I am studying laser physics at the moment and there is something that confuses me. The laser output consists of the central laser mode and some axial modes due to constructive interference in the optical cavity. At first there is a high enough population inversion and while the laser works this population inversion decreases until a threshold is reached defined by internal losses and the losses due to the mirrors. So at the steady state the relation g(v)= g_th holds , where g(v) is the gain curve of the laser. If the initial gain is decreased due to a decrease of the population inversion then at some point g(vo)=g_th, where vo is the central frequency. Shouldn't the other modes move under the gain threshold due to the gain decrease and thus disappear with time? If that's not true then the gain curve must become flatter when continues wave (CW) operation is reached. Thanks for your time.