增益(损耗)介质中高功率激光束的小尺度自聚焦理论研究

将Bespalov-Talanov理论推广到介质有增益(损耗)的情形，揭示了在介质有增益或损耗从而 光强随传输距离变化的情况下小尺度自聚焦的特性.主要是在一定的输入功率下，小尺度调 制增长的临界频率、最快增长频率及其对应的最大增益(B积分)均随介质增益或传输距离的 增大而增大；对于确定的输出功率，随介质增益的提高，B积分值变小，同时最快增长频率 向低频方向移动.利用局部能量守恒定律唯象地研究了增益和损耗对激光束成丝的影响，发 现增益使成丝距离延长而损耗使成丝距离缩短.讨论了介质增益和损耗对高功率激光放大器 设计的影响.尽量使激光器在小输入高增益状态下运行，可以从某种程度上抑制因小尺度自 聚焦而导致的介质破坏效应. 关键词： 自聚焦 成丝 强激光传输

THEORY OF SMALL-SCALE SELF-FOCUSING OF INTENSE LASER BEAMS IN MEDIA WITH GAIN AN D LOSS

The Bespalov-Talanov theory is extended to include the effects of gain and loss of the medium. An analytic expression for the growth rate of the ripples is obta ined and it reveals some new properties of small-scale self-focusing. For a give n input power, the cutoff spatial frequency, the fastest growing frequency and t he maximum perturbation growth, in an amplifying medium, increase with the incre ase of the gain of the medium and propagation distance. For a given output power , as the medium gain increases, the maximum perturbation growth, and thus the B- integral, decreases, while the fastest growing frequency shifts to lower frequen cy. Also, the expression for the distance at which the filaments are formed is o btained by means of a phenomenological approach, and it is found that the gain ( loss) of the medium extends (reduces) the distance. It is thus concluded that it is an effective way to suppress the small-scale self-focusing by increasing the gain of the medium.