The interaction of the laser radiaion with the active amplifying medium

After a short review of the tendency in the high power laser pulses and the limitations due to self-damage and nonlinear energy saturation effects in solid state amplifying media, the general equations of the two level laser amplifier, in the electric dipole transition approximation, are presented. The generalization in the three level approximation takes into account the lifetime of the lower laser level, for the four level laser media. (author)