This PDF file contains the front matter associated with SPIE Proceedings Volume 12347, including the Title Page, Copyright information, Table of Contents, and Conference Committee list.

Activities on diode pumped alkali laser (DPAL) in Japan are updated. Since the last presentation in Chengdu, we have continued our DPAL research mainly targeting the development of tools for future scaling-up and high-efficiency operation. In particular, the focus is on a numerical simulation that combines computational fluid dynamics (CFD) and wave optics. To confirm the agreement between the experimental and simulation results, we designed and built several experimental apparatuses. Our latest gas-flow type DPAL produced 15.5 W output power with 80% slope efficiency based on the absorbed pump light. The agreement with theoretical prediction was excellent not only for output power but also for transverse mode shape and beam quality. Additionally, to accurately predict the performance of Cs DPAL, we measured the main reaction cross sections, namely, mixing and quenching reaction cross sections of the two P states of Cs with various hydrocarbon buffer gases. The main result of the repetitive pulsed operation by the cavity dumping technique and relevant numerical simulation is also discussed.

Modeling of static K and Rb diode pumped alkali lasers (DPAL) and ion-electron recombination processes in these lasers is reported. 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)]. Calculations of potential energy curves of the 2 K + and 2 R_b + molecular ions and of the diabatic ¹ε⁺, ³ε⁺, ¹Δ, ³Δ, ¹ Π ³Π, ¹Φ and ³Φ valence states of 2 K + and 2 Rb + that provide the routes for dissociative recombination (DR of the ions are performed. These curves are required for subsequent calculations of DR rate constants. The excited states of K atoms produced by DR are 4²P and 5²P. Most of the Rb atoms produced by DR are in the 62P excited state. This conclusion contradicts the kinetic scheme for K and Rb DPAL proposed elsewhere, and thus the kinetic schemes of these DPALs should be modified according to the present results.

Parametric measurements of pulsed output energy from the four-level Cs-Ar exciplex pumped alkali laser (XPAL) as a function of input pump energy and temperature show a strong dependence on temperature. The data shows a D₂ line laser performance increase with temperature towards a peak efficiency, followed by a decrease as temperature is increased beyond the peak performance point. The efficiency was highest with Cs-Ar in the temperature range 493 – 513 K. Prior simulations of Cs-Ar XPAL measurements indicated that energy pooling from the 6p ²P_3/2 state of Cs was significant at higher temperature and it was hypothesized that the addition of temperature-dependent reaction rates may be important. This paper presents BLAZE Multiphysics™ simulations using temperature-dependent energy pooling reaction rates baselined to available experimental rate data. These calculations show that the temperature-dependent energy pooling rates explain the rise and fall of Cs-Ar XPAL performance with temperature with reasonable accuracy. Longer pulse simulations are also presented that show a significant increase in optical-to-optical efficiency, but also exhibit a faster decay at higher cell temperatures due to the increased impact from energy pooling. Estimates of temperature-dependent energy pooling reaction rates are presented for Rb and K.

This paper presents the development and initial experimental testing of a new compact, high-performance singlet oxygen generator (SOG). The generator uses a centrifugal design to assist in two-phase separation, a porous plate injection technique to create a froth and maximize chlorine utilization, and a closely coupled minimum length nozzle (MLN) to minimize transport losses. A high froth speed in the curved wall region is essential in order to attain a significant centrifugal force that pins the liquid component to the curved wall, which consequently results in a rapid and efficient separation of the vapor containing the excited oxygen that then flows into the nozzle. This generator operates without diluent and was demonstrated to perform at pressures greater than 350 Torr, which enables the technology to operate without the need for a pressure recovery system in an airborne chemical oxygen-iodine laser (COIL). The implementation of this froth SOG (FSOG) in combination with the use of a MLN allows the generation of large number densities of excited oxygen at a very high plenum pressure without the use of any diluent. Experimental results for this novel FSOG, along with the engineering logic behind it, are presented. Number densities greater than 5.3×10¹⁷ cm^–3 were measured via calibrated spectroscopic analysis of the O₂(¹ Δ) emission about 1268 nm at the exit of a supersonic nozzle. To the authors’ knowledge, the FSOG produced a higher pressure and larger excited oxygen number density than any other SOG system reported.

Silver halide polycrystalline infrared (PIR) fibers and hollow waveguides (HWGs) have high transmission at midinfrared wavelengths from 3 to 18 μm. Their applications include a flexible delivery of CO₂ and CO laser power. We investigated transmission of PIR fibers and HWGs at different bending radii using CO₂ laser radiation and studied the intensity distribution after the distal fiber. The PIR fibers show only a relatively weak decrease in transmission with increasing curvature 1/R or decreasing bending radius R. This is an advantage over hollow waveguides, where transmission decreases sharply with curvature. Disadvantages are high reflection losses at the PIR fiber end faces due to the high refractive index of 2.15 for wavelengths in the mid-infrared region. To reduce these losses, the surface of the fiber end faces must be treated with several special methods including microstructuring or coupling with an antireflective window. The measured near-field and far-field intensity distributions or beam profiles are highly inhomogeneous for both fiber types. For large core diameters of 0.9 or 1 mm, the beam profiles appear to be more homogeneous for the PIR fibers.

