Nonlinear theory of lasers without inversion with two upper lasing levels

The nonlinear equation of motion for the laser field is obtained by using a perturbation method up to third order. The linear gain and intensity of the laser field are obtained. A perfect lasing without inversion can be achieved when ${\mathrm{\delta }}_3$ ${\mathrm{\mu }}_2$ √${\mathrm{\Gamma }}_2$ =-${\mathrm{\delta }}_2$ ${\mathrm{\mu }}_3$ √${\mathrm{\Gamma }}_3$ .