WePolarizationof laser resonators will now use the differential Eq. (9.44) that describes the amplification of the intensity inside the active medium to derive the output power of stable resonators. In the resonator model used (Fig. 10.1) it is assumed that both the forward traveling beam with intensity I+(z) and the backward traveling beam with intensity I−(z) cover the same volume of the active medium. The complete overlap of the two counter-propagating beams is characteristic for stable resonators. During a round trip, the intensity is decreased due to diffraction lossesDiffraction losses (loss factors V1 – V4), scattering, and absorptionLossesabsorption inside the medium (loss factor VS), and by the transmission of the output coupling mirror. In steady state operation, these losses are compensated by the amplification process which is characterized by the small-signal gain coefficient g0.