We incorporate neural networks into the optical design of off-axis three-mirror reflective system, enabling us to achieve design outcomes without relying on iteration or ray tracing methods. Our approach involves combining analytical relations with neural networks during the design process, which yields results covering the entire parameter space with a single user input, and each design is scored simultaneously. Our results demonstrate that neural networks can simulate the complex relationship between performance requirements and structural parameters of an optical system. As such, the structural parameters can be directly obtained from the performance requirements, replacing the iterative optimization process traditionally used. This approach leads to relatively efficient and straightforward optical design. We anticipate that this method can be extended to various optical systems, reducing the experience threshold and difficulty of optical design.
We presented a novel scheme to improve the stability of the orbital angular momentum (OAM) modes transmission by adding a dip at the edge of the annular high-index region of the air-core fiber. The simulation indicated a larger effective index difference of the vector modes that composed OAM modes in the same order, promising a stable transmission of the OAM modes. The intensity of the modes was concentrated better in this scheme decreasing the crosstalk between adjacent fibers. The propagation properties of the OAM modes in bent fiber were investigated.
The polarization smoothing (PS) of the focal spot on target is a key technology for inertial confinement fusion (ICF) laser. A mathematical model is presented to analyze the polarization smoothing in a convergent beam. The relation between the separations (both transverse and longitudinal) of focal spots and the parameters of the crystal are established. Via numerical simulation, the three-dimensional distributions of the far-field with and without PS are demonstrated. The relation between the property of the focal spot and the crystal’s thickness and tilt angle are obtained. Best smoothing can be achieved with the optimized thickness and tilt angle of the PS crystal.
The laser pulse should be shaped to satisfy the ICF physical requirement and the profile should be flattened to increase the extraction efficiency of the disk amplifiers and to ensure system safety in ICF laser facility. The spatial-temporal distribution of the laser pulse is affected by the gain saturation, uniformity gain profile of the amplifiers, and the frequency conversion process. The pulse spatial-temporal distribution can’t be described by simply analytic expression, so new iteration algorithms are needed. We propose new inversion method and iteration algorithms in this paper. All of these algorithms have been integrated in SG99 software and the validity has been demonstrated. The result could guide the design of the ICF laser facility in the future.
Physical model was established to describe the pulse superposition in multi-pass amplification process when the pulse reflected from the cavity mirror and the front and the end of the pulse encountered. Theoretical analysis indicates that pulse superposition will consume more inversion population than that consumed without superposition. The standing wave field will be formed when the front and the end of the pulse is coherent overlapped. The inversion population density is spatial hole-burning by the standing wave field. The pulse gain and pulse are affected by superposition. Based on this physical model, three conditions, without superposition, coherent superposition and incoherent superposition were compared. This study will give instructions for high power solid laser design.
Multi-FM SSD and CPP was experimentally studied in high fluence and will be equipped on all the beams of SG-III laser facility. The output spectrum of the cascade phase modulators are stable and the residual amplitude modulation is small. FM-to-AM effect caused by free-space propagation after using smoothing by spectral dispersion is theoretically analyzed. Results indicate inserting a dispersion grating in places with larger beam aperture could alleviate the FM-to- AM effect, suggesting minimizing free-space propagation and adopting image relay. Experiments taken on SG-III laser facility indicate when the number of color cycles (Nc) adopts 1, imposing of SSD with 3.3 times diffraction limit (TDL) did not lead to pinhole closure in the spatial filters of the preamplifier and main amplifier with 30-TDL pinhole size. The nonuniformity of the focal spot using Multi-FM SSD and CPP drops to 0.26, comparing to 0.84 only using CPP. The experiments solve some key technical problems using SSD and CPP on SG-III laser facility, and provide a flexible platform for laser-plasma interaction experiments. Combined beam smoothing and polarization smoothing are also analyzed. Simulation results indicate through adjusting dispersion directions of one-dimensional SSD beams in a quad, two-dimensional SSD could be obtained. The near field and far field properties of beams using polarization smoothing were also studied, including birefringent wedge and polarization control plate (PCP). By using PCP, cylindrical vector beams could be obtained. New solutions will be provided to solve the LPI problem encountered in indirect drive laser fusion.
optical propagation simulation by SG99 code and invert algorithm has been made for two typical laser architecture,
namely the National Ignition Facility (model A) and SG-III laser facility (model B) based on measured 400mm aperture
Nd:glass slab gain distribution data on ITB system. When the gain nonuniformity is about 5%, 7%, and 9% respectively
within 395x395mm2 aperture and output beam aperture is 360x360mm2, and output energy is about 16kJ/5ns(square)
with B-integral limited, 1ω(1053nm) nearfield modulation is about 1.10, 1.15, and 1.30 respectively for model A (11+7
slab configuration), and 1.07, 1.08, and 1.17 respectively for model B (9+9 slab configuration) without spatial gain
compensation. With the above three gain nonuniformity and slab configuration unchanged, to achieve flat-in-top output
near field, the compensation depth of the input near field is about 1.5:1, 2.0:1, and 6.0:1 respectively for model A, and
1.3:1, 1.4:1, and 3.5:1 respectively for model B. Compared with model A (the beam aperture unchanged in multi-pass
amplification), the influence of slab gain nonuniformity on model B (beam aperture changed) is smaller. All the above
simulation results deserve further experiment study in the future.
This paper is focused on the research of SSD and CPP carried out on TIL. A bulk phase modulator with 9.2-GHz modulation frequency is adopted in SSD. The output spectrum of the phase modulator is stable and the residual amplitude modulation is small. FM-to-AM effect caused by free-space propagation after using smoothing by spectral dispersion is theoretically and experimentally studied. Results indicate inserting a dispersion grating in places with larger beam aperture alleviates the FM-to-AM effect, suggesting minimizing free-space propagation and adopting image relay. Experiments indicate when the number of color cycles (Nc) adopts 1, imposing of SSD with 4.26 times diffraction limit (TDL) did not lead to pinhole closure in the spatial filters of the preamplifier with 20 TDL and main amplifier with 26 TDL. Experimental results also indicate SSD didn’t influence the load capacity of the laser facility. The contrast of the 440-μm diameter focal spot with 95% energy included using SSD and CPP drops to 0.47, comparing to 1.71 not using SSD and CPP. When the pulse width of the third harmonic wave is 1 ns and the energy is 1115 J, no damage is found in CPP and other final optics. The experiments solves some key technical problems using SSD and CPP on high-power laser facilities, and provides a flexible platform for the laser-plasma interaction experiments.
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