Complex Nonlinear Propagation Effect of Modulated Beams in High-Power Laser Systems
Objective Currently,the development of high-power laser drivers is limited by the damage to terminal optical system components.Furthermore,self-focusing is an important nonlinear propagation effect that causes damage to components.As a special self-focusing phenomenon,a hot image,with extremely high intensity of light field,typically appears in the downstream conjugate position of the upstream defect relative to the nonlinear medium.Therefore,in the design of system,the nonlinear propagation effect is usually controlled by regulating the B integral and arranging the optical elements to avoid the conjugate plane of the hot image.This in turn reduces the damage threat to the components.However,in the operation experiment of the laser drivers,it is determined that the position and intensity of certain light field strength areas deviate from the existing theory.Hence,unexpected damage often occurs in the components.Therefore,laws and mechanisms relating to nonlinear propagation effects of high-power lasers should be further examined.Methods The formation of hot image can be divided into three processes.First,the beam modulated by defects reaches the nonlinear medium via linear propagation.Second,when the beam propagates in the nonlinear medium,the third-order nonlinear polarization of the medium is induced,and the beam gradually converges.Third,the beam propagates linearly after ejecting from the medium,and it finally converges to form hot images.Based on the thin slice approximation,the analytical expressions of the position and intensity of the hot image can be obtained by solving the nonlinear Schrodinger equation.However,in practical high-power laser systems,nonlinear media usually do not satisfy the condition of slice approximation.In this study,the angular spectrum method for the linear diffraction propagation and the split-step Fourier algorithm for the nonlinear propagation were used to explore the evolution of the hot image characteristics.Results and Discussions For the single defect,when considering the edge steepness,a single hot image caused by a single defect is divided into double-peak hot images located in front of and behind the conjugate plane(Fig.2).This breaks the longitudinal correspondence between the defect and hot image and can cause accidental damage to the optical elements downstream of the conjugate plane.The modulation degrees of double-peak hot images exceed that of single hot image,and the positions are affected by the size of defects(Fig.4).For double defects,when they are very close to each other in the same plane,two additional hot image peaks appear before and after the double-peak hot images.Specifically,both peaks are located in the central axis direction of the double defects(Fig.7).The positions and modulation degrees of these two peaks are affected by size,center spacing,and modulation depth(Fig.9).The existence of the new peaks indicates that defects and hot images no longer correspond in the transverse direction.When double defects are located in different planes,the coaxial double defects can produce an emphasis peak formed by the superposition of double-peak hot images(Fig.12).Furthermore,when the transverse distance ofdefects increases,thedownstream modulation decreases rapidly(Fig.14).Conclusions A systematic study is conducted on the deviation between the observed positions and intensities of hot images in high-power laser devices when compared to existing theories.The results show that fine morphology differences and mutual interference of the upstream defects can further enhance the optical field's intensity area and intensity downstream.This disrupts the correspondence between the defects and hot images in the horizontal and vertical directions,undermining the effectiveness of nonlinear control strategies for laser drivers that rely the original B-integral and hot image cognition.The results of this study deepen the understanding of the propagation law of complex nonlinear optical field.This understanding not only aids in optimizing the arrangement of terminal components in the system,but also highlights new requirements for the precise detection and evaluation of defects of large-aperture optical components in the high-power laser devices.Furthermore,this provides an important technical reference for improving the load capacity of the final optics system in the high-power laser drivers.
high power laser drivernonlinear propagation effectdefectoptical field modulationself-focusing
万方数据
维普
