Enhancing performance status of final optics assembly on high power laser at 351nm laser is experimentally studied. We experimentally demonstrate 61 shots of 310mm × 310mm laser. The maximum laser energy flux is 5.5J/cm<sup>2</sup>. The laser energy conversion efficiency is more than 72%. And the laser perforation efficiency across 800μm at 3000J is more than 96%. These results provide valuable information to improve final optics assembly performance research of high power laser.
A new high power laser facility with 8 beams and maximum output energy of one beam 5kJ/3.4ns/3ω has been performed and operated since 2015. Combined together the existing facilities have constructed a multifunction experimental platform including multi-pulse width of ns, ps and fs and active probing beam, which is an effective tool for Inertial Confinement Fusion (ICF) and High Energy Density (HED) researches. In addition another peculiar high power laser prototype pushes 1ω maximum output energy to 16kJ in 5ns and 17.5kJ in 20ns in flat-in-time pulse, this system is based on large aperture four-pass main amplifier architecture with 310mm×310mm output beam aperture. Meanwhile the near field and far field have good quality spanning large energy scope by use of a wide range of technologies, such as reasonable overall design technique, the integrated front end, cleanness class control, nonlinear laser propagation control, wave-front adaptive optics and precision measurement. Based on this excellent backup, 3ω damage research project is planning to be implemented. To realize the above aims, the beam expanding scheme in final transport spatial filter could be adopted considering tradeoff between the efficient utilization of 1ω output and 3ω damage threshold. Besides for deeply dissecting conversion process for beam characteristic influence of 1ω beam, WCI (Wave-front Code Image) instrument with refined structure would be used to measure optical field with simultaneous high precision amplitude and phase information, and what’s more WCI can measure the 1ω, 2ω and 3ω optical field in the same time at same position, so we can analyze the 3ω beam quality evolution systematically, and ultimately to improve the 3ω limited output. <p> </p>In a word, we need pay attention to some aspects contents with emphasis for future huger laser facility development. The first is to focus the new technology application. The second is to solve the matching problem between 1ω beam and the 3ω beam. The last is to build the whole effective design in order to improve efficiency and cost performance.
The high fluence performance of high-power laser systems is set by optical damage, especially in the final optics assembly (FOA). The flaws on the frequency converter surface can cause optical intensity intensification and, therefore, damage the downstream optical elements, such as the beam sampling grating (BSG), which is an important component in the FOA. Mitigation of BSG damage caused by flaws is discussed. Physical models are established to simulate the optical field enhancement on BSG modulated by the upstream flaw, considering both the linear and nonlinear propagation effects. Numerical calculations suggest that it is important to place the BSG in a properly selected position to mitigate the laser-induced damage. Furthermore, strict controls of flaw size, modulation depth, distance between frequency converter and focusing lens, and the thickness of the focusing lens are also significant to mitigate the BSG damage. The results obtained could also give some suggestions for damage mitigation of optical components and the layout design of the final optics assembly.
In high-power laser facilities for the inertial confinement fusion, there are many large-radius optical elements, which
inevitably have some flaws on the surface. The flaws can cause optical intensity intensification and therefore damage the
optical elements in the downstream, especially for the beam sampling grating (BSG), which is an important element in
the final optics assembly. In this paper, several physical models are established to study the optical field enhancement in
the BSG position modulated by upstream flaw. Firstly, when only the linear transportation is considered, it is found that
there is a peak or valley of the maximum intensity after the focus lens compared with the ideal wave front. Meanwhile
the influence of flaw has an effective range. Secondly, when the nonlinear effect of the focus lens is also considered, the
peak maximum downstream is much bigger than the one for the linear consideration and the damage risk of the BSG
there is much higher too. From the simulation, we can see that it is important to place the BSG in a properly selected
position to mitigate the laser induced damage. The results could give some references to the mitigation of BSG damage
caused by upstream flaws and the layout of the final optics assembly.
The wedged focus lens of fused silica, one of the final optics assembly’s optics, focuses the 351 nm beam onto target and separates the residual 1053 and 527 nm light with 351 nm light. After the experiment with beam energies at 3ω range from 3 to 5KJ, and pulse shapes about 3ns, the wedged focus lens has laser-induced damage at particular area. Analysis the damage result, there are three reasons to induce these damages. These reasons are beam intensity modulation, optics defect and contamination that cause different damage morphologies. The 3ω beam intensity modulation, one of three factors, is the mostly import factor to induce damage. Here, the n<sub>2</sub> nonlinear coefficient of fused silica material can lead to small-scale self-focusing filament because of optics thickness and beam intensity. And some damage-filaments’ tails are bulk damage spots because there are subsurface scratches or metal contaminations.
Laser-induced damage (LID) to optical glass has become a growing problem in high-power laser systems. It is well known that the main reason of glass being damaged is due to defects and impurities in the material. Damage caused by subsurface defects (SSDs) is especially common in actual system running. Accordingly, in the presence of SSDs, a simple and alternative calculation method is developed to evaluate the enhancement of light field around the incident and exit surface. This ray tracing approach, based on the classical optics theory, is very direct and clear to show the optical phenomena of light intensity enhancement. Some basic SSD shapes have been studied and investigated here, which reveals the importance and boundary condition of controlling the size and density of SSDs in grinding and polishing process. Finally, to achieve optimal breadth depth ratio, the least etching amounts by hydrofluoric (HF) acid is investigated. The theoretical analysis and simulation results provide an appropriate range of removal amounts, which is very important in the HF etching process.
In order to improve laser damage resistance of the Final Optics Assembly (FOA), simulation analysis have been done for
1ω, 2ω and 3ω laser beam considering ghost images to the 4th order. The panels of ground glass scatter ghost laser
around the FOA walls and the panels of architectural glass absorb the 1<sup>th</sup> order energy. The appearance of smoothing
fused silica surface defect and the effect of wiping off etching contamination are researched on HF-based etching
processes under ultrasonic. Now, 18 shots were executed using 310x310mm laser with 3ns pulse width. During the
experiment, the third harmonic laser terminal output energy is 1500J~3500J, and the maximum laser energy flux is about
4J/cm<sup>2</sup>. This presentation addresses the optical configuration of the FOA, the simulation analysis of ghost, the way of
ground glasses absorbing energy and the result of laser damage resistance of fused silica on HF-based etching processes