对B积分(自聚焦)的有效抑制和控制是高能激光系统发展过程中的一个焦点问题.传统的B积分控制技术(空间滤波器)在一定程度上提高了光束质量,但是随着激光功率的不断增大,空间滤波技术受到诸多因素限制,已经不能满足要求,因此还需发展新的B积分控制技术.基于非线性倍频过程的二阶级联非线性可等效于三阶非线性,它的符号可控(允许为负),量值可变且很大,并可消除双光子吸收.级联非线性实际上提供了一个新的自由度,它可以产生有用的非线性相移.初步实验表明,利用级联非线性产生的负非线性相移,在一定程度上可以补偿强激光束在非线性传输过程中积累的B积分,具有很好的应用前景.

It is a crucial problem in the field of high energy lasers to control Β integral (i.e. self-focusing) and to restrain it effectively. As conventional control techniques of Β integral (e.g. spatial filtering) nonetheless are far from solving the problem, novel techniques need to be developed. Based on nonlinear frequency doubling the cascaded second order nonlinearity, virtually equivalent to the third order nonlinearity, has a controllable sign (allowing of negative values), variable values that can be veiy large, and can eliminate two-photon absorption. The cascade nonlinearity provides a new degree of freedom for short-pulse sources to produce serviceable nonlinear phase shifts. The preliminary experiments show that using these negative phase shifts can to a certain extent compensate for the cumulative Β integral due to the nonlinear propagation of high power laser (pulse) beams.