Zhaoyang Jiao

Mitigation of beam sampling grating damage induced by upstream flaws in the final optics assembly

Abstract. 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.

High power glass laser research progresses in NLHPLP

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. 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.

A developmental perspective on high power laser facility technology for ICF

The latest progress on high power laser facilities in NLHPLP was reported. Based on a high power laser prototype, damage behavior of 3ω optics was experimentally tested, and the key influencing factors contributed to laser-induced damage in optics were deeply analyzed. The latest experimental results of advanced precision measurement for optical quality applied in the high power laser facility were introduced. At last, based on the accumulated works of 3ω elements damage behavior status in our laboratory, beam expanding scheme was presented to increase the total maximum output 3ω energy properly and decrease the laser induced damage risking of ω optics simultaneously.

Shape dependence of downstream light intensification caused by flaws

In high power laser system, the upstream flaw could induce light intensification in the downstream, thus damaging the optical component. In most of the research, the shape of the defect model is ideal, for example, Gaussian shape. However, the defect in the real system is non-ideal with different shapes. In this paper, the light intensification effect caused by defects with different shapes are compared by numerical simulation. Results show the shape dependence of downstream light intensification caused by flaws. When only the linear effect is considered, the change of defect shape could change the maximum light intensification factor and the downstream location for the maximum intensity. When the nonlinear effect is also considered, the light intensification effect will be more sensitive to the shape of defects. This research can provide some reference for the beam quality control and defect management in the high power laser systems.

Experimental study on final optics assembly at 351nm laser

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/cm2. 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.

Analysis of hot images in final optics assembly

Optical components are often damaged by hot images in high power laser system, especially for the final optics assembly. There are several nonlinear optical elements and a focusing lens. So both the hot images in free space propagation and in the focusing system are theoretically and numerically studied. We find that the focusing lens moves the hot images towards the lens. Through Fresnel number, the connection of hot image position in free space propagation and in the focusing system is discussed. What’s more, the nonlinear effect of the focusing lens is also considered for the hot images formation because the lens is sometimes very thick. At last, the influence of the size and modulation depth of scatter on the hot images position and intensity are also given. The hot images analysis is essential for the final optics assembly design, which can shed some light on avoiding the optical damage.

Enhancement of light intensity related to distribution of defects in the final optics assembly

Laser induced damage in the final optics assembly is one of the bottleneck problems in high power laser systems for the inertial confinement fusion. Defects on the optical elements can cause optical intensity intensification and therefore damage the optical elements in the downstream. However, only single defect is considered for most cases. In this paper, physical models are established to study enhancement of light intensity related to distribution of defects in the final optics assembly. Results show that, when the distance of two localized defects reduces to a certain distance, there will be a stronger light intensity intensification duo to the interference effect. What’s more, it will be much more serious when the nonlinear effect is taken into consideration. Meanwhile, the interaction of two kinds of different defects are also studied, i.e., the periodic defect and the localized defect. The optical field will be enhanced to a certain extent at the overlapped area. Thus, we can see that single defect may not cause optical damage. But when there are more than one defect with a certain distribution, light field may be further enhanced, thus damaging the optical element. As a conclusion, the distribution of defects also needs strict constraints. The results could give some references to the mitigation of damage caused by defects in the final optics assembly.

Mitigation of BSG damage caused by upstream flaw in 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.

Off-axis focusing and imaging of scaled zone plates and anamorphic photon sieves

Zone plates and photon sieves can be used to focus soft X-rays and hard X-rays. Relative to the parallel plane wave incidence and focusing on the optical axis, we here present two different models to describe the other kinds of focusing properties. The former, the scaled zone plates or photon sieves are appropriate for the titled plane wave to image, which can alter the propagation direction. The latter, the eccentric elliptical zone plates or photon sieves are appropriate for the point-to-point off-axis focusing. Based on the above-mentioned models, the different algorithms are discussed in detail under the condition of different numerical apertures. Furthermore, the correctness of our model has been verified through the commercial software VirtualLAB. The obtained results can be used for the analysis, design, and simulation of different zone plates and photon sieves, meanwhile the non-coaxial characteristics can increase the flexibility of the optical system.

Temporal characteristic analysis of stepped pulse on hohlraum wall

Temporal and spatial optical field distributions of a stepped pulse on the hohlraum wall are numerically analyzed using fast-Fourier-transform method combined with chromatographic theory. Changes in the rising edge of the stepped pulse caused by spatio-temporal effect are systematically studied. Results demonstrate that a certain time difference exists for the laser to reach the hohlraum wall because of oblique incidence, which causes distortion to the stepped pulse, especially to the rising edge. Influences of the incident angle, focal length, and width of the input rising edge on distortion degree of the stepped pulse are also discussed. This letter provides some guidance for targeting pulse shaping and synchronization.