This PDF file contains the front matter associated with SPIE Proceedings Volume 11777, including the Title Page, Copyright information, and Table of Contents.

Welcome and Introduction to SPIE Conference 11777

We are reviewing the high-average and high peak-power fs-laser sources and experimental areas currently in operation and preparation for user operation. This includes the 1 kHz, 15fs, 50mJ, Allegra laser based on OPCPA-technology. Short pulse 5ps-CPA thin disc lasers pump a series of OPCPA crystals ensuring a high contrast output. The Allegra laser enters the experimental area E1 with a number of end-stations for user experiments. The HAPLS (sub-30fs, Ti: Sapphire) laser pumped by a high-average power frequency converted DPSSL is currently delivering 500 TW, 3.3 Hz pulses via a stable vacuum beam transport system with a pointing stability around 1rad to the experimental areas for plasma physics experiments (E3) and ion acceleration (E4) with the ELIMAIA station. Both areas are fully equipped with target chambers and focusing optics for experimental operation and user assisted commissioning. The Nd:Glass laser Aton provides 1.5 kJ pulses and is currently being compressed to 10 PW in a large compressor tank. A second oscillator allows shaped pulse ns-operation at kJ level or future combination of 1 PW pulses and kJ shaped ns-pulses for advanced WDM or fusion experiments in the E3 area. A new laser disc liquid cooling technology enables repetition rates of 1 shot/minute allowing a much higher data acquisition for this kind of experiments. Furthermore we will report on the first experiments and the future experimental plans as well as on the prospects for user operation.

The Extreme Light Infrastructure – Attosecond Light Pulse Source (ELI-ALPS), the Hungarian pillar of ELI, is the first of its kind that operates by the principle of a user facility, supporting laser based fundamental and applied researches in physical, biological, chemical, medical and materials sciences at extreme short time scales. This goal is realized by the combination of specialized primary lasers which drive nonlinear frequency conversion and acceleration processes in more than twelve different secondary sources. Any light pulse source can act as a research tool by itself or, with femtosecond synchronization, in combination with any other of the sources. Thus a uniquely broad spectral range of the highest power and shortest light pulses becomes available for the study of dynamic processes on the attosecond time scale in atoms, molecules, condensed matter and plasmas. The ground-breaking laser systems together with the subsequent outstanding secondary sources generate the highest possible peak power at the highest possible repetition rate in a spectral range from the E-UV through visible and near infrared to THz. The facility – besides the regular scientific staff - will provide accessible research infrastructure for the international scientific community user groups from all around the world. The attosecond secondary sources are based on advanced techniques of Higher-order Harmonic Generation (HHG). Other secondary sources provide particle beams for plasma physics and radiobiology. A set of state-of-the-art endstations will be accessible to those users who do not have access or do not wish to bring along their own equipment. Step by step the lasers are now commissioned, trialed and handed over for user operation. References S. Kuhn et al., “The ELI-ALPS facility: the next generation of attosecond sources.”, Topical Review, Journal of Physics B, 50 (2017) 132002

Founded by the European Strategy Forum on Research Infrastructure (ESFRI), three state-of-art laser-based institutes in Romania, Hungary, and the Czech Republic were commissioned in the Extreme Light Infrastructure (ELI). Construction for the three sites started in 2012 and, as of 2020, all sites are operational. ELI-NP (Extreme Light Infrastructure: Nuclear Physics) is located 10km south of Bucharest in Romania. Its flagship installation is two beams of 10 PW, each providing 230 J output energy at a 23 fs laser pulse width. The capability to provide a 10 PW output was recently demonstrated in a live performance. We were able to show that the 10 PW laser shots can be delivered for 10 minutes at a rate of one shot every minute. A total of 230 Zoom participants worldwide, including Prof G Mourou and Prof D Strickland, the Physics Nobel Laureates in 2018, witnessed this breakthrough demonstration. An early experiment at the 100 TW laser station at ELI-NP has already been completed. We successfully demonstrated an electron acceleration of up to 300 MeV, either resulting in monoenergetic or broadband spectra, depending on the well controllable experimental conditions we set. Operations at the 1 PW and 10 PW experimental stations will start soon. External user access will be tested with the early and commissioning experiments and will be formulated coherently within the framework of the IMPULSE project guided by ELI-DC. Reference Current status and highlights of the ELI-NP program research program, KA Tanaka, K Spohr, D Balabanski, et al., Matter Rad. Extremes, 5, 024402 (2020): doi.10.1063/1.5093535

