Power Laser Systems Research Articles

Laser-induced damage threshold study on TiO2/SiO2 multilayer reflective coatings

Laser-induced damage threshold (LIDT) is a key parameter in high power laser systems. Highly reflective mirrors are made by the combination of high index and low index dielectric thin films of materials, usually oxides, having high damage threshold. The aim of the present investigation was to study the effect of multilayers on LIDT for a combination of high and low index material films with the increase in the number of layers. Firstly, we chose a combination of relatively high damage threshold high index (H) and low index (L) oxide materials, like TiO2 and SiO2. Then, we chose five reflective samples with increasing the number of layers starting with a TiO2 single quarter wave optical thick (QWOT) layer, three-QWOT layer (HL)1H, five-QWOT layer (HL)2H, seven-all QWOT layer (HL)3H and seven-layer (HL)2H 1.6L0.4H with upper two non-quarter layers for sample preparation using electron beam deposition. It has been found that LIDT measured at 1064 nm for single layer is large (2.09 J/cm2), decreases for three layers and remains nearly constant (1.51 J/cm2) as the number of multilayers increases further. When LIDT is measured at 532 nm, LIDT of the single layer and multilayers remains almost the same. However, in case of top two layers made of non-QWOT in seven-layer design the LIDT of the samples in both the cases improved.

Nm-sized cryogenic hydrogen clusters for a laser-driven proton source.

A cryogenic hydrogen cluster-jet target is described which has been used for laser-plasma interaction studies. Major advantages of the cluster-jet are, on the one hand, the compatibility to pulsed high repetition lasers as the target is operated continuously and, on the other hand, the absence of target debris. The cluster-jet target was characterized using the Mie-scattering technique allowing to determine the cluster size and to compare the measurements with an empirical formula. In addition, an estimation of the cluster beam density was performed. The system was implemented at the high power laser system ARCTURUS, and the measurements show the acceleration of protons after irradiation of the cluster target by high intensity laser pulses with a repetition rate of 5 Hz.

The influence of inhomogeneity of the optics material on the laser nearfield

The inhomogeneity of the refractive index distribution in optics material degrades the output laser nearfield beam quality. The modulation effect of the nearfield influenced by the refractive index distribution in the crystal material is studied theoretically in this work. The internal refractive index distribution of the transmissive optics is analyzed by measuring the transmitted wavefront and the output laser nearfield after passing through the defective crystal is measured in a high power laser system. The experimental results show that due to the diffraction effect during laser transmission, the internal refractive index defect of the optics material causes the intensity modulation in the nearfield of the output laser.

Stress compensated anti-reflection coating for high power laser deposited with IBS SiO2 and ALD Al2O3

Both low stress and high laser induced damage threshold are required in the development of high power large laser facilities. IBS and ALD films have potential in such applications in aspects of film thickness control and high LIDT, but their severe stresses cause an obstacle towards the widely application. The origin of compressive stress of IBS coating and tensile stress of ALD coating is analyzed with a model based on intermolecular potential. An anti-reflection coating having compensated stress is designed and fabricated with ALD Al2O3 and IBS SiO2. The LIDT of the anti-reflection coating as well as the single layers are tested at wavelength 532 nm. A transmittance of 96.5% at single surface and a stress as low as 38 MPa are achieved. The LIDT of the anti-reflection coating is limited by the additional defects introduced at the film interfaces. This design takes advantage of the power resistance of IBS/ALD coatings and their contrary stresses, providing a concept towards the application in high power laser systems.

Laser-induced damage threshold in HfO2/SiO2) multilayer films irradiated by β-ray**Project supported by the National Natural Science Foundation of China (Grant No. 11405085) and the Jiangsu Provincial Natural Science Fund, China (Grant No. BK20130789).

Post-processing can effectively improve the resistance to laser damage in multilayer films used in a high power laser system. In this work, HfO2/SiO2 multilayer films are prepared by e-beam evaporation and then β-ray irradiation is employed as the post-processing method. The particle irradiation affects the laser induced damage threshold (LIDT), which includes defects, surface roughness, packing density and residual stress. The residual stress that is relaxed during irradiation changes from compressive stress into tensile stress. Our results indicate that appropriate tensile stress can improve LIDT remarkably. In view of the fact that LIDT rises from 8 J/cm2 to 12 J/cm2, i.e., 50% increase, after the film has been irradiated by 2.2 × 1013/cm2 β-ray, the particle irradiation can be used as a controllable and desirable post-processing method to improve the resistance to laser induced damage.

