Optical pulse propagation in resonantly absorbing medium and a new definition of group velocity

本研究的目的是建立一种新的群速度定义，这种定义即使在传统的群速度定义失效的长脉冲传播距离下也有很好的意义，这种新的定义考虑了折射率虚部的影响。由于Kromas-Kronig关系，虚部不可避免地存在，虚部在脉冲在共振区的传播中起着至关重要的作用。啁啾脉冲由于折射率虚部引起的光谱位移而表现出较大的整形延迟。在非相干脉冲中，没有办法讨论脉冲速度。在本研究中，我们用重力中心来定义非相干脉冲的位置，并讨论了传播速度的意义。虽然各个非相干脉冲中的整形延迟是随机出现的，但该延迟是集合平均的，并且脉冲以净延迟传播。为了验证上述理论预测，我们构建了一个基于改进的迈克尔逊干涉仪和TiO2-Safire激光器的实验系统，并对飞秒脉冲传播进行了系统实验。根据这些结果，我们认为群速度的新定义即使在相位调制脉冲中也有很好的意义。

The purpose of the present research is to establish a new definition of a group velocity : which has a good meaning even in the long pulse propagation distance where the conventional definition of the group velocity breaks down, This new definition takes the effect of the imaginary part of the refractive index into account. The imaginary part, which inevitably exists owing to the Kromas-Kronig relation, is very essential in the pulse propagation in the resonant regions. Chirped pulses show a large Reshaping Delay owing to the spectral shift caused by the imaginary part of the refractive index. In incoherent pulses, there was no way to discuss the pulse velocity. In the present research, we define the position of the incoherent pulses by the centre of gravity, and discussed the meaning of the propagation velocity. While the Reshaping Delay in the individual incoherent pulses appears randomly, this delay is the ensemble averaged and the pulse propagates with a Net Delay.In order to verify the above theoretical predictions, we constructed an experiment system based on a modified Michelson interferometer and a TiO2-Safire laser, and performed systematic experiments on the femtosecond pulse propagation. Form these results, we consider that the new definition of the group velocity has well meaning even in the phase modulated pulses.