The Matter in Extreme Conditions (MEC) instrument at LCLS pioneered the use of the hard X-ray free electron laser (XFEL) in combination with high-power optical lasers to advance high energy density science. Commissioned in 2012 as an open-access scientific capability, this application of the powerful XFEL diagnostic has driven a rich array of high-profile scientific results, providing new insight into atomic and structural properties of dynamic plasma and high-pressure material states. Aided in part by the success of MEC and other high power laser facilities, there has been a strong call from the research community over the past 5 years for increased national investments in high power lasers combined with existing national lab infrastructure. In response to a mission need statement from the US Department of Energy, Fusion Energy Sciences, SLAC has developed a conceptual design for a project to build a new HED science facility combining high rep-rate (10Hz) petawatt laser systems and high energy (1kJ) long pulse lasers with the LCLS XFEL. Combined with flexible and high efficiency experimental systems, this facility will enable a world-unique set of scientific capabilities complementing the new emerging generation of high-power laser facilities, including the pillars of ELI and new HED end stations at European XFEL and SACLA. In this talk, I will present an overview of the facility conceptual design and place it in the context of the growing field of high-power laser science.

The temporal contrast of ultrashort pulses has been a major concern in applications as the light preceding the main pulse can significantly affect interaction properties. Consequently, nonlinear temporal filtering techniques such as cross-polarized phase generation [1], nonlinear ellipse rotation (NER) [2,3] or plasma mirrors [4] are often applied in high peak power laser systems. NER is an efficient post-compression technique to generate high contrast ultrashort pulses as it allows the simultaneous temporal cleaning and nonlinear spectral broadening. It also exhibits the highest conversion efficiency and best spatial beam characteristics. So far, this phenomenon has been used mostly in hollow core fibers. Recently, post-compression of ultrashort pulses in multipass cells are widely and efficiently used as they provide high transmission and excellent output beam quality. Cavity based setups could be also suitable for temporal cleaning by exploiting NER [5]. On the other hand, the possibilities for temporal cleaning in multipass cells have not been investigated in detail. Here, we report on the contrast enhancement of ultrashort pulses in gas-filled multipass cells by combining the post-compression technique with NER. The nonlinear spectral broadening and temporal cleaning were examined with the help of a 3-dimensional numerical model. Nonlinear propagation of pulses at mJ energy level with 800 nm or 1030 nm central wavelengths were simulated. We propose possible arrangements with optimized parameters for both cases. The results suggest that the proposed scheme is suitable to obtain a compression factor higher than five with 50% efficiency, meanwhile 10^3 contrast enhancement can be achieved. By customizing the cavity geometry in a way that the accumulation of B integral is slow and continuous during the propagation, the beam quality is preserved at the output and no space-time coupling could be observed. Pulses were also tested at 20 mJ energy, in that range where capillary-based setups are difficult to implement, and the results indicate that the technique allows the generation of high contrast, few-cycle pulses even with TW peak power. [1] Jullien A, Albert O, Burgy F, Hamoniaux G, Rousseau J P, Chambaret J P, Augé-Rochereau F, Chériaux G, Etchepare J, Minkovski N and Saltiel S M, 2005 Opt. Lett. 30 920–922 [2] Khodakovskiy NG, Kalashnikov MP, Pajer V, Blumenstein A, Simon P, Toktamis MM, Lozano M, Mercier B, Cheng Z, Nagy T and Lopez-Martens R, 2019 Laser Phys Lett. 16 095001 [3] Mijesh NS, Hang XZ, Ischer PF, Uschet AAM, Salh RS, Tajalli A, Orgner UM and Veisz L, 2019 Opt Lett. 44 4028-4031. [4] Kapteyn H C, Murnane M M, Szoke A and Falcone R W, 1991 Opt. Lett. 16 490–492 [5] Kalashnikov M P, Risse E, Schönnagel H, Husakou A, Herrmann J and Sandner W, 2004 Opt. Express 12 5088–5097

This paper presents effects of tilt and decenter caused by vibration on spot size and shape. The ideal spot shape for a laser source can be modeled as a Gaussian curve. In high-power laser systems, focusing the laser beam on a target is very crucial to increase the power density. The laser spot size should be small to focus the energy on a small area on the target adequately. Thus, it is necessary that the optomechanical system mustn’t be affected by environmental factors such as temperature, humidity, vibration, etc. In this study, the effect of vibration on optomechanical design is examined. Results of optomechanical design analysis performed by using the finite element method are take into account to get tilt and decenter values as an input. The analysis is based on the MIL-STD-810G standard. As a result of the analysis, tilt and decenter values exposed to optomechanical design are determined and these values were provided as the input to the optical design.