Enhancement of magneto-optical performance of Tb0.94Pr0.06VO4 single crystals by Pr doping

Tb0.94Pr0.06VO4 (TPV) magneto-optical crystal with sizes of 27 × 35 mm3 was grown by the Czochralski (Cz) method. Its structure was verified by Powder X-ray diffraction, demonstrating that TPV belongs to tetragonal system. The optical and magneto-optical properties were systematically investigated. TPV exhibits high transparency in the near-infrared region. The Verdet constants of TPV at 633 and 1064 nm are about 60% larger than that of TGG. The thermal conductivity and laser-induced damage threshold of TPV were also determined for evaluating its practical usage. All the results indicate that TPV is a promising Faraday rotation material for magneto-optical applications in high power laser systems.

Measurement of mid-frequency wavefront error for large optical components with ptychography.

In a high power laser system, the wavefront quality of large optical components in the mid spatial frequency band plays a critical role in system performance and safe operation. A simple and efficient method based on ptychography is used to measure mid-frequency wavefront error, which has the advantages of simple structure, strong anti-interference ability, flexible frequency-range selection, and low cost. It has excellent frequency response in the entire mid-frequency region. The transfer function is demonstrated to be greater than 0.7 at 1/2 Nyquist frequencies (8.33 mm-1) for 60mm field-of-view experimentally. The method has been successfully applied to the wedged focused lens (WFL) to achieve high-fidelity measurement. Without any auxiliary lens, the power spectrum of the WFL at the frequency of interest is obtained through large-aperture measurement. This technique is especially suitable for optical components difficult to be measured using interferometers and opens up a new perspective for measuring mid-frequency wavefronts.

Free space variable optical attenuator using frustrated total internal reflection with 70 dB dynamic range.

With the advance of high power laser systems, there is an increasing need to dim the corresponding light field power over larger dynamic ranges. The usual means to control the attenuation of directed light use optical filters or coatings with tailored reflectance and transmittance properties. They do not provide the variability that is often required in experiments. A system is reported here that demonstrated continuous variation of the transmittance over 70dB, using frustrated total internal reflection.

Full penetration hybrid laser arc welding of up to 28 mm thick S355 plates using electromagnetic weld pool support

The laser hybrid welding process offers many advantages regarding deep penetration, increased welding velocity and with the help of the supplied filler wire an improved bridgeability to gap and misalignment tolerances. High power laser systems with a power of approx. 30 kW are already available on the market. Nevertheless, multi-layer technology with an arc process is still used for welding of plates from a thickness from 20 mm. A potential cause is the process instability with increasing laser power. It is inevitable that gravity drop-out due to the high hydrostatic pressure at increasing wall thickness especially at welding in flat position and with a low welding speed. The surface tension decreases with increasing root width resulting from low welding velocities. To prevent such inadmissible defects of the seam a use of weld pool support is required. Usual weld pool support systems such as ceramic or powder supports require a mechanical detachment which is time-consuming. The electromagnetic weld pool support system described in this work shows an alternative weld pool support which works contactless. It is based on generating Lorentz forces in the weld pool due to oscillating magnetic field and induced eddy currents. This innovative technology offers single pass welds up to 28 mm in flat position and reduced welding velocity with a laser power of just 19 kW. It also leads to improved mechanical-technological properties of the seams because of the slow cooling rate. With usage of an electromagnetic weld pool support the limitation of the hybrid laser arc welding process in the thick sheet metal will be extend.

