Recently, a research team led by Prof. Zhu Jianqiang and Associate Researcher Jiao Zhaoyang from the Joint Laboratory on High-Power Laser Physics at the Shanghai Institute of Optics and Fine Mechanics (SIOM), Chinese Academy of Sciences, has made significant progress in understanding the nonlinear hot image effect induced by the cascade of multiple nonlinear processes in high-power laser facilities. The related findings were published in Optics Express under the title "Formation and mechanism of multiple peak hot images induced by the cascade effect of nonlinear frequency conversion and self-focusing."
Previous research has shown that an upstream defect can generate a single hot image at the conjugate plane downstream in a self-focusing medium. Under conditions involving cascaded multiple media, complex multi-peak hot images can even emerge. The intensity of a hot image can reach several times the average beam intensity, making it a key factor limiting the output capability of high-power lasers. Understanding the underlying principles of hot images is crucial for the overall design of laser drivers. However, the evolution mechanisms of hot images resulting from the combined effects of self-focusing and nonlinear frequency conversion processes within the final optics assembly remain unclear.
Addressing this challenge, the team discovered a novel phenomenon of dual-peak hot images induced by the cascade effect of nonlinear frequency conversion and self-focusing. Significantly, they uncovered a new principle where beam wavelength conversion causes a rearward shift of the hot image location. They also established an analytical formula for the axial position distribution of these dual-peak hot images. The first peak generated under cascade effects originates from the frequency conversion process, while the second peak is primarily caused by self-focusing. Furthermore, the team observed a new phenomenon: within a certain range under cascade conditions, the hot image intensity remains almost constant regardless of changes in the object distance. This finding suggests that the traditional strategy of increasing the spacing between optical components to suppress hot images may become ineffective. This work provides, for the first time, a clear explanation of the formation mechanism and evolution principles of hot images under cascaded complex nonlinear processes in final optics systems. It offers valuable insights for optimizing the design and layout of optical components in these systems,reducing component damage,and is of significant importance for enhancing the load capacity of high-power laser facilities.
Figure 1. Optical path model of dual-peak hot images under cascade effects.
Figure 2. Evolution of the transverse optical field downstream of the crystal with propagation distance. The red dashed line indicates the position of the hot image plane. (a) With cascade effects;(b) When considering the frequency conversion process exclusively.
Figure 3. Dependence of hot image characteristics on object distance. (a) Hot image location;(b) Hot image intensity.
This research received support from the National Natural Science Foundation of China,the CAS Strategic Priority Research Program,and other projects.