The measurement and manipulation of high-intensity femtosecond laser pulses in-situ enables pulse optimization and the quantification of strong-field processes within the same experiment and is also very important for testing and validating new models, but it has proven very difficult to achieve. Here we present an all-optical and fully in-line technique for the temporal characterization of ultrashort laser pulses directly on target and at full laser power during high-intensity laser-matter interactions, using the concurrent third harmonic (TH) emission from the target. This TH is generated at the interaction location and thus it depends on and carries information of the driving laser field. This technique permits the full (amplitude and phase), in situ, in-parallel characterization of the pulses in a gas or solid target over a very wide intensity range that encompasses the 1013–1015 Wcm−2 regime of high harmonic generation (HHG) and other important strong-field phenomena. Despite HHG being a non-perturbative process, the TH signal shows a perturbative-like behavior over a very large intensity range extending well into the relativistic regime. We demonstrated the technique by measuring intense 4-fs pulses in conditions optimized for HHG and show numerically its extension to relativistic intensities, from 1018 to at least 1021 Wcm−2, which are presently inaccessible to other diagnostics. By combining a pulse compressor (chirped mirrors and a pair of wedges) already present in typical HHG setups with the TH signal, we implemented a dispersion-scan (d-scan) variant, whereby with a single measurement we can then relate the pulse at any compressor position with its respective HHG spectrum, providing a way to quantify and optimize the process. We refer to this new method as THIS:d-scan (Third-Harmonic In-Situ d-scan). Its in-situ nature can account for the nonlinear phase accumulated by the high intensity pulse travelling to the experiment, and within the experiment itself, unlike conventional ex-situ techniques that require sampling a small fraction of the pulse for diagnostic purposes. Experiments were conducted in argon gas targets, where the retrieved spectrum and spectral phase compare very well with the ones obtained through an independent ex-situ SPIDER technique. The achieved low retrieval error and the excellent agreement between THIS:d-scan and SEA-F-SPIDER measurements provide further evidence for a perturbative cubic dependence of the TH on the laser field, even in a situation where laser intensities are clearly in the non-perturbative regime and can excite strong-field processes such as HHG. In neon targets and at higher pulse intensities, the THIS:d-scan measurement already shows signs of pulse reshaping within the target, which cannot be observed by standard ex-situ diagnostics. THIS:d-scan can be implemented with minor modifications to existing strong-field setups. The fact that the TH emission occurs in gases, solids, and plasmas over a broad range of target densities, laser pulse durations, central wavelengths and intensities extending well into the relativistic regime, makes THIS:d-scan a very promising technique for the in-situ temporal characterization of ultrashort laser pulses during strong-field laser–matter interaction at the extreme conditions presently attainable with ultra-high power lasers.

Two OPCPA-systems at central wavelengths of 2.1 μm (in operation) and 3.1 μµm (under construction) provide pulses with a temporal duration of 4 cycles at a pulse repetition rate of 10 kHz and an average power of about 30 W. Both systems feature a single pump laser design with a powerful thin disk Yb:YAG pumplaser. Concept, simulation and beam emission characteristics are discussed. Flux levels in high harmonic generation of ~10⁹ photons/eV/s @ 150 eV, ~10⁷ photons/eV/s @ 350 eV and ~10⁶ photons/eV/s @ 450 eV have been determined with absolutely calibrated measurement tools. An IR-pump - soft X-ray scattering probe experiment at the Gd N-edge (150 eV) employing the transversal magnetic optical Kerr effect (TMOKE) investigates dynamics of optical magnetic switching and serves as a benchmark for future studies.

Ultrafast and broadband laser sources in the mid-infrared (mid-IR) spectral domain (3-8 μm) find important applications in many fields, such as metrology, imaging and spectroscopy. Most of the available mid-IR sources operate at high repetition rates and high stability but they are limited either in their spectral range, peak power or pulse duration. Although the high repetition rate is ideal for investigation of processes with low cross sections, energies below the mJ-level are not optimal for driving processes such as soft x-ray high harmonic generation, and the scaling in energy of the mid-IR pulses is of high interest. KTA is a birefrigent material which has a high laser-induced damage threshold (LIDT) and a moderate nonlinear coefficient (deff ~ 2.1 pm/V). Another interesting material is magnesium oxide doped lithium niobate (MgO:LN) which has a high nonlinear coefficient (deff ~ 3.9 pm/V) allowing for large phase matching bandwidth and has been demonstrated at intensities as high as 25 GW/cm2. In this work we evaluate numerically the individual and combined use of two nonlinear crystals, KTA and MgO:LN to conceptually propose a 3 μm, multi-mJ-level (5 mJ), 10 Hz, sub-50 fs OPCPA laser pumped by a 1.03 μm, > 60 mJ, 10 Hz, ps-level Yb-based CPA and discuss its performance supported by comprehensive simulation data.