Surface error modeling, evaluation and optimization of large optics in inertial confinement fusion laser system

Surface errors of mounted large optics have significant impacts on laser performance in high power laser system. However, in SG-III inertial confinement fusion (ICF) laser system, it is really tough to realize the minimized surface error and satisfy the design criterions. In this paper, the compositions and corresponding causes of complicated surface topographies are analyzed systematically. Surface decoupling, reconstruction and fitting methods are proposed successively and constitute an integrated surface modeling methodology to visualize and evaluate both offline and online mirror surfaces. Further, we present a closed-loop “mounting – inspection – optimization” assembly process based on the combination of interferometric measuring, finite element simulation and digital reconstruction, which will lead the mounting induced surface error of the large optics to meet the wavefront criterions. Through a case study of the large aperture laser transport mirror, the proposed approach is proved to have good performance on both improving optical mounting performance and obtaining accurate surface features.

Simulational and experimental investigation on the actuator-corresponding high-frequency aberration of the piezoelectric stacked array deformable mirror

The high-frequency aberrations (HIFAs) that are corresponding to the actuator array have been reported to exist on the initial surface shape of many deformable mirrors (DMs), such as the bimorph DM, the unimorph DM, the monomorph DM, and the membrane DM. This actuator-corresponding high-frequency aberration (AC-HIFA) could not be corrected effectively by the DM and would limit the correction ability of the DM. In this paper, we presented the AC-HIFA in a stacked array piezoelectric (PZT) DM, which may result in ghost damage that is dangerous in the high power laser system. More importantly, we investigated a solution through simulation and experiment that by using a mirror plate and a long thin post array, which were machined integrally from a piece of BK7 glass, the AC-HIFA could be eliminated completely. In addition, the structural parameters' influences on the AC-HIFA were investigated in the simulation, which could help other researchers to determine appropriate parameters of the mirror and the posts and to make a fine surface shape in their own DMs.

Role of \u2018dusting and pop-dusting\u2019 using a high-powered (100\xa0W) laser machine in the treatment of large stones (\u2265\u200915\xa0mm): prospective outcomes over 16\xa0months

Ureteroscopy and laser stone fragmentation (URSL) has had recent advancements with the more powerful laser systems with the ability to ‘dust’ and ‘pop-dust’ the stone. We wanted to look at the outcomes of this method for large stones (≥ 15 mm) using our new 100 W holmium laser. Over a period of 16 months (January 2017–April 2018), 50 patients underwent URSL for minimum cumulative stone size of ≥ 15 mm. Data were collected prospectively on patient and stone demographics and outcomes of URSL. The laser setting used was a power of 0.3–0.6 J and a frequency of 20–50 Hz using a long-pulse setting with a 272-µm fiber. Fifty patients underwent 55 URSL procedures (5 bilateral procedures) using dusting and pop-dusting settings. The mean age was 58 years (range 2–88 years) with a male:female ratio of 35:15. The mean single and overall stone size were 10.3 mm (3–23 mm) and 21 mm (range 15–52 mm) with two-thirds of all patients (65%) having multiple stones. The stone location was in the kidney (n = 65, 78%), in the ureter (n = 19, 22%) and 5 patients had bilateral renal stones. With a mean operating time of 51 min, the initial and final SFR were 93 and 98%, respectively. A pre-operative stent, access sheath and a post-operative stent were present in 29 (53%), 34 (62%) and 51 (93%) procedures, respectively. Over a mean hospital stay of 0.6 days (74% day-case procedures), there was one Clavien IV complication related to urosepsis but without any other major or minor complications. Dusting and pop-dusting techniques achieve an excellent SFR with low risk of complications even for large stones. This might set a new benchmark for treating large stones, bilateral or multiple stones in a single setting, without the need for secondary procedures in most cases.

6th Target Fabrication Workshop (TFW6) and the Targetry for High Repetition Rate Laser-Driven Sources (Targ3) Conference

Conference Proceedings, Editor’s Preface2017 was an exciting year for the Target Fabrication community with both the 6th Target Fabrication Workshop (TFW) held in Greenwich London, UK and the 3rd Targetry for High Repetition Rate Laser Driven Sources (Targ) workshop held Salamanca, Spain taking place.These well-established workshops, with their focus on different but complimentary areas of Target Fabrication, cover the main areas of research providing novel and high specification samples for the next generation of high power laser systems. The TFW series, has developed to cover high repetition rate solid targets, cryogenic targets and microfluidics and had exciting talks from many PhD students from around the world. The TargN workshop series brings together experts and key players involved in the latest trends on intense laser-plasma research, requiring the design of specific targets aimed at particular requirements to exploit unique opportunities presented by such extreme peak laser powers.

