针对拉曼驱动场下的四能级理论模型，讨论了实现无吸收、高折射率的条件，并分析了探测场及相干激发场的线宽对介质的折射率及增益的影响。计算结果表明，改变相干激发场的场强，或改变相干场的频率与原子能级间的失谐均可获得介质的无吸收、高折射率。激发场线宽破坏了原子的相干性，使介质的增益下降并改变了折射率的大小，探测场的线宽使介质的探测增益谱及折射率谱的分辨率下降。

The conditions of the maximum refractive index with vanishing absorption and the effect of the finite bandidths of the probe and driving field on the index of refraction and the gain are analyzed based on a Raman driven four level system. The calculated results show that the maximum index of refraction with vanishing absorption can be achieved by changing the detuning between the frewuency of the driving field and the resonant atomic transition or by changing the intensity of the driving field. The finite bandwidths of the driving and the probe field decrease the gain and change the value of the index of refraction, especially of the driving field.