Study on nonlinear effects in propagation of frequency-chirped laser pulse in atomic laser isotope separation

在铀的原子蒸气激光同位素分离中，激光通过铀原子蒸气远距离传输，与U^&lt；235&gt；N发生共振反应，也与U^&lt；238&gt；N发生近共振反应。在传输过程中，近谐振相互作用会改变激光脉冲的形状和载频。我们开发了一个同时处理原子激发动力学和激光传输的模拟程序CEALIS-P，并从数值和实验上研究了近共振相互作用对分离特性的影响。1)在近谐振相互作用中，脉冲形状的改变和频率啁啾激光脉冲的传输速度很大程度上取决于扫频方向。在铀浓缩系统中，正频啁啾激光脉冲脉宽被压缩，传输速度降低。2)分析了正频啁啾激光脉冲与扫频宽度、漏率、入射激光强度和原子密度的依赖关系，讨论了获得高效激发和高浓缩的最佳激光条件。3)实验测量了在Sm原子蒸气中，由于近谐振效应引起的频率啁啾脉冲的脉冲压缩和脉冲延迟。实验和模拟之间的良好一致性导致了模拟在激光浓缩铀装置设计中的有效性。

In atomic vapor laser isotope separation of uranium, the laser beams propagate through the uranium atomic vapor for a long distance in reacting with U^<235>N on resonance and also U^<238>N in near-resonance. The near-resonant interaction will modify the laser pulse shape and the carrier frequency during the propagation. We have developed a simulation code named CEALIS-P which treats the atomic excitation dynamics and the laser beam propagation simultaneously and investigated the effect of near-resonant interaction on the separation characteristics numerically and experimentally. The following results have been obtained successfully.1)In the near-resonant interaction, the modification of pulse shape and the propagation velocity of frequency chirped laser pulse greatly depend on the direction of frequency sweep. In uranium enrichment system, the positively frequency chirped laser pulse in compressed in pulse width and is decreased in propagation velocity.2)The dependences of these modification on the frequency sweep width, the weep rate, the incident laser intensity and the atomic density were analyzed and the optimum laser condition were discussed to obtain the efficient excitation and high enrichment.3)In samarium atomic vapor the pulse compression and the pulse delay of frequency chirped pulse due to near-resonant effects have been measured experimentally. Good agreements between the experiments and simulations lead the usefulness of simulation for the design of uranium enrichment plant by lasers.