Development of a multi-shot experiment for proton acceleration

In this paper, we present a target system to produce protons from laser plasma interactions at high repetition rates. In order to increase the number of available shots needed for the foreseen application, the target system will help to improve the reproducibility and stability of the proton source. We describe the experiments that we will carry out with the 50 TW (1.2J, 25 fs, 10Hz) laser at the Laser Laboratory for Acceleration and Applications (L2A2) at the University of Santiago of Compostela to study the production of radioactive isotopes for medical imaging such as PET. We describe the multi-shot target that we have developed and the related diagnostic to characterize the focal spot and the relative distance between the target and the focus. The multi-shot target will permit the use of thin foils or nano-structured targets with repetition rates relevant to our high power laser system.

A New Candidate Laser Dye Based 1,4-Bis[β-(2-Naphthothiazolyle)Vinyl]Benzene. Spectroscopic Behavior, Laser Parameters and Excitation Energy Transfer.

A new candidate laser dye based 1,4-bis[β-(2-naphthothisolyl) vinyl] benzene (BNTVB) were prepared, and characterized in various organic solvents. The center polarity is less sensitive than electronic absorption. A red shift was noticed in the fluorescence spectra (ca. 40nm) with increment in the solvent's polarity, this means that BNTVB's polarity appreciates upon excitation. The dipole moment of ground state (μg) and the excited singlet state dipole moment (μe) are determined from Kawski - Chamma and Bakshiev-Viallet equations using the disparity of Stokes shift with solvent polarity function of ε (dielectric constant) and n (refractive index) of the solvent. The result was found to be 0.019D and 5.13D for ground and exited state, in succession. DFT/TD-DFT manners were used to understand the electronic structures and geometric of BNTVB in other solvents. The experimental and theoretical results showed a good agreement. The photochemical quantum yield (Фc) of BNTVB was calculated in variable organic reagents such as Dioxane, CHCl3, EtOH and MeOH at room temperature. The values of φc were calculated as 2.3 × 10-4, 3.3 × 10-3, 9.7 × 10-5 and 6.2 × 10-5 in Dioxane, CHCl3, EtOH and MeOH, respectively. The dye solutions (2 × 10-4M) in DMF, MeOH and EtOH give laser emission in the blue-green region. The green zone is excited by nitrogen pulse 337.1nm. The tuning range, gain coefficient (α) and cross - section emission (σe) of laser were also estimated. Excitation energy transfer from BNTVB to rhodamine-6G (R6G) and N,N-bis(2,6-dimethyphenyl)-3,4:9,10-perylenebis-(dicarboximide) (BDP) was also studied in EtOH to increase the laser emission output from R6G and BDP when excited by nitrogen laser. The dye-transfer power laser system (ETDL) obeys the Foster Power Transmission (FERT) mechanism with a critical transmission distance, Ro of 40 and 32 Ả and kET equals 2.6 × 1013 and 1.06 × 1013M-1s-1 for BNTVB / R6G and BNTVB / BDP pair, respectively.

Highly efficient, cascaded extraction optical parametric amplifier.

The scheme of cascaded extraction optical parametric amplifier (CE-OPA) has been proposed as a final amplifier for high peak power laser systems. 4D numerical simulations show that conversion efficiency of a CE-OPA system pumped with a temporal Gaussian pump pulse is as close to the theoretical limit of quantum efficiency as a conventional OPA pumped with temporal flat-top pump pulse. The CE-OPA system is also similar to the conventional scheme in output energy stability and alignment sensitivity of the phase-matching angles, too. However, with the use of the CE-OPA scheme, the requirement of pump pulse shaping can be relaxed, leading to an overall higher plug in efficiency as well as compact design.

Laser Induced Damage of Potassium Dihydrogen Phosphate (KDP) Optical Crystal Machined by Water Dissolution Ultra-Precision Polishing Method.

