Controllable cavity linewidth narrowing via spontaneously generated coherence in a four level atomic system

A scheme for cavity linewidth narrowing in a four-level atomic system with spontaneously generated coherence is proposed. The atomic system consists of three closely spaced excited levels, which decay to one common ground level. In such a system, spontaneously generated coherence can result in the appearance of two narrow transparency windows accomplished by steep normal dispersion. When the medium is embedded in a ring cavity, two ultranarrow transmission peaks locating close to the position of the transparency windows can be obtained simultaneously. The cavity linewidth narrowing is owing to the quantum interference between the three decay channels and can be controlled by the frequency splitting of the excited levels, requiring no coupling lasers.