中文摘要：

研究一个具有两个靠得足够近激发能级的双Lambda模型,在矩形脉冲中分离出光学前驱动场.当耦合场是大失谐时,该原子系统在自发辐射相干效应的影响下产生一个透明窗口并伴随着陡峭的色散曲线.因此,由于透明窗口中的慢光效应可以使光学前驱动场从主脉冲场中分离出来.另外,我们通过调控脉冲波形序列研究探测场相位和幅值的累积光学前驱动波.数值结果表明：由于累积光前驱动场与脉冲主场的相长干涉大大地提高瞬态脉冲的能量.

英文摘要：

Optical precursors were first studied by Sommerfield and Brillouin in 1914 to resolve the apparent contradictions between fast light propagation and the theory of relativity.They showed theoretically that the front edge of a stepmodulated pulse does not interact with the medium and always travels at c because the dispersive material has a finite response time to the optical pulse.The past experimental studies of precursors in classical pulse propagation were always focused on an opaque medium with single or multiple Lorentz absorption lines.In these cases,the precursor signal cannot be separated from the main pulse or otherwise the main field is absorbed.However,the electromagnetically induced transparency（EIT）technique was successfully used to separate precursors from the main pulse due to the slow-light effect in cold atoms.The EIT refers to the absorption suppression or elimination of a probe field through atomic coherence in a certain medium dressed by a strong coupling field.In this paper,a four-level double-lambda atomic system with two upper states coupled to the excited state is explored to separate optical precursors from a square-modulated laser pulse with the effect of spontaneously generated coherence（SGC）.The SGC effect occurs in the process of spontaneous emission,in which the atom decays from closely placed upper levels to a single ground level.The quantum interference between the decay channels takes place,which leads to decay induced transparency,thus enhancing the Kerr nonlinearity and amplification without inversion.With the assistance of spontaneously generated coherence,an EIT window appears with steep normal dispersion when the trigger field is far from resonance.Then we can obtain the optical precursors which are separated from the main pulse due to the slow-light effects in the EIT window.In the absence of SGC,the main pulse is absorbed by an opaque medium with Lorentz absorptive lines,so the slow-light effect could not take place.In addition,we obtain the stacked optical precursor