When a non- loss laser amplifier is considered, the exact solution of the rate equations can be obtained as Frantz & Nodvik's form, but in lossy case, the equations cannot be solved in closed form. We solved the equations of ruby laser amplifier with linear loss as marching problem of finite difference by a personal computer.
Effect of loss coeffcient γ on the output waveform of amplifier with length L is slightly different from Frantz & Nodvik's solution multiplied by exp (-γL). The computed results are useful in the simulation of nonlinear amplification, when the chromiumconcent-ration of ruby is determined by laser ray method.