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，它在长脉冲传播距离下具有良好的意义，而传统的群速度定义在长脉冲传播距离下是不成立的，新的定义考虑了折射率虚部的影响。由于Kromas-Kronig关系，虚部的存在是脉冲在谐振区传播的关键。啁啾脉冲由于折射率的虚部引起的光谱移动而表现出大的整形延迟。在非相干脉冲中，无法讨论脉冲速度。在本研究中，我们定义了重心的非相干脉冲的位置，并讨论了传播速度的含义。虽然单个非相干脉冲中的整形延迟是随机出现的，但该延迟是系综平均的，脉冲以净延迟传播。为了验证上述理论预测，我们构建了基于改进型迈克尔逊干涉仪和TiO 2-Safire激光器的实验系统，对飞秒脉冲的传播进行了系统的实验。从这些结果可以看出，新的群速度定义即使在相位调制脉冲中也是有意义的。

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.