Dependence of Electromagnetically Induced Transparency on Laser Linewidth

The influence of the linewidth of coupling laser on the electromagnetically induced transparency (EIT) spectral width is theoretically investigated. The model to describe the EIT spectral width is based on the standard semi-classical theory. The result shows that the effect of the linewidth of coupling laser is equivalent to an additional relaxation between two ground states in the Λ-type configuration. A broadening linewidth of coupling laser implies the increasing relaxation between the two ground states, which will make the wider EIT spectral linewidth.