Allegra laser system is one of the main laser sources of the ELI-Beamlines facility in the Czech Republic. The system is designed to operate at 1 kHz and >100 mJ output with the pulse duration of <15 fs at 820 nm central wavelength. The main role of the laser system includes driving plasma X-ray and high harmonic (HHG) secondary sources at ELI-Beamlines facility. In this submission we present the most recent results on the optimization of high-energy OPCPA amplification stages and discuss the operational performance of the laser system.

The L1 Allegra is an OPCPA-based, high average power, high repetition rate laser system pumped by thin-disc based regenerative amplifiers currently under development at the ELI-Beamlines center in Czech Republic. The repetition rate is 1 kHz, pulse duration is below 15 fs and the wavelength centered around 820 nm with a maximum design pulse energy of 100 mJ. To avoid problems with self-focusing, a large portion of the system was placed inside vacuum, including the compressors and second-harmonic crystals for the last three 1030 nm pump lasers, the final three OPA stages, and the chirped mirror compressor. In order to reach the designed output energy of the whole system, the pump lasers need to be efficient, stable, and providing enough pump power for each of the amplification stages. Pulse compression of the final three pump lasers as well as efficient conversion to the second harmonic frequency in vacuum has posed several challenges and we report on their solutions and results. The vacuum environment causes difficulties for two main reasons. The first one is laser-induced-contamination (LIC) degrading the optical surfaces of dielectric gratings, mirrors and crystals, due to the presence of degassing components contaminating the vacuum chambers. The second reason is second-harmonic generation crystal mounts heating up, requiring regular phase matching corrections by rotation of the crystal mounts. The LIC problem was solved by regular cleaning of the chambers by means of an RF-plasma source, and the heating problem was solved by implementing active temperature stabilization by means of installing thermo-electric coolers on the crystal holders. To increase the efficiency of the second-harmonic generation, beam profiles of the pump lasers had to be improved. The original Faraday rotators, present in the linearly-designed regenerative amplifiers, caused non-Gaussian beam profiles due to the self focusing inside the rotators. By using KTF crystals inside a new type of rotators, the spatial profile of the pump lasers is more Gaussian, allowing the efficiency of the SHG to be higher, almost by 25%. All the solved problems recently allowed the system to reach a short pulse output energy of 56 mJ, paving a way to reach 100 mJ successfuly in the future.

We present a novel approach regarding the design of OPAs in the NIR, where all the pulse chirp is controlled using bulk materials. This has a potential to increase the robustness and simplicity of such system.

We present our recent results obtained with single-crystal Fe:CdTe lasers. In particular, different pumping schemes and operation temperatures are investigated. Our high-quality Fe:CdTe single-crystal has been grown using the physical vaportransport technology with simultaneous Fe²⁺-ions doping. When pumped by 40-ns pulses from a Q-switched Er:YAG laser at 2.94 μm, the Fe:CdTe laser produces up to 2 mJ of output energy at room temperature (RT), with a slope efficiency of 16 %. A spectral tunability in the range of 5.1–6.3 μm is achieved. The lifetime of the upper laser level is measured to be 530 ns at RT. When using a 4.1-μm Fe:ZnSe laser (40-ns pulses) as a pump source instead of Er:YAG, the efficiency of the Fe:CdTe laser is increased to 30 % and we obtain an output energy of 5.8 mJ at RT. Furthermore, a record 2300-nm smooth and continuous wavelength tunability over 4.5–6.8 μm is achieved. With the latter system we demonstrate the first application of the Fe:CdTe laser to intracavity absorption spectroscopy. The experimental spectrum of atmospheric H₂O shows good agreement with a calculated spectrum from the HITRAN database. The obtained sensitivity corresponds to an effective absorption path length L_eff = 10 m, being limited only by the laser-pulse duration. However, by applying longer pump-pulses, the sensitivity can be enhanced by up to seven orders of magnitude, thus enabling extremely sensitive spectroscopic measurements. Furthermore, an energy up-scaling of the laser output is demonstrated at 77 K. The increased upper-state lifetime of 68 μs facilitates pumping by 250-μs pulses of a 4.1-μm Fe:ZnSe laser, and our Fe:CdTe laser produces a record-output of 0.35 J at 77K, with a slope efficiency of 44 %.

