Research On Nonlinear Hot Image Effect In Intense Laser Beams

In high-power lasers, the intense laser beam modulated by obscurations, such as contaminations and defects, on the upstream optics leads to hot image to be formed downstream through a nonlinear Kerr medium element, which may be intense enough to damage expensive optics and hinder the safe operation and the performance of lasers. The comprehensive study of the nonlinear hot image effect may greatly help in improving laser beam quality, and thereby improving the safety performance of the high-power laser system, so it is of theoretical significance and of value of practical application. We have investigated nonlinear hot image effect in detail and obtained some innovative results.Firstly, we have investigated hot image effect in an intense laser beam with limited spatial width and through a thick nonlinear medium slab. Previous studies of hot images have been concentrated on the case of an ideal plane wave and based on the thin-medium approximation. However, in the operating lasers, the light beam possesses a limited spatial width, and the nonlinear medium has a thickness additionally. Therefore, it is significant for safely operating lasers to practically reconsider the theories of hot image which are founded on the ideal cases. Taking Gaussian optical beam for an example, we have studied the hot image effect of laser beam with limited spatial width through a thin nonlinear self-focusing medium slab, and disclosed the dependence of the location and intensity of hot image on the spatial width of laser beam, at the same time, given the condition to analyze the nonlinear hot image effect of light beam with limited spatial width utilizing the plane-wave approximation. In addition, we have investigated hot image effect in an intense laser beam through a thick nonlinear medium slab with gain and loss or not. The expression for intensity of hot image through either a thick passive nonlinear medium or a thick active nonlinear medium is derived without thin-medium approximation. Further analysis demonstrated that size of the obscuration which leads to the most intense hot image is approximately equal to that of the efficient fastest growing modulation of the theory of small-scale self-focusing. At the same time, the condition for the thin-medium approximation is presented.Secondly, we present a theoretical and numerical investigation on the formation of hot image in an intense laser beam through cascaded Kerr medium disks, giving a clear and reasonable explanation for the curious damage pattern of hot images there and disclosing the variation of the location and intensity of hot image with parameters of the nonlinear mediums and the layout of elements. It is shown that, multiple hot images from an obscuration may be formed, instead of one hot image in the previous literature report. Further analysis demonstrates that, generally, the more nonlinear medium disks, the more hot images, the number of hot images closely depends on the amount of the nonlinear medium disks and the distance from the obscuration to the first disk and the separate space among the disks. The intensity of hot images increases with the B integral imposed by each disk increasing and decreases with the separate space among the disks widening. With the thickness of the nonlinear medium disk, for a give input intensity of the background beam, hot image intensifies, while for a give B integral imposed by each disk, it weakens due to the cutoff growing spatial frequency lowering.Thirdly, hot image through a thin self-defocusing medium and suppression effect of self-defocusing media on hot images through self-focusing media are investigated analytically and numerically. It is well known that hot image formation is a special case of the small-scale self-focusing which takes place only when an intense laser beam propagates through self-focusing media. We demonstrate that hot image may be formed through a thin self-defocusing medium for the first time, due to the creation of a Fresnel zone plates by the intensity-dependent refraction index of the self-defocusing medium acting like holographic medium. This implies self-defocusing media should be carefully used in an intense laser system to avoid damage to other optical components. In addition, provided appropriate parameters and layout, hot images through self-focusing media may be successfully suppressed by self-defocusing media.Finally, we present an experimental investigation on hot image formation in a broadband laser beam through Kerr media. Broadband laser is the direction of the development of the future high-power solid-state lasers. Previous studies of hot image have been concentrated on the monochromic laser beam. For the first time, we experimentally observed the second-order hot image and disclosed that second-order hot images can be formed through a thick or thin nonlinear medium slab. On the other hand, both of the first-order and the second hot image are intensified and eventually saturated with the power of the incident broadband laser beam; And with the thickness of the nonlinear medium increasing, the intensity of hot images increases for the laser beam of a given power, while it decreases for the case of a given B integral;With the pulse width increasing, the intensity of hot images decreases monotonously for the pulse of a given energy, whilst it increases for the pulse with a given peak power. In addition, we took two cascaded nonlinear Kerr medium slabs for example to investigate the hot image effect in brand-band laser beam through cascaded Kerr medium slabs, demonstrating that multiple hot images can be formed from a single obscuration and the intensity of the hot images increases with the input power increasing and decrease with the separate space between the two slabs widening.