Formation and mechanism of multiple peak hot images induced by the cascade effect of nonlinear frequency conversion...

In high-power laser systems, optical component damage in the final optics system remains a critical factor limiting output capability. One of the primary causes of this damage is the nonlinear enhancement of the optical field, particularly the hot-image effect. The hot image effect is formed downstream due to the interaction between diffracted light from a defect and background light in a nonlinear self-focusing medium. Typically, the optical arrangement of the final optics system is optimized to avoid hot images. However, both nonlinear frequency conversion and self-focusing occur in the final optics system, and their combined influence on hot-image formation remains unclear. In this study, we observed a novel phenomenon of double-peak hot images induced by the cascade effect of these nonlinear interactions. The first peak primarily results from frequency conversion, whereas the second peak-an effect overlooked in previous research whose intensity is stronger than that of the first peak-arises due to self-focusing. To analyze the spatial distribution of these hot images, we employed the concept of the equivalent Fresnel number, which provides simple formulas for predicting the peak positions based on the relative distances between the defect, crystal, and medium. Further analysis examined the effects of defect size and modulation depth on hot-image characteristics. The defect size significantly influences both the position and intensity of hot images, whereas modulation depth primarily affects intensity. Notably, when the spacing between the defect and the crystals increases, the intensities of the two hot images remain relatively strong within a certain range-contradicting the conventional belief that hot-image intensity diminishes with increasing distance. This finding suggests that conventional hot-image control strategies in isolated final optics systems may be insufficient. This work enhances the understanding of light-field enhancement in complex nonlinear systems, provides insights for optimizing component arrangements in final optics systems, and contributes to improving system load capacity.