We report on the conceptual design of a 2 μm, ultrashort pulse laser system including power amplification based [1] on Tm-doped gain medium. We consider ceramic [2] Tm:Lu₂O₃ with a direct diode-pumping architectures to develop a solid-state laser technology for high-efficiency, high repetition rate and ultra-short pulse laser, with high peak power and kWscale average power.. A CW pumping (duty cycle of the diodes 100%) and a multi-pulse extraction mode [3] scheme are considered with a multipass scheme with thin disk Tm:Lu₂O₃ doped at 4% and with lateral (edge) [5] pumping (EPDL). Here we show the preliminary results of the modelling of a two-stage amplifier operating at a repetition rate of 1 kHz, output energy of <500mJ, with a maximum fluence of about 1.5 J/cm2 for the first stage and of about 2 J/cm2 for the second stage. The optical model of the two-stages is based on a multi-pass configuration and designed for a gain of 50x and 12x respectively. Each stage consists of two gain media in active mirror configuration. This conceptual design represents the best balance between constructive complexity and efficiency in terms of energy gain.

This paper represents a number of fundamental problems related to resonant phenomena in QED processes in external strong electromagnetic fields. In particular, a systematic theoretical study of resonant QED processes in strong laser fields up to 10²⁶ W/cm² is planned, such as: the resonant laser-assisted spontaneous bremsstrahlung of ultrarelativistic electrons in the nucleus field, the resonant laser-assisted photoproduction of ultrarelativistic electron-positron pairs in the nucleus field, the resonant laser-stimulated generation of ultrarelativistic electron-positron pairs in the collision of high-energy electrons with a strong laser wave, the resonant laser-assisted Breit-Wheeler process, the resonant laser-assisted annihilation and production process of an ultrarelativistic electron-positron pair. The resonant kinematics of these processes in a strong laser field is studied in detail. A significant dependence of the energy of the resulting ultrarelativistic electron-positron pairs on their outgoing angles is shown. The presence of a threshold for the minimum number of photons of the laser wave is shown. This number of photons depends significantly on the energy of the initial particles and the laser intensity. Resonant differential cross-sections of these QED processes are obtained. It is shown that the resonant differential cross-section can exceed the corresponding non-resonant ones very significantly (by several orders of magnitude). This theoretical research predicts a number of new physical effects and can be tested in international research project ELI (Extreme Light Infrastructure, Czech Republic).

Combination of revolutionary high-repetition-rate high-power laser technology and the latest achievements in laser-plasma accelerators (LPAs) opens the way to develop a new generation of laser-driven compact free-electron lasers (FEL). The LPA electron beam brightness is extremely high due to a low normalized emittance (less than 0.5 µm) and high peak current (multi kA), resulting from an ultra-short e-beam duration (few fsec), which makes such beam extremely attractive for the production of high power pulses of the coherent photon radiation. In the frame of this report, we overview the laser-driven FEL research program at ELI-Beamlines (near Prague, Czech Republic), which aims to demonstrate the self-amplified spontaneous emission (SASE) regime with a generation of the coherent photon radiation starting from the extreme ultraviolet wavelength range. A possible extension of this research program to the X-ray FEL regime will be discussed.

Recently, we published a novel design approach for unstable laser cavities that overcomes the limitation of such a design to high gain media and presented first results of a laboratory prototype [1]. Based on this, we developed a larger prototype system suitable for applications in laser shock peening featuring a relatively simple layout and a compact footprint of about 0.8x0.6m². The presented laser is based on cryogenically cooled Yb:YAG ceramic equipped with a Cr:YAG absorber cladding to suppress amplified spontaneous emission. As the pump source we utilize a 7kW peak power laser diode module generating a high quality tophat beam with hexagonal lateral profile. With this the system achieves an output energy in excess of 1J in a 10ns to 20ns pulse in q-switch operation mode. The output beam profile adapts well to the hexagonal shape of the pump beam. The laser can be operated with up to 20Hz repetition rate limited only by the cooling capacity of the used cryostat. The application of the system in laser shock peening (LSP) was tested by treating aluminum plates within a test setup. The treated area was then examined regarding residual stress to verify the success of the LSP process. It was found that significant residual stress was generated with a penetration depth of up to 0.6mm and a pressure of 200MPa at the surface. [1] J. Körner, S. Zulić, D. Rostohar, A. Lucianetti, T. Mocek, “Novel unstable resonator configuration for highly efficient cryogenically cooled Yb: YAG Q-switched laser”, Optics Express, 27(15): 21622-21634, 2019