Laser induced damage threshold (LIDT) is an important optical indicator for nonlinear Potassium Dihydrogen Phosphate (KDP) crystal used in high power laser systems. In this study, KDP optical crystals are initially machined with single point diamond turning (SPDT), followed by water dissolution ultra-precision polishing (WDUP) and then tested with 355 nm nanosecond pulsed-lasers. Power spectral density (PSD) analysis shows that WDUP process eliminates the laser-detrimental spatial frequencies band of micro-waviness on SPDT machined surface and consequently decreases its modulation effect on the laser beams. The laser test results show that LIDT of WDUP machined crystal improves and its stability has a significant increase by 72.1% compared with that of SPDT. Moreover, a subsequent ultrasonic assisted solvent cleaning process is suggested to have a positive effect on the laser performance of machined KDP crystal. Damage crater investigation indicates that the damage morphologies exhibit highly thermal explosion features of melted cores and brittle fractures of periphery material, which can be described with the classic thermal explosion model. The comparison result demonstrates that damage mechanisms for SPDT and WDUP machined crystal are the same and WDUP process reveals the real bulk laser resistance of KDP optical crystal by removing the micro-waviness and subsurface damage on SPDT machined surface. This improvement of WDUP method makes the LIDT more accurate and will be beneficial to the laser performance of KDP crystal.

Lifetime laser damage performance of β-Ga2O3 for high power applications

Gallium oxide (Ga2O3) is an emerging wide bandgap semiconductor with potential applications in power electronics and high power optical systems where gallium nitride and silicon carbide have already demonstrated unique advantages compared to gallium arsenide and silicon-based devices. Establishing the stability and breakdown conditions of these next-generation materials is critical to assessing their potential performance in devices subjected to large electric fields. Here, using systematic laser damage performance tests, we establish that β-Ga2O3 has the highest lifetime optical damage performance of any conductive material measured to date, above 10 J/cm2 (1.4 GW/cm2). This has direct implications for its use as an active component in high power laser systems and may give insight into its utility for high-power switching applications. Both heteroepitaxial and bulk β-Ga2O3 samples were benchmarked against a heteroepitaxial gallium nitride sample, revealing an order of magnitude higher optical lifetime damage threshold for β-Ga2O3. Photoluminescence and Raman spectroscopy results suggest that the exceptional damage performance of β-Ga2O3 is due to lower absorptive defect concentrations and reduced epitaxial stress.

Spatio-spectral structures in high harmonic generation driven by tightly focused high repetition rate lasers

We investigate the spatio-spectral properties of extreme ultraviolet (XUV) high harmonic radiation driven by high repetition rate femtosecond laser systems. In the spatio-spectral domain, ring-shaped structures at each harmonic order associated with long-trajectory electrons are found to form arrow-shaped structures at the cutoff. These structures are observed with two different laser systems: an optical parametric chirped-pulse amplifier system at a central wavelength of 1.55 μm and 125 kHz repetition rate, and a temporally compressed femtosecond ytterbium fiber amplifier at 1.03 μm wavelength and 100 kHz repetition rate. As recently pointed out, the observed structures are well explained by considering the space–time atomic dipole-induced phase for short and long electron trajectories in the generation plane. The tighter focusing geometry and longer wavelength associated with these emerging driving laser systems increase the ring-like divergence and spectral broadening for high harmonics. Cutoff energies and photon fluxes obtained in argon and neon are also reported. Overall, these results shed new light on the properties of XUV radiation driven by these recently developed high average power laser systems, paving the way to high photon-flux XUV beamlines.

A GaN-Based Synchronous Buck Converter for High Power Laser Diode Drive Applications

Laser Diodes are the essential
 components for development of high power laser systems. To operate these
 devices, highly efficient and high power density, compact, current regulated
 switching converters are necessary. In this paper, Gallium Nitride (GaN) based synchronous
 buck converters are studied for the application of laser diode driver. For this
 purpose, two synchronous buck converters one with Si-based the other one with
 GaN-based are designed, simulated, tested and compared. With the application of
 GaN-based semiconductors, 96.6% efficiency and 9.1 W/cm3 power
 density is achieved for 11A, 28V load at 700 kHz operating frequency.