal fibers with mechanical drilling

A mechanical drilling method for the preparation of photonic crystal fibers (PCFs) is presented in this paper. Several PCFs were fabricated with Ge20Sb15S65 chalcogenide glasses, which have high transparency in the mid-infrared (IR) range. The mechanical drilling method has been identified as a powerful tool to prepare fibers with a variety of structures and to increase the transmission of the obtained fibers. For a three-ring PCF, the near-field intensity distribution and the transmission loss were measured. It was found that most of the optical energy is contained in the core of the PCF. The profile of the near-field intensity image shows that the prepared PCF can work with a large mode area, which is important in high-power laser propagation and fiber amplifiers.

High-power lasers for directed-energy applications.

In this article, we review and discuss the research programs at the Naval Research Laboratory (NRL) on high-power lasers for directed-energy (DE) applications in the atmosphere. Physical processes affecting propagation include absorption/scattering, turbulence, and thermal blooming. The power levels needed for DE applications require combining a number of lasers. In atmospheric turbulence, there is a maximum intensity that can be placed on a target that is independent of the initial beam spot size and laser beam quality. By combining a number of kW-class fiber lasers, scientists at the NRL have successfully demonstrated high-power laser propagation in a turbulent atmosphere and wireless recharging. In the NRL experiments, four incoherently combined fiber lasers having a total power of 5 kW were propagated to a target 3.2 km away. These successful high-power experiments in a realistic atmosphere formed the basis of the Navy's Laser Weapon System. We compare the propagation characteristics of coherently and incoherently combined beams without adaptive optics. There is little difference in the energy on target between coherently and incoherently combined laser beams for multi-km propagation ranges and moderate to high levels of turbulence. Unlike incoherent combining, coherent combining places severe constraints on the individual lasers. These include the requirement of narrow power spectral linewidths in order to have long coherence times as well as polarization alignment of all the lasers. These requirements are extremely difficult for high-power lasers.

The extreme nonlinear optics of gases and femtosecond optical filamentation

Under certain conditions, powerful ultrashort laser pulses can form greatly extended, propagating filaments of concentrated high intensity in gases, leaving behind a very long trail of plasma. Such filaments can be much longer than the longitudinal scale over which a laser beam typically diverges by diffraction, with possible applications ranging from laser-guided electrical discharges to high power laser propagation in the atmosphere. Understanding in detail the microscopic processes leading to filamentation requires ultrafast measurements of the strong field nonlinear response of gas phase atoms and molecules, including absolute measurements of nonlinear laser-induced polarization and high field ionization. Such measurements enable the assessment of filamentation models and make possible the design of experiments pursuing applications. In this paper, we review filamentation in gases and some applications, and discuss results from diagnostics developed at Maryland for ultrafast measurements of laser-gas interactions.