ISP SYSTEM designs and manufactures deformable mirrors dedicated to intense lasers for 15 years. The design patented by ISP uses electro-mechanical actuators and allows high order optical correction with a very high stability even unpowered. It can be adapted to any laser beam size and shape and membrane can be easily replaced. Laser peak-power increases continuously thanks to higher energy and shorter pulse duration, repetition rates are more and more important, leading to new technical challenges: high damage threshold for coatings, increased beam size, higher average power absorbed by the optics. ISP SYSTEM has improved continuously his deformable mirror in order to fulfill these requirements. Aperture has been increased using larger thin substrates and larger coating chambers for these substrates but keeping an acceptable actuator quantity. Optical test bench for calibration has also been upgraded for beams up to Ø500mm. Outgassing rates and cleanliness have been improved thanks to new materials, new lubricants and new cleaning processes coming from space and semiconductor industries. Outgassing of complete deformable mirror can be measured thanks to Rest Gas Analysis. Actuators have been improved in term of resolution, foot print and actuation speed. Actuators with very small diameter have been designed in order to increase the actuator density and improve the correction of high order Zernike. An active cooled deformable mirror is under development for applications with high average power of the laser beams up to 30 kW. This active cooled version is based on a technology similar to the standard deformable mirrors designed by ISP System and thus takes advantage of the electromechanical actuators benefits. It is adapted to CW and high repetition rate lasers and it is designed to able to dissipate continuously a thermal power of 300W. Optical correction quality is similar to the standard deformable mirror. Outgassing rates and cleanliness have been improved thanks to new materials, new lubricants and new cleaning processes coming from space and semiconductor industries. Outgassing of complete deformable mirror can be measured thanks to Rest Gas Analysis. Actuators have been improved in term of resolution, foot print and actuation speed. Actuators with very small diameter have been designed in order to increase the actuator density and improve the correction of high order Zernike. An active cooled deformable mirror is under development for applications with high average power of the laser beams up to 30 kW. This active cooled version is based on a technology similar to the standard deformable mirrors designed by ISP System and thus takes advantage of the electromechanical actuators benefits. It is adapted to CW and high repetition rate lasers and it is designed to able to dissipate continuously a thermal power of 300W. Optical correction quality is similar to the standard deformable mirror.

The demand for high-power vertical cavity surface emitting laser (VCSEL) with multiple-emitters is dramatically growing for 3D sensing application in consumable and automotive electronics. Oxide confinement technology is favorable for 2D VCSEL arrays with the advantage of small threshold current, low manufacture cost and high power conversion efficiency. Since both current and optical light is confined by the insulating oxide layer, oxide aperture delineation will be useful for VCSEL array characterization and failure analysis. In this work, electrical and physical failure analysis techniques are utilized for oxide aperture delineation for both good and dim emitters in 2D VCSEL array chip, which has been subjected to overstress test. Three different kinds of techniques, that is, near-field testing (NFT), infrared (IR) microcopy and planar-view transmission electron microscopy (PVTEM) are used to investigate the entire oxide aperture confined region, which is the essential region for VCSEL lasing. From functional view of point, NFT emission pattern below threshold current provide emission profile from each emitter, which is related with oxide aperture confinement. From physical view of point, IR and PVTEM images provide emitter structure analysis to correlate with emission pattern. For good emitter (Emitter 1), electrical near-field testing below threshold current shows circular shape, uniform emission pattern. Non-destructive IR images and destructive planar-view TEM confirm the round-shape oxide aperture. For bad emitters (Emitter 2 and Emitter 3), NFT images below threshold show the dark spot at the emission pattern. IR images is only able to provide oxide aperture shape and reveal oxidation defect. PVTEM is able to capture both dark-line defect and further oxidation failure, which correlate with the dark spot site in NFT emission pattern. This work demonstrates the effectiveness of the three techniques for oxide aperture delineation and provide good correlation between NFT emission profile and PVTEM aperture morphology. It also can link weak emission location with physical defect site in the dim emitter. These combined techniques can work together for oxide aperture delineation and fault isolation in 2D VCSEL arrays.

