In high-power laser systems, hot image is a phenomenon of drastic intensification on the downstream light field due to nonlinear Kerr self-focusing, which poses a threat to the safety of optical components. It is traditionally believed that a single defect will trigger a hot image, which appears at the downstream conjugate position of the defect relative to the nonlinear medium. Our previous study has shown that a phase defect can cause double-peak hot images when considering the defect edge steepness. When the number of defects increases, the downstream modulation becomes more complex. Here the evolution of the hot-image is analyzed when there are two symmetrical distributed super-Gaussian defects around the beam center for example. The results show that double defects will form five modulation peaks. The second and fourth peaks are the results of two single defects. Another three peaks are in the center of the beam, which do not appear in the case of single defect. The linear modulation peak is generated by the interaction of diffraction fringes from double defects and finally forms the first peak. The second and third strong peaks are caused by the interference between the second and fourth peaks, respectively. The characteristics of these three peaks are affected by the center spacing and size. The existence of these new strong modulation regions will pose a potential damage threat to the optical components at this position, so they need to be avoided when designing the arrangement of the components.
Phase imaging is important in biomedical, astronomy, defect detection, wavefront sensing and so on. Relative to amplitude, phase carries more information and is more difficult to detect. Most of traditional phase imaging methods are based on linear transmission, which high frequency information is severely lost. This greatly affects the resolution and contrast of imaging. Here we demonstrate an iterative phase imaging method based on nonlinear hot image. Hot image is a nonlinear holographic phenomenon due to the Kerr effect. It can transform weak phase to strong amplitude. The nonlinear feedback of hot image improves the response to high frequency information thus improves the imaging resolution. This method provides new directions to precise phase imaging.
Laser induced damage in the final optics system is one of the bottleneck problems in the high-power laser system. For almost all the eight beams of final optics assembly (FOA), there are usually damages in the middle areas of the focusing lens in SG-II laser facility. Through detailed analysis, we find a correlation between the damage in the focusing lens and defect in the continuous phase plate (CPP). The main mechanism for downstream damage is regarded as the defect induced light intensification. The phase distribution of the CPP is characterized by coherent diffraction imaging method. Through simulation, we can clearly see a much stronger light intensification in the middle of the beam caused by the real CPP than the theoretical designed CPP. Compared with the designed CPP, there are defects on the CPP causing the downstream intensification. Meanwhile, there are unexpected periodic modulation on the surface of CPP. We assume the central defect is a kind of laser induced defect because the defects caused by the optical processing are randomly distributed. Through ray tracing analysis, we find a ghost image near the center of the CPP position. So the CPP is slightly damaged or modified in the middle area of the ghost image ray, thus forming a defect with strong modulation. A stray light management is proposed base on ground glass to mitigate the ghost image problem. The periodic modulation is possibly formed by the manufacturing process of CPP. Small-period modulation can cause greater downstream modulation. It should be controlled with power spectral density specification in the manufacturing process. Once the laser induced defect problem is solved, the laser induced damage in the middle of focusing lens is greatly mitigated.
In this paper, a new mechanism of hot image formation introduced in frequency conversion process is introduced. Under the effect of second order nonlinear effect, defects can also cause downstream hot image, where the peak light intensity can reach several times of the average light intensity. The corresponding physical model was established, and the physical process and related laws of the upstream defect induced optical field enhancement under the second order nonlinear action were studied by theoretical analysis and numerical simulation. It is found that three wavelengths of laser will form corresponding hot images at different positions downstream of the nonlinear crystal. The effects of defect size and conversion efficiency on hot image strength are further studied. Finally it is found that the generation of hot image of triple frequency can be effectively inhibited by increasing the spacing between two crystals. This research can provide an important reference for beam quality control of final optical systems of high power laser deriver.
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