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

在铀原子蒸气激光同位素分离中，激光束在铀原子蒸气中传播很长一段距离，与U <235>^N发生共振<238>和近共振反应。近共振相互作用将改变激光脉冲的形状和传输过程中的载波频率。我们开发了一个名为CEALIS-P的模拟程序，它同时处理原子激发动力学和激光束传输，并研究了近共振相互作用对分离特性的影响。结果表明：1）在近共振相互作用中，频率啁啾激光脉冲的传播速度和脉冲形状的改变很大程度上取决于频率扫描的方向。在铀浓缩系统中，正频率啁啾激光脉冲的脉宽被压缩，传播速度被降低。2）这些变化与扫频宽度、扫频速率、分析了入射激光强度和原子密度的变化规律，讨论了获得高效激发和高富集的最佳激光条件在钐原子蒸气中，实验测量了近共振效应对频率啁啾脉冲的压缩和延迟。模拟结果与实验结果吻合较好，为激光铀浓缩装置的设计提供了有效的模拟方法。

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.