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.
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.
Deformation of the large aperture mirror caused by the external environment load seriously affects the optical performance of the optical system, and there is a limit to develop the shape quality of large aperture mirror with traditional mounting method. It is effective way to reduce the optical mirror distortion with active support method, and the structural-optical integrated method is the effective means to assess the merits of the mounting for large aperture mirror. Firstly, we proposes a new support scheme that uses specific boundary constraints on the large lens edges and imposes flexible torque to resist deformation induced by gravity to improve surface quantity of large aperture mirror. We calculate distortion of the large aperture mirror at the edges of the flexible torque respectively with the finite element method; secondly, we extract distortion value within clear aperture of the mirror with MATLAB, solve the corresponding Zernike polynomial coefficients; lastly, we obtain the peak-valley value (PV) and root mean square value (RMS) with optical-structural integrated analysis . The results for the 690x400x100mm mirror show that PV and RMS values within the clear aperture with 0.4MPa torques than the case without applying a flexible torque reduces 82.7% and 72.9% respectively. The active mounting on the edge of the large aperture mirror can greatly improve the surface quality of the large aperture mirror.
In high power laser facility, contaminants on optics surfaces reduce damage resistance of optical elements and then decrease their lifetime. By damage test experiments, laser damage induced by typical metal particles such as stainless steel 304 is studied. Optics samples with metal particles of different sizes on surfaces are prepared artificially based on the file and sieve. Damage test is implemented in air using a 1-on-1 mode. Results show that damage morphology and mechanism caused by particulate contamination on the incident and exit surfaces are quite different. Contaminants on the incident surface absorb laser energy and generate high temperature plasma during laser irradiation which can ablate optical surface. Metal particles melt and then the molten nano-particles redeposit around the initial particles. Central region of the damaged area bears the same outline as the initial particle because of the shielding effect. However, particles on the exit surface absorb a mass of energy, generate plasma and splash lots of smaller particles, only a few of them redeposit at the particle coverage area on the exit surface. Most of the laser energy is deposited at the interface of the metal particle and the sample surface, and thus damage size on the exit surface is larger than that on the incident surface. The areas covered by the metal particle are strongly damaged. And the damage sites are more serious than that on the incident surface. Besides damage phenomenon also depends on coating and substrate materials.
The deformation of KDP crystals caused by gravity and mounting force can induce the phase mismatch, resulting in the decrease of frequency conversion efficiency. Loading strips are used to reduce the distortion in some existing methods, but it is difficult to fabricate. In order to improve the surface shapes of KDP crystals, small loading plates are used instead of loading strips. The mounting configuration is analyzed by finite element methods (FEM) and the position of the loading plates is optimized by adaptive single-objective algorithm. The results show the effectiveness of the mounting configuration in reducing the gravitational sag of KDP crystals.
Approximate formulas of transient temperature and stress distributions in the slab of a two-sided pumped heat capacity
laser (HCL) were attained by solving the heat diffusion equation. The thermal effects in the slab HCL were taken into
consideration. By finite element analysis, the transient temperature and stress distributions in the slab medium were
simulated with various boundary conditions, and the ANSYS simulations showed that when the pumping beam aperture
was less than the pumping area, the peak thermal stress of the slab would increase to 80MPa. Then a new method for
reducing thermal gradients between the outer surface of the slab and the internal portions in the slab was proposed, that
was the temperature of the outer surfaces of the slab to be adjusted during the lasing stage for a HCL. This method is
helpful to reduce the thermal peak stress to 40MPa. It not only provided a compensation for thermal gradients resulting
from the limitation of the beam aperture during the pumping stage, but also further improved the output power and the
beam quality. Our simulations are meaningful for designing a SSHCL especially the thermal management system in a
Wavefront distortion induced by structure design must be minished to raise the quality of output beam in ICF facility.
The support system of large octagonal Nd:glass in main amplifiers of SG-II is optimized with finite element analysis
software ANSYS, and wavefront distortion in aperture of amplifier is calculated with Zernike polynomials fitting in
different parameters combination. The transmission wavefront distortion induced by optimal support system is less than
tenth wavelength and meets the requirement of system.
This paper proposes a new digital method to compensate for the aberration of electron-objective lens in electron holography. In this method, the object wavefront in the exit pupil plane is numerically reconstructed from digitized electron hologram, and is corrected by multiplying it with the conjugate aberration function of the electron-objective lens. Then, aberration-free image can be obtained by calculating the Fresnel integral of this corrected wavefront. In comparison with traditional methods, this method is much more convenient and accurate.