Large samples have been employed to obtain energy state distributions in products (translational, vibrational and rotational) for the OH (v_OH = 0; j_OH = 2) + D₂ (vD₂ = 0; jD₂) gas phase reaction initial conditions. It uses Quasi-Classical Trajectory calculations and the Gaussian Binning methodology on Wu-Schatz-Lendvay-Fang-Harding potential energy surface. As the author has already observed in a previous works, depending on a first selection, Vibrational-Gaussian Binning, a second selection, Rotational-Gaussian Binning, it induces a variable preference for odd or even values in the Total Angular Momentum of the HOD product molecule. The increase of the rotational level, D₂ (v = 0; j), induces a minor displacement of the distributions, P(J'), towards higher values of J' than when it increases the vibrational level, D₂ (v ; j = 2). Nevertheless, causes more anisotropy in those distributions, P(J').

3D laser metal printing is used to build up work pieces layer by layer from powderized or solid metal with the help of high-power laser welding. The latter process replaces conventional machining more and more due to the advantages of neglectible material loss and absence of tool wear,although disadvantages are a relatively low processing speed and sometimes necessary postprocessing. Two options are mainly used,namely the selective melting of preplaced metal powder with travelling laser (SLM) and also the melting down of a flow of powderized metal or solid metal wires with travelling nozzle and laser (LMD) where in the latter case the work piece volume is more or less unlimited but a health risks exists due to inhaled powder,both associated to the absence of housing . In the actual paper these two options are treated in detail where first of all the mechanical set up and its function are explained followed by a numerical analysis of the LMD process that can easily be used also for the SLM process and covers the phenomena ultimately necessary to run the processes .It demonstrates the influence of the main parameters as laser power,focus shape and size,travelling speed and others on the temperature of the molten material,layer width and thickness and in consequence on building rate and helps thus to optimize equipment and process. It is expected that these processes have a bright future especially if the development of enhanced processing speed will be successful.

The automotive industry, especially electric vehicles, opens a large variety of new laser-based applications. Particularly, the importance of high reflective materials such as copper and aluminum increased and lead to technology developments in laser devices and laser materials processing. The key factors are high brilliance, new wavelengths, and beam shaping, as well as new sensors. This contribution highlights the latest achievements and technology trends in the area of welding of copper components widely used in the E-Mobility industry. It is shown that the green wavelength supports the stability of the welding process. In addition, it is shown, that in thick materials, the NIR-laser in combination with beam shaping approaches, such as superimposed beams, can drastically decrease cycle times and increase welding depths with lasers of higher brilliance. In this contribution, laser applications for battery cell production and power electronics are demonstrated. It is shown how the applications can be sorted in those who benefit most from green and from NIR laser wavelength, both a domain of the disc laser. We show that advanced process strategies enable to use the full potential of those laser tools. Apart from welding applications, green wavelength enables unseen new applications in 3D printing of copper parts.

Coherent beam combining (CBC) by active phase control is an efficient technique to power scale fiber laser sources emitting in the near-infrared, between 1 and 2 μm, up to the multi-kilowatt level. Interestingly, it has been demonstrated by our team that CBC could also be used to power scale mid-infrared sources, frequency converters, generating a wavelength between 3 and 5 μm. We present our latest results on coherent combining of continuous-wave highefficiency mid-infrared sources: optical parametric oscillators (OPOs) and detail the difficulties encountered to achieve this combining, as well as the main limitations to efficient operation of CBC in this case. In a second part of this talk, we also present recent results on coherent combining of seven 1.5-μm fiber lasers through active phase control, using frequency-tagging, and operating efficiently on a remote target. A testbed has been designed to combine these 7 lasers on a remote surface, with phase-locking operating through analysis of the optical signal backscattered by the target, in a so-called target-in-the-loop (TIL) experiment. In such TIL configuration, CBC mitigates both laser-amplification-induced and atmospheric turbulence-induced phase fluctuations simultaneously. CBC demonstrated proper operation outdoors, on a target located up to 1 km from the laser and the results from this experimental campaign will be described.

Picosecond pulses at 515 nm, 343 nm, 257 nm, and 206 nm were generated in sum frequency processes in nonlinear crystals (LBO, CLBO) pumped by a high-power DPSSL Yb:YAG thin disk laser at 90 kHz repetition rate. Output power and M² dependencies on the fundamental input power have been measured.