The generation of single-mode sub-Poissonian light by lasers is studied. A general master equation is presented, which can be applied to any stationary atomic injection and allows for systems of finite cavity lifetime. The Mandel factor for the output photon number is derived. It is pointed out that on the basis of pump-noise-suppression, the fluctuation of the output photon number can be reduced further by shortening the cavity lifetime as well as suppressing the spontaneous emission.pump-noise, cavity lifetime as well as suppressing the spontaneous emission.