The issue of laser-induced damage on critical components emerges as a bottleneck that limits the high-power or high-energy laser systems, especially for the fused silica optics used in ultraviolet light. Sub-surface defects such as microcracks and impurities on fused silica optics have been discovered as damage precursors and determine the laser-induced damage threshold (LIDT) of the optics. Under the state-of-the-art advanced mitigation processes (AMP) and laser conditioning, only a few destructive damage sites that grow rapidly with successive shots still exist on a large-aperture fused silica optic. Therefore, we propose a method of selectively eliminating the destructive damage sites on fused silica optics by laser micromachining and consequently lead to a significant enhancement of LIDT in this paper. The removal of a damage site is implemented by precisely shaping the destructive damage site into an optically benign cone of special design using a femtosecond laser, with a subsequent CO₂-laser-polishing process to reduce the roughness. Compared with previous methods, the thermal effect on the processed region is dramatically reduced because of the nonthermal ablation by a femtosecond laser. Through optimizing the parameters of laser micromachining, a typical damage site is eliminated and replaced with a designed cone of excellent quality. The manufactured cone typically has a smooth wall with a slope angle of 12°, a diameter of 800 μm, and a negligible raised rim with a height of 14.5 nm (∼ λ/25 @ 355 nm). By employing the raster scan LIDT test procedure, several fused silica optics processed by laser micromachining are investigated and a laser-induced damage threshold (@ 355 nm, 1.6 ns) higher than 14 J/cm² and 10 J/cm² on the input surface and output surface are discovered, respectively. Furthermore, the downstream light intensification is proven to be trivial in the absence of a detrimental high-intensity central spot, owing to the ultra-low raised rim. These results demonstrate that rapid laser micromachining is an effective way to improve laser-induced damage resistance of fused silica optics and eventually enhance the performance of high-power or high-energy laser systems.

Three different fiber-coupled laser diode system of size 500 μm are designed and simulated using Zemax optical design software. The performance of each designed system is compared in terms of output power and coupling efficiency. At the wavelength of 808 nm, the output of a 10-watt single emitter laser diode is coupled into the multimode fiber. Since the laser diode beam is highly divergent, in order to reduce the divergence angle and collimate the beam, the ball, cylindrical, and toroidal lenses are used as a collimator. The focused beams are coupled into a 200 μm core diameter multimode fiber, which has a numerical aperture of 0.22. Beam parameter product is calculated as the product of half waist width and half far-field divergence angle to evaluate the beam quality. The ray simulation results revealed that the output power with the ball lens is 9 watts, corresponding to a total coupling efficiency of 90%. Similarly, the calculated output power is 9.2 watt from the cylindrical lens, resulting to a total coupling efficiency of 92%. The calculated output power with the toroidal lens is greater than 9.5 watts, corresponding to a total coupling efficiency of more than 95%. The comparative analysis of newly designed system shows that with a toroidal profile, fiber-coupled laser diode attained the highest coupling efficiency with minimum aberrations.

A compact Q-switched laser head emitting radiation at wavelength 1318 nm was designed and constructed. This laser was based on a separate Nd:YAG gain part and V:YAG saturable absorber. The dielectric mirrors of the laser resonator were directly deposited on the Nd:YAG crystal (pumping mirror highly reflecting at 1318 nm radiation; highly transparent for 808 nm and 1064 nm) and on the V:YAG crystal (output coupler with reflection 90 % at 1318 nm; highly transparent at 1064 nm). To decrease the influence of thermal effects and to reduce the residual unabsorbed pumping radiation, a 16 mm long low doped (0.85 at.% Nd/Y) Nd:YAG crystal was used. Similarly, the saturable absorber was based on V:YAG with low absorption coefficient (∼ 0.2 cm⁻¹), corresponding to an 8 mm long V:YAG crystal (initial transmission 85% at 1.3 μm). For longitudinal Nd:YAG pumping, a fibre coupled (core diameter 400 µm; NA = 0.22; wavelength 808 nm, P_max = 16W) laser diode was used. The pumping diode was operated in a pulsed regime with a repetition rate from 10 to 1000 Hz. The pumping pulse length was set to generate just one generated Q-switched pulse (420 μs for 10 Hz, 220 μs for 1 kHz). In the whole range of tested frequencies, the laser generated stable ∼ 15 ns long (FWHM) pulses with energy up to 160 µJ and peak power ∼ 10 kW. The output beam with TEM₀₀ cross-section was linearly polarized (degree of polarization < 99%). The relative deviation of pulse duration, pulse energy, and pulse peak power at a fixed pumping repetition rate was lower than 1 %. The change of these parameters for the maximum change of pumping repetition rate was not greater than 20%.

Two different mid-infrared (mid-IR) crystalline laser active media of Zn_1-xMn_xSe and Zn_1-xMg_xSe with similar manganese or magnesium ions amount of x ≈ 0.3 co-doped by divalent chromium (Cr²⁺) and iron (Fe²⁺) ions were investigated at cryogenic temperatures and compared for different excitation wavelengths used. Both single crystals were synthesized by high pressure Bridgman method and their thickness were 2.6 mm and 5 mm, respectively. Crystals were investigated under three excitation wavelengths of the Q-switched Er³⁺:YLF laser at ~1.73 μm, Q-switched Er³⁺:YAG laser at ~2.94 μm, and the gain-switched Fe²⁺:ZnSe laser operated at liquid nitrogen temperature of 78 K at a wavelength of ~4.05 μm. Spectroscopic and laser output characteristics were measured: absorption and fluorescence spectra, laser output pulse duration, mean output energy, and laser oscillation spectra. Both laser systems were able to generate radiation by Cr²⁺ or by Fe²⁺ ions under direct excitation, or by Fe²⁺ ions via the Cr²⁺ → Fe²⁺ energy transfer depending on the excitation wavelength and the output coupler conditions. Fe²⁺ ions in Cr²⁺,Fe²⁺:Zn_1-xMn_xSe and Cr²⁺,Fe²⁺:Zn_1-xMg_xSe (x ≈ 0.3) laser systems at 78 K pumped by Er³⁺:YLF laser radiation at ~1.73 μm via the energy transfer mechanism generated laser radiation at the wavelengths of ~4.4 μm and ~4.8 μm at 78 K, respectively. Obtained results have shown a possibility of developing novel coherent laser systems in mid-IR region (~2.3 – 2.5 μm and ~4.4 – 4.9 μm) based on AIIBVI matrices. Thus, possibility to excite the Fe²⁺ active ions in both samples directly by ~2.94 μm as well as ~4.05 μm radiation or eventually in a compact way through the Cr²⁺ → Fe²⁺ ions energy transfer-based mechanism by ~1.73 μm radiation was demonstrated.

An increasing intensity of the laser systems becomes available for experiments in the fields of particle acceleration, radiation sources and many other applications. The higher intensity of the laser irradiation is inherently accompanied by the growth of the amplified spontaneous emission (ASE) pedestal and other parasitic effects. Considering the forthcoming generation of multi-PW laser systems, the nanosecond and picosecond pedestals significantly exceed the respective plasma formation thresholds and may have detrimental effects on the laser--target interaction. One of the promising methods to mitigate these effects is the relativistic plasma shutter, where an ultra-thin foil is placed in front of the target. The initial interaction with the shutter increases the temporal contrast, where the pre-plasma is opaque for the pedestal, but relativistically transparent for the main pulse. Moreover, the created pre-plasma exhibits a focussing effect, increasing the effective intensity of the main pulse. Two-dimensional hydrodynamic simulations of the pedestal are performed, followed by PIC simulations of the main pulse. The parameters of the shutter, like density and thickness, are varied to optimize performance of the configuration.

The smaller lattice constant and high band gap quaternary – capped Al_xIn_yGa_1−x−yAs material acts as a confinement – enhancing (CE) barrier layer in type – I InAs/In_xGa_1−xAs sub-monolayer (SML) quantum dot (QD) optoelectronic devices. The QDs grown inside In_0.15Ga_0.85As matrix surrounded by Al_xGa_yIn_1−x−yAs/GaAs barriers, resembles of dot –in – a – well (DWELL) heterostructure to achieve higher device performance. A larger in – plane compressive strain gets induced due to smaller lattice constant of CE barrier layer and this effect can be probed for designing higher modal gain and low – threshold current density vertical cavity surface – emitting lasers (VCSELs). The decreasing In content in CE layers increases the compressive strain in QDs, leading to reduced threshold current by decreasing (increasing) the background carrier concentration (conduction band, CB offset). The increased CB offset with Al (0.21), In (0.21) contents will therefore prevent the carriers from escaping to quantum well (QW) or barrier layers, thus non – radiative leakage current effects can be reduced. We have analyzed the primary strain tensors: hydrostatic and biaxial strain, photoluminescence (PL) emission energies using Nextnano++ simulations based on 8 – band k.p theory. Both the hydrostatic and biaxial strain increases inside QD, capping layers for increasing In (Al) composition, facilitating the threshold current reduction. Through the strain analysis, the material gain to carrier characteristics can be figured out for the efficient design of VCSELs. The QD low (19 K) and room –temperature (300 K) PL eigen energies were found at 1.1805 (1.1759) eV and 1.111 (1.177) eV for Al_0.2Ga_0.8As/GaAs and Al_0.21Ga_0.58In_0.21As/GaAs CE layers.